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Conjugated polymers/semiconductor nanocrystals hybrid materials— preparation, electrical transport properties and applications Peter Reiss,* Elsa Couderc, Julia De Girolamo and Adam Pron* Received 14th June 2010, Accepted 6th October 2010 DOI: 10.1039/c0nr00403k This critical review discusses specific preparation and characterization methods applied to hybrid materials consisting of p-conjugated polymers (or oligomers) and semiconductor nanocrystals. These materials are of great importance in the quickly growing field of hybrid organic/inorganic electronics since they can serve as active components of photovoltaic cells, light emitting diodes, photodetectors and other devices. The electronic energy levels of the organic and inorganic components of the hybrid can be tuned individually and thin hybrid films can be processed using low cost solution based techniques. However, the interface between the hybrid components and the morphology of the hybrid directly influences the generation, separation and transport of charge carriers and those parameters are not easy to control. Therefore a large variety of different approaches for assembling the building blocks—conjugated polymers and semiconductor nanocrystals—has been developed. They range from their simple blending through various grafting procedures to methods exploiting specific non-covalent interactions between both components, induced by their tailor-made functionalization. In the first part of this review, we discuss the preparation of the building blocks (nanocrystals and polymers) and the strategies for their assembly into hybrid materials’ thin films. In the second part, we focus on the charge carriers’ generation and their transport within the hybrids. Finally, we summarize the performances of solar cells using conjugated polymer/semiconductor nanocrystals hybrids and give perspectives for future developments.
INAC/SPrAM (UMR 5819 CEA-CNRS-Univ. J. Fourier-Grenoble I), Laboratoire d’Electronique Mol eculaire Organique et Hybride, CEA Grenoble, 17 Rue des Martyrs, 38054 Grenoble Cedex 9, France. E-mail: [email protected]
; [email protected]
; Fax: +33 438 78 51 13
Peter Reiss graduated from the University of Karlsruhe (TH), Germany, in 1997 and received the PhD degree (under the supervision of Dieter Fenske) from the Institute of Inorganic Chemistry in 2000. In the same year, he joined the Commissariat a l0 Energie Atomique (CEA) in Grenoble, France, as a postdoctoral researcher and was hired in the Fundamental Research Department of Peter Reiss Condensed Matter in 2002. Since the end of 2008, he is the Head of the Laboratory of Molecular, Organic and Hybrid Electronics at the Institute of Nanoscience and Cryogenics, CEA-Grenoble. His research focuses on the chemical synthesis, functionalization and application of semiconductor nanocrystals and their hybrids with p-conjugated polymers. 446 | Nanoscale, 2011, 3, 446–489
The fields of conjugated polymers and semiconductor nanocrystals had been developing in parallel, with little overlap and interactions, till the early 1990s. At that time, the publication of the famous paper of Burroughes et al.1 on the electroluminescence of poly(p-phenylene vinylene) (PPV) triggered an
Elsa Couderc obtained her MSc in Physics from the Ecole Normale Sup erieure, Paris, France, in 2008. She is currently a PhD student at the Commissariat a l0 Energie Atomique (CEA) in Grenoble, France in Peter Reiss’s group, where she studies polymer/nanocrystals hybrids materials. Her research interests lie on energy conversion processes and on electrical transport phenomena in disordered materials.
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enormous interest in organic electronics. In the same period a significant progress was made in the preparation of semiconductor nanocrystals of controlled and tunable electronic and spectroscopic properties.2 Very quickly it turned out that a combination of these two components may lead to nanocomposites whose properties are unmatched by conventional materials.3 Since then, several applications of these new hybrid materials in various domains of electronics were developed, for example as active layers in light emitting diodes, photodiodes, and photovoltaic cells, to name a few. In essentially all applications described here conjugated polymers are used in their undoped i.e. semiconducting form. They can also be used in their doped (conducting) form, for example as components of electrodes or hole transporting layers but these applications are beyond the scope of this article. Recently, a few specific reviews have been published on the preparation of conjugated polymer/ nanocrystals hybrid materials.4–7 More general reviews on the synthetic and processing strategies used for the incorporation of these nanoparticles into polymer matrices of various types were also published.8–10 To specify the object of this critical review it is helpful to give the broadest definition of a hybrid since different (and sometimes contradictory) definitions of this term are found in the literature. In the context of this article, a ‘‘hybrid’’ is defined as the association of at least two components of distinctly different chemical nature, whose molecular level distribution is achieved either by simple mixing or by linking both components together via specific interactions. These can involve the formation of covalent, coordination, ionic or hydrogen bonds. Each hybrid component possesses its chemical identity and can exist independently of the hybrid. In conjugated polymers their processability and principal physicochemical properties can be tuned, in a wide range, by their pre- or post-polymerization functionalisation, controlling their macromolecular parameters and inducing appropriate supramolecular organization. In the case of nanocrystals, these
properties can be modified—due to quantum confinement—not only by changing their size, but also by varying the shape, composition and surface ligands. Thus, in hybrids discussed here the properties of their constituents can be tuned individually and adapted to each other. This particularly applies to the positions of the HOMO and LUMO energy levels, which determine their electronic, redox and luminescent properties. Therefore, conjugated polymers/nanocrystals hybrids can hardly be equaled by other systems as far as the design flexibility is concerned. The organization of this article is as follows: The first section reviews the progress in the chemical synthesis of semiconductor nanocrystals. Concerning their specific properties, and in particular the photophysical ones, the interested reader is referred to excellent reviews on this subject.11–15 In addition to the description of binary II–VI (for example CdSe, CdS, CdTe, ZnSe), III–V (InP, InAs) and IV–VI (PbS, PbSe, PbTe) semiconductor nanocrystals, we will also discuss recent examples of ternary and quaternary compounds. The surface functionalization of nanocrystals in view of their use in hybrid materials for (opto-)electronic devices will be presented next. The second section introduces the most important families of n- and p-type semiconducting polymers, their synthesis and electronic properties. Based on these organic and inorganic semiconductors, the third section focuses the preparation of organic/inorganic hybrid materials. We are attempting to categorize the large number of different chemistry and processing approaches into a few general strategies. The characterization of the physicochemical properties of the hybrids is added to this section, with special emphasis on (spectro-)electrochemical techniques. The following, extensive section is dedicated to the description of methods used for the study of the electronic and transport properties of hybrid materials, illustrated with a large number of recent examples. Finally, in the last section the device characteristics of hybrid solar cells will briefly be reviewed and compared to other types of photovoltaic devices.
Julia De Girolamo obtained her PhD degree in chemistry in 2007 from the University Joseph Fourier in Grenoble, France. The same year she started a post-doctoral position at the Centre National de la Recherche Scientifique (CNRS) in Grenoble dedicated to the elaboration of materials for nanoimprint lithography. From 2008 to 2010 she worked on the encapsulation and packaging of Julia De Girolamo OLED displays at the Commissariat a l0 Energie Atomique (CEA) in Grenoble. She was hired in the Laboratory for Innovation in New Energy Technologies and Nanomaterials (LITEN) of CEA in 2010 and her research focuses on the encapsulation and structure of organic electronic devices such as organic LEDs and photodetectors.
Adam Pron was born (1951) and educated in Poland. After obtaining his MSc in chemistry and chemical engineering he moved to the USA where in 1980 he completed his PhD at the University of Pennsylvania (under the supervision of Alan G. MacDiarmid). In the same year he started working at the Warsaw University of Technology where he became full professor in 1993. In 1998 he Adam Pron moved to the Commissariat a l0 Energie Atomique (CEA) in Grenoble (France), where he presently holds the position of research director (directeur de recherches). His research interests involve the preparation and characterisation of organic electroactive materials and hybrid organic–inorganic electroactive nanomaterials.
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2 Chemical synthesis of semiconductor nanocrystals A number of recent reviews deal in detail with the synthesis and physical properties of nanocrystals.16–20 Therefore this section aims at giving a brief, systematic overview of the different applied synthesis strategies for a large variety of materials. We include in particular novel types of ternary and quaternary semiconductor nanocrystals, such as chalcopyrite (CuInE2, E ¼ S, Se) and stannite (Cu2ZnSnE4, E ¼ S, Se) derivates. These compounds currently draw strongly increasing attention due to their low band gap (Eg,bulk # 1.5 eV) and the absence of toxic heavy metals, such as Cd, Pb or Hg.
2.1 Classification of the synthetic approaches We are dividing the solution phase or ‘‘wet-chemical’’ methods for synthesis of semiconductor nanocrystals into three categories, depending on the used reaction medium: (i) room temperature methods in aqueous media, either in the presence of stabilizers or within inverse micelles; (ii) reactions at elevated temperatures in organic solvents of hydrophobic or amphiphilic nature; (iii) synthesis at the oil–water interface. A variant of method (ii) is the solvothermal synthesis, which relies on high autogenous pressures afforded in a sealed autoclave by heating the solvents above their boiling point. This method has been so far of particular interest for the synthesis of group III nitride nanocrystals, whose preparation by conventional methods at standard pressure has not yet been achieved.21–25 Among other reaction media, not discussed here, ionic liquids26 and supercritical fluids27 have been proposed. For a comprehensive review of recent advances in the field of wet-chemical synthesis of inorganic nanostructures the reader is referred to ref. 28. Historically the synthesis in aqueous media was the first successful preparation method of colloidal semiconductor nanocrystals. Nanocrystals formation takes place in homogenous aqueous solutions containing appropriate stabilizers of surfactant- or polymer-type.29,30 The latter bind to the NC surface and stabilize the particles by steric hindrance and/or electrostatic repulsion. In parallel to this monophase synthesis, a bi-phase technique has been developed, which is based on the arrested precipitation of nanocrystals within inverse micelles.31–34 Here nanometre-sized water droplets (dispersed phase) are stabilized in an organic solvent (continuous phase) by an amphiphilic surfactant. They serve as nano-reactors for the NC growth and prevent at the same time from particle agglomeration. Both methods provide relatively simple experimental approaches using standard reagents as well as room temperature reactions and were of great importance for the development of NC synthesis. Furthermore, for some materials (e.g. mercury chalcogenides,35–38 iron sulfide39) aqueous synthesis is either the only or at least the best preparation method reported to date. On the other hand, the samples prepared by these synthetic routes usually exhibit size dispersions s on the order of 15% or more and therefore post-synthetic size fractionation procedures have to be applied, such as size-selective precipitation. More recently, Li and coworkers reported another bi-phase preparation method, also called liquid–solid–solution (LSS) method.40 Here metal and chalcogenide precursors are reacted in an ethanol/ 448 | Nanoscale, 2011, 3, 446–489
water mixture in the presence of fatty acid stabilizers.41 In addition to transition metal sulfide and selenide semiconductor nanocrystals, this process has been successfully applied to noble metal, magnetic and oxide nanoparticles.42 In 1993 a novel high temperature synthesis method in organic solvents yielded nearly monodiserse (s < 10%) CdS, CdSe and CdTe nanocrystals without size fractionation.2 As demonstrated in classical studies by LaMer and Dinegar,43 the synthesis of monodisperse colloids via homogeneous nucleation requires a temporal separation of nucleation and growth of the seeds. Initially the concentration of monomers, i.e. the minimum subunits of the crystal, constantly increases by their addition from exterior or by in situ generation within the reaction medium. It should be noted that in this stage no nucleation occurs even in supersaturated solution, due to the extremely high energy barrier for spontaneous homogeneous nucleation. The latter is overcome for a yet higher degree of supersaturation (S > Sc), where nucleation and formation of stable nuclei take place. As the rate of monomer consumption induced by the nucleation and growth processes exceeds the rate of monomer supply, the monomer concentration and hence the supersaturation decrease below Sc, the level at which the nucleation rate becomes zero. In the following stage, the particle growth continues under further monomer consumption as long as the system is in the supersaturated regime. 2.2 Overview of the synthesis methods used for different materials Experimentally, the separation of nucleation and growth can be achieved by rapid injection of the reagents into the hot solvent, which raises the precursor concentration in the reaction flask above the nucleation threshold (‘‘hot-injection method’’).44 The
Fig. 1 TEM images of Cu2S nanocrystals with diameters of 7 (a), 15 (b), 19 (c) and 20 (d) nm.45 Reproduced with permission from American Chemical Society.
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Table 1 Synthesis conditions used for various semiconductor nanocrystals in organic solvents Materials
Precursors and stabilizersb
CdS, CdSe, CdTe CdSe CdSe, CdTe CdSe CdS, CdSe CdSe CdSe CdSe CdSe CdSe CdS CdS, ZnS CdS, ZnS CdTe CdTe CdTe ZnS ZnS ZnSe ZnSe ZnTe HgSe HgTe Cd1xZnxSe Cd1xZnxSe Cd1xZnxS CdSe1xTex InP InP, InAs InP InP InAs GaP GaP GaP GaN, AlN, InN PbSe PbSe PbSe PbS PbS PbTe PbTe SnS (>7 nm) SnS (10 nm) CuInS2 (10 nm, trigonal pyramids) CuInSe2 ( 0, I < 0] corresponds to the electrical power extracted from the device. The parameters Rs, Rsh, I0 and n characterize the performances of the device (together with the fill factor, see Scheme 18, Voc and Isc) and give hints on physical processes at play within the active layer. Huynh et al. studied the resistivity of thin films of P3HT containing 90 wt% of 7 60 nm CdSe nanorods as a function of the film thickness.271 They found that a light-intensity dependent shunt resistor better described the system than an Ohmic one. The shunt photo-conductance (Rsh light Rsh dark)1 increased linearly with the light intensity from 107 U1 cm2 to 102 U1 cm2 for intensities varying from 0.01 mW cm2 to 500 mW cm2, indicating larger current leakages for higher light intensities.
Scheme 17 Typical sample architecture for photocurrent measurements.
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show an Ohmic behaviour. The presence of charges with poor mobility induces space-charge effects: they are not evacuated fast enough and accumulate, screening the electric field (reducing the effective field probed by the charges). This indicates that low mobilities of carriers limit the transport out of the film. This socalled ‘‘space-charge effect’’ implies a drop of the bulk heterojunction potential. The current in the non-Ohmic series resistor can be expressed as: I ¼ IRs ¼
Vb 9 þ Vb2 3 am3Ntr Rs 8d
where the first term of the sum is the Ohmic term (dominant at low voltages) and the second term takes into account the spacecharge-induced potential drop in the bulk. a is the electrode surface, m the carrier mobility, 3 the permittivity of the sample, Ntr the trap density, and d the film thickness. With the second term modified to account for the field dependence of the mobility, the previous equation becomes:271,302 I ¼ IR s ¼ Scheme 18 Electrical modelisation of hybrid devices: equivalent circuit of hybrid devices and current–voltage characteristics in the dark and under illumination (in linear and semi-logarithmic scales). The asymmetrical behaviour is observed only when the active layer or the electrodes present an asymmetry (see for example ref. 264 and 296). The fill factor is defined as the orange area (Isc Voc) divided by the purple area (the maximal rectangular area delimited by the I–V curve in the lower right quadrant).
5.3.2 Electrical transport regimes from the current–voltage characteristics. In thin films of nanocrystals, the transport is found to occur via hopping, which is a thermally activated process, and to present space-charge limitations due to slow electrons. For example, Ginger and Greenham found electron mobilities of 104 to 106 cm2 V1 s1 in thin films of TOPOcoated CdSe nanocrystals.298 As indicated in Section 2.3, the surface ligand exchange after deposition of nanocrystal multilayers has been used to increase the charge carrier mobilies. To give a recent example, Liu et al. studied charge carrier mobilities in field effect transistors whose channels consisted of thin films of PbSe nanocrystals treated chemically. For 6.1 nm nanocrystals treated with ethanedithiol, the electrons (holes) reached a mobility of 0.07 cm2 V1 s1 (0.03 cm2 V1 s1).124 In thin films of polymers, hole transport is also field-dependent and temperature activated. Typical mobilities measured for P3HT films are in the range of 101 to 104 cm2 V1 s1 and for MEH-PPV 106 to 107 cm2 V1 s1 (e.g. ref. 259,299 and 300). They are strongly influenced by the molecular mass of the polymer and the morphology of the deposited layer.301 Applied field dependence of dark and photo-currents. Applied field dependence of dark current. The seminal study of Alivisatos’ group on the dark current in thin films of 90% in weight 7 60 nm CdSe nanorods embedded in P3HT showed that the dark current analysis in terms of equivalent circuit is good enough for low applied voltages.271 For higher voltages (from 0.7 V for 7 60 nm CdSe nanorods in P3HT), the series resistance does not 474 | Nanoscale, 2011, 3, 446–489
pﬃﬃﬃﬃﬃﬃ Vb 9 þ Vb2 3 am3Ntr eC V =d 8d Rs
C is a temperature dependent parameter. In this case the Shockley equation (eqn (5)) can be rewritten as follows: V Vb IðV ; TÞ ¼ Id Ish ¼ I0 eqðV Vb =nkT Þ 1 Rsh
where Vb is defined as in eqn (8). Huynh et al. included high field effects i.e. field-dependent mobility and space-charge to fit dark current–voltage characteristics for P3HT/7 60 nm CdSe nanorod hybrids as well as for 8 13 nm CdSe nanorod/P3HT samples.303 In samples of CdS nanopillars embedded in a MEH-PPV matrix, Yang et al. found a good agreement between their current–voltage characteristics and eqn (6) for low voltages (0.1 < V < 1 V) and observed a large discrepancy between the calculated and measured data at higher voltages.267 Their analysis has also led to the conclusion that low mobility of holes in the polymer induces space-charge effects. Applied field dependence of photocurrent. The photocurrent corrected for dark current is also called ‘‘external quantum efficiency’’ (EQE, the number of collected electrons in the external circuit per incident photon). Huynh et al. measured the EQE as a function of applied voltage in hybrids of P3HT and 7 60 nm CdSe nanorods (90 wt% of nanocrystals).303 From relatively high values (from 60% to 50%) for reverse bias, it fell drastically to very low ones at forward bias. Forward bias creates a field that opposes the built-in electric field, and the effective field is lowered. Therefore the number of collected electrons is lower as a result of the diminution of their mobility and the increase of their recombination efficiency. Watt et al. studied the hole transport in blends of MEH-PPV and 4 nm spherical surfactant-free PbS nanocrystals, synthesized in situ in MEH-PPV, as a function of the applied field. They compared it to the hole transport in pristine MEH-PPV265 and found that the hole transport is much less field dependent in the blends than that in the pure polymer devices. This journal is ª The Royal Society of Chemistry 2011
Temperature dependence of dark and photocurrents. Temperature dependence of dark current. Huynh et al. studied the temperature dependence of dark current in the temperature range between 298 K and 453 K in a P3HT/CdSe nanorod hybrid containing 90 wt% of nanocrystals.271 The saturation current I0 increased with temperature and could be fitted by a thermally activated current I0 f ef/kT with two different activation energies, f ¼ 0.35 eV for lower temperatures and f ¼1.47 eV for higher temperatures. The latter roughly corresponds to the band gap of the used nanocrystals. Temperature dependence of photocurrent. Greenham et al. investigated the temperature dependence of the short circuit current for a device containing 5% in weight of 5 nm CdSe nanocrystals in P3HT using an illumination at 514 nm.112 The recorded curves presented a reproducible hysteresis between cooling and warming phases. The current at 298 K was of the order of 200 nA and decreased to 40 nA at 10 K. This rather small fivefold decrease indicates that the conduction is not strongly thermally activated up to room temperature. The authors argue that the recombination is governed by the morphology of the films. Dependence of photocurrent on the illumination parameters. Dependence of photocurrent on light intensity. Increasing the light intensity tends to increase the number of generated excitons and eventually of free charges. However, after an initial increase of the photocurrent, it starts to decrease. This phenomenon is attributed to recombination processes and will be discussed in the next paragraph.271 The increase of photocurrent with light intensity is accompanied by an increase of the open circuit voltage Voc.112,267,271 It reaches a saturation plateau at large illumination intensities. In ref. 112, the authors studied hybrids consisting of 5 nm CdSe nanocrystals (90% in weight) in MEH-PPV for different illumination intensities. For intensities higher than 500 mW cm2 (up to the highest measured intensity of 50 mW cm2), the reported saturation plateau of Voc was in the range 0.5–0.6 eV, and corresponded to the work function difference between the used Al (4.3 eV) and ITO (ca. 4.7–4.9 eV) electrodes. To the contrary, for 90% in weight 7 60 nm CdSe nanorods in P3HT illuminated by a 514 nm source,271 and for CdS nanopillar arrays embedded in MEH-PPV,267 the maximal Voc exceeds the differences in the work functions of the electrodes and also the difference between the electron affinity of the acceptor and the ionization potential of the donor. In both cases, the authors explain this discrepancy by the pinning of the Fermi level at an interface. In the case of P3HT/CdSe the calculated pinning of the Al electrode to the lowest unoccupied energy level of CdSe nanocrystals gave an effective maximal Voc of 0.85 eV, in agreement with the experimental data. In the case of MEH-PPV/ CdS the calculated value was 0.5 eV, also in agreement with the experimental data, but no saturation plateau was observed for Voc even at the highest measured light intensity of 100 mW cm2.267 Spectral dependence of photocurrent. Monitoring the photocurrent as a function of illumination wavelength gives clues This journal is ª The Royal Society of Chemistry 2011
about the relative efficiencies of light to current conversion for a photon of given energy. Depending on the absorbance spectrum of the active layer, photons of different wavelengths penetrate more or less deeply before being absorbed. In strongly absorbing layers, excitons are formed close to the transparent electrode/semiconducting layer interface. However, at the same time this process heightens the density of excitons and charges in this area, which leads to the enhanced recombination and lowering of the effective field. These phenomena impede, to a large extent, efficient exciton dissociation and transport of electrons towards the metallic electrode. Greenham et al. noticed that for low CdSe nanocrystals/MEHPPV mass ratios, a peak in the photocurrent occurs in a spectral region where the optical density of the semiconducting nanocomposite is low and light is thus absorbed far inside the layer.112 For higher ratios, the photocurrent spectrum follows the absorption spectrum, indicating that the percolating network of nanocrystals facilitates exciton dissociation and electrons transport to the metallic electrode. This parallelism between absorption and current action spectra is used as an indicator of efficient exciton dissociation and charge transport and may serve to optimize the hybrid counterpart contents and the film thickness.303 Convincing examples are found in the field of infrared photodetectors. McDonald et al. prepared hybrids of MEH-PPV and 5 nm PbS nanocrystals capped with octylamine.304 The maximum of the MEH-PPV absorption peak was located at 522 nm, whereas the nanocrystals first excitonic peak occurred at 1262 nm (but with a much lower optical density), consistent with the assumption of the superposition of the spectra of both components. Since the photocurrent of the composite started to increase steadily from 1400 nm with decreasing wavelength, it could be concluded that an efficient charge generation was induced in a spectral region not absorbed by the polymer (cf. Scheme 19). The nanocrystals served, in this case, as sensitizers and the polymeric phase as a charge transporting medium. Similar results were found by Rauch et al. who studied ternary blends of P3HT:PCBM and 4.5 nm PbS nanocrystals.149 A peak in the EQE spectrum was found at 1220 nm, attributed to the first excitonic nanocrystals peak (none of the other components absorbed at this wavelength). Electronic coupling between the hybrid components. Electrical transport in hybrids strongly depends on the coupling between the two phases constituting the hybrid and also between the individual nanocrystals. It is therefore influenced by the film supramolecular organisation and morphology. Furthermore, the electronic coupling between the polymer and nanocrystals is strongly affected by the nanocrystals capping ligands. Insulating ligands will tend to trap one type of charges in the inorganic island-like phase, thus limiting the recombination and transport. Smaller or conductive ligands offer a much smaller energy barrier to overcome and facilitate the separation and diffusion of charges. Charge retention: hysteresis in the current–voltage characteristics. The weak coupling between the hybrid components can lead to a hysteresis in the electrical characteristics of the hybrid. This phenomenon corresponds to an electrical bistability, which Nanoscale, 2011, 3, 446–489 | 475
is frequently exploited in memory devices. Recent papers of Wei and collaborators described memory effects due to the electrical bistability from charge retention in hybrids of polymers and nanocrystals. In ref. 275, hybrid thin films of P3HT/2.85 nm CdSe nanocrystals and P3HT-only devices were studied in gated geometries. The current–voltage characteristics presented a much larger hysteresis under illumination than in the dark. This behaviour was postulated to be a consequence of trapped charges. The same group pursued these studies with the goal to improve the charge retention. For this purpose they used type I CdSe/ ZnSe core/shell nanocrystals (2.85 nm core) embedded in a P3HT matrix.276 In these nanocrystals, the energy band alignment of the core and shell material induces confinement of the electrons and holes in the core (see Scheme 1). As expected, the current after illumination was higher than in the case of control samples containing 2.85 nm CdSe core nanocrystals without a shell. Ham et al. also presented the core/shell nanocrystals/polymer approach for memory devices.282 They measured current–voltage characteristics of 100 nm thick films of (insulating) PMMA containing 80 wt% of InP/ZnS nanocrystals but without any gate voltage. They observed low conductivity for low applied voltages (currents up to 1 nA for voltages between 0 and 1.5 V) and high conductivity when the voltage sweep reached 1.5 V. This change was not reversible for the applied voltages between 5 V and +5 V, with currents in the mA–mA range. The authors attributed the change in conductivity to trapped charge carriers acting as space charges. The high conductivity I(V) curve could be fitted by a space charge limited current I f V2. Current dependence on time after illumination changes. Thin films of P3HT/2.85 nm CdSe nanocrystals and P3HT-only devices in FET geometries were illuminated for several seconds.275 After turning off the light, the relaxation of charges was studied. In the control device (P3HT only), all charges relaxed quickly. On the other hand, in the hybrids, after a rapid relaxation phase, the current stabilized at a non-zero value. This behaviour seems to originate from a rapid de-trapping in the gate oxide and at the gate oxide–P3HT interface, followed by a second phase where charges stay trapped on nanocrystals, leaving the device in a metastable state of enhanced conductivity. Zhang et al. electrically characterized hybrid devices consisting of blends of MEH-PPV and octylamine treated PbS nanocrystals (80 wt% nanocrystals).305 They annealed their samples at different temperatures up to 220 C, exceeding both the octylamine boiling point (175 C) and the MEH-PPV glass transition
Scheme 19 Typical aspect of the EQE spectrum as compared to the absorption spectra of the polymeric and nanocrystals components of a hybrid containing low band gap nanocrystals (NCs).
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(215 C). The current intensity rose with increasing annealing temperature. Moreover, switching the laser illumination on (off) caused a rise (decay) of the measured short circuit current. The difference of temporal evolution of the dark and photocurrent at zero bias resulted from charge trapping and release. The rise (decay) was much shorter in the case of annealed samples than in the case of unannealed ones. For example, the characteristic rise time of the photocurrent was 1.7 ms for annealed hybrid thin films, more than ten times smaller than the 18 ms measured for unannealed ones. This was explained by the partial deprotection of nanocrystals. Similar charge retention phenomena leading to the electrical bistability have been widely studied in organic matrices containing metallic (e.g. gold nanoparticles306) or organic dopants. The performances of such memories, as well as the mechanisms of their functioning, have been reviewed by several groups.307–309 Bistability effects in materials implying the use of nanocrystals in non-polymer matrices have also been reported. Fischbein and Drndic310 and Mohanta et al.311 reported on the bistability in thin films of CdSe nanocrystals surrounded by their initial surface ligands. In the first example, the dependence of the electrical behaviour on the irradiation conditions was investigated. In the second one, the impedance spectroscopy allowed demonstrating that the effective dielectric constant increased during the transistion from low-to-high conduction states. Percolation. Increasing the efficiency of transport to the electrodes by increasing the nanocrystals/polymer weight ratio. As already indicated, in most studied hybrid materials, nanocrystals act as electron acceptors and the organic phase as the electron donor according to the staggered energy band alignment displayed in Scheme 15. The transport efficiency is limited by the lowest mobility carriers. Electrons inside individual nanocrystals have a larger mobility than holes in the polymeric phase. Nonetheless, the existence of an efficient transport route from the dissociation point to the electrode is not necessarily assured for electrons due to the granular and dispersed nature of the nanocrystal phase. Numerous experiments show that the transport efficiency increases with increasing nanocrystal proportion up to an optimal polymer/nanocrystals ratio. For hybrids with ‘‘standard’’ polymers such as P3HT, MEH-PPV, e.g. having no specific interaction with pyridine treated nanocrystals, the optimal weight ratio has been determined as 1 : 9 (polymer : nanocrystals).111,112,266,274,278 This improved efficiency is attributed to the formation of a percolating network within the nanocrystal phase. In the case of amine end-functionalized P3HT, the optimum ratio is much lower (around 1 : 1).206 Here the affinity of the amine group for the nanocrystals surface results in a better control of the phase segregation than in the case of the pristine polymer. Increasing the efficiency of transport to the electrodes by optimizing the shape and orientation of nanocrystals. Percolation is also improved by use of anisotropic nanocrystals such as nanorods,110,111 tetrapods,279,281 or hyperbranched nanocrystals.272 Very recently, Dayal et al. studied the influence of nanocrystals’ shape on device performances and on microwave This journal is ª The Royal Society of Chemistry 2011
photoconductivity measurements.312 Spherical, nanorod and tetrapod CdSe nanocrystals of comparable characteristic dimensions (the smallest dimension being 4.5 nm, the length of the rods’ and tetrapods’ arms being around 30–50 nm) were used in blends with P3HT. Concerning device performances, the short circuit current increased from spherical nanocrystals over nanorods to tetrapods, while the open circuit voltage decreased. The fill factor was superior in the case of tetrapods. This improvement in the device performance was correlated with time-resolved microwave conductivity measurements. In these measurements, the sample is placed in a microwave cavity at its resonance. The dielectrical properties of the sample, among others the photoconductivity of carriers, modify the cavity resonance. This pump-probe technique thus allows probing dynamically and locally the photogenerated carriers’ conductance by measuring the variation of the reflected power of the cavity. Rod- and tetrapod-based hybrid devices exhibited larger photoconductivity transients at long times than dot-based samples. This finding was ascribed to the existence of longerlived photogenerated carriers. The authors suggested that the efficiency of mobile carrier generation (in other words: the efficiency of charge separation or the efficiency of generation of long-lived, free charge carriers) increases the photocurrent in samples containing anisotropic nanocrystals, rather than the percolation itself. In the case of tetrapod-based samples, additional improvement results from a more efficient percolation. The problem of forming a percolating network for the charge carriers can be circumvented by building arrays of vertically aligned nanowires.267,280 Bicontinuous interpenetrating network shown by impedance spectroscopy. In impedance spectroscopy, real and imaginary parts of the effective complex impedance Z of the device are recorded as a function of frequency u. As was mentioned earlier, modeling this couple Re[Z(u) + jIm[Z(u)] by an equivalent circuit facilitates the description of the device in terms of resistors, capacitors and inductors. Huang et al. studied thin films of blends of MEH-PPV with different loadings of 5 nm spherical CdSe nanocrystals by impedance spectroscopy.266 At low frequencies for low nanocrystal concentrations, they observed negative imaginary impedance. This phenomenon is more generally associated with inductive effects and has been explained in the cited paper by the existence of transient space charge at the interface between MEH-PPV and CdSe caused by poor evacuation of electrons into the nanocrystal phase. The negative imaginary part of the impedance vanished at higher concentrations of nanocrystals, in parallel to a decrease of the real effective impedance (corresponding to the resistance). This finding seems to indicate that an efficient interpenetrating network has been formed by increasing the nanocrystals concentration from 83 wt% to 90 or 94 wt%. 5.3.3 Effective mobilities and densities of charge carriers. There exist several methods for the measurement of charge mobilities. The most frequently used in conjugated polymers are: space charge limited current (SCLC), time of flight (TOF) or measurements of field effect transistor whose channel consists of the semiconductor being studied (FET configuration).313 The This journal is ª The Royal Society of Chemistry 2011
latter techniques will briefly be described here as they have also been applied to hybrids. Time-of-flight experiments imply the use of a sample such as the one presented in Scheme 17. The device is illuminated by means of a laser pulse and the type of charge-carrier is selected by applying a bias on the sample. The recorded transient current exhibits a maximum at a ‘‘transfer’’ time ttr. The mobility can then be calculated using eqn (10): m¼
where d is the film thickness and E is the applied field. Charge-carrier mobilities from time-of-flight measurements. Choudhury et al. used the TOF technique to determine the hole mobility as a function of electric field and film thickness in blends of PVK and pyridine treated CdS nanocrystals (diameter < 14 nm).286 The obtained values lied between 107 and 3 106 cm2 V1 s1, i.e. close to the values obtained in TOF measurements of pristine PVK.314 Watt et al. studied hole and electron mobilities in thin films of a hybrid consisting of 4 nm spherical surfactant-free PbS nanocrystals and MEH-PPV—the nanocrystals are synthesized in situ in the polymer—and found that electron and hole mobilities are well balanced in these blends. Furthermore, blending enhances both mobilities, which have been determined by TOF in pure MEH-PPV as mh MEH-PPV ¼ 2.83 (0.11) 103 cm2 V1 s1 and me MEH-PPV ¼ 1.8 (0.1) 105 cm2 V1 s1 (values obtained for 4945 V cm1 electric field). A FET device has three distinct electrodes, as shown in Scheme 20. The drain and source electrodes are used to apply voltage and measure current. The gate electrode is located on the whole area of the active layer (in this case called ‘‘channel’’) and separated from it by a thin dielectric layer. The application of a gate voltage allows depletion or accumulation of one type of carriers in the channel of the active layer. Charge-carrier mobilities from the transfer characteristics of gated devices. The drain–source current Ids is expressed as a function of the gate–source voltage Vgs using eqn (11): W 2 mh C Vgs Vth Ids Vgs ¼ 2L
where W is the width and L the length of the channel, C the gate oxide capacitance and Vth the threshold voltage. From this socalled ‘‘transfer characteristics’’ in the saturated regime, the mobility of holes can be found. Chen et al. reported typical p-channel FET characteristics for P3HT/3.5 nm CdSe nanocrystals hybrids:275 the current was dominated by hole transport. The same group found hole mobilities of 4 104 cm2 V1 s1 for P3HT-only devices, 8 104 cm2 V1 s1 for P3HT/2.85 nm core CdSe nanocrystals devices (90 wt% nanocrystals) and 3 103 cm2 V1 s1 for devices containing P3HT and CdSe/ZnSe core/shell nanocrystals (core diameter 2.85 nm, 90 wt% nanocrystals).276 Rauch et al. investigated ternary mixtures of P3HT, PCBM and 4.5 nm PbS nanocrystals.149 Their devices exhibited ambipolar transport with mobilities up to mh ¼ 4.3 104 cm2 V1 s1 and me ¼ 4.6 105 cm2 V1 s1 found in FET geometry Nanoscale, 2011, 3, 446–489 | 477
measurements. They showed that both P3HT and PCBM were necessary to achieve efficient nanocrystals exciton dissociation and charge-carrier transport. PCBM is the electron transporter (its LUMO level matches that of the nanocrystals) and P3HT is the hole-transporting phase (its HOMO level matches that of the nanocrystals). Therefore the obtained devices showed the transport features of all-organic heterojunctions with additional sensitivity to infrared light due to the presence of PbS nanocrystals. Charge-carrier densities from the transfer characteristics of gated devices. Chen et al. estimated the increase DN in the carrier density between dark and light characteristics using eqn (12):275 DN ¼
Ci DVth e
with Ci the capacitance per unit area of the dielectric layer, e the electron charge and DVth the shift of Vth between dark and light transfer characteristics. The obtained values were DN ¼ 4.51 1011 cm2 for P3HT only devices and DN ¼ 1.39 1012 cm2 for P3HT/3.5 nm CdSe nanocrystals devices.
5.4 Recombination processes When describing experimental techniques used to determine the exciton and photo-excitation lifetimes in previous sections, we gave hints on how to identify the types of recombinations that occur in different types of hybrid materials. Indeed, two processes shorten the photo-excitation lifetimes: recombination processes and current collection at the electrodes. The latter process is, in itself, a recombination process that permits charge neutrality conservation in the device. It is not distinguishable from trap-assisted recombination processes or from those taking place at the heterojunction interface. To the contrary, spectroscopic characterisations necessitate neither current collection nor the use of electrodes. Thus they allow studying recombination processes independently from current collection. Still, it must be noted that the interface between the active layer and electrodes can be considered as a preferable site for traps and thus might give rise to trap-assisted recombinations. Recombination is the main process limiting efficient charge transport in the active layer. One distinguishes between different types of recombination: (i) monomolecular (geminate) recombination implies that an electron and a hole, originating from the same exciton, recombine as favored by Coulomb attraction; (ii) bimolecular recombination (also called Langevin recombination) involves an electron and a hole originating from different
Scheme 20 Device architecture for field effect transistor measurements.
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excitons; (iii) trap assisted recombination arises when an electron (hole) recombines at a defect state lying lower (higher) in energy. Recombination should thus depend on densities of defects and charge-carriers. The monomolecular (geminate) recombination rate is proportional to the density of excitons Nexc (i.e. Rmono ¼ kmono$Nexc). The bimolecular (Langevin) recombination rate, Rbim, is proportional to the product of the densities of electrons and holes, NP or equivalently to the square of the exciton density Nexc2. The trap assisted recombination rate, Rtr, depends on the trap density, Ntr, the hole and electron densities.267 Consequently, the exciton density evolves like eqn (13): dNexc ¼ kgen i ðkmono þ kdiss ÞNexc þ Rbim ðN; PÞ þ Rtr ðNtr ; N; PÞ dt (13) where kgen is the generation rate of excitons, i is the light intensity and kdiss the dissociation rate of excitons. The elements of this formula are taken from the different domains of organic and inorganic semiconductors and the authors suggest that the validity of all hypotheses should be checked thoroughly.267 For example, in hybrids, the chargecarrier densities are not uniform throughout the device: N is the electron density in the nanocrystal phase, P the hole density in the polymer, in the vicinity of inorganic interfaces for bimolecular recombination and in the vicinity of traps for trap-assisted recombination. 5.4.1 Types of defects Photoluminescence measurements. Some types of defects in nanocrystals or in the polymer can be detected by photoluminescence measurements. Yang et al. observed two photoluminescence peaks at 1.8 eV and 2.4 eV in a CdS nanopillar array without polymer.267 These peaks are present at room temperature as well as at 12 K and were attributed to sulfur vacancy defects and intersticial sulfur defects, respectively. Electron spin resonance. An electron spin resonance signal in the dark also indicates the presence of unpaired spins in the material. While Pientka et al. reported no dark signals in pure MDMO-PPV, CdSe nanocrystals or InP nanocrystals, indicating quasi-defect-free organic and inorganic components,268 Dietmueller et al. observed unpaired spins due to dangling bonds of 33 nm Si nanocrystals in disordered medium and positive polarons in P3HT, even in the dark.277 5.4.2 Electroluminescence. The high colour purity and accordable luminescence of nanocrystals make them promising candidates for electroluminescent devices: electrically injected charges recombine radiatively under photon emission. Temperature dependence of electroluminescence. Dabbousi et al. studied the electroluminescence of blends of MEH-PPV and t-Bu-PBD (60 wt% of MEH-PPV) with 3.2, 4.0 and 6.0 nm CdSe nanocrystals (5–10% volume fraction of nanocrystals) as a function of temperature.296 The electroluminescence as well as the photoluminescence increased with decreasing temperature. This effect was attributed to the lower efficiency of non-radiative recombination pathways when the temperature drops. The electroluminescence increase saturated between 50 K and 10 K, This journal is ª The Royal Society of Chemistry 2011
contrarily to the photoluminescence that increased steadily. The electroluminescence saturation arised probably from the drop of the charge mobilities in MEH-PPV (hole transporting component) and t-Bu-PBD (electron transporting component) with decreasing temperature. Applied voltage dependence of electroluminescence. Gao et al. studied thin films prepared by alternate deposition of soluble PPV precursor layers and CdSe/CdS core/shell nanocrystals (core diameter 3.7 nm), followed by thermal conversion of the polymeric precursor to PPV.261 The electroluminescence recorded for this composite showed a peak around 660 nm, a signature of the nanocrystals. No contribution from the polymer was observed. The threshold voltages for electroluminescence were between 3.5 V and 5 V. The use of insulating PAH, instead of PPV as the organic component, allowed better understanding of the role of the polymer. The electroluminescence was identical but the threshold voltage was shifted to a value of 10 V. Thus, it was demonstrated that the polymer film served mostly as a charge transporting layer rather than a photon emitting one. Nanocrystals’ loading dependence of electroluminescence. Tessler et al. described near-infrared electroluminescent devices made of blends of InAs/ZnSe core/shell (core diameter 4 nm) nanocrystals with MEH-PPV or F6BT.263 Similar to in the previous case, the luminescence of hybrids originated mainly from the nanocrystal component of the hybrid. The threshold voltage and the intensity of the electroluminescence peak increased with the content of nanocrystals in the hybrid. The photoluminescence of the polymer was quenched by a factor of 5 in blends of a 1 : 1 volume ratio, indicating an energy transfer from the polymer to the nanocrystals. Both observed phenomena were attributed to bipolar charge trapping on the nanocrystals. 5.4.3 Recombination of photogenerated charges at energetic interfaces. Since the densities of photogenerated excitons, electrons and holes can be related to the number of absorbed photons, recombination processes must depend on the light intensity. Photocurrent dependence on light intensity. One way to study recombination processes is to record current–voltage characteristics as a function of the illumination intensity. As predicted by eqn (13), the linear dependence of the short circuit photocurrent Isc as a function of light intensity i can be treated as an indication of a monomolecular recombination, whereas sublinear dependence indicates the domination of bimolecular processes.267,271 The simple model outlined above indeed suggests that higher intensities imply higher charge densities due to low mobilities and thus an increased probability of electrostatic interaction between charges. Greenham et al. found an approximately linear dependence of Isc on the light intensity i for low intensities (Isc f i0.9) in samples of MEH-PPV blended with 90 wt% of 5 nm CdSe nanocrystals.112 They attributed this finding to a fixed number of recombination centers, identified as ‘‘dead ends’’ of the nanocrystals’ percolation paths. At higher light intensities, Isc became sublinear (Isc f i0.65). This journal is ª The Royal Society of Chemistry 2011
In blends of 7 60 nm CdSe nanorods and P3HT, illuminated with 514 nm light, the current density as a function of light intensity was found to be slightly sublinear, especially at intensities larger than 10 mW cm2.271 Also in composites of CdS nanopillars and MEH-PPV a sublinear Isc dependence on light intensity was found for all intensities.267 This observation is consistent with the presence of traps in the system inducing large trap-assisted (thus bimolecular) recombinations. Moreover, the fact that bimolecular recombination occurs by confinement at a two-dimensional interface should be taken into account. Yang et al. suggested consequently the application of a modified Langevin-like formula for two-dimensional bimolecular recombination.267,315 The open circuit voltage Voc also depends on the light intensity. This intensity dependence can be included in the short circuit Isc(i), so that Voc can be written as: nkT Isc ðiÞ (14) Voc ¼ ln q I0 þ 1 where, as indicated before, n is the ideality factor and Isc and I0 respectively the short circuit current and the saturation current. Fitting Voc to the data gives an ideality factor of 1.57 for the 7 60 nm CdSe nanorods/P3HT hybrids271 and an ideality factor of 2 for the CdS nanopillars/MEH-PPV composite.267 These high ideality factor values confirm the domination of loss mechanisms. PIA dependence on light intensity. To distinguish between mono- and bi-molecular recombinations, one should consider the low-frequency range of PIA signals (see eqn (2) and (3)). In this regime, monomolecular recombination pathways yield linear dependence on light intensity whereas bimolecular recombination processes imply a square-root dependence on light intensity. In blends of MDMO-PPV and 4.1 nm CdSe nanocrystals as well as in blends or MDMO-PPV and 4.2 nm InP nanocrystals, Pientka et al. found a monomolecular recombination regime for low illumination intensities (up to 1 mW cm2) and a bimolecular recombination regime for larger light intensities.268 This finding was consistent with larger charge-carrier densities at higher illumination intensities, giving rise to an increased probability of meeting the opposite charge. In hybrids of PFE and 3.2 nm CdSe nanocrystals, Tseng et al. found a DT/T f i0.61 relationship which they interpreted as a signature of bimolecular recombination regime with a distribution of lifetimes.283 5.5 Summary and discussion In summary, the electronic properties of hybrid materials consisting of conjugated polymers and semiconducting nanocrystals have been studied with various experimental techniques in the last fifteen years. Spectroscopic studies allow the characterisation of the hybrid materials on their own, whereas electrical measurements strongly depend on the film morphology, on the interfaces with the electrodes and on the electrode materials. Exciton generation upon illumination has been proved by the appearance of photo-induced absorption bands.268 The lifetimes of these excitons have been studied by photoluminescence decay experiments266,270 and PIA frequency measurements.262,268 Their Nanoscale, 2011, 3, 446–489 | 479
diffusion seems to be thermally activated.284 Whereas photoluminescence quenching indicates the dissociation of excitons into free charge carriers or energy transfer (i.e. creation of an exciton on the lower gap component of the hybrid),262,270,273 only positive polarons on the polymers have been observed by lightinduced electron spin resonance268,273 and photo-induced absorption262,268,270,283 and no sign of their negative counterparts has been detected yet. The positive polarons exhibited long lifetimes that are distributed in such wide ranges as the micro- to milli-second range or longer, as found by LESR signal decay studies and PIA frequency measurements. The latter tend to indicate dispersive recombination processes in the most recent studies.273,285 Electrical characterisations exploring a wide range of parameters such as temperature and light intensity have been rather rare until now.112,267,271 Dark and photocurrents have been described by a space charge limited current model associated with a field dependent mobility. Temperature dependence indicated the thermal activation for energies correponding to material parameters. Studies of the current dependence on the illumination wavelength gave mostly an absorbance sensitizer role to the nanocrystals rather than the desired electron transport role.149,264 Current hystereses have been reported when scanning the applied voltage back and forth.275,276,282 These hystereses are more important when nanocrystals are decoupled, either by the use of type I core/shell nanocrystals or of insulating surface ligands, or by blending them with an insulating polymer and are attributed to nanocrystals’ charging. Finally, mobilities have been measured by time-of-flight measurements265,286 and with field effect transistor configuration.149,275,276 The obtained hole mobilities were relatively close to those measured in pristine polymer films, with the exception of ref. 265, where the nanocrystals were directly synthetized in the polymer matrix. The transport was dominated by holes in the hybrids investigated in these reports. Recombination is likely to be the dominating efficiency-limiting process in hybrid materials. It has been shown to be dispersive in many samples by studying the short circuit photocurrent as a function of light intensity, either monomolecular at low light intensities (low charge-carrier density) or bimolecular at higher light intensities. These findings are coherent with the widely distributed lifetimes of free charge carriers mentioned above, as well as with the idea that hybrid materials are of highly disordered nature. In contrast with allorganic blends (e.g. with PCBM as an electron acceptor), electrons have neither been detected spectroscopically so far nor have charge transport studies shown their large contribution. Many efforts are underway to improve the transport by enhancing mobilities through the use of anisotropic nanocrystals and crystalline polymers and through reducing trapping and recombinations. However, these novel and hopefully higherperformance hybrid materials have to be submitted to the various studies described above in order to be able to understand the factors governing their electrical properties. Doing so, one should be aware of the limitations of each type of experiments and of the fragility of the asumptions made in usual models. Indeed, two characteristics of hybrids influence largely charge transport and thus device efficiency: (i) they are made of two or more components of very different chemical nature that are electronically coupled and (ii) they are at least partly disordered. One should thus bear in mind that the usual 480 | Nanoscale, 2011, 3, 446–489
asumptions that were made to develop the most common models of transport might not be verified in the case of hybrids. For example, Drude’s conductivity model was developped for ordered inorganic materials in the framework of the drift-diffusion model, where charge carriers are considered to be free or ‘‘delocalized’’ (i.e. the mean free path between collisions is much larger than Bloch’s wavelength of the carrier). In organic semiconductors, disorder leads to spatially localized states.316 These are described by non-uniform densities of states from the fluctuations in state energies and by electronic coupling distributions from the fluctuations in the interactions between sites via various wavefunction overlaps. Although the mobility concept arose from the analysis of ordered materials, it is widely applied to organic semiconductors, nanocrystal assemblies and hybrid thin films. It has been shown that the mobility in organic semiconductors depends strongly on diverse parameters: temperature, disorder and impurities, electrical field, charge carrier densities, and so on (see ref. 313 for a review of mobility measurements and calculations in organic semiconductors). For this reason, it should be noted that mobilities measured by different techniques are supposed to be different. For example, in ref. 317 it is shown that the hole mobilities in PPV derivatives obtained via field effect transistor measurements are very different from those derived from space charge limited current (diode configuration). One reason for this difference is the strong possibility of increasing the charge carrier densities in the FET configuration. Moreover, the larger the surface of the investigated sample, the stronger the mobility depends on material’s order and purity.313 These discrepancies arise from different measurement configurations. On the other hand, when a quantity is measured many times in similar experimental conditions, the variability of its value often leads to averaging procedures. Averaging over many events is not convincing if the dispersion of the studied quantity values is large compared to the quantity itself.260 For this reason, studies in the field of organic semiconductors led to suggest the use of mobility distribution functions to reflect spatially dispersive transport processes.318,319 Finally, the apparition of localized states having energies inside the forbidden energy band, blurs the strict notion of the energy band gap. The localized states exhibit very restricted mobility in comparison to delocalized states, so it has been proposed to use a forbidden mobility region rather than a strict energy band gap concept.320,321 These considerations were formulated in the quest of understanding disordered systems, more particularly in the case of organic semiconductor thin films, and should most probably also be relevant for future studies of hybrid materials.
Application of hybrids in solar cells
Solar cells are devices, which transform solar radiation energy into electricity. In Section 5 the electrical properties of related devices, namely photodetectors, were also presented. These devices change their conductivity depending on the illumination conditions and will not be discussed here. For more details on this topic, we refer the reader to Section 7.2 of ref. 258. The simplest organic solar cell consists of a layer of an appropriate conjugated polymer (oligomer) sandwiched between This journal is ª The Royal Society of Chemistry 2011
two electrodes of different work functions. One of these electrodes must be transparent since the radiation must reach the semiconductor layer. Upon radiation excitons are formed in this layer which then dissociate. The holes and electrons formed in the dissociation process migrate to the electrodes creating the difference of potentials. In this simple device, however, the photoinduced charge generation—the key parameter of the cell operation—is extremely inefficient.322 This is caused by the fact that the conjugated polymers, used for the fabrication of photovoltaic cells, are good electron donors but poor electron acceptors. As a consequence the formed excitons can dissociate to liberate the charges only in the vicinity of the electrode since the electrode is the only electron accepting area in the device. The photoinduced charge generation is facilitated if, in addition to the conjugated polymer, an electron-accepting component is dispersed in the active layer. In this case the large interface area between the electron donating and the electron accepting components promotes the photoinduced charge generation. If both phases form interpenetrating, percolating networks, the created electrons and holes can be transported to the respective electrodes. This briefly described concept of ‘‘a bulk heterojunction’’ was first experimentally proven by Heeger and co-workers who fabricated bulk heterojunction solar cells consisting of MEH-PPV (poly(2-methoxy-5-(20 -ethyl-hexyloxy)-1,4phenylene-vinylene)) as an electron donating semiconductor and fullerenes (C60) as electron acceptors.323 In more recent works C60 was replaced by its soluble derivative—methanofullerene [6,6]-phenyl C61 butyric acid methyl ester (PCBM). Some of the solar cells based on regioregular poly(3-hexylthiophene) and PCBM show power conversion efficiences approaching 5%.324 Even higher values were reported recently with lower band gap polymers and PC70BM:poly(2,7-carbazole) derivatives yielded 6.1%325 and thieno[3,4-b]thiophene—benzodithiophene derivatives 7.4%326 under AM 1.5 conditions (100 mW cm2). Photovoltaic cells containing nanocrystals as electron accepting components are based on the same principle of bulk heterojunction. In addition to their appropriate electronic structure, nanocrystals offer some advantages over other electron accepting components. First, the shape of nanocrystals can be varied (spheres, rods, tetrapods, hyperbranched) and by changing their shape it is possible to tune the percolation threshold since the dispersion of objects of higher aspect ratios results in its lowering. Second, the absorption spectrum of the crystals and their HOMO and LUMO levels can also be tuned by changing their size or their composition. Thus, it is possible to fabricate nanocrystals, which—in addition to the conjugated polymer— strongly contribute to the collection of photons under solar irradiation. The principles of the device functioning of photovoltaic cells with semiconductor nanocrystals as electron acceptors are illustrated in Fig. 3 (Section 3.1). For proper operation the staggered (type II) alignement of the HOMO and LUMO levels of the bulk heterojunction components must be accomplished, compatible with the electrodes work functions. If an exciton is formed in the polymer phase, then the electron is transferred to the nanocrystals phase and reaches the aluminium electrode via its percolating pathway. The remaining hole is transported to the ITO electrode through the polymer phase. In the alternative case, This journal is ª The Royal Society of Chemistry 2011
i.e. the formation of an exciton in the nanocrystals phase, the hole is transferred to the polymer phase and then transported to the ITO electrode, whereas the electron reaches the aluminium electrode through the nanocrystals phase. A typical conjugated polymer/semiconductor nanocrystals bulk heterojunction type photovoltaic cell contains two additional layers (PEDOT:PSS and LiF), which improve the contact between the active layer and the electrodes and facilitate the extraction of holes and electrons, respectively. The first bulk heterojuction-type solar cells containing semiconductor nanocrystals as electron accepting components were reported by Greenham et al. who used spherical CdSe or CdS nanocrystals in nanocomposites with MEH-PPV.112 The low efficiency of the obtained solar cells (0.2% under AM 1.5 conditions, 0.5 mW cm2) is in accordance with the observed incomplete photoluminescence quenching. The obtained morphology suffered from uncontrolled phase separation of the active layer components. It should be underlined that already in this early study pyridine has been suggested for replacing the initial surface ligands and improving the charge transfer and charge transport properties of the hybrid. A significant progress in the cell efficiency is generally achieved by replacing spherical nanocrystals with rod-like ones.110 Solar cells fabricated from 7 60 nm pyridine-treated CdSe nanorods and regioregular poly(3-hexylthiophene) showed an improved power conversion efficiency of 1.7% under AM 1.5 conditions, 100 mW cm2.111 Other types of anisotropic nanostructures, such as tetrapods or hyperbranched CdSe nanocrystals, were also tested as components for hybrid photovoltaic cells. CdSe tetrapods with poly(p-phenylene vinylene) exhibited an energy conversion efficiency of 2.8%.327 Using a low band gap thiophene–benzothiadiazole based copolymer, 3.1% have been reached.281 Hybrid solar cells consisting of hyperbranched CdSe nanocrystals and poly(3-hexylthiophene) showed an efficiency of 2.2%.272 While the shape and size of the nanocrystals are controlled during their synthesis, the supramolecular organization and morphology of the polymer phase strongly depend on the hybrid thin film processing conditions. Mixing poly(3-hexylthiophene) with rod-like CdSe nanocrystals from 1,2,4-trichlorobenzene leads to a fibrilar morphology of the polymeric phase, facilitating charge carriers transport. The resulting interpenetrating network of two high aspect ratio components is very favorable for photovoltaic applications. Although for the nanocrystals’ contents required for the functioning of the cell the fibrilar morphology is partially lost, still high cell efficiencies, exceeding 2.5%, can be obtained.109 A clear evidence of the existence of preformed poly(alkylthiophene) nanofibers after processing has been obtained by Jiu et al. who prepared hybrids with pyridinetreated CdSe nanorods.328 In this study, a better device performance was achieved with poly(butylthiophene) instead of poly(hexylthiophene) nanofibers, in accordance with the observed improved nanoscale organization of the hybrid thin film. As discussed in Section 2.3, surface ligands are strongly influencing charge transfer and transport processes in the hybrid and thus directly influence the solar cell device performance. Pyridine, widely used in the processing of bulk heterojunction layers, has several disadvantages: (i) the ligand exchange is not complete; (ii) the solubility of the pyridine-treated nanocrystals is Nanoscale, 2011, 3, 446–489 | 481
reduced, and the addition of pyridine to the nanocrystal/polymer co-solvent is required in most cases to obtain stable colloidal solutions; (iii) pyridine used as a co-solvent influences the supramolecular organization of conjugated polymers, it is for example a non-solvent for P3HT. Therefore an increasing number of studies concerns the search of alternative nanocrystals surface ligands for hybrids. Aldakov and coworkers investigated a number of small sulfurcontaining conjugated heterocycles (3-methylthiophene, 3octylthiophene, ethylenedioxythiophene, and 1,3-thiazole) in hybrids with CdSe nanorods.256 However, in no case an improvement of the photovoltaic device performances with respect to pyridine could be obtained, which was attributed to the observed phase segregation of the hybrid’s components into large polymer-rich and nanocrystal-rich domains. In a similar study, Olson et al. examined stearic acid, oleic acid, butylamine and tributylamine in hybrids with spherical CdSe nanocrystals.113 The use of butylamine in combination with an annealing step at around 110 C yielded devices reaching an efficiency of 1.8% under AM 1.5 illumination (100 mW cm2). On the other hand, the authors point out a somehow limited reproducibility of the results due to inefficiencies in the ligand exchange procedure. Zhou et al. reported recently a PCE of 2% for hybrids consisting of small spherical CdSe nanocrystals and P3HT, which is an unprecedented high value for this system. In their approach, the authors used hexadecylamine (HDA) capped CdSe nanocrystals, which were washed with hexanoic acid (Fig. 16). TEM and dynamic light scattering revealed a significantly reduced interparticle distance and effective particle size, respectively, pointing at the replacement of the bulky, insulating HDA ligands. It is instructive to compare the data obtained for conjugated polymers/semiconductor nanocrystals hybrid solar cells with those reported for other types of these devices investigated in the recent years (see Table 4). Given a large number of different parameters, which characterize the performance of photovoltaic cells, it is sometimes difficult to compare literature values. For these reasons all data collected in Table 4 represent energy conversion efficiencies under air mass (AM) 1.5 1 sun conditions
(typical solar spectrum, 100 mW cm2), according to the following relationships: Power conversion efficiency; h ¼
Fill factor; FF ¼
Pmax Isc Voc FF ¼ (15) Pin Pin
Pmax Isc Voc
Pmax ¼ largest power output Isc ¼ short circuit current Voc ¼ open circuit voltage FF ¼ fill factor Pin ¼ incident light power intensity We purposely added in Table 4 a column specifying the active surface of the measured devices. In the listed all-organic and hybrid solar cells, it strongly varies from 0.045 to 0.28 cm2 with most of the devices having an area of around 0.1 cm2. When comparing the obtained efficiency values, one should bear in mind that these are not independent from the device active surface. Pandey and coworkers showed that the measured efficiency increases strongly with decreasing device area, especially if the latter becomes smaller than 0.3 cm2.336 As can be seen from Fig. 17, reducing the area from around 0.3 cm2 to 0.03 cm2 leads to an approximately double value of device efficiency. Simulations revealing that the loss of performance of larger solar cells originated from the higher series resistance, which reduces FF and Jsc, have consolidated these results obtained with an organic model system. Therefore the performance loss of larger devices cannot simply be attributed to poorer film quality, but to the sheet resistance of ITO used as anode. To conclude, although the power conversion efficiency of conjugated polymer/semiconductor nanocrystals hybrid photovoltaic cells is still significantly lower than those measured for silicon based or Gr€ atzel type cells, the described new nanomaterials show several promising features. Their technologically most important advantages are: (i) they are easy to process, using low cost methods of low energy consumption; (ii) they can be deposited on large surface, cheap substrates which can partially compensate their lower performance. Of course, still much research is needed to improve their efficiency and long-term stability.
Fig. 16 TEM image of hexadecylamine-capped CdSe nanocrystals before (a) and after washing with hexanoic acid.329 (c) Proposed process for the surface ligand removal. (d) J/V characteristics of a device containing 87 wt% of nanocrystals under AM 1.5 illumination (100 mW cm2). Reprinted with permission from American Institute of Physics.
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7 Perspectives Research on hybrids consisting of semiconductor nanocrystals in a matrix of a conjugated polymers was originally motivated by the appealing possibility of combining in a synergic way two individually tunable semiconductors with complementary properties. In particular, conjugated polymers provide solution processibility in the form of thin films on large and flexible substrates, while semiconductor nanocrystals exhibit optical and electronic properties, which can be easily adjusted with size and composition. Among the possible applications of hybrids, solar cells and photodetectors have been mostly investigated so far. The majority of reported results concern n-type cadmium chalcogenides in combination with p-type poly(alkylthiophene) or poly(phenylene vinylnene) derivatives. Motivated on the one hand by environmental and toxicological issues and on the other hand by the lack of covering a spectral range above 800 nm, a series of novel nanocrystals has been developed
during the last few years, mostly copper-containing binary and multinary sulfides and selenides. Due to the different electronic properties of these novel compounds, also other families of polymers will have to be investigated in order to obtain the desired energy band (or HOMO–LUMO level) alignment of the organic and inorganic semiconductors. Moreover, better matching of the solar spectrum by conjugated polymers’ absorption spectra is still possible. Although the building block approach and the donor–acceptor concept significantly contributed to the preparation of several dozens of low band gap polymeric semiconductors, this synthetic pathway is still far from being exhausted. To this end electrochemical, spectroelectrochemical and photoelectron spectroscopic studies have to be performed in order to assess the energy level positions of the hybrids constituents. Nanocrystals’ surface chemistry remains a critical point for assuring compatibility with the polymer matrix as well as for enabling efficient charge transfer and charge transport processes.
Table 4 Power conversion efficiencies reported for photovoltaic devices of various types. In addition to the highest reported value, the numbers in brackets indicate typical values observed for the corresponding device type Materiala
Device active surface area/cm2
h (AM 1.5), 100 mW cm2
Monocrystalline silicon Polycrystalline silicon Amorphous silicon Dye-sensitized mesoporous TiO2/redox couple in liquid electrolyte (Gr€ atzel cell) Dye-sensitized mesoporous TiO2/MeOTAD (solid state) MDMO-PPV/PC70BM P3HT/PCBM PCPDTBT/PC70BM PTB7/PC70BM P3HT/CdSe nanocrystals (spherical) P3HT/CdSe nanorods P3HT/CdSe nanorods P3HT/CdS nanorods MDMO-PPV/CdSe tetrapods PCPDTBT/CdSe tetrapods
4 1 22–24 0.186
24.4% (12–16%) 20.3% (9–12%) 9.5% (4–8%) 10.4%
330 331 332 333
0.16 0.1 0.19 0.127 0.1 0.08 0.28 0.045 0.1 0.045 0.11
5.0% 3.0% 4.9% 6.1% 7.4% 2.0% 1.6% 2.6% 2.9% 2.8% 3.1%
334 335 324 325 326 329 328 109 243 327 281
a MeOTAD ¼ 2,20 ,7,70 -tetrakis(N,N-di-p-methoxyphenylamine)-9,90 -spirobifluorene, MDMO-PPV ¼ poly[2-methyloxy-5-(30 ,70 -dimethyloctyloxy)p-phenylenevinylene], P3HT ¼ poly(3-hexylthiophene), PCPDTBT ¼ poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4b0 ]dithiophene)alt-4,7-(2,1,3-benzothiadiazole)], PTB7 ¼ poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b0 ]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]].
Fig. 17 Variation of Voc and FF (a) and of Jsc and efficiency (b) with the area of a device consisting of ITO/40 nm PEDOT:PSS/55 nm pentacene/25 nm C60/8 nm bathocuproine (BCP)/60 nm Al.336 Reprinted with permission from Elsevier.
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For the time being, a rather empirical choice of surface ligands has been practiced, looking for ‘‘small’’ molecules to replace the initial ones containing long, insulating alkyl chains. A concomitant requirement is the solubility of the functionalized nanocrystals in a common solvent with the polymer. At the same time the composition of the multicomponent mixture of nanocrystals, polymer and at least one solvent strongly influences the morphology and phase separation of the thin films obtained in the processing step. Morphology control in turn is of crucial importance for the electronic properties of the hybrid film. Phase separation should occur on the length scale of the exciton diffusion length, and continuous percolation pathways for the charge carriers to the electrodes have to be provided. It has recently been demonstrated that complementary solution NMR techniques can give valuable information about dynamics of ligand exchange processes337 and in fine about the composition of the hybrid mixture in solution. Further progress in the hybrid processing techniques will require wider use of these techniques. Concerning the application of hybrids in solar cells, the results obtained to date—although being promising in view of the rather short time of research and few teams involved—are far from the expectations. While all-organic devices reach now more than 8% of power conversion efficiency, the best values obtained with hybrid devices have stagnated around 2–3% for the last five years. It is obvious that the cited parameters (nanocrystals’ surface chemistry, morphology control, processing parameters) are key issues to be addressed. On the other hand, the approaches developed so far only give an incremental improvement, while a ‘‘quantum leap’’ would be needed to make technologically competitive hybrid-based photovoltaic devices. Another issue, which has not to be forgotten in the quest of better efficiencies, is the problem of device stability. Some devices based on hybrids of PbS and P3HT have shown promising stability under ambient conditions.120,337 Nevertheless, prolonged exposure to air for non-encapsulated devices generally leads to degradation of their performance due to oxygen and water diffusion at grain boundaries and through pinholes of the back electrode and/or oxidation of the electrode itself. An exciting approach to overcome this problem, not applied to hybrids so far, is the preparation of solar cells with an inverted structure. Instructive examples of this approach can be found in ref. 338–340. Instead of using a high work function transparent semiconducting oxide (e.g. ITO) and a low work function metal cathode (e.g. Ca, Al, Mg), inverted cells apply a less air sensitive high work function metal (e.g. Ag, Au) as the back electrode for hole collection and a metal oxide (e.g. TiO2, ZnO) as the electron collector at the ITO interface. To give an example, a solar cell consisting of a plastic/ITO/ZnO NPs/P3HT:PCBM/PEDOT:PSS/Ag stack yielded a power conversion efficiency of 3.3% and 80% of this efficiency was retained after 40 days of operation under ambient conditions.339 Furthermore, this strategy avoids problems related to the diffusion of metal into the active layer during the electrode deposition process. Concluding, in order to meet the high expectations set on hybrids and to make them a real alternative to competing approaches, such as all-organic electronic devices, two points seem to be of particular importance. First, a better analysis of the limiting factors in the different hybrid solar cells is needed, since these devices are complex systems with multiple interfaces. As 484 | Nanoscale, 2011, 3, 446–489
has been shown in this review, special spectroscopic (e.g. photoinduced absorption measurements) and electrical (e.g. TOF measurements) studies can help to extract such information. These studies should be carried out on model systems of precisely defined properties and in parallel on real test devices. The structure of planar heterojunctions, for example, can be much better controlled than that of bulk heterojunctions. Second, a much stronger implication of theorists in this research is highly desirable. The predictive role of such studies cannot be overestimated. DFT calculations carried out for model compounds have already turned out to be very useful guidelines for the preparation of pre- or post-functionalization of semiconducting polymers. Similarly, modeling of the electronic structure of nanocrystal–surface ligands complexes and of the electrical transport at the interfaces and within the hybrid would be extremely helpful for designing more rational approaches towards hybrids with improved optoelectronic properties.
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