Highly efficient bulk heterojunction photovoltaic cell ...

Highly efficient bulk heterojunction photovoltaic cell based on tris[4-(5phenylthiophen-2-yl)phenyl]amine and C70 combined with optimized electron transport layer Yan-qiong Zheng, William J. Potscavage, Takeshi Komino, and Chihaya Adachi Citation: Appl. Phys. Lett. 102, 153302 (2013); doi: 10.1063/1.4801954 View online: http://dx.doi.org/10.1063/1.4801954 View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v102/i15 Published by the AIP Publishing LLC.

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APPLIED PHYSICS LETTERS 102, 153302 (2013)

Highly efficient bulk heterojunction photovoltaic cell based on tris[4-(5-phenylthiophen-2-yl)phenyl]amine and C70 combined with optimized electron transport layer Yan-qiong Zheng,1,2 William J. Potscavage, Jr.,3 Takeshi Komino,3 and Chihaya Adachi1,2,3,a) 1

Center for Future Chemistry, Kyushu University, Fukuoka 819-0395, Japan Life Beans Center Kyushu, Bio Electromechanical Autonomous Nano-Systems (BEANS) Laboratory, 744 Motooka, Nishi, Fukuoka 819-0395, Japan 3 Center for Organic Photonics and Electronics Research (OPERA) and International Institute for Carbon Neutral Energy Research (WPI-I2CNER), Kyushu University, 744 Motooka, Nishi, Fukuoka 819-0395, Japan 2

(Received 26 January 2013; accepted 2 April 2013; published online 16 April 2013) Efficient bulk heterojunction (BHJ) photovoltaic cells (PVs) based on 5 wt. % donors and C70 were fabricated. Tris[4-(5-phenylthiophen-2-yl)phenyl]-amine (TPTPA)-based BHJ PVs show higher power conversion efficiency (gPCE) than aluminum phthalocyanine chloride-based BHJ PVs. Although the absorption of AlPcCl is complementary to that of C70, TPTPA’s high hole mobility and symmetrical molecular structure are likely to be crucial contributing factors to the higher gPCE. Phase separation occurs in the 5%-TPTPA blend. The device was optimized via replacement of the bathocuproine buffer by a combination of 3,4,9,10-perylenetetracarboxylic bis-benzimidazole C 2013 and bathocuproine. gPCE of 5.96% is achieved because of the decreased series resistance. V AIP Publishing LLC [http://dx.doi.org/10.1063/1.4801954] Organic photovoltaic cells (OPVs) have been widely studied in recent years because of their tremendous potential to provide flexible, inexhaustible, and clean sources of electricity in the future. Apart from the synthesis of new lightabsorbing semiconductors to act as donors and acceptors in OPVs, improvement of the cell architecture provides an important approach to produce efficient OPVs. Tris[4-(5phenylthiophen-2-yl)phenyl]amine (TPTPA) shows significant horizontal molecular orientation and exhibits the highest recorded hole drift mobility (l ¼ 1.0 102 cm2 V1 s1) among the reported organic disordered systems.1,2 The driving voltage of organic electroluminescence devices (OEs) decreased because of the high carrier mobility of TPTPA.3,4 Also, planar heterojunction (PHJ) PVs fabricated with TPTPA as the donor obtained power conversion efficiencies (gPCE) of 1.7%–2.2%.5 When C70 was used as the acceptor, a TPTPA-based PHJ cell with MoO3 as a cathode interlayer achieved a gPCE of 3.3%.6 Recently, TPTPA was used as an exciton blocking layer to increase the gPCE of tetraphenyldibenzoperiflanthene-based PHJ cells from 3.9% to 5.2%.7 Therefore, TPTPA-based OPVs have the potential to achieve high gPCE. Because of the weak visible absorption of TPTPA, the photocurrent from TPTPA is limited. However, the properties of TPTPA provide the possibility of fabrication of highly efficient bulk heterojunctions (BHJs) with small amounts of donor.8 To date, there have been no reports of BHJs containing TPTPA. In this study, we focus on BHJs fabricated with C70 and a small amount of TPTPA and discuss the possible operating mechanisms. Finally, we optimize the gPCE by improving the cathode buffer layer. All chemicals used were purchased commercially and were used without further purification, except for TPTPA a)

Author to whom correspondence should be addressed. Electronic mail: [email protected]

0003-6951/2013/102(15)/153302/4/$30.00

and aluminum phthalocyanine chloride (AlPcCl). The organic and metal layers were deposited sequentially on clean ITO (indium tin oxide) glass substrates with a sheet resistance of 25 X cm2 through shadow masks, to form devices with an active area of 0.024 cm2. The ITO substrates were treated for 15 min in an ultraviolet-ozone chamber before being placed in the deposition chamber. The blend layer was deposited by co-evaporation of the donor and acceptor at a rate of 0.005:0.095 nm/s for BHJs with 5 wt. % donor. All the devices were measured under a vacuum immediately after fabrication. Current density-voltage (J-V) characteristics and the incident-photon-to-current-conversion efficiency (IPCE) were measured by using an AM1.5G sunlight simulator (Bunko Keiki OTENTO-SUN II, Bunkokeiki Co.) and a Xe arc lamp in conjunction with a monochromator (Bunko Keiki SM-10P). Transmission electron microscopy (TEM) images were measured with an acceleration voltage of 200 kV (TITAN80-300, FEI Co.). All of the highest occupied molecular orbital (HOMO) levels were measured by ultra-violet photoelectron spectroscopy (AC-2, Riken Keiki Co.). The lowest unoccupied molecular orbital (LUMO) energies were determined by measuring the low-energy optical absorption edge positions. The refractive indices were measured by variable angle spectroscopic ellipsometry using a fast spectroscopic ellipsometer (M-2000U, JA Woollam Co.). A device with a configuration of ITO/MoO3 (6 nm)/ TPTPA:C70 (5 wt. %, 40 nm)/bathocuproine (BCP) (10 nm)/ Ag (100 nm) was fabricated. For comparison, another device using AlPcCl as the donor was also prepared, because AlPcCl is a representative metal-phthalocyanine with a similar HOMO level of 5.3 eV. Figure 1(a) depicts the absorption spectra of the donors and C70. TPTPA shows narrow absorption from 350 to 460 nm, while AlPcCl presents wide and strong absorption from 600 to 830 nm. Interestingly, C70 has

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Appl. Phys. Lett. 102, 153302 (2013) TABLE I. Organic photovoltaic characteristics.

Active layers

FIG. 1. Absorption spectra and refractive indices of C70, TPTPA, and AlPcCl.

an absorption spectrum that is complementary to AlPcCl, with a peak of 520 nm. The refractive indices of TPTPA, AlPcCl, and C70 are shown in Fig. 1(b). AlPcCl shows a complementary refractive index to C70 from 350 to 720 nm, which is important because refractive index matching could increase the light absorption of devices.9 To investigate the effects of the TPTPA content in the C70 matrix, BHJs with TPTPA content ranging from 5% to 40% were fabricated. The J-V characteristics and IPCE spectra are shown in Figs. 2(a) and 2(c), and the photovoltaic properties of these BHJs are summarized in Table I. The

FIG. 2. J–V characteristics (a), (b) and IPCE spectra (c), (d) of TPTPAbased BHJs (a), (c) and AlPcCl-based BHJs (b), (d) with C60 or C70 as the acceptor at various donor concentrations (indicated by X% acceptor).

5% TPTPA-C60 5% TPTPA-C70 20% TPTPA-C70 40% TPTPA-C70 5% AlPcCl-C60 5% AlPcCl-C70 20% AlPcCl-C70 40% AlPcCl-C70

JSC (mA cm2)

VOC (V)

FF

gPCE (%)

5.5 11.6 10.1 5.9 5.6 10.0 11.0 6.2

0.92 0.91 0.82 0.79 1.01 1.02 0.92 0.81

0.520 0.496 0.490 0.336 0.387 0.397 0.386 0.278

2.64 5.23 4.05 1.57 2.18 4.07 3.91 1.39

highest gPCE was obtained with 5% TPTPA, and the VOC and gPCE decreased distinctly with increasing TPTPA content. The relatively high fill factor (FF) of TPTPA-based BHJs is beneficial to the overall gPCE. One possible reason for this is that the high hole drift mobility guarantees fluent hole extraction before the holes recombine with electrons. Another possibility is that the symmetrical molecular configuration of TPTPA is good for close packing with the fullerenes and supports uniform hole transport. This suggests that easy hole transport in the blend layer with a small amount of donor is important to obtain a high gPCE.10 The IPCE spectra suggest that the photocurrent mainly comes from C70, and the maximum IPCE value reaches approximately 80% for the 5%-TPTPA BHJ. BHJs with C60 as the acceptor were also investigated, as shown in Fig. 2 and Table I, and the JSC and overall gPCE were almost half of those of C70 BHJs with the same architecture. AlPcCl BHJs with various donor concentrations were fabricated for comparison, and Figs. 2(b) and 2(d) show the corresponding J-V curves and IPCE spectra. All of the characteristics are summarized in Table I. The gPCE of the 5%-AlPcCl:C70 BHJ is only 78% of that of the 5%-TPTPA BHJ. With an increase in the AlPcCl concentration, the JSC values are higher than those of TPTPA BHJs with the same donor content, which is because of the matching of the absorption and refractive index characteristics of AlPcCl with C70. However, the lower FF limits the overall gPCE of AlPcCl BHJs in all the architectures. The low FF may originate from the relatively low hole mobility or the lower molecular compatibility with fullerenes.11 To determine the mechanism leading to the high gPCE of the 5%-TPTPA BHJ, the morphology of the 5% TPTPA blend layer was investigated. Figure 3 shows the cross

FIG. 3. Cross sectional TEM images of 5% TPTPA:C70 device by (a) energy-field mode and (b) bright-field mode.


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sectional TEM images of the 5%-TPTPA:C70 device in two TEM modes. The dispersed lattice structures of C70 are observed by the bright-field mode (b), implying the formation of a C70 crystal lattice with a size of about 20 nm in the blend. Simultaneously, a small number of TPTPA clusters of less than 4 nm (black dots) disperse in the C70 matrix, as shown in Fig. 3(a). By considering this in combination with the lattice structure of C70 in the blend layer, we can conclude that phase separation of TPTPA and C70 occurs and that hole and electron penetration paths exist in the blend layer, as deduced from the comparable FF and the ultrahigh JSC, respectively. Along with the active layer, we also investigated the effects of the anode interlayer. The MoO3 layer was replaced by N,N0 -bis(naphthalen-1-yl)-N,N0 -bis (phenyl)-benzidine (a-NPD) because it has a similar HOMO level (5.4 eV) to that of TPTPA (5.4 eV) together with a relatively high hole mobility (l ¼ 4.8 104 cm2 V1 s1).12 The device configuration is ITO/a-NPD (10 nm)/TPTPA:C70 (5 wt.%, 40 nm)/ BCP (10 nm)/Ag (100 nm). The photovoltaic parameters are listed in Table II. However, the gPCE of the BHJ with the 10 nm a-NPD layer is much lower than that with a 6 nm MoO3 layer, and the maximum IPCE decreases from 80% (MoO3) to 70% (a-NPD) because of the additional bandbending effect13 and superior hole-injection ability of MoO3 in comparison to a-NPD. It was also found that the built-in field caused by band bending induced by the MoO3 interlayer enhances hole extraction from AlPcCl to the anode.14,15 The BHJ without any anode buffer layer shows a lower VOC of 0.62 V but a high JSC, suggesting that MoO3 has a greater impact on VOC than on JSC. To further improve the gPCE of the 5%-TPTPA:C70 device with a 6 nm MoO3 interlayer, we optimized the cathode buffer layer. 3,4,9,10-perylenetetracarboxylic bisbenzimidazole (PTCBI), which is usually used as an acceptor layer in OPVs,16,17 and 1,4,5,8-napthalene-tetracarboxylicdianhydride (NTCDA) were used as a cathode buffer layer to replace the commonly used BCP.18 Because their LUMO levels are well aligned with those of C70, PTCBI and NTCDA allow low-resistance transport of electrons directly from C70 to the cathode. The configurations of the cathode buffer layers are listed in Table II, and Fig. 4 shows the J-V characteristics. The combination of PTCBI and transparent NTCDA maintains the JSC but lowers the FF considerably, while 10 nm of PTCBI alone enhances the FF. The AFM

FIG. 4. J–V characteristics of ITO/MoO3 (6 nm)/TPTPA:C70 (5 wt. %, 40 nm)/ cathode buffer/Ag (100 nm). The cathode buffers are shown in Table II.

images of neat NTCDA and PTCBI films on quartz substrates were measured, and the root mean square (RMS) roughness values of the 23 nm films were 14.1 nm and 1.3 nm, respectively. Therefore, one possible explanation for the high FF of the BHJ with PTCBI is its smooth surface. Compared with the 10 nm BCP cathode buffer layer, a combination of 5 nm PTCBI and 5 nm BCP achieves higher JSC and FF, and the possible mechanism for this is depicted in Fig. 5. Electron transport through BCP is via cathode-metal deposition-induced damage that results in a high density of conducting trap states,19 so the BCP thickness is therefore limited by the depth of the damage (no thicker than 10 nm). PTCBI can enhance the FF, but the devices with PTCBI alone as buffer show a low yield of 50% because of some shortage, which probably arises from penetration of the deposited Ag. Insertion of a thin BCP layer achieves a high yield of 100%. When using PTCBI and BCP in a compound blocking layer, as shown in Fig. 5(b), the electrons can be transported with low resistance in the PTCBI layer in the absence of damage20 and are spontaneously extracted via the Ag-deposition-induced damage in BCP. Simultaneously, BCP acts as a hole-blocking layer. Thus, as shown in Table II, the improved FF of the BHJ cell with a PTCBI (5 nm)/BCP (5 nm) cathode interlayer can be attributed to reduction of the series resistance (RS). The device with PTCBI/BCP as

TABLE II. Photovoltaic characteristics of 5%-TPTPA BHJs with various anode and cathode buffer layers.

Buffer layers No MoO3 a-NPD (10 nm)a BCP (10 nm)b PTCBI (10 nm)b NTCDA (15 nm)/PTCBI (5 nm)b PTCBI (5 nm)/BCP (5 nm)b a

Anode buffer. Cathode buffer.

b

JSC (mA cm2)

VOC (V)

FF

gPCE (%)

9.6 6.6 11.6 11.4 11.5 12.2

0.62 0.81 0.91 0.92 0.88 0.91

0.480 0.206 0.496 0.537 0.481 0.538

2.85 1.10 5.23 5.62 4.86 5.96

FIG. 5. Schematic energy band diagrams of hole blocking layers that transport electrons via (a) BCP (via damage-induced trap states) and (b) PTCBI/ BCP (through the LUMO level directly, and then via damage-induced trap states); (c) the J-V curves of 5% TPTPA BHJs with BCP or PTCBI/BCP buffers under dark conditions at voltages over VOC.


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the buffer layer has a much higher J at voltages over VOC (see Fig. 5(c)) than the device with BCP as the buffer, which implies the lower RS. The RS was estimated from the inverse of the slope of the J-V curve at high forward bias, where series resistance dominates.21 By this means, the RS values are estimated to be 50.6 and 289.8 X cm2 for the PTCBI/BCP and BCP devices, respectively. The interface between PTCBI and the blend layer can contribute slightly to the JSC because of the acceptor properties and higher absorption coefficient of PTCBI in comparison to C70 over 590 nm. Also, the thin PTCBI layer may optimize the internal electric field, as the traps in BCP induce an electric field which is opposite to that from the heterojunction.18 By incorporating the advantages of PTCBI and BCP, the overall gPCE reaches 5.96%, which is one of the highest values of gPCE for a single PV reported to date.8,10,22 In conclusion, we prepared BHJs with 5 wt. % donors and C70 as the active layer. TPTPA BHJs show higher gPCE values than those of the AlPcCl BHJs because of their higher hole drift mobility. We deduce from the TEM images that phase separation occurs in the blend layer. A compound cathode interlayer of PTCBI/BCP obviously enhances the FF in comparison to BCP because of the decreased series resistance, and thus the overall gPCE is optimized to 5.96%. The authors would like to thank Sumika Chemical Analysis Center for the TEM measurements and are sincerely grateful for the financial support of the New Energy and Industrial Technology Development Organization (NEDO) and the BEANS Lab, Japan. This work was supported in part by the Funding Program for World-Leading Innovative R&D on Science and Technology (FIRST) and by the International Institute for Carbon Neutral Energy Research (WPI-I2CNER), sponsored by MEXT.

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