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Oct 11, 2013 - Adv. Energy Mater. 2014, 4, 1301226. A Depletion-Free, Ionic, Self-Assembled Recombination. Layer for Tandem Polymer Solar Cells.
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Jinho Lee, Hongkyu Kang, Jaemin Kong, and Kwanghee Lee* To increase the device efficiency[1–3] and stability[3–5] of polymer solar cells (PSCs), intensive studies have been conducted on the development of novel device configurations that utilize the unique features of PSCs. These device geometries include tandem architectures[6–9] and inverted structures.[7–9] Tandem PSCs are composed of two serially connected sub-cells that use low- and high-bandgap conjugated polymers with complementary absorption spectra to harvest the full solar spectrum. In contrast, inverted PSCs allow for the use of air-stable, printable metal electrodes (e.g., Ag ink) and thus exhibit much better device stability and printability than conventional PSCs that use low-work-function (WF) metal electrodes. Hence, inverted tandem PSCs (I-TPSCs) that combine the advantages of both device geometries are considered to be ideal for PSCs. Despite these excellent air stability and printability of I-TPSCs, the performance of these devices has been limited by a lack of efficient recombination layers (RLs) that electrically connect the two sub-cells. To realize an inverted tandem configuration, a RL comprising a bilayer of p-type and n-type interfacial materials must be introduced between the front and back subcells. Currently, combinations of p-type metallic poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and n-type semiconducting metal oxides (e.g., zinc oxide (ZnO) or titanium sub-oxide (TiOx)) are widely used in RLs due to their appropriate mechanical, optical, and electrical properties.[7–9] However, a significant mismatch between the Fermi levels (i.e., WFs) of PEDOT:PSS and metal oxides results in the charge carrier exchange between two materials for aligning their Fermi levels, thus forming depletion-induced Schottky barrier at the RL interface. Furthermore, sol-gel-processed metal oxides possess many charge trap sites. Conventional RLs require photo-[9–11] or chemical[12,13] doping to reduce the length of the depletion region and to fill the trap sites. As a result, metallic PEDOT:PSS/semiconducting metal oxide junctions may not be the best candidates for use in high-efficiency I-TPSCs. As an alternative, the metal oxides can be replaced with polyelectrolytes (PEs).[14–16] By applying thin PE films with thicknesses of only few nanometers, the energy level of the metallic electrodes can be modified without creating unwanted energetic barriers or charge traps. This results in remarkable WF J. Lee, H. Kang, J. Kong, Prof. K. Lee Department of Nanobio Materials and Electronics School of Materials Science and Engineering Heeger Center for Advanced Materials (HCAM) Research Institute for Solar and Sustainable Energies (RISE) Gwangju Institute of Science and Technology (GIST) Gwangju, 500–712, Republic of Korea E-mail: [email protected]

DOI: 10.1002/aenm.201301226

Adv. Energy Mater. 2014, 4, 1301226

reductions in the electrodes through the formation of interfacial dipoles.[14–16] Here, by using ionic self-assembly of a nonconjugated polyelectrolyte (NPE) on a metallic indium tin oxide (ITO) cathode for the front cell and a PEDOT:PSS cathode for the back cell, we demonstrate an innovative method for achieving depletion-free, ionic self-assembled RLs that enable highly efficient I-TPSC operation. Particularly, the PEDOT:PSS/ NPE RLs form tunnel junctions with different work functions of 5.1 eV and 3.9 eV on each of their surfaces. This allows for loss-free charge recombination at the single metallic polymer layer. Our tandem solar cells containing these RLs exhibit solar power conversion efficiencies exceeding 8.3%. Figure 1a shows the chemical structures of the three NPEs (polyethyleneimine (PEI), polyallylamine (PAA), and polylysine (PLS)) used in this study. These polymers all have functional amines that can be partially protonated in aqueous solution, resulting in cationic NPEs. These cationic NPEs can easily form ionic self-assembled NPE dipole layers on negatively charged surfaces.[15,17] In our I-TPSCs, NPEs are coated on PEDOT:PSS and ITO surfaces to create PEDOT:PSS/NPE RLs (PNRLs) and ITO/NPE cathodes (Figure 1b,c). To easily explain the mechanism for the dipole formation of NPEs with ITO and PSS, we simplified the chemical structures of the NPEs as a typical polyamine with the simplest repeating unit as shown in Figure 1b. The protonated amines of the NPEs electrostatically interact with the sulfonate anions (–SO3−) of the PEDOT:PSS surface and the oxygen anions (–O−) of the ITO surface, significantly reducing the surface WF of the PEDOT:PSS and ITO through a downward vacuum level shift (Figure 1d). As a result, the PNRLs and ITO/NPE cathodes can form tunnel junctions with the fullerenes of the sub-cells. The ionic self-assembly of the NPEs on the PEDOT:PSS and ITO surfaces is based on two acid-base reactions,[18–21] a proton transfer reaction and a neutralization reaction (Figure 1b). The PEDOT:PSS and ITO are sources of anions, sulfonic acids (–SO3H), and terminal hydroxyls (–OH), respectively. Therefore, when the basic NPE (pH ≈ 10.1 for PEI, 9.8 for PAA, and 7.6 for PLS) solutions consisting of neutral amines, cationic protonated amines, and counter hydroxyl anions are deposited on the surfaces, the neutral amines and hydroxyl anions react with the protons (H+) arising from the deprotonation (–SO3H ⇒ –SO3− + H+, –OH ⇒ –O− + H+) of the surfaces. These reactions produce additional protonated amines through a proton transfer reaction (–N– + H+ ⇒ –HN+–) and eliminate counter hydroxyl anions by way of a neutralization reaction (OH− + H+ ⇒ H2O), significantly increasing the number of protonated amines that participate in ionic self-assembly with the surface anions at the expense of interactions with counter hydroxyl anions. The sulfonic acid of the PEDOT:PSS (pH = 2.0) can be more easily deprotonated than the terminal hydroxyls

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A Depletion-Free, Ionic, Self-Assembled Recombination Layer for Tandem Polymer Solar Cells

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Figure 1. a) Chemical structures of the NPEs. b) Chemical interactions of the NPE with the PSS and the ITO. c) Structure of the I-TPSC. d) Energy level diagram of the I-TPSC.

of the ITO; therefore, the reactivity of the NPEs on the acidic PEDOT:PSS is much stronger than on the ITO, leading to more protonated amines on the PEDOT:PSS surface than on the ITO surface. The PNRLs thus have more protonated amines than the ITO/NPE cathodes and yield larger WF reductions through stronger dipole formation. To validate our assumption, we compared the surface chemical composition of the PNRLs and the ITO/NPE cathodes using X-ray photoelectron spectroscopy (XPS). The survey XPS spectra of the PNRLs and the ITO/NPE cathodes clearly show N1s peaks at a binding energy of 400 eV (see Supporting Information, Figure S1). In all of the high-resolution XPS spectra, we observed two asymmetric N1s peaks that are attributed to neutral amines at low binding energies and protonated amines at high binding energies (see Figure 2a). We note that the peak intensities of the PNRLs at high binding energies are significantly higher than those of the ITO/NPE cathodes,

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indicating that more amines of the NPEs are protonated on the PEDOT:PSS surface than on the ITO surface.[22–24] In addition, we confirmed that a glass/PSS/NPE sample has a higher concentration of protonated amines than a glass/NPE sample, implying that the acidic PSS of the PEDOT:PSS is the main contributor of additional protonation (Supporting Information, Figure S2). Consequently, these results support our assumption that the neutralization and proton transfer reactions between the basic NPEs and the acidic PEDOT:PSS increase the number of protonated amines within the PNRLs. Using the Kelvin probe (KP) method, we investigated the correlation between the level of protonated amines and WF changes (ΔWF) for the PNRLs and the ITO/NPE cathodes. Based on our previous report,[15] [N+]/[C] ratios determine the intensity of the interfacial dipoles (i.e., ΔWF) in the ITO/NPE cathodes, where carbon atoms hinder the formation of the NPE dipole layers. As shown in Table 1, we confirmed that the [N+]/[C] ratios (14.12 for

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(b) N1s

Intensity (a.u.)

ITO/PEI

On ITO

PEDOT:PSS/PAA

ITO/PAA

PEDOT:PSS/PLS

ITO/PLS

3.6

PEDOT:PSS/PEI

Work function (eV)

N1s

On PEDOT:PSS

4.0

4.4

4.8

5.2

397 398 399 400 401 402 403 398 399 400 401 402 403 404

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(a)

ITO

Binding Energy (eV)

PEI

PAA PLS PEO PEDOT PEI :PSS

PAA PLS PEO

Figure 2. a) High-resolution XPS spectra of the ITO/NPEs and PNRLs and b) KP data for ITO and PEDOT:PSS modified with NPE.

PEI, 10.79 for PAA, and 5.22 for PLS) of the PNRLs are much larger than the corresponding [N+]/[C] ratios (5.76 for PEI, 4.37 for PAA, and 3.62 for PLS) of the ITO/NPE cathodes. Thus, the ΔWF values (1.22 eV for PEI, 1.11 eV for PAA, and 0.66 eV for PLS) of the PNRLs are larger than the ΔWF values (0.85 eV for PEI, 0.60 eV for PAA, and 0.40 eV for PLS) of the ITO/NPE cathodes (Figure 2b). We also measured the ΔWF values of ITO/ PEI and PEDOT:PSS/PEI using ultraviolet photoelectron spectroscopy (UPS) and confirmed that the results are very similar with the KP data as shown in Figure S3 (Supporting Information). To clarify the role of the protonated amines, we also investigated polyethylene oxide (PEO), which is one of the NPE containing neutral ethers in its backbone. In contrast to NPE with amine groups, PEO on PEDOT:PSS and ITO did not make any difference between the ΔWF (0.38 eV) of PEDOT:PSS/PEO and the ΔWF (0.36 eV) of ITO/PEO. This result implies that the neutral ethers of the PEO are unable to induce additional dipole moments, even on the acidic PEDOT:PSS surface. Thus, these observations demonstrate that the protonated amines of NPEs, rather than neutral amines, play an essential role in reducing the WFs of ITO and PEDOT:PSS. The WF values are an important criterion for fabricating efficient I-TPSCs. Therefore, we selected PEI, as it showed had the lowest WF (3.88 eV in the PNRL and 3.95 eV in the ITO/NPE cathode) and the highest performance in the single inverted PSC (Table 1 and Supporting Information, Figure S4). In addition, we confirmed that PEI shows homogeneous surface and morphology on ITO and PEDOT:PSS by using atomic force

microscopy (AFM) and scanning probe microscopy (SPM) (Supporting Information, Figure S5,S6). The front sub-cell is composed of poly(3-hexylthiophene) (P3HT) and indene-C60 bisadduct (IC60BA). The back sub-cell consists of poly(thieno[3,4-b] thiophene-alt-benzodithiophene) (PTB) and [6,6]-phenyl C71butyric acid methyl ester (PC71BM). The current density–voltage (J–V) characteristics and performance parameters of the singlejunction devices for the front and back sub-cells and the I-TPSC using the PNRLs were characterized under standard irradiation conditions, as shown in Figure 3a and Table 2. It is worth noting that we fabricated more than 100 devices and all data shown represent average values for 20 devices. The best performance of the P3HT:IC60BA single cell yields a PCE of 5.58%, with an open circuit voltage (Voc) of 0.85 V, a short circuit current (Jsc) of 9.38 mA cm−2, and a fill factor (FF) of 0.70. The PTB:PC71BM single cell yields a PCE of 6.36%, with a Voc of 0.72 V, a Jsc of 14.25 mA cm−2, and a FF of 0.62. These single cell results indicate that inverted structures can be well established using NPE interlayers. By carefully controlling the thicknesses of the photoactive layers, the optimized I-TPSC exhibits a PCE of 8.32%, with a Voc of 1.57 V, a Jsc of 7.46 mA cm−2, and a FF of 0.71. We note that the Voc of the I-TPSC is exactly equal to the sum of the Voc values of the front and back sub-cells. Moreover, the high FF of the I-TPSC is comparable to the FF of a currentlimiting P3HT:IC60BA single cell.[6,25–27] Based on Kirchhoff's law,[9,27,28] the exact sum of the Voc values is determined by potential-loss-free series connections.[29,30] This indicates that the PNRL can act as an efficient recombination center for the

Table 1. Ratio of protonated amines to carbons and the corresponding ΔWFs for ITO and PEDOT:PSS modified with NPEs.

ITO

WF [eV]

ΔWF [eV]

4.80 (± 0.05)





PEDOT:PSS PEDOT:PSS/PEI

3.88 (± 0.05)

[N+]/[C] [×10−2]

WF [eV]

ΔWF [eV]

5.10 (± 0.05)





[N+]/[C] [×10−2]

ITO/PEI

3.95 (± 0.05)

0.85 (± 0.05)

5.76

1.22 (± 0.05)

14.12

ITO/PAA

4.20 (± 0.05)

0.60 (± 0.05)

4.37

PEDOT:PSS/PAA

3.99 (± 0.05)

1.11 (± 0.05)

10.79

ITO/PLS

4.40 (± 0.05)

0.40 (± 0.05)

3.62

PEDOT:PSS/PLS

4.44 (± 0.05)

0.66 (± 0.05)

5.22

ITO/PEO

4.44 (± 0.05)

0.36 (± 0.05)



PEDOT:PSS/PEO

4.72 (± 0.05)

0.38 (± 0.05)



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www.MaterialsViews.com Table 2. Cell parameters of the front cell, the back cell, and the tandem cell. Voc [V]

Jsc [mA cm-2]

FF

PCE [%]

Front cell

0.84 ± 0.01

9.17 ± 0.21

0.69 ± 0.01

5.28 ± 0.3

Back cell

0.71 ± 0.01

14.00 ± 0.25

0.60 ± 0.02

6.16 ± 0.2

Tandem cell

1.55 ± 0.02

7.26 ± 0.42

0.69 ± 0.02

8.02 ± 0.3

1.57

7.46

0.71

8.32

Best

lowest unoccupied molecular orbital level of the fullerene for the back sub-cell (Figure 1d). Moreover, the high FF indicates that the ultrathin NPEs can efficiently form metal/semiconductor tunnel junctions to facilitate trap-free electron quantum tunneling from the PC71BM to the PEDOT:PSS. Indeed, the PNRL shows extremely low resistance compared with conventional RLs. For example, the much thicker PEDOT:PSS/ZnO RLs possess many trap sites and a Schottky barrier (0.7 eV), which result in high resistance without UV treatment. After UV treatment, the trap sites of the ZnO become filled with photoinduced charges, reducing the width of the energetic barrier (Supporting Information, Figure S7–S9 and Table S2).[9–11] Considering the inevitable decrease in the incident photon-tocurrent efficiency of the sub-cells due to a substantial absorption overlap between the front and back photoactive materials (Figure 3b,c), we believe that the PCE of 8.32% is the maximum efficiency achievable using these polymer donor systems. In conclusion, we have described an efficient I-TPSC using a combination of PEDOT:PSS and NPEs as an efficient RL. Furthermore, we have clearly proposed the working mechanism of the PNRLs in the I-TPSC. Using XPS and KP analysis, we confirmed that the PNRLs show much larger ΔWF values than the ITO/NPE cathodes. This finding is attributed to the increased concentration of protonated amines caused by neutralization and proton transfer reactions between the basic NPEs and the acidic PEDOT:PSS. Due to the presence of dual WFs and formation of tunnel junctions, the PNRLs act as efficient recombination centers for I-TPSCs, resulting in high-efficiency I-TPSC operation with a PCE of 8.32%. These findings offer a strategic RL design rule for achieving more efficient solar-light power generation from I-TPSCs.

Experimental Section

Figure 3. a) J–V characteristics under Air Mass 1.5 global (AM 1.5G) illumination for the front cell, the back cell, and the tandem cell. b) IPCE spectra of single cells and tandem cells. c) Spectrum from the AM 1.5G and absorption spectra of the front cell, the back cell, and the homopolymers.

holes (from the front sub-cell) and the electrons (from the back sub-cell) due to its two WF values (5.1 eV and 3.88 eV), which induce ohmic contacts with the highest occupied molecular orbital level of the P3HT for the front sub-cell and the

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Materials and Preparation: PEI (Aldrich, 50 wt% in H2O), PAA (Aldrich, 20 wt% in H2O), and PLS (Sigma, 0.1% (w/v) in H2O) were diluted in isopropanol (IPA) to 0.1 wt%, 0.1 wt%, and 0.05 wt%, respectively. PEO was diluted in IPA to 0.4 wt% after being dissolved in deionized (DI)-water to 1 wt%. The sol-gel-based ZnO precursor solution was prepared by dissolving 100 mg of zinc acetate dehydrate (Aldrich, reagent grade) in 50 mg of ethanolamine (Aldrich, 99.5%) and 5000 mg of 2-methoxyethanol (Aldrich, anhydrous, 99.8%) by stirring under ambient conditions. Device Fabrication: I-TPSCs were fabricated with a structure of ITO/PEI/ P3HT:IC60BA/PEDOT:PSS/PEI/PTB:PC71BM/PEDOT:PSS/Ag. The precleaned ITO-coated glass substrates were treated with UV-ozone before device fabrication. PEI was spin-casted at 5000 rpm from dilute solutions and then dried at 100 °C for 5 min. In single photoactive layer devices, PAA, PLS, PEO, and ZnO were spin-casted at 5000 rpm, 2000 rpm, 5000 rpm, and 2000 rpm, respectively. They were then annealed at 100 °C, 120 °C, 120 °C, and 200 °C, respectively, for 10 min. The samples were then transferred into a glove box. For fabrication of

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Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements J.L. and H.K. contributed equally to this work. The authors thank the Heeger Center for Advanced Materials (HCAM) and the Research Institute of Solar and Sustainable Energies (RISE) at the Gwangju Institute of Science and Technology (GIST) of Korea for device fabrication and measurements. This research was supported by a National Research Foundation (NRF) of Korea grant funded by the Korea government (MSIP) (No. 2008-0093869 and No. 2008-0062606, CELANCRC). K.L. acknowledges support provided by the Core Technology Development Program for Next-generation Solar Cells of RISE, GIST.

Adv. Energy Mater. 2014, 4, 1301226

The authors thank Dr. Jouhahn Lee at the Korea Basic Science Institute (KBSI) for UPS measurements. Received: August 13, 2013 Revised: September 12, 2013 Published online: October 11, 2013

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the front cell, a solution containing P3HT:IC60BA (1:1 weight ratio) in 1,2-dichlorobenzene solvent with a concentration of 1.4 wt% was spincoated at 800 rpm for 30 s and then solvent annealed in a glass petri dish for 3 h. Before coating the conducting PEDOT:PSS (Clevios P) on the P3HT:IC60BA film, the surface was modified by spin-casting a dilute solution of PEDOT:PSS (AI 4083) with IPA at a 1:7 volume ratio at 2000 rpm to prevent dewetting. Subsequently, PEDOT:PSS (Clevios P) was spin-casted at 5000 rpm for 80 s and then annealed at 150 °C for 10 min. For fabrication of the back cell, PEI and PAA were applied as described for the front cell. Next, the PEDOT:PSS/PEI film on the glass area without ITO was removed to avoid direct connections between the top electrode and the RL during measurement. PTB:PC71BM (1:1.5) in 0.7 wt% chlorobenzene/1,8-diiodooctane (DIO) (97:3 vol%) solution was spin-coated at 2000 rpm and then baked at 80 °C for 10 min. Then, a PEDOT:PSS (AI 4083) solution mixed with IPA at a 1:6 volume ratio was directly spin-casted on the PTB:PC71BM film at 2000 rpm and dried at 100 °C for 5 min. Finally, the silver (Ag, 120 nm) electrode material was deposited by thermal evaporation in a 1 × 10−6 torr vacuum. The Ag electrode area (14.5 mm2) defined the active area of the device. Calibration and Measurement: The intensity of the solar spectrum obtained from the xenon (Xe) lamp (150 W Oriel) of the solar simulator was calibrated using a calibrated standard silicon solar cell certified by the National Renewable Energy Laboratory (NREL). The J–V characteristics of the devices were measured using a Keithley 236 Source Measure Unit (SMU), under Air Mass 1.5G global (AM 1.5G) illumination with a 100 mW cm−2 irradiation intensity. To avoid overestimation during measurements from parallel transport through the highly conductive PEDOT:PSS (Clevios P), the sample was illuminated through a mask with a defined area of 14.5 mm2. For more accurate measurements, the IPCE spectra of individual sub-cells in the I-TPSC were measured by selective excitation using a monochromatic light bias (532 nm and 700 nm). To measure the thicknesses of the NPE films, the absorption coefficients of the NPEs were first derived from the optical absorbance of the NPE films with mechanically measurable thicknesses (by Alpha-Step equipment). Then, assuming a linear proportion between the absorption intensity and the film thickness, the NPE film thicknesses were calculated from the optical absorbance of the ultrathin NPE films. XPS and KP Measurement: The XPS measurements were performed in a MultiLab 2000 (Thermo electron corporation, England, a base pressure of 1 × 10−9 Torr) using monochromatized Al K α X-ray photons (hv = 14.9 keV for XPS). The pass energy and a step size were 50 eV and 0.5 eV for survey spectra and 20 eV and 0.05 eV for core level spectra. All samples were kept inside a high vacuum chamber overnight to remove solvent. The contact potential difference (CPD) of each sample was detected by a Kelvin probe (KP 6500 Digital Kelvin probe, McAllister Technical Services. Co. Ltd) under dry nitrogen atmosphere. The CPD was calibrated to highly ordered pyrolytic graphite (HOPG) at 4.58 ± 0.03 eV. To obtain the accurate data, measurements were performed for each KP and XPS analysis.