Tandem Solar Cells from Accessible Low Band-Gap Polymers Using

Dec 23, 2015 - ... Laboratory, Wright-Patterson Air Force Base, Dayton, Ohio 45433, United States ... Tandem solar cell architectures are designed to ...
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Tandem Solar Cells from Accessible Low Band-Gap Polymers Using an Efficient Interconnecting Layer Santanu Bag,*,§,‡ Romesh J. Patel,§,‡ Ajaykumar Bunha,§,‡ Caroline Grand,⊥ J. Daniel Berrigan,§ Matthew J. Dalton,§ Benjamin J. Leever,§ John R. Reynolds,⊥ and Michael F. Durstock*,§ §

Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright-Patterson Air Force Base, Dayton, Ohio 45433, United States ‡ Biological and Nanoscale Technologies Division, UES Inc. Dayton, Ohio 45432, United States ⊥ School of Chemistry and Biochemistry, School of Materials Science and Engineering, Center for Organic Photonics and Electronics, Georgia Tech Polymer Network, Georgia Institute of Technology, Atlanta, Georgia 30332, United States S Supporting Information *

ABSTRACT: Tandem solar cell architectures are designed to improve device photoresponse by enabling the capture of wider range of solar spectrum as compared to single-junction device. However, the practical realization of this concept in bulkheterojunction polymer systems requires the judicious design of a transparent interconnecting layer compatible with both polymers. Moreover, the polymers selected should be readily synthesized at large scale (>1 kg) and high performance. In this work, we demonstrate a novel tandem polymer solar cell that combines low band gap poly isoindigo [P(T3-iI)-2], which is easily synthesized in kilogram quantities, with a novel Cr/MoO3 interconnecting layer. Cr/MoO3 is shown to be greater than 80% transparent above 375 nm and an efficient interconnecting layer for P(T3-iI)-2 and PCDTBT, leading to 6% power conversion efficiencies under AM 1.5G illumination. These results serve to extend the range of interconnecting layer materials for tandem cell fabrication by establishing, for the first time, that a thin, evaporated layer of Cr/MoO3 can work as an effective interconnecting layer in a tandem polymer solar cells made with scalable photoactive materials. KEYWORDS: polymers, tandem solar cells, scalable, isoindigo, interconnecting layer, solution processing

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a wide band gap material as a front cell for high energy photon absorption, an interconnecting layer in the middle for charge recombination, and a smaller band gap material in the back for low energy photon absorption. The entire device is then sandwiched between two electrodes to complete the circuit. Typically, the open circuit voltage (Voc) in such systems equals the sum of those of the individual subcells, thus enhancing the overall PCE, whereas the total current density is limited by the subcell with lower current density.8 The tandem architecture, therefore, offers an exceptional opportunity for overcoming efficiency limits that create a barrier toward commercialization of PSCs. Additionally, the low cost, solution-processed, high-throughput production of PSCs requires the development of a technology that is unlimited by materials supply.10 In this respect, isoindigo based conjugated polymers are attractive due to their simple, high yield and scalable synthetic routes, as well as the natural occurrence of their fundamental building units.11 The strong electron-withdrawing nature of the isoindigo

olution-processed polymer solar cells (PSCs) are some of the most actively studied devices among the photovoltaic (PV) community due to their potential for fabricating lightweight and large-area flexible PV devices by low-cost, roll-to-roll compatible deposition techniques.1,2 The adoption of a bulk-heterojunction (BHJ) concept,3 where an interpenetrating blend of conjugated polymers as electron donors and fullerene derivatives as electron acceptors serves as the photoactive layer, has been the key to continuous development of this field over the past several years. However, the inherent material properties of the conjugated polymers, such as low charge carrier mobility and lack of a broad absorption in the solar spectrum with proper charge-transport properties, constitute a bottleneck to their continued development and it is quite challenging to achieve greater than 10% power conversion efficiency (PCE) from single junction based PSCs.4,5 Multilayered tandem cell architectures composed of two or more complementary photoactive layers, stacked on top of each other in series and separated by a thin interconnecting layer (IL), utilize a wider range of the solar spectrum for photon absorption and provide a viable route to increase the overall PCEs, even surpassing 10%.6−9 So far, most tandem PSC devices are made from double-junction cells and consist of © XXXX American Chemical Society

Received: October 23, 2015 Accepted: December 23, 2015

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DOI: 10.1021/acsami.5b10170 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Letter

ACS Applied Materials & Interfaces (acceptor) moiety coupled with different electron rich (donor) units in repeating donor−acceptor type polymeric structures has led to the development of a variety of low-band gap polymers with promising PV properties. PSCs based on this class of polymeric materials have achieved PCEs close to 6% in single junction cells, with their absorption spectra extending to the near-infrared (NIR) region.11 The photoresponse can be further broadened by integrating complementary wide band gap polymers into tandem architectures. The isoindigo based polymers could, therefore, enable low-cost, high-throughput PSC production capability due to their high yield synthesis, scalability, and low band gap properties, once further efficiency enhancements are achieved. Besides an abundant supply of photoactive material, seriesconnected monolithic integration of two or more subcells in a tandem configuration requires an IL that joins the vertically stacked single junction subcells. Ideally, an efficient IL must have the proper band alignment with those of the donor and acceptor molecules in the active layers, high optical transparency, low electrical resistivity, easy processabilty, and good chemical and mechanical stability.8 Although several ILs have been developed in recent years, it has been shown that it is hard to satisfy all of these desired properties with a single material. Typically, an IL consists of a combination of nonabsorbing electron and hole transport layers in contact with two separate subcells and serves as a charge recombination zone for electrons from one cell and holes from the other.8 A major challenge faced during the integration of ILs with solution processed photoactive components is the difficulty in controlling the process such that the underlying layers are not destroyed upon further processing. Thus, the physical and chemical robustness of the IL is of great importance in tandem solar cell fabrication.3,8 In this work, we have developed a new, chemically robust tandem cell IL composed of a thermally evaporated Cr and MoO3 bilayer, and we show the utility of this layer through the fabrication of monolithic tandem solar cells from an easily accessible, isoindigo based low band gap polymer. A tandem device structure consisting of glass/ITO/PEDOT:PSS/ PCDTBT:PC71BM/Cr/MoO3/P(T3-il)-2:PC71BM/Ca/Al is fabricated and demonstrates successful series connection of subcells with cumulative Voc. The PCEs of these isoindigo based polymer tandem devices reached up to 6%, suggesting great promise toward low-cost polymer PV. The scope of this communication is to report proof-of-concept studies using Cr/ MoO3 as a new composite interlayer for tandem PSCs composed of a low band gap polymer which can be synthesized in kilogram quantities. It is presumed that further efficiency gains can be realized through optimization of polymer layer thicknesses to match the current densities of each photoactive layer and thereby minimize losses due to electron−hole recombination. Figure 1 shows the chemical structure of an isoindigo based low band gap (Eg = 1.6 eV) polymer, P(T3-iI)-2 synthesized in kilogram quantities. Its broad absorption spectra up to 800 nm, high extinction coefficient (∼1.2 × 105 cm−1 in 550−700 nm region), favorable energy levels and high solubility in organic solvents are advantageous for construction of BHJ PV devices. Optimized single junction solar cells were made from a 1:2 weight ratio [P(T3-il)-2:PC71BM] active layer deposited from chloroform with 2 vol % of 1,8-diiodooctane (DIO) (see Supporting Information). A regular device structure (Figure 2a) of glass/ITO/PEDOT:PSS (40 nm)/active layer/Ca (2 nm)/

Figure 1. (a,b) Molecular structures and (c) UV−visible-near-infrared absorption spectra of the (a) donor organic polymers, P(T3-il)-2 and PCDTBT, and (b) acceptor PC71BM molecule used in this work. Solid lines in panel c represent thin film samples, whereas dotted lines are from samples in CHCl3 solution. A small red shift in the absorption spectra, moving from solution to film, can be related to polymer aggregation.

Figure 2. (a) Single junction device structure and (b) J−V curves of P(T3-il)-2:PC71BM solar cells with different active layer thickness.

Al (100 nm) was used to make single active layer control devices, and the device performance shows a strong dependence on the active layer thicknesses. Here, a thin poly(3,4ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) layer was used as a hole conductor and calcium as an electron-buffer layer. Addition of 2 vol % DIO processing solvent additive in the blend significantly improved the morphology (Figure S1) of the active layer and consistently enhanced the PV device performance. The average PCE of the optimized P(T3-il)-2:PC71BM solar cell device reaches about 6.1% with a short-circuit current density (Jsc) of 14.0 ± 0.4 mA cm−2, a Voc of 0.72 ± 0.02 V, and a fill factor (FF) of 61 ± 4% when the thickness of the active layer is about 100 nm (Table S1). Increasing the thickness further to 130 nm results in a drop in FF and an improvement in Jsc, whereas Voc remains almost similar. On the other hand, for a slightly thinner active layer, 70 nm, Jsc decreases and FF slightly increases due to reduced light absorption and low charge recombination. In this case, the decreased J sc significantly outweighs the slight increase in FF and a concomitant loss in average PCE to 5.4% is observed. Figure 2b shows the current density−voltage (J−V) characteristics of the highest efficiency single junction P(T3-il)-2:PC71BM solar cells with various thicknesses (i.e., from 70 to 130 nm). The ultraviolet visible (UV−vis) spectrum of the 100 nm thick P(T3-il)-2:PC71BM film in Figure 3a shows nonuniform spectral response across its absorption range. Further enhancement in the light absorption, therefore, is highly desirable. Indeed, integration of this low band gap polymer with a wider band gap polymeric material in a tandem structure is a potential method to improve the photo response of the solar cell. B

DOI: 10.1021/acsami.5b10170 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Alternatively, we found depositing a thin bilayer of thermally evaporated Cr (∼2 nm) and ∼12 nm MoO3 yields16 a robust IL that gives consistently better device performance. Cr being a chemically inert element effectively protects the underlayers from subsequent device processing, and has recently been used to improve the stability of solution processed solar cells.17 In addition to its chemical stability, this newly developed thermally deposited Cr(∼2 nm)/MoO3(∼12 nm) IL is highly transparent (over 80%) in the range of 400−1000 nm (Figure S3), ensuring minimum optical loss when light passes from one subcell to another. Tandem solar cells are built from a PCDTBT:PC71BM front cell and a P(T3-il)-2:PC71BM back cell using this Cr/MoO3 recombination layer. Figure 3b,c shows the structure of the multilayer tandem cell together with the energy band diagram of the different layers, as obtained from literature values.11,12,16 The work functions of Cr and MoO3 are well aligned with the respective conduction band of PC71BM and the valence band of P(T3-il)-2 and therefore electrons from the front PCDTBT subcell combine with holes from the back P(T3-il)-2 subcell at the Cr/MoO3 interface. Since the total photocurrent from a series connected tandem cell is limited by the subcell with the lowest photocurrent, the PCDTBT wide band gap front cell is the limiting cell in this case. We chose an 80 nm thick PCDTBT front subcell with a high Jsc and FF to maintain sufficient transmission to the back subcell in order to achieve high performance in our tandem cell architectures. Low band gap P(T3-il)-2 rear cells with three different thickness, ranging from 70 to 130 nm, were studied, and the details of the tandem cell device performances are summarized in Table 1 (Table S3). Figure 4a shows the corresponding J−V curves of the tandem cells measured under simulated 100 mW/cm2 solar illumina-

Figure 3. (a) UV−vis NIR absorption spectra of a 80 nm PCDTBT:PC71BM BHJ composite film, a 100 nm P(T3-il)2:PC71BM BHJ composite film, and a bilayer of the two with 2 nm Cr/12 nm MoO3 in between. (b) Schematic of the tandem device structure in which a Cr/MoO3 interconnecting layer was employed. (c) Energy diagram for the various layers in an optimized tandem device (values are in eV).

However, an efficient, transparent recombination layer must be developed to facilitate integration. Poly [N-9′-heptadecanyl-2,7carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT) (Figure 1a),12,13 one of the most efficient and stable wide band gap conjugated polymers, was chosen as a visible absorber (Figure 1c) to enhance the solar absorption of the P(T3-il)-2:PC71BM PV device. Optimized single junction solar cells, made from a 1:2 weight ratio of in-house synthesized PCDTBT with PC71BM in chloroform, have a thickness of 80 nm with the best device efficiency reaching 5.2% under Air Mass 1.5 Global (AM1.5G) illumination (100 mW/cm2) from a calibrated solar simulator. Our device optimization results follow the same trend as seen in the majority of the literature (Table S2, Figure S2).12,13 The UV−vis absorption spectrum of this 80 nm thick film of PCDTBT:PC71BM (Figure 3a) shows slightly higher absorption in the 500−600 nm spectral range than that of a 100 nm thick optimized P(T3-il)-2:PC71BM film. The absorption spectrum of a bilayer film of 100 nm P(T3-il)2:PC71BM and 80 nm PCDTBT:PC71BM also shows a simple superposition of those of the two individual composites. Thus, the construction of tandem cells with these two BHJ material combinations is advantageous. To build efficient polymer tandem solar cells based on P(T3il)-2 materials, proper choice of the IL is critical. Our initial attempts to use one of the most common solution processable IL,14,15 consisting of ZnO nanoparticles as the electron transport layer and pH-neutral PEDOT:PSS as the hole transport layer, did not work well likely due to the complexity of ink formulation and aqueous nature of commercial PEDOT:PSS which potentially damaged the underlayers.

Figure 4. J−V curves of (a) tandem PSCs with various thicknesses of rear P(T3-il)-2:PC71BM cells; and (b) the subcells and the best tandem polymer cell.

tions. It can be seen that the tandem cell with an 80 nm front cell and a 100 nm back cell showed the highest performance with the best cell reaching 6.0% PCE. Atomic force microscopy (AFM) images of the thin-film surface topography at different stages of progression during the tandem cell fabrication process

Table 1. Summary of Averagea PV Parameters of the Optimized Solar Cells

a

sampleb

d (nm)c

PCDTBT P(T3-iI)-2 tandem tandem tandem

80 100 80/70 80/100 80/130

Jsc (mA·cm−2)

Voc (V) 0.88 0.72 1.48 1.51 1.42

(0.88) (0.74) (1.50) (1.53) (1.45)

9.9 (10.2) 14.0 (13.6) 7.2 (7.5) 8.3 (8.6) 7.6 (8.0)

FF (%)

PCE (%)

57 61 47 44 36

5.0 6.1 5.0 5.5 3.9

(58) (65) (50) (46) (40)

(5.2) (6.5) (5.6) (6.0) (4.6)

Values in parentheses are for the best performing devices. bBlend with PC71BM. cThickness of the active layer. C

DOI: 10.1021/acsami.5b10170 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Li, N.; Machui, F.; Spyropoulos, G. D.; Ameri, T.; Lemaĭtre, N.; Legros, M.; Scheel, A.; Gaiser, D.; Kreul, K.; Berny, S.; Lozman, O. R.; Nordman, S.; Välimäki, M.; Vilkman, M.; Søndergaard, R. R.; Jørgensen, M.; Brabec, C. J.; Krebs, F. C. Scalable, Ambient Atmosphere Roll-to-Roll Manufacture of Encapsulated Large Area, Flexible Organic Tandem Solar Cell Modules. Energy Environ. Sci. 2014, 7, 2925−2933. (3) Dou, L.; You, J.; Hong, Z.; Xu, Z.; Li, G.; Street, R. A.; Yang, Y. 25th Anniversary Article: A Decade of Organic/Polymeric Photovoltaic Research. Adv. Mater. 2013, 25, 6642−6671. (4) Huo, L.; Liu, T.; Sun, X.; Cai, Y.; Heeger, A. J.; Sun, Y. Singlejunction Organic Solar Cells Based on a Novel Wide-bandgap Polymer with Efficiency of 9.7%. Adv. Mater. 2015, 27, 2938−2944. (5) He, Z.; Zhong, C.; Su, S.; Xu, M.; Wu, H.; Cao, Y. Enhanced Power-conversion Efficiency in Polymer Solar Cells Using an Inverted Device Structure. Nat. Photonics 2012, 6, 593−597. (6) Zhou, H.; Zhang, Y.; Mai, C.-K.; Collins, S. D.; Bazan, G. C.; Nguyen, T.-Q.; Heeger, A. J. Polymer Homo-tandem Solar Cells with Best Efficiency of 11.3%. Adv. Mater. 2015, 27, 1767−1773. (7) Chen, C. C.; Chang, W.-H.; Yoshimura, K.; Ohya, K.; You, J.; Gao, J.; Hong, Z.; Yang, Y. An Efficient Triple-junction Polymer Solar Cell Having a Power Conversion Efficiency Exceeding 11%. Adv. Mater. 2014, 26, 5670−5677. (8) You, J.; Dou, L.; Hong, Z.; Li, G.; Yang, Y. Recent Trends in Polymer Tandem Solar Cells Research. Prog. Polym. Sci. 2013, 38, 1909−1928. (9) You, J.; Dou, L.; Yoshimura, K.; Kato, T.; Ohya, K.; Moriarty, T.; Emery, K.; Chen, C.-C.; Gao, J.; Li, G.; Yang, Y. A Polymer Tandem Solar Cell with 10.6% Power Conversion Efficiency. Nat. Commun. 2013, 4, 1446−1455. (10) Li, N.; Baran, D.; Spyropoulos, G. D.; Zhang, H.; Berny, S.; Turbiez, M.; Ameri, T.; Krebs, F. C.; Brabec, C. J. Environmentally Printing Efficient Organic Tandem Solar Cells with High Fill Factors: A Guideline Towards 20% Power Conversion Efficiency. Adv. Energy Mater. 2014, 4, 1400084-1−1400084-14. (11) Stalder, R.; Mei, J.; Graham, K. R.; Estrada, L. A.; Reynolds, J. R. Isoindigo, a Versatile Electron-deficient Unit for High-performance Organic Electronics. Chem. Mater. 2014, 26, 664−678. (12) Beaupré, S.; Leclerc, M. PCDTBT: En Route for Low Cost Plastic Solar Cells. J. Mater. Chem. A 2013, 1, 11097−11105. (13) Synooka, O.; Eberhardt, K.-R.; Singh, C. R.; Hermann, F.; Ecke, G.; Ecker, B.; von Hauff, E.; Gobsch, G.; Hoppe, H. Influence of Thermal Annealing on PCDTBT: PCBM Composition Profiles. Adv. Energy Mater. 2014, 4, 1300981-1−1300981-14. (14) You, J.; Chen, C.-C.; Hong, Z.; Yoshimura, K.; Ohya, K.; Xu, R.; Ye, S.; Gao, J.; Li, G.; Yang, Y. 10.2% Power Conversion Efficiency Polymer Tandem Solar Cells Consisting of Two Identical Sub-cells. Adv. Mater. 2013, 25, 3973−3978. (15) Kim, J. Y.; Lee, K.; Coates, N. E.; Moses, D.; Nguyen, T.-Q.; Dante, M.; Heeger, A. J. Efficient Tandem Polymer Solar Cells Fabricated by All-solution Processing. Science 2007, 317, 222−225. (16) Zhao, D. W.; Sun, X. W.; Jiang, C. Y.; Kyaw, A. K. K.; Lo, G. Q.; Kwong, D. L. Efficient Tandem Organic Solar Cells with an Al/MoO3 Intermediate Layer. Appl. Phys. Lett. 2008, 93, 083305−1. (17) Kaltenbrunner, M.; Adam, G.; Glowacki, E. D.; Drack, M.; Schwodiauer, R.; Leonat, L.; Apaydin, D. H.; Groiss, H.; Scharber, M. C.; White, M. S.; Sariciftci, N. S.; Baur, S. Flexible High Power-perweight Perovskite Solar Cells with Chromium-oxide-metal Contacts for Improved Stability in Air. Nat. Mater. 2015, 14, 1032−1039.

showed very small surface roughness values all within 15 nm (Figure S4). The J−V characteristics of the best performing tandem cells are replotted in Figure 4b and compared with those of the best performing single junction subcells of similar thicknesses. These results clearly show that the Voc of ≈1.53 V of the tandem device with Jsc of 8.6 mA/cm2 and FF of 46%, is approximately equal to the sum of Voc values of the front (0.88 V) and back (0.74 V) single junction cells, which confirms an effective series connection by the Cr/MoO3 IL. Importantly, with thinner (70 nm) rear cells the Jsc of tandem devices decrease to an average value of 7.2 mA/cm2 due to low photon absorption, and increasing rear cell thickness to 130 nm reduced the FF of tandem devices due to increased charge recombination losses (Table 1). However, in all of our tandem devices, very close to additive voltages are obtained, which suggests a great opportunity for isoindigo based low band gap conjugated polymers for integration in tandem architectures. In conclusion, we have successfully demonstrated construction of high performance polymer tandem solar cells from an isoindigo based low band gap polymer that could be easily obtainable from renewable and sustainable synthetic sources. The importance of a new, chemically robust Cr/MoO3 IL is also illustrated for consistent tandem cell device performance. The constructed tandem cells reached promising PCEs (5.5% average, 6.0% for a champion cell) with Voc of 1.51 ± 0.02 V, approaching 94% of the sum of the single junction subcells. Optimization of current balance in each subcell along with choice of proper wide-band gap polymer with minimum spectral overlap is expected to push the efficiency even further. With the expected high efficiency tandem cell, isoindigo based polymers hold tremendous potential for the commercialization of PSCs, overcoming the critical barrier of materials availability.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b10170. Experimental and measurement details, solar cell device performances (J−V curves, EQE, and table), AFM images, and spectroscopic data (PDF).

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AUTHOR INFORMATION

Corresponding Authors

*S. Bag. Email: [email protected]. *M. F. Durstock. Email: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial supports from the Air Force Office of Scientific Research (AFSOR) and the Air Force Research Laboratory (AFRL) are highly appreciated. J.R.R. acknowledges funding of this work from the Office of Naval Research (N00014-140173).

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REFERENCES

(1) Po, R.; Bernardi, A.; Calabrese, A.; Carbonera, C.; Corso, G.; Pellegrino, A. From Lab to Fab: How Must the Polymer Solar Cell Materials Design Change?−An Industrial Perspective. Energy Environ. Sci. 2014, 7, 925−943. (2) Andersen, T. R.; Dam, H. F.; Hösel, M.; Helgesen, M.; Carlė, J. E.; Larsen-Olsen, T. T.; Gevorgyan, S. A.; Andreasen, J. W.; Adams, J.; D

DOI: 10.1021/acsami.5b10170 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX