Exceeding 14% Efficiency for Solution-Processed Tandem Organic

Sep 25, 2018 - For a highly efficient tandem organic solar cell, it is important for the subcells to minimize the absorption overlap and generate high...
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Exceeding 14% Efficiency for Solution-Processed Tandem Organic Solar Cells Combining Fullerene- and Nonfullerene-Based Subcells with Complementary Absorption Bing Guo,†,§ Wanbin Li,†,§ Guoping Luo,‡,§ Xia Guo,*,†,∥ Huifeng Yao,# Maojie Zhang,*,† Jianhui Hou,# Yongfang Li,†,⊥ and Wai-Yeung Wong*,∥ †

Laboratory of Advanced Optoelectronic Materials, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, China ‡ School of Science, Guangdong University of Petrochemical Technology, Maoming 525000, China ∥ Institute of Molecular Functional Materials and Department of Applied Biology and Chemical Technology, The Hong Kong Polytechnic University, Hung Hom, Hong Kong, China # Beijing National Laboratory for Molecular Sciences, State Key Laboratory of Polymer Physics and Chemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China ⊥ CAS Research/Education Center for Excellence in Molecular Sciences, CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China S Supporting Information *

ABSTRACT: For a highly efficient tandem organic solar cell, it is important for the subcells to minimize the absorption overlap and generate high and balanced currents. Considering the strong absorption and high external quantum efficiency at the short wavelength, developing a highly efficient blend system with a wide-bandgap (WBG) polymer as the donor and a fullerene derivative as the acceptor in the front cell would be an effective strategy. However, it is a challenge to obtain a high short-current density (Jsc) for this blend system. Here, we develop a WBG polymer (PBD1) with an optical bandgap of 1.88 eV. The PBD1:PC71BM blend system with a thickness of 230 nm achieves a power conversion efficiency (PCE) of 9.8% with a high Jsc of 14.6 mA cm−2. When tandem devices are fabricated with PBD1:PC71BM in the front cell, a PCE of 14.2% with a high Jsc of 12.3 mA cm−2 is achieved. layers (ICLs),13−27 where PCEs of over 13% were achieved recently.28−33 The most common tandem OSC devices are based on a two-terminal monolithic geometry, where two subcells are connected in series by highly transparent ICLs and photogenerated electrons from one subcell recombine with holes that are arriving from the subcell on the opposite side of the ICL.34 The open-circuit voltage (Voc) of a tandem device is determined by the sum of all of their subcell voltages, whereas the short-circuit current density (Jsc) is limited by the smaller Jsc

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olution-processed organic solar cells (OSCs) enable the fabrication of low-cost, lightweight, large-area, and flexible solar modules through roll-to-roll processing.1−5 There has great progress made in this field, and power conversion efficiencies (PCEs) of over 13% in the single-junction OSCs have been reported.6−8 The full solar spectrum is difficult to obtain, considering the limited absorption range and the thickness of the active layer in single-junction cells. Tandem OSCs composed of two or more subcells with complementary absorption stacked together have been designed to utilize the solar radiation more effectively.9−11 Since the breakthrough in all solution-processed tandem OSCs was reported by Heeger and co-workers in 2007,12 additional research has been devoted to develop novel photovoltaic materials and interconnecting © XXXX American Chemical Society

Received: August 8, 2018 Accepted: September 25, 2018 Published: September 25, 2018 2566

DOI: 10.1021/acsenergylett.8b01448 ACS Energy Lett. 2018, 3, 2566−2572

Letter

Cite This: ACS Energy Lett. 2018, 3, 2566−2572

Letter

ACS Energy Letters

Figure 1. (a) Chemical structure of PBD1. (b) GIXD profile for a pure film of PBD1. (c) UV−vis absorption spectra of PBD1 in solution and thin film. (d) CV of the PBD1 film on a platinum electrode, which was measured in a 0.1 mol L−1 Bu4NPF6 acetonitrile solution at a scan rate of 10 mV s−1.

Hence, in this work, we focused on developing a new WBG conjugated polymer with thick films for highly efficient tandem OSCs. This WBG polymer, referred to as PBD1 (Figure 1a), exhibited a wide optical bandgap (Egopt) of 1.88 eV, a low-lying highest occupied molecular orbital (HOMO) energy level of −5.39 eV, and a high hole mobility of 1.4 × 10−3 cm2 V−1 s−1. The OSC device based on PBD1:[6,6]-phenyl-C71-butyric acid methyl ester (PC71BM), with a thickness of over 200 nm, exhibited excellent photovoltaic performance with an outstanding PCE of 9.8%. The device also afforded a high Jsc of 14.6 mA cm−2 and a fill factor (FF) of 74%. Considering that the blend film of PBD1:PC71BM exhibited strong absorption at a short wavelength of 300−650 nm and the corresponding high external quantum efficiency (EQE) values of over 70% in this region, the tandem devices were fabricated by using PTB7-Th:IEICO4F as the LBG active layer for the complementary absorption (see Figure 2a). The best performance of the tandem cell gave a Jsc of 12.3 mA cm−2, a Voc of 1.61 V, and a high FF of 72%, resulting in an overall PCE of 14.2%, as it benefited from the high EQE and FF of the WBG subcell. The values were among the highest values reported in solution-processed tandem OSCs to date. The new polymer PBD1 was synthesized by a Stille coupling reaction, as shown in Scheme S1 in the Supporting Information. The polymer exhibited a number-average molecular weight (Mn) of 57.5 kDa and polydispersity index (PDI) of 1.95. As shown in Figure 1c, the UV−vis absorption spectra of PBD1 in the chloroform solution and the thin film displayed a strong absorption between 400 and 650 nm. The thin-film absorption spectrum of PBD1 shifted slightly to the red compared to that in solution. It is worth noting that obvious vibronic shoulder peaks at 600 nm were observed in both the solution and the thin film, which suggested that strong aggregation of the polymer chains already existed in the solution.

throughout the two subcells, indicating that the balanced and high possible current between the subcells is essential when achieving a high Jsc and PCE in tandem devices.35 Therefore, minimizing the overlap between the absorption spectra of the front and rear cells is critical. It would be feasible to use the active layers with the wide-bandgap (WBG) and the low-bandgap (LBG) photoactive materials in the front and rear cells, respectively. The front cell is responsible for utilizing the high-energy photons and obtaining a high Voc, while the rear cell collects the low-energy photons and provides a matched Jsc with the front cell. Recently, nonfullerene (NF) OSCs were developed very quickly,36−38 and the rapid development of NF acceptors with a low bandgap of ∼1.3 eV provides new opportunities to achieve high efficiency for tandem OSCs.39−43 Notably, the single-junction devices based on PTB7-Th44 as the donor material and 2,2′-((2Z,2′Z)-(((4,4,9,9-tetrakis(4-hexylphenyl)-4,9-dihydros-indaceno[1,2-b:5,6-b′]dithiophene-2,7-diyl)bis(4-((2-ethylhexyl)oxy)thiophene-5,2-diyl))bis(methanylylidene))bis(5,6-difluoro-3oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile (IEICO-4F)31 or (4,4,10,10-tetrakis(4-hexylphenyl)-5,11-(2-ethylhexyloxy)-4,10-dihydro-dithienyl[1,2-b:4,5b′]benzodithiophene-2, 8-diyl)bis(2-(3-oxo-2,3-dihydroinden-5,6-dichloro-1-ylidene)malononitrile) (BT-CIC)40 as acceptor materials can yield PCEs of over 11% with a high Jsc of over 20 mA cm−2. Furthermore, the tandem devices with PTB7-Th:IEICO-4F as the LBG active layer reported by Hou and co-workers gave a high PCE exceeding 14%.31 Considering the strong absorption and high external quantum efficiency at the short wavelength, a blend system of WBG polymers and fullerene derivatives as the active layer in the front cell should be an effective strategy to minimize the absorption overlap of the subcells. However, it is difficult to obtain a high Jsc for the WBG polymer:fullerene derivative systems, which is generally limited by narrow absorption spectra and the small thickness of the blend film. 2567

DOI: 10.1021/acsenergylett.8b01448 ACS Energy Lett. 2018, 3, 2566−2572

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Figure 2. (a) Chemical structures of PC71BM, PTB7-Th, and IEICO-4F. (b) Absorption spectra of the active layers for the front and rear cells. (c) J−V characteristics and (d) EQE curves of the front and rear cells. (e) Device structure of the tandem solar cells. (f) Energy level diagram of the materials used in this study (front cell: PBD1:PC71BM; rear cell: PTB7-Th:IEICO-4F).

(PEDOT:PSS)/PBD1:PC 71 BM/zinc oxide nanoparticles (ZnO-NPs)/Al. The photovoltaic performance can be enhanced with 1,8-diiodooctane (DIO) additive and thermal annealing treatment (Figure S2 and Table S1, Supporting Information). As shown in Table 1, the optimized device that was based on PBD1:PC71BM with a thickness of 230 nm exhibited a high PCE of 9.8%, with a high Jsc of 14.6 mA cm−2, a Voc of 0.91 V, and a FF of 74%. This efficiency was among the highest values reported so far for the WBG polymer:fullerene-based singlejunction OSCs. Figure 2c shows the current density−voltage (J−V) characteristics of PBD1:PC71BM and PTB7-Th:IEICO4F-based single-junction devices. The corresponding EQE curve of the optimized device is shown in Figure 2d. The PBD1:PC71BM-based device exhibited a high response in the range of 300−700 nm, and a maximum EQE of 78.8% at 490 nm was recorded. The morphologies of the blend films were investigated using atomic force microscopy (AFM) and transmission electron microscopy (TEM). The root-mean-square (RMS) roughness increased to 1.44 nm after the DIO and thermal annealing treatment (Figure S3, Supporting Information). Compared to the as-cast blend film, the film with DIO and thermal annealing treatment showed a continuous interpenetrating

The result further indicated a significant order in the polymer structure. The Egopt that was estimated from the absorption edge (λedge) of the thin film was 1.88 eV (Egopt = 1240/λedge). The HOMO energy level of PBD1 was determined by cyclic voltammetry (CV), as shown in Figure 1d. The onset oxidation potential (φox) of PBD1 was 0.68 eV, corresponding to a HOMO energy level value of −5.39 eV, according to the equation HOMO = −e(φox + 4.71) (eV). The deep HOMO value was beneficial for a high Voc in OSCs. The crystallinity and the molecular orientation of the PBD1 film were studied by two-dimensional grazing-incidence X-ray diffraction (GIXD) measurements.45 As shown in Figure 1b, a sharp and intensive (010) diffraction peak was found in the out-ofplane direction at ∼1.69 Å, suggesting that the polymer had a preferential face-on orientation relative to the substrate with a π−π stacking distance of 3.73 Å. In addition, the hole mobility of PBD1 was measured by the space-charge-limited current (SCLC) method, where a value of 1.4 × 10−3 cm2 V−1 s−1 was obtained (Figure S1, Supporting Information). Photovoltaic properties of the new polymer PBD1 were investigated by fabricating devices with a structure of indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene):polystyrenesulfonate 2568

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Table 1. Photovoltaic Performance Parameters of Optimized Single-Junction Solar Cells under AM 1.5G, 100 mW cm−2 Illumination active layer PBD1:PC71BM

PTB7-Th:IEICO-4F

Voc (V) 0.91 0.91 0.91 0.68 0.71 0.70

Jsca (mA cm−2) 14.1 14.6 14.1 21.8 21.3 20.6

FF (%)

(13.5) (13.9) (13.4) (21.0) (20.2) (19.6)

72 74 75 66 68 68

PCEb (%) 9.2 9.8 9.6 9.8 10.2 9.9

(9.0 ± 0.1) (9.5 ± 0.1) (9.4 ± 0.2) (9.5 ± 0.2) (10.0 ± 0.1) (9.7 ± 0.1)

thickness (nm) 250 230 200 120 110 100

a

Values calculated from EQE are in parentheses. bAverage PCEs in parentheses were obtained from over 30 devices.

Figure 3. (a) Simulated Jsc generated in the tandem solar cells as a function of the active layer thickness. (b) Simulated energy distribution in the tandem device with a front active layer (PBD1:PC71BM) thickness of 230 nm and a rear active layer (PTB7-Th:IEICO-4F) thickness of 110 nm.

network with fibrous features, which is beneficial for exciton dissociation and charge transport in the devices (Figure S4, Supporting Information). Clearly, PBD1 is a promising photoactive material that produced a high Jsc and FF and has good potential for making WBG subcells in the tandem OSCs. The PTB7-Th:IEICO-4F39 blend system was used as the LBG active layer to make full use of the solar spectrum (Figure 2a). As seen in Figure 2b, the PTB7-Th:IEICO-4F film extended the absorption response to the near-infrared region of ∼1000 nm. The PBD1:PC71BM film showed a relatively narrow absorption spectrum, covering the visible region within 300−650 nm. Therefore, the two active layers nearly covered the sunlight spectrum between 300 and 1000 nm. More importantly, there was only a slight overlap between the absorption spectra in the two subcells, which was beneficial when obtaining high and balanced current densities within each subcell. The PTB7Th:IEICO-4F-based device gave a PCE of 10.2% with a high Jsc of 21.3 mA cm−2 (Table 1). As shown in Figure 2d, the PTB7-Th:IEICO-4F-based device exhibited a broad response in the range of 300−1000 nm. Importantly, the PTB7-Th:IEICO4F-based device exhibited excellent EQE values within 650−1000 nm, where the PBD1:PC71BM-based device had its most diminished EQE. These results indicated that if these two active layers were used in the tandem cells, one could expect high EQE values in a wide range. The PBD1:PC71BM and PTB7-Th:IEICO-4F blend systems were used as the WBG and the LBG active layers to fabricate tandem OSCs. As shown in Figure 2e, the tandem devices were fabricated using solution processing, except for the Al electrode. The corresponding energy level diagram of the materials is presented in Figure 2f. The ICL consisted of a 30 nm layer of ZnO-NPs46 processed from n-butanol and a 30 nm layer of

pH-neutral poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (N-PEDOT:PSS)21 spin coated by water/isopropanol. The thickness adjustment of the active layers in the subcells is essential for overall Jsc enhancement of the tandem solar cells. We initially conducted optical simulations of the electric field distribution within the device by using the transfer-matrix modeling method, which provided guidance for selection of the optimal thicknesses of the subcells.47,48 The refractive index (n) and the extinction coefficient (k) values for the materials in each layer that was used in our tandem device were obtained using variable-angle spectroscopic ellipsometry (Figure S5, Supporting Information). The spectrally averaged internal quantum efficiency (IQE) values for each active layer were 95% for PBD1:PC71BM and 85% for PTB7-Th:IEICO-4F, which were determined as the ratio between the experimental Jsc and the absorbed photon flux.49 It was assumed that there were no losses in the interconnecting layer. The modeling results of the simulated Jsc that was generated in the tandem solar cells as a function of the active layer thickness are shown in Figure 3a. The optical simulation showed that an optimum value of over 12 mA cm−2 was obtained when the optimal thicknesses of the active layers in the front and rear cells were 230 and 110 nm, respectively. The simulated distribution of the normalized optical electric field and the energy dissipation rate in the tandem device with an optimized active layer thickness of 230/110 nm in the front and rear cells are displayed in Figures S6 and 3b. The simulation showed that the energy distribution in the front cell was primarily located at a wavelength of 300−650 nm, while the rear cell had a small fraction of energy that dissipated in that region but was primarily located in the region of 650−1000 nm. The results from the single-junction device performance and the optical simulation described above were used to fabricate a 2569

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Figure 4. (a) J−V characteristics of tandem solar cells with various film thicknesses of the subcells. (b) EQE curves of the optimized tandem solar cells.

cells obtained from three batches (Figure S7, Supporting Information). Moreover, tandem devices with a large active area were fabricated, and a PCE of 13.0% was obtained for devices with a 1.12 cm2 area (Figure S8, Supporting Information, and Table 2). The results measured under the AM 1.5G condition were verified by taking EQE measurements of the front cell and the rear cell in the tandem device. As shown in Figure 4b, the front cell absorbed most of the high-energy photons within the range of 300−650 nm, and a maximum EQE value of 75.2% at 490 nm was recorded. The rear cell exhibited a broad spectrum with a relatively low response in the visible region and a high response in the near-infrared region within 650−1000 nm, which agreed well with the simulated results. The integral current densities obtained from the EQE measurements were 11.9 mA cm−2 for the front cell and 11.7 mA cm−2 for the rear cell, which were balanced and consistent with the tandem device in the J−V measurement. In summary, we introduced a highly efficient WBG polymer PBD1 as the donor material in the front cell of the tandem device. Through optical simulation and device optimization, a remarkably high PCE of 14.2% with a Voc of 1.61 V, a Jsc of 12.3 mA cm−2, and a high FF of 72% was observed, which was among the highest values reported in the solutionprocessed tandem OSCs. These results indicated that PBD1 is a promising WBG polymer donor for applications in tandem OSCs. The overall performance could be further enhanced by selecting better-suited photoactive materials in the near future.

series of tandem solar cells with different active layer thicknesses of the subcells. The active layer thickness of the front cell was increased from 200 to 250 nm while keeping the thickness of the rear cell at 110 nm. The thickness of the rear cell ranging from 100 to 120 nm was studied by fixing the thickness in the front cell to be 230 nm. As shown in Figure 4a and Table 2, these tandem devices afforded PCEs of over Table 2. Photovoltaic Performance Parameters of the Tandem Solar Cells with Various Film Thicknesses of the Subcells under AM 1.5G, 100 mW cm−2 Illumination thickness of the front/ rear active layer (nm)

Voc (V)

Jsc (mA cm−2)

FF (%)

250/110 230/110 200/110 230/120 230/100 230/110a

1.60 1.61 1.61 1.59 1.60 1.60

12.1 12.3 11.7 12.1 11.7 12.0

71 72 73 72 72 68

PCE (%) 13.7 14.2 13.8 13.8 13.5 13.0

(13.4 (13.8 (13.4 (13.5 (13.2 (12.7

± ± ± ± ± ±

0.1)b 0.2)b 0.2)b 0.2)b 0.1)b 0.2)c

a The cell area is 1.12 cm2, and the aperture area of the shadow mask is 0.64 cm2. bAverage PCEs in parentheses were obtained from over 30 devices. cAverage PCEs in parentheses were obtained from over 10 devices.

13% and Voc values of ∼1.60 V, which was nearly equal to the sum of the values for the subcells. When the active layer thickness of the rear cell was fixed at 110 nm and the thickness of the front cell was gradually increased from 200 to 230 nm, the Voc and FF values showed no apparent changes, while the Jsc value was increased from 11.7 to 12.3 mA cm−2. When the thickness of the front cell was maintained at 230 nm and the thickness of the rear cell varied from 100 to 120 nm, the Jsc reached a maximum at 110 nm with little change for the Voc and FF. The experimental results matched well with the optical simulation. The tandem device with an optimized active layer thickness of 230/110 nm in the front and the rear subcell exhibited a high PCE of 14.2% with a Voc of 1.61 V, a Jsc of 12.3 mA cm−2, and a high FF of 72%, which is 31% higher than that of the front subcell and 28% higher than that of the rear cell. The efficiency enhancement of the tandem devices relative to their subcells is higher than most of the reported results with a PCE over 12% (Table S2, Supporting Information). It is worth mentioning that the optimized tandem device performance was reproducible with low variations for the key photovoltaic parameters, where an average PCE of 13.8 ± 0.2% was obtained for the 30 tandem

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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/acsenergylett.8b01448. Synthetic route of PBD1, device fabrication and measurement, mobility measurement, AFM and TEM images, and photovoltaic data and simulated distribution of the normalized optical electric field in a tandem device (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (X.G.). *E-mail: [email protected] (M.Z.). *E-mail: [email protected] (W.-Y.W.). 2570

DOI: 10.1021/acsenergylett.8b01448 ACS Energy Lett. 2018, 3, 2566−2572

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Huifeng Yao: 0000-0003-2814-4850 Maojie Zhang: 0000-0002-6102-5856 Jianhui Hou: 0000-0002-2105-6922 Yongfang Li: 0000-0002-2565-2748 Wai-Yeung Wong: 0000-0002-9949-7525 Author Contributions §

B.G., W.L., and G.L. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (NSFC) (No. 51503135, 51573120, 51773142, 91633301), the Jiangsu Provincial Natural Science Foundation (Grant No. BK20150332), and the Natural Science Foundation of Guangdong University of Petrochemical Technology, China (Grant No. 2017rc20). W.-Y.W. acknowledges financial support from the Areas of Excellence Scheme, University Grants Committee, HKSAR (AoE/P-03/08), Hong Kong Research Grants Council (PolyU123384/16P), Hong Kong Polytechnic University (1-ZE1C), and the Endowment fund from Ms. Clarea Au (847S). X.G. also acknowledges support from the Hong Kong Scholars Program.

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