A Wide-Band Gap Copolymer Donor for Efficient Fullerene

Jun 5, 2019 - A Wide-Band Gap Copolymer Donor for Efficient Fullerene-Free. Solar Cells ... nonfullerene organic solar cells.1 The state-of-the-art si...
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A Wide-Band Gap Copolymer Donor for Efficient Fullerene-Free Solar Cells Jingyi Zhu,†,‡ Qishi Liu,‡ Dan Li,‡ Zuo Xiao,‡ Yu Chen,*,† Yong Hua,*,§ Shangfeng Yang,*,∥ and Liming Ding*,‡

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†

Key Laboratory of Macromolecular Science of Shaanxi Province, School of Materials Science and Engineering, Shaanxi Normal University, Xi’an 710062, China ‡ Center for Excellence in Nanoscience (CAS), Key Laboratory of Nanosystem and Hierarchical Fabrication (CAS), National Center for Nanoscience and Technology, Beijing 100190, China § School of Materials Science and Engineering, Yunnan University, Kunming 650091, China ∥ Department of Materials Science and Engineering, University of Science and Technology of China, Hefei 230026, China S Supporting Information *

ABSTRACT: The performance of a wide-band gap copolymer donor PDTPO-BDTT in nonfullerene solar cells was investigated. These solar cells presented broad photoresponse and high short-circuit current density. PDTPO-BDTT:IT-4F and PDTPO-BDTT:NNFA-4F solar cells with more efficient photoluminescence quenching and better film morphology gave decent power conversion efficiencies of 10.96 and 10.04%, respectively, which are much higher than those of the previously reported PDTPO-BDTT:fullerene solar cells.

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INTRODUCTION The past few years have witnessed the rapid progress of nonfullerene organic solar cells.1 The state-of-the-art singlejunction and tandem cells based on nonfullerene acceptors (NFAs) afford 14−17% power conversion efficiencies (PCEs).2 The super advantage of NFAs versus fullerene acceptors is their strong visible to near-infrared light-harvesting capability, which greatly enhances the short-circuit current density (Jsc).3 For high-performance NFA cells, NFAcompatible donor materials are equally important. Due to complementary light absorption, the wide-band gap donors are ideal partners for the low-band gap NFAs. The blend of a wideband gap donor and an NFA can harvest a broad range of solar irradiation and give high PCEs.4 Recent studies indicate that some wide-band gap donors, which performed not so well in fullerene cells, inversely show high performance in NFA cells.5 In this regard, wide-band gap donor materials, especially those already applied in fullerene cells, deserve more studies in NFA cells. PDTPO-BDTT is a reported donor−acceptor (D−A) copolymer donor with a large optical band gap (Egopt) of 1.95 eV (λon = 636 nm).6 It has many advantages, like nontedious synthesis, deep highest occupied molecular orbital (HOMO) level, and high hole mobility. However, it delivered only 6.84% PCE in fullerene cells due to low Jsc. In this work, we investigated the performance of PDTPO-BDTT in NFA cells by blending it with IT-4F,7 NNFA-4F,8 and COi8DFIC,9 respectively. All PDTPO-BDTT:NFA solar cells gave broad © XXXX American Chemical Society

external quantum efficiency (EQE) spectra and high Jsc. PDTPO-BDTT:IT-4F and PDTPO-BDTT:NNFA-4F cells gave decent PCEs of 10.96 and 10.04%, respectively, much better than those reported fullerene cells.

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RESULTS AND DISCUSSION The structures of PDTPO-BDTT, IT-4F, NNFA-4F, and COi8DFIC are shown in Figure 1. PDTPO-BDTT,6 IT-4F,7 and COi8DFIC9 were synthesized according to the literature. The synthesis for the unpublished acceptor NNFA-4F is described in the Supporting Information. The absorption spectra for PDTPO-BDTT, IT-4F, NNFA-4F, and COi8DFIC films are shown in Figure 2a. The donor PDTPO-BDTT shows intensive absorption at 400−636 nm. The absorption spectra of the NFAs are complementary with those of PDTPOBDTT. IT-4F, NNFA-4F, and COi8DFIC show a strong intramolecular charge transfer (ICT) band in the long wavelength region, with a peak at 728, 793, and 830 nm, respectively. The optical band gaps (Egopt) estimated from the absorption onsets of IT-4F, NNFA-4F, and COi8DFIC films are 1.54, 1.37, and 1.26 eV, respectively. The band gap shrink from IT-4F to NNFA-4F to COi8DFIC can be attributed to Received: May 11, 2019 Accepted: June 5, 2019

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Figure 1. Structures of PDTPO-BDTT, IT-4F, NNFA-4F, and COi8DFIC.

respectively, suggesting that hole transfer from the excited acceptor to the donor could be less efficient. We studied the photoluminescence (PL) quenching between PDTPO-BDTT and the NFAs (Figure 3). Each NFA quenched over 99% PL of PDTPO-BDTT (Figure 3a,c,e), whereas PDTPO-BDTT quenched 97, 93, and 47% PL of IT-4F, NNFA-4F, and COi8DFIC, respectively (Figure 3b,d,f). The inefficient quenching of COi8DFIC’s PL could be due to the smaller HOMO offset (0.06 eV) between PDTPO-BDTT and COi8DFIC, which is unfavorable for photocurrent generation in solar cells. The performance of PDTPO-BDTT in NFA solar cells was investigated by making solar cells with a structure of indium tin oxide (ITO)/ZnO/PDTPO-BDTT:NFA/MoO3/Ag.13 The D/A ratio, active layer thickness, additive content, and solvent annealing time were optimized (Tables S2−S13). J−V curves and external quantum efficiency (EQE) spectra for the best cells are shown in Figure 4a,b, respectively, and the performance data are listed in Table 1. The best PDTPOBDTT:IT-4F, PDTPO-BDTT:NNFA-4F, and PDTPOBDTT:COi8DFIC solar cells afforded 10.96, 10.04, and 6.46% PCEs, respectively. With similar LUMO levels, IT-4F, NNFA-4F, and COi8DFIC solar cells gave similar open-circuit voltages (Voc) of ∼0.8 V.14 The Jsc for PDTPO-BDTT:IT-4F, PDTPO-BDTT:NNFA-4F, and PDTPO-BDTT:COi8DFIC solar cells are 19.18, 19.30, and 16.96 mA cm−2, respectively, which are much higher than that of the reported PDTPOBDTT:PC71BM cells (∼10 mA cm−2).6 This is due to the broader EQE spectra of NFA cells. As shown in Figure 4b, all NFAs contributed considerable EQE response at the long wavelength region (λ > 636 nm). The maximum EQE (EQEmax) for IT-4F, NNFA-4F, and COi8DFIC cells are 79.6, 71.5, and 57.2%, respectively, suggesting that charge generation is more efficient in IT-4F and NNFA-4F cells but less efficient in CO i8DFIC cells. Exciton dissociation probabilities (Pdiss) were evaluated (Figure S4).15 Pdiss for IT4F, NNFA-4F, and COi8DFIC cells are 95.4, 93.6, and 87.6%, respectively, consistent with EQEmax. The fill factors (FF) for IT-4F, NNFA-4F, and COi8DFIC cells are 70.4, 62.8, and 46.8%, respectively. Charge transport and recombination affect FF significantly. We measured hole and electron mobilities (μh and μe) in PDTPO-BDTT:NFA blend films by using the space charge-limited current (SCLC) method (Figures S5 and S6, Table S14).16 From IT-4F to NNFA-4F and COi8DFIC cells, μh increases whereas μe decreases. The μh/μe is 2.5, 3.3, and

Figure 2. (a) Absorption spectra for PDTPO-BDTT, IT-4F, NNFA4F, and COi8DFIC films; (b) energy level diagram.

the gradually increased electron-donating capability of the core unit, which strengthened the ICT.10 The energy levels for PDTPO-BDTT, IT-4F, NNFA-4F, and COi8DFIC were evaluated by cyclic voltammetry (CV) (Figure S3 and Table S1).11 The energy level diagram is given in Figure 2b. The HOMO and the lowest unoccupied molecular orbital (LUMO) energy levels for the donor PDTPO-BDTT are −5.20 and −2.93 eV, respectively. The LUMO levels for all NFAs are almost the same (∼−3.9 eV), whereas the HOMO levels are −5.49, −5.29, and −5.26 eV for IT-4F, NNFA-4F, and COi8DFIC, respectively. The electrochemical band gaps (Egec) for IT-4F, NNFA-4F, and COi8DFIC are 1.60, 1.39, and 1.36 eV, respectively, which are close to their Egopt. The LUMO offsets between PDTPO-BDTT and the NFAs are ∼1 eV, suggesting sufficient driving force for electron transfer from the excited donor to the acceptor.12 The HOMO offsets for PDTPO-BDTT:IT-4F, PDTPO-BDTT:NNFA-4F, and PDTPO-BDTT:COi8DFIC are 0.29, 0.09, and 0.06 eV, B

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Figure 3. PL quenching between PDTPO-BDTT and NFA. (a, b) PDTPO-BDTT and IT-4F; (c, d), PDTPO-BDTT and NNFA-4F; (e, f) PDTPO-BDTT and COi8DFIC.

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CONCLUSIONS We studied the photovoltaic performance of a wide-band gap polymer donor PDTPO-BDTT by blending it with three lowband gap NFAs, respectively. All PDTPO-BDTT:NFA solar cells presented broad EQE spectra and high Jsc. PDTPOBDTT:IT-4F and PDTPO-BDTT:NNFA-4F blend films with more efficient PL quenching and better morphology yielded solar cells giving decent PCEs of 10.96 and 10.04%, respectively, which are much higher than that of those reported PDTPO-BDTT:fullerene solar cells. This work suggests that those wide-band gap polymer donors with poor performance in fullerene solar cells could present high performance in nonfullerene solar cells. The donor/acceptor partnership is very crucial in developing high-performance organic solar cells.

3.8 for IT-4F, NNFA-4F, and COi8DFIC cells, respectively, suggesting that charge transport is the most balanced in IT-4F cells but the least balanced in COi8DFIC cells. Balanced charge transport can enhance FF. We also studied bimolecular recombination by plotting Jsc against light intensity (Plight) (Figure S7).17 The data were fitted to a power law: Jsc ∝ Plightα. The α values for IT-4F, NNFA-4F, and COi8DFIC cells are 0.970, 0.958, and 0.933, respectively, suggesting that bimolecular recombination increases in the order of IT-4F, NNFA-4F, and COi8DFIC cells. The increased charge recombination also leads to lower FFs in NNFA-4F and COi8DFIC cells. The morphology of the active layers was studied by using atomic force microscope (AFM) (Figure S8). The root-mean-square roughnesses for PDTPO-BDTT:IT-4F, PDTPO-BDTT:NNFA-4F, and PDTPO-BDTT:COi8DFIC blend films are 1.25, 1.73, and 0.34 nm, respectively. Compared with PDTPO-BDTT:COi8DFIC film, PDTPOBDTT:IT-4F and PDTPO-BDTT:NNFA-4F films present clearer nanofiber structures. The better morphology for PDTPO-BDTT:IT-4F and PDTPO-BDTT:NNFA-4F films favor exciton dissociation and charge carrier transport, thus leading to higher Jsc and FF.

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EXPERIMENTAL SECTION Inverted Solar Cells. A ZnO precursor solution18 was spin-coated onto ITO glass (4000 rpm for 30 s). The films were annealed at 200 °C in air for 30 min. The thickness is ∼30 nm. A PDTPO-BDTT:NFA blend in chlorobenzene (or chloroform) with (or without) DIO additive was spin-coated onto ZnO. Finally, MoO3 (∼6 nm) and Ag (∼80 nm) was successively deposited onto the active layer through a shadow C

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active layer through a shadow mask (pressure ca. 10−4 Pa). J− V curves were measured by using a Keithley 2400 SourceMeter in the dark.

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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/acsomega.9b01363. General characterization, synthesis, NMR, CV, optimization of device performance, exciton dissociation probabilities, SCLC, bimolecular recombination, AFM (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: *E-mail: *E-mail: *E-mail:

[email protected] (Y.C.). [email protected] (Y.H.). [email protected] (S.Y.). [email protected] (L.D.).

ORCID

Zuo Xiao: 0000-0003-3705-7716 Yu Chen: 0000-0001-9545-6761 Yong Hua: 0000-0003-4799-2871 Shangfeng Yang: 0000-0002-6931-9613 Liming Ding: 0000-0001-6437-9150

Figure 4. J−V curves (a) and EQE spectra (b) for PDTPOBDTT:NFA solar cells. The integrated current densities from EQE spectra for PDTPO-BDTT:IT-4F, PDTPO-BDTT:NNFA-4F, and PDTPO-BDTT:COi8DFIC cells are 18.93, 18.87, and 16.41 mA cm−2, respectively.

Notes

The authors declare no competing financial interest.

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mask (pressure ca. 10−4 Pa). The effective area is 4 mm2. The active layer thicknesses were measured by using a KLA Tencor D-120 profilometer. J−V curves were measured by using a Keithley 2400 SourceMeter and an Enli Tech solar simulator (AM 1.5G, 100 mW cm−2). The light intensity of solar simulator was determined by using a monocrystalline silicon solar cell (Enli SRC2020, 2 cm × 2 cm) calibrated by NIM. EQE spectra were measured by using an Enli Tech QE-R3011 measurement system. Hole-Only Devices. The device structure is ITO/ PEDOT:PSS/PDTPO-BDTT:NFA/MoO 3 /Al. A PEDOT:PSS aqueous dispersion was spin-coated onto ITO glass (4000 rpm for 30 s). The substrates were dried at 150 °C for 10 min. The thickness of PEDOT:PSS is ∼30 nm. A PDTPO-BDTT:NFA blend was spin-coated onto PEDOT:PSS. Finally, MoO3 (∼6 nm) and Al (∼100 nm) were successively deposited onto the active layer through a shadow mask (pressure ca. 10−4 Pa). J−V curves were measured by using a Keithley 2400 SourceMeter in the dark. Electron-Only Devices. The device structure is Al/ PDTPO-BDTT:NFA/Ca/Al. Al (∼80 nm) was deposited onto glass through a shadow mask (pressure ca. 10−4 Pa). A PDTPO-BDTT:NFA blend was spin-coated onto Al. Ca (∼5 nm) and Al (∼80 nm) were successively deposited onto the

ACKNOWLEDGMENTS L.D. thanks the National Key Research and Development Program of China (2017YFA0206600), the National Natural Science Foundation of China (21572041, 51773045, 21772030, and 21704021), and the Youth Association for Promoting Innovation (CAS) for financial support.

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REFERENCES

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Table 1. Performance Data for the Solar Cells active layer

Voc [V]

PDTPO-BDTT:IT-4F PDTPO-BDTT:NNFA-4F PDTPO-BDTT:COi8DFIC

0.81 (0.81 ± 0.004) 0.83 (0.83 ± 0.001) 0.81 (0.81 ± 0.012)

a

Jsc [mA cm−2]

FF [%]

PCE [%]

19.18 (18.94 ± 0.14) 19.30 (18.72 ± 0.82) 16.96 (16.26 ± 0.42)

70.4 (68.3 ± 1.7) 62.8 (62.3 ± 0.7) 46.8 (46.5 ± 0.7)

10.96 (10.45 ± 0.27) 10.04 (9.66 ± 0.55) 6.46 (6.12 ± 0.17)

a

The data in the parentheses are averages for eight cells. D

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