A Wide-Band Gap Copolymer Donor for Efficient Fullerene-Free Solar Cells.

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.

Introduction

The past few years have witnessed the rapid progress of nonfullerene organic solar cells.[1] The state-of-the-art single-junction 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, NFA-compatible 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 wide-band 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 CO8DFIC,[9] respectively. All PDTPO-BDTT:NFA solar cells gave broad 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.

Results and Discussion

The structures of PDTPO-BDTT, IT-4F, NNFA-4F, and CO8DFIC are shown in Figure . PDTPO-BDTT,[6] IT-4F,[7] and CO8DFIC[9] 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 CO8DFIC films are shown in Figure a. The donor PDTPO-BDTT shows intensive absorption at 400–636 nm. The absorption spectra of the NFAs are complementary with those of PDTPO-BDTT. IT-4F, NNFA-4F, and CO8DFIC 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 CO8DFIC films are 1.54, 1.37, and 1.26 eV, respectively. The band gap shrink from IT-4F to NNFA-4F to CO8DFIC can be attributed to 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 CO8DFIC were evaluated by cyclic voltammetry (CV) (Figure S3 and Table S1).[11] The energy level diagram is given in Figure b. 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 CO8DFIC, respectively. The electrochemical band gaps (Egec) for IT-4F, NNFA-4F, and CO8DFIC 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:CO8DFIC are 0.29, 0.09, and 0.06 eV, 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 ). Each NFA quenched over 99% PL of PDTPO-BDTT (Figure a,c,e), whereas PDTPO-BDTT quenched 97, 93, and 47% PL of IT-4F, NNFA-4F, and CO8DFIC, respectively (Figure b,d,f). The inefficient quenching of CO8DFIC’s PL could be due to the smaller HOMO offset (0.06 eV) between PDTPO-BDTT and CO8DFIC, which is unfavorable for photocurrent generation in solar cells.

Figure 1

Structures of PDTPO-BDTT, IT-4F, NNFA-4F, and CO8DFIC.

Figure 2

(a) Absorption spectra for PDTPO-BDTT, IT-4F, NNFA-4F, and CO8DFIC films; (b) energy level diagram.

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 CO8DFIC.

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 a,b, respectively, and the performance data are listed in Table . The best PDTPO-BDTT:IT-4F, PDTPO-BDTT:NNFA-4F, and PDTPO-BDTT:CO8DFIC solar cells afforded 10.96, 10.04, and 6.46% PCEs, respectively. With similar LUMO levels, IT-4F, NNFA-4F, and CO8DFIC 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:CO8DFIC solar cells are 19.18, 19.30, and 16.96 mA cm–2, respectively, which are much higher than that of the reported PDTPO-BDTT:PC71BM cells (∼10 mA cm–2).[6] This is due to the broader EQE spectra of NFA cells. As shown in Figure b, all NFAs contributed considerable EQE response at the long wavelength region (λ > 636 nm). The maximum EQE (EQEmax) for IT-4F, NNFA-4F, and CO8DFIC 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 CO8DFIC cells. Exciton dissociation probabilities (Pdiss) were evaluated (Figure S4).[15]Pdiss for IT-4F, NNFA-4F, and CO8DFIC cells are 95.4, 93.6, and 87.6%, respectively, consistent with EQEmax. The fill factors (FF) for IT-4F, NNFA-4F, and CO8DFIC 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 CO8DFIC cells, μh increases whereas μe decreases. The μh/μe is 2.5, 3.3, and 3.8 for IT-4F, NNFA-4F, and CO8DFIC cells, respectively, suggesting that charge transport is the most balanced in IT-4F cells but the least balanced in CO8DFIC 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 CO8DFIC cells are 0.970, 0.958, and 0.933, respectively, suggesting that bimolecular recombination increases in the order of IT-4F, NNFA-4F, and CO8DFIC cells. The increased charge recombination also leads to lower FFs in NNFA-4F and CO8DFIC 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:CO8DFIC blend films are 1.25, 1.73, and 0.34 nm, respectively. Compared with PDTPO-BDTT:CO8DFIC film, PDTPO-BDTT: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.

Figure 4

J–V curves (a) and EQE spectra (b) for PDTPO-BDTT:NFA solar cells. The integrated current densities from EQE spectra for PDTPO-BDTT:IT-4F, PDTPO-BDTT:NNFA-4F, and PDTPO-BDTT:CO8DFIC cells are 18.93, 18.87, and 16.41 mA cm–2, respectively.

Table 1

Performance Data for the Solar Cells

active layer	Voc [V]	Jsc [mA cm–2]	FF [%]	PCE [%]
PDTPO-BDTT:IT-4F	0.81 (0.81 ± 0.004)a	19.18 (18.94 ± 0.14)	70.4 (68.3 ± 1.7)	10.96 (10.45 ± 0.27)
PDTPO-BDTT:NNFA-4F	0.83 (0.83 ± 0.001)	19.30 (18.72 ± 0.82)	62.8 (62.3 ± 0.7)	10.04 (9.66 ± 0.55)
PDTPO-BDTT:COi8DFIC	0.81 (0.81 ± 0.012)	16.96 (16.26 ± 0.42)	46.8 (46.5 ± 0.7)	6.46 (6.12 ± 0.17)

The data in the parentheses are averages for eight cells.

Conclusions

We studied the photovoltaic performance of a wide-band gap polymer donor PDTPO-BDTT by blending it with three low-band gap NFAs, respectively. All PDTPO-BDTT:NFA solar cells presented broad EQE spectra and high Jsc. PDTPO-BDTT: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.

Experimental Section

Inverted Solar Cells

A ZnO precursor solution[18] 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 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/MoO3/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 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.