Qunping Fan

Efficient ternary blend all-polymer solar cells with a polythiophene derivative as a hole-cascade material

Ternary blending is one of the effective strategies to broaden the complementary absorption range and smooth the energy level at the donor/acceptor interface for achieving high efficiency bulk heterojunction (BHJ) polymer solar cells (PSCs). In this study, we report efficient ternary blend all-polymer solar cells (all-PSCs) with complementary absorption bands based on two polymer donors PTB7-Th and PBDD-ff4T and one polymer acceptor N2200. The polythiophene derivative PBDD-ff4T as a hole-cascade material plays a bridging role in energy levels between PTB7-Th and N2200, and thus provides more efficient channels for charge transfer. The ternary all-PSCs with 10 wt% PBDD-ff4T content show efficient photon harvesting, enhanced charge mobility and better active layer morphology due to the induced crystallization of PTB7-Th by the inserted PBDD-ff4T in the donor domains. As a result, the device without any extra treatments exhibits an optimized power conversion efficiency (PCE) of 7.2% with an open circuit voltage (Voc) of 0.82 V, a short circuit current density (Jsc) of 15.7 mA cm−2, and a fill factor (FF) of 56%. While the PCEs are 5.9% and 4.2% for the all-PSCs based on the binary blends PTB7-Th:N2200 and PBDD-ff4T:N2200, respectively. This PCE of 7.2% is one of the highest values reported in the literature so far for ternary all-PSCs and polythiophene derivative-based all-PSCs.

Synergistic effect of fluorination on both donor and acceptor materials for high performance non-fullerene polymer solar cells with 13.5% efficiency

A high performance polymer solar cells (PSCs) based on polymer donor PM6 containing fluorinated thienyl benzodithiophene unit and n-type organic semiconductor acceptor IT-4F containing fluorinated end-groups were developed. In addition to complementary absorption spectra (300–830 nm) with IT-4F, the PM6 also has a deep HOMO (the highest occupied molecular) level (−5.50 eV), which will lower the open-circuit voltage (Voc) sacrifice and reduce the Vloss of the IT-4F-based PSCs. Moreover, the strong crystallinity of PM6 is beneficial to form favorable blend morphology and hence to suppress recombination. As a result, in comparison with the PSCs based on a non-fluorinated D/A pair of PBDB-T:ITIC with a medium PCE of 11.2%, the PM6:IT-4Fbased PSCs yielded an impressive PCE of 13.5% due to the synergistic effect of fluorination on both donor and acceptor, which is among the highest values recorded in the literatures for PSCs to date. Furthermore, a PCE of 12.2% was remained with the active layer thickness of up to 285 nm and a high PCE of 11.4% was also obtained with a large device area of 1 cm2. In addition, the devices also showed good storage, thermal and illumination stabilities with respect to the efficiency. These results indicate that fluorination is an effective strategy to improve the photovoltaic performance of materials, as well as the both fluorinated donor and acceptor pair-PM6:IT-4F is an ideal candidate for the large scale roll-to-roll production of efficient PSCs in the future.

Side-chain engineering for efficient non-fullerene polymer solar cells based on a wide-bandgap polymer donor

In this work, a new wide-bandgap polymer, PSBZ, based on thienyl substituted benzodithiophene (BDTT) as the donor unit and difluorobenzotriazole (BTz-2F) as the acceptor unit was synthesized for photovoltaic applications. Compared to the analogous polymer J61 with linear dodecylthio side chains in the BDTT unit and a long 2-hexyldecyl side chain in BTz-2F, PSBZ possesses branched 2-butyloctyl side chains to increase steric hindrance of the BDTT unit and a short 2-butyloctyl side chain to decrease steric hindrance of the BTz-2F unit for more efficient charge separation and transport in the devices. As a result, PSBZ exhibited stronger π–π interaction and smaller stacking spacing leading to a higher extinction coefficient of 1.48 × 105 cm−1 and a high hole mobility of 8.56 × 10−3 cm2 V−1 s−1. Compared to the analogous polymer J61 with a power conversion efficiency (PCE) of 9.53% and a short-circuit current density (Jsc) of 17.43 mA cm−2, the PSBZ:ITIC-based polymer solar cells yielded a higher PCE of 10.5% with a higher Jsc of 19.0 mA cm−2. The results show that our design strategy is successful for improving photovoltaic performance by side chain engineering.

A 1,1′-vinylene-fused indacenodithiophene-based low bandgap polymer for efficient polymer solar cells

A 1,1′-vinylene-fused indacenodithiophene (IDTV) donor unit with 22 π-conjugated electrons was synthesized. A ladder-type D–A copolymer PIDTV-ffBT using IDTV as the donor unit and 5,6-difluorobenzothiadiazole (ffBT) as the acceptor unit was developed for application as a donor material in polymer solar cells (PSCs). Compared to other analogue polymers, PIDTV-ffBT possesses a two-dimensional conjugated multi-electron fused ring, excellent planarity and close π–π stacking, leading to a higher light harvesting coefficient, an enhanced charge carrier mobility of 0.032 cm2 V−1 s−1 and improved photovoltaic performance. The PSCs based on PIDTV-ffBT:PC71BM achieved a promising power conversion efficiency (PCE) of 7.3% with a high short-circuit current density (Jsc) of 17.1 mA cm−2. These results indicate that the introduction of the 1,1′-vinylene-fused system into IDTV for ladder-type polymers is an effective strategy to enhance the light absorption coefficient and improve charge carrier mobility for high efficiency PSCs.

High-performance nonfullerene polymer solar cells based on a fluorinated wide bandgap copolymer with a high open-circuit voltage of 1.04 V

In this work, efficient nonfullerene (NF) polymer solar cells (PSCs) based on a polymer donor PM6 containing a fluorinated-thienyl benzodithiophene unit and a small molecule acceptor ITIC were developed. PM6 possesses a strong absorption in the short wavelength region of 300–685 nm with a large bandgap of 1.80 eV, which is complementary to that of ITIC (1.55 eV) and facilitates achieving high short-circuit current (Jsc) in PSCs. Moreover, PM6 shows a deep HOMO level of −5.50 eV, a strong crystallinity and a dominant face on packing, which helps to achieve a high open-circuit voltage (Voc) and fill factor (FF) in PSCs. As a result, the PM6:ITIC-based PSCs obtained a power conversion efficiency (PCE) of 9.7% with a Voc of up to 1.04 V, a Jsc of 16.0 mA cm−2 and a FF of 58%, under the illumination of AM 1.5G, 100 mW cm−2. Notably, the energy loss (Eloss) of the PSCs is as low as 0.51 eV, which is smaller than the empirically low threshold of 0.6 eV. The PCE of 9.7% is one of the highest values reported in the literature for PSCs with a Voc over 1.0 V and an Eloss less than 0.55 eV.

Significant enhancement of the photovoltaic performance of organic small molecule acceptors via side-chain engineering

To achieve efficient polymer solar cells (PSCs), it is important to increase the optical absorption coefficient and charge mobility of photovoltaic materials for obtaining a high short-circuit current density (Jsc) and fill factor (FF) in the devices without sacrificing the open-circuit voltage (Voc). Herein, we designed and synthesized two novel narrow bandgap n-type organic semiconductor (n-OS) acceptors named POIT-M and MOIT-M by modifying the side-chains of IT-M from para-hexylphenyl to para-hexyloxylphenyl and then to meta-hexyloxylphenyl. Due to the synergistic effects of introducing oxygen atoms and varying substitution positions on the phenyl side-chains, MOIT-M shows a significantly improved absorption coefficient, stronger intermolecular π–π stacking interaction, increased crystallinity and higher electron mobility in comparison with IT-M and POIT-M, which helps to gain higher Jsc and FF in PSCs. These special features combined with the complementary absorption of the MOIT-M acceptor and wide bandgap polymer PTZ1 donor resulted in a high power conversion efficiency (PCE) of 11.6% with a Voc of 0.96 V, a Jsc of 17.5 mA cm−2 and a FF of 68.8% for the PSCs processed with simple thermal annealing at 120 °C for 10 min, which is one of the highest PCEs reported for additive-free PSCs and significantly higher than those of the PSCs based on PTZ1:IT-M (9.1%) or PTZ1:POIT-M (9.7%). Our results indicate that side-chain engineering is an effective way to further improve the photovoltaic performance of n-OS acceptors in PSCs.

Use of two structurally similar small molecular acceptors enabling ternary organic solar cells with high efficiencies and fill factors

Ternary blends have shown great potential to increase the power conversion efficiency (PCE) of organic solar cells (OSCs). In this work, we studied a ternary OSC system with a donor polymer (PM6) and two structurally similar non-fullerene acceptors (named ITCPTC and MeIC). Although these two small molecular acceptors (SMAs) exhibit similar absorption spectra, they introduce a surprising synergistic effect on tuning the domain size and crystallinity of the OSC blend. More specifically, MeIC is a SMA with strong crystallinity, which results in excessive phase segregation and large domain size for the PM6:MeIC binary blend. By adding a structurally similar and less crystalline SMA (ITCPTC) into the binary blend, the domain size and morphology of the blend are much improved without sacrificing the electron mobility of the blend. As a result, the optimal blend ratio of PM6 : ITCPTC : MeIC (1 : 0.4 : 0.6) led to an impressive FF of 78.2% and PCE of 14.13%, which are the highest values reported for ternary non-fullerene OSCs reported to date.