Xin Ke

Fine-Tuning the Energy Levels of a Nonfullerene Small-Molecule Acceptor to Achieve a High Short-Circuit Current and a Power Conversion Efficiency over 12% in Organic Solar Cells.

Organic solar cell optimization requires careful balancing of current-voltage output of the materials system. Here, such optimization using ultrafast spectroscopy as a tool to optimize the material bandgap without altering ultrafast photophysics is reported. A new acceptor-donor-acceptor (A-D-A)-type small-molecule acceptor NCBDT is designed by modification of the D and A units of NFBDT. Compared to NFBDT, NCBDT exhibits upshifted highest occupied molecular orbital (HOMO) energy level mainly due to the additional octyl on the D unit and downshifted lowest unoccupied molecular orbital (LUMO) energy level due to the fluorination of A units. NCBDT has a low optical bandgap of 1.45 eV which extends the absorption range toward near-IR region, down to ≈860 nm. However, the 60 meV lowered LUMO level of NCBDT hardly changes the Voc level, and the elevation of the NCBDT HOMO does not have a substantial influence on the photophysics of the materials. Thus, for both NCBDT- and NFBDT-based systems, an unusually slow (≈400 ps) but ultimately efficient charge generation mediated by interfacial charge-pair states is observed, followed by effective charge extraction. As a result, the PBDB-T:NCBDT devices demonstrate an impressive power conversion efficiency over 12%-among the best for solution-processed organic solar cells.

Organic and solution-processed tandem solar cells with 17.3% efficiency

Tailoring tandem organics Tandem solar cells can boost efficiency by using a wider range of the solar spectrum. The bandgap of organic semiconductors can be tuned over a wide range, but, for a two-terminal device that directly connects the cells, the currents produced must be nearly equal. Meng et al. used a semiempirical analysis to choose well-matched top- and bottom-cell active layers. They used solution processing to fabricate an inverted tandem device that has a power conversion efficiency as high as 17.4%. Science, this issue p. 1094 A semi-empirical analysis helped to optimize materials for a tandem organic solar cell with high power conversion efficiency. Although organic photovoltaic (OPV) cells have many advantages, their performance still lags far behind that of other photovoltaic platforms. A fundamental reason for their low performance is the low charge mobility of organic materials, leading to a limit on the active-layer thickness and efficient light absorption. In this work, guided by a semi-empirical model analysis and using the tandem cell strategy to overcome such issues, and taking advantage of the high diversity and easily tunable band structure of organic materials, a record and certified 17.29% power conversion efficiency for a two-terminal monolithic solution-processed tandem OPV is achieved.

A New Nonfullerene Acceptor with Near Infrared Absorption for High Performance Ternary‐Blend Organic Solar Cells with Efficiency over 13%

Abstract A new acceptor–donor–acceptor (A–D–A) type nonfullerene acceptor, 3TT‐FIC, which has three fused thieno[3,2‐b]thiophene as the central core and difluoro substituted indanone as the end groups, is designed and synthesized. 3TT‐FIC exhibits broad and strong absorption with extended onset absorption to 995 nm and a low optical bandgap of 1.25 eV. The binary device based on 3TT‐FIC and the polymer PTB7‐Th exhibits a power conversion efficiency (PCE) of 12.21% with a high short circuit current density (   J sc) of 25.89 mA cm−2. To fine‐tune the morphology and make full use of the visible region sunlight, phenyl‐C71‐butyricacid‐methyl ester (PC71BM) is used as the third component to fabricate ternary devices. In contrast to the binary devices, the ternary blend organic solar cells show significantly enhanced EQE ranging from 300 to 700 nm and thus an improved  J sc with a high value of 27.73 mA cm−2. A high PCE with a value of 13.54% is achieved for the ternary devices, which is one of the highest efficiencies in single junction organic solar cells reported to date. The results provide valuable insight for the ternary devices in which the external quantum efficiency (EQE) induced by the third component is evidently observed and directly contributed to the enhancement of the device efficiency.

Fine-tuning the side-chains of non-fullerene small molecule acceptors to match with appropriate polymer donors

Side-chain engineering of donor and acceptor materials is an important topic in the field of organic photovoltaics. The influence of side-chains in active layer molecules on the corresponding photovoltaic device performances is still elusive, especially for the devices based on non-fullerene small molecule acceptors. In this work, we designed and synthesized two non-fullerene acceptors (IDTT-BH and IDTT-OBH) using an indacenodithieno[3,2-b]thiophene (IDTT) moiety as the central building block and (2-3-oxo-2,3-dihydro-1H-cyclopenta[b]naphthalen-1-ylidene)malononitrile (NINCN) as the end-group, which have similar energy levels and optical absorption spectra, and differ in the side-chains. We systematically investigated the effect of the side-chains on the device performance based on these two non-fullerene acceptors pairing with different donor materials. For the devices with J71 and PDCBT as donor materials, IDTT-BH showed better PCEs of 11.05% and 10.35%, respectively. Notably, 10.35% efficiency is among the top values in PDCBT-based OSCs. While, the devices based on PBDB-T:IDTT-OBH showed a better PCE of 10.93% than that of IDTT-BH based devices. It was found that the side-chains of non-fullerene acceptors have an effect on tuning the morphology of blend films and thus affect the photovoltaic performance, and different donor materials should be paired with the acceptors with the most suitable side-chains to achieve the best photovoltaic performance.