After the first report in 2008, diketopyrrolopyrrole (DPP)-based small-molecule photovoltaic materials have been intensively explored. The power conversion efficiencies (PCEs) for the DPP-based small-molecule donors have been improved up to 8%. Furthermore, through judicious structure modification, DPP-based small molecules can also be converted into electron-acceptor materials, and, recently, some exciting progress has been achieved. The development of DPP-based photovoltaic small molecules is summarized here, and the photovoltaic performance is discussed in relation to structural modifications, such as the variations of donor-acceptor building blocks, alkyl substitutions, and the type of conjugated bridges, as well as end-capped groups. It is expected that the discussion will provide a guideline in the exploration of novel and promising DPP-containing photovoltaic small molecules.

The last two years have witnessed the rapid progress of organic solar cells (OSCs), driven by the newly developed nonfullerene acceptor (NFA) Y6, which contains an electron-deficient-core-based central fused ring....

Herein, a simple “Same-A-Strategy” (SAS), constructing p-type and n-type photovoltaic materials with the same electron-accepting (A) unit of benzotriazole, is adopted to initially control the energy offsets. Then, chlorine atoms are introduced into the conjugated side chain of the benzo­[1,2-b:4,5-b′]­dithiophene (BDT) donor unit of the p-type polymer to fine-tune the optoelectronic properties. The chlorinated polymer J52-Cl, blended with a non-fullerene small molecule acceptor BTA3, yields the very small energy offsets (ΔE HOMO = 0.10 eV, ΔE LUMO = 0.28 eV) and the decreased nonradiative recombination loss of 0.24 eV. Benefiting from the strong molecular aggregation, ordered molecular orientation, and fine film morphology, J52-Cl:BTA3 device delivers balanced carriers mobilities and also suppressed charge recombination losses. Consequently, the obtained device yields a very high open-circuit voltage (V OC) of 1.24 V, a short-circuit current (J SC) of 13.16 mA cm–2, and a fill factor of 66.62%, giving rise to a promising power conversion efficiency (PCE) of 10.5%, which is a large breakthrough for organic solar cells with high V OC beyond 1.20 V. Our results provide a rare opportunity to break through the limitation of the problematic trade-off between energy loss and PCE and show a great potential for the application in tandem solar cells.

To maximize the short-circuit current density (J SC ) and the open circuit voltage (V OC ) simultaneously is a highly important but challenging issue in organic solar cells (OSCs). In this study, a benzotriazole-based p-type polymer (J61) and three benzotriazole-based nonfullerene small molecule acceptors (BTA1-3) are chosen to investigate the energetic driving force for the efficient charge transfer. The lowest unoccupied molecular orbital (LUMO) energy levels of small molecule acceptors can be fine-tuned by modifying the endcapping units, leading to high V OC (1.15-1.30 V) of OSCs. Particularly, the LUMO energy level of BTA3 satisfies the criteria for efficient charge generation, which results in a high V OC of 1.15 V, nearly 65% external quantum efficiency, and a high power conversion efficiency (PCE) of 8.25%. This is one of the highest V OC in the high-performance OSCs reported to date. The results imply that it is promising to achieve both high J SC and V OC to realize high PCE with the carefully designed nonfullerene acceptors.

A solution-processed diketopyrrolopyrrole (DPP)-based small molecule, namely BDT-DPP, with broad absorption and suitable energy levels has been synthesized. The widely used solvents of chloroform (CF) and o-dichlorobenzene (o-DCB) were used as the spin-coating solvent, respectively, and 1,8-diiodooctane (DIO) was used as additive to fabricate efficient photovoltaic devices with BDT-DPP as the donor material and PC71BM as the acceptor material. Devices fabricated from CF exhibit poor fill factor (FF) of 43%, low short-circuit current density (Jsc) of 6.86 mA/cm(2), and moderate power conversion efficiency (PCE) of 2.4%, due to rapid evaporation of CF, leading to poor morphology of the active layer. When 0.3% DIO was added, the FF and Jsc were improved to 60% and 8.49 mA/cm(2), respectively, because of the better film morphology. Active layer spin-coated from the high-boiling-point solvent of o-DCB shows better phase separation than that from CF, because of the slow drying nature of o-DCB, offering sufficient time for the self-organization of active-layer. Finally, using o-DCB as the parent solvent and 0.7% DIO as the cosolvent, we obtained optimized devices with continuous interpenetrating network films, affording a Jsc of 11.86 mA/cm(2), an open-circuit voltage (Voc) of 0.72 V, an FF of 62%, and a PCE of 5.29%. This PCE is, to the best of our knowledge, the highest efficiency reported to date for devices prepared from the solution-processed DPP-based small molecules.