• Energy Research

AbstractGenerally, highly efficient organic solar cells require both a high open‐circuit voltage (VOC) and a high short‐circuit current density (JSC). Reducing the energy loss (Eloss) is an effective way to achieve a high VOC without compromising the photocurrent, which is ideal for enhancing the power conversion efficiencies (PCEs). Herein, a new chlorinated nonfullerene acceptor (ITC‐2Cl) with chlorinated thiophene‐fused end groups is developed. In comparison with the unchlorinated counterpart (ITCPTC), the introduction of Cl improves not only the electronic properties by redshifting the absorption spectra and deepening the lowest unoccupied molecular orbital energy levels, but also the molecular packing and thus thin‐film morphology. The PM6:ITC‐2Cl‐based device yields a significantly higher PCE (13.6%) with a lower Eloss (0.67 eV) than the ITCPTC‐based device (PCE of 12.3% with Eloss of 0.70 eV). More importantly, compared to the archetypal nonfullerene acceptors such as IT‐4F (PCE of 12.9% with Eloss of 0.73 eV) and IT‐4Cl (PCE of 12.7% with Eloss of 0.76 eV), the ITC‐2Cl‐based device shows a higher PCE and a lower Eloss. These results demonstrate that the chlorinated thiophene‐fused end group is a promising candidate for a high‐performance nonfullerene acceptors with low energy loss.

A novel small molecule donor based on thiazolo[5,4-d]thiazole (TTz) unit is developed for OSCs application with 13.9% power conversion efficiency and a small non-radiative energy loss of 0.18 eV.

AbstractOver the past decade, organic solar cells (OSCs) have achieved a dramatic boost in their power conversion efficiencies from about 6 % to over 16 %. In addition to developments in device engineering, innovative photovoltaic materials, especially fluorinated donors and acceptors, have become the dominant factor for improved device performance. This minireview highlights fluorinated photovoltaic materials that enable efficient OSCs. Impressive OSCs have been obtained by developing some important molds of fluorinated donor and acceptor systems. The molecular design strategy and the matching principle of fluorinated donors and acceptors in OSCs are discussed. Finally, a concise summary and outlook are presented for advances in fluorinated materials to realize the practical application of OSCs.

Ternary OSCs fabricated with two acceptors with similar absorption spectra achieved the best PCE of 14.13% with an impressive FF of 78.2%.

AbstractThe excited‐state properties and chain conformations of a new low‐bandgap copolymer based on benzo[1,2‐b:4,5‐b′]dithiophene (BDT) and thieno[3,4‐b]thiophene with meta‐alkoxyphenyl‐substituted side chains in solution were investigated comprehensively. Time‐resolved spectroscopy suggested that the excited‐state properties were sensitive to the conformations of the copolymer in solution. In addition, excited‐state dynamics analyses revealed the photogeneration of triplet excited states by intersystem crossing (ISC) at a rate constant of ∼0.4×109 s−1 as a result of direct meta‐alkoxyphenyl connection to the donor unit BDT irrespective to the macromolecular conformations. According to El‐Sayed's rule, the fast ISC herein is correlated with the change of orbital types between singlet and triplet excited states as also shown by quantum chemical calculations. Our studies may shed light on the structure–property relationships of photovoltaic materials.

Understanding effect of morphology on charge carrier transport within polymer/fullerene bulk heterojunction is necessary to develop high-performance polymer solar cells. In this work, we synthesized a new benzodithiophene-based polymer with good self-organization behavior as well as favorable morphology evolution of its blend films with PC(71)BM under improved processing conditions. Charge carrier transport behavior of blend films was characterized by space charge limited current method. Evolved blend film morphology by controlling blend composition and additive content gradually reaches an optimized state, featured with nanoscale fibrilla polymer phase in moderate size and balanced mobility ratio close to 1:1 for hole and electron. This optimized morphology toward more balanced charge carrier transport accounts for the best power conversion efficiency of 3.2%, measured under simulated AM 1.5 solar irradiation 100 mW/cm(2), through enhancing short circuit current and reducing geminate recombination loss.

A kinetics-controlling strategy is used to regulate molecular crystallization and vertical phase distribution, where a third small molecule donor is utilized to advance the aggregation of polymer donor and prolong its self-assembly time.

The PM7:ITC-2Cl:IXIC-4Cl-based ternary device achieved an excellent PCE of 15.37% with a energy loss of 0.42 eV.