Organic photovoltaics (OPVs) have attracted considerable attention in the last two decades to overcome the terawatt energy challenge and serious environmental problems. During their early development, only wide-band-gap organic semiconductors were synthesized and employed as the active layer, mainly utilizing photons in the UV–visible region and yielding power conversion efficiencies (PCEs) lower than 5%. Afterward, considerable efforts were made to narrow the polymer donor band gap in order to utilize the infrared photons, which led to the enhancement of the PCE from 5% to 12% in about a decade. Since 2017, the study of narrow-band-gap non-fullerene acceptors helped usher in a new era in OPV research and boosted the achievable the PCE to 17% in only 3 years. In essence, the history of OPV development in the last 15 years can be summarized as an attempt to narrow the band gap of organic semiconductors and better position the energy levels. There are multiple benefits of a narrower band gap: (1) considerable infrared photons can be utilized, and as a result, the short-circuit current density can increase significantly; (2) the energy offset of the lowest unoccupied molecular orbital energy levels or highest occupied molecular orbital energy levels between the donor and acceptor can be reduced, which will reduce the open-circuit voltage loss by minimizing the loss caused by the donor/acceptor charge transfer state; (3) because of the unique molecular orbitals of organic semiconductors, the red-shifted absorption will induce high transmittance in the visible region, which is ideal for the rear subcells in tandem-junction OPVs and transparent OPVs.

中文翻译：

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缩小带隙：高性能有机光伏的关键。

在过去的二十年中，有机光伏（OPV）受到了相当大的关注，以克服兆瓦级能源挑战和严重的环境问题。在它们的早期开发中，仅宽带隙有机半导体被合成并用作活性层，主要利用紫外可见区域中的光子，其功率转换效率（PCE）低于5％。之后，为了利用红外光子，人们进行了相当大的努力来缩小聚合物供体的能带隙，这导致PCE在大约十年中从5％提高到12％。自2017年以来，对窄带隙非富勒烯受体的研究帮助开启了OPV研究的新纪元，并且仅3年就将可实现的PCE提升至17％。在本质上，过去15年中OPV发展的历史可以概括为试图缩小有机半导体的带隙并更好地确定能级的尝试。较窄的带隙具有多个好处：（1）可以利用大量的红外光子，结果，短路电流密度会大大增加；（2）可以减少供体和受体之间的最低未占据分子轨道能级或最高占据分子轨道能级的能量偏移，这将通过最大程度地减少由施主/受体电荷引起的损耗来减少开路电压损耗。转移状态 （3）由于有机半导体具有独特的分子轨道，红移吸收将在可见光区引起高透射率，