Solar energy is an ample renewable energy resource, with photovoltaic (PV) technology enabling a direct route from light to electricity. Currently, PVs are limited in photon conversion efficiency, due in major part to spectral losses. Mitigation of these losses is therefore important, economically and environmentally. Two processes that aim to increase solar light utilisation are described herein. The first is triplet–triplet annihilation upconversion (TTA-UC), through which two incoherent photons of low energy can produce one of higher energy, reducing below bandgap losses. Secondly, singlet fission (SF), through which two triplet states may be obtained from one initial singlet excited state, in theory allowing two electrons per photon in a PV, reducing thermalisation losses. These fields are often covered seperately, despite being the reverse processes of one another. This work aims to consolidate research in the two fields and highlight their similarities and common challenges, specifically those relevant to PV applications. Herein, we cover systems primarily based on organic small molecules (anthracene, rubrene, tetracene, pentacene), and detail the fabrication of functional materials containing them (MOFs, gels, SAMs on TiO₂, thin evaporated and solution cast films, and cavities). We further offer our recommendations for the focus of future work in both the TTA and SF fields, and discuss the need to address current limitations such as poor triplet diffusion, limited charge injection to PVs, and material stability. Specifically, one could do this by cherry picking ideas from other research fields, for example photosensitisers for photodynamic therapy could be used as TTA sensitisers, and molecules having a considerable excited state aromaticity could be considered as SF materials. We hope this review may aid development towards the end goal of an efficient PV, incorporating either, or both, SF and TTA-UC materials.

中文翻译：

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三重态-三重态湮没上转换和单重态裂变在太阳能应用方面的最新进展

太阳能是一种丰富的可再生能源，光伏 (PV) 技术可实现从光到电的直接路径。目前，PV 的光子转换效率受到限制，主要是由于光谱损失。因此，减轻这些损失在经济和环境方面都很重要。本文描述了旨在提高太阳光利用率的两种方法。第一个是三重态-三重态湮没上转换（TTA-UC），通过它，两个低能量的非相干光子可以产生一个更高的能量，减少低于带隙的损耗。其次，单重裂变 (SF)，通过它可以从一个初始单重激发态获得两个三重态，理论上允许 PV 中每个光子有两个电子，从而减少热化损失。这些领域通常被单独覆盖，尽管是彼此的逆过程。这项工作旨在巩固这两个领域的研究，并突出它们的相似之处和共同挑战，特别是那些与光伏应用相关的挑战。在这里，我们介绍了主要基于有机小分子（蒽、红荧烯、并四苯、并五苯）的系统，并详细介绍了包含它们的功能材料（MOF、凝胶、二氧化钛上的 SAM）的制造。₂，薄的蒸发和溶液流延薄膜，和空腔）。我们进一步对 TTA 和 SF 领域未来工作的重点提出建议，并讨论解决当前限制的必要性，例如三线态扩散差、对 PV 的电荷注入有限和材料稳定性。具体来说，可以通过从其他研究领域中挑选想法来做到这一点，例如用于光动力治疗的光敏剂可以用作 TTA 敏化剂，具有相当大的激发态芳香性的分子可以被认为是 SF 材料。我们希望这篇综述可以帮助实现高效 PV 的最终目标，结合 SF 和 TTA-UC 材料中的一种或两种。