• Energy Research

Ferrocene was introduced as a solid additive in organic solar cells (OSCs). The use of ferrocene provides PM6:Y6 based device with improved performance and stability, demonstrating its great potential in the fabrication of efficient and stable OSCs.

Non-fused ring electron acceptors (NFREAs) potentially have lower synthetic costs than their fused counterparts. However, the low backbone planarity and the presence of bulky substituents adversely affect the crystallinity of NFREAs, impeding charge transport and the formation of bicontinuous morphology in organic solar cells. Here we show that a binary solvent system can individually control the crystallization and phase separation of the donor polymer (for example, D18) and the NFREA (for example, 2BTh-2F-C2). We select solvents such as chloroform and o-xylene that evaporate at different temperatures and rates and have different solubility for D18. Upon evaporation of chloroform, D18 starts to assemble into fibrils. Then, the evaporation of o-xylene induces the rapid formation of a fibril network that phase segregates 2BTh-2F-C2 into pure domains and leads to a bicontinuous morphology. The well-defined interpenetrating network morphology affords an efficiency of 19.02% on small-area cells and 17.28% on 1 cm2 devices.

AbstractThe donor/acceptor interaction in non‐fullerene organic photovoltaics leads to the mixing domain that dictates the morphology and electronic structure of the blended thin film. Initiative effort is paid to understand how these domain properties affect the device performances on high‐efficiency PM6:Y6 blends. Different fullerenes acceptors are used to manipulate the feature of mixing domain. It is seen that a tight packing in the mixing region is critical, which could effectively enhance the hole transfer and lead to the enlarged and narrow electron density of state (DOS). As a result, short‐circuit current (JSC) and fill factor (FF) are improved. The distribution of DOS and energy levels strongly influences open‐circuit voltage (VOC). The raised filling state of electron Fermi level is seen to be key in determining deviceVOC. Energy disorder is found to be a key factor to energy loss, which is highly correlated with the intermolecular distance in the mixing region. A 17.53% efficiency is obtained for optimized ternary devices, which is the highest value for similar systems. The current results indicate that a delicate optimization of the mixing domain property is an effective route to improve theVOC,JSC, and FF simultaneously, which provides new guidelines for morphology control toward high‐performance organic solar cells.

An asymmetric wide-bandgap acceptor, namely AITC, is synthesized. The large dipole moment of AITC reinforces the molecular packing in ternary blends. Single-junction and tandem OSCs yielded a remarkable PCE of 18.8% and 19.4%, respectively.

This review summarizes the important morphological characteristics and recent research progress of non-fullerene acceptor based organic solar cells, as well as provides insights and perspectives on this topic.

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.

The spontaneous carrier generation process was identified in the Y6 acceptor with moderate quantum yields and extremely low recombination in optimized blends, which result in high efficiency and morphology tolerance.

Molecular design of non-fullerene acceptors is of vital importance for high-efficiency organic solar cells. The branched alkyl chain modification is often regarded as a counter-intuitive approach, as it may introduce an undesirable steric hindrance that reduces charge transport in non-fullerene acceptors. Here we show the design and synthesis of a highly efficient non-fullerene acceptor family by substituting the beta position of the thiophene unit on a Y6-based dithienothiophen[3,2-b]-pyrrolobenzothiadiazole core with branched alkyl chains. It was found that such a modification to a different alkyl chain length could completely change the molecular packing behaviour of non-fullerene acceptors, leading to improved structural order and charge transport in thin films. An unprecedented efficiency of 18.32% (certified value of 17.9%) with a fill factor of 81.5% is achieved for single-junction organic solar cells. This work reveals the importance of the branched alkyl chain topology in tuning the molecular packing and blend morphology, which leads to improved organic photovoltaic performance. Molecular design of acceptor and donor molecules has enabled major progress in organic photovoltaics. Li et al. show that branched alkyl chains in non-fullerene acceptors allow favourable morphology in the active layer, enabling a certified device efficiency of 17.9%.