Tao Wang

The ternary strategy has been widely identified as an effective approach to obtain high-efficiency organic solar cells (OSCs). However, for most ternary OSCs, the nonradiative voltage loss lies between those of the two binary devices, which limits further efficiency improvements. Herein, an asymmetric guest acceptor BTP-2F2Cl is designed and incorporated into a PM1:L8-BO host blend. Compared with the L8-BO neat film, the L8-BO:BTP-2F2Cl blend film shows higher photoluminescence quantum yield and larger exciton diffusion length. Introducing BTP-2F2Cl into the host blend extends its absorption spectrum, improves the molecular packing of host materials, and suppresses the nonradiative charge recombination of the ternary OSCs. Consequently, the power conversion efficiency is improved up to 19.17% (certified value 18.7%), which represents the highest efficiency value reported for single-junction OSCs so far. The results show that improving the exciton behaviors is a promising approach to reducing the nonradiative voltage loss and realizing high-performance OSCs.

Abstract Organic solar cells are composed of electron donating and accepting organic semiconductors. Whilst a significant palette of donors has been developed over three decades, until recently only a small number of acceptors have proven capable of delivering high power conversion efficiencies. In particular the fullerenes have dominated the landscape. In this perspective, the emergence of a family of materials–the non‐fullerene acceptors (NFAs) is described. These have delivered a discontinuous advance in cell efficiencies, with the significant milestone of 20% now in sight. Intensive international efforts in synthetic chemistry have established clear design rules for molecular engineering enabling an ever‐expanding number of high efficiency candidates. However, these materials challenge the accepted wisdom of how organic solar cells work and force new thinking in areas such as morphology, charge generation and recombination. This perspective provides a historical context for the development of NFAs, and also addresses current thinking in these areas plus considers important manufacturability criteria. There is no doubt that the NFAs have propelled organic solar cell technology to the efficiencies necessary for a viable commercial technology–but how far can they be pushed, and will they also deliver on equally important metrics such as stability?

Organic photovoltaics (OPVs) suffer from a trade-off between efficient charge transport and suppressed nonradiative recombination due to the aggregation-induced luminance quenching of organic semiconductors. To resolve this grand challenge, a π-extended nonfullerene acceptor (NFA) B6Cl with large voids among the honeycomb network is designed and introduced into photovoltaic systems. We find that the presence of a small amount of (i.e., 0.5 or 1 wt %) B6Cl can compress the molecular packing of the host acceptor L8-BO, leading to shortened π–π stacking distance from 3.59 to 3.50 Å (that will improve charge transport) together with ordered alkyl chain packing (that will inhibit nonradiative energy loss due to the suppressed C–C and C–H bonds vibrations), as validated by high-energy X-ray scattering measurements. This morphology transformation ultimately results in simultaneously improved JSC, FF, and VOC of OPVs. As a result, the maximum PCEs of PM6:L8-BO and D18:L8-BO are increased from 19.1 and 19.3% to 19.8 and 20.2%, respectively, which are among the highest values for single-junction OPVs. The university of B6Cl to increase the performance of OPVs is further evidenced in a range of polymer:NFA OPVs.

Abstract With a certified efficiency as high as 25.2%, perovskite has taken the crown as the highest efficiency thin film solar cell material. Unfortunately, serious instability issues must be resolved before perovskite solar cells (PSCs) are commercialized. Aided by theoretical calculation, an appropriate multifunctional molecule, 2,2‐difluoropropanediamide (DFPDA), is selected to ameliorate all the instability issues. Specifically, the carbonyl groups in DFPDA form chemical bonds with Pb 2+ and passivate under‐coordinated Pb 2+ defects. Consequently, the perovskite crystallization rate is reduced and high‐quality films are produced with fewer defects. The amino groups not only bind with iodide to suppress ion migration but also increase the electron density on the carbonyl groups to further enhance their passivation effect. Furthermore, the fluorine groups in DFPDA form both an effective barrier on the perovskite to improve its moisture stability and a bridge between the perovskite and HTL for effective charge transport. In addition, they show an effective doping effect in the HTL to improve its carrier mobility. With the help of the combined effects of these groups in DFPDA, the PSCs with DFPDA additive achieve a champion efficiency of 22.21% and a substantially improved stability against moisture, heat, and light.

The nanoscale fibrillar morphology, featuring long-range structural order, provides abundant interfaces for efficient exciton dissociation and high-quality pathways for effective charge transport, is a promising morphology for high performance organic solar cells. Here, we synthesize a thiophene terminated non-fullerene acceptor, L8-ThCl, to induce the fibrillization of both polymer donor and host acceptor, that surpasses the 20% efficiency milestone of organic solar cells. After adding L8-ThCl, the original weak and less continuous nanofibrils of polymer donors, i.e. PM6 or D18, are well enlarged and refined, whilst the host acceptor L8-BO also assembles into nanofibrils with enhanced structural order. By adapting the layer-by-layer deposition method, the enhanced structural order can be retained to significantly boost the power conversion efficiency, with specific values of 19.4% and 20.1% for the PM6:L8-ThCl/L8-BO:L8-ThCl and D18:L8-ThCl/L8-BO:L8-ThCl devices, with the latter being certified 20.0%, which is the highest certified efficiency reported so far for single-junction organic solar cells. The nanoscale fibrillar morphology of the photoactive layer is critical to improve performance of organic solar cells. Here, the authors incorporate thiophene terminal groups in the non-fullerene acceptor, realizing nanofibrils with enhanced structural order and certified device efficiency of 20%.