High-efficiency organic solar cells with low non-radiative recombination loss and low energetic disorder

Abstract

Energy loss within organic solar cells (OSCs) is undesirable as it reduces cell efficiency 1-4. In particular, non-radiative recombination loss 3 and energetic disorder 5 , which are closely related to the tail states below the band edge and the overall photon energy loss, need to be minimized to improve cell performance. Here, we report how the use of a small-molecule acceptor with torsion-free molecular conformation can achieve a very low degree of energetic disorder and mitigate energy loss in OSCs. The resulting single-junction OSC has an energy loss due to non-radiative recombination of just 0.17 eV and a high power conversion efficiency of up to 16.54% (certified as 15.89% by the National Renewable Energy Laboratory). The findings take studies of organic photovoltaics deeper into a new regime, beyond the limits of energetic disorder and large energy offset for charge generation. Recent developments in organic/polymer bulk heterojunction solar cells (OSCs/PSCs) have led to tremendous advances in power conversion efficiency (PCE), with current leading certified efficiencies of over 15-16% for single-junction devices 6-9 and over 17% for multi-junction devices 10. However, the PCEs of OSCs still lag behind their inorganic semiconductors and perovskite counterparts, in most part due to the modest open-circuit voltage (V OC) imposed by the relatively large photon energy loss 1-4. The photon energy loss in solar cells, ΔE loss ¼ E g � qV OC I (q is the elementary charge, E g is the optical gap of the absorber; Fig. 1), represents the lower limit of energy loss during conversion of photon energy to electrical potential. So far, the best performing inorganic crystalline solar cells show a lower ΔE loss in gallium arsenide (0.32 eV) and crystalline silicon (0.38 eV) 11 , while most of the highly efficient perovskite solar cells have a ΔE loss in the range of 0.4−0.5 eV (ref. 4). Recently, there has been rapid progress in the reduction of photon energy loss in high-performance perovskite solar cells, leading to a record value of 0.34 eV (ref. 12). In contrast, state-of-the-art OSCs usually suffer from high ΔE loss in the range of 0.6−1.1 eV (ref. 13), which is much higher than the theoretical value of 0.25−0.30 eV predicted by Shockley-Queisser (SQ) theory 14. This is because of the relatively high radiative recombination loss due to absorption edge broadening effects 4 and the strong non-radiative recombination loss 1,2,15. Hence, it is clear that further improvement of V OC in OSCs requires a significant reduction of both radiative and non-radiative recombination loss. Although there is no intuitively simple approach to reduce the overall photon energy loss in OSCs, the energetic disorder 5 , which

D‐A Polymer with a Donor Backbone ‐ Acceptor‐side‐chain Structure for Organic Solar Cells

“…Compared to the E loss of 0.5-0.6 eV reported for the best OSC systems, [51] the rather large energy loss of our OSC devices might be ascribed to the non-radiative recombination, [51][52][53][54][55] which is caused by the molecular disordering, low crystallinity, suboptimal nanophase separation, and the existence of charge carrier traps in the active blend layer. [53,55,56] Particularly, the existence of a large number of traps in the polymer phase was confirmed by the relatively large threshold voltage (À 25-À 30 V) of the PTIBTbased OTFT devices. The geometrically random side chain configurations (E and Z isomers), regiorandom backbone as well as the poor crystallinity of the polymer films would contribute to the existence of a large amount of charge carrier traps.…”

“…[49,50] Based on the definition of the energy loss (E loss ), E loss = E g -eV OC , where E g is the lowest optical bandgap among the donor and acceptor components and V OC is the open circuit voltage of the OSC device, [32] the E loss of the PTIBT: ITIC device was calculated to 1.04 eV. Compared to the E loss of 0.5-0.6 eV reported for the best OSC systems, [51] the rather large energy loss of our OSC devices might be ascribed to the non-radiative recombination, [51][52][53][54][55] which is caused by the molecular disordering, low crystallinity, suboptimal nanophase separation, and the existence of charge carrier traps in the active blend layer. [53,55,56] Particularly, the existence of a large number of traps in the polymer phase was confirmed by the relatively large threshold voltage (À 25-À 30 V) of the PTIBTbased OTFT devices.…”

10.13% Efficiency All‐Polymer Solar Cells Enabled by Improving the Optical Absorption of Polymer Acceptors

“…During the past five years, polymer solar cells (PSCs) based on narrow bandgap (NBG) fused‐ring small molecule (SM) acceptors have made considerable progress, among which the state‐of‐the‐art PSCs have achieved power conversion efficiencies (PCEs) of 16–18% . Regarding such SM acceptor‐based PSCs, the all‐polymer solar cells (all‐PSCs) consisting of a polymer donor and a polymer acceptor show unique advantages in the flexible large‐scale and wearable energy generators due to their excellent morphology stability and mechanical robustness .…”

Asymmetric Electron Acceptors for High‐Efficiency and Low‐Energy‐Loss Organic Photovoltaics

“…[ 6–12 ] BTP‐based NFAs originated by Zou's group, such as Y6 and its derivatives, possess unique features of dominant face‐on orientation, high mobility, and low energy loss with high electroluminescence quantum efficiency (EQE EL ) in devices, thus enabling OPVs with over 17% efficiencies. [ 13–19 ] It is interesting to note that efficient NFAs could exhibit relatively low energy loss and fast charge separation under small driving forces, compared with fullerene‐based acceptors. [ 20–22 ] Although some progresses have been made, energy loss in OPVs based on NFAs is still obviously large, compared to Si and perovskite counterparts.…”