Rational molecular and device design enables organic solar cells approaching 20% efficiency.

For organic solar cells to be competitive, the light-absorbing molecules should simultaneously satisfy multiple key requirements, including weak-absorption charge transfer state, high dielectric constant, suitable surface energy, proper crystallinity, etc. However, the systematic design rule in molecules to achieve the abovementioned goals is rarely studied. In this work, guided by theoretical calculation, we present a rational design of non-fullerene acceptor o-BTP-eC9, with distinct photoelectric properties compared to benchmark BTP-eC9. o-BTP-eC9 based device has uplifted charge transfer state, therefore significantly reducing the energy loss by 41 meV and showing excellent power conversion efficiency of 18.7%. Moreover, the new guest acceptor o-BTP-eC9 has excellent miscibility, crystallinity, and energy level compatibility with BTP-eC9, which enables an efficiency of 19.9% (19.5% certified) in PM6:BTP-C9:o-BTP-eC9 based ternary system with enhanced operational stability.

Balancing the Efficiency and Synthetic Accessibility of Organic Solar Cells with Isomeric Acceptor Engineering.

With the continuous development of organic semiconductor materials and on-going improvement of device technology, the power conversion efficiencies (PCEs) of organic solar cells (OSCs) have surpassed the threshold of 19%. Now, the low production cost of organic photovoltaic materials and devices have become an imperative demand for its practical application and future commercialization. Herein, the feasibility of simplified synthesis for cost-effective small-molecule acceptors via end-cap isomeric engineering is demonstrated, and two constitutional isomers, BTP-m-4Cl and BTP-o-4Cl, are synthesized and compared in parallel. These two non-fullerene acceptors (NFAs) have very similar optoelectronic properties but nonuniform morphological and crystallographic characteristics. Consequently, the OSCs composed of PM6:BTP-m-4Cl realize PCE of 17.2%, higher than that of the OSCs with PM6:BTP-o-4Cl (≈16%). When ternary OSCs are fabricated with PM6:BTP-m-4Cl:BTP-o-4Cl, the averaged PCE value reaches 17.95%, presenting outstanding photovoltaic performance. Most excitingly, the figure of merit (FOM) values of PM6:BTP-m-4Cl, PM6:BTP-o-4Cl, and PM6:BTP-m-4Cl:BTP-o-4Cl based devices are 0.190, 0.178, and 0.202 respectively. The FOM values of these systems are all among the top ones of the current high-efficiency OSC systems, revealing high cost-effectiveness of the two NFAs. This work provides a general but accessible strategy to minimize the efficiency-cost gap and promises the economic prospects of OSCs.

Volatile Solid Additive-Assisted Sequential Deposition Enables 18.42% Efficiency in Organic Solar Cells.

Morphology optimization of active layer plays a critical role in improving the performance of organic solar cells (OSCs). In this work, a volatile solid additive-assisted sequential deposition (SD) strategy is reported to regulate the molecular order and phase separation in solid state. The OSC adopts polymer donor D18-Cl and acceptor N3 as active layer, as well as 1,4-diiodobenzene (DIB) as volatile additive. Compared to the D18-Cl:N3 (one-time deposition of mixture) and D18-Cl/N3 (SD) platforms, the D18-Cl/N3(DIB) device based on DIB-assisted SD method exhibits a finer phase separation with greatly enhanced molecular crystallinity. The optimal morphology delivers superior charge transport and extraction, offering a champion power conversion efficiency of 18.42% with significantly enhanced short-circuit current density (Jsc ) of 27.18 mA cm-2 and fill factor of 78.8%. This is one of the best performances in binary SD OSCs to date. Angle-dependent grazing-incidence wide-angle X-ray scattering technique effectively reveals the vertical phase separation and molecular crystallinity of the active layer. This work demonstrates the combination of volatile solid additive and sequential deposition is an effective method to develop high-performance OSCs.

15.3% Efficiency All-Small-Molecule Organic Solar Cells Achieved by a Locally Asymmetric F, Cl Disubstitution Strategy.

Single junction binary all-small-molecule (ASM) organic solar cells (OSCs) with power conversion efficiency (PCE) beyond 14% are achieved by using non-fullerene acceptor Y6 as the electron acceptor, but still lag behind that of polymer OSCs. Herein, an asymmetric Y6-like acceptor, BTP-FCl-FCl, is designed and synthesized to match the recently reported high performance small molecule donor BTR-Cl, and a record efficiency of 15.3% for single-junction binary ASM OSCs is achieved. BTP-FCl-FCl features a F,Cl disubstitution on the same end group affording locally asymmetric structures, and so has a lower total dipole moment, larger average electronic static potential, and lower distribution disorder than those of the globally asymmetric isomer BTP-2F-2Cl, resulting in improved charge generation and extraction. In addition, BTP-FCl-FCl based active layer presents more favorable domain size and finer phase separation contributing to the faster charge extraction, longer charge carrier lifetime, and much lower recombination rate. Therefore, compared with BTP-2F-2Cl, BTP-FCl-FCl based devices provide better performance with FF enhanced from 71.41% to 75.36% and J sc increased from 22.35 to 24.58 mA cm-2, leading to a higher PCE of 15.3%. The locally asymmetric F, Cl disubstitution on the same end group is a new strategy to achieve high performance ASM OSCs.

An asymmetric end-capping strategy enables a new non-fullerene acceptor for organic solar cells with efficiency over 10.

Two different terminal groups, rhodanine-flanked benzo[c][1,2,5]thiadiazole (BR) and 2-(5,6-difluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (IM2F), were connected to an indaceno[1,2-b:5,6-b']dithiophene (IDT) core to construct a new non-fullerene acceptor (IDTBF). Solar cells based on this acceptor exhibited promising photovoltaic performances with a power conversion efficiency (PCE) of up to 10.43%.

A "σ-Hole"-Containing Volatile Solid Additive Enabling 16.5% Efficiency Organic Solar Cells.

Here we introduce a σ-hole-containing volatile solid additive, 1, 4-diiodotetrafluorobenzene (A3), in PM6:Y6-based OSCs. Aside from the appropriate volatility of A3 additive, the synergetic halogen interactions between A3 and photoactive matrix contribute to more condensed and ordered molecular arrangement in the favorable interpenetrating donor/acceptor domains. As a result, greatly accelerated charge transport process with suppressed charge recombination possibility is observed and ultimately a champion PCE value of 16.5% is achieved. Notably, the A3 treated OSCs can maintain a high efficiency of over 16.0% in a wide concentration range of A3 additive between 10 and 35 mg/mL. The A3-treated device shows excellent stability with an efficiency of 15.9% after 360-h storage. This work demonstrates that the σ-hole interaction can be applied to enhance the OSC performance and highlights the importance of non-covalent interactions in the optoelectronic materials.

Simple near-Infrared Nonfullerene Acceptors Enable Organic Solar Cells with >9% Efficiency.

Nonfullerene acceptors (NFAs) based on calamitic-shaped small molecules are being developed rapidly to improve the photoelectron conversion efficiencies (PCEs) of organic solar cells. NFAs with light absorption extended to the near-infrared (NIR) region are of interest because they play a pivotal role in both organic tandem cells and semitransparent devices. In this work, two simple acceptor-donor-acceptor-structured NFAs (CPDT-4Cl and CPDT-4F) have been designed and synthesized. Featured with dimerized 4H-cyclopenta[1,2-b:5,4-b']dithiophene (CPDT) as the electron-donating core and Cl- or F-substituted 2-(3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile as the electron-accepting unit, the absorption spectra of two NFAs are extended to the NIR region with an absorption edge at approximately 910 nm. In conjunction with the polymer donor material PBDB-T, a PCE of 9.47% was achieved by using a CPDT-4F-based device with a short-circuit current density of up to 20.1 mA/cm2, which slightly outperforms its counterpart CPDT-4Cl (PCE = 9.28%) under the same condition. This work broadens the scope of developing new NIR NFAs with both high efficiency and easy accessibility.