Electron Transfer Enhanced by a Minimal Energetic Driving Force at the Organic-Semiconductor Interface.

Energetic driving force for electron transfer must be minimized to realize efficient optoelectronic devices including organic light-emitting diodes (OLEDs) and organic photovoltaics (OPVs). Exploring dynamics of a charge transfer (CT) state at an interface leads to a comprehension of the relationship between energetics, electron transfer efficiency, and device performance. Here, we investigate the electron transfer from the CT state to the triplet excited state (T1) in upconversion OLEDs with 45 material combinations. By analyzing the CT emission and the singlet excited state emission from triplet-triplet annihilation via dark T1, their energetics and electron transfer efficiencies are extracted. We demonstrate that the CT→T1 electron transfer is enhanced by the stronger CT interaction and a minimal energetic driving force (<0.1 eV), which is explained using the Marcus theory with a small reorganization energy of <0.1 eV. Through our analysis, a novel donor-acceptor combination for the OLED is developed and shows an efficient blue emission with an extremely low turn-on voltage of 1.57 V. This work provides a solution to control interfacial CT state toward efficient optoelectronic devices without energy loss.

Evaporable Fullerene Indanones with Controlled Amorphous Morphology as Electron Transport Layers for Inverted Perovskite Solar Cells.

Fullerenes are among the most commonly used electron-transporting materials (ETMs) in inverted perovskite solar cells (IPSCs). Although versatile functionalized fullerene derivatives have shown excellent performance in IPSCs, pristine [60]fullerene (C60) is still the most widely used in devices mainly because of its uniform morphology by thermal deposition. However, thermally evaporable fullerene derivatives have not yet been achieved. Herein, we developed a series of evaporable fullerene derivatives, referred to as fullerene indanones (FIDOs), affording IPSCs with high power conversion efficiency (PCE) and long-term storage stability. The FIDOs were designed with a unique architecture in which the fullerene moiety and a benzene ring moiety are linked via a five-membered carbon ring in benzene ring plane. This molecular arrangement affords exceptional thermal stability, allowing the FIDOs to withstand harsh thermal deposition conditions. Moreover, by manipulating the steric bulk of the functional groups, we could control the state of the organic film from crystalline to amorphous. Subsequently, we used FIDOs as an electron transport layer (ETL) in IPSCs. Thanks to the suitable energy level and dual-passivation effect of FIDOs compared with a reference ETL using C60, the device using FIDOs achieved an open-circuit voltage of 1.16 V and a fill factor of 0.77. As a result, the PCE reached 22.11%, which is superior to 20.45% of the best-performing reference device. Most importantly, the FIDO-based IPSC devices exhibited exceptional stability in comparison to the reference device due to the stability of the amorphous ETL films.