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Achieving long-term stability in organic solar cells is a remaining bottleneck for the commercialization of this otherwise highly appealing technology. In this work, we study the performance and stability differences in standard and inverted DBP/C-70 based organic solar cells. Differences in the charge-transfer state properties of inverted and standard configuration DBP/C-70 solar cells are revealed by sensitive external quantum efficiency measurements, leading to differences in the open-circuit voltages of the devices. The degradation of standard and inverted solar cell configurations at ISOS aging test conditions (ISOS-D-3 and ISOS-T-3) was investigated and compared. The results indicate that the performance drop in the small molecule bilayer solar cells is less related to changes at the D-A interface, suggesting also a pronounced morphological stability, and instead, in the case of inverted cells, dominated by degradation at the electron transport layer (ETL) bathocuproine (BCP). Photoluminescence measurements, electron-only-device characteristics, and stability measurements show improved exciton blocking, electron transport properties and a higher stability for BCP/Ag ETL stacks, giving rise to inverted devices with enhanced performance and device stability. The research leading to these results has received funding from the People Programme (Marie Curie Actions) of the European Union's Seventh Framework Programme FP7/2007-2013/under REA Grant Agreement No. 607232, THINFACE. J.B., D.S. and K.V. acknowledge the German Federal Ministry for Education and Research (BMBF) for funding the project through the InnoProfile project "Organische p-i-n Bauelemente 2.2".

Intermolecular charge-transfer states at the interface between electron donating (D) and accepting (A) materials are crucial for the operation of organic solar cells but can also be exploited for organic light-emitting diodes(1,2). Non-radiative charge-transfer state decay is dominant in state-of-the-art D-A-based organic solar cells and is responsible for large voltage losses and relatively low power-conversion efficiencies as well as electroluminescence external quantum yields in the 0.01-0.0001% range(3,4). In contrast, the electroluminescence external quantum yield reaches up to 16% in D-A-based organic light-emitting diodes(5-7). Here, we show that proper control of charge-transfer state properties allows simultaneous occurrence of a high photovoltaic and emission quantum yield within a single, visible-light-emitting D-A system. This leads to ultralow-emission turn-on voltages as well as significantly reduced voltage losses upon solar illumination. These results unify the description of the electro-optical properties of charge-transfer states in organic optoelectronic devices and foster the use of organic D-A blends in energy conversion applications involving visible and ultraviolet photons(8-11). This work was supported by the German Federal Ministry for Education and Research (BMBF) through the InnoProfile project 'Organische p-i-n Bauelemente 2.2' (03IPT602X) and by the German Research Foundation (DFG) project Photogen (VA 1035/5-1). X.J. and Y.L. acknowledge support from the China Scholarship Council (nos. 201706140127 and 201506920047, respectively). The authors also acknowledge the DFG for supporting K.T. (project 382633022 'RECOLPER'), F.P., S.Ro. and D.N. (SFB 951 'HIOS') and A.F. (RE 3198/6-1 'EFOD').