Highly efficient inverted BHJ solar cells were demonstrated using a wet-chemically prepared doped ZnO layer with a self-organized ripple nanostructure. The solar cell based on PTB7 and PC71BM with Li2CO3-doped ZnO layer yielded a maximum efficiency of 10.08%.

The enhancement of interfacial charge collection efficiency using buffer layers is a cost‐effective way to improve the performance of organic photovoltaic devices (OPVs) because they are often universally applicable regardless of the active materials. However, the availability of high‐performance buffer materials, which are solution‐processable at low temperature, are limited and they often require burdensome additional surface modifications. Herein, high‐performance ZnO based electron transporting layers (ETLs) for OPVs are developed with a novel g‐ray‐assisted solution process. Through careful formulation of the ZnO precursor and g‐ray irradiation, the pre‐formation of ZnO nanoparticles occurs in the precursor solutions, which enables the preparation of high quality ZnO films. The g‐ray assisted ZnO (ZnO‐G) films possess a remarkably low defect density compared to the conventionally prepared ZnO films. The low‐defect ZnO‐G films can improve charge extraction efficiency of ETL without any additional treatment. The power conversion efficiency (PCE) of the device using the ZnO‐G ETLs is 11.09% with an open‐circuit voltage (V OC), short‐circuit current density ( J SC), and fill factor (FF) of 0.80 V, 19.54 mA cm‐2, and 0.71, respectively, which is one of the best values among widely studied poly[4,8‐bis(5‐(2‐ethylhexyl)thiophen‐2‐yl)benzo[1,2‐b;4,5‐b′]dithiophene‐2,6‐diyl‐alt‐(4‐(2‐ethylhexyl)‐3‐fluorothieno[3,4‐b]thiophene‐)‐2‐carboxylate‐2‐6‐diyl)]: [6,6]‐phenyl‐C71‐butyric acid methyl ester (PTB7‐Th:PC71BM)‐based devices.

In spite of the successful enhancement of the power‐conversion efficiency (PCE) in organic bulk heterojunction (BHJ) solar cells by surface plasmon resonance (SPR), the incorporation of several tens of nanometer‐sized (25–50 nm) metal nanoparticles (NPs) has some limitations to further enhancing the PCE due to concerns related to possibly transferring nonradiative energy and disturbing the interface morphology. Instead of tens of nanometer‐sized metal NPs, here, dodecanethiol stabilized Au nanoclusters (Au:SR, R = the tail of thiolate) with sub‐nm‐sized Au38 cores are incorporated on inverted BHJ solar cells. Although metal NPs less than 5 nm in size do not show any scattering or electric field enhancement of incident light by SPR effects, the incorporation of emissive Au:SR nanoclusters provides effects that are quite similar to those of tens of nanometer‐sized plasmonic metal NPs. Due to effective energy transfer, based on the protoplasmonic fluorescence of Au:SR, the highest performing solar cells fabricated with Au:SR clusters yield a PCE of 9.15%; this value represents an ≈20% increase in the efficiency compared to solar cells without Au:SR nanoclusters.