11% Organic Photovoltaic Devices Based on PTB7-Th: PC₇₁BM Photoactive Layers and Irradiation-Assisted ZnO Electron Transport Layers

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.