Enhanced efficiency of inverted polymer solar cells using two-step sputtered ZnO as cathode interfacial layer

Efficient inverted polymer solar cells (PSCs) with two-step deposited zinc oxide (ZnO) cathode interfacial layer are developed by magnetron sputtering method. The first-step ZnO layer sputtered with Ar gas has lower bulk resistance due to increasing oxygen vacancy. The second-step ZnO layer sputtered with O2/Ar ratio (1:7) gas can obtain lower contact barrier and superior interface properties at ZnO/photoactive layer interface. The power conversion efficiencies of inverted poly(3-hexylthiophene) and phenyl-C61-butyric acid methylester (P3HT:PCBM) based PSCs with 11 nm/4 nm and 50 nm/10 nm ZnO layers are, respectively, 4.08% and 3.13%, which are obviously higher than that of corresponding PSCs with O2/Ar gas sputtered 15 nm and 60 nm ZnO layers. The conduction band level of ZnO film with O2/Ar sputtering gas is higher, 0.14 eV, than that of ZnO film with only Ar layer derived from ultraviolet photoelectron spectroscopy and optical band gap of ZnO films. The capacitive behavior of PSCs with 30 nm and 60 nm layers indicates that two-step sputtered ZnO layer can efficiently enhance electron transport ability of bulk ZnO layer and attenuate electron accumulation at ZnO/P3HT:PCBM interface. Lower reverse saturation current and higher shunt resistant of PSCs with two-step sputtered ZnO layer demonstrated that the defects penetrating into the whole ZnO layer can be inhibited.