Undoped ZnO electrodes for low-cost indoor organic photovoltaics

Abstract

The unique features of organic photovoltaics (OPVs) offer great potential for their use as indoor energy harvesters. Here, we demonstrate for the first time the use of a low-cost undoped ZnO film for the transparent conducting electrode (TCE) of OPVs. Atomic layer deposition was employed to produce potentially defect-free and crystalline ZnO films. Poly(3-hexylthiophene) (P3HT):indene-C₆₀ bisadduct (ICBA)-based OPVs with 200 nm-thick ZnO TCEs were shown to hardly operate under 1 sun conditions due to the high sheet resistance (R_SH) of the ZnO film (260 Ω sq⁻¹). Surprisingly, the indoor performance of the OPVs was found to be barely dependent on the R_SH of the TCEs; rather, other factors such as leakage components associated with the shunt resistance were found to have a greater influence on the performance. The OPVs exhibited excellent indoor performance with a power-conversion efficiency (PCE) of 9.5 ± 0.3% under a light emitting diode lamp with a luminance of 500 lux. The measured PCE was slightly higher than or comparable to that of reference devices with a 200 nm-thick ITO TCE. Finally, the ZnO TCE is expected to result in more than 20% cost reduction of the OPVs when compared with the ITO TCE.