Improved Light Utilization Efficiency for an ITO-Free Semitransparent Organic Solar Cell Using a Multilayer Silver Back Electrode as Infrared Mirror

Abstract

Semitransparent organic solar cells (STOSCs) exhibit promising application as power-generating windows in buildings and agricultural greenhouses. Due to unique optical properties of organic semiconductors, they can efficiently absorb near-infrared light while maintaining a high degree semitransparency in the visible range. Since power conversion efficiency (PCE) and average visible transmission (AVT) frequently stand in a trade-off relationship, a major challenge in improving the overall performance of STOSCs is maximizing the product of both, called light utilization efficiency (AVT × PCE = LUE). Herein, using multiple layers of aluminium-doped ZnO (AZO) and silver as an infrared reflecting back electrode, in order to increase current generation while maintaining high visible transparency, is proposed. Using optical modeling, the optimal layer thickness of the AZO layer sandwiched between two Ag layers is determined, leading to an increased photocurrent generation of up to 10%. Simultaneously, experimental findings show that the fill factor decreases with an increasing AZO layer thickness. By adjusting the thickness of the photoactive layer, the blend concentration, and improving the top electrode material the thus-far highest reported LUE for indium tin oxide-free STOSCs is attained, reaching 4.0% with a PCE of 8.7% and an AVT of 46.3%.