Improved Current Generation for an ITO-free Semitransparent Organic Solar Cell Using a Multilayer Silver Electrode as Distributed Bragg Reflector
Creators: name: Pap, Leonie
name: Glissmann-Jahnke, Nico
name: Zimmermann, Birger
name: Würfel, Uli
name: :Unav
Publisher: name: Fraunhofer-Gesellschaft
Year: 2022

description: Semitransparent organic solar cells (STOSC) exhibit promising applications as power-generating windows in buildings and agricultural greenhouses. Due to their widely adjustable optical band gap, organic semiconducting materials have the potential to efficiently utilize near infrared light while maintaining semitransparency in the visible light range. One challenge in improving the overall performance of STOSCs is maximizing both power conversion efficiency (PCE) and average visible transmittance (AVT) simultaneously, since intrinsically they are often in a trade-off relationship.[1] A key factor in optimizing both values, is choosing the right material for each intermediate layer of the cell. One well-established material used for electrodes in semitransparent cells is Indium tin oxide (ITO). Although known for its high transparency and good conductivity, it is considered brittle, and its production is cost- and energy-intensive. Furthermore, Indium is a metal that rarely occurs in nature.[2] Lastly ITO exhibits a comparably low reflection in the infrared region.[3] One alternative for these problems is using multiple interlayers of silver and Al-doped ZnO (AZO) as a distributed Bragg reflector on the backside of the solar cell. By varying the layer thicknesses, the optical properties of this electrode can be modified to maximize reflection in the infrared range. This way the current generation of the photoactive layer can be improved while simultaneously maintaining a high transparency for the cell. Herein we report recent progress in ITO-free semitransparent organic solar cells by implementing a multilayer silver back electrode as an infrared mirror, achieving a power conversion efficiency of 10.2% for a cell stack based on PV-X Plus with an average visible transmittance of 33.1%. Both optical modelling and experimental findings were used to maximize the photocurrent generation and the average visible transmission of the solar cell. The back electrode consists of alternating layers of Al-doped ZnO and thin silver. As the reflected part of the light passes the photoactive layer a second time, an increase in absorption is achieved. Optical simulations show that the distance between the two silver layers has a strong influence on the wavelength range of the reflected light and thus determines the generation of the photocurrent in the cell. By optimizing the spacing between the two silver layers, it was possible to increase the photocurrent for a solar cell based on PV-X Plus by 9.8% (from 19.3 mA/cm2 to 21.2 mA/cm2). At the same time, it was found that the fill factor decreases with an increase in the thickness of the interlayer. When using a single silver layer of 14 nm, the fill factor could be kept reasonably high (71.3%) that despite a comparably lower current generation (17.0 mA/cm2) a LUE (PCE x AVT) of 3.38% was achieved.