2, California Institute of Technology, Pasadena, California, United States
We have developed superstrates for perovskite solar cells that feature increased transparency and conductivity due to the incorporation of effectively transparent contacts (ETCs). They increase the short circuit current density by more than 1 mA/cm2 compared to standard indium tin oxide (ITO) on glass superstrates. Our superstrates feature effectively transparent grid fingers, which enable a significant reduction in the ITO thickness required for current extraction with a high fill factor. They are composed of soda-lime glass with a thin (~40 µm) layer of polydimethylsiloxane (PDMS) that features triangular cross-section microscale grooves, which are infilled with a conductive silver ink and subsequently coated by a thin (~30 nm) ITO layer such that high lateral conductivity (< 5 Ω/sq) is achieved without altering the surface properties of standard perovskite superstrates. Due to the reduction of the ITO thickness, parasitic absorption is greatly reduced and antireflection properties are optimized leading to up to a 1 mA/cm2 increase in short circuit current density. High lateral conductivity is obtained by spacing the triangular silver lines closely (~80 µm distance). Whereas such densely-spaced grid fingers would normally cause excessive shading losses, here, their triangular cross-section and high aspect ratio serve to reflect all incident light to metal-free areas of the superstrate, leading to >99% effective transparency as demonstrated in our previous work for such contacts applied to other solar cells (Adv. Optical Mater. 4 (10), 1470-1474 (2016); Photovoltaic Specialists Conference (PVSC) IEEE 43rd, 3612-3615, (2016); Sustainable Energy and Fuels, 1 (3), 593-598, (2017)). FACsPbI3 perovskite solar cells were fabricated on these superstrates and showed improved external quantum efficiency, with an average integrated short circuit current increase of 1 mA/cm2. Computational modelling of optical and electrical properties guided the device design. Here, we will present computational simulations, fabrication methods and experimental results on increased perovskite solar cell performance.
The above described approach constitutes an effective and scalable way of enhancing the short-circuit current density in perovskite solar cells, and incorporates only materials that are widely used in the photovoltaic industry. The area fraction devoted to macroscopic grid fingers and busbars can be further reduced on large scale solar cells and modules, as compared with conventional designs. Furthermore, our superstrates may find application in thin film tandem solar cell architectures as well as in other optoelectronic devices.