Optical design of spectrally selective interlayers for perovskite/silicon heterojunction tandem solar cells.

Monolithic perovskite/c-Si tandem solar cells have the potential to exceed the Shockley-Queisser limit for single junction solar cells. However, reflection losses at internal interfaces play a crucial role for the overall efficiency of the tandem devices. Significant reflection losses are caused by the charge selective contacts which have a significantly lower refractive index compared to the absorber materials. Here, we present an approach to overcome a significant part of these reflection losses by introducing a multilayer stack between the top and bottom cell which shows spectrally selective transmission/reflection behavior. The layer stack is designed and optimized by optical simulations using transfer matrix method and a genetic algorithm. The incident sun light is split into a direct part and an isotropic diffuse part. The tandem solar cell with interlayer shows an absolute improvement of short-circuit current density of 0.82 mA/cm2.

Liquid hydridosilane precursor prepared from cyclopentasilane via sonication at low temperatures without the action of light.

We report on a liquid hydridosilane precursor ink prepared via the ultrasonically induced ring-opening polymerisation of cyclopentasilane (Si5H10) without irradiation by ultraviolet light. The sonication is carried out in N2 atmosphere at temperatures between 20 and 75°C. We use size exclusion chromatography (SEC) to show polymer growth and estimate molecular mass with increasing sonication time. In combination with UV-vis transmission measurements, further SEC analysis is used to compare solutions subjected to either purely thermal or ultrasonic treatment at the same process temperature and for the same duration. Our findings provide strong evidence showing that the initiation of the polymerisation is sonocatalytic in nature and not thermic due to the macroscopic temperature of the solution. The liquid precursor is used to produce homogeneous hydrogenated amorphous silicon (a-Si:H) thin films via spin coating and pyrolytic conversion. The optoelectronic properties of the films are subsequently improved by hydrogen radical treatment. Fourier transform infrared spectroscopy (FTIR) is used to determine a compact film morphology and electrical conductivity measurements show that the layers attain a light-to-dark photosensitivity ratio of 2×103 making them suitable for application in optoelectronic devices.

Post passivation light trapping back contacts for silicon heterojunction solar cells.

Light trapping in crystalline silicon (c-Si) solar cells is an essential building block for high efficiency solar cells targeting low material consumption and low costs. In this study, we present the successful implementation of highly efficient light-trapping back contacts, subsequent to the passivation of Si heterojunction solar cells. The back contacts are realized by texturing an amorphous silicon layer with a refractive index close to the one of crystalline silicon at the back side of the silicon wafer. As a result, decoupling of optically active and electrically active layers is introduced. In the long run, the presented concept has the potential to improve light trapping in monolithic Si multijunction solar cells as well as solar cell configurations where texturing of the Si absorber surfaces usually results in a deterioration of the electrical properties. As part of this study, different light-trapping textures were applied to prototype silicon heterojunction solar cells. The best path length enhancement factors, at high passivation quality, were obtained with light-trapping textures based on randomly distributed craters. Comparing a planar reference solar cell with an absorber thickness of 280 µm and additional anti-reflection coating, the short-circuit current density (JSC) improves for a similar solar cell with light-trapping back contact. Due to the light trapping back contact, the JSC is enhanced around 1.8 mA cm-2 to 38.5 mA cm-2 due to light trapping in the wavelength range between 1000 nm and 1150 nm.

Comparison and optimization of randomly textured surfaces in thin-film solar cells.

Using rigorous diffraction theory we investigate the scattering properties of various random textures currently used for photon management in thin-film solar cells. We relate the haze and the angularly resolved scattering function of these cells to the enhancement of light absorption. A simple criterion is derived that provides an explanation why certain textures operate more beneficially than others. Using this criterion we propose a generic surface profile that outperforms the available substrates. This work facilitates the understanding of the effect of randomly textured surfaces and provides guidelines towards their optimization.