Zone-Melting Recrystallization for Crystalline Silicon Thin-Film Solar Cells

Abstract

Thin-film solar cells from crystalline silicon require only a fraction of the high purity absorber material needed for wafer based solar cells. Due to the crystalline structure, the silicon film can be processed into solar cells analogous to multicrystalline silicon wafers - and the cells can reach similar conversion efficiencies.<br /><br />This work investigates a crystalline silicon thin-film solar cell technology that is based on a so-called high temperature approache. A thin silicon film is applied onto a substrate by chemical vapor deposition (CVD) and subsequently transferred into a coarse-grained structure by zone-melting recrystallization (ZMR). This film acts as a seed film, which is afterwards epitaxially thickened. The final silicon thin film can mainly be processed into solar cells with standard technology.<br /><br />The ZMR process largely determines the conversion efficiency of the final solar cell, since crystal defects like dislocations or grain boundaries are replicated by the subsequent epitaxial growth. Of special importance are low angle grain boundaries, which are an inevitable feature of the ZMR process and in this connection are called subgrain boundaries. This work examines how dislocations in the epitaxial film are related to subgrain boundaries in the seed film. Experimental results confirm a theory that explained subgrain boundary formation by tilting of subgrains and polygonization of dislocations. Further, the effect of different material and process parameters on dislocation density is examined, like film thickness and ZMR scan speed.<br /><br />For a quantitative description of the relation between dislocations and electronic film properties, a theoretical model is developed, based upon the work of Donolato (J. Appl. Phys. 84, 2656 - 1998). For the purpose of this work Donolato´s model is extended regarding the use of the quantum efficiency effective diffusion length and the appropriate description of thin devices with a finite thickness and a finite back surface recombination velocity. A technique for automated etch pit density (EPD) mapping has been developed for application of the model to experimental data. Normalized recombination strength values are determined from EPD data and measurements of effective diffusion lengths by spectrally resolved light beam induced current (SR-LBIC). Further, the model can be used to describe the dependence of open circuit voltage on dislocation density.<br /><br />The last part of this work demonstrates how solar cell conversion efficiency can be improved by a process that is especially adapted to the silicon thin films. With such an optimized process, conversion efficiencies up to 13.5 % (Voc = 610 mV, Jsc = 30.9 mA/cm², and FF = 71.7 %) are demonstrated, for a model substrate with a ~31 µm thick silicon film.