Isotextured Silicon Solar Cell Analysis and Modeling 2: Recombination and Device Modeling

Abstract

We extend our analysis of isotextured silicon solar cells by 1) examining experimentally the role played by isotexture in determining the surface recombination velocity at silicon surfaces and 2) combining these experimental results with our model for photogeneration in order to simulate in one dimension typical solar cell devices with isotextured surfaces. We examine both undiffused and diffused n-type isotextured silicon surfaces, and we find that the rate of surface recombination usually decreases with increasing isotexture etch depth. However, when undiffused surfaces are passivated with hydrogenated SiO 2 or SiN x , surface recombination velocity is, counterintuitively perhaps, found to be independent of surface texture-this is despite a surface area that is up to 1.9-fold larger than a planar equivalent. We demonstrate the utility of our analysis of isotextured surfaces by simulating various device structures in one dimension. In one case, where device parameters are chosen to approximate a typical screen-printed cell with full-area back surface field, simulation results indicate that the optimal isotexture etch depth is 1-3 μm. This optimum etch depth is slightly below the one deduced from published experimental results, indicating that surface recombination on samples observed in this study is uniquely independent of isotexture morphology.

Modelling isotextured silicon solar cells and modules

“…First, it is typically hypothesised that the texture etch progressively removes a 'saw-damaged,' defect-rich near surface layer, and that more complete removal (via deeper etching) reduces surface recombination, increasing v oc and slightly reducing j sc (due to reduced collection efficiency near the surface) [1,2]. However, we note that in our previous study, recombination at isotextured surfaces was found to be relatively constant with etch depth, and hence insensitive to the impact of any possible damaged region [4]. Second, as etching proceeds to greater depths, the surface texture morphology progresses from highly concave pits to mild undulations.…”

“…Shunt resistance R sh extracted from IV curves increases with etch depth. It is likely that diffusion sheet resistance is decreased at low etch depths [4]; shallower junctions are more susceptible to firing through of contacts. Series resistance R s is independent of etch depth, suggesting, for example, that contact resistance varies negligibly with morphology.…”

“…Here, we apply the models established previously [3,4] to predict the performance of standard industrial multicrystalline silicon solar cells described in Section II -in particular we attempt to replicate the texture etch depth of j sc , v oc and η. Each component is an input to PC1D [5] in a model of an isotextured cell having a certain morphology (etch depth).…”

“…In our previous work [4], monocrystalline substrates were isotextured in identical equipment to that applied to cell substrates in Section II. Symmetrical test structures were either left undiffused (1.8 Ω cm p-type) or received n + diffusion from a POCl 3 source in a quartz tube to a target sheet resistance (on planar wafers) of 110 Ω/□.…”

“…In this work, we apply recently developed analyses [3,4] of the optical and electrical properties of isotextured silicon to arrive at a one-dimensional analysis of commercial multicrystalline silicon solar cells. We calculate the front surface reflectance and the profile of photogeneration in isotextured cells, approximate the recombination at the textured surface, then combine optical and recombination models to calculate device performance parameters to predict an optimal isotexture etch depth of ~1 µm.…”