Towards the efficiency limits of silicon solar cells: How thin is too thin?

Abstract

a b s t r a c tIt is currently possible to fabricate crystalline silicon solar cells with the absorber thickness ranging from a few hundreds of micrometres (conventional wafer-based cells) to devices as thin as 1 μm. In this work, we use a model single-junction solar cell to calculate the limits of energy conversion efficiency and estimate the optimal absorber thickness. We have found that the limiting efficiency for cells in the thickness range between 40 and 500 μm is very similar and close to 29%. In this regard, we argue that decreasing the thickness below around 40 μm is counter-productive, as it significantly reduces the maximum achievable efficiency, even when an optimal light trapping is implemented. We analyse the roles of incomplete light trapping and extrinsic (bulk and surface) recombination mechanisms. For a reasonably high material quality, consistent with present-day fabrication techniques, the optimal thickness is always higher than a few tens of micrometres. We identify incomplete light trapping and parasitic losses as a major roadblock for improving the efficiency upon the current record of 25.6% for silicon solar cells. Finally, considering the main parameters that impact the solar cell performance, we quantify the requirements for achieving a given efficiency, which helps us to establish a proper design strategy for high efficiency silicon solar cells.

Revisiting thin silicon for photovoltaics: a technoeconomic perspective

“…The recent industrial transition to PERC aims to reduce rear surface recombination. With further advancement of surface passivation technology, recent studies [12], [13] suggest that wafer thinning may no longer be as detrimental for conversion efficiency. Better passivation in the rear surface also coincides with an improved optical performance, which also contributes to higher efficiency.…”

Interdigitated back‐contact heterojunction solar cell concept for liquid phase crystallized thin‐film silicon on glass

“…The main limit for higher efficiency is the FF on which further investigations will have to focus on. Additionally, recent investigations on optimum Si absorber thicknesses point out that 40 µm is the lowest thickness not significantly reducing maximum possible efficiency . Hence, considering to go to thicker absorbers than 10 µm might be an option.…”

Silicon solar cells: toward the efficiency limits

“…The diffusion coefficients are taken to be D E = 12.5 cm 2 /s in the n-type emitter and D B = 25 cm 2 /s in the p-type base. We assume a solar cell structure with a 5-nm thick n-type emitter, to minimize recombination losses in this heavily doped layer, and we take optimal doping values [102]: the emitter doping is equal to N d ¼ 1:5 Â 10 18 cm À3 , while the base doping is equal to N a ¼ 10 16 cm À3 .…”

“…A more complete analysis of the optimal thickness for a given material quality also in the presence of parasitic losses, and of the requirements to reach a given efficiency target, is given in Ref. [102]. Notice that the short-circuit current is unchanged, unless the diffusion length becomes comparable to or smaller than the cell thickness.…”

“…The effect is largest at small thicknesses, which is physically intuitive, as the Efficiency role of surfaces is expected to become more important for thinner samples. However the effect of surface recombination is nonnegligible at any thickness: for example, assuming S ¼ 1 cm/s leads to a maximum efficiency η max ¼ 28:73% [102]. The reason for this high sensitivity to surface recombination lies in the fact that c-Si solar cells close to the efficiency limits are in the high-injection regime, in which the photogenerated carrier density becomes larger than the doping.…”