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

It 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.