Development of four-terminal devices utilising thin-film solar cells

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Single junction solar cells suffer from two primary loss mechanisms due to spectral mismatch: thermalisation and non-absorption losses. A photon with energy greater than the bandgap of the solar cell can be absorbed to excite an electron-hole pair and the excess energy received is released as heat constituting thermalisation losses. On the other hand, a photon with energy lower than the bandgap of the solar cell is not absorbed and leads to non-absorption losses. William B. Shockley and Hans-Joachim Queisser worked out the theoretical efficiency limit of the single junction solar cell based on the spectral mismatch and assuming band-to-band recombination, arrived at a conclusion that the maximum possible theoretical efficiency of a single junction solar cell is 33.1%.

To go beyond the theoretical efficiency limit, a high bandgap solar cell (top cell) can be optically coupled to a low bandgap solar cell (bottom cell) such that the high energetic photons are absorbed in the top cell and the non-absorbed photons from the top cell in the bottom cell. This explains the principle of tandem solar cells. In this work, a tandem solar cell structure known as four terminal device (4TD) is utilised. In a four terminal device tandem cell, the top and bottom cells are mechanically stacked together to have the same optical path, but are electrically isolated to ensure independent performance. This reduces the fabrication complexity involved in a two terminal tandem device structure.

To access the efficiency potential of various 4TD configurations before the actual fabrication of such devices, a theoretical model was derived in this work. The model predicted an absolute efficiency gain of 2% in the bottom cell for a combination of a-SiOx:H-CIGS (top cell-bottom cell) and a 3.5% efficiency gain for a a-SiOx:H-CIS devices. Based on the theoretical model, different combinations of 4TD’s were fabricated. All such devices showed a gain in efficiency in the bottom cell and the combination of a lower bandgap a-SiOx:H-CIS devices showed a maximum efficiency gain of 4.40%.

The performance analysis of different combinations of 4TD’s revealed that the extent of efficiency gain in the bottom cell is higher for a low performing bottom cell and the potential of the current 4TD configuration is limited by the transmission of the top cell. Using GenPro4 simulations, the optical system of the 4TD were analysed and possible solutions were explored. The infrared (IR) transmission of the top cell showed a drastic improvement from 55% to 80% by replacing the Asahi substrate with textured IO:H substrate. The new optimised top cell possess a combination of excellent spectral response in shorter wavelength and a good transparency in the IR region, thus making it more suitable for high efficiency 4TD configuration.