We propose embedment of a fishnet metastructure in the back passivation layer for enhancing the efficiency of hydrogenated amorphous silicon (a-Si:H) thin film solar cells. Incident light excites a plasmon resonance that results in frequency dependent effective impedance for the embedding layer so that the input impedance satisfies impedance matching condition. Reflection is very low under this condition. A planar 20 nm-thick fishnet structure is embedded in the back passivation layer of the solar cell to enhance light absorption near the bandgap of a-Si:H. This enhancement remains over a wide range of incident angles. Detailed electromagnetic modeling of the absorption in different layers of the solar cell is performed. Only absorption in the a-Si:H is included in computing the photocurrent generation. 64% of the total absorbed energy at resonance is in the silicon layer and this absorption is uniformly distributed inside the silicon. Based on the enhancement of photocurrent density near the bandgap of a-Si:H, we obtained 14.8% enhancement in total short circuit current at normal incidence and the estimated PV efficiency of the solar cell with the fishnet is 7.43% at normal incidence compared to 6.36% without fishnet. The fishnet can be tuned to provide absorption enhancement at any desired frequency where the intrinsic absorption of the semiconductor is low.
Superstrate a-Si:H solar cells incorporating a nano-column array for light and photocarrier collection have been fabricated and evaluated. It is found that the short circuit current density (JSC) is significantly increased while the open circuit voltage and fill factor are not detrimentally affected by this architecture. Numerical analysis of JSC matches experiment and shows that the enhanced JSC observed is due to both effective absorber thickness and photonic-plasmonic effects. Further analysis shows that this nano-column architecture can lead to a 42% increase in conversion efficiency over that of the planar control for a 200 nm absorber thickness cell.
Incorporating plasmonic structures into the back spacer layer of thin film solar cells (TFSCs) is an efficient way to improve their performance. The fishnet structure is used to enhance light trapping. Unlike other previously suggested discrete plasmonic particles, the fishnet is an electrically connected wire mesh that does not result in light field localization, which leads to high absorption losses. The design was verified experimentally. A silver fishnet structure was fabricated using electron beam lithography (EBL) and thermal evaporation. The final fabricated structure optically resembles a TFSC. The results predicted by numerical simulations were reproduced experimentally on a fabricated sample. We show that light absorption in the a-Si absorber layer is enhanced by a factor of 10.6 at the design wavelength of 690 nm due to the presence of the fishnet structure. Furthermore, the total absorption over all wavelengths was increased by a factor of 3.2. The short-circuit current of the TFSC was increased by 30% as a result of including the fishnet.
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