Ultra-thin film microcrystalline silicon with high deposition rate and its application in tandem silicon solar cells

Abstract

Reducing production cost to accelerate the industrialization process of thin film solar cells (TFSCs) makes it urgently demanded to elevate the deposition rate and reduce the needed thickness of absorbers in addition to the prerequisite performance improvement. Based on very high frequency plasma enhanced chemical vapor deposition process with a low bombardment energy and large ion flux, ultra-thin, high-deposition-rate, and high-performing hydrogenated microcrystalline silicon (c-Si:H) single- and related hydrogenated amorphous silicon (a-Si:H)/c-Si:H double-junction TFSCs are developed in this study. By tuning various process parameters (silane concentration, power, and pressure), the deposition rates and electrical properties of c-Si:H materials are studied in detail. Device-level c-Si:H intrinsic materials with a deposition rate of 10.57 /s and photosensitivity of 7.54102 can be obtained when depositing with a silane concentration of 9%, a power of 70 W, and a pressure of 2.5 Torr. By further applying device-level high-deposition-rate c-Si:H intrinsic materials in c-Si:H single-junction TFSCs on magnetron-sputtered and wet-etched aluminum-doped zinc oxide (ZnO:Al) substrates with optimized surface morphologies and photoelectrical properties, and by combining advanced device designs, an initial conversion efficiency of 7.49% can be achieved for pin-type ultra-thin and high-deposition-rate c-Si:H single-junction TFSCs (the thickness values of intrinsic layers are 1.1~m). To further improve the conversion efficiency of TFSCs, pin-type a-Si:H/c-Si:H tandem TFSCs are fabricated by using n-a-Si/n-c-Si/n-nc-SiOx:H/p-nc-SiOx:H as the tunnel recombination junctions (TRJs), which, however, have unaddressed issues that the wide band-gap nc-SiOx:H materials with a low conductivity strongly reduce the recombination rate of carriers, thereby resulting in the photo-generated carriers accumulating near the TRJs, weakening the built-in electric field in the top sub-cells and leading to an open circuit voltage (Voc) loss in a-Si:H/c-Si:H tandem TFSCs up to 115~mV far above average values. By simultaneously inserting the p- and n-type narrow-gap c-Si:H materials, which are highly defective and narrower than the band gap of nc-SiOx:H materials, into the TRJs to implement the electrically lossless interconnection between the a-Si:H top and c-Si:H bottom sub-cells, the Voc loss is successfully reduced to 43~mV and an initial efficiency of 12.03% (Voc=1.48~eV, Jsc=11.67~mA/cm2, FF=69.59%) is achieved for ultra-thin pin-type a-Si:H/c-Si:H tandem TFSCs with a total thickness of 1.48~m, thus paving the way for the low-cost production of TFSCs.