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Abstract
Polycrystalline silicon (poly-Si) thin-film solar cells are receiving attention by many researchers in recent times. The focus of this thesis is the evaporated ALICE solar cell, a thin-film poly-Si solar cell fabricated on a glass superstrate by e-beam evaporation. The acronymn ALICE comes from - ALuminium Induced Crystallization Solid Phase Epitaxy. The concept is first to form a high-quality crystalline silicon layer on glass by Aluminium Induced Crystallization (AIC). The AIC seed layer (grain size>20 µm) acts as the template from which the crystalline information is transferred into the silicon over-layer by solid-phase epitaxy (SPE). As a result, the ALICE solar cells have much larger grain size compared to the poly-Si thin-film solar cell (2~3 µm) by random nucleation and growth (RNG). This leads to the minimized grain boundary recombination and hence potential improved conversion efficiency.
The temperature of 200°C is found to be optimal for the deposition of amorphous Si (a-Si) precursor thin films. The epitaxy process of the ALICE cell is successful, proving the feasibility and reliability of the deposition and post-treatment processes.
The ALICE cell is successfully metallized using a bifacial interdigitated scheme. Wet etching using KOH is introduced to realize the uniform Si etching, and phosphoric acid etching is introduced to remove the local shunts in the ALICE cell.
The results show that the material quality of ALICE solar cells are much worse than that of the AIC seed layer, which is related to the poor epitaxy quality on (111) planes grown from the AIC seed layer. Additional experiments show that the fraction of (100) oriented grains in AIC is the main factor in determining the material quality and the resulting solar cell performance, rather than grain size. Therefore, both a high fraction of (100) oriented grains and large grain size are required for AIC seed layers to achieve the ALICE solar cells with superior performance.
Comparison of the ALICE cells prepared at different base pressures and deposition rates show that the base pressure is much less important than the deposition rate. Therefore, the capital cost of the evaporator system can be reduced and hence potentially the manufacturing cost of solar cells.
The densification anneal was introduced to improve the crystal quality of poly-Si thin films by SPE. It is shown that the cause is the structural relaxation induced into the a-Si film, instead of the prevention of the oxygen percolation. The crystal quality of c-Si films obtained from low-rate (50 nm/min) evaporated a-Si is considerably improved by densification anneal, whereas densification has no beneficial effect on c-Si films obtained from high-rate (300 nm/min) evaporated a-Si. However, the densification anneal has no improvement on the electrical performance of ALICE solar cell.
The ALICE solar cell performances are strongly related to the doping level in the absorber layer. The optimal doping density needs to be determined to achieve the best performance. The highest Voc and Jsc are simultaneously achieved when the minimum phosphorous doping density of ~5.5×1015 cm-3 (unintentionally doped) is applied for the evaporated ALICE solar cells.
Since silicon is a weak absorber and ALICE solar cell has only ~1.5 µm thickness, light trapping is applied to enhance the light absorption of the visible and the red light. Three different approaches are applied: ALICE cells on textured glass sheet, back surface reflector and thicker Si film. The ALICE cells on textured glass suffer from a significant loss of performance. The only successful approach to improve the light trapping in this thesis is to apply white paint as back surface reflector, which increases the Jsc drastically (~60%) compared to a planar sample.
Analysis of the optical properties of poly-Si thin films is important as it assists the design of the thin-film solar cells. It is found that there is enhanced absorption in the visible wavelengths. This is mainly attributed to defected a-Si material at the grain boundaries. The hydrogenation process does not affect this enhanced absorption. The optical analysis proves that large grain size is desired to obtain high performance poly-Si thin-film solar cell, e.g. ALICE solar cell.
At the end of this research, ALICE cells with η~3.83%, Voc~485 mV, Jsc~17.75 mA/cm2 have been achieved.