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Abstract
Weak absorption of near infrared light and a goal of 2μm absorber thickness necessitate efficient light management, incoupling and trapping of light, to make poly-Si thin-film solar cells more competitive in the photovoltaic industry.
This thesis targets breaking the record short-circuit current density (Jsc) of 29.7mA/cm2 for a silicon cell of this thickness via alternative light management approaches. HF is used to thin the borosilicate glass (BSG) superstrate from 3.3mm to 0.5mm. This enhances Jsc by 8%, even higher than the value of 4.6% found in the literature. Also, a subwavelength (SW) structured, broadband antireflection coating is produced on the air side of the glass superstrate by a novel mask forming technique. This SW structure enhances Jsc by 3.4%. Additionally, an Al Induced Texturing (AIT) glass process is optimised and the conventional AIT etch solution, a mix of 49%HF+69%HNO3, is replaced by NaOH and dilute HF solutions. The new etch demonstrates better control of optical and electrical properties than the conventional etch. The optimal back reflector is ≥500nm MgF2+Ag for a randomising back surface as determined by simulation.
Through the methods above, a Jsc of 28mA/cm2 is attained for a ~2 μm thick cell which outperforms the record cell in the 800-1200nm wavelength range. The underperformance in the 300-800nm range mainly comes from inferior metallisation and unsatisfactory passivation.
Estimation of the light trapping properties of textured glass prior to depositing anything on it is desirable. Conventional methods such as haze classification or angularly resolved scattering are found not as effective as a novel method -calculating the correlation coefficient between the interfaces of a Si film based on a self-coding Si film growth model.
Attempts to improve the light trapping of electron-beam (e-beam) poly-Si solar cells are also reported. It is found that only smooth submicron and SW structures do not degrade the open-circuit voltage (Voc) of e-beam poly-Si solar cells and produce a Jsc enhancement of 21% compared to a planar control. The optimal back surface plasmon configuration is found to be paint directly applied on Ag nanoparticles. Three back texturing methods are also developed.
Nano-imprinting is developed to transfer the texture from an AIT glass template to sol-gel coated planar glass. A baseline process is established and proved to successfully complete the pattern transfer. However the patterned sol-gel is found to be not compatible with a rapid thermal annealing process.