Parasitic loss mitigation and photocurrent enhancement in amorphous silicon solar cells by using phosphorous-doped fluorinated µc-SiO:H back reflector

Abstract

Investigation of the light trapping effect and optical loss mitigation by incorporation of a back reflector in the single junction p-i-n solar cell has been reported. Phosphorous-doped fluorinated microcrystalline silicon oxide (n-µc-SiO:F:H) based back reflector has been developed by radio frequency plasma enhanced chemical vapor deposition process (RF-PECVD at 13.56 MHz) using silicon tetrafluoride (SiF4) in the gas phase reaction. The optical analysis of n-µc-SiO:F:H back reflector layer (BRL) demonstrates that it has a low refractive index and higher reflection in long wavelength region compared to conventional oxide-based (n-µc-SiO:H) back reflector. HRTEM and Raman spectroscopy analysis of the deposited n-µc-SiO:F:H film confirms that the use of SiF4 in the plasma provide better microcrystalline silicon growth without any seed layer. The lower refractive index (n) of n-µc-SiO:F:H back reflector layer effectively reflects the higher wavelength light to absorber layer. Its lower value of extinction coefficient (k) significantly minimizes the parasitic absorption in the film. As a result, the photogenerated current and red response of the solar cell got enhanced significantly. The optically calibrated simulations supported by experimental results elucidate that the improvement in device performance originates from enhanced photon absorption and higher charge carrier generation. The a-Si:H solar cell fabricated with fluorinated microcrystalline SiO:H back reflector provides high current density of 15.63 mA/cm2. The power conversion efficiency of the n-µc-SiO:F:H back reflector cell is 9.72% which is 6.85% higher than n-µc-SiO:H based back reflector cell and 17.30% higher than the solar cell without any back reflector.