Fabrication and Analysis of a-Si:H/μc-Si:H Tandem Solar Cell on Flexible Al Substrates

Abstract

Our world is on its way to re-engineer the daily life based on renewable clean energy. And this multilateral attempt to combat environmental and climate change has its focus perfectly poised to embrace the infinite energy source - solar energy as an alternative. Thin-film photovoltaic technology also referred to as second-generation technology is an alternative family of solar technology in the market which holds rich market prospects such as potential advantages of cost-effective substrates, low transportation-installation costs due to its lightweight property and the possibility of direct integration in building materials. HyET solar is focusing its production on Roll-to-Roll manufacturing of thin films. The utilisation of R2R production with efficient deposition in substrate configuration, and the serial interconnections of cells done during production can bring down the cost of production and contribute to manufacturing flexible solar cells on polymers. The thin-film solar energy technology is looking at a possible re-entry to the market with these advantages. Integrating the micromorph configuration into this technology is challenging. To start we have taken two base models and tried to fabricate tandem samples on the aluminium substrate. These substrates are temporary foils with modulated surface texturing implemented on them for better light utilization. The initial lab-scale samples yielded less than 35%of the total. The lab-scale samples showed highly defective μc-Si :H subcells. An investigation on the layers used in the samples was conducted and it suggested conclusively that the μc-SiOx n layers with low activation energies were the main reason for the very low shunt resistances in the cells. The power of optoelectronic modelling is taken advantage of to improve the performances is done. This study resulted in a model of optimised efficiency of 12.2%. A study on doping of the tunnelling recombination junctions suggested that with poorly doped layers, the objective of TRJ to facilitate tunnelling recombination at the junction is not satisfied and ultimately affect the cell behaviour forming a reverse biased n-p junction. The use of a highly reflective intermediate reflective layer for the tandems is detrimental to the bottom cell performance. But at the same time, a reflective behaviour in the TRJ p-doped layer, when using a p-doped μc-SiOx layer fabricated at 3.2 sccm CO2 flow rate, the reflection is moderate and can be proved beneficial resulting in a thinner top cell. The possibility of modelling to predict the performance, out of standard test conditions suggests a drop in the open-circuit voltage of our cell by 4.9mV per Kelvin increase and a fall in the efficiency by 0.058% per Kelvin.
In summary, the tandem flexible solar cells once overcome the challenges in fabrications, it can facilitate cheap, high-efficiency modules. The simulations on this same structure point towards several potential spaces for the micromorph configuration for improvement within itself and certainly a bright future ahead of us powered by thin-film flexible tandem solar cells.