Laser Annealing of Carrier-Selective Layers in High-Efficiency Photovoltaic Devices
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To realize efficient photovoltaic devices, carrier-selective layers are utilized to effectively separate photogenerated charge carriers and collect them at the electrodes. These layers allow the transport of one type of charge carrier while presenting a barrier to the other, preventing recombination within the layer. Typically, in both high-efficiency perovskite and silicon solar cells, carrier-selective layers require high-temperature treatments to achieve enhanced efficiency, which hinders scalability, increases thermal manufacturing budgets, and causes other detrimental effects to the underlying layers. Traditional furnace annealing processes have been used for high-temperature treatment. This work proposes the investigation of high-temperature treatment using short-pulse laser heating to improve photovoltaic device performance, overcome some of the limitations of the furnace annealing process, and generate a fundamental understanding of the laser annealing process.
By utilizing pulsed laser annealing, energy can be injected at short timespans (nanoseconds) with localized energy deposition due to the shallow absorption depth of the laser light wavelength. This prevents the aforementioned detrimental effects by having a lower thermal budget, higher throughput, and increasing the overall device efficiencies. Additionally, laser processing is advantageous due to the ability to control the laser fluence, repetition rate, scan speed, pulse overlap, beam shape, and pulse width, allowing for fine-tuning of laser-material interaction.
To achieve this goal, the following research work has been accomplished: (1) investigations on the laser annealing of low-temperature TiO2 electron transport layers (ETLs) for flexible perovskite solar cells, with results published in the ACS Applied Materials & Interfaces journal and (2) investigations of laser annealing for selective activation of dopants and crystallization of a/poly-Si carrier-selective passivating contacts (CSPC) for high-efficiency silicon solar cells and a manuscript has been prepared for submission to the IEEE Journal of Photovoltaics. The main results demonstrate the use of laser raster scanning of the thin layers enables (1) conversion of chemical-solution deposited film to an efficient TiO2 electron transporting layer without damaging underlying flexible substrates and (2) partial crystallization and dopant activation of n+ doped a-Si:H thin films without detrimental effects to the underlying tunnel oxide or bulk wafer.
The proposed plan is to utilize the findings and experience from the above studies to fabricate laser annealed TiOx and MoOx carrier-selective layer-based full Si solar cell devices. These layers will be deposited via a sol-gel method or with low-temperature evaporation and annealed via pulsed laser to hydrolyze, oxidize, crystallize, or otherwise activate the layer for low contact resistance, carrier selectivity, and improved passivation. By utilizing the experience with sol-gel carrier selective TiO2 layers in perovskite solar cells, n+ poly-Si carrier-selective passivating contacts, and laser processing of ultrathin films, this device fabrication goal will be achieved. This proposed area is of widespread interest to the photovoltaic community and essential to improving efficiency and reducing the fabrication cost of photovoltaic devices.
Additional investigations will include: (1) the impact of pulsed laser annealing parameters on the loss of passivation and defect generation in well-passivated silicon solar cells and (2) thru-wafer laser bonding of well-passivated n- and p-type wafers for the purpose of junction formation without significant charge-carrier recombination. The former is necessary to understand the impact of the proposed pulsed laser annealing on the silicon substrate, while the latter is an innovative concept for the formation of Si-based p-n junctions and understanding laser-material interactions.