Laser Annealing of Carrier-Selective Layers in High-Efficiency Photovoltaic Devices

To realize efficient photovoltaic devices, charge carrier-selective layers are utilized to effectively separate 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 budgets, and causes other detrimental effects to the underlying layers. Traditionally, furnace annealing processes have been used for high-temperature treatment. This thesis investigates 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 (10s of nanometers) of the laser light wavelength. This prevents the detrimental effects by having selective energy deposition, lower thermal budget, higher throughput, and by increasing the overall device efficiencies. In addition, 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.
This work has investigated: (1) the laser annealing of low-temperature TiO2 electron transport layers (ETLs) used in flexible perovskite solar cells; (2) laser annealing for selective activation of dopants and crystallization of a/poly-Si carrier-selective passivating contacts (CSPC) for high-efficiency; (3) laser annealing of Al2O3, MoOx, and TiOx passivating and carrier-selective layers for Si solar cell devices; (4) laser-induced defects on a well-passivated silicon solar cell to evaluate the impact of laser annealing and high fluence laser processing on defect formation; and (5) the use of lasers to bond well-passivated wafers of opposite doping type to form a p-n junction and solar cell device by transmitting a focused beam through one of the wafers.
The main results demonstrate the use of laser annealing of the thin layers that enables (1) conversion of chemical solution-deposited film to an efficient TiO2 electron transporting layer without damaging underlying flexible substrates; (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; (3) improved performance of passivating and carrier-selective metal oxides for silicon solar cells through interface improvements and changes in stoichiometry. The results also demonstrate (4) the impact of laser fluence and the percentage of laser process area on defect formation and subsequent impact on device performance; and (5) that wafer bonding of well-passivated Si wafers is feasible and shows promise for future high-efficiency devices.
This work is of widespread interest to the photovoltaic community and essential to improving efficiency and reducing the fabrication cost of photovoltaic devices. By demonstrating laser annealing methods for various carrier-selective layers and demonstrating an innovative method for p-n junction formation, this work provides viable avenues for further improvements to device efficiency and fabrication cost reduction.