Deposition, performance, and stability of halide perovskite solar cells [electronic resource]

Halide perovskites have had unprecedented improvements since they first were used as a solar cell absorber in 2009. These materials have the formulation ABX3 and their properties can be tuned by changing the specific stoichiometry, while keeping the same crystal structure. Halide perovskites can be deposited many different ways and despite being polycrystalline they have excellent opto-electronic properties. We performed an in depth study of one of the highest performance deposition methods using lead chloride and three molar equivalents of methylammonium iodide to understand how it formed high quality films. Using x-ray photoelectron spectroscopy and x-ray fluorescence, we determined that the final film contains less than 1% chlorine. We also ascertained that the chlorine and excess methylammonium sublimated during annealing. Finally, using in-situ UV-vis absorption and x-ray diffraction measurements, we examined the kinetics of film formation and discuss possible structures for the crystalline precursor phase. Sensitive optical absorption measurements can help determine what limits solar cell efficiency and identify sub-bandgap trap states. We characterized several perovskite compositions with photothermal deflection spectroscopy and Fourier transform photocurrent spectroscopy. Using these techniques we identify Sn4+ as a trap state in lead tin mixed perovskites. We discuss the challenges of using these techniques as well as those associated with using these measurements to determine the radiatively limitied voltage. Finally, we discuss the stability of the solar cells in reverse bias. In a solar module, a shaded cell is forced to pass the photocurrent of its unshaded neighbors. This forces the cell into reverse bias where it dissipates power which can cause damaging heating. Halide perovskite solar cells breakdown or allow current to pass in reverse bias between -1V and -4V and the breakdown current is passed uniformly across the entire device. Holding a cell in reverse bias causes a significant decrease in efficiency, but this is due to a drop in open-circuit voltage rather than a decrease in shunt resistance seen in other technologies. Additionally, this efficiency degradation is mostly reversible. Our proposed mechanism supported with drift-diffusion modeling is that under reverse bias mobile ions move towards the electrodes causing band bending which allows for tunneling of carriers. The decrease in efficiency appears to be due to some reaction of these ions which causes them to be charge compensated. The degradation that takes longer and longer to recover each time, so there may be permanent damage. We propose two solutions. First, if the degradation reaction could be prevented, we would actually want a lower breakdown voltage to lower the dissipated power and heating. Second, if the degradation cannot be prevented, bypass diodes will need to be used and in that case we would want to increase the breakdown voltage.

Keywords: Materials Science and Engineering.
Advisor: McGehee Michael
McIntyre Paul Cameron advisor. Thesis advisor
Salleo Alberto