Interfacial Analysis and Charge Transfer in Solar Cells

Abstract: Harnessing sunlight through solar cells is vital for sustainable energy production. The conventional architecture of a solar cell consists of multilayers of materials, each serving a particular function. The absorbing layer converts the solar energy into valuable electricity by the photovoltaic effect. Positioned on either side of the absorbing layer are charge transport layers, which collect the generated electricity.  Charge transfer and recombination occur at interfaces, affecting the solar cells' open-circuit voltage (Voc). This doctoral thesis focuses on the analysis of the perovskite/n-type semiconductor interface and plasmonic/p-type semiconductor interface, provides an understanding of the charge generation and transfer in solar cells, and helps to establish a selection of the ideal charge transport layer for efficient charge extraction.Initially, photogenerated charge carriers were investigated by time- and energy-resolved photoluminescence (TER-PL) from the thin formamidinium lead-bromide (FAPbBr3) perovskite absorber film. Both exciton and free charge dynamics under various excitation power intensities were taken into consideration. Following studies on the charge transfer at interfaces were performed by incorporating n-type semiconductors (TiO2 and SnO2) as the charge transport layer. The results revealed that the TiO2 is more efficient as a fluorescence quencher due to the higher density of states in its conduction band. While conductivity measurements suggested that SnO2 was better for avoiding the accumulation of charges. Thus, solely charge extraction efficiency is not sufficient for determining the solar cell performance.To alleviate the energy losses in solar cells, extraction of hot electrons would be a viable way. Then, hot carriers generated through plasmon excitation in metal nanoparticles, which is different from semiconductors, were collected in the solid-state direct plasmonic solar cell (DPSC). Also, plasmonics show high absorption across the visible–infrared spectrum. Early progress mainly focused on hot electron transfer. The second part of the thesis investigated the effect of Li-TFSI dopant concentration in the hole transport layer (spiro-OMeTAD) on solar cell performance. 33% oxidised species (spiro+TFSI-) gave the optimal efficiency. By using transient absorption spectroscopy, the injection of hot electrons and holes was verified in a complete solar cell device as well.