| Summary: | The stunning rise of methylammonium lead iodide perovskite material as light harvester in recent years has drawn numerous attentions in the photovoltaic community, owning to its good intrinsic properties of suitable bandgap, high absorption coefficients and long diffusion lengths. The advantages of high performance surpassing 20%, easy solution process and low fabrication cost have made it a promising candidate to challenge silicon solar cells. However, despite of the continuous improvement of deposition techniques and film treatment processes which have led to rapid device efficiency increase, the impact of perovskite thin film morphology on device performance, charge recombination process of the solar cells and the anomalous hysteresis effect in current-voltage measurement still require deeper understanding.
In this thesis, the surface morphology of perovskite layer within the device has been confirmed to play a major role to affect device performance and charge recombination process in mesoscopic devices. Selection of deposition process (i.e. single and sequential deposition process) and modulation of precursor conditions can greatly alter surface morphology of perovskite films within the mesoscopic solar cells. Sparse distribution of micrometer size perovskite bundles over the mesoscopic TiO2 film with pin-holes and low loading of perovskite nanocrystal into the mesoscopic scaffold were observed in single deposition process. On the other hand, sequential deposition resulted in much more uniform surface morphology of perovskite capping layer with less pin-holes and homogeneous distribution in the TiO2 scaffold. Correspondingly, photovoltaic device parameters from sequential deposition process are much improved.
The impact of perovskite film surface morphology on the charge recombination processes within the mesoscopic perovskite solar cells is further scrutinized via electroluminescence and charge extraction measurement. More uniform surface morphology of perovskite capping layer with less pin-holes from sequential deposition process by modulation of precursor concentrations led to increase in electroluminescence quantum efficiency, owing to suppressed interfacial non-radiative charge recombination process. Moreover, two-stage second-order charge recombination process was also revealed in mesoscopic perovskite solar cells by charge extraction measurement. With the more uniform surface coverage of perovskite thin film layer with less pin-holes by sequential deposition process, inhibited charge recombination in solar cells matches with the higher charge density and slower photovoltage decay profiles measured, which also correlates well with the enhanced devices’ photovoltaic parameters as well.
Besides the charge recombination process, anomalous hysteresis effect in mesoscopic perovskite solar cells, made from both single and sequential deposition processes, has also been investigated. Scan rate, scan direction, light intensity and electric field dependence have been observed in hysteresis effect. Through further electrical polarization experiment, we attributed the origin of hysteresis effect to ionic migration within the bulk and charge accumulation at interfacial contacts, which leads to interfacial band bending and switchable polarity. Perovskite film thickness increase, compositional tuning and the corresponding stronger polarization effects confirmed bulk migration of nonstoichiometric ionic charge carriers. We also showed temperature dependent behavior of the measured polarization effect under electric field, in agreement with our previous results.
In summary, this thesis provides a detailed study of the morphological impact of perovskite thin film on the device performance, charge recombination processes and anomalous hysteresis effect within the mesoscopic perovskite device. Based on this, further optimizing the surface morphology of perovskite, inhibiting interfacial charge recombination and reducing bulk defects and mobile ions to suppress hysteresis effect should be pursued in future. We believe the understanding obtained based on this CH3NH3PbI3 perovskite material could also be extended to other types of new perovskite materials that can be potentially employed in mesoscopic solar cells.
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