Additive Engineering for High-Performance Perovskite Photovoltaics
Perovskite photovoltaics has attracted tremendous attention recently due to the advance in the device performance. However, it is still challenging to effectively commercialize the perovskite technology due to several issues including current-voltage hysteresis, stability, complicated device architectures, etc. In this dissertation, we use the additive to tailor the properties of the functional layers in perovskite photovoltaic devices, aiming to engineer the interface, film morphology, carrier dynamics and film crystallization process. By using the additive engineering approaches, our goal is to achieve high-performance perovskite photovoltaics with reduced hysteresis, improved stability, versatile processing methods and simplified device architectures.
Perovskite solar cells usually employ p-i-n device architectures and TiO2 is a typical n-type semiconductor widely used in perovskite solar cells. However, perovskite/TiO2 interface is not preferable for the photo-excited carrier collection due to the energy band misalignment, conductivity mismatch, etc. In chapter 2, additive was used to tailor the properties of TiO2 and enable improved interface for perovskite solar cells. With Nb5+ as additive in TiO2, the conductivity of TiO2 and interface band alignment were simultaneously improved. Consequently, high-performance perovskite solar cells were successfully obtained with reduced hysteresis by using the Nb-TiO2.
In addition to the interface, we explored the impact of morphology and carrier dynamics of perovskite films on solar cell performance. In chapter 3, NH4SCN and PbI2 were used as additives to tune the morphology and charge carrier dynamics of perovskite films. Using NH4SCN additive could significantly enlarge the grain size of the polymorph perovskite films while using PbI2 additive could increase charge carrier lifetime of perovskite films. It was found that the open-circuit voltage and fill factor of perovskite photovoltaics were correlative with charge carrier lifetime while short-circuit current density of perovskite photovoltaics were correlative with grain sizes. Using both PbI2 and NH4SCN simultaneously could synergistically improve the quality of perovskite films and performance of perovskite solar cells.
Based on the understanding from chapter 3, a room-temperature process was developed to deposit high-quality perovskite films by using PbI2 and methylammonium thiocyanate (MASCN) as additives in chapter 4. Due to the synergistic effects of the additives, room-temperature-processed perovskite films with micron-size grains and microsecond-range carrier lifetime were successfully obtained for high-performance devices. More importantly, we established the correlation between the crystal grain size in resultant perovskite films and the precursor aggregate size in precursor solutions. The correlation suggested that the perovskite grain sizes from solution process depended on the precursor aggregate sizes.
Following the understanding built in chapter 3, we used the additive engineering method to impact the performance of ETL-free perovskite solar cells. In chapter 5, we found out that the photo-excited carrier injection at the interface was significantly inhibited without the assistance of an ETL, which would compromise the collection of the photo-excited carriers. By using PbI2 as additive to tune the carrier lifetimes in perovskite films, it was found out that increased carrier lifetimes in perovskite films could effectively counterbalance the inferior interface without ETLs and enabled high performance for ETL-free perovskite solar cells. By using perovskite with microsecond carrier lifetime, ETL-free perovskite solar cells were successfully realized with performance comparable to that of ETL-containing perovskite devices. Such results offer the opportunity for the perovskite devices with simplified device architecture.
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