Faiazul Haque

Simultaneous enhancement in stability and efficiency of low-temperature processed perovskite solar cells

Mixed ion based perovskite solar cells (PSCs) have recently emerged as a promising photoactive material owing to their augmented electronic and light harvesting properties combined with stability enhancing characteristics. However, to date most of the high performing perovskite devices employ a high temperature (∼500° C) sintering process for depositing a conventional titanium oxide (TiO2) based electron transport layer (ETL), which is a serious bottleneck towards roll-to-roll processing with flexible substrates, large scale manufacturability and also results in high energy consumption. The present work demonstrates simultaneous enhancement in efficiency and stability in the perovskite solar cell that is totally fabricated using low temperature methods with the synthesis process temperature not exceeding 150 °C at any stage. The perovskite devices, thus fabricated, exhibited high power conversion efficiency of ∼14.5% and device stability > 570 hours (normalized PCE to reach 80% of its original value), which is the first of this kind of accomplishment ever reported in entirely low temperature processed PSCs. It is noteworthy to mention that the presented devices utilize a ∼360 °C lower temperature than required for the conventional TiO2 based PSCs to achieve similar enhancements in terms of stability and efficiency simultaneously. The high performing PSCs reported in this work incorporate mixed organic perovskite (MA0.6FA0.4PbI3) as the light absorber and aluminium-doped zinc oxide (AZO) as the electron transport layer. Adding to the merits, the MA0.6FA0.4PbI3/AZO devices exhibited a substantially low photocurrent hysteresis phenomenon. In order to examine the underlying causes of the efficiency and stability enhancements in AZO based devices, a low temperature processed MA0.6FA0.4PbI3/ZnO device was also fabricated and comparatively studied. Investigations reveal that the improved dark carrier mobility and superior interfacial electronic properties at the perovskite/AZO interface are attributed to their enriched device performance. Slow perovskite decomposition rate/high device stability with AZO based perovskite devices was found to be associated with the more hydrophobic and acidic nature of the AZO surface and the related interfacial interactions with the adjacent perovskite layer.

Analysis of burn-in photo degradation in low bandgap polymer PTB7 using photothermal deflection spectroscopy

The efficiency of organic photovoltaic devices continues to increase; however, their limited stability is currently a barrier to the commercial prospects of the technology. Burn-in photo degradation, caused by continuous illumination under a light source, can cause a significant reduction in device performance. Our aim was to investigate this degradation pathway for the high-efficiency polymer PTB7, which was compared to the well-studied P3HT:PC71BM material system. In this study, we compared the burn-in aging profile for organic solar cells containing either P3HT or PTB7 as the donor polymer. This showed that PTB7:PC71BM solar cells exhibit a severe initial reduction in performance, due mainly to reduced short circuit current density (Jsc), during the 5 hour test period. P3HT:PC71BM cells were relatively stable during this test. Photothermal deflection spectroscopy (PDS), which provides sensitive measurement of sub bandgap absorption, was employed to discover the underlying mechanism causing this discrepancy. In PTB7-based devices, a significant increase in sub bandgap absorption was observed after illumination, which was attributed to the formation of sub bandgap trap states. This mechanism was identified as a contributing factor to the severe burn-in for PTB7-based organic solar cells. No such increase was observed for P3HT:PC71BM films.

Solution-Processed Lithium-Doped ZnO Electron Transport Layer for Efficient Triple Cation (Rb, MA, FA) Perovskite Solar Cells

The current work reports the lithium (Li) doping of a low-temperature processed zinc oxide (ZnO) electron transport layer (ETL) for highly efficient, triple-cation-based MA0.57FA0.38Rb0.05PbI3 (MA: methylammonium, FA: formamidinium, Rb: rubidium) perovskite solar cells (PSCs). Lithium intercalation in the host ZnO lattice structure is dominated by interstitial doping phenomena, which passivates the intrinsic defects in ZnO film. In addition, interstitial Li doping also downshifts the Fermi energy position of Li-doped ETL by 30 meV, which contributes to the reduction of the electron injection barrier from the photoactive perovskite layer. Compared to the pristine ZnO, the power conversion efficiency (PCE) of the PSCs incorporating lithium-doped ZnO (Li-doped) is raised from 14.07 to 16.14%. The superior performance is attributed to the reduced current leakage, enhanced charge extraction characteristics, and mitigated trap-assisted recombination phenomena in Li-doped devices, thoroughly investigated by means of electrochemical impedance spectroscopy (EIS) analysis. Li-doped PSCs also exhibit lower photocurrent hysteresis than ZnO devices, which is investigated with regard to the electrode polarization phenomena of the fabricated devices.