Improvement in Performance of Hybrid Perovskite FAPI Solar Cell by Adlayer of Surface Engineered FAPI₃ Quantum Dots Layer

Formamidinium lead iodide (FAPI) thin films in the perovskite crystalline phase have recently attracted attention for single junction solar cells owing to their ideal bandgap, long photocarrier lifetime and intrinsic structural stability in respect to all-inorganic based perovskites of interest for solar cells. However, the performance and stability of cells based on FAPI bulk crystalline films is limited by the high density of trap states on the surface and within the lattice, which determine the appearance of deep levels acting as non-radiative recombination centers and result in the deterioration of device performance over time. Various strategies have been explored in to overcome these limitations such as film crystallization and deposition optimization, precursor engineering, tailoring the interface properties and energy gap. The formation of distinct bulk heterojunctions has also been proven to be experimentally non-trivial due to the inherent instability of these materials surface towards exposure to polar solvents. Nonetheless the integration of low dimensional perovskites¹ and the incorporation of nanocrystals with quantum confinement has proven to be a feasible way to ameliorate stability, boost carrier collection and tailor energy band gap at the same time.²<br/><br/>In this study we explore the potential of using adlayers (50 to 100 nm) of formamidinium lead iodide (FAPbI₃) quantum dot (QDs) on FAPI bulk films, to explore integration of QDs and fs laser induced surface engineered (SE) FAPI₃ QDs with FAPI bulk films, tailor energy gap, boost the carrier collection and increase the light absorption efficiency at shorter photon wavelengths. These FAPbI₃QDs have been synthesized with a well-established hot injection method and surface-engineered QDs (SE-QDs) are obtained by processing the FAPbI₃QDs with a fs-laser generated plasma treatment in liquid. We report that SE-QDs are affected both in particle size and surface state while retaining their structural and colloidal stability. We then proceed making solar cell devices based on SnO₂ and Spiro-OMeTAD as electron- and hole- transport layers and consider two different cases to distinguish the effect of QDs on the device performance. QD-only devices that are meant to understand the effects of surface engineering alone and devices with a thick microcrystalline FAPI₃ film (~600 nm) and thin adlayers of QDs (100 nm) both as synthesized and surface engineered and compare them against a control FAPI film solar cell.<br/>In the former case (QD-only cells) we notice important relative improvements of average IV parameters of solar cells, above all bigger short circuit current and PCE (+20%) whilst only minor improvements in open circuit voltage and field factor, and overall figures in line with all-QDs based FAPI₃ cells. In the latter case (thin heterojunction cells) we observe a slight deterioration of IV characteristics with as-synthesized QDs adlayers, whereas marked improvements for most of the average IV parameters with SE-QDs adlayers. Notably a 7% increase in power conversion efficiency, with the record cell topping at 20.7% PCE, J_sc=25.6 mA/cm², V_OC=1.13 V and FF=70%. Additionally, when measuring again the IV characteristics after two months, a less severe decay in performance in respect to the control device is observed (e.g.: V_OC -4% for the control vs -2.5% for the SE-QDs device). We notice also that thin layers of QDs up to 100 nm have only minor influence on the optical absorption characteristics of the thicker FAPI film, hence we infer the improved performance as the result of more efficient photocarrier extraction. The thin adlayers of SE-QDs improve interface and determine a more favorable electron energy alignment between the FAPI film and the spiro-OMeTAD.<br/><br/>1. Zhang, C. <i>et al.</i> <i>Adv. Energy Mater.</i> <b>10</b>, 2002004 (2020).<br/>2. Rocks, C. <i>et al.</i> <i>Nano Energy</i> <b>50</b>, 245–255 (2018).