Kiran Ghimire

Fabrication of Efficient Low-Bandgap Perovskite Solar Cells by Combining Formamidinium Tin Iodide with Methylammonium Lead Iodide

Mixed tin (Sn)-lead (Pb) perovskites with high Sn content exhibit low bandgaps suitable for fabricating the bottom cell of perovskite-based tandem solar cells. In this work, we report on the fabrication of efficient mixed Sn-Pb perovskite solar cells using precursors combining formamidinium tin iodide (FASnI3) and methylammonium lead iodide (MAPbI3). The best-performing cell fabricated using a (FASnI3)0.6(MAPbI3)0.4 absorber with an absorption edge of ∼1.2 eV achieved a power conversion efficiency (PCE) of 15.08 (15.00)% with an open-circuit voltage of 0.795 (0.799) V, a short-circuit current density of 26.86(26.82) mA/cm(2), and a fill factor of 70.6(70.0)% when measured under forward (reverse) voltage scan. The average PCE of 50 cells we have fabricated is 14.39 ± 0.33%, indicating good reproducibility.

Optical response of mixed methylammonium lead iodide and formamidinium tin iodide perovskite thin films

Mixed tin (Sn) and lead (Pb) based perovskite thin films have been prepared by solution processing combining methylammonium lead iodide (MAPbI3) and formamidinium tin iodide (FASnI3) precursors. Optical response in the form of complex dielectric function (e = e1 + ie2) spectra and absorption coefficient (α) spectra of (FASnI3)1-x(MAPbI3)x based perovskite films have been extracted over a spectral range 0.74 to 5.89 eV using spectroscopic ellipsometry. Absorption band edge energy changes as a function of composition for films including FASnI3, MAPbI3, and mixed x = 0.20, 0.35, 0.40, and 0.6 (FASnI3)1-x(MAPbI3)x perovskites. (FASnI3)0.60(MAPbI3)0.4 is found to have the minimum absorption band edge energy near ∼1.2 eV.

Optical monitoring of CH3NH3PbI3 thin films upon atmospheric exposure

CH3NH3PbI3 perovskite films of interest for photovoltaic (PV) devices have been prepared by (i) vapor deposition and (ii) solution processing. Complex dielectric function (e = e 1 + ie 2) spectra and structural parameters of the films have been extracted using near infrared to ultraviolet spectroscopic ellipsometry. In situ real time spectroscopic ellipsometry (RTSE) over a 48 h period has been performed on vapor deposited CH3NH3PbI3 after the deposition in normal atmospheric laboratory ambient conditions. Analysis of RTSE data for vapor deposited CH3NH3PbI3 film prepared under un-optimized conditions identifies phase segregated PbI2 and CH3NH3I at the substrate/film interface and unreacted PbI2 and CH3NH3I on the film surface. This analysis also provides the time dependence of the effective thicknesses of perovskite film, unreacted components, and phase segregated layers to track CH3NH3PbI3 decomposition.

Spectroscopic ellipsometry studies of CH3NH3PbX3 thin films and their growth evolution

CH3NH3PbX3 (X=I, Cl) perovskite films of interest for photovoltaics (PV) devices have been deposited by (i) evaporation followed by vapor annealing and (ii) co-evaporation. Near infrared to ultraviolet spectroscopic ellipsometry has been used to determine the spectroscopic optical response for vapor-annealed material ex situ and co-evaporated material in situ during growth. Real time spectroscopic ellipsometry (RTSE) applied during co-evaporation has tracked the formation and time evolution of perovskite and phase segregated layers at the substrate and surface interfaces. Good agreement between density functional theory (DFT) calculations of optical response and experimental measurements has also been demonstrated.

Optical properties and degradation monitoring of CH 3 NH 3 PbI 3

CH₃NH₃PbI₃ perovskite films for photovoltaics (PV) have been deposited by vapor deposition and solution processing. Spectroscopic ellipsometry from the near infrared to ultraviolet (0.75 to 5.89 eV) is used to study the optical response of CH₃NH₃PbI₃. In-situ spectroscopic ellipsometry is applied to monitor CH₃NH₃PbI₃ undergoing degradation in laboratory ambient conditions to study the degradation / decomposition of CH₃NH₃PbI₃ and evolution of unreacted and phase segregated materials (PbI₂, CH₃NH₃I) found at surface / ambient and substrate / film interfaces.