(Invited) Development of High-Performance Tin Perovskite Solar Cells Via a Sequential Two-Step Deposition Approach

Abstract

The performance of a tin-based perovskite solar cell (PSC) has undergone rapid progress in recent years but most high-performing devices were fabricated according to a traditional one-step method with the key anti-oxidizer or passivator treated as an additive in the perovskite precursor solution. Herein we present a sequential deposition procedure to passivate the surface of a hybrid mixed cationic perovskite with phenylhydrazinium halide (PHX, X = Cl, Br, I and SCN) dissolved in trifluoroethanol (TFE) solvent. TFE plays an important role not only to give PHX sufficient solubility but also to retain the tin-perovskite films intact for an effective surface passivation characterized by SEM, XRD, GIWAX, SIMS, XPS, UPS, c-AMF and TCSPC. A slow surface passivation was observed; the performance of the PHSCN device improved upon increasing the storage period to attain the best efficiency 13.5 % for a current-voltage scan in the forward bias direction. An inverted effect of hysteresis was observed in that the efficiency of the forward scan was greater than of the reverse scan. An ion-migration model due to the effect of PH surface passivation was proposed to account for the observed phenomena. Furthermore, High-performance tin-based PSC have been extensively explored using PEDOT:PSS as a hole-transport material (HTM) layer according to a one-step fabrication method. In the second part of the work we report tin-based PSC using the concept of a self-assembled monolayer (SAM) to modify the ITO surface with deposition in two steps; a traditional one-step method failed to produce a uniform tin perovskite layer on the ITO/SAM surface because of insufficient surface hydrophilicity. Herein we developed a preheating procedure to diminish effectively the amounts of hydroxyl groups and oxygen vacancies on the ITO surface for a uniform SAM to form, which has been characterized with X-ray photoelectron spectra. We found that the ITO substrate preheated at 400 °C gave the best device performance for an efficiency of power conversion attaining 6.5 % with excellent enduring stability in a glove box for ~1900 h without encapsulation. The present work thus provides a new direction for the development of HTM-free lead-free perovskite solar cells for their future up-scale production.