Advancing Photovoltaic Performance and Stability of Perovskite Solar Cells Using a Solid-State Multi-Modal Electrochemical Approach

Analysis and characterization of buried interfaces in metal halide perovskite (MHP) devices has always been a major challenge in advancing device performance and stability, particularly under conditions of operation including light bias and charge transport. Recently, our group has developed a solid-state electrolyte “peel and stick” spectro-electrochemistry methodology in order to directly study the energy levels and charge extraction capabilities of the buried interface between ITO/MHP under <i>operando</i> conditions. Moreover, this tool allows including redox probes in the solid electrolyte in order to measure near-gap and mid-gap defect densities of MHP with quantification limits around ~10¹⁴ cm^-3, which is below the space-charge limited current methodology or conventional spectroscopic techniques.<br/><br/>Our approach overcomes a major challenge to industrial adaptation and manufacturing at scale of printable electronics. Generally, interface modifications at charge-selective-layer/MHP bottom contacts are speculated to improve performance through correlations with indirect measurements and a demonstration of a change in power conversion efficiency. For instance, photoelectron spectroscopies can only determine the chemical environment and energetics at the MHP surface, which is widely recognized to be dependent on buried interface chemistries that cannot be probed with subsequent layer deposition. Similarly, time-resolved photoluminescence spectroscopy is restricted to measuring the net total change in carrier lifetime but lacks exact information on charge extraction/injection at the buried interface with the MHP.<br/><br/>In this context, herein we extend the use of our “peel and stick” electrochemistry platform to assess the effect of partial device stacks comprised to transparent conductive oxide/hole-transporting-layer (HTL)/MHP interfaces, systematically modified using ionic liquids and phosphonic acids. Our approach is exquisitely sensitive to small changes in energetics, monitored through injection barriers, and charge extraction capabilities of HTL/MHP interface in a p-i-n Perovskite Solar Cells (PSCs) configuration. Remarkably, we demonstrate the correlation between interface charge extraction “quality” and photovoltaic response of p-i-n PSCs so that our solid-state electrochemical platform may advance the device performance by probing the buried interface.<br/><br/>Similarly, we take advantage of this powerful tool to explore MHP stability under thermal and illumination stress conditions and to measure the effect of HTL nature by quantifying the near-gap defect densities. Ultimately, by combining this approach with complementary spectroscopies, we are able to provide new insights into the degradation processes occurring in the MHP and corresponding devices. Thus, we highlight the potential of our solid-state electrochemistry methodology to become a very useful tool for advancing the behavior and stability of MHP and PSCs, which is necessary in order to find proper materials and interface treatments to increase PSCs stability to commercial level requirements.