Interrogating Electronic and Ionic Carrier Motion in Halide Perovskites with Scanning Probe Microscopy

Abstract

In December of 2015, 194 states and the European Union reached The Paris Agreement – an ambitious global goal asking all countries to undertake efforts to limit global temperatures from exceeding 2 °C above pre-industrial levels. Ultimately, this has led to many countries, including the United States, to set a milestone of achieving net-zero greenhouse gas (GHG) emissions by the year 2050. Replacing fossil fuel usage with clean energy technology to generate electricity is one primary way to reduce GHG emissions. Lead halide perovskite semiconductors have emerged as highly efficient photovoltaic active layers, with power conversion efficiencies (PCE) on par with silicon – the industry standard. However, perovskites lag behind silicon in terms of long-term and operating stability. The ability of perovskite materials to conduct ions through their lattice is considered to be a large contributor to the lowered stability. We used scanning probe microscopy (SPM) to further our understanding of ionic motion in lead halide perovskites. We first used scanning kelvin probe microscopy (SKPM) to investigate how ionic motion varies with dimensionality in 2D perovskite layers undergoing an applied electric field. In this study, we investigated BA2PbI4 (n=1) and BA2MA3Pb4I13 (~<n>=4), where BA is butylammonium and MA is methylammonium. We demonstrated that 2D perovskite materials undergo ionic migration, similar to their 3D counterparts, and investigated how those dynamics change under illumination. We extracted the rate of change in the contact potential difference (CPD) at temperatures ranging from 283 – 313 K and generated Arrhenius plots to calculated activation energies. In the dark, we calculated activation energies ~0.61 eV, which agrees well with the activation energy attributed to iodide ionic migration. Next, we modified the SKPM technique to probe the shift in the contact potential difference (CPD) between the AFM tip and perovskite sample immediately after applying an electric field between the sample substrate and tip. We used this technique to determine the suppression in ion motion after passivation with (3-aminopropyl)trimethoxy silane (APTMS). We found that APTMS defect passivation led to a reduction in CPD shift resulting from ionic migration from 100 to 20 mV. We further confirmed the success of APTMS defect passivation by showing a suppression in halide phase segregation using hyperspectral photoluminescence microscopy. Finally, we employed time-resolved electrostatic force microscopy (trEFM) to probe carrier recombination dynamics with sub-diffraction-limited spatial resolution. We hypothesize that trEFM is measuring the time it takes for the surface potential to equilibrate during photoexcitation. We showed that the photo-induced dynamics measured with trEFM correlate strongly with both carrier lifetimes and surface recombination velocities (SRV) measured by time-resolved photoluminescnece through the use of various surface passivating agents. Using drift-diffusion simulations, we demonstrated that the surface potential equilibration time is primarily influenced by the SRV and ion motion. On the nanoscale, trEFM revealed slower surface potential equilibration times at grain boundaries, which we propose is due to locally higher concentration of defects, such as halide vacancies, that mediate ion migration. Furthermore, we observed overall slower dynamics after passivation due to the suppression of SRV. However, we still observed heterogeneity in the equilibration times measured after passivation, which we hypothesize is a result of local variations in the effectiveness and uniformity of passivation. We further supported our findings through a combination of intensity dependent and wavelength dependent measurements.