Effect of Annealing on the Morphological and Optoelectronic Properties of Ch 3 Nh 3 Pb I 3 Thin Films for Perovskite Solar Cell Applications
Organolead Trihalide Perovskites (OTPs) is one of the third generations’ photovoltaic technologies which have achieved excellent power conversion efficiency (PCE) of 3.8% in 2009 to 23.7% in 2018, just within nine years of its advent. High extinction coefficient, high charge mobility, long carrier diffusion distance and long carrier lifetime are some essential factors that enable perovskite materials to be used as electron/hole transport layer aside from acting as a light-absorbing layer. However, numerous factors such as annealing temperature, solution concentration, precursor composition, and the choice of solvent affect the film morphology. In this work, methylammonium lead iodide thin films were prepared using two different deposition techniques, one-step spin-coating technique (OSSC) and two-step spin coating technique (TSSC). The variation of the temperature and the type of fabrication approach affects the surface morphology of the films. The surface morphology obtained from SEM revealed that perovskite films produced by OSSC had large pinholes and non-uniform coverage, while thin films obtained by the TSSC method exhibited full-coverage with little or no pinholes implying enhanced film performance which is consistent with that which is reported by Swati (Chaudhary et al., 2020). As the temperature changes, the morphology was observed to yield full coverage at 100 0 C than at lower (80 0 C) or higher (120 0 C) temperatures. By comparing device performance for OSSC and TSSC methods, it was found that TSSC deposited-based perovskite thin films demonstrated a significant enhancement in performance. This could be as a result of high absorbance, lesser trap density, low current leakage, larger carrier mobility arising from moderate temperature and better surface morphology. Characterization obtained from the UV/VIS Spectrometry of the perovskite thin films showed high absorbance for TSSC than for OSSC at all temperatures. It also indicated that for both TSSC and OSSC higher absorbance was observed at 100 0 C. This could be attributed to the fairly moderate temperature that does not cause film degradation leading to strong and broad spectra. Sheet Resistance measurements using the Four-Point Probe indicated lower resistivity at 100 0 C (7.0556Ω/ sq and 7.9816Ω/ sq) for both TSSC and TSSC techniques respectively which resulted in the high current generation. However, the sheet resistance for TSSC was fairly lower than that of the OSSC at the same temperature yielding high device (thin films) performance which is consistent to what was observed by Mwende Mbilol et al., Barnett et al., 2016 (11.66% PCE).