Abstract
Effect of defects mitigation on the photovoltaic (PV) performance of three different types of perovskite solar cells (PSCs), namely, single halide (methylammonium lead triiodide, MAPbI3), mixed halide (MAPbIxCl3-x) and phenyl-C71-butyric acid methyl ester (PC71BM) based bulk heterojunction (MAPbI3-PC71BM) PSCs is studied using impedance and transient spectroscopy as a function of dc voltage bias. Parameters extracted from impedance spectra and transient signals are quantitatively analysed to understand the operation mechanism of each PSC. Among the PSCs, the low frequency capacitance (Clf) values obtained near to the open circuit voltage (VOC), where charge carriers are restricted inside the device, are found to be minimum for MAPbI3 based PSCs. Whereas, bulk capacitance (Chf), high frequency resistance (Rhf) and low frequency resistance (Rlf) values are obtained to be highest for the MAPbI3 based device which exhibited the best power conversion efficiency (PCE) of 16%. The extracted impedance parameters suggest lessened defect or ion accumulation, improved charge storage capacity/charge life-time, reduced recombination losses and ion transport for the MAPbI3 device, favoring performance enhancement. The characteristic time constants for the high frequency and low frequency processes also specify longer charge carrier lifetime and faster interfacial charge transfer process near to VOC in MAPbI3 based PSCs. In addition, transient photocurrent and photovoltage analysis confirms the existence and interplay of ion induced electric field, built-in and applied external field. Clf, Chf, Rlf and Rhf values as well as transient responses are observed to be greatly dependent on the chemical composition, grain size, grain boundaries and pinholes formed in the perovskite active layer. Based on the results from impedance measurements and transient spectroscopy, the working mechanism of each PSC is proposed in view of the interplay among ion induced electric field, built-in-field and applied electric field.
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