Deep Level Transient Spectroscopy of Methylammonium lead bromide perovskite solar cells
Mmantsae Diale a, Matshisa Legodi a, David Cahen b, John Rosenberg b
a University of Pretoria, South Africa, Hatfield, Pretoria, 28, Pretoria, South Africa
b Weizmann Institute of Science, Herzl St. 234, Rehovot, Israel
NIPHO
Proceedings of Perovskite Thin Film Photovoltaics (ABXPV17)
València, Spain, 2017 March 1st - 2nd
Organizers: Henk Bolink and David Cahen
Poster, Mmantsae Diale, 047
Publication date: 18th December 2016

Significant progress has been made in power conversion efficiency (PCE) and device stabilization of organoplumbate-trihalide (MAPbX3 perovskite solar cells. From the humble 4% reported in 2009 to a certified 22% efficiency in 2015; these developments are spectacular.  Interest in the transport properties of the MAPbX3’s was triggered as soon as the first high efficiency devices appeared. A remarkable feature in MAPbX3 devices is the low diffusion coefficients and mobility: 0.05–0.2 cm2s-1 and 1–30 cm2V-1s-1, respectively, in polycrystalline material. Mobilities in single crystalline material are marginally higher than 100 cm2V-1s-1. This is in spite of the low effective masses that are comparable to those in prototypical Si and GaAs solar cells. Structural defects and not impurities are thought to be responsible for the low mobility figures. Furthermore, defects are also implicated in the hysteresis typical of these materials and in the intrinsic doping. Bandgap engineering has been demonstrated by introducing Sn and Ge to partially substitute Pb in this materials system. Again, the effect of defects introduced in this manner on the transport properties is still unknown. Knowledge about defects is critical for further improvements in materials properties. We have studied highly resistive single crystal light p-type methylammonium lead-bromide perovskite using thermal admittance spectroscopy (TAS) and current-mode deep levels transient spectroscopy (I-DLTS). Multy-frquency capacitance-temperature (C-T) scans reveal a phase transition around 150 K and the presence of a frequency dispersion around 275 K; the same dispersion does not appear in the C-T spectra when the sample is cooled and measured under forward-bias. The C-T spectra also reveal anomalous capacitance profiles; capacitance decrease with increasing temperature, suggesting deep-lying defects. Light on/off C-T measurements supports this characterization. Using Laplace-DLTS at various biases at 2000 s-1 rate window, we detected an electron/hole traps at Ev+282 eV and Ev+0.174 eV with apparent cross section ∼10-18cm-2. There are two other defects which are still under investigation so that we can reveal their signatures.The defect-complex becomes almost undetectable by I-DLTS post vacuum-annealing at 380 K. 



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