Abstract
Electronic transport properties of methylammonium lead iodide perovskite single crystals prepared by inverse temperature crystallisation have been investigated using dark current, steady state and transient photocurrent measurements. Above 250 K the dark conductivity at electric fields below 100 V/cm is thermally activated, with single activation energy 0.7 eV and magnitude 4×10−8 S/cm at 300 K. At higher electric fields, conductivity increases as a power law suggesting space charge limitation. Steady state photoconductivity exhibits a roughly linear dependence on photon flux below 1015 cm−2s−1 and square root dependence above this, consistent with trap-limited and band-to-band recombination mechanisms respectively. Photocurrent overshoot and undershoot on a timescale of 10-100 s are observed on application and removal of a steady light source. Transient photocurrent measurements with temperature reveal a peak in the localised density of states 0.26 eV into the band gap, with an attempt to escape frequency of 1011 s−1. Deeper states are distributed exponentially with characteristic energy 54 meV, decreasing towards mid-gap.
Highlights
Over the past decade, methylammonium lead tri-halide perovskite solar cells have maintained a spectacular year-on-year increase in headline photovoltaic conversion efficiency, surpassing 22% [1, 2], but their tendency to degrade and become unstable on exposure to light, heat and ambient atmosphere is well-known [3]
Electronic transport properties of methylammonium lead iodide perovskite single crystals prepared by inverse temperature crystallisation have been investigated using dark current, steady state and transient photocurrent measurements
The photocurrent overshoot observed on application and removal of light shown in figure 5 is typical of a variety of longer time-scale phenomena reported for bulk MAPI samples and solar cells, which may involve ionic redistribution [14]
Summary
Methylammonium lead tri-halide perovskite solar cells have maintained a spectacular year-on-year increase in headline photovoltaic conversion efficiency, surpassing 22% [1, 2], but their tendency to degrade and become unstable on exposure to light, heat and ambient atmosphere is well-known [3] The member of this family studied most extensively is methylammonium lead tri-iodide CH3NH3PbI3, commonly referred to by the acronym MAPI. Published under licence by IOP Publishing Ltd on/switch-off photoconductivity, and short pulse transient photoconductivity (TPC) From this we seek to quantify their electronic transport properties, in particular intrinsic electronic defect density of states (DOS) and its energetic distribution, and to gain insight into why these semiconductors are such promising photovoltaic (PV) absorbers and show a strong tendency towards instability
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