Extremely Slow Photoconductivity Response of CH₃NH₃PbI₃ Perovskites Suggesting Structural Changes under Working Conditions

Abstract

Photoconductivity measurements of CH3NH3PbI3 deposited between two dielectric-protected Au electrodes show extremely slow response. The CH3NH3PbI3, bridging a gap of ∼2000 nm, was subjected to a DC bias and cycles of 5 min illumination and varying dark duration. The approach to steady -state photocurrent lasted tens of seconds with a strong dependence on the dark duration preceding the illumination. On the basis of DFT calculations, we propose that under light + bias the methylammonium ions are freed to rotate and align along the electric field, thus modifying the structure of the inorganic scaffold. While ions alignment is expected to be fast, the adjustment of the inorganic scaffold seems to last seconds as reflected in the extremely slow photoconductivity response. We propose that under working conditions a modified, photostable, perovskite structure is formed, depending on the bias and illumination parameters. Our findings seem to clarify the origin of the well-known hysteresis in perovskite solar cells.

Light Intensity Dependence of Performance of Lead Halide Perovskite Solar Cells

“…The reverse scan increases FF, compared to the forward scan. This is consistent with the previous observation reported for any type of MAPbI 3 -based solar cells [28,31,32].…”

Ionic Reactivity at Contacts and Aging of Methylammonium Lead Triiodide Perovskite Solar Cells

“…Particularly intriguing is the hysteresis features in the response of the current density-voltage ( V J − ) curves [10][11][12][13] that have been related to a number of different explanations, as ferroelectric properties of the perovskite materials, [14][15][16] delayed electronic trapping processes, [17] or slow ion migration. [11,18,19] The recognition that the solar cell performance is heavily affected by time scale of the measurement and preconditioning procedures [9,17,20,21] has raised many concerns about the stability of the device and reliability for long time operation.…”

Optoelectronic Studies of Methylammonium Lead Iodide Perovskite Solar Cells with Mesoporous TiO2: Separation of Electronic and Chemical Charge Storage, Understanding Two Recombination Lifetimes, and the Evolution of Band Offsets during J–V Hysteresis

“…Several authors have reported on this effect and speculated about the causes. [18][19][20][21][22] Some recent articles present cell designs that minimize this hysteresis. 23,24 In this article, we describe the hysteresis effect in additional detail.…”

“…18 All three of these possibilities remain in the debate. 5,19,20,20,25 Along with the addition of crystal size effects 21 . It has been proposed that an optimum mesoporous TiO 2 film thickness minimizes the JV hysteresis.…”