Charge Accumulation and Hysteresis in Perovskite‐Based Solar Cells: An Electro‐Optical Analysis

Abstract

cell (i.e., mesoscopic, meso-superstructured and planar heterojunctions), the power conversion effi ciencies (PCEs) of these cells have improved tremendously from 3.8% to 20% within a few years. [1][2][3][4][5][6][7] The high effi ciencies obtained with the hybrid perovskites are attributed to the high absorbance and long-range balanced charge transport lengths within the hybrid perovskites. [ 8,9 ] While most studies focus on improving the device performance, equal emphasis should also be given to the fundamental device physics. Among the several open questions on perovskite solar cells, the most challenging issue to date is the hysteresis effect (or dynamic lag) in current -voltage ( I-V ) measurements. [ 10,11 ] It was found that the PCEs measured is highly dependent on scan rate, scan direction, scan history, and light exposure. This could lead to the inaccurate reporting of PCEs, which would undermine the credibility and progress of this nascent photovoltaic technology. Consensus on the origin(s) of the hysteresis has proven elusive. Proposed origins include slow trapping and detrapping of charges due to subgap traps of solution-processed perovskites; changes to the ferroelectric structure and ion migration, etc. [10][11][12][13] Detailed investigations are therefore urgently needed to unravel their complicated mechanisms and elucidate their physical origins. Such fi ndings would be highly essential for establishing clear design rules needed for further performance improvements in halide organic-inorganic perovskite solar cells.The electrical properties and optical properties of an optoelectronic material are intimately coupled; both are the macroscopic refl ection of the intrinsic electronic physics. Studying both the electrical and optical behavior in a photovoltaic device is an ideal approach to uncover the physics shared by the two. Till now, very few reports have concurrently studied the optical and electrical phenomena that occur in perovskite solar cells with hysteresis. Herein, through versatile combined electrical and optical measurements, we uncover that the hysteresis effect in CH 3 NH 3 PbI 3 (MAPbI 3 ):TiO 2 -based perovskite solar cells is dominated by distinct slow processes persisting from hundreds of milliseconds to tens of seconds. These processes originate from the dynamic rearrangement of the perovskite structure that is mediated by applied electric fi elds and accumulated

Capturing the Sun: A Review of the Challenges and Perspectives of Perovskite Solar Cells

“…[189] Li et al [115] also mentioned the often-observed kinks illustrated in Figure 6 in the J-V curve measurement. Several groups have observed a similar behavior, [15,117,128,190,191] and this phenomenon especially prevails when using fast scan rates. In the context of the ion-migration model, the authors found an explanation for this kink in the J-V curve.…”

Efficient and Air‐Stable Mixed‐Cation Lead Mixed‐Halide Perovskite Solar Cells with n‐Doped Organic Electron Extraction Layers

“…Both the perovskite absorber layer and the interfaces between the perovskite and the charge extraction layers can have a significant impact upon the long-term stability of the solar cell. [11][12][13][14][15][16] All commercial solar modules are encapsulated in order to enable 25 years of operation. Silicon modules have historically been encapsulated with a polymer back sheet laminated with a hot-melt polymer foil to a glass front sheet, and are moving toward glass-glass laminates to deliver improved stability.…”

“…Previous studies suggest that electron accumulation, arising from inefficient charge transfer at perovskite interfaces, exaggerates current voltage hysteresis and that the main origin is a combination of both defect sites responsible for trap-assisted recombination and mobile ionic species. [13,15,16] C 60 or other fullerene derivatives are already known to be able to effectively passivate or inhibit the formation of defect states at the perovskite surface. [43,44] It may be that the n-doping of C 60 further assists, by reducing the density of "vacant" trap sites in the perovskite absorber layer in the vicinity of this interface, by inducing electron accumulation within the perovskite film in this region by surface charge transfer.…”

Reduction in the Interfacial Trap Density of Mechanochemically Synthesized MAPbI3

“…35,36 According to recent reports, the charge/ion accumulation at TiO 2 /perovskite interface is intimately responsible for the hysteresis response of the devices. 37,38 Before exploring the charge/ion accumulation mechanism in the devices, the key parameters of solar cells such as series resistance (R S ), diode ideality factor (m) and dark reverse saturation Considering the fact that hysteresis phenomena may be related to the interfacial charge accumulation and is more prominent in MAPbI 3 (s) than MAPbI 3 (m) devices, impedance spectroscopy (IS) measurements were performed at zero bias under dark and illumination to investigate the ionic and electronic charge accumulation. As shown in Figure S8, Nyquist spectra in the dark exhibit a high frequency semicircle related to the electronic transport followed by the low frequency line due to ionic diffusion or ionic charge accumulation.…”