Origin of long lifetime of band-edge charge carriers in organic–inorganic lead iodide perovskites

Abstract

Long carrier lifetime is what makes hybrid organic-inorganic perovskites high-performance photovoltaic materials. Several microscopic mechanisms behind the unusually long carrier lifetime have been proposed, such as formation of large polarons, Rashba effect, ferroelectric domains, and photon recycling. Here, we show that the screening of band-edge charge carriers by rotation of organic cation molecules can be a major contribution to the prolonged carrier lifetime. Our results reveal that the band-edge carrier lifetime increases when the system enters from a phase with lower rotational entropy to another phase with higher entropy. These results imply that the recombination of the photoexcited electrons and holes is suppressed by the screening, leading to the formation of polarons and thereby extending the lifetime. Thus, searching for organic-inorganic perovskites with high rotational entropy over a wide range of temperature may be a key to achieve superior solar cell performance.organic-inorganic hybrid perovskite | carrier lifetime | photoluminescence | polaron T he record efficiency of hybrid organic-inorganic perovskite (HOIP)-based solar cells has reached above 22% (1-4), which is comparable to that of silicon solar cells. The most dominant contribution to the high photovoltaic performance of HOIPs comes from their long carrier lifetimes (≥ 1 μs), which translates to large carrier diffusion lengths despite their modest charge mobilities (5). Several microscopic mechanisms behind the unusually long carrier lifetime have been proposed, such as formation of ferroelectric domains (6-9), Rashba effect (10-12), photon recycling (13), and large polarons (14-16). When the HOIPs are replaced with all inorganic perovskites in the solar cell architecture, the device can still function as a solar cell. This indicates that the photons excite electrons and holes out of the inorganic metal halide atoms, which is consistent with the density functional theory (DFT) calculations that the corner interstitial cations, whether organic or inorganic, do not directly contribute to the band-edge states (17). However, the efficiency of the purely inorganic perovskites is currently at ∼11% (18-20), which is far below 22% of HOIP-based solar cells. This suggests that the presence of organic cation may be the key for achieving high solar cell efficiency. It is, however, yet to be understood how the organic cations enhance the efficiency.Among the aforementioned microscopic mechanisms, three are based on the role of organic cations. First, in the ferroelectric domain theory, nanoscale ferroelectric domains are formed due to alignment of organic cations (6-9). Such domains can spatially separate the photoexcited electron and holes and thereby reduce their recombination. Second, in the Rashba effect theory (10-12), the spin and orbit degrees of freedom of the inorganic atoms are coupled with the electric field generated by the organic cations. This results in the electronic band splitting for different spins and leads to an effectively in...

Properties of Excitons and Photogenerated Charge Carriers in Metal Halide Perovskites

“…At present, it is commonly accepted that the extremely efficient charge collection in lead‐based perovskite solar cells strongly relies on the long charge‐carrier diffusion lengths (in the micrometer range) that normally result from the sufficient long carrier lifetimes in perovskites . Whereas, the charge‐carrier mobility that contributes to the charge collection efficiency in perovskites are modest as compared to other PV‐relevant inorganic semiconductors, the origin of which will be explained in the following.…”

“…Since 2009, enormous attention has been paid to MHPs due to their great success in photovoltaics (PVs), with a phenomenally rapid rise of the power conversion efficiency (PCE) from 3.8% to over 23% in the past few years. Such high PCE of perovskite solar cells has been ascribed to the ultralong carrier lifetimes, long carrier diffusion lengths, and the extraordinarily defect tolerance . Following the success of perovskites in photovoltaics, research on light‐emitting diodes (LEDs), amplified spontaneous emission (ASE) or lasers, and photodetectors have also gained substantial interests.…”

Polaronic Charge Carrier–Lattice Interactions in Lead Halide Perovskites

“…It is generally accepted that excited charges do not exist as a bound electron-hole state in MHPs, with the exception of a transientexciton, which causes excitonic features in absorption measurements. [49][50][51] OPTP experiments on solution-grown single crystals of MAPbI 3 give the dissociation timescale of excitons < 1ps, [44,52] attributedt of ast screening by the dielectric background. Ap olaron model (Frçhlich polaron) is ideally suited to describe the equilibrium state, as will be discussed in the following.…”

Real‐Time Observation of Order‐Disorder Transformation of Organic Cations Induced Phase Transition and Anomalous Photoluminescence in Hybrid Perovskites

“…Furthermore, hydrogen (H) is a negative scatterer for neutrons (−3.74 fm). The difference in scattering lengths of the organic component in lead halide perovskites provides high sensitivity to the change in the orientation and reversal of organic cations (e.g., methylammonium (MA) or formamidinium (FA) ions) in hybrid perovskite structure . The TOPAZ single crystal neutron diffractometer at the Spallation Neutron Source uses neutron time‐of‐flight Laue technique for mapping 3D volumes of diffraction patterns from a stationary single crystal sample .…”