Unravelling the Effects of Grain Boundary and Chemical Doping on Electron–Hole Recombination in CH₃NH₃PbI₃ Perovskite by Time-Domain Atomistic Simulation

Abstract

Advancing organohalide perovskite solar cells requires understanding of carrier dynamics. Electron-hole recombination is a particularly important process because it constitutes a major pathway of energy and current losses. Grain boundaries (GBs) are common in methylammonium lead iodine CH3NH3PbI3 (MAPbI3) perovskite polycrystalline films. First-principles calculations have suggested that GBs have little effect on the recombination; however, experiments defy this prediction. Using nonadiabatic (NA) molecular dynamics combined with time-domain density functional theory, we show that GBs notably accelerate the electron-hole recombination in MAPbI3. First, GBs enhance the electron-phonon NA coupling by localizing and contributing to the electron and hole wave functions and by creating additional phonon modes that couple to the electronic degrees of freedom. Second, GBs decrease the MAPbI3 bandgap, reducing the number of vibrational quanta needed to accommodate the electronic energy loss. Third, the phonon-induced loss of electronic coherence remains largely unchanged and not accelerated, as one may expect from increased electron-phonon coupling. Further, replacing iodines by chlorines at GBs reduces the electron-hole recombination. By pushing the highest occupied molecular orbital (HOMO) density away from the boundary, chlorines restore the NA coupling close to the value observed in pristine MAPbI3. By introducing higher-frequency phonons and increasing fluctuation of the electronic gap, chlorines shorten electronic coherence. Both factors compete successfully with the reduced bandgap relative to pristine MAPbI3 and favor long excited-state lifetimes. The simulations show excellent agreement with experiment and characterize how GBs and chlorine dopants affect electron-hole recombination in perovskite solar cells. The simulations suggest a route to increased photon-to-electron conversion efficiencies through rational GB passivation.

Highly Efficient Perovskite Solar Cells Processed Under Ambient Conditions Using In Situ Substrate‐Heating‐Assisted Deposition

“…It should be pointed out that only the spin‐coating speed at the first step is altered during the two‐step deposition process. The thick perovskite film based on 1500 rpm embodies numerous grain boundaries, which are demonstrated to contain impurities and defects, resulting in serious carrier recombination . Small grains at the bottom indicate the probable unreacted PbI 2 , resulting in more trap‐states and unfavorable band alignment in devices, which would be detrimental to device performance.…”

“…The thick perovskite film based on 1500 rpm embodies numerous grain boundaries, which are demonstrated to contain impurities and defects, resulting in serious carrier recombination. [42] Small grains at the bottom indicate the probable unreacted PbI 2 , resulting in more trap-states and unfavorable band alignment in devices, which would be detrimental to device performance. On the contrary, the thinner perovskite film based on 3000 rpm seems to be homogeneous and compact.…”

Μethylammonium Chloride: A Key Additive for Highly Efficient, Stable, and Up‐Scalable Perovskite Solar Cells

“…Strikingly, these efficiencies were further increased up to~21.5% by incorporation of other additives/passivation agents such as CsCl or I 2 . [81] The use of Cl À (which is more electronegative than I À ) shifts the density of holes and electrons far from the defect centers, [82,83] while simultaneously facilitating the removal of excess MA + at a certain annealing temperature. For instance, 1.47moL•L -1 (FAPbI 3 ) 0.95 (MAPbBr 3 ) 0.05 perovskite with excess of 0.49moL•L -1 MACl led to record PCEs of~23.3% (certified at around 22.7%) by adopting alternative organic, small molecule or polymeric, semiconductors as hole transport materials.…”

Improved charge carrier lifetime in planar perovskite solar cells by bromine doping

“…In general, defects in the crystal structure are expected to decrease the lifetime of photogenerated charge carriers in the absorber, as additional recombination pathways speed up the loss of excess carriers. Recent publications indicate that such defects are mostly located at the grain boundaries67 or at the surface of the perovskite crystal8, therefore morphology engineering by film processing and chemical composition can influence the charge carrier recombination in the absorber, helping to achieve high performance perovskite solar cells91011. Most of the recombination studies presented so far have been focusing on perovskite films or crystals using photoluminescence (PL)71213, Terahertz (THz) spectroscopy14 or microwave photoconductivity (TRMC)141516, but only a few have studied charge carrier lifetimes incomplete solar cell devices1718.…”