A New Perspective on the Role of A‐Site Cations in Perovskite Solar Cells

Abstract

As the race towards higher efficiency for inorganic/organic hybrid perovskite solar cells (PSCs) is becoming highly competitive, a design scheme to maximize carrier transport towards higher power efficiency has been urgently demanded. Here, we unravel a hidden role of A-site cation of PSCs in carrier transport which has been largely neglected, i.e., tuning the Frhlich electron-phonon (e-ph) coupling of longitudinal optical (LO) phonon by A-site cations. The key for steering Frhlich polaron is to control the interaction strength and the number of proton (or lithium) coordination to halide ion. The coordination to I − alleviates electron-phonon scattering by either decreasing the Born effective charge or absorbing the LO motion of I. This novel principle discloses lower electron-phonon coupling by several promising organic cations including hydroxyl-ammonium cation (NH3OH + ) and possibly Li + solvating methylamine (Li + ···NH2CH3) than methyl-ammonium cation. A new perspective on the role of A-site cation could help in improving power efficiency and accelerating the application of PSCs.Solar energy is a highly efficient and eco-friendly source for future energy harvesting. In particular, inorganic/organic PSCs of ABX3 (A = Cs+, CH3NH3+, etc.; B = Pb2+; X = Cl-, Br-or I-) show extraordinary solar cell efficiencies exceeding 22 % [1] with unusual characteristics, [2][3][4] which attracts tremendous attention as most promising large-scale solar energy conversion materials.[5] One key origin of high efficiency arises from high carrier mobility (µ) 10 -8] even in the presence of defects.[9] While the carrier transport exhibits remarkable features in experiments, there is still a gap of understanding. One aspect of this difficulty stems from significant electron-phonon (e-ph) interactions, [2,8,[10][11][12][13] which complicates band pictures. Another aspect is due to the A-site cation that rotates [14] or even diffuses across the material, causing I-V hysteresis.[15] Although recent discovery of various types of cations has greatly advanced the efficiency of PSCs, [16] it also has brought about more ambiguity on the role of cations.In general, electrons in polar crystal experience the deformation potential in addition to Bloch potential due to large polarity of ionic bonding. The charge carriers are then described by polaron quasiparticles originating from coupling between electrons and phonons. For PSCs, there has been a critical debate on the source of e-ph coupling, acoustic vs. longitudinal. From the temperature dependence of µ ∼ T −3/2 around room temperature, acoustic phonons have been considered as the main source of e-ph coupling. [11,17] In this study, we unveil a hidden role of A-site cation in polaron picture of PSCs by accounting for the e-ph interactions regarding both A-site cation and LO phonon scattering at the first principles level. [18,19] We used the Frohlich polaron model [23] which is suitable for large bandwidth (W ) limit W ω ph by considering Frohlich vertex. [24] We cast light on the role o...

Polarons in Metal Halide Perovskites

“…Overall, remarkable advances have been achieved in reconciling the polaronic nature of charge carriers that have been recently demonstrated by both theory and experiments, with the previous and vast literature in which the peculiar optoelectronic properties of LHPs were showcased but only partially explained. In a recent contribution, Myung et al have proposed that the coordination of organic cations to I atoms can be used to tune the properties of polarons in HPs . However, the semilocal functionals employed to describe the electronic structure might not capture completely the physics of polarons in perovskites.…”

Intrinsic Carrier Mobility of Cesium Lead Halide Perovskites

“…Theoretically, the transport property of CsPbX 3 has been studied with various approaches. However, the calculated carrier mobility varies widely in the literature; electron (hole) mobilities for CsPbI 3 are 258 [13] and 74 (98) [14] cm 2 /Vs and those for CsPbBr 3 are 48 (41) [15] and 679 (52) [16] cm 2 /Vs. We note that every method involves empirical parameters within the model, which may undermine consistency among the computational results.…”

A‐site Cation Engineering for Highly Efficient MAPbI3 Single‐Crystal X‐ray Detector

“…[33][34][35][36] On one hand, large cation incorporation is expected to reduce the degree of octahedral tilting of the PbI 6 À framework to stabilize the structure,w hich called the "size effect". Theoretical study suggests incorporating additional large cations at A-site to overcome the shortcomings,w hich have already been adopted in MAPbI 3 thin-film photovoltaics.…”

“…Theoretical study suggests incorporating additional large cations at A-site to overcome the shortcomings,w hich have already been adopted in MAPbI 3 thin-film photovoltaics. [33][34][35][36] On one hand, large cation incorporation is expected to reduce the degree of octahedral tilting of the PbI 6 À framework to stabilize the structure,w hich called the "size effect". On the…”