Recent Progress on Formamidinium‐Dominated Perovskite Photovoltaics

Abstract

In the early days, MAPbI 3 is widely used for single-junction perovskite photovoltaics, and the PCE of these devices has achieved over 20%. [8] However, the large bandgap of MAPbI 3 (1.57 eV) limits its further improvement toward higher performance. Density functional theory (DFT) calculations show the octahedral tilting of FAPbI 3 is smaller than MAPbI 3 , which thus results in a narrower bandgap (1.43 eV) for FAPbI 3 . [9] This value fits better with the optimal bandgap for single-junction photovoltaics according to "Shockley-Queisser" (S-Q) limits. [10] In addition, FAPbI 3 exhibits smaller free volume, which will result in a weaker electron-photon coupling and longer carrier life time. [11] Moreover, the calculated effective mass of FAPbI 3 is also smaller than MAPbI 3 , indicating outstanding semiconducting properties. [12] Therefore, superior carrier transport properties and optimal bandgap endow FAPbI 3 as the most promising perovskite material for high-performing single-junction photovoltaic applications. [13] However, the α-FAPbI 3 is a metastable phase at room temperature and can easily transform into the insulating δ phase, which is a fatal problem for realizing high-efficiency photovoltaics. [14] Pure α-FAPbI 3 shows large anisotropic lattice strain in different crystallographic planes. [15] Besides, the cation rotation disorder for pure α-FAPbI 3 is also fairly large. [16] These features result in larger formation energy for α-FAPbI 3 compared to δ-FAPbI 3 , [17] which account for the fundamental reasons that lead to thermodynamic unstable of α-FAPbI 3 at room temperature. In addition, the formation of α-FAPbI 3 and δ-FAPbI 3 are competing with each other during crystallization. [18] This means it is difficult to fabricate phase-pure α-FAPbI 3 films spontaneously. Therefore, fabrication and stabilization of phase-pure α-FAPbI 3 perovskite films represent an important challenge for the whole community.Herein, we summarized the approaches that have been used to fabricate high-quality FA-dominated perovskite films. First, we highlight the reasons that made high-quality α-FAPbI 3 films difficult to fabricate from the viewpoint of thermodynamics. Then, the strategies for stabilizing α-FAPbI 3 are discussed in detail, including composition engineering, dimensionality engineering, substrate strain relaxation and crystallization regulation. On the other hand, various approaches have been adopted to passivate the defect states to improve the carrier transport/extraction efficiency, which is another important Organic-inorganic hybrid perovskite materials have attracted widespread attention in the photovoltaic field. The best-certified perovskite single-junction photovoltaics have achieved an impressive power conversion efficiency of 25.5%. Particularly, formamidinium lead triiodide (FAPbI 3 ) perovskite material has been considered to be one of the most promising materials for fabricating highly efficient single-junction solar cells due to its suitable bandgap (1.43 eV). However, the metastable α-FAPbI 3 per...

Low-Temperature Fabrication of Phase-Pure α-FAPbI3 Films by Cation Exchange from Two-Dimensional Perovskites for Solar Cell Applications

“…Among them, formamidinium lead iodide (FAPbI 3 ) perovskite exhibits an ideal bandgap (∼1.48 eV) 4 with significantly enhanced thermal stability compared to a MA-based perovskite. 5 As recently reported, the major perovskite solar cells (PSCs) with outstanding efficiency and stability were fabricated with a FA-based perovskite. 6−8 Despite clear advantages, FAPbI 3 favors a yellow-phase structure (hexagonal δ-FAPbI 3 ) instead of the light-absorbing perovskite structure (cubic, α-FAPbI 3 ) under ambient conditions, 9,10 which is the main impediment to the formation of α-FAPbI 3 films and also long-time stability of FA-based devices.…”

“…Organic–inorganic halide perovskite materials have attracted tremendous interest as light-absorbing materials in solar cells as a result of the remarkable progress in power conversion efficiency (PCE) increasing from 3.8 to 25.5% in the recent decade. − ABX 3 perovskite materials are composition-tunable, with methylammonium (MA + ), formamidinium (FA + ), Cs + , or Rb + occupying at the A site, Pb 2+ or Sn 2+ at the B site, and I – or Br – at the X site. Among them, formamidinium lead iodide (FAPbI 3 ) perovskite exhibits an ideal bandgap (∼1.48 eV) with significantly enhanced thermal stability compared to a MA-based perovskite . As recently reported, the major perovskite solar cells (PSCs) with outstanding efficiency and stability were fabricated with a FA-based perovskite. − Despite clear advantages, FAPbI 3 favors a yellow-phase structure (hexagonal δ-FAPbI 3 ) instead of the light-absorbing perovskite structure (cubic, α-FAPbI 3 ) under ambient conditions, , which is the main impediment to the formation of α-FAPbI 3 films and also long-time stability of FA-based devices .…”

Wide‐Bandgap Organic–Inorganic Lead Halide Perovskite Solar Cells

“…[10,11] The TBG perovskites have conferred PSCs with outstanding advancements, which have been summarized from various perspectives. [12][13][14][15] The WBG perovskites are of significance in semitransparent devices, dim indoor light conversion, photoelectrochemistry conversions generating energy-storing chemicals, tandem devices, spectrum-splitting systems, and light emitting diodes.…”

Study of Electron Transport Layer‐Free and Hole Transport Layer‐Free Inverted Perovskite Solar Cells

“…The power conversion efficiency (PCE) of perovskite-based solar cells (SCs) has increased dramatically in the last 10 years to more than 25%, competing well with other exciting SC technologies. [1][2][3] OrganicÀinorganic perovskite has unique and superior properties such as broad absorption, a long diffusion length, and it can be composed of a solution that makes it very suitable to function as a light harvester in SCs. [4-7 ' 8] Perovskite solar cells (PSCs) have three main architectures: 1) a planar structure; 2) a mesoporous structure; and 3) an inverted structure.…”