its volatility and reversible/irreversible decomposition reaction even at low temperature, MA is gradually avoided in the material designs. [9] At the same time, FA is more thermally stable than MA due to its stronger hydrogen bonding with PbX 6 octahedra and benign reversible decomposition reaction below 85 °C, and it is the primary cation in practically all current high-performance PSCs. [10,11] Also, FA has no irreversible (nonselective) back reaction. [12] At the same time, in order to approach the bandgap of 1.34 eV according to the Schottky-Queisser (S-Q) limit, from initially MAPbI 3 to double cation (MAFA or FACs), [10,13,14] triple cation (CsMAFA) based perovskites, [5] eventually to quadruple (CsRbMAFA) based perovskites, [4,[15][16][17] and recently FAPbI 3 are dominated ones. [18][19][20] FAPbI 3 has an ideal, narrow bandgap since FA remains the largest organic cation that fits into a 3D perovskite crystal structure. [21] The implicit or explicit goal of perovskite research is to obtain a black FA-based (stable phase) perovskite at room temperature. Avoiding yellow phase impurities encouraged the advancement of processing techniques and elaborate multication, multi-halide mixtures.It has been proved that the invasion of Br anion at the X site is effective for stabilizing the black phase FA-based and other perovskites. But it leads to an adverse blue-shift of the bandgap disproportionately. For example, there is a spanning of 700 meV persisting in MAPbI x Br 1-x from 2.28 eV (MAPbBr 3 ) to 1.58 eV (MAPbI 3 ). [22] Furthermore, from a stability perspective, introducing Br anion in iodide-based perovskites is unfavorable since Br/I mixtures will undergo severe anion segregation Lead halide-based perovskite solar cells (PSCs) are intriguing candidates for photovoltaic technology because of their high efficiency, low cost, and simple process advantages. Owing to lead toxicity, PSCs based on partially/fully substituted Pb with tin have attracted tremendous attention, which would enable the ideal bandgap to approach the Shockley-Queisser (S-Q) limit. Especially, methylammonium (MA), bromide-free, tin-based perovskites are striking, because of the intrinsic poor stability of MA and blue shift caused by the incorporation of Br − . The first section of this review emphasizes the motivation for studying single-junction MA, Br-free, and Sn-based perovskites. The film quality improvement strategies of Sn-based perovskites, including additive, composition, dimensional, and interface engineering toward high-efficiency devices are comprehensively overviewed. Moreover, strategies to improve stability, where shelf, thermal and operational stabilities of the devices are summarized. Finally, this review concludes with a discussion of actual limitations and future prospects for Sn-based PSCs.
PSCs have sparked widespread concern with their quick advancement in photovoltaic performance. Their PCEs have advanced from 3.8% to 25.7% in recent years. [1,2] In modern PSCs, methylammonium (MA) and formamidinium (FA) are two prevalent organic cations. [3][4][5][6] Recently lead and lead/tin mixed perovskite solar cells are attaining immense attraction due to their excellent band tunability. [7] Although FAPbI 3 's optical bandgap (1.45-1.48 eV) [8,9] is closer to the predicted bandgap (1.34 eV) than MAP-bI 3 's (1.58 eV). [10] There is a complication with using FAPbI 3 directly in PSCs since it converts to a photo inactive (δ-FAPbI 3 ) phase at ambient temperature. [11,12] In a thorough study, the α-to-δ phase transition was effectively inhibited by Rb + , [13,14] Cs + , [11,15] MA + , [5,16] and Br − incorporation [15,16] into the FAPbI 3 crystal lattice to form a more stale cubic perovskite structure. To prevent δ-FAPbI 3 formation, MA cations are detrimental to the device's thermal stability, [13] while adding Br − will cause a blue shift in the bandgap. [10] As a result, the favored cations are the thermally stable Cs + , Rb + , FA + , and I − as the chosen halide anion.Rubidium (Rb) is a member of the alkali metal family with a smaller ionic radius (1.52 Å) and is located above cesium in the alkali group. However, the side phase, that is, RbPbI 3 melted and disintegrated at 714 K without going through a phase shift to black perovskite when heated ≈298 and 634 K. It preserved yellow-δ-phase orthorhombic structure at such conditions. [17] To determine how the gradual replacement of Rb for Cs impacts the structure, thermodynamics, and electrical characteristics, Jung et al. used calculations based on a density-functional theory of the first principle. [18] Rubidium and guanidinium (GA) as A-site cations were used by Prochowicz et al. to study the effect of open-circuit voltage (V OC ) in PSCs, [19] determining that the type of the A-site cation controls the V OC of PSCs. By constructing a 2D Ruddlesden-Popper layer on top of a 3D Rb + doped triple cation perovskite, researchers have created a novel 3D/2D planar bi-layer perovskite. [20] The inclusion of Rb + cation lowers the work function, resulting in slightly higher PCE (>20%) than its 3D equivalents (19.5%). As a result of their Lead halide-based perovskites solar cells (PSCs) are intriguing candidates for photovoltaic technology due to their high efficiency, low cost, and simple fabrication processes. Currently, PSCs with efficiencies of >25% are mainly based on methylammonium (MA)-free and bromide (Br) free, formamide lead iodide (FAPbI 3 )-based perovskites, because MA is thermally unstable due to its volatile nature and Br incorporation will induce blue shift in the absorption spectrum. Therefore, MA-free, Br-free formamidine-based perovskites are drawing huge research attention in recent years. The hole transporting layer (HTL) is crucial in fabricating highly efficient and stable inverted p-i-n structured PSCs by enhancing charge extraction, lower...
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