Design of Low Bandgap CsPb1- x Snx I2 Br Perovskite Solar Cells with Excellent Phase Stability

Toward durable all-inorganic perovskite solar cells: from lead-based to lead-free

Organic-inorganic metal halide perovskite solar cells (PSCs) have attracted extensive attention from the photovoltaic (PV) community due to their fast-growing power conversion efficiency from 3.8% to 26.7% in only 15 years. However, these organic-inorganic hybrid PSCs suffer from inferior long-term operational stability under thermal and light stress, due to the fragile hydrogen bonds between organic cations and inorganic slabs. This motivates the exploration of more robust all-inorganic alternatives against external stimuli, by substituting inorganic cesium (Cs) cations for volatile organic cations. Despite reinforced ionic interaction between Cs cations and metal halide frameworks, these Cs-based all-inorganic perovskites tend to undergo spontaneous phase transition from photoactive black phases to non-perovskite yellow phases at room temperature, significantly deteriorating their optoelectronic performance. Thus, tremendous efforts have been made to stabilize the black phase of CsPbI3, while the phase instability issue of the tin-based analogue of CsSnI3 has not been resolved yet. This highlight article summarizes the empirical advances in stabilizing the metastable phases of CsPbI3, aiming to provide useful guidelines to accelerate the development of phase-stable CsSnI3 for durable lead-free PV applications. Finally, the remaining challenges and future research opportunities are outlined, providing a road map to realize efficient and durable all-inorganic perovskite solar cells towards practical applications.

Glutathione-Coated Gold Nanoparticles Enabling Bifunctional Therapy at the Buried Interface for Efficient and UV-Resistant Perovskite Solar Cells

Very recently, the poor contact between the perovskite and carrier selective layer has been regarded as a critical issue for improving the performance and stability of perovskite solar cells (PSCs). In this study, the buried interface of regularly structured PSCs has been targeted. Glutathione-coated gold nanoparticles (GSH-AuNPs) are used as double-sided passivating agents to improve the quality of the perovskite films. It has been demonstrated that the GSH-AuNPs interact strongly with the SnO2 underlayer and the upper perovskite layer, significantly reducing the defect densities of this interface. Thus, the power conversion efficiency (PCE) of the PSCs can be increased from 20.46% (control, 19.38%, IPCE corrected) to 22.22% (GSH-AuNPs modified, 21.10%, IPCE corrected) with notable enhancement in Voc and FF. Moreover, the strong interaction between the C═O groups of GSH-AuNPs and the undercoordinated Pb2+ species of the perovskite films inhibits the formation of metallic Pb0. As a result, the unencapsulated GSH-AuNPs-modified devices retained 80% of their initial PCEs after 1000 h at ambient conditions, with a relative humidity (RH) of 60 ± 5%. UV-resistant PSCs have also been demonstrated after introducing GSH-AuNPs. Therefore, our findings demonstrate the bidirectional therapy strategy as a feasible approach for achieving efficient and UV-resistant PSCs.

All-inorganic perovskite solar cells featuring mixed group IVA cations

All-inorganic perovskites are promising for solar cells owing to their potentially superior tolerance to environmental factors, as compared with their hybrid organic-inorganic counterparts. Over the past few years, all-inorganic perovskite solar cells (PSCs) have seen a dramatic improvement in certified power conversion efficiencies (PCEs), demonstrating their great potential for practical applications. Pb, Sn, and Ge are the most studied group IVA elements for perovskites. These group IVA cations share the same number of valence electrons and similarly exhibit the beneficial antibonding properties of lone-pair electrons when incorporated in the perovskite structure. Meanwhile, mixing these cations in all-inorganic perovskites provides opportunities for stabilizing the photoactive phase and tailoring the bandgap structure. In this mini-review, we analyze the structural and bandgap design principles for all-inorganic perovskites featuring mixed group IVA cations, discuss the updated progress in the corresponding PSCs, and finally provide perspectives on future research efforts faciliating the continued development of high-performance Pb-less and Pb-free all-inorganic PSCs.

Improving Out-of-Plane Charge Mobility and Phase Stability of Dion-Jacobson Lead-Free Perovskites via Intercalating π-Conjugated Aromatic Spacers

The layered quasi-2D perovskites are recognized as one of the effective strategies to resolve the big problem of intrinsic phase instability of the perovskites. However, in such configurations, their performance is fundamentally limited due to the correspondingly weakened out-of-plane charge mobility. Herein, the π-conjugated p-phenylenediamine (PPDA) is introduced as organic ligand ions for rationally designing lead-free and tin-based 2D perovskites with the aid of theoretical computation. It is evidenced that both out-of-plane charge transport capacity and stability can be significantly enhanced within as-established quasi-2D Dion-Jacobson (DJ) (PPDA)Cs_{n -1} Sn_n I_{3 n +1} perovskites. The obviously increased electrical conductivity and reduced carrier effective masses are attributed to the enhanced interlayer interactions, limited structural distortions of diamine cations, as well as improved orbital coupling between Sn²⁺ and I^- ions of (PPDA)Cs_{n -1} Sn_n I_{3 n +1} perovskites. Accordingly, by dimension engineering of the inorganic layer (n), the bandgap (E_g ) of quasi-2D perovskites can be linearly tailored toward the suitable E_g (1.387 eV) with optimal photoelectric conversion efficiency (PCE) of 18.52%, representing their great potential toward promising applications in advanced solar cells.

4-Trifluorophenylammonium Iodide-Based Dual Interfacial Modification Engineering toward Improved Efficiency and Stability of SnO2-Based Perovskite Solar Cells

Passivation engineering has been identified as an effective strategy to eliminate the targeted interfacial defects for improving the efficiency and stability of perovskite solar cells (PSCs). Herein, 4-trifluorophenylammonium iodide (CF₃PhAI) is presented as a multifunctional passivation agent to modify buried SnO₂/perovskite and perovskite/hole transport layer (HTL) interfaces. Upon incorporation of CF₃PhAI between SnO₂ and perovskite, CF₃PhAI can chemically link to SnO₂ via Lewis coordination and electrostatic coupling, thereby effectively passivating under-coordinated Sn and filling the oxygen vacancy. Meanwhile, CF₃PhAI helps anchor PbI₂ and organic cations (MA⁺/FA⁺) to control the crystallization of the perovskite. Consequently, reduced interfacial defects, homogeneous perovskite crystallites, and better energetic alignment can be simultaneously achieved. When CF₃PhAI was further used to modify the perovskite/HTL interface, the fabricated PSCs yielded an impressive power conversion efficiency of 23.06% together with negligible J-V hysteresis. The unencapsulated devices exhibited long-term stability in wet conditions (91.8% efficiency retention after 1000 h) due to the water-resistant CF₃PhAI. We also achieved good light soaking stability, maintaining 86.1% of its initial efficiency after aging for 720 h. Overall, our finding provides a promising strategy for modifying the dual contact interfaces of PSCs toward improved efficiency and stability.

Enhancing the Efficiency of Perovskite Solar Cells by Bidirectional Modification of the Perovskite and Electron Transport Layer

In perovskite solar cells (PSCs), the numerous defects present on the surface of the SnO₂ electron transport layer (ETL) and the bottom of the perovskite film limit their power conversion efficiency (PCE) and stability. In view of this, a bidirectional modification strategy is designed using formamidine acetate (FAAc) to passivate the defects on the SnO₂ ETL surface and bottom of the perovskite simultaneously. FA⁺ cations act on the harmful hydroxyl groups on the SnO₂ ETL surface, whereas Ac^- anions act on the iodine vacancy defect at the bottom of the perovskite. Because the interface defect is well passivated by FAAc, the interfacial charge recombination is restrained. This results in a significant increase in the filling factor of the PSC to ∼0.83 and the consequent increase in PCE to 23.05%, which considerably improves the stability. Bidirectional modification technology is an effective strategy for improving the PCE and stability of PSCs.

From Structural Design to Functional Construction: Amine Molecules in High-Performance Formamidinium-Based Perovskite Solar Cells

Formamidinium (FA) based perovskites are considered as one of the most promising light-absorbing perovskite materials owing to their narrower band gap and better thermal stability compared to conventional methylammonium-based perovskites. Constant improvement by using various additives stimulates the potential application of these perovskites. Amine molecules with different structures have been widely used as typical additives in FA-based perovskite solar cells, and decent performances have been achieved. Thus, a systematic review focusing on structural regulation and functional construction of amines in FA-based perovskites is of significance. Herein, we analyze the construction mechanism of different structural amines on the functional perovskite crystals. The influence of amine molecules on specific perovskite properties including defect conditions, charge transfer, and moisture resistance are evaluated. Finally, we summarize the design rules of amine molecules for the application in high-performance FA-based perovskites and propose directions for the future development of additive molecules.

Femtosecond Exciton and Carrier Relaxation Dynamics of Two-Dimensional (2D) and Quasi-2D Tin Perovskites

The dynamics of exciton and free-carrier relaxation of low-dimensional tin iodide perovskites, BA₂FA_n-1Sn_nI_3n+1, where n = 1 (N1), 2 (N2), 5 (N5), and 10 (N10), were investigated with femtosecond transient absorption spectra (TAS). The absorption and photoluminescence spectra of N1 and N2 show exciton characteristics due to quantum confinement, whereas N5 and N10 display a free-carrier nature, the same as for bulk three-dimensional (3D) films. The TAS profiles were fitted according to a global kinetic model with three time coefficients representing the interactions of biexcitons, trions, and excitons for N1 and N2 and hot carriers, cold carriers, and shallow trap carriers for N5 and N10. The carrier relaxation dynamics of N5 and N10 were similar to those of 3D FASnI₃ except for the absence of surface recombination in the deep-trap states due to passivation of the grain surfaces by the long alkyl chain for these quasi-2D samples (N5/N10 vs 3D).

Defect Passivation for Perovskite Solar Cells: from Molecule Design to Device Performance

Perovskite solar cells (PSCs) are a promising third-generation photovoltaic (PV) technology developed rapidly in recent years. Further improvement of their power conversion efficiency is focusing on reducing the non-radiative charge recombination induced by the defects in metal halide perovskites. So far, defect passivation by the organic small molecule has been considered as a promising approach for boosting the PSC performance owing to their large structure flexibility adapting to passivating variable kinds of defect states and perovskite compositions. Here, the recent progress of defect passivation toward efficient and stable PSCs was reviewed from the viewpoint of molecular structure design and device performance. To comprehensively reveal the structure-performance correlation of passivation molecules, it was separately discussed how the functional groups, organic frameworks, and side chains affect the corresponding PV parameters of PSCs. Finally, a guideline was provided for researchers to select more suitable passivation agents, and a perspective was given on future trends in development of passivation strategies.

Interfaces and Interfacial Layers in Inorganic Perovskite Solar Cells

Owing to their superior thermal stability, metal halide inorganic perovskite materials continue to attract interest for photovoltaics applications. The highest reported power conversion efficiency (PCE) for solar cells based on inorganic perovskites is over 20 %. As this PCE corresponds to 73 % of the theoretical limit, there remains more room for further improving the device PCEs than for improving organic-inorganic hybrid perovskite solar cells (PSCs). The main loss is in the photovoltage, which is limited by interfaces in terms of non-radiative recombination caused by traps and energy-level mismatch. Furthermore, inefficient charge extraction at interfacial contacts reduces the photocurrent and fill factor. This Minireview summarizes the recent developments in the fundamental understanding of how the interfaces and interfacial layers influence the performance of solar cells based on inorganic perovskite absorbers. An outlook for the development of highly efficient and stable inorganic PSCs from the interface point of view is also given.