A Review on Additives for Halide Perovskite Solar Cells

3D perovskite solar cells (PSCs) have shown great promise for use in next-generation photovoltaic devices. However, some challenges need to be addressed before their commercial production, such as enormous defects formed on the surface, which result in severe SRH recombination, and inadequate material interplay between the composition, leading to thermal-, moisture-, and light-induced degradation. 2D perovskites, in which the organic layer functions as a protective barrier to block the erosion of moisture or ions, have recently emerged and attracted increasing attention because they exhibit significant robustness. Inspired by this, surface passivation by employing 2D perovskites deposited on the top of 3D counterparts has triggered a new wave of research to simultaneously achieve higher efficiency and stability. Herein, we exploited a vast amount of literature to comprehensively summarize the recent progress on 2D/3D heterostructure PSCs using surface passivation. The review begins with an introduction of the crystal structure, followed by the advantages of the combination of 2D and 3D perovskites. Then, the surface passivation strategies, optoelectronic properties, enhanced stability, and photovoltaic performance of 2D/3D PSCs are systematically discussed. Finally, the perspectives of passivation techniques using 2D perovskites to offer insight into further improved photovoltaic performance in the future are proposed. 

Surface Passivation Using 2D Perovskites toward Efficient and Stable Perovskite Solar Cells

Understanding the origin of defects in lead halide perovskite nanocrystals is paramount to attaining long-term structural stability and improved optical efficiency, key features for their successful implementation in optoelectronic devices. Unlike other studies, we explore the possible formation of trap states in explicit, nonperiodic CsPbBr3 nanocrystal models about 3 nm in size. Using density functional theory, we compute the defect formation energies of interstitial, vacancy, and antisite defects in different regions of the nanocrystal (center, surface center, and surface edge), demonstrating that the most stable defect position is found at the surface. We ascribe the high defect tolerance of CsPbBr3 nanocrystals to the fact that vacancies, i.e. the loss of surface ligands as ion pairs, are energetically difficult to form and only excessive stripping of surface ligands might be problematic, as their detachment leaves undercoordinated Br- on the crystal surface that only in this case translates into deep traps.

Defects in lead halide perovskite nanocrystals: Analogies and (many) differences with the bulk

TiO2/WO3 Bilayer as Electron Transport Layer for Efficient Planar Perovskite Solar Cell with Efficiency Exceeding 20%

PEG Modified CsPbIBr2 Perovskite Film for Efficient and Stable Solar Cells

Reduced Defects of MAPbI3 Thin Films Treated by FAI for High-Performance Planar Perovskite Solar Cells

Two-dimensional (2D) perovskites, which have excellent stability compared with three-dimensional (3D) perovskites owing to the effective protection of the hydrophobic organic ligands, have become a research hotspot and have made great developmental progress in recent years. Herein, an n-butylammonium iodide (BAI) post-treatment process was developed to fabricate a 2D-3D hybrid perovskite with a thin layer of 2D perovskite covered on the surface of the 3D CH3NH3PbI3 perovskite. The growth process of 2D perovskite is formed through the chemical reaction between BAI and the residual PbI2, which improves stability and reduces the number of crystal defects of 3D perovskite by optimizing stoichiometry. Compared with the 3D counterpart, the 2D-3D hybrid perovskite shows outstanding light and air stability when exposed to external environments. Moreover, structure conversion from 3D to 2D-3D can induce the passivation of defects in the 3D films. The power conversion efficiency of the 2D-3D solar cell exceeds 18% and retains 80% of the initial value after more than 2000 h of storage without encapsulation.

https://iopscience.iop.org/article/10.1088/1361-6528/ab10f3/pdf

Highly efficient and stable 2D-3D perovskite solar cells fabricated by interfacial modification.

Various approaches of interface engineering are shown to be effective in improving the device performance of organic–inorganic hybrid perovskite solar cells (PSCs). The modification of the photoactive layer of PSC, CH 3 NH 3 PbI 3 (MAPbI 3 ), by spin-coating a layer of p-aminobenzoic acid (PABA), which can significantly enhance the open-circuit voltage (V OC ), the fill factor (FF), and the power conversion efficiency (PCE) of PSCs, is herein reported. The champion device shows a short-circuit current (J SC ) of 22.83 mA cm−2, V OC of 1.167 V, FF of 0.768, and PCE of 20.47%. The improvement in photovoltaic performance is attributed to the suppression of carrier trap states and the improvement in the morphologies of perovskite films. This work demonstrates a simple and effective protocol to enhance the device performance, and provides an insight into the influence of PABA post-treatment on the charge carrier dynamics. © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Enhancement of Open‐Circuit Voltage of Perovskite Solar Cells by Interfacial Modification with p‐Aminobenzoic Acid

Crystal orientations are closely related to the behavior of photogenerated charge carriers and are vital for controlling the optoelectronic properties of perovskite solar cells. Herein, we propose a facile approach to reveal the effect of lattice plane orientation distribution on the charge carrier kinetics via constructing CsBr-doped mixed cation perovskite phases. With grazing-incidence wide-angle X-ray scattering measurements, we investigate the crystallographic properties of mixed perovskite films at the microscopic scale and reveal the effect of the extrinsic CsBr doping on the stacking behavior of the lattice planes. Combined with transient photocurrent, transient photovoltage, and space-charge-limited current measurements, the transport dynamics and recombination of the photogenerated charge carriers are characterized. It is demonstrated that CsBr compositional engineering can significantly affect the perovskite crystal structure in terms of the orientation distribution of crystal planes and passivation of trap-state densities, as well as simultaneously facilitate the photogenerated charge carrier transport across the absorber and its interfaces. This strategy provides unique insight into the underlying relationship between the stacking pattern of crystal planes, photogenerated charge carrier transport, and optoelectronic properties of solar cells.

The Influence of CsBr on Crystal Orientation and Optoelectronic Properties of MAPbI3-Based Solar Cells.

Heterostructures constructed from graphene and transition metal dichalcogenides (TMDs) have established a new platform for optoelectronic applications. After a large number of studies, one intriguing debate is the existence of the interfacial exciton in graphene/TMDs. Hereby, by combined optical pump-terahertz probe spectroscopy and transient absorption spectroscopy, we report the observation of the interfacial exciton in graphene/MoS2 heterostructure. With the photon energy well below the band gap of monolayer MoS2, the hot electrons of graphene are transferred to MoS2 within 0.5 ps; subsequently, the relaxation of the holes in graphene and electrons in MoS2 shows an identical time scale of 15-18 ps, which manifests the formation and relaxation of the interfacial exciton in the heterostructure following photoexcitation. Moreover, a model of the carrier heating and photogating effect in graphene is proposed to estimate the amount of transferred charge, which agrees well with the experimental results. Our study provides insights into the dynamics of graphene-based heterostructure interfacial non-equilibrium carriers.

Yuqin Zou

Papers

Reduced Defects of MAPbI3 Thin Films Treated by FAI for High-Performance Planar Perovskite Solar Cells

Highly efficient and stable 2D-3D perovskite solar cells fabricated by interfacial modification.

Enhancement of Open‐Circuit Voltage of Perovskite Solar Cells by Interfacial Modification with p‐Aminobenzoic Acid

The Influence of CsBr on Crystal Orientation and Optoelectronic Properties of MAPbI3-Based Solar Cells.

Observation of Ultrafast Interfacial Exciton Formation and Relaxation in Graphene/MoS2 Heterostructure.