Metal–halide hybrid perovskites have prompted the prosperity of the sustainable energy field and simultaneously demonstrated their great potential in meeting both the growing consumption of energy and the increasing social development requirements. Their inimitable features such as strong absorption ability, direct photogeneration of free carriers, long carrier diffusion lengths, ease of fabrication, and low production cost triggered the development of perovskite solar cells (PSCs) at an incredible rate, which soon reached power conversion efficiencies up to the commercialized level. During their evolution process, it has been witnessed that alkali metal cations play a pivotal role in the crystal structure as well as intrinsic properties of hybrid perovskites, thus enabling the unique positioning of the correlated doping strategy in the development history of PSCs in the past decade. Herein, we summarize the growth and progress of the state-of-the-art alkali metal cation (Cs+, Rb+, K+, Na+, Li+) doping in the field of hybrid perovskite-based photovoltaics. To start with, the accurate identification of different alkali metal-occupied locations in the perovskite crystal lattice are discussed in detail with highlighted advanced characterization methods. Beyond that, the location-dependent functions induced by alkali metal doping are intensely focused upon and comprehensively assessed, indicating their versatile and special effects on perovskites in terms of bottleneck issues such as crystallinity modulation, crystal structure stabilization, defect passivation, and ion-migration inhibition. Thereafter, we are committed to analyze their responsible working mechanisms so as to unveil the relationship between occupied locations and crucial roles for each doped cation. The systematical overview and in-depth understanding of the superiorities of such strategies together with their future challenges and prospects would further boost the advancement of perovskite-related fields.

Advent of alkali metal doping: a roadmap for the evolution of perovskite solar cells.

Developing transparent p-type semiconductors and conductors has attracted significant interest in both academia and industry because metal oxides only show efficient n-type characteristics at room temperature. Among the different candidates, copper iodide (CuI) is one of the most promising p-type materials because of its widely adjustable conductivity from transparent electrodes to semiconducting layers in transistors. CuI can form thin films with high transparency in the visible light region using various low-temperature deposition techniques. This progress report aims to provide a basic understanding of CuI-based materials and recent progress in the development of various devices. The first section provides a brief introduction to CuI with respect to electronic structure, defect states, charge transport physics, and overviews the CuI film deposition methods. The material design concepts through doping/alloying approaches to adjust the optoelectrical properties are also discussed in the first section. The following section presents recent advances in state-of-the-art CuI-based devices, including transparent electrodes, thermoelectric devices, p-n diodes, p-channel transistors, light emitting diodes, and solar cells. In conclusion, current challenges and perspective opportunities are highlighted.

https://www.onlinelibrary.wiley.com/doi/pdf/10.1002/advs.202100546

Engineering Copper Iodide (CuI) for Multifunctional p-Type Transparent Semiconductors and Conductors.

Delafossites like CuGaO2 have appeared as promising p-type semiconductor materials for opto-electronic applications mainly due to their high optical transparency and electrical conductivity. However, existing synthetic efforts usually result in particles with large diameter limiting their performance relevant to functional electronic applications. In this article, we report a novel surfactant-assisted hydrothermal synthesis method, which allows the development of ultrafine (~5 nm) monodispersed p-type CuGaO2 nanoparticles (NPs). We show that DMSO can be used as a ligand and dispersing solvent for stabilizing the CuGaO2 NPs. The resulting dispersion is used for the fabrication of dense, compact functional CuGaO2 electronic layer with properties relevant to advanced optoelectronic applications. As a proof of concept, the surfactant-assisted hydrothermal synthesized CuGaO2 is incorporated as a hole transporting layer (HTL) in the inverted p-i-n perovskite solar cell device architecture providing improved hole carrier selectivity and power conversion efficiency compared to conventional PEDOT:PSS HTL based perovskite solar cells.

Employing surfactant-assisted hydrothermal synthesis to control CuGaO2 nanoparticle formation and improved carrier selectivity of perovskite solar cells

We present a solvothermal synthetic route to produce monodispersed CuO nanoparticles (NPs) in the range of 5-10 nm that can be used as hole selective interfacial layer between indium tin oxide (ITO) and perovskite active layer for p-i-n perovskite solar cells by a spin casting the dispersions at room temperature. The bottom electrode interface modification provided by spherical CuO-NPs at room temperature promotes the formation of high quality perovskite photoactive layers with large crystal size and strong optical absorption. Furthermore, it is shown that the nanoparticulate nature of the CuO hole transporting interfacial layer can be used to improve light manipulation within perovskite solar cell device structure. The corresponding p-i-n CH3NH3PbI3-based solar cells show high Voc values of 1.09 V, which is significantly higher compared to the Voc values obtained with conventional PEDOT:PSS hole selective contact based perovskite solar cells.

/pdf/room-temperature-nanoparticulate-interfacial-layers-for-3qff84o0jo.pdf

Room Temperature Nanoparticulate Interfacial Layers for Perovskite Solar Cells via solvothermal synthesis

Abstract Different 2D and quasi-2D perovskite materials have demonstrated significant improvements in the device stability compared to 3D perovskites due to their increased hydrophobicity and suppressed ion migration. However, fundamental investigations of these materials have been scarce and consequently detailed understanding of the processes responsible for experimental phenomena are often lacking despite huge interest in these materials. Even more importantly, there have been a limited number of structure-property studies for different material compositions, and research is generally by trial and error rather than by design. Here we discuss different stability issues in these materials and identify questions which need to be answered to design materials with further stability improvements. 

https://www.nature.com/articles/s43246-022-00285-9.pdf

Stability of 2D and quasi-2D perovskite materials and devices

Optimization of Bulk Defects in Sn/Pb Mixed Perovskite Solar Cells through Synergistic Effect of Potassium Thiocyanate

Inorganic copper-based hole transport materials for perovskite photovoltaics: Challenges in normally structured cells, advances in photovoltaic performance and device stability

Facilitate hole transport with thin 2D perovskite capping layer to passivate interface defects of 3D perovskite solar cells using PEABr

Degradation mechanism and addressing techniques of thermal instability in halide perovskite solar cells

Perovskite solar cells need a long-term thermal stability and high efficiencies for commercial viability. Unfortunately, perovskites tend to have more defects making them vulnerable to thermal decomposition. Previous research studies have explored different strategies such as surface passivation, interfacial modification, post-treatments of perovskite films with Lewis bases and acids, and photo-curing to address the challenges due to thermal instability; however, these techniques face obstacles such as the rapid phase segregation of I−Br− based perovskites, poor light and moisture stability, and low efficiency of devices. This review, therefore, discusses the technique of compositional engineering to suppress the defect states to improve the thermal stability of the device.

Luyun Bai

Papers

Optimization of Bulk Defects in Sn/Pb Mixed Perovskite Solar Cells through Synergistic Effect of Potassium Thiocyanate

Inorganic copper-based hole transport materials for perovskite photovoltaics: Challenges in normally structured cells, advances in photovoltaic performance and device stability

Facilitate hole transport with thin 2D perovskite capping layer to passivate interface defects of 3D perovskite solar cells using PEABr

Degradation mechanism and addressing techniques of thermal instability in halide perovskite solar cells

Compositional engineering solutions for decreasing trap state density and improving thermal stability in perovskite solar cells