Chen Weijie

In the past decade, the perovskite solar cell (PSC) has attracted tremendous attention. The electron transport layer (ETL) is one of the most important functional layers in PSCs. This review provides an up‐to‐date summary of the developments in inorganic electron transport materials (ETMs) for PSCs. Strategies to optimize ETL, an outlook on current challenges and further development are discussed.

Abstract

In the past decade, the perovskite solar cell (PSC) has attracted tremendous attention thanks to the substantial efforts in improving the power conversion efficiency from 3.8% to 25.5% for single‐junction devices and even perovskite‐silicon tandems have reached 29.15%. This is a result of improvement in composition, solvent, interface, and dimensionality engineering. Furthermore, the long‐term stability of PSCs has also been significantly improved. Such rapid developments have made PSCs a competitive candidate for next‐generation photovoltaics. The electron transport layer (ETL) is one of the most important functional layers in PSCs, due to its crucial role in contributing to the overall performance of devices. This review provides an up‐to‐date summary of the developments in inorganic electron transport materials (ETMs) for PSCs. The three most prevalent inorganic ETMs (TiO₂, SnO_2, and ZnO) are examined with a focus on the effects of synthesis and preparation methods, as well as an introduction to their application in tandem devices. The emerging trends in inorganic ETMs used for PSC research are also reviewed. Finally, strategies to optimize the performance of ETL in PSCs, effects the ETL has on J–V hysteresis phenomenon and long‐term stability with an outlook on current challenges and further development are discussed.

This work proposes an efficient method to produce tri‐iodide ions, which has been known as an efficient additive that improves the crystallinity, grain size, and morphology of perovskite films in a precursor solution using a photoassited process within short time, resulting in achieving the device performance up to 22%.

Abstract

One of the most effective methods to achieve high‐performance perovskite solar cells (PSCs) is to employ additives as crystallization agents or to passivate defects. Tri‐iodide ion has been known as an efficient additive to improve the crystallinity, grain size, and morphology of perovskite films. However, the generation and control of this tri‐iodide ion are challenging. Herein, an efficient method to produce tri‐iodide ion in a precursor solution using a photoassisted process for application in PSCs is developed. Results suggest that the tri‐iodide ion can be synthesized rapidly when formamidinium iodide (FAI) dissolved isopropyl alcohol (IPA) solution is exposed to LED light. Specifically, the photoassisted FAI–IPA solution facilitates the formation of fine perovskite films with high crystallinity, large grain size, and low trap density, thereby improving the device performance up to 22%. This study demonstrates that the photoassisted process in FAI dissolved IPA solution can be an alternative strategy to fabricate highly efficient PSCs with significantly reduced processing times.

Scaling up perovskite solar cells (PSCs) to fabricate efficient perovskite solar modules (PSMs) is the fundamental for application. To make a goal of PSC commercialization achievable, device architecture designs, scalable deposition methods, perovskite morphology modulation, charge transport materials, electrode materials, and encapsulation methods are all important to fabricate stable, low‐cost, and high‐efficiency PSMs.

Abstract

Power conversion efficiency of perovskite solar cells (PSCs) has been boosted to 25.5% among the highest efficiency for single‐junction solar cells, making PSCs extremely promising to realize industrial production and commercialization. Scaling up PSCs to fabricate efficient perovskite solar modules (PSMs) is the fundamental for applications. Here, present progresses on scaling up PSCs are reviewed. The structure design for PSMs is discussed. Various scalable methods and related morphology control strategies for large‐area uniform perovskite films are summarized. Potential charge transport materials and electrode materials together with their scalable methods for low‐cost, efficient, and stable PSMs are also summarized. Besides, current attempts on encapsulation for improving stability and reducing lead leakage are introduced, and the calculated cost and environment influence of PSMs are also outlined.

The first survey of the emerging photovoltaic reports initiative summarizes the best achievements published in academic journals in the research of emerging photovoltaic materials, e.g., organic, perovskite, and dye sensitized solar cells. The reports are presented as a function of the bandgap energy for different categories such as transparency, flexibility, and stability, and compared to the Shockley–Queisser limit.

Abstract

Emerging photovoltaics (PVs) focus on a variety of applications complementing large scale electricity generation. Organic, dye‐sensitized, and some perovskite solar cells are considered in building integration, greenhouses, wearable, and indoor applications, thereby motivating research on flexible, transparent, semitransparent, and multi‐junction PVs. Nevertheless, it can be very time consuming to find or develop an up‐to‐date overview of the state‐of‐the‐art performance for these systems and applications. Two important resources for recording research cells efficiencies are the National Renewable Energy Laboratory chart and the efficiency tables compiled biannually by Martin Green and colleagues. Both publications provide an effective coverage over the established technologies, bridging research and industry. An alternative approach is proposed here summarizing the best reports in the diverse research subjects for emerging PVs. Best performance parameters are provided as a function of the photovoltaic bandgap energy for each technology and application, and are put into perspective using, e.g., the Shockley–Queisser limit. In all cases, the reported data correspond to published and/or properly described certified results, with enough details provided for prospective data reproduction. Additionally, the stability test energy yield is included as an analysis parameter among state‐of‐the‐art emerging PVs.

A dissymmetric fused‐ring acceptor BTIC‐2Cl‐γCF₃ with chlorine and trifluoromethyl end groups give a power conversion efficiency (PCE) of over 17 % which is the highest among polymer solar cells processed by halogen‐free solvents. Dissymmetric chlorination and trifluoromethylation is a practical approach towards a low band‐gap acceptor for eco‐compatible processed photovoltaic applications.

Abstract

To elevate the performance of polymer solar cells (PSC) processed by non‐halogenated solvents, a dissymmetric fused‐ring acceptor BTIC‐2Cl‐γCF₃ with chlorine and trifluoromethyl end groups has been designed and synthesized. X‐ray crystallographic data suggests that BTIC‐2Cl‐γCF₃ has a 3D network packing structure as a result of H‐ and J‐aggregations between adjacent molecules, which will strengthen its charge transport as an acceptor material. When PBDB‐TF was used as a donor, the toluene‐processed binary device realized a high power conversion efficiency (PCE) of 16.31 %, which improved to 17.12 % when PC71ThBM was added as the third component. Its efficiency of over 17 % is currently the highest among polymer solar cells processed by non‐halogenated solvents. Compared to its symmetric counterparts BTIC‐4Cl and BTIC‐CF₃‐γ, the dissymmetric BTIC‐2Cl‐γCF₃ integrates their merits, and has optimized the molecular aggregations with excellent storage and photo‐stability, and also extending the maximum absorption peak in film to 852 nm. The devices exhibit good transparency indicating a potential utilization in semi‐transparent building integrated photovoltaics (ST‐BIPV).

Utilizing NP‐SC₆‐TiOPc and NP‐SC₆‐ZnPc as passivating agents on perovskite thin film through an antisolvent, improved performance and stability are achieved for perovskite solar cells. The highest power conversion efficiencies (PCEs) of 19.39% and 18.04% are obtained for NP‐SC₆‐TiOPc and NP‐SC₆‐ZnPc passivated devices, which is higher than that of the control devices without post‐treating the MAPbI₃ films (PCE of 17.67%).

Abstract

Semiconducting molecules have been employed to passivate traps extant in the perovskite film for enhancement of perovskite solar cells (PSCs) efficiency and stability. A molecular design strategy to passivate the defects both on the surface and interior of the CH₃NH₃PbI₃ perovskite layer, using two phthalocyanine (Pc) molecules (NP‐SC₆‐ZnPc and NP‐SC₆‐TiOPc) is demonstrated. The presence of lone electron pairs on S, N, and O atoms of the Pc molecular structures provides the opportunity for Lewis acid–base interactions with under‐coordinated Pb²⁺ sites, leading to efficient defect passivation of the perovskite layer. The tendency of both NP‐SC₆‐ZnPc and NP‐SC₆‐TiOPc to relax on the PbI₂ terminated surface of the perovskite layer is also studied using density functional theory (DFT) calculations. The morphology of the perovskite layer is improved due to employing the Pc passivation strategy, resulting in high‐quality thin films with a dense and compact structure and lower surface roughness. Using NP‐SC₆‐ZnPc and NP‐SC₆‐TiOPc as passivating agents, it is observed considerably enhanced power conversion efficiencies (PCEs), from 17.67% for the PSCs based on the pristine perovskite film to 19.39% for NP‐SC₆‐TiOPc passivated devices. Moreover, PSCs fabricated based on the Pc passivation method present a remarkable stability under conditions of high moisture and temperature levels.

Publication date: March 2021

Source: Nano Energy, Volume 81

Author(s): Chengxi Zhang, Yan-Na Lu, Wu-Qiang Wu, Lianzhou Wang

Publication date: 20 January 2021

Source: Joule, Volume 5, Issue 1

Author(s): So Me Yoon, Hanul Min, Jong Beom Kim, Gwisu Kim, Kyoung Su Lee, Sang Il Seok

The role of methylammonium chloride (MACl) in sequentially deposited bromine (Br)‐free formamidinium lead iodide (FAPbI₃)‐based perovskite is systematically demonstrated to regulate the PbI₂/FAI reaction, tune the phase transition at room temperature, and adjust the PbI₂ residual through an intermediate‐related perovskite decomposition during thermal annealing. The resulting optimized solar cells achieve a remarkable efficiency of 23.1% with considerably improved photostability.

Abstract

So far, the combination of methylammonium bromide/methylammonium chloride (MABr/MACl) or methylammonium iodide (MAI)/MACl is the most frequently used additives to stabilize formamidinium lead iodide (FAPbI₃) fabricated by the sequential deposition method. However, the enlarged bandgap due to the addition of bromide and the ambiguous functions of these additives in lead iodide (PbI₂) transformation are still worth considering. Herein, the roles of MACl in sequentially deposited Br‐free FA‐based perovskites are systematically investigated. It is found that MACl can finely regulate the PbI₂/FAI reaction, tune the phase transition at room temperature, and adjust intermediate‐related perovskite crystallization and decomposition during thermal annealing. Compared to FAPbI₃, the perovskite with MACl exhibits larger grain, longer carrier lifetime, and reduced trap density. The resultant solar cell therefore achieves a champion power conversion efficiency (PCE) of 23.1% under reverse scan with a stabilized power output of 23.0%. In addition, it shows much improved photostability under 100 mW cm⁻² white illumination (xenon lamp) in nitrogen atmosphere without encapsulation.

Highly sensitive sub‐bandgap external quantum efficiency (s‐EQE) spectroscopy for in situ monitoring of the ion dissociation process is developed to reveal the mechanism of intrinsic perovskite material degradation.

Abstract

Ion dissociation has been identified to determine the intrinsic stability of perovskite solar cells (PVSCs), but the underlying degradation mechanism is still elusive. Herein, by combining highly sensitive sub‐bandgap external quantum efficiency (s‐EQE) spectroscopy, impedance analysis, and theoretical calculations, the evolution of defect states in PVSCs during the degradation can be monitored. It is found that the degradation of PVSCs can be divided into three steps: 1) dissociation of ions from perovskite lattices, 2) migration of dissociated ions, and 3) consumption of I⁻ by reacting with metal electrode. Importantly, step (3) is found to be crucial as it will accelerate the first two steps and lead to continuous degradation. By replacing the metal with more chemically robust indium tin oxide (ITO), it is found that the dissociated ions under light soaking will only saturate at the perovskite/ITO interface. Importantly, the dissociated ions will subsequently restore to the corresponding vacancies under dark condition to heal the perovskite and photovoltaic performance. Such shuttling of mobile ions without consumption in the ITO‐contact PVSCs results in harvesting–rest–recovery cycles in natural day/night operation. It is envisioned that the mechanism of the intrinsic perovskite material degradation reported here will lead to clearer research directions toward highly stable PVSCs.

2D hybrid organic–inorganic metal halide perovskites are introduced into the field of semiconducting glasses through deliberate synthetic design of the hybrid structure, resulting in facile and reversible glass–crystalline switching under moderate thermal cycling. Such glasses can find prospective application in, for example, memory, computing, nonlinear optics, communication, catalysis, sensing, and batteries.

Abstract

Crystalline metal halide perovskites (MHPs) have provided unprecedented advances in interdisciplinary fields of materials, electronics, and photonics. While crystallinity offers numerous advantages, the ability to access a glassy state with distinct properties provides unique opportunities to extend the associated structure–property relationship, as well as broaden the application space for MHPs. Amorphous analogs for MHPs have so far been restricted to high pressures, limiting detailed studies and applications. Here, a 2D MHP is structurally tailored using bulky chiral organic cations to exhibit an unusual confluence of exceptionally low melting temperature (175 °C) and inhibited crystallization. The chiral MHP can thus be melt‐quenched into a stable glassy state, otherwise inhibited in the analogous racemic MHP. Facile and reversible switching between glassy and crystalline states is demonstrated for the chiral MHP, each with distinct optoelectronic character, opening new opportunities for applications including, for example nonvolatile memory, optical communication, and neuromorphic computing.

The interface property of perovskite is governed by the surface termination. The combination of ultraviolet photoelectron and metastable‐atom electron spectroscopies is demonstrated as a versatile technique to prove the surface termination. This method is applied to a solution‐processed CH₃NH₃PbI₃ perovskite film to show that the surface is terminated with a layer consisting of CH₃NH₃ and I.

Abstract

The interfaces of a perovskite solar cell significantly influence the charge processes in the cell, which contributes to the device performance with direct implication for surface potential, electronic structure, and chemical reactivity. The properties of the interface are strongly affected by the surface termination. In this work, the combination of ultraviolet photoelectron spectroscopy (UPS) and metastable‐atom electron spectroscopy is demonstrated, to examine the surface termination of a solution‐processed CH₃NH₃PbI₃ perovskite film. The results show that the surface of the CH₃NH₃PbI₃ perovskite film is terminated with a layer consisting of CH₃NH₃ and I. The interface energy level alignment for both occupied and unoccupied levels between CH₃NH₃PbI₃ and C60 is also examined using UPS and low‐energy inverse photoelectron spectroscopy. It turns out that an ideal energy level alignment is established for the electron collection and hole block at the perovskite and C60 interface.