zhezhi huang

Pb source plays a critical role in determining the solution-processed perovskite film's crystallization, structural, and optoelectronic properties. This review comprehensively summarized the current understanding and advanced development of Pb source engineering in perovskite solar cells (PSCs), which is hoped to motivate more ideas toward further development of high-performing PSCs with controllable, sustainable, and large-scale production.

Organic–inorganic hybrid Pb halide perovskites have gained much attention as the most promising next generation photovoltaics, and the certificated power conversion efficiency of perovskite solar cells (PSCs) has recently reached 25.5%. For the typical solution-processed film, the features of solutes in the precursor solution greatly influence the characteristics of the deposited film. While for Pb-based perovskites, PbI₆ octahedral is the key component of the perovskite framework, and thus the Pb source particularly plays a significant role in determining the crystallization, structural, and optoelectronic properties of the solution-processed perovskite film. In this review, the state-of-the-art studies that focus on disclosing the key role of Pb source in the performance improvement of PSCs are systematically summarized. In addition, a comprehensive discussion on the effect of various Pb sources (e.g., Pb halides, Pb salts, Pb chalcogens, metallic Pb, and recycled Pb compounds) on the crystallization kinetics and photovoltaic characteristics of perovskite film is given. Also, the significant role of Pb source in producing large-scale PSCs in a controllable and sustainable manner is highlighted. This review is expected not only to put steps forward for the future commercialization of PSCs, but also to inspire more ideas in many other optoelectronic devices regarding raw material engineering.

The acetate (Ac⁻) ions occupy lattice sites in the process of nucleation and crystallization of the perovskite, which effectively promotes the entry of formamidinium (FA⁺) into the lead halide octahedra to stabilize the α-phase of FAPbI₃. The solar cell based on the α-FAPbI₃ film presented a power conversion efficiency of 21.24% with negligible hysteresis.

The α-phase formamidinium lead triiodide (α-FAPbI₃)-based perovskite solar cells (PSCs) exhibit potential high efficiency due to their narrow bandgap, but the fabrication of a stable α-FAPbI₃ film still is challenging. Herein, a strategy is devised to achieve a stable α-FAPbI₃ film, in which lead acetate (PbAc₂) is added to the perovskite precursor solution. The Ac⁻ ions are involved in the formation of the lead halide octahedra, which effectively promotes the entry of FA⁺ into the lead halide octahedra to stabilize the α-phase of FAPbI₃. Furthermore, the Ac⁻ will gradually leave during the annealing process, thus the addition of PbAc₂ cannot introduce other components in the FAPbI₃. The crystallinity and crystal orientation of the perovskite films are also improved by the PbAc₂ additive to obtain low trap density films, leading to an increase in charge carrier collection. The champion solar cell based on the α-FAPbI₃ film presented a power conversion efficiency (PCE) of 21.24% with negligible hysteresis. After 500 h of storage under ambient conditions, the devices still maintained more than 90% of their initial efficiency.

Formamidinium (FA)-based additives in precursor solutions suppressed the formation of the undesired δ phase during the crystallization of FAPbI₃ perovskites, and heat-induced permeation of 4-tert-butylbenzylammonium iodide (tBBAI) into inner perovskite grains stabilized the α structure. Solar cells assembled from this material exhibited improved power conversion efficiency and stability.

Abstract

α-Formamidinium lead iodide (α-FAPbI₃) is one of the most promising candidate materials for high-efficiency and thermally stable perovskite solar cells (PSCs) owing to its outstanding optoelectrical properties and high thermal stability. However, achieving a stable form of α-FAPbI₃ where both the composition and the phase are pure is very challenging. Herein, we report on a combined strategy of precursor engineering and grain anchoring to successfully prepare methylammonium (MA)-free and phase-pure stable α-FAPbI₃ films. The incorporation of volatile FA-based additives in the precursor solutions completely suppresses the formation of non-perovskite δ-FAPbI₃ during film crystallization. Grains of the desired α-phase are anchored together and stabilized when 4-tert-butylbenzylammonium iodide is permeated into the α-FAPbI₃ film interior via grain boundaries. This cooperative scheme leads to a significantly increased efficiency close to 21 % for FAPbI₃ perovskite solar cells. Moreover, the stabilized PSCs exhibit improved thermal stability and maintained ≈90 % of their initial efficiency after storage at 50 °C for over 1600 hours.

Nature Energy, Published online: 01 November 2021; doi:10.1038/s41560-021-00923-5

Here comes the sun: A comprehensive Review on the defect passivation of perovskite solar cells (PSCs) from a molecule design viewpoint is reported. First, the influence of defects on the photovoltaic parameters of PSCs is demonstrated. Then, the structure-performance correlation of the passivation molecule is investigated. Finally, a perspective on future trends of passivation strategies is provided.

Abstract

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.

Nature Communications, Published online: 23 August 2021; doi:10.1038/s41467-021-25407-8

Herein, a novel approach, a reactive post-treatment technique using guanidine acetate, to enhance the grain size and introduce a secondary phase simultaneously is applied in a perovskite interface. The enhanced grain size helps to reduce the defect densities at the grain boundaries, while the secondary phases make the perovskite surface more n-type, enhancing the device efficiency and stability.

Organic–inorganic lead halide perovskites (OIHPs) have emerged as promising materials for next-generation photovoltaics. However, performance improvements in the perovskite-based device are still limited due to defects that exist more intensively on the surface as well as grain boundaries (GBs) and mismatching energy levels at the interface. Herein, a reactive post-treatment process (RPP) using guanidine acetate (GA) is adopted to address defects and interfacial energy level matching at the perovskite surface. The RPP with GA (GA-RPP) results in the formation of an improved perovskite layer with large grain size and low GB density, leading to the formation of secondary phases on the perovskite surface with appropriate energy levels, resulting in reduced defect density and charge recombination. Furthermore, density functional theory analysis reveals that the Pb-rich secondary phase could improve the conduction of electrons at the perovskite interface. Therefore, the GA-RPP-based perovskite-based solar cell (PSC) shows enhanced performance with 20.4% efficiency and long-term stability.

Siding with another: Side-chain-engineered benzodithiopheneammonium (BDT) molecules are shown to be excellent ligands for layered halide perovskites. Tailoring the side chain was found to be crucial in order to obtain crystalline structures whereas the highly conjugated BDT core and its sulfur-containing moieties afford enhanced photovoltaic charge collection, conversion efficiency, and stability when juxtaposed with conventional phenylethylammonium-based layered perovskites.

Abstract

Incorporating extended pi-conjugated organic cations in layered lead halide perovskites is a recent trend promising to merge the fields of organic semiconductors and lead halide perovskites. Herein, we integrate benzodithiophene (BDT) into Ruddlesden–Popper (RP) layered and quasi-layered lead iodide thin films (with methylammonium, MA) of the form (BDT)₂MA _n−1Pb _n I_3n+1. The importance of tuning the ligand chemical structure is shown as an alkyl chain length of at least six carbon atoms is required to form a photoactive RP (n=1) phase. With N=20 or 100, as prepared in the precursor solution following the formula (BDT)₂MA _N−1Pb _N I_3N+1, the performance and stability of devices surpassed those with phenylethylammonium (PEA). For N=100, the BDT cation gave a power conversion efficiency of up to 14.7 % vs. 13.7 % with PEA. Transient photocurrent, UV photoelectron spectroscopy, and Fourier transform infrared spectroscopy point to improved charge transport in the device active layer and additional electronic states close to the valence band, suggesting the formation of a Lewis adduct between the BDT and surface iodide vacancies.

A new urea-ammonium thiocyanate (UAT) molten salt was introduced as the additive in all-inorganic cesium lead triiodide solar cell, as a modification strategy to fully release and exploit coordination activities of SCN⁻ to deposit high-quality CsPbI₃ film. Thus, the UAT-based devices can provide an encouraging PCE up to 20.08 % with excellent operational stability of over 1000 h.

Abstract

Besides widely used surface passivation, engineering the film crystallization is an important and more fundamental route to improve the performance of all-inorganic perovskite solar cells. Herein, we have developed a urea-ammonium thiocyanate (UAT) molten salt modification strategy to fully release and exploit coordination activities of SCN⁻ to deposit high-quality CsPbI₃ film for efficient and stable all-inorganic solar cells. The UAT is derived by the hydrogen bond interactions between urea and NH₄ ⁺ from NH₄SCN. With the UAT, the crystal quality of the CsPbI₃ film has been significantly improved and a long single-exponential charge recombination lifetime of over 30 ns has been achieved. With these benefits, the cell efficiency has been promoted to over 20 % (steady-state efficiency of 19.2 %) with excellent operational stability over 1000 h. These results demonstrate a promising development route of the CsPbI₃ related photoelectric devices.

Nature Energy, Published online: 08 April 2021; doi:10.1038/s41560-021-00802-z

The defect landscape in metal–halide perovskites is described. This Review highlights the promise of the compounds, explains defects as an outstanding problem, and discusses the background of defects, methods to probe defects, and various passivation strategies used successfully to date.

The rise of hybrid metal–halide perovskites as potential solar energy materials has revolutionized research on next‐generation solar cells. According to recent studies, the rationale behind such success is the rich defect physics of materials. Studies on the origin of different types of prevailing defects, their formation, and mechanism of defect passivation have hence become decisive avenues. Herein, the possible origins of defects and different defect analysis techniques in hybrid halide perovskites are discussed. While initiating the discussion with the archetypal methylammonium lead halide, perovskites beyond the conventional ABX₃ structure are included. In this direction, some major advancements to date on defect formation in the bulk of hybrid halide perovskites, at the grains and grain boundaries, are summarized. Numerous effective methods to passivate the defects and the adverse effect of defects on device efficiency are further highlighted. Hence, the prospect of defect engineering in perovskite materials is pointed toward improving the power conversion efficiency and long‐term stability of perovskite solar cells (PSCs). The discussion rightfully addresses that the in‐depth exploration of defect engineering is anticipated to have a gigantic impact toward the achievement of predicted efficiency in metal–halide PSCs.

Passive attack: The α‐FAPbI₃ perovskite layer in a solar cell is stabilized without deteriorating the spectral features by passivating with 2,3,4,5,6‐pentafluorobenzyl phosphonic acid (PFBPA). High‐quality perovskite solar cells with an improved efficiency of 22.25 % was achieved with excellent moisture stability maintaining >90 % of its initial efficiency at high humidity levels.

Abstract

Perovskite solar cells (PSCs) have shown great promise for photovoltaic applications, owing to their low‐cost assembly, exceptional performance, and low‐temperature solution processing. However, the advancement of PSCs towards commercialization requires improvements in efficiency and long‐term stability. The surface and grain boundaries of perovskite layer, as well as interfaces, are critical factors in determining the performance of the assembled cells. Defects, which are mainly located at perovskite surfaces, can trigger hysteresis, carrier recombination, and degradation, which diminish the power conversion efficiencies (PCEs) of the resultant cells. This study concerns the stabilization of the α‐FAPbI₃ perovskite phase without negatively affecting the spectral features by using 2,3,4,5,6‐pentafluorobenzyl phosphonic acid (PFBPA) as a passivation agent. Accordingly, high‐quality PSCs are attained with an improved PCE of 22.25 % and respectable cell parameters compared to the pristine cells without the passivation layer. The thin PFBPA passivation layer effectively protects the perovskite layer from moisture, resulting in better long‐term stability for unsealed PSCs, which maintain >90 % of the original efficiency under different humidity levels (40–75 %) after 600 h. PFBPA passivation is found to have a considerable impact in obtaining high‐quality and stable FAPbI₃ films to benefit both the efficiency and the stability of PSCs.