Chen Weijie

Publication date: February 2023

Source: Nano Energy, Volume 106

Author(s): Dilpreet Singh Mann, Sung-Nam Kwon, Pramila Patil, Seok-In Na

Nature Communications, Published online: 06 December 2022; doi:10.1038/s41467-022-35255-9

It is requisite to develop dopant-free hole transporting materials (HTMs) to avoid formidable engineering and instability in perovskite solar cells. Organic HTMs, especially small molecules are easily reproducible. While considering the stability and efficiencies of perovskite solar cells, this review reveals the role of different molecular design strategies applied to small molecule HTMs.

Abstract

Perovskite solar cells (PSCs) have grabbed much attention of researchers owing to their quick rise in power conversion efficiency (PCE). However, long-term stability remains a hurdle in commercialization, partly due to the inclusion of necessary hygroscopic dopants in hole transporting materials, enhancing the complexity and total cost. Generally, the efforts in designing dopant-free hole transporting materials (HTMs) are devoted toward small molecule and polymeric HTMs, where small molecule based HTMs (SM-HTMs) are dominant due to their reproducibility, facile synthesis, and low cost. Still, the state-of-art dopant-free SM-HTM has not been achieved yet, mainly because of the knowledge gap between device engineering and molecular designs. From a molecular engineering perspective, this article reviews dopant-free SM-HTMs for PSCs, outlining analyses of chemical structures with promising properties toward achieving effective, low-cost, and scalable materials for devices with higher stability. Finally, an outlook of dopant-free SM-HTMs toward commercial application and insight into the development of long-term stability PSCs devices is provided.

Grid-connected perovskite solar cells (PSCs) must frequently work under non-ideal voltages. During surges or lightning strikes, they may also experience instantaneous extremely high voltages (IEHVs). IEHVs physically and chemically deteriorate the surface of perovskite films, resulting in increased recombination loss in the PSCs. Surface passivation can make PSCs durable to IEHV and keeps their efficiency after IEHV stress.

Abstract

The electrical stability of perovskite solar cells (PSCs) will play an essential role in their commercialization because field-installed PSCs frequently operate under non-ideal voltages. Particularly, an instantaneous extremely high voltage (IEHVs) from electro-static discharge will be applied to PSCs due to friction in roll-to-roll processes. In addition, lightning strikes and surges from grids are plausible sources of IEHVs to field-installed PSCs. Hence, the effect of IEHVs on PSCs is systematically investigated and a robust device structure is suggested. An IEHV severely deteriorates PSCs by destroying their diode characteristics. Physical and chemical damage from IEHVs to the interface between the perovskite film and buffer layers causes increased recombination losses and series resistance. To reinforce the heterointerface, a well-known surface defect passivation method is adopted, adding excessive PbI₂ to perovskite films. The excessive PbI₂, mainly located at the interface, successfully protects PSCs from IEHV. Moreover, inserting well-established defect passivation layers, C₆₀, and phenethylammonium iodide into the interface of a perovskite film improves the device's stability against IEHV. Therefore, interface defect passivation is viable for stable PSCs against abnormal electrical stress. It is believed that this study will provide fundamental insights for designing electrically reliable PSCs, which is crucial for grid-connected, field-installed energy generation sources.

Highly efficient state-of-the-art perovskite solar cells via a “three birds with one stone” strategy are investigated. This strategy boosts power conversion efficiency and operational stability of the device.

Abstract

Suppressing nonradiative recombination in perovskite solar cells (PSCs) is crucial for increases in their power conversion efficiency and operational stability. Here, it is reported that the synchronous use of a molecule daminozide (DA), as an interlayer and additive to judiciously construct a PTAA:F4TCNQ/DA/perovskite:DA hole-selective heterojunction that diminishes thermionic losses for collecting holes at the buried interface between perovskites and PTAA:F4TCNQ, and reduces defect sites at such buried interfaces as well as in the perovskite film. The proposed “three birds with one stone” strategy significantly promotes charge transport, and both the interface carrier recombination and defect-assisted recombination are suppressed. As a result, a remarkably improved efficiency of 22.15% and an impressive fill factor of 83.92% are achieved with excellent device stability compared to 19.04% of the control device. The two values are the highest records for polycrystalline MAPbI₃-based p-i-n structural PSCs reported to date. The work provides a promising approach of three birds with one stone, employing a functional material for further improvement of PSC performance.

Here, the plant-derived natural green additive l-Theanine (Thea) is selected to improve the crystal quality of the perovskite absorber and obtain high-performance perovskite solar cells (PSCs) with UV/O₃ resistance. Thea significantly alleviates the perovskite phase transition and film decomposition induced by UV/O₃ treatment. This study provides exploratory research for the application of plant-derived green additives in the UV/O₃ resistance field of perovskite photovoltaics.

Abstract

As the efficiency of perovskite solar cell has skyrocketed to as high as 25.7%, their stability has become the biggest obstacle to commercialization. Preliminary analyses suggest that additive engineering may be effective in improving both solar cell efficiency and its stability. Herein, the plant-derived natural green additive of l-Theanine (Thea) is selected to improve the crystal quality of the perovskite absorber and obtain high-performance perovskite solar cells (PSCs) with ultraviolet/ozone (UV/O₃) resistance. The characterization results reveal that the CO group in Thea can effectively inhibit the precipitation of metal Pb⁰, passivate undercoordinated Pb²⁺ ions, and promote the nucleation and crystallization of perovskite. In addition, the combination of the NH group and I⁻ in the form of a hydrogen bond cooperatively reduce the probability of nonradiative recombination of photogenerated carriers and effectively improves the extraction ability of carriers from perovskite absorber. With the cooperation of CO and NH₂ groups in Thea, the champion efficiency is improved from 22.29% in the control device to 24.58%. More importantly, Thea significantly alleviates the perovskite phase transition and film decomposition induced by UV/O₃ treatment. The study provides exploratory research for the application of plant-derived green additives in the UV/O₃ resistance field of perovskite photovoltaics.

Understanding and minimizing performance losses in all-perovskite tandem solar cells is crucial to accelerate advancements toward commercialization, yet challenging, since the individual subcells cannot be assessed directly in a monolithic interconnection. Thorough subcell-selective characterizations provide crucial feedback to improve monolithic all-perovskite tandem solar cells with optimized subcells and a lossless interconnect toward their true material potential.

Abstract

Understanding performance losses in all-perovskite tandem photovoltaics is crucial to accelerate advancements toward commercialization, especially since these tandem devices generally underperform in comparison to what is expected from isolated layers and single junction devices. Here, the individual sub-cells in all-perovskite tandem stacks are selectively characterized to disentangle the various losses. It is found that non-radiative losses in the high-gap subcell dominate the overall recombination in the baseline system, as well as in the majority of literature reports. Through a multi-faceted approach, the open-circuit voltage (V _OC) of the high-gap perovskite subcell is enhanced by 120 mV. Employing a novel (quasi) lossless indium oxide interconnect, this enables all-perovskite tandem solar cells with 2.00 V V _OC and 23.7% stabilized efficiency. Reducing transport losses as well as imperfect energy-alignments boosts efficiencies to 25.2% and 27.0% as identified via subcell selective electro- and photo-luminescence. Finally, it is shown how, having improved the V _OC, improving the current density of the low-gap absorber pushes efficiencies even further, reaching 25.9% efficiency stabilized, with an ultimate potential of 30.0% considering the bulk quality of both absorbers measured using photo-luminescence. These insights not only show an optimization example but also a generalizable evidence-based optimization strategy utilizing optoelectronic sub-cell characterization.

The most widely used lead-free perovskites are based on tin, however shortcomings in their stability prevent further developments of this technology. State-of-the-art avenues to overcome these shortcomings include composition engineering, additive engineering, and interface engineering. This review reveals a set of design rules for the development of stable tin-halide perovskite solar cells by collectively analyzing a decade of materials chemistry.

Abstract

Substituting toxic lead with tin (Sn) in perovskite solar cells (PSCs) is the most promising route toward the development of high-efficiency lead-free devices. Despite the encouraging efficiencies of Sn-PSCs, they are still yet to surpass 15% and suffer detrimental oxidation of Sn(II) to Sn(IV). Since their first application in 2014, investigations into the properties of Sn-PSCs have contributed to a growing understanding of the mechanisms, both detrimental and complementary to their stability. This review summarizes the evolution of Sn-PSCs, including early developments to the latest state-of-the-art approaches benefitting the stability of devices. The degradation pathways associated with Sn-PSCs are first outlined, followed by describing how composition engineering (A, B site modifications), additive engineering (oxidation prevention), and interface engineering (passivation strategies) can be employed as different avenues to improve the stability of devices. The knowledge about these properties is also not limited to PSCs and also applicable to other types of devices now employing Sn-based perovskite absorber layers. A detailed analysis of the properties and materials chemistry reveals a clear set of design rules for the development of stable Sn-PSCs. Applying the design strategies highlighted in this review will be essential to further improve both the efficiency and stability of Sn-PSCs.

The morphology and the molecular crystallinity of an all-polymer blend have been finely modulated by using a ternary strategy. Benefiting from the favorable miscibility of the two acceptors and the higher regularity of PY-DT, the ternary blend features a well-defined fibrillar morphology and improved molecular ordering, which leads to an efficiency of 18.03%, representing the highest efficiency for all-polymer solar cells thus far.

Abstract

Although all-polymer solar cells (all-PSCs) show great commercialization prospects, their power conversion efficiencies (PCEs) still fall behind their small molecule acceptor-based counterparts. In all-polymer blends, the optimized morphology and high molecular ordering are difficult to achieve since there is troublesome competition between the crystallinity of the polymer donor and acceptor during the film-formation process. Therefore, it is challenging to improve the performance of all-PSCs. Herein, a ternary strategy is adopted to modulate the morphology and the molecular crystallinity of an all-polymer blend, in which PM6:PY-82 is selected as the host blend and PY-DT is employed as a guest component. Benefiting from the favorable miscibility of the two acceptors and the higher regularity of PY-DT, the ternary matrix features a well-defined fibrillar morphology and improved molecular ordering. Consequently, the champion PM6:PY-82:PY-DT device produces a record-high PCE of 18.03%, with simultaneously improved open-circuit voltage, short-circuit current and fill factor in comparison with the binary devices. High-performance large-area (1 cm²) and thick-film (300 nm) all-PSCs are also successfully fabricated with PCEs of 16.35% and 15.70%, respectively.Moreover, 16.5 cm² organic solar module affords an encouraging PCE of 13.84% when using the non-halogenated solvent , showing the great potential of “Lab-to-Fab” transition of all-PSCs.

Publication date: February 2023

Source: Nano Energy, Volume 106

Author(s): You-Sun Lee, Sera Jeon, Dabin Kim, Dong-Min Lee, Dohyung Kim, Sang-Woo Kim

Publication date: February 2023

Source: Nano Energy, Volume 106

Author(s): Hanrui Xiao, Chuantian Zuo, Lixiu Zhang, Wenhua Zhang, Feng Hao, Chenyi Yi, Fangyang Liu, Huile Jin, Liming Ding

Publication date: February 2023

Source: Nano Energy, Volume 106

Author(s): Sungwoo Jung, Yongjoon Cho, Yutong Ji, Jiyeon Oh, Geunhyung Park, Wonjun Kim, Seonghun Jeong, Sang Myeon Lee, Shanshan Chen, Youdi Zhang, Changduk Yang

The processing environment of perovskite solar cells in pure inert gas significantly increases the practical production cost. The thermal annealing atmosphere of quasi-2D perovskite films can affect the self-assembly behavior of bulky organic cations, which modulates the phase distribution of 2D species with different n values. Air annealing modulates the crystallization process, resulting in high-quality quasi-2D perovskite films.

Although the photovoltaic performance of perovskite solar cells (PSCs) has reached commercial standards, the processing environment of a purely inert gas atmosphere significantly increases the cost of installing manufacturing facilities in actual manufacturing plants and hinders their commercial mass production. The thermal annealing condition of quasi-2D perovskite films can significantly affect the self-assembly behavior of bulky organic cations, which modulates the phase distribution of 2D species with different n values. Herein, the properties of quasi-2D perovskite annealing in a N₂ versus an environment representative of industrial conditions, i.e., open-air with 30% humidity are compared. It is found that the crystallization process of perovskites is regulated by air annealing, leading to high-quality 2D perovskite films with reduced trap density, well-aligned phase distribution, and released residual strain. The resulting devices prepared by air annealing achieve a power conversion efficiency of 18% with high reproducibility and suppressed voltage loss. The enhanced thermal stability of the air-annealed films is proved by in situ grazing incidence wide-angle X-ray scattering. The combination of air annealing and simple planar structures would facilitate the mass production of PSCs.

Indium zinc oxide is used for the interfacial layer to minimize ohmic resistance as well as the transparent conducting oxide in the semitransparent solar cell. The wide-bandgap (1.71 eV) perovskite solar cell delivers an efficiency to 19.26%, and a four-terminal perovskite/CdTe tandem solar cell and two-terminal perovskite/silicon tandem solar cell achieve efficiencies of 22.59% and 26.34%.

To overcome the efficiency limit of perovskite single-junction solar cells, it is vital to develop various types of tandem solar cells. Especially, wide-bandgap (WBG) perovskite solar cells (PSCs) have played an important role in high-efficiency tandem solar cells. Herein, an indium zinc oxide-based interfacial structure is developed to improve the performance of a WBG PSC and used as the transparent electrode for semitransparent (ST) PSCs. This approach minimizes ohmic contact between the electron-transport layer and metallic electrode, which also accelerates electron transfer and suppresses trap-assisted carrier recombination. As a result, the WBG PSC (1.71 eV) shows the best power conversion efficiency of 19.26% and improves operational stability. When the optimized ST-PSC is used as the ST-top cell, perovskite/CdTe four-terminal and perovskite/silicon (double-side polished) two-terminal tandem solar cells achieve a maximum efficiency of 22.59% and 26.34%, respectively.

A pure 2D perovskite and a 2D/3D perovskite mixture are formed on the MAPbI₃ film after surface treatments. This bilayer structure gives rise to a reduced trap density and an improved energy level alignment between the perovskite and HTL. Via simulation, these two positive effects are verified to be the cause of V _oc and fill factor (FF) improvement, respectively.

Abstract

Organic–inorganic perovskite solar cells (PSCs) have achieved great attention due to their expressive power conversion efficiency (PCE) up to 25.7%. To improve the photovoltaic performance of PSCs, interface engineering between the perovskite and hole transport layer (HTL) is a widely used strategy. Following this concept, benzyl trimethyl ammonium chlorides (BTACls) are used to modify the wet chemical processed perovskite film in this work. The BTACl-induced low dimensional perovskite is found to have a bilayer structure, which efficiently decreases the trap density and improves the energy level alignment at the perovskite/HTL interface. As a result, the BTACl-modified PSCs show an improved PCE compared to the control devices. From device modeling, the reduced charge carrier recombination and promoted charge carrier transfer at the perovskite/HTL interface are the cause of the open-circuit (V _oc) and fill factor (FF) improvement, respectively. This study gives a deep understanding for surface modification of perovskite films from a perspective of the morphology and the function of enhancing photovoltaic performance.

Nature Communications, Published online: 02 December 2022; doi:10.1038/s41467-022-35092-w

High-dimensionality Ruddlesden–Popper perovskites are investigated by considering 2D domains formation, thickness, distribution along the active layer, and the solar cells' performance. Combined advanced structural/morphological analyses are performed, finding that the 2D phase segregates at the interface with the top electrode, acting as a barrier for charge extraction, overall decreasing the short-circuit current.

High-dimensionality Ruddlesden–Popper (RP) perovskites, with general formula R₂A _n−1B _n X_3n+1 and high n values (n ≥ 5), are regarded as viable materials for photovoltaics because they feature higher stability if compared to the 3D perovskite, i.e., ABX₃, still maintaining good charge absorption and transport properties. When integrated into the actual solar cells, however, scattered, sometimes contradictory results are reported among different deposition procedures and different cations, especially for higher n resulting in not uniform morphology and mixed composition. Herein, high-dimensionality RP perovskites with n = 1, 4, 10, 20, and 40 values are systematically investigated considering the interplay between the formation of 2D domains, their distribution along the active layer, the active layer thickness, and the solar cells’ performance. Given the complexity of the investigated system, combined advanced structural/morphological analyses are performed to explain solar cells’ performance, finding that the 2D phase segregates at the interface with the top electrode, acting as a barrier for charge extraction, overall decreasing the short-circuit current (J _sc). Reducing the relative amount of bulky alkylammonium cation with respect to the methylammonium, the 2D perovskite overlayer is intentionally decreased leading to a recovery of the J _sc values, corroborating the hypothesis.

The ZnO–SnO₂ bilayer deposited by atomic layer deposition does not only have a good band alignment with the wide-bandgap perovskite and suppressed nonradiative recombination, but also an efficient passivation to crystalline silicon (c-Si) surface, thus making it a competitive candidate for preparing large-size perovskite/c-Si monolithic tandem cells to achieve a high open-circuit voltage.

High-quality electron transport layer (ETL) is a prerequisite for high-performance wide-bandgap mixed-halide perovskite solar cells (PSCs), which is critical for efficient perovskite/silicon tandem solar cells. Herein, an atomic layer deposited ZnO–SnO₂ bilayer ETL for wide-bandgap PSCs is reported, featuring a high uniformity and conformality over a large area. The ZnO–SnO₂ bilayer shows a matched band alignment with wide-bandgap perovskite for efficient electron extraction and transport, with a lower nonradiative recombination. As a result, a champion power conversion efficiency of 18.1% is achieved on the ZnO–SnO₂ bilayer-based wide-bandgap PSCs featuring an ultrahigh open-circuit voltage (V _oc) of 1.233 V, which is the highest value for wide-bandgap PSCs without any surface passivation. In addition, the atomic layer deposition ZnO–SnO₂ bilayer exhibits very good surface passivation and conformality on crystalline silicon surfaces, which makes it attractive to be applied for perovskite/silicon tandem solar cells with a higher V _oc and textured surfaces.

Organic chloride additives are explored in methylammonium-free perovskite solar cells. The additive with short ammonium chain (n = 2) offers improved morphology, i.e., larger grains and reduced grain boundaries, but evaporates during thermal annealing, while additives with longer chain (n = 3 and n = 4) moderately influence the morphology but passivate the grain boundaries of perovskite layers, enabling higher photovoltaic performance.

The use of chloride additives to achieve high performance in perovskite solar cells (PSCs) is extensively reported in perovskite research. However, few studies are dedicated to understanding and comparing the underlying effects of ammonium cations in organic chloride additives. Herein, the effect of ACl additives (A: ethylammonium [EA], propylammonium [PA], butylammonium [BA]), with increasing ammonium size, on the performance of MA-free Cs_0.1FA_0.9PbI₃ PSCs is investigated. The obtained results indicate that Cl promotes the growth of large grains, while organic cations facilitate grain favorable orientation. Moreover, it is observed that EACl evaporates during thermal annealing despite the similar ionic size of EA to that of FA, which would lead to A-site incorporation. However, PACl and BACl remain in final films, passivating grain boundaries. Moreover, by using double BACl–EACl additive, the films show considerably increased grain dimensions, passivated grain boundaries, and longer carrier lifetime. As a result, planar PSCs based on engineered Cs_0.1FA_0.9PbI₃ layers achieve higher power conversion efficiency of 20.98%, 20.07%, 19.64%, and 19.03% for BACl–EACl, BACl, PACl, and EACl, respectively, compared to 17.58%-efficiency for control PSCs. Moreover, this additive-based approach also increases moisture resistance of the Cs_0.1FA_0.9PbI₃ films, which consequently enhances the device ambient stability.

Nature Materials, Published online: 01 December 2022; doi:10.1038/s41563-022-01399-8