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Nature Energy, Published online: 14 September 2020; doi:10.1038/s41560-020-00687-4

The incorporation of various lanthanides ions in perovskite films (perovskite solar cells (PSCs)) and Ce³⁺ doping achieves the best performance, with a champion power conversion efficiency of 21.67% in contrast to 18.50% for pristine PSCs and outstanding long‐term and UV stability that originates from special Ce³⁺/Ce⁴⁺ redox pair and the unique 4f‐5d absorption in the UV region.

Abstract

Since Yan's work, incorporation of some lanthanide elements, such as Eu and Nd, into MAPbI₃ layer has been proven to be a powerful strategy on improving the permanence of the perovskite solar cells (PSCs). However, a comprehensive configuration has not been given for different lanthanide elements doping while the mechanism has not been clarified. Herein, the incorporation of various lanthanides ions (Ln³⁺ = Ce³⁺, Eu³⁺, Nd³⁺, Sm³⁺, or Yb³⁺) into perovskite films to largely enhance the performance of PSCs is presented. Arising from the enlarged grain size and crystallinity of perovskite film upon Ln³⁺ ions doping, the efficiency and stability of PSCs are significantly improved. Extraordinarily, PSCs with Ce³⁺ doping achieve the best performance, with a champion power conversion efficiency (PCE) of 21.67% in contrast to 18.50% for pristine PSCs, and outstanding long‐term and UV irradiation stability. Such high performance of PSCs after Ce³⁺ doping originates from special Ce³⁺/Ce⁴⁺ redox pair and the unique 4f‐5d absorption in the UV region. Finally, the flexible PSCs with low‐temperature preparation are explored. Considering the richer deposition of cerium element in the earth and lower price, the findings may provide new opportunities for developing low‐cost, highly efficient, air/UV stable, and flexible PSCs.

Sodium dodecyl benzene sulfonate (SDBS) is used as a multifunctional chemical additive for efficient and stable planar fully air‐processed perovskite solar cells (PSCs). The introduction of SDBS can promote the preferential growth of crystal orientation, reduce defects, inhibit the migration of iodide ions, enhance the built‐in potential, and improve the water resistance of perovskite films.

The device performance of organic–inorganic hybrid halide perovskite solar cells (PSCs) is highly dependent on the quality of perovskite layer. Herein, a multifunctional chemical additive strategy is reported to simultaneously improve the efficiency and stability of fully air‐processed PSCs. The planner methylammonium lead trihalide (MAPbI₃)‐based PSCs incorporating sodium dodecyl benzene sulfonate (SDBS) exhibit a champion power conversion efficiency (PCE) of 19.20% and negligible hysteresis, which is one of the top efficiencies of MAPbI₃‐based PSCs made in air. The increased efficiency is due to the reduction of defects and inhibition of ion migration in the perovskite films. Furthermore, the enhancement of device performance and stability can also be ascribed to highly preferred and efficient perovskite crystals protecting the perovskite films from humidity. The corresponding unencapsulated device retains 92.34% of its initial efficiency after 90 days (>2100 h) storage in air and maintains 85.20% of its original PCE after being exposed to 85 °C for 27 h. The results indicate that SDBS is a promising chemical additive to enhance the performance of air‐processed PSCs for future applications.

Pinhole‐free perovskite films with large grains are fabricated in ambient air by a spinning–bathing–spinning method. The effects of moisture on the formation of I‐dominant grain and Cl‐enriched boundaries and surfaces in the perovskite films are revealed, which enable the air‐processed perovskite solar cells with a high efficiency of more than 20%.

Metallic halide perovskite films are usually fabricated in inert environment due to their high sensitivity to moisture and oxygen. However, the fabrication process in the strictly controlled environment is not economical for mass production. Therefore, the fabrication of high‐quality perovskite films in ambient air is more practical for optoelectronic devices. Herein, a spinning–bathing–spinning (SBS) method is demonstrated to deposit pinhole‐free perovskite films with large grains in ambient air for solar cells. The effect of moisture on the rapid crystallization and grain coarsening can be suppressed using this SBS method. Furthermore, the moisture is found to encourage the halogen separation in the perovskite films when using PbI₂–PbCl₂ as the lead halide precursor, resulting in the formation of I‐dominant perovskite grains and Cl‐enriched boundaries and surface in the films. The Cl‐enriched grain boundaries and film surface, which mainly originate from the confined methylammonium chloride (MACl), can passivate defects and prevent further damage from moisture and oxygen. This spontaneous inner‐to‐outside passivation enables the air‐processed perovskite solar cells with the high power conversion efficiencies of more than 20% and improved stability.

A bipolar organic material 1,4‐bis(perfluorophenyl)‐2,5‐di(pyridin‐4‐yl)‐1,4‐dihydropyrrolo [3,2‐b] pyrrole (PFPPY) is utilized to passivate perovskite surface and boundary defects via solvent engineering approach, generating an impressive power conversion efficiency (PCE) of 19.62% and greatly enhanced stability.

At the device operating conditions, defects such as interstitials, vacancies, and impurities at the grain boundary and surface of photoactive layer have great impact on the power conversion efficiency (PCE) and device stability. To better passivate the surface and boundary defects, and further enhance the PCE and device stability, herein, a bipolar organic material termed 1,4‐bis(perfluorophenyl)‐2,5‐di(pyridin‐4‐yl)‐1,4‐dihydropyrrolo [3,2‐b] pyrrole (PFPPY) is introduced as modifier in antisolvent. The effects of PFPPY on perovskite film quality, photovoltaic performance, and charge transfer properties are systematically investigated. Under the optimized conditions, the PFPPY‐treated device shows an impressive PCE of 19.62%, which is 12% higher than the reference device (17.59%), and greatly enhanced stability, maintaining 95% of its initial efficiency under room temperature (RT) and relative humidity (RH) 30% condition for 650 h without encapsulation.

The latest research advancements of all‐inorganic perovskite solar cells (PSCs) are summarized systematically and discussed deeply from the perspective of phase stability, effective inorganic charge transport materials, device structures, and interfacial engineering.

The large‐scale commercial application of organic–inorganic hybrid perovskite solar cells (PSCs) based on organic hole transport material (HTM) is still hindered by poor long‐term operational stability, although a certified record power conversion efficiency (PCE) as high as 25.2% can be achieved. In the recent several years, all‐inorganic PSCs have received tremendous attention due to their superb thermal and moisture stability and considerable progresses have been witnessed. Herein, the recent advancements of all‐inorganic PSCs are reviewed comprehensively. First, the recent progresses of the strategies for stabilizing the black phase of inorganic perovskites through either increasing tolerance factor or enhancing the energy barrier of phase transition from black to yellow phase are summarized and discussed. Second, the deposition and growth techniques of inorganic perovskite films are discussed. Third, the effective inorganic HTMs in normal all‐inorganic PSCs are described. Fourth, HTM‐free normal all‐inorganic PSCs are discussed. Afterward, the effective inorganic electron transport materials in inverted all‐inorganic PSCs are discussed. Subsequently, the advancements of interface engineering for increasing the PCE and stability of all‐inorganic PSCs are reviewed. Finally, a brief summary and outlook are presented to push up the PCE of all‐inorganic PSCs to over 20% in the near future.

A solution‐processed calcium acetylacetone film is used as an electron‐selective contact in crystalline silicon solar cells to reduce the rear‐side contact resistance loss. Combined with an intrinsic amorphous silicon passivating layer, a stable power conversion efficiency of 21.6% is realized with full area rear contact.

Abstract

Crystalline silicon (c‐Si) solar cells featuring carrier‐selective passivating contacts have become a prominent path to develop highly efficient photovoltaic devices. Development of electron‐selective materials that can provide excellent surface passivation and low contact resistivity to c‐Si substrates while presenting good environmental stability is crucial for practical implementation. Here, an easy approach is demonstrated to achieve low resistivity Ohmic contacts between slightly doped n‐type c‐Si and aluminum electrodes via simple spin‐coating of metal acetylacetone (MAcac) film on a c‐Si surface. Contact resistivity of 1.3 mΩ cm² (18.2 mΩ cm² with an a‐Si:H(i) passivating layer) is realized when a thin calcium acetylacetone (CaAcac) interlayer is introduced between c‐Si and Al. An n‐Type c‐Si solar cell with a full area rear a‐Si:H(i)/CaAcac/Al electron‐selective contact is demonstrated with a power conversion efficiency of 21.6%. This work not only demonstrates an approach to develop highly efficient n‐type c‐Si solar cells with effective electron‐selective passivating contacts, but also contributes toward accomplishing a simplified fabrication process for photovoltaic devices, from vacuum to solution processing.

High‐performance quasi‐2D perovskite solar cells (PVSCs) are demonstrated via heat–light co‐treatment. The optimized quasi‐2D PVSC presents a maximum power conversion efficiency of 18.24% with excellent stability. The underlying mechanism of the light and heat co‐treatment in improving the device performance lies in its synergistic effect in reducing the trap states and improving the charge transport.

Abstract

2D perovskite solar cells (2D PVSCs) show good stability for commercialization. However, their power conversion efficiency (PCE) is relatively low. In this work, a post‐treatment strategy by simultaneously applying light and heat to quasi‐2D PVSCs, obtaining a record PCE of 18.24% is developed. It is found that heat‐treating PVSCs in the dark slightly increases the device performance over time at temperatures below 75 °C, whereas the performance deteriorates rapidly at temperatures above 100 °C. Upon illumination, the device efficiency is significantly improved, particularly when the thermal‐treatment temperature is increased to 100 °C. A comprehensive carrier dynamic study reveals that the enhanced performance can be attributed to the reduced quasi‐2D perovskite defect states and improved charge collection. In addition, this strategy enables better stability, and an unencapsulated device can retain 90% of its original PCE after 1340 h of direct exposure to air with a humidity of 50 ± 5%. Thus, the strategy paves the way for the commercialization of quasi‐2D PVSCs.

The T2‐CNORH organic passivation layer (OPL) is used to obtain low energy loss organic photovoltaics. The T2‐CNORH‐deposited PM6:Y6 device exhibits a power conversion efficiency (PCE) of 15.5% with low non‐radiative energy loss (0.203 eV). Furthermore, the OPL improves various photoactive layer systems with a best PCE of 16.4% for the PM6:Y7 system.

Abstract

Despite the tremendous development of various high‐performing photoactive layers in organic photovoltaic (OPVs) cells, improving their performance remains the most important challenge in the field. Here, an effective and compatible strategy (i.e., the concept of vacuum deposition of an organic passivation layer (OPL) on the photoactive layer) is presented to enhance the efficiency of the state‐of‐the‐art photoactive systems, where easy‐deposition processable T2‐ORH and T2‐CNORH OPLs are used. After the deposition process, T2‐ORH forms 2D‐like edge‐on crystalline structure, and the 3D‐like face‐on crystalline growth is induced in T2‐CNORH. Resulting from its relatively higher crystalline features and increased wettability with the cathode interfacial material, the performance of T2‐CNORH‐deposited OPVs with both small and the scaled‐up areas surpass devices without OPL and with T2‐ORH. Experimental studies are conducted linking conductivity, electroluminescence quantum efficiency, carrier transport, and recombination dynamics to find the reasons for the performance difference. Furthermore, by applying the T2‐CNORH to other photoactive platforms, the efficiencies are enhanced by 4.4–9.0% relative to those of the corresponding control devices; an optimal 16.4% efficiency is achieved, which validates its great applicability for photoactive layers that will be developed in the near future.

Metal halide perovskite (MHP)‐based tandem solar cells, including MHP/silicon, MHP/CuInGa, MHP/organic photovoltaic, MHP/quantum dot, and all‐perovskite tandem cells, which are boosting the development of cost‐effective and high‐performance, next‐generation solar cells than can compete with fossil fuels, are reviewed.

Abstract

Metal halide perovskite (MHP)‐based tandem solar cells are a promising candidate for use in cost‐effective and high‐performance solar cells that can compete with fossil fuels. To understand the research trends for MHP‐based tandem solar cells, a general introduction to single‐junction and multiple‐junction MHP solar cells and the configuration of tandem devices is provided, along with an overview of the recent progress regarding various MHP‐based tandem cells, including MHP/crystalline silicon, MHP/CuInGaS, MHP/organic photovoltaic, MHP/quantum dot, and all‐perovskite tandem cell. Future research directions for MHP‐based tandem solar cells are also discussed.

An ultrathin poly(ethylenimine) ethoxylated (PEIE) layer inserted underneath alters the work function and surface energy of the top SnO₂ surface. This PEIE/SnO₂ electron transport composite structure demonstrates negligible energy mismatch at the interface leading to high performance perovskite photovoltaic devices. More details can be found in article number https://doi.org/10.1002/admi.2020004122000412 by Chunjun Liang, Zhiqun He, and co‐workers.

Perovskite Solar Cells

In article number 2000149, Jun Song, Qing Shen, and co‐workers establish a new strategy to improve the performance of perovskite solar cells, which sheds more light on the currently proposed mechanism governing the action of moisture on the quality of perovskite film. Self‐passivated perovskite solar cells show an extraordinary V_OC of 1.17 V and highest efficiency of 21.38 %.

Perovskite Solar Cells

In article number 2000197, Wei‐Fang Su and co‐workers incorporate a cation of acetamidinium (Aa⁺) into conventional perovskite layer (MAPbI₃). The Aa⁺ cation effectively hinders the ion migration and enhances the long‐term stability of perovskite solar cells. The champion device achieves 20.68% efficiency. More than 80% of initial power conversion efficiency is maintained after 1300 h of 85°C/85 RH% test as encapsulated.

An N‐type semiconductor material, (CH₃)₂Sn(COOH)₂ (CSCO), is prepared for the first time as an electron transport layer for n‐i‐p planar perovskite solar cells, which leads to one of the highest power conversion efficiencies of 22.21%, and to remarkable stability, retaining over 83% of its initial power conversion efficiency without encapsulation after 130 days of storage in ambient conditions.

Abstract

The electron transport layer (ETL) has an important influence on the power conversion efficiency (PCE) and stability of n‐i‐p planar perovskite solar cells (PSCs). This paper presents an N‐type semiconductor material, (CH₃)₂Sn(COOH)₂ (abbreviated as CSCO) that is synthesized and prepared for the first time as an ETL for n‐i‐p planar PSCs, which leads to a high PCE of 22.21% after KCl treatment, one of the highest PCEs of n‐i‐p planar PSCs to date. Further analysis reveals that the high PCE is attributed to the excellent conductivity of CSCO because of its more delocalized electron cloud distribution due to its unique −O=C−O− group, and to the defect passivation of the Cs_0.05(FA_0.85MA_0.15)_0.95Pb(I_0.85Br_0.15)₃ (denoted as CsFAMA) perovskite through the interaction between the O (Sn) atoms of CSCO and the Pb (halogen) atoms of CsFAMA at CSCO/CsFAMA interface, while the traditional ETL materials such as SnO₂ film lack this function. In addition to the high PCE, the optimal PSCs using CSCO as ETL show remarkable stability, retaining over 83% of its initial PCE without encapsulation after 130 days of storage in ambient conditions (≈25 °C at ≈40% humidity), much better than the traditional SnO₂‐based n‐i‐p PSCs.

SnO₂ has been applied as an efficient electron transport layer for perovskite solar cells over the past few years. In this progress report, recent advances in SnO₂ modification toward high efficiency and stability are summarized from the perspective of the optimization strategies, and the remaining challenges as well as opportunities for future research are also discussed.

Abstract

The electron transport layer plays a key role in affecting the charge dynamics and photovoltaic parameters in perovskite solar cells. Compared to other counterparts, SnO₂ has unique advantages such as low temperature fabrication and high electron extraction ability, and it receives extra attentions from the research community since the first report. Planar‐type perovskite solar cells based on SnO₂ exhibit a simple architecture and state of art device can achieve a power conversion efficiency of over 23%, which can compete with traditional devices using mesoporous TiO₂. The modification engineering of SnO₂ has contributed significantly to the enhanced device performance during the past years. There is still great potential for further improvement in the efficiency and long‐term stability. Herein recent advances toward modifying the optoelectronic properties of SnO₂ from the perspective of the optimization strategies are summarized and the remaining challenges as well as opportunities for future research are discussed. The continuous efforts dedicated to this exciting field may pave the way for developing commercial perovskite solar cells.

Publication date: November 2020

Source: Nano Energy, Volume 77

Author(s): Seojun Lee, Janghyuk Moon, Jun Ryu, Bhaskar Parida, Saemon Yoon, Dong-Gun Lee, Jung Sang Cho, Shuzi Hayase, Dong-Won Kang

The charge‐extracting properties of PC₆₀BM, the electron transporting layer (ETL) widely used in perovskite solar cells, are greatly enhanced by complementing with Al:ZnO and triphenyl‐phosphine oxide films. Using these triple‐ETL results in a major improvement in device performance in terms of both efficiency and stability, due to better energy alignment, reduced trap‐assisted recombination, and higher built‐in voltage.

Abstract

Power conversion efficiencies of perovskite solar cells (PSCs) have rapidly increased from 3.8% to a certified 25.2% within only a decade. Eliminating possible recombination losses at the interfaces is essential to further improve both efficiency and stability of this class of emerging photovoltaic technology. Herein, a simple approach for improving the electron extraction of the PC₆₀BM electron transport layer (ETL) is presented by sequentially depositing Al:ZnO (AZO) and triphenyl‐phosphine oxide (TPPO) on top of it, in a p–i–n device configuration. The efficiency of the resulting CH₃NH₃PbI₃‐based solar cell is shown to improve from 14.6%, measured for the control PC₆₀BM‐only cell, to 17.9% for double‐ETL (PC₆₀BM/AZO) and 19.2% for triple‐ETL (PC₆₀BM/AZO/TPPO)‐based devices, respectively. Optimized triple‐ETL‐based cells exhibit high fill factor of 82%. The combination of electrical and quantum mechanical calculations shows that efficiency improvement is attributed to reduced trap‐assisted recombination at the interface and better energy level alignment due to chemical interactions between PC₆₀BM, AZO, and TPPO. Moreover, it is shown that the use of multilayer ETL results in better device stability (T ₈₀ ≈ 800 h) under constant illumination. This work opens new possibilities for inexpensive highly efficient and stable multilayered contacts for PSCs by combining organic small molecules and metal oxides.