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Inorganic CsPbI3 perovskite is the most competitive candidate to hybrid perovskites. However, its poor phase stability, hydrophobicity and high‐density defects have limited the development of CsPbI3 perovskite solar cells (PSCs). To overcome these obstacles for achieving high‐performance CsPbI3 PSCs, additive engineering has been widely employed. Herein, the progress of additive engineering in CsPbI3 PSCs is systematically reviewed.

All‐inorganic perovskite solar cells (PSCs) have attracted a lot of attention in the past few years because of their preeminent thermal stability compared with organic–inorganic hybrid PSCs. Among all kinds of all‐inorganic perovskites, CsPbI₃ perovskite with a proper bandgap of ≈1.7 eV becomes the most competitive candidate. However, its poor phase stability, hydrophobicity, and high‐density defects have limited the development of CsPbI₃ PSCs. To overcome these obstacles for achieving high‐performance CsPbI₃ PSCs, additive engineering has been widely used, which has rapidly promoted the power conversion efficiency (PCE) to over 19%. Herein, the progress of additive engineering in CsPbI₃ PSCs is systematically reviewed. First, the roles of additives in CsPbI₃ PSCs are introduced, including improving phase stability, increasing moisture resistance, and passivating defects. Then, the additive engineering is categorized (additive engineering in perovskites and at perovskite/hole transport layer interfaces) and reviewed in detail. Finally, future research directions on additive engineering are suggested for further enhancing stability and improving PCE.

A biological polymer is employed to regulate the arrangement of SnO₂ nanocrystals on a substrate and induce vertical crystal growth of a perovskite layer on top. The enhanced interface contact between the electron‐transport layer and the perovskite layer significantly contributes to the improvement of efficiency and stability of derived planar perovskite solar cells.

Abstract

Perovskite solar cells (PSCs) have rapidly developed and achieved power conversion efficiencies of over 20% with diverse technical routes. Particularly, planar‐structured PSCs can be fabricated with low‐temperature (≤150 °C) solution‐based processes, which is energy efficient and compatible with flexible substrates. Here, the efficiency and stability of planar PSCs are enhanced by improving the interface contact between the SnO₂ electron‐transport layer (ETL) and the perovskite layer. A biological polymer (heparin potassium, HP) is introduced to regulate the arrangement of SnO₂ nanocrystals, and induce vertically aligned crystal growth of perovskites on top. Correspondingly, SnO₂–HP‐based devices can demonstrate an average efficiency of 23.03% on rigid substrates with enhanced open‐circuit voltage (V _OC) of 1.162 V and high reproducibility. Attributed to the strengthened interface binding, the devices obtain high operational stability, retaining 97% of their initial performance (power conversion efficiency, PCE > 22%) after 1000 h operation at their maximum power point under 1 sun illumination. Besides, the HP‐modified SnO₂ ETL exhibits promising potential for application in flexible and large‐area devices.

The film quality of perovskite active layer is crucial for achieving high efficiency of perovskite solar cells. The antisolvent treatment method is a successful technique to improve the film quality. The fundamentals of antisolvent treatment, various antisolvent treatment methods, issues with antisolvents, and alternative methods are discussed.

Abstract

Organic–inorganic metal halide perovskite solar cells are emerging as potential solar energy harvesting tools and can be a tough competitor to already matured solar cell technologies. The success of perovskite solar cells is attributed to superior optoelectronic properties of perovskites, feasible synthesis process, and low fabrication cost. Though perovskite solar cells confront perovskite film quality related issues, such as rough surface, pinholes (which result in poor device performance) at the initial stages, many techniques have been developed to improve the perovskite film quality. Among these developed techniques, the antisolvent treatment method is certainly one of the most successful techniques till date. Antisolvent treatment increases the nucleus density during film formation to produce uniform and pinhole‐free perovskite film, which facilitates improved solar cell efficiency, low hysteresis, and stability. Interestingly, many of the best efficiency perovskite solar cells till date have been produced by the antisolvent treatment. This review discusses the fundamentals of antisolvent treatment, various aspects of antisolvent application on perovskite film, different issues with antisolvent usage, and alternatives techniques for perovskite film quality improvement.

The development of the first high‐performance, printable CsPbI₃ solar cells via an ambient blade‐coating technique is reported. High‐quality CsPbI₃ films are grown via the introduction of a low concentration of the multifunctional molecular additive Zn(C₆F₅)₂. As a result, the additive‐treated perovskite solar cell delivers a power conversion efficiency (PCE) of 19%.

Abstract

All‐inorganic CsPbI₃ holds promise for efficient tandem solar cells, but reported fabrication techniques are not transferrable to scalable manufacturing methods. Herein, printable CsPbI₃ solar cells are reported, in which the charge transporting layers and photoactive layer are deposited by fast blade‐coating at a low temperature (≤100 °C) in ambient conditions. High‐quality CsPbI₃ films are grown via introducing a low concentration of the multifunctional molecular additive Zn(C₆F₅)₂, which reconciles the conflict between air‐flow‐assisted fast drying and low‐quality film including energy misalignment and trap formation. Material analysis reveals a preferential accumulation of the additive close to the perovskite/SnO₂ interface and strong chemisorption on the perovskite surface, which leads to the formation of energy gradients and suppressed trap formation within the perovskite film, as well as a 150 meV improvement of the energetic alignment at the perovskite/SnO₂ interface. The combined benefits translate into significant enhancement of the power conversion efficiency to 19% for printable solar cells. The devices without encapsulation degrade only by ≈2% after 700 h in air conditions.

A new fluorinated organic ammonium halide salt, 4‐trifluoromethyl phenethylammonium iodide (CFPEAI), is utilized to passivate the surface of CsPbI₂Br perovskite for solar cells with enhanced efficiency as well as improved stability.

Surface modification is demonstrated as an efficient strategy to enhance the efficiency and stability of perovskite solar cells (PVSCs). Fluorinated organic ammonium salts featuring a strong hydrophobic nature are seldom used as passivation agents for the surface modification of CsPbI₂Br perovskites. Herein, a fluorinated organic ammonium halide salt, 4‐trifluoromethyl phenethylammonium iodide (CFPEAI), is incorporated into the surface of CsPbI₂Br perovskite for the first time. After the CFPEAI modification, the defects of CsPbI₂Br perovskite are significantly passivated with reduced trap densities. The best‐performance PVSC with CFPEAI modification shows an excellent power conversion efficiency (PCE) of 16.07% with a high fill factor (FF) of 84.65%, a short‐circuit current density (J _SC) of 15.45 mA cm⁻², and an open‐circuit voltage (V _OC) of 1.23 V. In contrast, the control PVSCs without the surface modification exhibit a lower PCE of 14.50% with a FF of 80.56%, a J _SC of 15.05 mA cm⁻², and a V _OC of 1.20 V. With CFPEAI passivation, the CsPbI₂Br perovskite film exhibits enhanced hydrophobicity, thereby leading to improved stability for the corresponding PVSC in comparison with the control PVSC without any surface modification.

Publication date: 16 September 2020

Source: Joule, Volume 4, Issue 9

Author(s): Yehao Deng, Zhenyi Ni, Axel F. Palmstrom, Jingjing Zhao, Shuang Xu, Charles H. Van Brackle, Xun Xiao, Kai Zhu, Jinsong Huang

The temperature dependence of the local and long‐range structures of the halide perovskite CsPbI₃ is studied. The Cs atom splits between two sites with the second site having a lower effective coordination number, suggesting that Cs rattles within the structure. Rattling, few CsI contacts, and the high degree of octahedral distortion explain the thermodynamic instability of perovskite‐phase CsPbI₃ .

Abstract

Despite the tremendous interest in halide perovskite solar cells, the structural reasons that cause the all‐inorganic perovskite CsPbI₃ to be unstable at room temperature remain mysterious, especially since many tolerance‐factor‐based approaches predict CsPbI₃ should be stable as a perovskite. Here single‐crystal X‐ray diffraction and X‐ray pair distribution function (PDF) measurements characterize bulk perovskite CsPbI₃ from 100 to 295 K to elucidate its thermodynamic instability. While Cs occupies a single site from 100 to 150 K, it splits between two sites from 175 to 295 K with the second site having a lower effective coordination number, which, along with other structural parameters, suggests that Cs rattles in its coordination polyhedron. PDF measurements reveal that on the length scale of the unit cell, the PbI octahedra concurrently become greatly distorted, with one of the IPbI angles approaching 82° compared to the ideal 90°. The rattling of Cs, low number of CsI contacts, and high degree of octahedral distortion cause the instability of perovskite‐phase CsPbI_3. These results reveal the limitations of tolerance factors in predicting perovskite stability and provide detailed structural information that suggests methods to engineer stable CsPbI₃‐based solar cells.

A correlative study investigates the influence of the novel KI‐passivation treatment on halide perovskite solar cell materials. By comparing the local electrical and chemical properties using an array of high‐spatial resolution imaging techniques, this research shows the spatial distribution of excess passivating material and links the nanoscale properties with macroscopic device performance.

Abstract

Perovskite semiconductors are an exciting class of materials due to their promising performance outputs in photovoltaic devices. To boost their efficiency further, researchers introduce additives during sample synthesis, such as KI. However, it is not well understood how KI changes the material and, often, leaves precipitants. To fully resolve the role of KI, multiple microscopy techniques are applied and the electrical and chemical behavior of a Reference (untreated) and a KI‐treated perovskite are compared. Upon correlation between electrical and chemical nanoimaging techniques, it is discovered that these local properties are linked to the macroscopic voltage enhancement of the KI‐treated perovskite. The heterogeneity revealed in both the local electrical and chemical responses indicates that the additive partially migrates to the surface, yet surprisingly does not deteriorate the performance locally, rather, the voltage response homogeneously increases. The research presented within provides a diagnostic methodology, which connects the nanoscale electrical and chemical properties of materials, relevant to other perovskites, including multication and Pb‐free alternatives.

Publication date: 19 August 2020

Source: Joule, Volume 4, Issue 8

Author(s): Caleb C. Boyd, R. Clayton Shallcross, Taylor Moot, Ross Kerner, Luca Bertoluzzi, Arthur Onno, Shalinee Kavadiya, Cullen Chosy, Eli J. Wolf, Jérémie Werner, James A. Raiford, Camila de Paula, Axel F. Palmstrom, Zhengshan J. Yu, Joseph J. Berry, Stacey F. Bent, Zachary C. Holman, Joseph M. Luther, Erin L. Ratcliff, Neal R. Armstrong

Publication date: 19 February 2020

Source: Joule, Volume 4, Issue 2

Author(s): Jiahuan Zhang, Zaiwei Wang, Aditya Mishra, Maolin Yu, Mona Shasti, Wolfgang Tress, Dominik Józef Kubicki, Claudia Esther Avalos, Haizhou Lu, Yuhang Liu, Brian Irving Carlsen, Anand Agarwalla, Zishuai Wang, Wanchun Xiang, Lyndon Emsley, Zhuhua Zhang, Michael Grätzel, Wanlin Guo, Anders Hagfeldt

By using a solvent‐mediated phase transformation process, a record certified 21.8% power conversion efficiency in pure‐iodide, alkaline‐metal‐free MA_0.5FA_0.5PbI₃ perovskite‐based solar cells is achieved.

Abstract

Composition and film quality of perovskite are crucial for the further improvement of perovskite solar cells (PSCs), including efficiency, reproducibility, and stability. Here, it is demonstrated that by simply mixing 50% of formamidinium (FA⁺) into methylammonium lead iodide (MAPbI₃), a highly crystalline, stable phase, and compact, polycrystalline grain morphology perovskite is formed by using a solvent‐mediated phase transformation process via the synergism of dimethyl sulfoxide and diethyl ether, which shows long carrier lifetime, low trap state density, and a record certified 21.8% power conversion efficiency (PCE) in pure‐iodide, alkaline‐metal‐free MA_0.5FA_0.5PbI₃ perovskite‐based PSCs. These PSCs show very high operational stability, with 85% PCE retention upon 1000 h 1 Sun intensity illumination. A 17.33% PCE module (6.5 × 7 cm²) is also demonstrated, attesting to the scalability of such devices.

Publication date: 15 July 2020

Source: Joule, Volume 4, Issue 7

Author(s): Qin Hu, Wei Chen, Wenqiang Yang, Yu Li, Yecheng Zhou, Bryon W. Larson, Justin C. Johnson, Yi-Hsien Lu, Wenkai Zhong, Jinqiu Xu, Liana Klivansky, Cheng Wang, Miquel Salmeron, Aleksandra B. Djurišić, Feng Liu, Zhubing He, Rui Zhu, Thomas P. Russell

CsPbI₃, with its excellent chemical stability, possesses a suitable bandgap for single‐junction and tandem solar cells, yet the poor phase stability hinders its application. In article https://doi.org/10.1002/adma.2020001862000186, Wan‐Jian Yin, Wallace C. H. Choy, and co‐workers reveal the nature of the photoactive CsPbI₃ phase transition from the perspective of PbI₆ octahedral rotation and develop a facile method to simultaneously stabilize the photoactive phase and reduce the defect density of the CsPbI₃.

Phenylhydrazine hydrochloride is introduced into FASnI₃‐based perovskite solar cells (where FA = NH₂CHNH₂ ⁺) in order to reduce the existing Sn⁴⁺ and prevent the further degradation of the FASnI₃. Consequently, the champion device shows a high power conversion efficiency up to 11.4%, a long‐term storage stability over 2300 h, and an efficiency recovery capability after being exposed to air.

Abstract

The development of tin (Sn)‐based perovskite solar cells (PSCs) is hindered by their lower power conversion efficiency and poorer stability compared to the lead‐based ones, which arise from the easy oxidation of Sn²⁺ to Sn⁴⁺. Herein, phenylhydrazine hydrochloride (PHCl) is introduced into FASnI₃ (FA = NH₂CH  NH₂ ⁺) perovskite films to reduce the existing Sn⁴⁺ and prevent the further degradation of FASnI₃, since PHCl has a reductive hydrazino group and a hydrophobic phenyl group. Consequently, the device achieves a record power conversion efficiency of 11.4% for lead‐free PSCs. Besides, the unencapsulated device displays almost no efficiency reduction in a glove box over 110 days and shows efficiency recovery after being exposed to air, due to a proposed self‐repairing trap state passivation process.

Colloidal synthesis of all inorganic single‐crystalline β‐CsPbI₃ nanorods with an excellent photostability under 45–55% humidity displays the superior characteristics of fabricated inverted perovskite solar cells without any device passivation. Atomic resolution transmission electron micrography reveals the probable distribution of Cs, Pb, and I atoms in a single β‐phase CsPbI₃ nanorod.

Abstract

The synthesis of single‐crystalline β‐CsPbI₃ perovskite nanorods (NRs) using a colloidal process is reported, exhibiting their improved photostability under 45–55% humidity. The crystal structure of CsPbI₃ NRs films is investigated using Rietveld refined X‐ray diffraction (XRD) patterns to determine crystallographic parameters and the phase transformation from orthorhombic (γ‐CsPbI₃) to tetragonal (β‐CsPbI₃) on annealing at 150 °C. Atomic resolution transmission electron microscopy images are utilized to determine the probable atomic distribution of Cs, Pb, and I atoms in a single β‐phase CsPbI₃ NR, in agreement with the XRD structure and selected area electron diffraction pattern, indicating the growth of single crystalline β‐CsPbI₃ NR. The calculation of the electronic band structure of tetragonal β‐CsPbI₃ using density functional theory (DFT) reveals a direct transition with a lower band gap and a higher absorption coefficient in the solar spectrum, as compared to its γ‐phase. An air‐stable (45–55% humidity) inverted perovskite solar cell, employing β‐CsPbI₃ NRs without any encapsulation, yields an efficiency of 7.3% with 78% enhancement over the γ‐phase, showing its potential for future low cost photovoltaic devices.

The surface and bulk defects of perovskite films are simultaneously passivated through the treatment of CsBr/methanol solution, in which the methanol helps CsBr penetrate the depth of the perovskite and reconstruct high‐quality films. This strategy can effectively improve the photovoltaic performance and operational stability of the resultant devices.

It is challenging to passivate defects that are buried in the depth of perovskite films; most of the reported passivation methods cannot reach the deep defects. Herein, methanol is adopted as a dual‐functional reagent to not only act as a solvent but also help the dissolved ions penetrate the depth of perovskite films. By treating the as‐prepared perovskite films with CsBr/methanol solution, Br⁻ ions can react with the undercoordinated Pb²⁺, and Cs⁺ ions can fill in the cation vacancies. This strategy enables surface and bulk defects passivation to be achieved simultaneously. The nonradiative recombination of the double‐passivated devices is significantly suppressed and the migration of charged defects is remarkably hindered. As a result, an improved power conversion efficiency of 19.5% and an open‐circuit voltage of 1.183 V is achieved. Moreover, the passivated device can retain ≈80% of the initial efficiency after working for 500 h at maximum power point under 1‐sun illumination, whereas the pristine device reaches 80% of the initial efficiency after only 90 h. This work demonstrates that surface and bulk defects passivation is critical to improve the efficiency and long‐term operational stability of the perovskite solar cells.

The power conversion efficiency of inorganic perovskite solar cells with compositions CsPbI_1.8Br_1.2, CsPbI_2.0Br_1.0, and CsPbI_2.2Br_0.8 exhibits a high dependence on the initial annealing step that is found to significantly affect the crystallization and texture behavior of the final perovskite film. This work brings new thoughts on the critical factors that lead to high efficiency in inorganic perovskite solar cells.

Inorganic perovskites with cesium (Cs⁺) as the cation have great potential as photovoltaic materials if their phase purity and stability can be addressed. Herein, a series of inorganic perovskites is studied, and it is found that the power conversion efficiency of solar cells with compositions CsPbI_1.8Br_1.2, CsPbI_2.0Br_1.0, and CsPbI_2.2Br_0.8 exhibits a high dependence on the initial annealing step that is found to significantly affect the crystallization and texture behavior of the final perovskite film. At its optimized annealing temperature, CsPbI_1.8Br_1.2 exhibits a pure orthorhombic phase and only one crystal orientation of the (110) plane. Consequently, this allows for the best efficiency of up to 14.6% and the longest operational lifetime, T _S80, of ≈300 h, averaged of over six solar cells, during the maximum power point tracking measurement under continuous light illumination and nitrogen atmosphere. This work provides essential progress on the enhancement of photovoltaic performance and stability of CsPbI_3 − x Br _x perovskite solar cells.

An effective strategy is demostrated to create a double barrier that not only blocks the invasion of the moisture but also takes advantage of the permeated moisture to increase the moisture durability of perovskite films, which results in an n–i–p perovskite solar cell with moisture stability over 115 days in a relative humidity of 70% and a champion efficiency up to 21.34%.

Abstract

The rapid growth in the device efficiency of perovskite solar cells (PSCs) has raised great demands for tackling their long‐term stability upon external environmental stimuli that restricts the commercialization of PSCs, in which the instability upon exposure to moisture has been one of the major obstacles. Herein, an effective way of building up double barriers for moisture degradation of the perovskite films is demonstrated by modifying them with rationally selected hydrolyzable hydrophobic molecules (1H,1H,2H,2H‐perfluorooctyl trichlorosilane, PFTS). The layer of oligomer derived from the hydrolyzed PFTS at the surface that increases the hydrophobicity of perovskite film could serve as an efficient wall preventing the moisture invasion. The long‐term exposure of the film upon moisture allows for the formation of a secondary wall that employs the hydrolyzation of PFTS at grain boundaries, favoring defects passivation to further improve the humidity stability. Such gradual hydrolyzation is encouragingly helpful for the enhancement of the open‐circuit voltage of the PSCs from the original 1.136 up to 1.205 V. The PSCs constructed with the double barriers demonstrate excellent stability upon moisture and improved thermal and light stabilities, as well as a champion power conversion efficiency up to 21.34%.