shuhui

A rutile TiO₂ electron transport layer (ETL) was prepared. The thickness and crystallinity can be controlled by deposition time and sintering temperature. Rutile TiO₂ has higher conductivity than anatase for faster electron transfer, better interface contact with the perovskite layer, and a lower trap density. These facilitate the charge extraction and collection and reducing carrier recombination.

Abstract

Interfacial charge collection efficiency has demonstrated significant effects on the power conversion efficiency (PCE) of perovskite solar cells (PSCs). Herein, crystalline phase‐dependent charge collection is investigated by using rutile and anatase TiO₂ electron transport layer (ETL) to fabricate PSCs. The results show that rutile TiO₂ ETL enhances the extraction and transportation of electrons to FTO and reduces the recombination, thanks to its better conductivity and improved interface with the CH₃NH₃PbI₃ (MAPbI₃) layer. Moreover, this may be also attributed to the fact that rutile TiO₂ has better match with perovskite grains, and less trap density. As a result, comparing with anatase TiO₂ ETL, MAPbI₃ PSCs with rutile TiO₂ ETL delivers significantly enhanced performance with a champion PCE of 20.9 % and a large open circuit voltage (V _OC) of 1.17 V.

In article no. 1900032, Lei Ying, Feng Liu, Fei Huang, Thomas P. Russell, and co‐workers study the multiple crystallization kinetics during bulk‐heterojunction film drying for all‐polymer solar cells by using in situ grazing incidence wide‐angle X‐ray scattering. Printing with 1,8‐diiodooctance is helpful for the formation of a multi‐length‐scale phase separation, and ultimately improves the solar cell performance.

Three enamine hole‐transporting materials (HTMs) based on Tröger's base scaffold were synthesized. These compounds are obtained in a three‐step facile synthesis from commercially available materials without the need of expensive catalysts, inert conditions or time‐consuming purification steps.

Abstract

The synthesis of three enamine hole‐transporting materials (HTMs) based on Tröger's base scaffold are reported. These compounds are obtained in a three‐step facile synthesis from commercially available materials without the need of expensive catalysts, inert conditions or time‐consuming purification steps. The best performing material, HTM3, demonstrated 18.62 % PCE in PSCs, rivaling spiro‐OMeTAD in efficiency, and showing markedly superior long‐term stability in non‐encapsulated devices. In dopant‐free PSCs, HTM3 outperformed spiro‐OMeTAD by a factror of 1.6. The high glass‐transition temperature (T _g=176 °C) of HTM3 also suggests promising perspectives in device applications.

Molecular doping is demonstrated as a powerful strategy for enhancing electrical conductivity in arrays of electronically coupled CsPbI₃ nanocrystals. The high surface‐area‐to‐volume ratio enables physisorbed organic redox molecules to inject charge carriers into a nanocrystal array. p‐Type doping improves both ground‐state conductivity and photoconductivity, enabling pronounced performance enhancements to both transistors and phototransistors.

Abstract

Doping of semiconductors enables fine control over the excess charge carriers, and thus the overall electronic properties, crucial to many technologies. Controlled doping in lead‐halide perovskite semiconductors has thus far proven to be difficult. However, lower dimensional perovskites such as nanocrystals, with their high surface‐area‐to‐volume ratio, are particularly well‐suited for doping via ground‐state molecular charge transfer. Here, the tunability of the electronic properties of perovskite nanocrystal arrays is detailed using physically adsorbed molecular dopants. Incorporation of the dopant molecules into electronically coupled CsPbI₃ nanocrystal arrays is confirmed via infrared and photoelectron spectroscopies. Untreated CsPbI₃ nanocrystal films are found to be slightly p‐type with increasing conductivity achieved by incorporating the electron‐accepting dopant 2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquinodimethane (F₄TCNQ) and decreasing conductivity for the electron‐donating dopant benzyl viologen. Time‐resolved spectroscopic measurements reveal the time scales of Auger‐mediated recombination in the presence of excess electrons or holes. Microwave conductance and field‐effect transistor measurements demonstrate that both the local and long‐range hole mobility are improved by F₄TCNQ doping of the nanocrystal arrays. The improved hole mobility in photoexcited p‐type arrays leads to a pronounced enhancement in phototransistors.

Electrohydrodynamic (EHD) printing with the unique advantages of high‐resolution patterning and large scalability is introduced to fabricate full‐color perovskite patterns. Perovskite inks via simple precursor mixing are prepared to in situ crystallize tunable‐ and bright‐photoluminescence perovskite arrays without adding antisolvent. Through optimizing the EHD printing process, a high‐resolution dot matrix of 5 µm is achieved.

Abstract

Hybrid perovskites show enormous potential for display due to their tunable emission, high color purity, strong photoluminescence and electroluminescence. For display applications, full‐color and high‐resolution patterning is compulsory, however, current perovskite processing such as spin‐coating fails to meet these requirements. Here, electrohydrodynamic (EHD) printing, with the unique advantages of high‐resolution patterning and large scalability, is introduced to fabricate full‐color perovskite patterns. Perovskite inks via simple precursor mixing are prepared to in situ crystallize tunable‐ and bright‐photoluminescence perovskite arrays without adding antisolvent. Through optimizing the EHD printing process, a high‐resolution dot matrix of 5 µm is achieved. The as‐printed patterns and pictures show full color and high controllability in micrometer dimension, indicating that the EHD printing is a competitive technique for future halide perovskite‐based high‐quality display.

Imaging and mapping characterization techniques are used to understand the fundamental properties that allow lead halide perovskites to have excellent performance metrics. In this work, commonly‐used and specialized tools that are used characterize halide perovskite materials and solar cells, including electron microscopy, atomic force microscopy, synchrotron‐based X‐ray mapping, and ultrafast and photoluminescence mapping are reviewed.

Abstract

Perovskite solar cells (PSCs) have attracted much attention as efficiencies have gone beyond 24%. To achieve these impressive numbers, the PSC scientific community is working to improve the perovskite optoelectronic properties. Imaging and mapping characterization techniques have been widely used to understand the fundamental properties that allow lead halide perovskites to achieve high performance. In this review, these techniques are evaluated, from simple tools, such as electron microscopy, to more complex systems that include atomic force microscopy, synchrotron‐based X‐ray mapping, and ultrafast and photoluminescence mapping. These tools have helped understand lead halide perovskites and their impressive optoelectronic properties, which make them outstanding materials for solar cell applications.

Trihydrazine dihydriodide is successfully used as an additive for solution deposition of a formamidinium tin iodide (FASnI₃) perovskite layer, resulting in improved surface morphology and reduced carrier concentration. Using the derived FASnI₃ layer as a light absorber, a maximum power conversion efficiency of 8.48% is achieved in a planar‐heterojunction solar cell using common precursor SnI₂ with 99% purity.

The deposition of a uniform and dense tin‐based perovskite layer with low defect‐caused background carrier density is crucial for achieving efficient tin perovskite solar cells (PSCs). These defects are mainly caused by the rapid oxidation of Sn²⁺ to Sn⁴⁺ in tin perovskite during device fabrication. Herein, trihydrazine dihydriodide ((N₂H₄)₃(HI)₂) is used as an additive for solution deposition of a formamidinium tin iodide (FASnI₃) perovskite layer. The resultant FASnI₃ layer is homogeneous with full surface coverage; moreover, the content of Sn⁴⁺ is significantly reduced in the film from the SnI₂ precursor owing to the reductive property of (N₂H₄)₃(HI)₂. With the high‐quality FASnI₃ layer as a light absorber, planar‐heterojunction perovskite solar cells are fabricated, exhibiting a maximum power conversion efficiency of 8.48% and good reproducibility. This work opens new possibilities for achieving efficient lead‐free tin‐based perovskite solar cells.

The power conversion efficiency of the carbon‐based perovskite solar cells is enhanced by 21.4% simply by interfacial post‐treatment with cesium acetate. The nonencapsulated device can remain stable for 4 months without observable degradation. The improved performance is attributed to the better matched energy levels and the reduced defect density.

The interface between the perovskite layer and carbon electrode is important for printable carbon‐based perovskite solar cells (PSCs) to improve the power conversion efficiency (PCE) and device stability. A series of acetate salts are employed to in situ post‐modify the interface between the perovskite layer and carbon electrode for printable carbon‐based PSCs by the post‐treatment method. Cesium acetate (CsAc) is identified to enhance the average PCE from 12.6% to 15.3%. The stabilized output PCE reaches 15.6%, and the highest open‐circuit voltage (V _OC) is 1.1 V, representing a new milestone in increasing the ratio of V _OC/E _g (E _g: bandgap of perovskite) to be 0.67 for the printable carbon‐based PSCs without hole transporting materials. Moreover, the device stability in air is also improved by CsAc post‐modification. The improved performance is attributed to the better matching of energy levels of the perovskite layer with a carbon electrode and reduced defect density in the perovskite layer via in situ produced methylammonium acetate and ion replacement. This simple and effective CsAc post‐treatment method opens a new promising direction for developing scalable carbon‐based PSCs.

A simple coalloying strategy is applied to partly substitute HC(NH₂)₂/CH₃NH₃ (FA/MA) and I⁻ in FA_0.85MA_0.15PbI₃ perovskite by Cs⁺ and Ac⁻ respectively, which is an effective way to improve the tolerance factor, crystallinity, electronic properties, and band structure of FA_0.85MA_0.15PbI₃ materials. Consequently, the coalloyed perovskite solar cells yield a champion power conversion efficiency of 21.95% with negligible hysteresis and high stability.

A simple coalloying strategy is applied to improve the efficiency and stability of FA_0.85MA_0.15PbI₃ perovskite solar cells (PSCs) by using cesium acetate (CsAc) as an additive. It is found that the simultaneous incorporation of cation (Cs⁺) and anion (Ac⁻) into the FA_0.85MA_0.15PbI₃ film is an effective approach to realize lattice contraction, grain size enlargement, photoelectric properties improvement, band structure modulation, and therefore the optimization of the efficiency and stability of PSCs. At optimal CsAc alloying, the FA_0.85MA_0.15PbI₃ PSCs achieve a maximum power conversion efficiency (PCE) of 21.95% and an average of over 21%. In addition, the alloyed PSCs retain 97% of their initial PCE values after aging for 55 days in air without encapsulation.

In this review, multifunctional molecules for perovskite solar cells (PSCs) are introduced. All the molecules can help to improve the performance of PSCs, such as forming low‐dimensional or dimensionally mixed perovskites and passivating defects, thus inducing good crystal growth behavior, improving the morphology of perovskite films, and facilitating charge transport. Eventually, PSCs with superior photoelectric properties and better stability can be obtained.

Organic–inorganic halide perovskite solar cells (PSCs) have recently attracted much attention with the recent certified power conversion efficiency (PCE) record exceeding 24%. To date, many approaches have been developed for producing high‐performance PSCs, in which the application of multifunctional molecules plays an important role. The multifunctional molecules can modify the morphology of perovskite films and/or passivate the surface defects through interactions with the perovskites' boundaries and/or the charge carrier extraction interfaces. As a result, both the PCEs and the stability of PSCs are improved. The recent progress in the development of multifunctional molecules‐incorporated PSCs is reviewed. The importance of further understanding of the role of the multifunctional molecules in the perovskite film formation process and defect passivation mechanism is discussed. Further research in terms of multifunctional molecules can help to develop high‐performance devices with long‐term stability for future practical applications of PSCs.

Publication date: September 2019

Source: Nano Energy, Volume 63

Author(s): Lin Xu, Xinfu Chen, Junjie Jin, Wei Liu, Biao Dong, Xue Bai, Hongwei Song, Peter Reiss

Graphical abstract

Research on planar perovskite solar cells (PSCs) in (inverted) p–i–n configuration, using transparent p-type front-electrodes, is strongly emerging. NiO_x has been demonstrated to be one of the most promising candidates to be employed as a hole transport layer (HTL) in these devices, however, its low intrinsic conductivity and unmatched Fermi level with respect to the perovskite layer limit the performance of the PSCs. Extrinsic doping of NiO_x HTLs is a versatile and powerful strategy to mitigate these shortcomings, which, within the past three years, led to significantly enhanced power conversion efficiencies (exceeding 20%). In this review, we present a comprehensive overview of the strategies applied to improve the performance of NiO_x HTLs used in inverted PSCs with special emphasis on the properties modulation induced by extrinsic doping. Current challenges and perspectives for exploiting these HTLs in high-efficiency inverted PSCs are also given.

Publication date: September 2019

Source: Nano Energy, Volume 63

Author(s): Xianyong Zhou, Manman Hu, Chang Liu, Luozheng Zhang, Xiongwei Zhong, Xiangnan Li, Yanqing Tian, Chun Cheng, Baomin Xu

Graphical abstract

Nondoped and Ca²⁺‐doped γ ‐CsPbI₃ films are prepared at low temperature (60 °C). The theoretical simulation and experimental results testify that adding Ca²⁺ can lower the total cohesive energy of γ‐CsPbI₃ and yield a more stable γ‐CsPbI₃ film. The Ca²⁺‐doped γ‐CsPbI₃ perovskite solar cells achieve a hysteresis‐free J–V curve and a maximum power conversion efficiency of 9.20%.

Abstract

Inorganic cubic CsPbI₃ perovskite (α‐CsPbI₃) has been widely explored for perovskite solar cells (PSCs) due to its thermal stability and suitable bandgap of 1.73 eV. However, α‐CsPbI₃ usually requires high synthesis temperatures (>320 °C). Additionally, it usually undergoes phase transition to the nonperovskite structure phase (β‐CsPbI₃), which results in poor photoelectric performance in devices. In this study, it is first found that the tortuous 3D CsPbI₃ phase (γ‐CsPbI₃) can be prepared and used for PSCs by solution process without any additive at low temperature (60 °C). The γ‐CsPbI₃ exhibits suitable bandgap of 1.75 eV and favorable photoelectric properties. However, γ‐CsPbI₃ is a metastable phase and easily transforms into β‐CsPbI₃ in ambient moisture. In order to improve the stability of γ‐CsPbI₃, calcium ions (Ca²⁺) with a relatively small radius of 100 pm are used to partially substitute lead ions (119 pm). This research proves that Ca²⁺ can effectively improve the stability of the γ‐CsPbI₃ at room temperature. By optimizing the doping concentration of Ca²⁺ (CsPb_{1− x} Ca _x I₃, x is from 0% to 2%), the Ca²⁺‐doped γ‐CsPbI₃ PSCs achieve a hysteresis‐free J–V curve and a maximum power conversion efficiency (PCE) of 9.20%.

A semiconducting conjugated polymer, poly(bithiophene imide), is successfully introduced to perovskite grain boundaries along with augmented grain sizes. This results in effective defect passivation and hence reduced recombination losses and increased efficiency, as well as reduced ion migration and improved stability.

Abstract

Grain boundaries in lead halide perovskite films lead to increased recombination losses and decreased device stability under illumination due to defect‐mediated ion migration. The effect of a conjugated polymer additive, poly(bithiophene imide) (PBTI), is investigated in the antisolvent treatment step in the perovskite film deposition by comprehensive characterization of perovskite film properties and the performance of inverted planar perovskite solar cells (PSCs). PBTI is found to be incorporated within grain boundaries, which results in an improvement in perovskite film crystallinity and reduced defects. The successful defect passivation by PBTI yields reduces recombination losses and consequently increases power conversion efficiency (PCE). In addition, it gives rise to improved photoluminescence stability and improved PSC stability under illumination which can be attributed to reduced ion migration. The optimal devices exhibit a PCE of 20.67% compared to 18.89% of control devices without PBTI, while they retain over 70% of the initial efficiency after 600 h under 1 sun illumination compared to 56% for the control devices.

Perovskite solar cells with ZnO exhibit greatly improved stability when the methylammonium cation is excluded. The interfacial acid‐base reactions between methylammonium and ZnO are probed and the degradation kinetics are modulated by the acidity of the organic cation. Solar cells on ZnO films provide improved open circuit voltage, lower series resistance, and lower processing temperatures than those on SnO₂.

Abstract

Perovskite solar cells have achieved the highest power conversion efficiencies on metal oxide n‐type layers, including SnO₂ and TiO₂. Despite ZnO having superior optoelectronic properties to these metal oxides, such as improved transmittance, higher conductivity, and closer conduction band alignment to methylammonium (MA)PbI₃, ZnO is largely overlooked due to a chemical instability when in contact with metal halide perovskites, which leads to rapid decomposition of the perovskite. While surface passivation techniques have somewhat mitigated this instability, investigations as to whether all metal halide perovskites exhibit this instability with ZnO are yet to be undertaken. Experimental methods to elucidate the degradation mechanisms at ZnO–MAPbI₃ interfaces are developed. By substituting MA with formamidinium (FA) and cesium (Cs), the stability of the perovskite–ZnO interface is greatly enhanced and it is found that stability compares favorably with SnO₂‐based devices after high‐intensity UV irradiation and 85 °C thermal stressing. For devices comprising FA‐ and Cs‐based metal halide perovskite absorber layers on ZnO, a 21.1% scanned power conversion efficiency and 18% steady‐state power output are achieved. This work demonstrates that ZnO appears to be as feasible an n‐type charge extraction layer as SnO₂, with many foreseeable advantages, provided that MA cations are avoided.

A new azeotropic promoted approach is proposed to successfully synthesize In doped CuCrO₂ under low temperatures in a short time. This In doped CuCrO₂ HTL has thermal stability up to 200 °C, and exhibits improved optical transmission and carrier mobility, which is beneficial for achieving high performance perovskite solar cells.

Abstract

While there are very limited studies of doped ternary metal oxide based hole transport materials, a multifunctional synthesis approach of In doped CuCrO₂ nanoparticles (NPs) as efficient hole transport layers (HTLs) including simplifying the synthesis requirements is proposed, enabling doping and achievement of treatment‐free HTLs. Remarkably, compared with conventional methods for synthesizing CuCrO₂ NPs, the newly proposed azeotropic promoted approach dramatically reduces the reaction time by 90% and the calcination temperature by one‐third, which not only promotes high throughput production but also reduces power consumption and cost in synthesis. Equally important, indium is successfully doped into CuCrO₂, which is fundamentally difficult in low temperature processes. The In doping offers less d–d transition of Cr³⁺ and p‐type doping characteristics for improving HTL transmittance and conductivity, respectively. Interestingly, In doped CuCrO₂ HTL with these improvements can be achieved by a simple ambient‐condition process and exhibits thermal stability up to 200 °C, which allows perovskite solar cells (PSCs) to achieve a power conversion efficiency of 20.54%. Meanwhile, the devices show good repeatability and photostability. Consequently, the work contributes to establishing a simple approach to realize pristine and doped multinary oxides based HTL for the development of practical and high performing PSCs.

A high quality FAPbI₃ ‐based perovskite film is successfully developed via a ligand and additive synergetic process. The planar flexible solar cell based on this film shows a record power conversion efficiency of 19.38%. This device exhibits excellent ambient stability and mechanical stability.

Abstract

Compared with silicon‐based solar cells, organic–inorganic hybrid perovskite solar cells (PSCs) possess a distinct advantage, i.e., its application in the flexible field. However, the efficiency of the flexible device is still lower than that of the rigid one. First, it is found that the dense formamidinium (FA)‐based perovskite film can be obtained with the help of N‐methyl‐2‐pyrrolidone (NMP) via low pressure‐assisted method. In addition, CH₃NH₃Cl (MACl) as the additive can preferentially form MAPbCl_{3− x} I _x perovskite seeds to induce perovskite phase transition and crystal growth. Finally, by using FAI·PbI₂·NMP+x%MACl as the precursor, i.e., ligand and additive synergetic process, a FA‐based perovskite film with a large grain size, high crystallinity, and low trap density is obtained on a flexible substrate under ambient conditions due to the synergetic effect, e.g., MACl can enhance the crystallization of the intermediate phase of FAI·PbI₂·NMP. As a result, a record efficiency of 19.38% in flexible planar PSCs is achieved, and it can retain about 89% of its initial power conversion efficiency (PCE) after 230 days without encapsulation under ambient conditions. The PCE retains 92% of the initial value after 500 bending cycles with a bending radii of 10 mm. The results show a robust way to fabricate highly efficient flexible PSCs.

A novel glued poly(ethylene‐co‐vinyl acetate) (EVA) interfacial layer is used to fabricate highly efficient and stable perovskite solar cells (PVSCs) with excellent waterproofness and flexibility. The EVA‐treated PVSCs exhibit superior power conversion efficiency values of 19.31% for a rigid device (0.1 cm²) and 11.73% for a solar module (25 cm²), as well as over 85% retention for a flexible device after 5000 bending cycles.

Abstract

Perovskite solar cells (PVSCs) are promising photovoltaic technologies for realizing power sources with outstanding power conversion efficiency (PCE) and low‐cost properties. However, the extraordinary photovoltaic performance can be maximized only if an extremely stabilized device structure is developed. Here, a novel glued poly(ethylene‐co‐vinyl acetate) (EVA) interfacial layer is introduced to fabricate highly efficient and stable PVSCs with excellent waterproofness and flexibility. This strategy can effectively passivate the perovskite surface, reduce defect density, and balance charge transfer, which leads to a champion PCE of 19.31% for a 0.1 cm² device and 11.73% for a 25 cm² solar module. More importantly, the formation of a glued EVA thin layer on the surface of perovskite can inhibit ionic migration to the Ag electrode, form favorable interfacial contact and adhesive interaction with the perovskite/[6,6]‐phenyl‐C₆₁‐butyric acid methyl ester to sustain mechanical bending, and produce significant waterproofness from moisture invasion, thus facilitating improvement in the operational stability of the PVSCs. The EVA‐treated PVSCs exhibit superior PCE values of 15.12% for a flexible device (0.1 cm²) and 8.95% for a flexible module (25 cm²), as well as over 85% retention after 5000 bending cycles, which opens up a new strategy for the practical application of PVSCs in portable and wearable electronics.

The initial stages of MAPbI₃ photodegradation prior to any significant change in light absorption are studied, with independent control of sample temperature and sunlight intensity (1–500 suns). Under the combined action of light and heat, a strong reduction of photoluminescence (PL) is observed. In contrast, illumination of perovskite films (with an intensity up to 500 suns) without heating induces considerable PL enhancement.

The initial stages of photo‐degradation of CH₃NH₃PbI₃ (MAPbI₃) thin films prior to any significant change in light absorption are studied in experiments with independent control of sample temperature and intensity of concentrated sunlight from 50 to 500 suns. Photo‐stability of the MAPbI₃ film is revealed to be extremely sensitive to the sample temperature. Under the combined action of light and heat (either by concentrated sunlight or by external heating), a strong reduction of the film photoluminescence (PL) without changes in the perovskite light absorption can be observed during the initial stages of degradation. In contrast, illumination of perovskite films (with intensity up to 500 suns) without heating (using chopped concentrated sunlight) induces considerable PL enhancement while the optical absorption spectrum remains unchanged. With accurate temperature control, aging under concentrated sunlight results in similar instability trends as that under 1 sun.

A simple dynamic antisolvent quenching process is used for the efficient and reliable fabrications of uniform and high‐quality 10 × 10 cm² large‐area perovskite films. The perovskite module fabricated using this technique achieves an efficiency approaching 18% and a certified efficiency of 17.4% with the aperture area of 53.64 cm².

Perovskite solar cells represent a promising photovoltaic technology, which achieves record power conversion efficiencies over 24%. However, a problem on the commercial processing is the unavoidable efficiency loss during the scalable fabrication of perovskite solar module. The efficient and reliable fabrications of high‐quality large‐area perovskite films guarantee commercialized up‐scaling of perovskite solar cells with high efficiency. Herein, a simple dynamic antisolvent quenching (DAS) process is presented to understand large‐area uniform perovskite films to obtain an efficient perovskite solar module. This method provides a facile and universal approach to fabricate cracks‐free and uniform large‐area mixed‐cation perovskite films. A champion module device (10 × 10 cm²) with efficiency of 17.82% (another module with certified efficiency of 17.4%) is obtained using DAS process.

Publication date: 18 September 2019

Source: Joule, Volume 3, Issue 9

Author(s): Xiaotian Hu, Xiangchuan Meng, Lin Zhang, Yanyan Zhang, Zheren Cai, Zengqi Huang, Meng Su, Yang Wang, Mingzhu Li, Fengyu Li, Xi Yao, Fuyi Wang, Wei Ma, Yiwang Chen, Yanlin Song

Context & Scale

Summary

Graphical Abstract

Publication date: September 2019

Source: Nano Energy, Volume 63

Author(s): Fengyou Wang, Yuhong Zhang, Meifang Yang, Jinyue Du, Leilei Xue, Lili Yang, Lin Fan, Yingrui Sui, Jinghai Yang, Xiaodan Zhang

Graphical abstract

Perovskite solar cells with ZnO exhibit greatly improved stability when the methylammonium cation is excluded. The interfacial acid‐base reactions between methylammonium and ZnO are probed and the degradation kinetics are modulated by the acidity of the organic cation. Solar cells on ZnO films provide improved open circuit voltage, lower series resistance, and lower processing temperatures than those on SnO₂.

Abstract

Perovskite solar cells have achieved the highest power conversion efficiencies on metal oxide n‐type layers, including SnO₂ and TiO₂. Despite ZnO having superior optoelectronic properties to these metal oxides, such as improved transmittance, higher conductivity, and closer conduction band alignment to methylammonium (MA)PbI₃, ZnO is largely overlooked due to a chemical instability when in contact with metal halide perovskites, which leads to rapid decomposition of the perovskite. While surface passivation techniques have somewhat mitigated this instability, investigations as to whether all metal halide perovskites exhibit this instability with ZnO are yet to be undertaken. Experimental methods to elucidate the degradation mechanisms at ZnO–MAPbI₃ interfaces are developed. By substituting MA with formamidinium (FA) and cesium (Cs), the stability of the perovskite–ZnO interface is greatly enhanced and it is found that stability compares favorably with SnO₂‐based devices after high‐intensity UV irradiation and 85 °C thermal stressing. For devices comprising FA‐ and Cs‐based metal halide perovskite absorber layers on ZnO, a 21.1% scanned power conversion efficiency and 18% steady‐state power output are achieved. This work demonstrates that ZnO appears to be as feasible an n‐type charge extraction layer as SnO₂, with many foreseeable advantages, provided that MA cations are avoided.

Biodegradable and biocompatible transparent conductive electrodes are fabricated from bamboo for flexible perovskite solar cells. After extensive mechanical tests, including bending and crumpling tests, they still exhibit excellent electrical performance and negligible decay. The bamboo‐based bioelectrode perovskite solar cell shows a record power conversion efficiency of 11.68%, maintaining over 70% of initial power conversion efficiency after the bending tests.

Abstract

Wearable devices are mainly based on plastic substrates, such as polyethylene terephthalate and polyethylene naphthalate, which causes environmental pollution after use due to the long decomposition periods. This work reports on the fabrication of a biodegradable and biocompatible transparent conductive electrode derived from bamboo for flexible perovskite solar cells. The conductive bioelectrode exhibits extremely flexible and light‐weight properties. After bending 3000 times at a 4 mm curvature radius or even undergoing a crumpling test, it still shows excellent electrical performance and negligible decay. The performance of the bamboo‐based bioelectrode perovskite solar cell exhibits a record power conversion efficiency (PCE) of 11.68%, showing the highest efficiency among all reported biomass‐based perovskite solar cells. It is remarkable that this flexible device has a highly bendable mechanical stability, maintaining over 70% of its original PCE during 1000 bending cycles at a 4 mm curvature radius. This work paves the way for perovskite solar cells toward comfortable and environmentally friendly wearable devices.

Three novel polytriphenylamine‐based polymers (H‐Z1, H‐Z2, and H‐Z3) are designed and applied as hole‐transport layers in all‐inorganic perovskite solar cells. Due to the gradual deepening of the highest occupied molecular orbital energy levels from H‐Z1, H‐Z2 to H‐Z3, the energy loss (E _loss) can be decreased from 0.69, 0.64, to 0.62 eV for H‐Z1, H‐Z2, and H‐Z3, respectively.

The energy loss (E _loss) control via interfacial engineering is a significant indispensible methodology to realize high‐performance all‐inorganic perovskite solar cells (PVSCs). Herein, three novel polytriphenylamine‐based polymer derivatives (H‐Z1, H‐Z2, and H‐Z3) are synthesized, and the energy levels of these polymers are tuned feasibly through introducing the electron‐withdrawing group of trifluoromethyl in the triphenylamine (TPA) unit. These very deep HOMO energy levels are very beneficial for improving the open‐circuit voltages (V_ocs) in PVSCs with the potentially decreased E _losss. Due to the gradual deepening of HOMO energy levels from H‐Z1, H‐Z2 to H‐Z3, the V_ocs are elevated from 1.23, 1.28 to 1.30 V, respectively, where the E _loss s are decreased from 0.69, 0.64, to 0.62 eV for H‐Z1, H‐Z2, and H‐Z3, respectively. Interestingly, both of the H‐Z1‐ and H‐Z2‐based devices show the highest PCEs, over 14%, in all‐inorganic PVSCs, which are effectively comparable to the results of reference device using Spiro‐OMeTAD as HTL. Thus, through the efficient atomic engineering and chemical modification in corresponding p‐typed polymers, excellent hole transport polymers are achieved for high‐performance and stable PVSCs with very low E _loss.

Publication date: September 2019

Source: Nano Energy, Volume 63

Author(s): Meng Zhang, Jueming Bing, Yongyoon Cho, Yong Li, Jianghui Zheng, Cho Fai Jonathan Lau, Martin A. Green, Shujuan Huang, Anita W.Y. Ho-Baillie

Graphical abstract

KI₃ complex as an additive stabilizes perovskite precursors as well as improving perovskite solar cell device performance.