Chen Weijie

Perovskite solar cells contain various defects within the perovskite absorber and the corresponding interfaces, affecting device performance and stability. Fortunately, there have been tremendous efforts in advancing passivation techniques contributing to high‐efficiency devices with improved stability. Here, the state‐of‐the‐art passivation approaches for each layer of the perovskite cell with the aim of improving carrier extraction, reducing carrier recombination and improving cell stability and performance are reviewed.

Perovskite solar cells contain various defects within the perovskite absorber and the corresponding interfaces, affecting device performance and stability. Fortunately, there have been tremendous efforts in advancing passivation techniques contributing to high‐efficiency perovskite solar cell with improved stability. Here, the state‐of‐the‐art passivation approaches for each layer of the perovskite cell with the aim of improving carrier extraction, reducing carrier recombination, and/or improving cell stability are reviewed. Passivation of the electron transport layer can improve the stability of perovskite solar cells by reducing trap states or by physically separating the transport layer from contacting perovskite. Controlling the amount of PbI₂ in the perovskite precursor has been found to be effective in passivating defect states at the grain boundaries and on the surface. Additives such as elemental iodine, organic surfactants, and Group 1 metal compounds incorporated in perovskite precursors have been reported to passivate recombination trap centers. These approaches have also contributed to improved energy band alignment between carrier transport layers and perovskite absorber improving device performance. An effective strategy to improve moisture stability is the use of 2D perovskites or hydrophobic large cation molecules forming 2D or quasi‐2D phases at grain boundaries or film surfaces providing passivation and preventing moisture ingress.

Bi‐doped MAPbI₃ perovskite can simultaneously improve the Seebeck coefficient and electrical conductivity. It not only promotes the charge transport through carrier channels near grain boundaries, but can also passivate the defects, increasing the stability of MASnI₃.

Abstract

In this article, the thermoelectric properties of a Bi‐doped CH₃NH₃PbI₃ (MAPbI₃) perovskite thin film are studied. Bi‐doped MAPbI₃ thin film samples are fabricated, and it is found that Bi doping could greatly enhance the stability and thermoelectric properties of MAPbI₃. The Bi dopant located at the grain boundaries to modify the carrier channel near grain boundaries, which is observed via scanning electron microscopy and atomic force microscopy, efficiently reduces ion migration and facilitates charge transport. In addition, the Bi dopant can also passivate the defects in bulk MAPbI₃, increasing the polarization effect of MAPbI₃ which is demonstrated by the capacitance‐frequency measurement, thus greatly enhancing the mobility of Bi‐doped MAPbI₃. In addition, Bi‐doped MAPbI₃ leads to grain size reduction; the small size effect not only effectively hinders the MAPbI₃'s crystal phase transition from the tetragonal phase to the cubic phase, but it could also make the structure of MAPbI₃ more stable. Especially, the Seebeck voltage variation of Bi‐doped perovskite was less than that of the undoped one, meaning Bi doping would lead to a much more stable state in MAPbI₃ thin films. The results show that Bi‐doped MAPbI₃ is a promising approach to develop high stable thermoelectric and photovoltaic properties in organic–inorganic hybrid perovskite materials.

An effective approach to low temperature, solution‐processed ZnO electron‐transport layers (ETLs) for perovskite solar cells by combustion synthesis is developed. Due to the intrinsic passivation effects, high crystallinity, matched energy levels, ideal surface topography, and good chemical compatibility with the perovskite layer, combustion‐processed ZnO electron transport layers enable power conversion efficiencies approaching 17–20% for three representative perovskite systems without ETL doping or surface functionalization.

Abstract

Perovskite solar cells (PSCs) have advanced rapidly with power conversion efficiencies (PCEs) now exceeding 22%. Due to the long diffusion lengths of charge carriers in the photoactive layer, a PSC device architecture comprising an electron‐ transporting layer (ETL) is essential to optimize charge flow and collection for maximum performance. Here, a novel approach is reported to low temperature, solution‐processed ZnO ETLs for PSCs using combustion synthesis. Due to the intrinsic passivation effects, high crystallinity, matched energy levels, ideal surface topography, and good chemical compatibility with the perovskite layer, this combustion‐derived ZnO enables PCEs approaching 17–20% for three types of perovskite materials systems with no need for ETL doping or surface functionalization.

A simple‐to‐apply antireflective coating, immersion oil application, and chromophore selection enable a processable, ambiently stable solar‐to‐fuel electrolysis device at 6.6% efficiency with no added bias. The sequential series multijunction dye‐sensitized solar cell technology is improved in solar‐to‐electric conversion efficiency substantially by controlling optical loss pathways with a >10% efficiency and 2.3 V output from a single illuminated area.

Abstract

Sequential series multijunction dye‐sensitized solar cells (SSM‐DSCs) can power solar‐to‐fuel processes with a single illuminated area device. Dye selection and strategies limiting photon losses are critical in SSM‐DSC devices for higher performance systems. Herein, an efficient and readily applicable spin coating protocol on glass surfaces with an antireflective fluoropolymer (CYTOP) is applied to an SSM‐DSC architecture. Combining CYTOP with the use of an immersion oil between glass spacers in a three subcell SSM‐DSC with judiciously selected TiO₂ photoanode sensitizers and thicknesses, an overall power conversion efficiency (PCE) of 10.1% is obtained with an output of 2.3 V. Without external bias, this SSM‐DSC configuration shows an impressive overall solar‐to‐fuel conversion efficiency of 6% when powering IrO₂ and Au₂O₃ electrocatalysts for CO₂ and H₂O to CO and H₂ conversion in aqueous solution. The role of CYTOP, immersion oil, sensitizer selection, and film thickness on SSM‐DSC devices is discussed along with the stability of this system.

Through the rational molecular design of fluorination, the work function of the conjugated polymer (CP) is enhanced from 4.83 to 5.00 eV. Consequently, the CP can be used to modify efficient active layers consisting of polymer donors with a deep HOMO level, such as PBDB‐T‐2F:IT‐4F, and an outstanding power conversion efficiency of 12.7% is achieved in the corresponding device without V _oc loss.

Abstract

Since the highest occupied molecular orbital (HOMO) level of donors in organic solar cells (OSCs) is being constantly downshifted for achieving high open‐circuit voltage (V _oc), a further enhancement of the anode work function (WF) is required. Herein, an effective approach of fluorination is demonstrated to simultaneously improve the WF and transparency for anode interlayer (AIL) material. By fluorination, in combination with the dialysis treatment in LiCl solution, the WF of PCP‐2F‐Li could be significantly enhanced from 4.86 to 5.0 eV, as compared to PCP‐Na. Meanwhile, the transparency of the polymer is also improved. As a result, PCP‐2F‐Li can be used to modify efficient active layers consisting of polymer donors with deep HOMO levels, such as PBDB‐T‐2F:IT‐4F, and an outstanding power conversion efficiency (PCE) of 12.7% is achieved in the corresponding device with a high V _oc of 0.84 V. This result represents the highest efficiency for the OSCs using a solution‐processed pH‐neutral AIL, which is beneficial to the low‐cost fabrication of high‐performance OSCs with improved stability. More importantly, PCP‐2F‐Li could be processed by blade coating for making large‐area device of 1 cm², and a PCE of 10.6% is achieved, bringing a promising prospect for the large‐area device fabrication.

Leadless hybrid perovskites are obtained by the epitaxial synthesis of CsPbX₃ (X = Cl, Br, I) perovskite quantum dots through surface chemical conversion of Cs₂GeF₆ double perovskites with PbX₂ (X = Cl, Br, I). The obtained CsPbBr₃/Cs₂GeF₆ products show high color purity and enhanced stability, indicating their potential application in lighting devices.

Abstract

Lead halide perovskites (LHPs) have received increased attention owing to their intriguing optoelectronic and photonic properties. However, the toxicity of lead and the lack of long‐term stability are potential obstacles for the application of LHPs. Herein, the epitaxial synthesis of CsPbX₃ (X = Cl, Br, I) perovskite quantum dots (QDs) by surface chemical conversion of Cs₂GeF₆ double perovskites with PbX₂ (X = Cl, Br, I) is reported. The experimental results show that the surface of the Cs₂GeF₆ double perovskites is partially converted into CsPbX₃ perovskite QDs and forms a CsPbX₃/Cs₂GeF₆ hybrid structure. The theoretical calculations reveal that the CsPbBr₃ conversion proceeds at the Cs₂GeF₆ edge through sequential growth of multiple PbBr₆ ⁴⁻ layers. Through the conversion strategy, luminescent and color‐tunable CsPbX₃ QDs can be obtained, and these products present high stability against decomposition due to anchoring effects. Moreover, by partially converting red emissive Cs₂GeF₆:Mn⁴⁺ to green emissive CsPbBr₃, the CsPbBr₃/Cs₂GeF₆:Mn⁴⁺ hybrid can be employed as a low‐lead hybrid perovskite phosphor on blue LED chips to produce white light. The leadless CsPbX₃/Cs₂GeF₆ hybrid structure with stable photoluminescence opens new paths for the rational design of efficient emission phosphors and may stimulate the design of other functional CsPbX₃/Cs‐containing hybrid structures.

CZTS quantum dots are employed as hole transporting materials for CsPbBr₃ inorganic perovskite solar cells. More effective hole extraction and transfer properties, as well as stability have been demonstrated after introducing CZTS as the hole selective contact. This work reveals the great promise of CZTS as hole acceptors within inorganic perovskite‐based devices.

All‐inorganic CsPbBr₃ perovskite solar cells (PSCs) have recently generated tremendous interest in next‐generation cost‐effective and stable photovoltaic devices. However, the commonly used costly and unstable organic hole transporting material (HTM) has so far prevented the further development and large‐scale application of PSCs. In this work, Cu₂ZnSnS₄ quantum dots (CZTS QDs) are exploited as a novel inorganic HTM for CsPbBr₃ PSCs. Due to the well‐matched energy levels with the inorganic perovskite layer, a decent power conversion efficiency of 4.84% is achieved, which is quite comparable to the efficiency of the traditional device based on spiro‐OMeTAD HTM (5.36%). Moreover, the photoluminescence (PL) and impedance spectroscopy further demonstrate the more effective hole extraction and transfer properties of the CZTS QDs interface layer, making it a promising material for fabricating efficient and stable PSCs toward practical applications.

Publication date: 20 March 2019

Source: Joule, Volume 3, Issue 3

Author(s): Xin Song, Nicola Gasparini, Masrur Morshed Nahid, Sri Harish Kumar Paleti, Jin-Liang Wang, Harald Ade, Derya Baran

Context & Scale

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The rapid development of n‐type polymers has boosted the efficiency of all‐polymer solar cells, which has improved from 2% to 10 % in only seven years. There is a strong need to summarize the design criteria, synthesis, structure–property relationships and recent advances of n‐type polymers, which is addressed in this review. Moreover, the challenges and prospects for further development of all‐PSCs are briefly discussed.

Abstract

All‐polymer solar cells (all‐PSCs) based on n‐ and p‐type polymers have emerged as promising alternatives to fullerene‐based solar cells due to their unique advantages such as good chemical and electronic adjustability, and better thermal and photochemical stabilities. Rapid advances have been made in the development of n‐type polymers consisting of various electron acceptor units for all‐PSCs. So far, more than 200 n‐type polymer acceptors have been reported. In the last seven years, the power conversion efficiency (PCE) of all‐PSCs rapidly increased and has now surpassed 10%, meaning they are approaching the performance of state‐of‐the‐art solar cells using fullerene derivatives as acceptors. This review discusses the design criteria, synthesis, and structure–property relationships of n‐type polymers that have been used in all‐PSCs. Additionally, it highlights the recent progress toward photovoltaic performance enhancement of binary, ternary, and tandem all‐PSCs. Finally, the challenges and prospects for further development of all‐PSCs are briefly considered.

A quantum of solace: Quantum dots (QDs) of lead chalcogenides have potential in a wide range of applications, such as, photovoltaics (PV), optoelectronics, sensors, and bio‐electronics. The surface ligand plays an essential role in the production of QDs, post‐synthesis modification, and their integration to practical applications. This Review demonstrates the application of colloidal synthesis techniques for the preparation of lead chalcogenide based QDs.

Abstract

Quantum dots (QDs) of lead chalcogenides (e.g. PbS, PbSe, and PbTe) are attractive near‐infrared (NIR) active materials that show great potential in a wide range of applications, such as, photovoltaics (PV), optoelectronics, sensors, and bio‐electronics. The surface ligand plays an essential role in the production of QDs, post‐synthesis modification, and their integration to practical applications. Therefore, it is critically important that the influence of surface ligands on the synthesis and properties of QDs is well understood for their applications in various devices. In this Review we elaborate the application of colloidal synthesis techniques for the preparation of lead chalcogenide based QDs. We specifically focus on the influence of surface ligands on the synthesis of QDs and their solution‐phase ligand exchange. Given the importance of lead chalcogenide QDs as potential light harvesters, we also pay particular attention to the current progress of these QDs in photovoltaic applications.

Publication date: May 2019

Source: Nano Energy, Volume 59

Author(s): Sirazul Haque, Manuel J. Mendes, Olalla Sanchez-Sobrado, Hugo Águas, Elvira Fortunato, Rodrigo Martins

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Publication date: May 2019

Source: Nano Energy, Volume 59

Author(s): Miao Zhang, Ruijie Ming, Wei Gao, Qiaoshi An, Xiaoling Ma, Zhenghao Hu, Chuluo Yang, Fujun Zhang

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