Chen Weijie

The time‐resolved grazing‐incidence wide‐angle X‐ray scattering technique provides real‐time insights on the phase‐transition during the organic cation coating and perovskite quantum wells (PQWs)/3D architecture formation mechanism. With fluorinated poly(triarylamine) (PTAA) as a dopant‐free hole‐transport layer, this PQWs/3D architecture leads to stable perovskite photovoltaics with power conversion efficiency of >22%.

Abstract

The combination of a bulk 3D perovskite layer and a reduced dimensional perovskite layer (perovskite quantum wells (PQWs)) is demonstrated to enhance the performance of perovskite solar cells (PSCs) significantly in terms of stability and efficiency. This perovskite hierarchy has attracted intensive research interest; however, the in‐depth formation mechanism of perovskite quantum wells on top of a 3D perovskite layer is not clearly understood and is therefore the focus of this study. Along with ex situ morphology and photophysical characterization, the time‐resolved grazing‐incidence wide‐angle X‐ray scattering (TS‐GIWAXS) technique performed in this study provides real‐time insights on the phase‐transition during the organic cation (HTAB ligand molecule) coating and PQWs/3D architecture formation process. A strikingly strong ionic reaction between the 3D perovskite and the long‐chain organic cation leads to the quick formation of an ordered intermediate phase within only a few seconds. The optimal PQWs/3D architecture is achieved by controlling the HTAB casting, which is assisted by time‐of‐flight SIMS characterization. By controlling the second ionic reaction during the long‐chain cation coating process, along with the fluorinated poly(triarylamine) (PTAA) as a hole‐transport layer, the perovskite solar cells demonstrate efficiencies exceeding 22% along with drastically improved device stability.

Currently, the further performance improvement for inverted perovskite solar cells (PVSCs) is mainly limited by the high open circuit voltage ( V OC ) loss caused by the detrimental non‐radiative recombination (NRR) processes. Herein, we report a simple and efficient way to simultaneously reduce the NRR processes inside perovskite and at the interface by rationally designing a new pyridine‐based polymer hole transporting material (HTM), i. e. PPY2 , which exhibits suitable energy levels with perovskites, high hole mobility, effective passivation of the uncoordinated Pb 2+ and iodide defects, as well as the capability of promoting the formation of high quality polycrystalline perovskite film. In absence of any dopants, the inverted PVSCs using PPY2 as the HTM deliver an encouraging PCE up to 22.41% with a small V OC loss (0.40 eV), among the best device performance for inverted PVSCs reported so far. Furthermore, PPY2 ‐based unencapsulated devices show an excellent long‐term photostability, and over 97% of its initial PCE can be maintained after one sun constant illumination for 500 h.

Publication date: April 2021

Source: Nano Energy, Volume 82

Author(s): Jingnan Wu, Qunping Fan, Minghai Xiong, Qiutang Wang, Kai Chen, Haiqin Liu, Mengyuan Gao, Long Ye, Xia Guo, Jin Fang, Qing Guo, Wenyan Su, Zaifei Ma, Zheng Tang, Ergang Wang, Harald Ade, Maojie Zhang

Two sequentially deposited SnO₂ layers doped with a low and a high amount of ammonium chloride, respectively, boost the open‐circuit voltage and fill factor of perovskite solar cells. The main effect of the novel electron transport layer is a change in the energy level alignment with the perovskite interface leading to decreased carrier recombination.

Abstract

Tin oxide (SnO₂) is an emerging electron transport layer (ETL) material in halide perovskite solar cells (PSCs). Among current limitations, open‐circuit voltage (V _OC) loss is one of the major factors to be addressed for further improvement. Here a bilayer ETL consisting of two SnO₂ nanoparticle layers doped with different amounts of ammonium chloride is proposed. As demonstrated by photoelectron spectroscopy and photophysical studies, the main effect of the novel ETL is to modify the energy level alignment at the SnO₂/perovskite interface, which leads to decreased carrier recombination, enhanced electron transfer, and reduced voltage loss. Moreover, X‐ray diffraction reveals reduced strain in perovskite layers grown on bilayer ETLs with respect to single‐layer ETLs, further contributing to a decrease of carrier recombination processes. Finally, the bilayer approach enables the more reproducible preparation of smooth and pinhole‐free ETLs as compared to single‐step deposition ETLs. PSCs with the doped bilayer SnO₂ ETL demonstrate strongly increased V _OC values of up to 1.21 V with a power conversion efficiency of 21.75% while showing negligible hysteresis and enhanced stability. Moreover, the SnO₂ bilayer can be processed at low temperature (70 °C), and has therefore a high potential for use in tandem devices or flexible PSCs.

Nonfullerene acceptors dominate organic solar cell research due to their promising high device efficiencies. However, key challenges for achieving high stability in commercially viable devices still remain. In this review, recent progress and challenges toward stable organic solar cells are discussed correlating molecular design and device engineering to device stability.

Abstract

Organic solar cells (OSCs) based on nonfullerene acceptors (NFAs) have made significant breakthrough in their device performance, now achieving a power conversion efficiency of ≈18% for single junction devices, driven by the rapid development in their molecular design and device engineering in recent years. However, achieving long‐term stability remains a major challenge to overcome for their commercialization, due in large part to the current lack of understanding of their degradation mechanisms as well as the design rules for enhancing their stability. In this review, the recent progress in understanding the degradation mechanisms and enhancing the stability of high performance NFA‐based OSCs is a specific focus. First, an overview of the recent advances in the molecular design and device engineering of several classes of high performance NFA‐based OSCs for various targeted applications is provided, before presenting a critical review of the different degradation mechanisms identified through photochemical‐, photo‐, and morphological degradation pathways. Potential strategies to address these degradation mechanisms for further stability enhancement, from molecular design, interfacial engineering, and morphology control perspectives, are also discussed. Finally, an outlook is given highlighting the remaining key challenges toward achieving the long‐term stability of NFA‐OSCs.

Highly efficient CsPbI_1.5Br_1.5 perovskite solar cells (PSCs) are achieved via introducing fluorescein isothiocyanate (FITC) organic dye as passivator. FITC not only reduces the metal ion related trap states but also improves film crystallinity, resulting in an enhancement of device efficiency from 12.3% to 14.05%. In addition, it is demonstrated that CsPbI_1.5Br_1.5 perovskite shows the optimal halide composition for inorganic PSCs.

Abstract

All‐inorganic perovskite solar cells (PSCs) have recently received growing attention as a promising template to solve the thermal instability of organic–inorganic PSCs. However, the thermodynamic phase instability and relatively low device efficiency pose challenges. Herein, highly efficient and stable CsPbI_1.5Br_1.5 compositional perovskite‐based inorganic PSCs are fabricated using an organic dye, fluorescein isothiocyanate (FITC), as a passivator. The carboxyl and thiocyanate groups of FITC not only minimize the trap states by forming interactions with the under‐coordinated Pb²⁺ ions but also significantly increase the grain size and improve the crystallinity of the perovskite films during annealing. Consequently, perovskite films with superior optoelectronic properties, prolonged carrier lifetime, reduced trap density, and improved stability are obtained. The resulting device yields a champion efficiency of 14.05% with negligible hysteresis, which presents the highest reported efficiency for inorganic CsPbI_1.5Br_1.5 solar cells reported thus far. In addition, FITC can be generally adopted as attractive passivator to improve the performance of CsPbI₂Br‐ and CsPbIBr₂‐based PSCs. Furthermore, with a comprehensive comparison of mixed‐halide inorganic perovskites, it is demonstrated that CsPbI_1.5Br_1.5 compositional perovskite is a promising candidate with the optimal halide composition for high‐performance inorganic PSCs.

Field‐induced formation of dopant‐free radial junctions at the Al₂O₃/n‐c‐Si (crystalline silicon) interface is demonstrated. Atomic layer deposition of Al₂O₃ conformally coats tapered c‐Si microwire arrays to form the radial junctions. A dopant‐free radial junction solar cell is fabricated based on this technique. At 20.1%, the device obtains the highest efficiency compared with that of previously reported radial junction solar cells.

Abstract

Radial junctions on crystalline silicon (c‐Si) microwire structures considerably reduce the diffusion length of photoinduced minority carriers required for energy generation by decoupling light absorption and carrier separation in orthogonal spatial directions. Hence, radial junctions mitigate the need for high‐purity materials, and thus reduce the fabrication cost of c‐Si solar cells. In this study, the formation of dopant‐free radial junctions from atomic layer deposition (ALD) of Al₂O₃ on an n‐c‐Si microwire surface is reported. ALD‐Al₂O₃ generates a p⁺ inversion layer, which eventually forms the radial junction on the n‐c‐Si surface. The width of depletion region induced by the p⁺ inversion layer is calculated from PC1D simulation as 900 nm. The fabricated dopant‐free radial junction c‐Si solar cells exhibit a power conversion efficiency of 20.1%, which is higher than those of previously reported microwire‐based radial junction solar cells. Notably, internal quantum efficiencies of over 90% are obtained in the 300–980 nm wavelength region, thereby verifying the successful formation of radial junctions.

A defect‐resolved mobility measurement (DRMM) method is developed, enabling the evaluation of effective majority carrier mobility of highly anisotropic materials such as antimony chalcogenides directly from a working device. This method provides a reliable approach to the investigation of the carrier transport mechanisms of Sb₂Se₃ and Sb₂S₃ solar cells and other anisotropic low‐dimensional crystal semiconductor devices.

Majority carrier mobility is one of the most fundamental and yet important carrier transport parameters determining the optimal device architecture and performance of the emerging antimony chalcogenide solar cells. However, carrier mobility measurements based on the Hall effect have limitations for these highly anisotropy materials due to the discrepancy of transport directions under Hall measurement and device operation. Herein, a defect‐resolved mobility measurement (DRMM) method enabling the evaluation of effective majority carrier mobility from a working device without such limitations is presented. Using this method, comprehensive information about the carrier transport in representative Sb₂S₃ and Sb₂Se₃ solar cells is extracted. Though with preferred [hk1]‐crystalline orientation, Sb₂S₃ and Sb₂Se₃ still suffer from extremely low carrier mobility and low carrier density, respectively, resulting in large bulk resistance and poor carrier collection efficiency. Further crystalline structure analysis discloses that crystalline defects such as dislocations may significantly constrain carrier transport in these low‐dimensional materials. These results suggest that a p‐i‐n device architecture with fully depleted absorber is a promising optimization approach for further efficiency advances of antimony chalcogenide solar cells.

This Minireview describes developments in all‐polymer solar cells containing a new type of n‐type conjugated polymer, polymerized small‐molecule acceptors (PSMAs). PSMAs combine the merits of small‐molecule acceptors (narrow band gap, strong absorption, and suitable electronic energy levels) with the good film formation, higher morphology and light‐irradiation stability of polymers.

Abstract

All‐polymer solar cells (all‐PSCs) have drawn tremendous research interest in recent years, due to their inherent advantages of good film formation, stable morphology, and mechanical flexibility. The most representative and most widely used n‐CP acceptor was the naphthalene diimide based D‐A copolymer N2200 before 2017, and the power conversion efficiency (PCE) of the all‐PSCs based on N2200 reached over 8% in 2016. However, the low absorption coefficient of N2200 in the near‐infrared (NIR) region limits the further increase of its PCE. In 2017, we proposed a strategy of polymerizing small‐molecule acceptors (SMAs) to construct new‐generation polymer acceptors. The polymerized SMAs (PSMAs) possess low band gap and strong absorption in the NIR region, which attracted great attention and drove the PCE of the all‐PSCs to over 15% recently. In this Minireview we explain the design strategies of the molecular structure of PSMAs and describe recent research progress. Finally, current challenges and future prospects of the PSMAs are analyzed and discussed.

The impact of SnF₂ on FA_{0 . 83}Cs_{0 . 17}Sn _x Pb_{1− x} I₃ perovskite thin films is examined for additive amounts varying between 0.1% and 20%. Structural distortion from lattice strain is reduced by SnF₂ addition. Lower background hole doping, longer photoluminescence lifetimes, and higher charge‐carrier mobilities are observed with as little as 1% SnF₂ added. Larger quantities of the additive introduce defects alongside these beneficial effects.

Abstract

Mixed tin‐lead halide perovskites are promising low‐bandgap absorbers for all‐perovskite tandem solar cells that offer higher efficiencies than single‐junction devices. A significant barrier to higher performance and stability is the ready oxidation of tin, commonly mitigated by various additives whose impact is still poorly understood for mixed tin‐lead perovskites. Here, the effects of the commonly used SnF₂ additive are revealed for FA_{0 . 83}Cs_{0 . 17}Sn _x Pb_{1− x} I₃ perovskites across the full compositional lead‐tin range and SnF₂ percentages of 0.1–20% of precursor tin content. SnF₂ addition causes a significant reduction in the background hole density associated with tin vacancies, yielding longer photoluminescence lifetimes, decreased energetic disorder, reduced Burstein–Moss shifts, and higher charge‐carrier mobilities. Such effects are optimized for SnF₂ addition of 1%, while for 5% SnF₂ and above, additional nonradiative recombination pathways begin to appear. It is further found that the addition of SnF₂ reduces a tetragonal distortion in the perovskite structure deriving from the presence of tin vacancies that cause strain, particularly for high tin content. The optical phonon response associated with inorganic lattice vibrations is further explored, exhibiting a shift to higher frequency and significant broadening with increasing tin fraction, in accordance with lower effective atomic metal masses and shorter phonon lifetimes.

Half‐mixed Pb‐Sn perovskite solar cells with significantly improved performance and stability are prepared by introducing an ionic imidazolium tetrafluoroborate additive. The synergistic effects of IM cation and tetrafluoroborate anion enable efficient defect passivation at grain boundaries, reducing leakage current, and enlargement in grain size with relaxed lattice strain simultaneously, thereby exerting a remarkable impact on device performance and stability.

Abstract

Narrow‐bandgap mixed Pb‐Sn perovskite solar cells (PSCs) have great feasibility for constructing efficient all‐perovskite tandem solar cells, in combination with wide‐bandgap lead halide PSCs. However, the power conversion efficiency of mixed Pb‐Sn PSCs still lags behind lead‐based counterparts. Here, additive engineering using ionic imidazolium tetrafluoroborate (IMBF₄) is proposed, where the imidazolium (IM) cation and tetrafluoroborate (BF₄) anion efficiently passivate defects at grain boundaries and improve crystallinity, simultaneously relaxing lattice strain, respectively. Defect passivation is achieved by the chemical interaction between the IM cation and the positively charged under‐coordinated Pb²⁺ or Sn²⁺ ions, and lattice strain relaxation is realized by lattice expansion with the intercalation of BF₄ anions into the perovskite lattice. As a result, the synergistic effects of the cation and anion in the IMBF₄ additive greatly enhance the optoelectronic performance of half‐mixed Pb‐Sn perovskites, leading to much longer carrier lifetimes. The best‐performing half‐mixed Pb‐Sn PSC shows an efficiency above 19% with negligible hysteresis, while retaining over 90% of its initial efficiency after 1000 h in a nitrogen‐filled glovebox and showing a lifetime to 80% degradation of 53.5 h under continuous illumination.

A novel strategy to neutralize charged point defects in organic‐inorganic hybrid perovskite materials is proposed for highly efficient and stable perovskite solar cells by using a zwitterionic L‐alanine additive, which can be passivated simultaneously with both positively and negatively charged defects because it contains both anion and cation functional groups in one molecule.

Abstract

Ionic defects (e.g., organic cations and halide anions), preferably residing along grain boundaries (GBs) and on perovskite film surfaces, are known to be a major source of the notorious environmental instability of perovskite solar cells (PeSCs). Although passivating ionic defects is desirable, previous approaches using Lewis base or acid molecules as additives suppress only the negatively or positively charged defects, thus leaving oppositely charged defects. In this work, both the cationic and anionic defects inside methyl ammonium lead tri‐iodide (MAPbI₃) are simultaneously passivated by introducing a zwitterionic form of the amino acid, L‐alanine, into the precursor solution as an additive. L‐alanine has both positive (NH₃ ⁺) and negative (COO⁻) functional groups at a specific solvent pH, thereby passivating both the cation and anion defects in MAPbI₃. The addition of L‐alanine increases the grain size of the perovskite crystals and lengthens the charge carrier lifetime (τ > 1 µs), leading to improved power conversion efficiencies (PCEs) of 20.3% (from 18.3% without an additive) for small‐area (4.64 mm²) devices and 15.6% (from 13.5%) for large‐area submodules (9.06 cm²). More importantly, the authors’ approach also significantly enhances the shelf storage and photoirradiation stabilities of PeSCs.