Chen Weijie

The effects of electronic property and backbone configuration of the imide-functionalized ladder-type heteroarenes-based polymer acceptors on performance of all-polymer solar cells are investigated. Heteroarene size extension and polymer backbone fluorination not only lower the frontier molecular orbital energy levels but also affect symmetric characteristics of PBTIn-(F)T. The highest efficiencies are attained from PBTI3-(F)T polymers with a curved backbone configuration.

Abstract

Electron-deficient ladder-type π-conjugated systems are highly desired for constructing polymer acceptors due to their unique electronic properties. Herein, two series of polymer acceptors PBTIn-(F)T (n = 1–4) based on imide-functionalized ladder-type heteroarenes (BTIn) with tunable conjugation length are synthesized. Effects of their backbone configuration and electronic properties on film morphology and performance of all-polymer solar cells (all-PSCs) are systematically investigated through theoretical computation, Raman spectroscopy, grazing incidence wide-angle X-ray scattering, etc. It is found that the ladder-type heteroarene size extension and polymer backbone fluorination gradually lower the frontier molecular orbital energy levels, leading to progressive bandgap narrowing with more efficient exciton dissociation. Furthermore, the centrosymmetric and axisymmetric characteristics of BTIn result in distinct backbone configuration with varied self-aggregation and crystalline phases, hence determining the blend film morphology. The highest efficiencies in these two series are attained from PBTI3-T and PBTI3-FT with a curved backbone configuration. PBTI4-(F)T with further extended heteroarenes shows linear backbone, negatively affecting film morphology and efficiency. This study provides fundamental material structure-device performance correlations for ladder-type heteroarenes-based polymer acceptors for the first time and demonstrates that more extended ladder-type backbones do not necessarily improve the device performance, offering guidelines for designing polymer acceptors to maximize all-PSC performance.

A reversible mechanochromism of Dion–Jacobson and Ruddlesden–Popper layered hybrid perovskites based on 1,4-phenylenedimethylammonium and benzylammonium spacers is demonstrated in the 0–0.35 GPa pressure range. X-ray scattering and density functional theory calculations under pressure reveal that this is related to the structural interplay between the spacer and the inorganic framework, providing critical insights that expand the perspectives for future applications.

Abstract

Layered Dion–Jacobson (DJ) and Ruddlesden–Popper (RP) hybrid perovskites are promising materials for optoelectronic applications due to their modular structure. To fully exploit their functionality, mechanical stimuli can be used to control their properties without changing the composition. However, the responsiveness of these systems to pressure compatible with practical applications (<1 GPa) remains unexploited. Hydrostatic pressure is used to investigate the structure–property relationships in representative iodide and bromide DJ and RP 2D perovskites based on 1,4-phenylenedimethylammonium (PDMA) and benzylammonium (BzA) spacers in the 0–0.35 GPa pressure range. Pressure-dependent X-ray scattering measurements reveal that lattices of these compositions monotonically shrink and density functional theory calculations provide insights into the structural changes within the organic spacer layer. These structural changes affect the optical properties; the most significant shift in the optical absorption is observed in (BzA)₂PbBr₄ under 0.35 GPa pressure, which is attributed to an isostructural phase transition. Surprisingly, the RP and DJ perovskites behave similarly under pressure, despite the different binding modes of the spacer molecules. This study provides important insights into how the manipulation of the crystal structure affects the optoelectronic properties of such materials, whereas the reversibility of their response expands the perspectives for future applications.

High-quality NiO _x nanoparticles have been synthesized via an ionic liquid-assisted synthesis method (NiO _x -IL). The inverted perovskite solar cell based on a NiO _x -IL hole transport layer exhibits a striking efficiency exceeding 22.62 %, and a superior operational stability maintaining 92 % of its initial efficiency at a maximum power point tracking under one sun illumination for 1000 h.

Abstract

The performance enhancement of inverted perovskite solar cells applying nickel oxide (NiO _x ) as the hole transport layer (HTL) has been limited by impurity ions (such as nitrate ions). Herein, we have proposed a strategy to obtain high-quality NiO _x nanoparticles via an ionic liquid-assisted synthesis method (NiO _x -IL). Experimental and theoretical results illustrate that the cation of the ionic liquid can inhibit the adsorption of impurity ions on nickel hydroxide through a strong hydrogen bond and low adsorption energy, thereby obtaining NiO _x -IL HTL with high conductivity and strong hole-extraction ability. Importantly, the removal of impurity ions can effectively suppress the redox reaction between the NiO _x film and the perovskite film, thus slowing down the deterioration of device performance. Consequently, the modified inverted device shows a striking efficiency exceeding 22.62 %, and superior stability maintaining 92 % efficiency at a maximum power point tracking under one sun illumination for 1000 h.

An intermediate phase-assisted sequential deposition strategy is demonstrated to prepare a CsPbI₂Br film composed of CsPbI₂Br grains and CsBr species. CsPbI₂Br film possesses full coverage, micro-sized grains, and excellent phase stability. Meanwhile, CsBr species can passivate defects. Carbon-electrode perovskite solar cell CsPbI₂Br film yields the record-high efficiency of 15.24%, coupled with a remarkable photovoltage of 1.312 V and excellent humidity stability.

All-inorganic perovskite CsPbI₂Br is emerging as a promising absorber material for perovskite solar cells (PSCs) due to its superior photophysical properties and thermal stability. However, there are still many great challenges to obtaining high-quality, phase-stable, thick CsPbI₂Br films in ambient air to promote further development of the PSCs. Herein, for the first time, an intermediate phase-assisted sequential deposition for desired CsPbI₂Br films is proposed. It is carried out by sequentially spin-coating PbBr₂ and CsI precursors onto the substrate in ambient air, during which a Ruddlesden–Popper (R–P) perovskite intermediate phase film composed of a Cs-Pb-I-Br complex is produced. After annealing, the intermediate phase film is transformed into a CsPbI₂Br film consisting of CsPbI₂Br grains and CsBr species through a spinodal decomposition reaction. The as-obtained CsPbI₂Br film holds full coverage, micro-sized grains, and excellent phase stability. Moreover, the CsBr species located at grain boundaries can effectively passivate the defects. Therefore, a carbon-electrode PSC with such a desired CsPbI₂Br film yields the optimized efficiency of 15.24%, coupled with a remarkable photovoltage of 1.312 V and excellent stability in ambient air with relative humidity of 60–70%. The efficiency achieved herein is among the record efficiencies for carbon-electrode PSCs based on various all-inorganic perovskites reported currently.

New Spiro-OMeTAD analogues and simpler “half” compounds, bearing methoxyphenyl and/or carbazolyl chromophores, are synthesized and evaluated as hole-transporting materials for perovskite solar cells. Their power conversion efficiencies range from 16.1% to 18.3%. Devices containing “half” compounds, bearing methoxyphenyl and carbazolyl chromophores (V1222, V1226, V1238), present with almost no degradation after 450 h under continuous illumination and outperform Spiro-OMeTAD.

New Spiro-OMeTAD analogues and the simpler “half” structures with the terminated methoxyphenyl and/or carbazolyl chromophores are successfully synthesized under Hartwig–Buchwald amination conditions using commercially available starting materials. New fluorene-based hole-transporting materials combined with suitable ionization energies properly align with the valence band of the perovskite absorber. Additionally, these compounds are amorphous, which is an advantage for the formation of homogenous films, as well as eliminate the possibility for films to crystallize during operation of the devices. The most efficient perovskite solar cells devices contain carbazolyl-terminated Spiro-OMeTAD analogue V1267 and reach a power conversion efficiency of 18.3%, along with a short-circuit current density, open-circuit voltage, and fill factor of 23.41 mA cm⁻², 1.06 V, and 74.0%, respectively. Moreover, “half” structures with methoxyphenyl/carbazolyl fragments show excellent long-term stability and outperform Spiro-OMeTAD and, therefore, hold a great prospect for practical wide-scale applications in optoelectronic devices.

Barium ions are reported to effectively n-dope perovskites. Distribution of the barium ions and related n-doping effect is in a gradient across the perovskite film, resulting in a homojunction, which facilitates the separation and transport of the photogenerated carriers. Carrier diffusion length >2 µm and boosted efficiencies of 21.2% for single-junction cell and 25.3% for all-perovskite tandem cell are achieved.

Abstract

Narrow-bandgap (NBG) tin (Sn)–lead (Pb) perovskites generally have a high density of unintentional p-type self-doping, which reduces the charge-carrier lifetimes, diffusion lengths, and device efficiencies. Here, a p–n homojunction across the Sn–Pb perovskite is demonstrated, which results from a gradient doping by barium ions (Ba²⁺). It is reported that 0.1 mol% Ba²⁺ can effectively compensate the p-doping of Sn–Pb perovskites or even turns it to n-type without changing its bandgap. Ba²⁺ cations are found to stay at the interstitial sites and work as shallow electron donor. In addition, Ba²⁺ cations show a unique heterogeneous distribution in perovskite film. Most of the barium ions stay in the top 600 nm region of the perovskite films and turn it into weakly n-type, while the bottom portion of the film remains as p-type. The gradient doping forms a homojunction from top to bottom of the perovskite films with a built-in field that facilitates extraction of photogenerated carriers, resulting in an increased carrier extraction length. This strategy enhances the efficiency of Sn–Pb perovskite single-junction solar cells to over 21.0% and boosts the efficiencies of monolithic perovskite–perovskite tandem solar cells to 25.3% and 24.1%, for active areas of 5.9 mm² and 0.94 cm², respectively.

1,4-Butanediamine dihydroiodide not only effectively reduces the defect density but forms a Dion-Jacobson 2D structure to enhance interfacial charge extraction and suppresses surface charge recombination, leading to a nearly zero-degradation of the efficiency after 800 h of continuous thermal aging (60 °C) with a high open-circuit voltage of 1.24 V and power conversion efficiency of 18.34%.

Abstract

While 2D Ruddlesden-Popper (RP) perovskites exhibit attractive opto-electronic properties and stability for use in perovskite solar cells (PSCs), their complicated film-forming processes often induce a non-negligible level of defects that significantly undermine the power conversion efficiency (PCE) and stability of PSCs. Here, the use of two organic ammonium salts with the same chain length, namely monoammonium (butylammonium iodide, BAI) and diammonium (1,4-butanediamine dihydroiodide, BDAI₂) for surface defect passivation of RP-2D perovskite films of (AA)₂MA₄Pb₅I₁₆ (n = 5) are reported. It is found that the diammonium BDAI₂ not only effectively reduces the defect density (similarly to using monoammonium BAI) but forms a Dion-Jacobson (DJ) 2D structure to enhance interfacial charge extraction and suppress surface charge recombination. As a result, a boosted PCE of 18.34% has been obtained with a high open-circuit voltage of 1.24 V. Owing to the enhanced structural integrity of the DJ phase, the RP-2D/DJ-2D perovskite heterojunction films exhibit supreme material robustness, which translates to the impressive environmental stability of devices, showing nearly zero-degradation of the efficiency after 800 h of continuous thermal aging (60 °C) for 800 h. This work enriches the fundamental understanding of the impacts of the DJ-2D structure on the surface properties of 2D perovskites.

It is shown that an ammonium chloride additive in precursor solutions governs the even depth distribution of alkali metal (group 1A) elements in methylammonium-free perovskites, favors crystallinity, and induces a lateral growth of large monolithic film grains. Combined with the surface treatment by n-propylammonium iodide, it results in solar cells with high photovoltaic performance and improved stability.

Abstract

Incorporating multiple cations of the 1A alkali metal column of the periodic table (K⁺/Rb⁺/Cs⁺) to prepare perovskite films is promising for boosting photovoltaic properties but requires a uniform distribution. The effects of NH₄Cl additives and alkali metal cations (K⁺/Rb⁺/Cs⁺) on the one-step formation process of methylammonium-free, formamidinium-based, iodide perovskite films are analyzed in a step-by-step manner. NH₄Cl improves the solubility of PbI₂ in solution by forming an intermediate and then favors the perovskite phase formation. Moreover, during the annealing process, this additive is shown to increase grain size, to improve crystallinity and to suppress PbI₂ formation. K at low concentration is always homogeneously distributed across the film thickness. On the other hand, Cs is more concentrated at the surface and Rb in the depths of pristine films. With NH₄Cl additives, these two alkali metals are more homogeneously distributed because NH₄Cl slows down the movement of Cs⁺ and Rb⁺, it changes the growth direction of the perovskite film, making the overall crystallization quality improved and the distribution more uniform. It results in perovskite films with large monolithic grains. Combined with a perovskite film surface treatment with n-propylammonium iodide, a high stabilized power conversion efficiency of 22.04% is reached.

The film quality is vital to the technological breakthrough of perovskite solar cells (PSCs). Herein, the two-step method including two-step immersion and spin coating method for PSCs is summarized. Strategies such as component, solvent, and additive engineering are analyzed. Besides, applications in lead-free PSCs, all-inorganic PSCs, large areas, and tandems are introduced.

The quality of the perovskite film is crucial to the technological breakthrough of perovskite solar cells (PSCs). The two-step method can facilitate the formation of a perovskite film with high quality and reproducibility. Many milestones have been made in the development of hybrid lead halide PSCs by using the two-step method, which has a significant impact on their practical application. Herein, the reaction mechanism of the two-step method including two-step immersion method and two-step spin coating method is summarized. The strategies such as component engineering, solvent engineering, and additive engineering of the two-step method in preparing high-quality films are introduced systematically and in detail. In addition, current issues of the two-step method and its applications in lead-free PSCs, all-inorganic PSCs, large areas, and tandems are introduced and some suggestions are put forward for future research.

In this article, state-of-the-art perovskite solar cells were reported with a small amount of incorporated indium (In³⁺) ions. The In³⁺ ions accumulate at the buried interface and align the energy level to the electron transport layer. Additionally, the charge carrier dynamics at and above the band edge in mixed cation and halide perovskites can be greatly affected.

Abstract

Perovskite solar cells (PSCs) have been propelled into the limelight over the past decade due to the rapid-growing power conversion efficiency (PCE). However, the internal defects and the interfacial energy level mismatch are detrimental to the device performance and stability. In this study, it is demonstrated that a small amount of indium (In³⁺) ions in mixed cation and halide perovskites can effectively passivate the defects, improve the energy-level alignment, and reduce the exciton binding energy. Additionally, it is confirmed that In³⁺ ions can significantly elevate the initial carrier temperature, slow down the hot-carrier cooling rate, and reduce the heat loss before carrier extraction. The device with 1.5% of incorporated In³⁺ achieves a PCE of 22.4% with a negligible hysteresis, which is significantly higher than that of undoped PSCs (20.3%). In addition, the unencapsulated PSCs achieve long-term stability, which retain 85% of the original PCE after 3,000 h of aging in dry air. The obtained results demonstrate and promote the development of practical, highly efficient, and stable hot-carrier-enhanced PSCs.

This work provides a high temperarure layer-by-layer inkjet printing strategy to achieve the balance of vertical phase separation and molecular aggregation for printed photoactive films. A record efficiency of 13.09% is achieved.

Abstract

Drop-on demand inkjet-printing (IJP) is a deposition technique with promise in the context of fabricating large-area organic solar cells (OSCs) because of its high material usage, direct-pattern, and large-area roll-to-roll printing compatibility. But its feature of drop-to-drop deposition during IJP makes the film's drying and phase separation process different from spin-coating, and forms different nanophase separation and vertical phase separation morphology. In this work, the nanophase separation of the inkjet-printed organic blend films is systematically studied at different substrate temperatures. The results reveal that increasing the substrate temperature can suppress excess molecules aggregation owing to the high drying speed, leading to improved exciton dissociation efficiency in the blend films. However, the quick drying process at high temperature also leads to a homogenous vertical phase separation, which is not ideal for charge collection. Instead of printing the mixture of donor and acceptor solution directly to form the bulk-heterojunction structure, the polymer donor is printed on the top of the acceptor surface, a so-called layer-by-layer inkjet printing (LBL-IJP) process. By using this LBL-IJP route, balanced nanoscale phase aggregation and gradient vertical phase separation morphology are achieved, which leads to a record power conversion efficiency of 13.09% for the OSCs with an inkjet-printed active layer.

Publication date: May 2022

Source: Nano Energy, Volume 95

Author(s): Junjie Zhou, Minghao Li, Siyang Wang, Liguo Tan, Yue Liu, Chaofan Jiang, Xing Zhao, Liming Ding, Chenyi Yi