Chen Weijie

The polycyclic aromatic lactams (PAL)-based small-molecule donors are synthesized by atom-economic direct C─H activation strategy. They feature low-lying highest occupied molecular orbital energy levels, wide optical bandgap, and high crystallinity. The optimized ternary devices achieve a higher power conversion efficiency (17.01% for Ph-RDN and 17.00% for TT-RDN). This study may stimulate further research interest for PAL-based small-molecule materials.

The rapidly developed organic solar cells (OSCs) are largely driven by π-conjugated small-molecule and polymer materials in recent years. However, the organic photovoltaic materials served as the active layer are typically synthesized by multistep Stille or Suzuki cross-coupling reaction, which are neither cost-efficient nor atom-economic, and may bring toxicity concerns. Thus, it is imperative to develop easily accessible materials that do not involve organotin and organoboron reagents to promote the commercial application of OSCs. Herein, three small-molecule donors (Ph-RDN, N-RDN, and TT-RDN) comprising polycyclic aromatic lactams (PAL) core are designed and synthesized by direct C─H activation strategy and employed as the third components into the model PM6:Y6 photovoltaic systems. These donors show low-lying highest occupied molecular orbital energy levels, good crystallinity, and complementary absorption with the host system (PM6:Y6). The optimized ternary devices achieve higher power conversion efficiency (PCE) (17.01% for PM6:Ph-RDN:Y6, 16.58% for PM6:N-RDN:Y6, and 17.00% for PM6:TT-RDN:Y6) as compared with PM6:Y6 binary devices (15.67%), which are attributed to the higher charge mobilities, the more balanced charge transport, and the optimized morphology with a more well-defined fibrillar network. Therefore, PAL-based small-molecule donors, as a class of “greener” organic photovoltaic materials, can enhance the PCE of state-of-the-art OSCs.

In this work, a multifunctional magnetic field-assisted interface embedding strategy is developed to construct 2D/3D composite perovskite films. The composite structure can improve crystallinity, passivate defects, promote vertical hole transport, and form a good surface and internal encapsulation of 3D perovskite. As a result, the fabricated rigid and flexible devices achieve the high efficiency of 24.21% and 21.23%, respectively.

Abstract

Perovskite solar cells (PSCs) based on 2D/3D composite structure have shown enormous potential to combine high efficiency of 3D perovskite with high stability of 2D perovskite. However, there are still substantial non-radiative losses produced from trap states at grain boundaries or on the surface of conventional 2D/3D composite structure perovskite film, which limits device performance and stability. In this work, a multifunctional magnetic field-assisted interfacial embedding strategy is developed to construct 2D/3D composite structure. The composite structure not only improves crystallinity and passivates defects of perovskite layer, but also can efficiently promote vertical hole transport and provide lateral barrier effect. Meanwhile, the composite structure also forms a good surface and internal encapsulation of 3D perovskite to inhibit water diffusion. As a result, the multifunctional effect effectively improves open-circuit voltage and fill factor, reaching maximum values of 1.246 V and 81.36%, respectively, and finally achieves power conversion efficiency (PCE) of 24.21%. The unencapsulated devices also demonstrate highly improved long-term stability and humidity stability. Furthermore, an augmented performance of 21.23% is achieved, which is the highest PCE of flexible device based on 2D/3D composite perovskite films coupled with the best mechanical stability due to the 2D/3D alternating structure.

Publication date: September 2023

Source: Nano Energy, Volume 114

Author(s): Dahyun Jeong, Jin-Woo Lee, Seungjin Lee, Geon-U Kim, Hyesu Jeon, Seoyoung Kim, Changduk Yang, Changyeon Lee, Bumjoon J. Kim

Publication date: August 2023

Source: Nano Energy, Volume 113

Author(s): Hui Chen, Hanjian Lai, Qiuju Jiang, Xue Lai, Yulin Zhu, Jianfei Qu, Qinghe Wu, Feng He

Publication date: September 2023

Source: Nano Energy, Volume 114

Author(s): Xiaoguo Li, Fengming Xie, Saqib Rafique, Haoliang Wang, Liangliang Deng, Zejiao Shi, Yaxin Wang, Xin Zhang, Kai Liu, Yanyan Wang, Yiyi Pan, Fengcai Liu, Chongyuan Li, Tianxiang Hu, Jiao Wang, Anran Yu, Jianxin Tang, Yiqiang Zhan

Unencapsulated inverted organo-tin halide perovskite photovoltaics using a bismuth capped copper cathode achieve record stability when tested in ambient air under 1 sun simulated solar illumination and electrical load. The bismuth layer blocks corrosion of the copper electrode by iodine gas and sequesters iodine gas by seeding its condensation on top of the device.

Abstract

An effective approach is reported to enhance the stability of inverted organo-tin halide perovskite photovoltaics based on capping the cathode with a thin layer of bismuth. Using this simple approach, unencapsulated devices retain up to 70% of their peak power conversion efficiency after up to 100 h testing under continuous one sun solar illumination in ambient air and under electrical load, which is exceptional stability for an unencapsulated organo-tin halide perovskite photovoltaic device tested in ambient air. The bismuth capping layer is shown to have two functions: First, it blocks corrosion of the metal cathode by iodine gas formed when those parts of the perovskite layer not protected by the cathode degrade. Second, it sequesters iodine gas by seeding its deposition on top of the bismuth capping layer, thereby keeping it away from the electro-active parts of the device. The high affinity of iodine for bismuth is shown to correlate with the high polarizability of bismuth and the prevalence of the (012) crystal face at its surface. Bismuth is ideal for this purpose, because it is environmentally benign, non-toxic, stable, cheap, and can be deposited by simple thermal evaporation at low temperature immediately after deposition of the cathode.

A facile preburied cesium formate (CsFo) strategy is developed to stabilize the bottom side of perovskite by fully releasing the in-plane tensile strain at buried interface. Meanwhile, the CsFo eliminates the voids and reduces the defects as well as facilitates the charge extraction. The optimized formamidinium (FA)-based perovskite solar cell (PSC) exhibits impressive efficiency and stability.

Abstract

The fragile bottom side of perovskite films is demonstrated to be harmful to the efficiency and stability of perovskite solar cells (PSCs) because the carrier extraction and recombination can be significantly influenced by the easily formed strain, voids, and defects on the bottom side. Nevertheless, the bottom side of perovskite films is usually overlooked because it remains a challenge to directly characterize and modify the bottom side. Herein, a facile and effective strategy is reported to stabilize the bottom side via preburying cesium formate (CsFo) into the SnO₂ electron transport layer (ETL). It is found that the synergistic effect of cesium cation (Cs⁺) and formate anion (HCOO⁻) causes strain relaxation, void elimination, and defects’ reduction, which further facilitate the charge extraction. Consequently, the champion power conversion efficiency (PCE) of formamidinium (FA)-based PSCs is increased from 23.34% to 24.50%. Meanwhile, the ultraviolet (UV), thermal, and operational stability are also enhanced. Finally, formamidinium–cesium (FACs)-based PSCs are investigated to confirm the effectiveness of this preburied CsFo strategy, and the optimal device exhibits a champion PCE of 25.03% and a remarkably high fill factor (FF) of 85.65%.

Publication date: 19 July 2023

Source: Joule, Volume 7, Issue 7

Author(s): Xiaofeng Huang, Fang Cao, Shaoqi Zhan, Qifan Feng, Mengsi Zhu, Zhenhuang Su, Xingyu Gao, Jun Yin, Jing Li, Nanfeng Zheng, Binghui Wu

Low-temperature sol-gel prepared ZnCo₂O₄ spinel-based films are developed as high-performance hole transporting layer for depositing perovskite films from the basic MAPbI₃/ACN/CH₃NH₂ solution in air without using anti-solvent. Inverted solar cells and modules based on 2 mole% (vs Zn) Cu²⁺ doped ZnCo₂O₄ HTLs and NA-Psk absorbers exhibit the maximum power conversion efficiency of 20.0%–15%, respectively, and very stable.

Abstract

Low-temperature sol-gel prepared ZnCo₂O₄ spinel-based thin films are developed as high-performance hole transporting layer (HTL) for coating perovskite film (NA-Psk) from the basic MAPbI₃/ACN/CH₃NH₂ solution in air without using anti-solvent. Inverted PSC based on 2 mole% (vs Zn) Cu²⁺ doped ZnCo₂O₄ (2%Cu@ZnCo₂O₄) HTL and NA-Psk absorber exhibit the maximum power conversion efficiency (PCE) of 20.0% with no current hysteresis while the cell based on ZnCo₂O₄ and PEDOT:PSS HTL (using NA-Psk absorber) achieves the PCE of 15.79% and 12.3% with a current hysteresis index of 9.8% and 32.4%, respectively. Without encapsulation, PSCs based on 2%Cu@ZnCo₂O₄, ZnCo₂O₄, and PEDOT:PSS HTLs maintain 90%, 77%, and 12%, respectively of the original efficiency by standing in ambient atmosphere (temperature: 20–25 °C, RH:30%–40%) for 1800 h. Large area (10 cm × 10 cm substrate) perovskite mini-module (PSM) with PCE over 15% is also demonstrated by using sol-gel prepared 2%Cu@ZnCo₂O₄ HTL. The poor photovoltaic performance of PEDOT:PSS HTL is due to the basic MAPbI₃/ACN/CH₃NH₂ solution will deprotonate the acidic PEDOT:PSS to reduce its conductivity whereas ZnCo₂O₄ HTL are not affected by basic perovskite precursor solution.

Herein, an overview of the recent advancements in perovskite-based 2-terminal tandem solar cells by focusing on the key challenges is provided. Subsequently, they focused on optoelectronics, experimental spectroscopic, and theoretical characterizations to figure-out the charge-carrier dynamics regarding current-matching issues faced by the photovoltaic-society. Importantly, they elucidated the experimental spectroscopic and optical-simulations with the photovoltaic performance of tandem solar cells.

Abstract

Recently, multijunction tandem solar cells (TSCs) have presented high power conversion efficiency and revealed their immense potential in photovoltaic evolution. It is demonstrated that multiple light absorbers with various bandgap energies overcome the Shockley–Queisser limit of single-junction solar cells by absorbing the wide-range wavelength photons. Here, the main key challenges are reviewed, especially the charge carrier dynamics in perovskite-based 2-terminal (2-T) TSCs in terms of current matching, and how to manage these issues from a vantage point of characterization. To do this, the effect of recombination layers, optical and fabrication hurdles, and the impact of wide bandgap perovskite solar cells are discussed extensively. Afterward, this review focuses on various optoelectronics, spectroscopic, and theoretical (optical simulation) characterizations to figure out those issues, especially current-matching issues faced by the photovoltaic society. This review comprehensively provides deep insights into the relationship between the current-matching problems and the photovoltaic performance of TSCs through a variety of perspectives. Consequently, it is believed that this review is essential to address the main problems of 2-T TSCs, and the suggestions to elucidate the charge carrier dynamics and its characterization may pave the way to overcome such obstacles to further improve the development of 2-T TSCs in relation to the current-matching problems.

In this article, phenylpropylammonium bromine (PPABr) is used both as precursor additive and surface passivation additive, reducing the carrier recombination at the bulk phase and interface, forming a 2D interface that protects 3D perovskite and enhances the carrier transmission. After treatment, the open-circuit voltage and power conversion efficiency of perovskite solar cells are significantly improved.

Abstract

Inverted perovskite solar cells (PSCs) are a promising technology for commercialization due to their reliable operation and scalable fabrication. However, in inverted PSCs, depositing a high-quality perovskite layer comparable to those realized in normal structures still presents some challenges. Defects at grain boundaries and interfaces between the active layer and carrier extraction layer seriously hinder the power conversion efficiency (PCE) and stability of these cells. In this work, it is shown that synergistic bulk doping and surface treatment of triple-cation mixed-halide perovskites with phenylpropylammonium bromine (PPABr) can improve the efficiency and stability of inverted PSCs. The PPABr ligand is effective in eliminating halide vacancy defects and uncoordinated Pb²⁺ ions at both grain boundaries and interfaces. In addition, a 2D Ruddlesden–Popper (2D-RP) perovskite capping layer is formed on the surface of 3D perovskite by using PPABr post-treatment. This 2D-RP perovskite capping layer possesses a concentrated phase distribution ≈n = 2. This capping layer not only reduces interfacial non-radiative recombination loss and improves carrier extraction ability but also promotes stability and efficiency. As a result, the inverted PSCs achieve a champion PCE of over 23%, with an open-circuit voltage as high as 1.15 V and a fill factor of over 83%.

Organic bulk-heterojunction photoanodes with an additional polymer layer generate long-lived hole polaron states on the timescale of seconds under photoelectrochemical operational conditions. As a result, the organic photoanode with the polymer overlayer shows enhanced photocatalytic performance, reaching ≈7 mA cm⁻² at 1.23 V_RHE and external quantum efficiency of 18% at 850 nm excitation without a co-catalyst.

Abstract

Photogenerating charges with long lifetimes to drive catalysis is challenging in organic semiconductors. Here, the role of a PM6 polymer overlayer on the photoexcited carrier dynamics is investigated in a Y6:PM6 bulk-heterojunction (BHJ) photoanode undergoing ascorbic acid oxidation. With the additional polymer layer, the hole lifetime is increased in the solid state BHJ film. When the photoanode is electrically coupled to a hydrogen-evolving platinum cathode, remarkably long-lived hole polaron states are observed on the timescale of seconds under operational conditions. It is demonstrated that these long-lived holes enable the organic photoanode with the polymer overlayer to show enhanced ascorbic acid oxidation performance, reaching ≈7 mA cm⁻² at 1.23 V_RHE without a co-catalyst. An external quantum efficiency of 18% is observed using 850 nm excitation. It is proposed that the use of an organic overlayer can be an effective design strategy for generating longer charge carrier lifetimes in organic photoanodes for efficient oxidation catalysis.

Synergistic utilization of fluorine and ester-substituted monothiophene and vinyl-bridges yields a novel thiophene derivative FETVT, a desirable building block for developing new low-cost polythiophene derivatives. The first polymer PFETVT-T derived from FETVT shows a record power conversion efficiency of 11.81% with a large open-circuit voltage of 0.93 eV in all-polymer solar cells.

Abstract

Polymerized small molecule acceptors have recently greatly facilitated the development of all-polymer solar cells (All-PSCs) with respect to the power conversion efficiencies (PCEs). However, high-performance and low-cost polymer donors for All-PSCs are still lacking, limiting further large-scale manufacturing of All-PSCs. Herein, this work designs and synthesizes a new thiophene derivative, FETVT, featuring vinyl-bridged fluorine and ester-substituted monothiophene. Incorporation of FETVT into a polymer yields a high-performance polythiophene derivative PFETVT-T, which exhibits deep-lying HOMO level, suitable solution pre-aggregation ability, finely-tuned polymer crystallinity, and appropriate thermodynamic miscibility with the polymer acceptor L15. As a result, binary based on PFETVT-T achieves a record PCE of 11.81% with agood stability, representing a breakthrough for polythiophenes and their derivatives-based All-PSCs, which is also significantly higher than that (1.92%) of All-PSCs based on its isomerized analog. Remarkably, PFETVT-T achieves an impressive PCE exceeding 16% via the implementation of a ternary blend design. These findings offer a hopeful pathway toward attaining high-performance, stable, and cost-effective PSCs.

We have successfully identified the distribution of Sn⁴⁺ on both the top and bottom surfaces of the perovskite layer. A thin layer of Sn metal was introduced at the surfaces. The implementation of this strategic insertion resulted in a significant reduction in the levels of Sn⁴⁺ and trap densities. The device was able to attain a power conversion efficiency of 14.31 % through the optimization of carrier transportation.

Abstract

The photoelectric properties of nontoxic Sn-based perovskite make it a promising alternative to toxic Pb-based perovskite. It has superior photovoltaic performance in comparison to other Pb-free counterparts. The facile oxidation of Sn²⁺ to Sn⁴⁺ presents a notable obstacle in the advancement of perovskite solar cells that utilize Sn, as it adversely affects their stability and performance. The study revealed the presence of a Sn⁴⁺ concentration on both the upper and lower surfaces of the perovskite layer. This discovery led to the adoption of a bi-interface optimization approach. A thin layer of Sn metal was inserted at the two surfaces of the perovskite layer. The implementation of this intervention yielded a significant decrease in the levels of Sn⁴⁺ and trap densities. The power conversion efficiency of the device was achieved at 14.31 % through the optimization of carrier transportation. The device exhibited operational and long-term stability.

A multifunctional elastomer with abundant hydrogen bonds and carbonyl groups was designed and successfully incorporated into perovskite film. The multiple chemical bonding interactions between the elastomer and perovskite lead to high-quality perovskite film and efficient perovskite solar cells with improved stability and reduced lead leakage.

Abstract

Perovskite solar cells (PSCs) are considered as a promising photovoltaic technology due to their high efficiency and low cost. However, their long-term stability, mechanical durability, and environmental risks are still unable to meet practical needs. To overcome these issues, we designed a multifunctional elastomer with abundant hydrogen bonds and carbonyl groups. The chemical bonding between polymer and perovskite could increase the growth activation energy of perovskite film and promote the preferential growth of high-quality perovskite film. Owing to the low defect density and gradient energy-level alignment, the corresponding device exhibited a champion efficiency of 23.10 %. Furthermore, due to the formation of the hydrogen-bonded polymer network in the perovskite film, the target devices demonstrated excellent air stability and enhanced flexibility for the flexible PSCs. More importantly, the polymer network could coordinate with Pb²⁺ ions, immobilizing lead atoms to reduce their release into the environment. This strategy paves the way for the industrialization of high-performance flexible PSCs.

The concept of “quantum confinement breaking” in 2D perovskites is proposed based on theoretical calculation and experimental results. An intensive orbital coupling between the bithiophenemethylammonium (BThMA) spacer and adjacent inorganic layers in (BThMA)₂PbI₄ is observed, whereas no such interactions exist when using a biphenemethylammonium-based spacer. The reducing or eliminating of the quantum confinement in 2D perovskite promotes efficient charge transport.

Abstract

2D Ruddlesden–Popper perovskites have become emerging photovoltaic materials due to their intrinsic structure stability. Here, a concept of “quantum confinement breaking” in 2D perovskites is proposed using organic semiconductor spacers with suitable energy levels based on theoretical calculation and experimental results. An interesting finding is that there is intensive orbital coupling between the bithiophenemethylammonium (BThMA) spacer and adjacent inorganic layers in (BThMA)₂PbI₄, resulting in the breaking of the multiple quantum well structure. In comparison, no orbital interactions exist in (BPhMA)₂PbI₄ due to the wide bandgap of the biphenemethylammonium (BPhMA) spacer. Benefitting from the improved film quality, increased dielectric constant, and reduced binding energy, the (BThMA)₂MA _{n −1}Pb _n I_{3 n +1} (n = 5) perovskite-based device displays an outstanding power conversion efficiency (PCE) of 18.05%, which is much higher than that of the BPhMA-based device (PCE = 12.69%) and among the best efficiency in 2D PSCs based on long conjugated spacers. The results provide an important implication for the effects of orbital interactions between organic semiconductor spacers and the adjacent [PbI₆]⁴⁻ octahedron layer on the performance of 2D perovskite solar cells and other optoelectronic devices.

The PYBA cation is used as a template for the crystallization of FAPbI₃ perovskite, which induces a highly oriented crystal and a pure α-phase film. The external compression strain provided by the PYBA pairs compensates for the inherent tension strain of the FAPbI₃ crystals. As a result, the strain-regulated formamidinium PSC exhibits an improved PCE.

Abstract

Formamidinium lead iodide (FAPbI₃) perovskite possesses an ideal optical bandgap and is a potential material for fabricating the most efficient single-junction perovskite solar cells (PSCs). Nevertheless, large formamidinium (FA) cations result in residual lattice strain, which reduces the power conversion efficiency (PCE) and operational stability of PSCs. Herein, the modulation of lattice strain in FAPbI₃ crystals via a π-conjugated organic amine, i.e., 4-pyrene oxy butylamine (PYBA), is proposed. PYBA pairs at the grain boundary serve as a template for the crystallization of FAPbI₃ perovskite, thereby inducing a highly oriented crystal and a pure α-phase film. The PYBA pairs with strong π–π interactions provide a solid fulcrum for external compression strain, thus compensating for the inherent tension strain of FAPbI₃ crystals. The strain release elevates the valence band of the perovskite crystals, thereby decreasing the bandgap and trap density. Consequently, the PYBA-regulated FAPbI₃ PSC achieves an excellent PCE of 24.76%. Moreover, the resulting device exhibits improves operational stability and maintains over 80% of its initial PCE after 1500 h under maximum power point tracking conditions.

A simple and cost-effective approach to accessing giant molecule acceptors is presented, employing a Lewis acid-catalyzed Knoevenagel condensation (or its silyl-mediated version). Notably, this new approach significantly expands the range of substrates that enables the use of readily available diboronated linkers. This methodology also offers a modular and robust synthesis of tailor-made donor-acceptors for a wide range of emerging technologies.

Abstract

The operational stability of polymer solar cells is a critical concern with respect to the thermodynamic relaxation of acceptor-donor-acceptor (A-D-A) or A-DA'D-A structured small-molecule acceptors (SMAs) within their blends with polymer donors. Giant molecule acceptors (GMAs) bearing SMAs as subunits offer a solution to this issue, while their classical synthesis via the Stille coupling suffers from low reaction efficiency and difficulty in obtaining mono-brominated SMA, rendering the approach impractical for their large-scale and low-cost preparation. In this study, we present a simple and cost-effective solution to this challenge through Lewis acid-catalyzed Knoevenagel condensation with boron trifluoride etherate (BF₃ ⋅ OEt₂) as catalyst. We demonstrated that the coupling of the monoaldehyde-terminated A-D-CHO unit and the methylene-based A-link-A (or its silyl enol ether counterpart) substrates can be quantitatively achieved within 30 minutes in the presence of acetic anhydride, affording a variety of GMAs connected via the flexible and conjugated linkers. The photophysical properties was fully studied, yielding a high device efficiency of over 18 %. Our findings offer a promising alternative for the modular synthesis of GMAs with high yields, easier work up, and the widespread application of such methodology will undoubtedly accelerate the progress of stable polymer solar cells.

Nature Communications, Published online: 16 June 2023; doi:10.1038/s41467-023-39223-9

Two novel selenium substitution asymmetric acceptors, AsymSSe-2F and AsymSSe-2Cl, are synthesized to investigate the synergistic modification effects on device performance compared with symmetric Y6. When blending AsymSSe-2F with the wide-bandgap polymer D18, a remarkable power conversion efficiency of 18.31% is yielded, which is attributed to the broadened absorption, enhanced π–π stacking, balanced carrier mobilities and fine phase-separation morphology.

Abstract

Substantial efforts of A–DA′D–A type non-fullerene acceptors (NFAs) molecular design have impelled power conversion efficiency (PCE) of single junction organic solar cells (OSCs) to exceed 19%. Asymmetric geometry strategy, selenium-substitution, and end-group engineering are proven to be effective modification methods. Here, two novel selenium substitution asymmetric NFAs, AsymSSe-2F, and AsymSSe-2Cl, are synthesized to investigate the synergistic modification effects on device performance compared with symmetric Y6. When blending AsymSSe-2F with the wide-bandgap and high crystallinity polymer D18, a remarkable PCE of 18.31% is yielded, and an excellent fill factor of 79.46% is achieved, which is attributed to the broadened absorption, enhanced π–π stacking, balanced carrier mobilities, and fine phase-separation morphology. Notably, among the reported selenium-substituted asymmetric NFAs based OSCs, especially combined with the seldom-reported D18, this PCE is top-ranked in binary bulk heterojunction organic solar cells. This work indicates that the combined modification of asymmetric geometry and selenium substitution in NFAs is a promising strategy for fabricating high performance OSCs.