Chen Weijie

Publication date: 15 December 2023

Source: Nano Energy, Volume 118, Part B

Author(s): Zhaochen Suo, Zheng Xiao, Shitong Li, Jian Liu, Yufei Xin, Lingxian Meng, Huazhe Liang, Bin Kan, Zhaoyang Yao, Chenxi Li, Xiangjian Wan, Yongsheng Chen

Vapor-transport deposition (VTD) is a solvent-free processing method that is operated at moderate vacuum. Herein, it is demonstrated that VTD offers control over perovskite (methylammonium lead iodide [MAPbI₃]) film composition and morphology. Photovoltaic devices incorporating VTD-processed, lead iodide-excessive MAPbI₃ film achieve efficiencies exceeding 12%. This is the highest performing cell demonstrated using moderate vacuum processing.

Photovoltaic cells based on metal-halide perovskites have exceeded the performance of other thin film technologies and rival the performance of devices based on archetypical silicon. Attractively, the perovskite active layer can be processed via a variety of solution- and vapor-based methods. Herein, emphasis is on the use of vapor transport codeposition (VTD) to process efficient n–i–p photovoltaic cells based on methylammonium lead iodide (MAPbI₃). VTD utilizes a hot-walled reactor operated under moderate vacuum in the range of 0.5–10 Torr. The organic and metal-halide precursors are heated with the resulting vapor transported by a N₂ carrier gas to a cooled substrate where they condense and react to form a perovskite film. The efficiency of photovoltaic devices based on VTD-processed MAPbI₃ is found to be highest in films with excess lead iodide content, with champion devices realizing exceeding 12%.

Integration of benzothiazole and imide functionalized aromatics for enhanced mobility, thermal resilience, and energy level alignment is studied. An inverted perovskite solar cell exhibiting an exceptional 22.02% efficiency and excellent stability is unveiled.

Dopant-free hole transport layers (HTLs) play a crucial role in achieving high-efficiency and stable perovskite solar cells (PSCs). However, only a limited number of these HTLs have demonstrated power conversion efficiencies (PCEs) surpassing 21%. Herein, the design and synthesis of two polymeric HTLs with a novel DA’D–A backbone are presented. The incorporation of two A units (benzothiazole and imide functionalized aromatics) in the polymer imparts a rigid backbone, high mobility, appropriate film morphology, excellent thermal stability, and a deeper highest occupied molecular orbital energy level, crucial for achieving a more suitable energy-level alignment between HTL and perovskite layer. Notably, the champion PSC based on our HTLs exhibits a remarkable PCE of 22.02% with minimal hysteresis and excellent thermal stability, surpassing the performance of devices based on the benchmark polymeric HTL PTAA under identical conditions. These findings underscore the immense potential of our DA’D-A backbone strategy in developing high-performance dopant-free polymer HTLs for enhancing the PCE of PSCs.

Tailoring crystallization dynamics of cesium lead triiodide (CsPbI₃) via lead acetate substitution is a holistic strategy for scalable production of efficient and stable all-inorganic perovskite photovoltaic devices. The reactions between dimethylammonium and acetate are key to regulating the uniform nucleation and growth of the CsPbI₃ perovskite films for large-area perovskite solar cells and modules.

Abstract

All-inorganic perovskite cesium lead triiodide (CsPbI₃) with inorganic nature, low-temperature synthesis, and a suitable bandgap is desirable for high-performance photovoltaics. However, the scalable production of CsPbI₃ photovoltaics is still challenging due to a large nucleation energy barrier and slow phase transition during unassisted natural crystallization. Here, the crystallization dynamics of CsPbI₃ thin films is tailored via lead acetate (PbAc₂) substitution in the perovskite precursor ink, allowing the scalable fabrication of efficient all-inorganic perovskite solar cells and minimodules. Introducing PbAc₂ enlarges CsPbI₃ colloid size in the precursor and reduces the nucleation energy barrier. Additionally, reactions between acetate and dimethylammonium in the wet film accelerate the removal of dimethylammonium additives and generate solvent vapors for self-regulate internal solvent annealing, resulting in densely packed, uniform, and pinhole-free CsPbI₃ perovskite films over large areas. This strategy demonstrates inverted CsPbI₃ solar cells with 20.17% efficiency and good operational stability (retaining 95.5% of initial efficiency after continuous operation for 1800 h) and 15.1%-efficient CsPbI₃ minimodules with an active area of 26.8 cm².

This roadmap review summarizes the latest progress, outstanding challenges, and future directions of Pb-free halide perovskites regarding their synthesis, optical spectroscopy, and optoelectronic devices.

Abstract

Halide perovskites, in the form of thin films and colloidal nanocrystals, have recently taken semiconductor optoelectronics research by storm, and have emerged as promising candidates for high-performance solar cells, light-emitting diodes (LEDs), lasers, photodetectors, and radiation detectors. The impressive optical and optoelectronic properties, along with the rapid increase in efficiencies of solar cells and LEDs, have greatly attracted researchers across many disciplines. However, most advances made so far in terms of preparation (colloidal nanocrystals and thin films), and the devices with highest efficiencies are based on Pb-based halide perovskites, which have raised concerns over their commercialization due to the toxicity of Pb. This has triggered the search for lower-toxicity Pb-free halide perovskites and has led to significant progress in the last few years. In this roadmap review, researchers of different expertise have joined together to summarize the latest progress, outstanding challenges, and future directions of Pb-free halide perovskite thin films and nanocrystals, regarding their synthesis, optical spectroscopy, and optoelectronic devices, to guide the researchers currently working in this area as well as those that will join the field in the future.

A remarkable power conversion efficiency (PCE) of 17.96% is accomplished by enhanced morphology in air-processed OPVs fabricated using halogen-free solvent (o-xylene) and π–bridge-assisted π-conjugated polymer donors. This notable performance is achieved by improving nano-scale film morphology, intra- and inter-molecular interactions, and dipole moments. A phenomenal PCE of 13.88% is achieved using large-area halogen-free sub-modules (55 cm²).

Abstract

Organic photovoltaic (OPV) sub-modules shall be feasible for production using halogen-free solvents in air,high power conversion efficiencies (PCEs), and long-term stability, which are challenging requirements. To achieve this goal, air-processed OPVs are fabricated by employing a synthesized set of π-conjugated polymers, NAP-T-SiBTZ (P1) and NAP-TT-SiBTZ (P2), with thiophene and thienothiophene π-spacers, respectively. P1 and P2 incorporated ternary OPVs show excellent PCEs and are used to produce small-area, sub-module air-processed devices using o-xylene as the solvent. Interestingly, P2-added ternary devices has remarkable PCEs of 17.62% (PM6:P2:Y7) and 17.96% (PM6:P2:L8-BO), which is the highest reported for air-processed OPVs. . Notably, P2-associated ternary blends exhibit a nano-morphology, increased charge carrier mobilities, exciton dissociation, and decreased non-geminate recombination, which are deemed responsible for the enhanced PCEs observed. In addition, P2 demonstrates high efficiency for a thick-film device (>300 nm), with a PCE of >16.50%. Notably, a 55 cm² sub-module produced by bar coating using o-xylene in open air has a PCE of 13.88%. Additionally, P2-containing devices demonstrate impressive thermal and photo-stabilities. This study shows the potential of an OPV that may be used to produce low-cost solar cell sub-module at low cost with exceptional commercial value.

In the preparation of formamidinium perovskite solar cells, use of the 1-butyl-3-methylimidazolium methanesulfonate (MS) precursor additive results in the spontaneous aggregation of MS molecules at both sides of the interface following perovskite crystallization. This process significantly mitigates the defect density at the interface and curbs the V _OC loss. The optimal device prepared using the MS strategy attains a noteworthy power conversion efficiency of 25.12%.

Abstract

Perovskite solar cells (PSCs) are promising candidates for next-generation photovoltaics owing to their unparalleled power conversion efficiencies (PCEs). Currently, approaches to further improve device efficiencies tend to focus on the passivation of interfacial defects. Although various strategies have been developed to mitigate these defects, many involve complex and time-consuming post-treatment processes, thereby hindering their widespread adoption in commercial applications. In this work, a concise but efficient in situ dual-interface passivation strategy is developed wherein 1-butyl-3-methylimidazolium methanesulfonate (MS) is employed as a precursor additive. During perovskite crystallization, MS can either be enriched downward through precipitation with SnO₂, or can be aggregated upward through lattice extrusion. These self-assembled MS species play a significant role in passivating the defect interfaces, thereby reducing nonradiative recombination losses, and promoting more efficient charge extraction. As a result, a PCE >25% (certified PCE of 24.84%) is achieved with substantially improved long-term storage and photothermal stabilities. This strategy provides valuable insights into interfacial passivation and holds promise for the industrialization of PSCs.

Publication date: March 2024

Source: Journal of Energy Chemistry, Volume 90

Author(s): Qingyan Chang, Yidan An, Huaiman Cao, Yuzhen Pan, Liangyu Zhao, Yulong Chen, Yi We, Sai-Wing Tsang, Hin-Lap Yip, Licheng Sun, Ze Yu

Publication date: 15 December 2023

Source: Nano Energy, Volume 118, Part B

Author(s): Boyuan Hu, Jian Zhang, Yulin Yang, Yayu Dong, Jiaqi Wang, Wei Wang, Kaifeng Lin, Debin Xia, Ruiqing Fan

Publication date: January 2024

Source: Nano Energy, Volume 119

Author(s): Heyi Yang, Yunxiu Shen, Guiying Xu, Fu Yang, Xiaoxiao Wu, Junyuan Ding, Haiyang Chen, Weijie Chen, Yeyong Wu, Qinrong Cheng, Chuang Jin, Yaowen Li, Yongfang Li

The two-step method, which is industrially preferred over the single-step anti-solvent technique, faces two critical issues, i.e., unnecessary PbI₂ residue and uncontrollable perovskite growth. The burying of RbPbI₃ crystal seeds in PbI₂ substrate solves both of these issues, and a PCE over 24% is achieved with a superior long-life span in perovskite solar cells.

Abstract

High power conversion efficiencies (PCEs) in perovskite solar cells (PSCs) have always been awe-inspiring, but perovskite films scalability is an exacting precondition for PSCs commercial deployment, generally unachievable through the antisolvent technique. On the contrary, in the two-step sequential method, the perovskite's uncontrolled crystallization and unnecessary PbI₂ residue impede the device's performance. These two issues motivated to empower the PbI₂ substrate with orthorhombic RbPbI₃ crystal seeds, which act as grown nuclei and develop orientated perovskites lattice stacks, improving the perovskite films morphologically and reducing the PbI₂ content in eventual perovskite films. Thence, achieving a PCE of 24.17% with suppressed voltage losses and an impressive life span of 1140 h in the open air.

This mini-review highlights the great potential of solution-processed semiconductor (SPS) materials as cathode interlayers (CILs) in organic solar cells. The working mechanism and material design strategy of SPS-based CIL materials are elucidated. The SPS-based CIL materials, including organic small molecules, conjugated polymers, non-conjugated polymers, and transition metal oxides, are summarized and the structure-property-performance relationship of SPS-based CIL materials is revealed. After a brief summary, the remaining issues and the prospects of SPS-based CILs for organic solar cells are suggested.

Abstract

Cathode interlayers (CILs) play a crucial role in improving the photovoltaic efficiency and stability of OSCs. CILs generally consists of two kinds of materials, interfacial dipole-based CILs and SPS-based CILs. With good charge transporting ability, excellent compatibility with large-area processing methods, and highly tunable optoelectronic properties, the SPS-based CILs exhibit remarkable superiorities to their interfacial dipole-based counterparts in practical use, making them promising candidate in developing efficient CILs for OSCs. This mini-review highlights the great potential of SPS-based CILs in OSC applications and elucidates the working mechanism and material design strategy of SPS materials. Afterward, the SPS-based CIL materials are summarized and discussed in four sections, including organic small molecules, conjugated polymers, nonconjugated polymers, and TMOs. The structure-property-performance relationship of SPS-based CIL materials is revealed, which may provide readers new insight into the molecular design of SPS-based CILs. The mechanisms to endow SPS-based CILs with thickness insensitivity, resistance to environmental erosion, and photo-electric conversion ability are also elucidated. Finally, after a brief summary, the remaining issues and the prospects of SPS-based CILs are suggested.

The authors use magneto-optical spectroscopy to demonstrate the importance of organic spacer cations on exciton fine structure in 2D perovskites. The dark exciton state can be manipulated by applying a magnetic field, producing a >30% increase in collected photoluminescence. This highlights the critical role of the dark exciton state for light-emitting applications, assisting the development of optoelectronic technologies based on 2D perovskites.

Abstract

The organic spacer cation plays a crucial role in determining the exciton fine structure in 2D perovskites. Here, magneto-optical spectroscopy is used to gain insight into the influence of the organic spacer on dark excitons in Ruddlesden–Popper (RP) perovskites. Using modest magnetic field strengths (<1.5 T), the optically forbidden dark exciton state can be identified and its emission properties significantly modulated via application of in-plane magnetic fields, up to temperatures of 15 K. At low temperatures, an increase in collected photoluminescence efficiency of >30% is demonstrated, signifying the critical role of the dark exciton state for light-emitting applications of 2D perovskites. The exciton fine structure and the degree of magnetic-field-induced mixing are significantly impacted by the choice of organic spacer cation, with 4–methoxyphenylethylammonium (MeO-PEA) showing the largest effect due to larger bright–dark exciton splitting. This study distinguishes between interior (bulk) and surface dark-exciton emission, showing that bright–dark exciton splitting differs between the interior and surface. The results emphasize the significance of the organic spacer cation in controlling the exciton fine structure in 2D perovskites and have important implications for the development of optoelectronic technology based on 2D perovskites.

Reconstructing Sn-based perovskite's surface is able to form a protective gradient layer with exceptional electron transfer, defect passivation, and suppressed Sn(II) oxidation. This strategy enhances perovskite film's and device's environmental endurance even after prolonged exposure to moisture and ambient conditions, offering valuable insights for fabricating robust lead-free perovskite photovoltaic devices.

Abstract

Tin halide perovskites are an appealing alternative to lead perovskites. However, owing to the lower redox potential of Sn(II)/Sn(IV), particularly under the presence of oxygen and water, the accumulation of Sn(IV) at the surface layer will negatively impact the device's performance and stability. To this end, this work has introduced a novel multifunctional molecule, 1,4-phenyldimethylammonium dibromide diamine (phDMADBr), to form a protective layer on the surface of Sn-based perovskite films. Strong interactions between phDMADBr and the perovskite surface improve electron transfer, passivating uncoordinated Sn(II), and fortify against water and oxygen. In situ grazing incidence wide-angle X-ray scattering (GIWAXS) analysis confirms the enhanced thermal stability of the quasi-2D phase, and hence the overall enhanced stability of the perovskite. Long-term stability in devices is achieved, retaining over 90% of the original efficiency for more than 200 hours in a 10% RH moisture N₂ environment. These findings propose a new approach to enhance the operational stability of Sn-based perovskite devices, offering a strategy in advancing lead-free optoelectronic applications.

Inspired by pressure cooker principles, we employ high-pressure methods to address challenges in processing high molecular weight polymers. Successfully dissolving HW-D18 in chloroform at 100 °C within pressure vials, heightened vapor pressure elevates boiling point and solubility of chloroform. This enables the creation of blend films with superior properties, ultimately achieving an excellent PCE of 19.65 %, setting a new record for binary OSCs.

Abstract

In this work, inspired by the principles of a pressure cooker, we utilized a high-pressure method to address the processing challenges associated with high molecular weight polymers. Through this approach, we successfully dissolved high molecular weight D18 in chloroform at 100 °C within a pressure-tight vial. The increased steam pressure raised the boiling point and dissolving capacity of chloroform, enabling the creation of a hybrid film with superior properties, including more ordered molecular arrangement, increased crystallinity, extended exciton diffusion length, and improved phase morphology. Organic solar cells (OSCs) based on D18 : L8-BO prepared using this high-pressure method achieved an outstanding power conversion efficiency of 19.65 %, setting a new record for binary devices to date. Furthermore, this high-pressure method was successfully applied to fabricate OSCs based on other common systems, leading to significant enhancements in device performance. In summary, this research introduces a universal method for processing high molecular weight D18 materials, ultimately resulting in the highest performance reported for binary organic solar cells.

A polymerizable organic molecule monomer, N-carbamoyl-2-propan-2-ylpent-4-enamide (Apronal), is introduced into the perovskite precursor to form a polymer (P-Apronal) through thermal crosslinking, which can effectively interact with shallow defects, leading to enhance crystallinity and reduced lattice strain, especially inhibiting the migration of I⁻, thus resulting in enhanced long-term stability and performance of devices.

Abstract

The migration of ions is known to be associated with various detrimental phenomena, including current density-voltage hysteresis, phase segregation, etc., which significantly limit the stability and performance of perovskite solar cells, impeding their progress toward commercial applications. To address these challenges, we propose incorporating a polymerizable organic small molecule monomer, N-carbamoyl-2-propan-2-ylpent-4-enamide (Apronal), into the perovskite film to form a crosslinked polymer (P-Apronal) through thermal crosslinking. The carbonyl and amino groups in Apronal effectively interact with shallow defects, such as uncoordinated Pb²⁺ and iodide vacancies, leading to the formation of high-quality films with enhanced crystallinity and reduced lattice strain. Furthermore, the introduction of P-Apronal improves energy level alignment, and facilitates charge carrier extraction and transport, resulting in a champion efficiency of 25.09 %. Importantly, P-Apronal can effectively suppress the migration of I⁻ ions and improve the long-term stability of the devices. The present strategy sets forth a path to attain long-term stability and enhanced efficiency in perovskite solar cells.

Two formamidinium (FA)-based semiconductor spacers, namely TTFA and BTFA, respectively, were successfully developed for two-dimensional (2D) Ruddlesden-Popper perovskite solar cells. The nucleation and crystallization kinetics of 2D perovskite films were systematically investigated and understood using in situ optical microscopy and in situ grazing incidence wide-angle X-ray scattering measurements. The device based on (TTFA)₂MA_n−1PbnI_3n+1 (n=5) achieved a record efficiency of 19.41 %.

Abstract

The conjugated organic semiconductor spacers have drawn wide attention in two-dimensional (2D) perovskites and formamidinium (FA) has been widely used as A-site cation in high-performance 3D perovskite solar cells (PSCs). However, the FA-based semiconductor spacers have rarely been investigated in 2D Ruddlesden-Popper (RP) perovskites. Here, we developed two FA-based spacers containing thieno[3,2-b]thiophene (TT) and 2,2′-bithiophene (BT) units, namely TTFA and BTFA, respectively, for 2D RP PSCs. The nucleation and crystallization kinetics of TTFA-Pb and BTFA-Pb from sol-gel to film were investigated using in situ optical microscopy and in situ grazing incidence wide-angle X-ray scattering (GIWAXS) measurements. It is found that the TTFA spacer could reduce the energy barrier of nucleation and induces crystal vertical orientation of 2D perovskite by forming larger clusters in precursor solution, resulting in much improved film quality. Benefiting from the enlarged crystal grains, reduced exciton binding energy, and decreased electron-phonon coupling coefficient, the photovoltaic device based on (TTFA)₂MA_n−1Pb_nI_3n+1 (n=5) achieved a champion efficiency of 19.41 %, which is a record for 2D RP PSCs with FA-based spacers. Our work provides deep understanding of the nucleation and crystallization process of 2D RP perovskite films and highlights the great potential of FA-based semiconductor spacers in highly efficient 2D PSCs.

Publication date: January 2024

Source: Nano Energy, Volume 119

Author(s): Jiawei Deng, Jiabin Liu, Wenhao Li, Xiaokang Geng, Jiaping Xie, Sang Young Jeong, Bin Huang, Dan Zhou, Feiyan Wu, Han Young Woo, Lie Chen

CsBr nanocrystals (NCs) are introduced on the surface of FAPbI₃ perovskite film by harmless non-polar solvent n-hexane. Full depth doping of Cs⁺ and Br⁻ are realized via surface treatment with CsBr NCs. Gradient potential distribution from surface into bulk optimizes energy level alignment and suppressed n-type doping at the surface to improve hole extraction, resulting in a superior PCE of 23.22% and improved long-term stability.

Abstract

Achieving longitudinal doping of specific ions by surface treatment remains a challenge for perovskite solar cells, which are often limited by dopant and solvent compatibility. Here, with the flowing environment created by CsBr colloidal nanocrystals, ion exchange is induced on the surface of the perovskite film to enable the homogeneous distribution of Cs⁺ and gradient distribution of Br⁻ simultaneously at whole depth of the film. Meanwhile, assisted by long-chain organic ligands, the excess PbI₂ on the surface of perovskite film is converted to a more stable quasi-2D perovskite, which realizes effective passivation of defects on the surface. As a result, the unfavorable n-type doping on the top surface is suppressed, so that the energy level alignment between perovskite and hole transport layer is optimized. On the basis of co-modification of the surface and the bulk, the PCE of champion device reaches 23.22% with enhanced V _OC of 1.12 V. Device maintains 97.12% of the initial PCE in dark ambient air at 1% RH after 1056 h without encapsulation, and 91.56% of the initial PCE under light illumination of 1 sun in N₂ atmosphere for more than 200 h. The approach demonstrated here provides an effective strategy for the nondestructive introduction of inorganic ions in perovskite film.

Two benzodithiophene-thienothiadiazole copolymers have been developed as dopant-free hole-transporting layers, resulting in 22.20% efficiency perovskite solar cells along with outstanding long-term operational stability.

The exploration of dopant-free hole-transporting materials (HTMs) with excellent optoelectronic properties and defect passivation ability is of great significance for simultaneously improving the efficiency and stability of perovskite solar cells (PSCs). Herein, two donor–acceptor type conjugated polymers PTTDZ-F and PTTDZ-Cl with alternating benzodithiophene (BDT) and thienothiadiazole (TTD) units are successfully developed with desirable hole mobilities and conductivities. The formation of non-covalent interaction (S···N) between thiophene bridge and TTD units significantly improves the co-planarity of the molecular backbone and promotes the intramolecular charge transfer from BDT to TTD. The hydrophobicity and energy levels of the polymers are finely regulated via introducing different halogen atoms on the BDT moiety. Moreover, the functional groups in polymers efficiently passivate the surface charged traps in perovskite. As a result, champion power conversion efficiencies of 21.50% and 22.20% along with negligible hysteresis have been achieved for devices based on dopant-free PTTDZ-F and PTTDZ-Cl, respectively. The unencapsulated devices also demonstrate excellent long-term operational stability. Over 93% of their initial efficiencies can be retained after 320 h of maximum output power point tracking under 1 sun illumination. Herein, it paves a new avenue for developing highly stable and efficient HTMs for PSCs.

Polyvinylidene difluoride (PVDF)-based perovskite solar cells (PSCs) have high efficiency due to polarization. The polarization-enhanced solar cells offer a simpler strategy to achieve stable polarization to increase efficiency. Polarization can be induced by piezo-phototronic effect, spontaneous and external electric field poling. The fundamental research on polarization-enhanced built-in field in PVDF-based PSCs is reviewed.

Polyvinylidene difluoride (PVDF)-based perovskite solar cells (PSCs) have led to continuous improvements in efficiency of up to 24.23%. These types of polarization-enhanced solar cells offer a simpler strategy to achieve stable polarization to increase efficiency. The fundamental research progress in polarization-enhanced built-in fields in PVDF-based PSCs is reviewed. Herein, it is discussed how polarization can be induced by piezo-phototronic effect and spontaneous and external electric field poling. Finally, the directions for high-efficiency PSCs based on PVDF are outlined.

Nature, Published online: 23 October 2023; doi:10.1038/s41586-023-06745-7