Chen Weijie

Insulating polymers with low costs play a vital role in organic semiconductors. Herein, the optimization strategies with insulating polymers for organic solar cells (OSCs) are focused upon. The various applications of insulating polymers in OSCs optimize and stabilize morphology and enhance energy of charge transfer state and dipole interaction, which provide a brand new perspective for the development of OSCs.

Organic solar cells (OSCs), as a promising photovoltaic technology, have made great progress recently due to the various optimization methods and designs of new materials. However, the general strategies show some restrictions on photovoltaic systems and commercialization of OSCs. Insulating polymers with low costs exhibit rich functionality, enhance device performance, and stabilize film morphology in air or at a high temperature. Herein, the strategies of the ternary, interface layer, and block copolymer in insulator‐modified OSCs (i‐OSCs) reported in recent years are highlighted. In addition, the corresponding underlying mechanisms, such as crystallinity, self‐assembly, dipole interaction, and charge transfer, are also discussed. The applications of insulating polymers in OSCs open a novel avenue for optimizing device performance and bestow a potential pathway to achieve large‐scale industrial production in the future.

A novel donor polymer PBTVT comprising a bis(carboxylic ester)‐substituted thiophene–vinylene–thiophene acceptor unit and a benzodithiophene donor unit is designed and synthesized. It displays a wide bandgap of 1.87 eV and good solubility in halogen‐free solvents such as o‐xylene. The corresponding devices exhibit a high PCE of 11.04%, though the highest occupied molecular orbital (HOMO) level offset is small (0.08 eV).

Herein, a novel wide‐band‐gap donor polymer, PBTVT, which comprises a bis(carboxylic ester)‐substituted thiophene–vinylene–thiophene (TVT) acceptor unit and benzodithiophene donor unit, is designed and synthesized. PBTVT displays good solubility in halogen‐free green solvents such as o‐xylene at elevated temperature. Grazing‐incidence wide‐angle X‐ray scattering (GIWAXS) measurement revealed that PBTVT can form the desired face‐on orientation in the blend films. Organic solar cells are fabricated with PBTVT as the donor and ITOIC‐2F as the acceptor. Although the highest occupied molecular orbital (HOMO) level offset between PBTVT and 2,2′‐((2Z,2′Z)‐(((4,4,9,9‐tetrakis(4‐hexylphenyl)‐4,9‐dihydro‐s‐indaceno[1,2‐b:5,6‐b′]dithiophene‐2,7‐diyl)bis(3,4‐bis(hexyloxy)thiophene‐5,2‐diyl))bis(methanylylidene))bis(5,6‐difluoro‐3‐oxo‐2,3‐dihydro‐1H‐indene‐2,1‐diylidene))dimalononitrile (ITOIC‐2F) is only 0.08 eV, devices still exhibit an overall power conversion efficiency (PCE) of 11.04%. Especially, devices fabricated with the green solvent o‐xylene outperform those fabricated with halogenated solvents such as 1,2‐dichlorobenzene (o‐DCB).

Solution‐processed cesium halides (CsX, X represents F, Cl, Br, I) are investigated as electron‐selective contacts for c‐Si solar cells. The CsF/Al contact with a thin intrinsic amorphous silicon passivating layer has both high‐quality surface passivation and low contact resistivity. With this full‐area rear‐side contact, record power conversion efficiencies of about 21.8% are demonstrated for n‐type c‐Si solar cells.

Crystalline silicon (c‐Si) solar cells with carrier‐selective passivating contacts have been prosperously developed over the past few years, showing fundamental advantages, e.g., simpler configurations and higher potential efficiencies, compared with conventional c‐Si solar cells using highly doped emitters. Herein, solution‐processed cesium halides (CsX, X represents F, Cl, Br, I) are investigated as electron‐selective contacts for c‐Si solar cells, enabling lowest contact resistivity down to about 1 mΩ cm² for slightly doped n‐type c‐Si/CsF/Al contact. After inserting a thin intrinsic amorphous silicon (a‐Si:H(i)) passivating layer, the contact resistivity can still be kept in a low value, about 10 mΩ cm². With full area rear‐side a‐Si:H(i)/CsF/Al electron‐selective passivating contacts, record power conversion efficiencies of about 21.8% are finally demonstrated for n‐type c‐Si solar cells, showing a simple approach to realize high‐efficiency c‐Si solar cells.

TiO₂ microspheres with high crystallinity and purity are synthesized by a single source, surfactant‐free, and one‐step solvothermal reaction. The morphology character of TiO₂ mesoporous layers is conducive to good permeation and interfacial contact of perovskite. After adjusting the colloidal concentration, a well‐matched energy level is obtained and an efficiency of 20.95% with slight hysteresis and satisfactory reproducibility is achieved.

The instant extraction and transmission of photogenerated charge carriers is the key factor to obtain efficient perovskite solar cells (PSCs). The mesoporous electronic transport layer can provide heterogeneous nucleation sites for high‐quality perovskite films and expand the charge separation region to better balance charge transport. Therefore, compared with planar PSCs, most mesoporous PSCs usually exhibit superior photovoltaic performance. Herein, TiO₂ microspheres with high crystallinity and purity are synthesized by a single source, surfactant‐free, and one‐step solvothermal reaction strategy. The morphology character of TiO₂ mesoporous layers is conducive to good permeation and interfacial contact of perovskite. The gradient adjustment of energy level by the insertion of TiO₂ mesoporous layer in the device is beneficial for the effective extraction of electrons. Finally, the optimized mesoporous PSCs can achieve an efficiency of 20.95% with slight hysteresis and satisfactory reproducibility.

The self‐assembled monolayer (SAM) has emerged as a powerful nanomaterial for improving performance of perovskite solar cells (PSCs). This review article covers recent studies that demonstrate direct benefits of SAM‐based interfacial engineering on the mechanistic understanding of the electronic functions of PSCs and their power conversion efficiency.

Abstract

Self‐assembled monolayers (SAMs), owing to their unique and versatile abilities to manipulate chemical and physical interfacial properties, have emerged as powerful nanomaterials for improving the performance of perovskite solar cells (PSCs). Indeed, in the last six years, a collection of studies has shown that the application of SAMs to PSCs boosts the performance of devices compared to the pristine PSCs. This review describes recent studies that demonstrate the direct advantages of SAM‐based interfacial engineering to power conversion efficiency (PCE) of PSCs. This review includes 1) a brief introduction on SAMs as interfacial engineering nanomaterials; 2) a thorough survey of molecules used in SAM‐engineered PSCs and analysis of chemical structures; 3) an extensive discussion on how SAMs affect the morphology of perovskite film and the electronic function of devices; and 4) a comprehensive summary of various types of approaches for producing SAM‐engineered PSCs. This review provides an insightful perspective to stimulate new ideas and innovation in the development of PSCs for the next‐generation photovoltaics and beyond.

Three isomeric small‐molecule acceptors (SMAs) are developed by altering the substitution site of Cl and Br on the benzene‐fused end group, namely, BTP‐ClBr, BTP‐ClBr1, and BTP‐ClBr2, and the effects of substitution position in the SMAs on the photoelectric properties and photovoltaic performance are systematically investigated.

Abstract

It is widely recognized that subtle changes in the chemical structure of organic semiconductors can induce dramatic variations in their optoelectronic properties and device performance, especially for the nonfullerene small‐molecule acceptors (SMAs). For instance, halogenation of the end groups in the acceptor–donor–acceptor‐type SMAs is an effective strategy to modulate the properties of the end group and thus the entire SMA. While previous position modulations have focused on only one substituent, this study shows the development of three isomeric SMAs (BTP‐ClBr, BTP‐ClBr1, and BTP‐ClBr2) via manipulating the position of two halogen substituents (chlorine and bromine) on the terminal unit. BTP‐ClBr exhibits a blueshifted absorption, a shallower lowest unoccupied molecular orbital energy level, and a weaker crystallization tendency relative to BTP‐ClBr1 and BTP‐ClBr2. A power conversion efficiency (16.82%) and an excellent fill factor (FF) (0.79) are realized in the optimal PM6:BTP‐ClBr organic solar cell device. The higher FF is consistent with the results of the characterization including a longer charge recombination lifetime, a faster photocurrent decay, a weaker bimolecular recombination, and a more favorable domain size for PM6:BTP‐ClBr, which all originate from a subtle change in the substitution sites that strongly influences the physicochemical properties of the SMA.

In this review, the crystallization kinetics and their effects on the performance of various types of 2D perovskite solar cells (PSCs) up to now are discussed. The crystal/natural quantum well structures and original stability for 2D perovskite are also clearly summarized. Finally, remaining challenges are discussed and possible solutions are proposed in terms of development bottlenecks for 2D PSCs.

Abstract

2D perovskites demonstrate higher moisture stability, oxygen content, thermal stability, and a significantly lower ion migration/phase transition occurrence in comparison to 3D perovskite. These advantages imply huge potential for 2D perovskite in commercial applications in the photovoltaic field. However, the horizontal arrangement of the organic layer severely hinders the transport of carriers, and thus, the power conversion efficiency of 2D perovskite solar cells (PSCs) is significantly lower than that of 3D. Controlling the crystallization orientation to achieve rapid carrier transport can effectively avoid or reduce such adverse effects. Hence, an in‐depth understanding of the formation mechanism and crystallization kinetics of 2D perovskite films is crucial to the development of high‐performing 2D PSCs. This review explores the studies conducted on crystallization kinetics, which is the key issue for 2D perovskite, and discusses their effects on the performance of various types of 2D PSCs to date. The crystal/natural quantum well structures and origin of the stability for 2D perovskite are also summarized. Finally, the remaining challenges in terms of development bottlenecks for 2D PSCs are discussed, alongside the proposal of possible solutions.

Thermal annealing of 2D/3D perovskite heterostructures leads to beneficial diffusion passivation; however, it also causes lattice expansion of the 2D perovskite. Here a novel preparation strategy, simultaneously inhibiting lattice expansion, compensating the large tensile stress of 2D perovskite, and inducing diffusion passivation, is introduced. As a result, a certified efficiency of 20.22% is obtained.

Abstract

Lattice matching and passivation are generally seen as the main beneficial effects in 2D/3D perovskite heterostructured solar cells, but the understanding of the mechanisms involved is still incomplete. In this work, it is shown that 2D/3D heterostructure are unstable under common thermal processing conditions, caused by the lattice expansion of strained 2D perovskite. Therefore an innovative fabrication technology involving a compressively strained PEA₂PbI₄ layer is proposed to compensate the internal tensile strain and stabilize the 2D/3D heterostructure. Moreover, a small amount of PEA⁺ diffusing into the grain boundaries of 3D perovskite forms 2D perovskite and passivates the defects there. Combining the effects of strain compensation and diffusion passivation, the stabilized 2D/3D perovskite solar cells deliver a reproducible and robust laboratory power conversion efficiency (PCE) of 21.31% for the p‐i‐n type devices, along with a high V _OC of 1.18 V. A certified PCE of 20.22% is confirmed by an independent national metrology institute.

A carbon‐encapsulated (Cs_0.15FA_0.85)Pb(I_0.9Br_0.1)₃ photocathode with a sandwich‐like structure is prepared and demonstrates state‐of‐the‐art performance for photo‐electrochemical (PEC) CO₂ reduction among organic–inorganic hybrid perovskite‐based PEC devices. A tandem device consisting of this photocathode and a Si photoanode further realizes unbiased PEC CO₂ reduction with an outstanding solar‐to‐CO energy conversion efficiency of 3.34%.

Abstract

Photo‐electrochemical (PEC) carbon dioxide reduction to chemicals or fuels has been regarded as an attractive strategy that can close the anthropogenic carbon cycle. However, identifying a PEC system capable of driving efficient and durable CO₂ conversion remains a critical challenge. Herein, the fabrication of a sandwich‐like organic–inorganic hybrid perovskite‐based photocathode with carbon encapsulation for PEC CO₂ reduction is reported. The carbon encapsulation not only affords protection to the perovskite, but also allows for efficient conductance of photogenerated electrons. When decorated with a cobalt phthalocyanine molecular catalyst, the photocathode shows an onset potential at 0.58 V versus reversible hydrogen electrode (RHE) and a high photocurrent density of −15.5 mA cm⁻² at −0.11 V versus RHE in CO₂‐saturated 0.5 m KHCO₃ under AM 1.5G illumination (100 mW cm⁻²), which represents state‐of‐the‐art performance in this field. Moreover, the photocathode remains stable during a continuous reaction that lasted for 25 h. Unbiased PEC CO₂ reduction is further realized by integrating the photocathode with an amorphous Si photoanode in tandem, delivering a solar‐to‐CO energy conversion efficiency of 3.34% and a total solar‐to‐fuel energy conversion efficiency of 3.85%.

The development of the first high‐performance, printable CsPbI₃ solar cells via an ambient blade‐coating technique is reported. High‐quality CsPbI₃ films are grown via the introduction of a low concentration of the multifunctional molecular additive Zn(C₆F₅)₂. As a result, the additive‐treated perovskite solar cell delivers a power conversion efficiency (PCE) of 19%.

Abstract

All‐inorganic CsPbI₃ holds promise for efficient tandem solar cells, but reported fabrication techniques are not transferrable to scalable manufacturing methods. Herein, printable CsPbI₃ solar cells are reported, in which the charge transporting layers and photoactive layer are deposited by fast blade‐coating at a low temperature (≤100 °C) in ambient conditions. High‐quality CsPbI₃ films are grown via introducing a low concentration of the multifunctional molecular additive Zn(C₆F₅)₂, which reconciles the conflict between air‐flow‐assisted fast drying and low‐quality film including energy misalignment and trap formation. Material analysis reveals a preferential accumulation of the additive close to the perovskite/SnO₂ interface and strong chemisorption on the perovskite surface, which leads to the formation of energy gradients and suppressed trap formation within the perovskite film, as well as a 150 meV improvement of the energetic alignment at the perovskite/SnO₂ interface. The combined benefits translate into significant enhancement of the power conversion efficiency to 19% for printable solar cells. The devices without encapsulation degrade only by ≈2% after 700 h in air conditions.

Lessons learned from the historical analysis of diverse solar cells provide a fundamental diagnosis of the relative and absolute development status of perovskite solar cells. The upscaling of perovskite solar cells and commercialization of various solar cells are comparatively analyzed and feasible technologies that can be applied to the perovskite upscaling process, both now and in the future, are suggested.

Abstract

The status and problems of upscaling research on perovskite solar cells, which must be addressed for commercialization efforts to be successful, are investigated. An 804 cm² perovskite solar module has been reported with 17.9% efficiency, which is significantly lower than the champion perovskite solar cell efficiency of 25.2% reported for a 0.09 cm² aperture area. For the realization of upscaling high‐quality perovskite solar cells, the upscaling and development history of conventional silicon, copper indium gallium sulfur/selenide and CdTe solar cells, which are already commercialized with modules of sizes up to ≈25 000 cm², are reviewed. GaAs, organic, dye‐sensitized solar cells and perovskite/silicon tandem solar cells are also reviewed. The similarities of the operating mechanisms between the various solar cells and the origin of different development pathway are investigated, and the ideal upscaling direction of perovskite solar cells is subsequently proposed. It is believed that lessons learned from the historical analysis of various solar cells provide a fundamental diagnosis of relative and absolute development status of perovskite solar cells. The unique perspective proposed here can pave the way toward the upscaling of perovskite solar cells.

Two fully non‐fused acceptors are precisely designed and easily prepared. The side chain encapsulation can induce a planar molecular backbone conformation, endowing the acceptor with broad light absorption. Thermal annealing promotes molecular rearrangement to form J‐aggregates with even broader absorption and higher absorption coefficient. A PCE over 10 % is one of the highest PCE for fully non‐fused ring acceptors.

Abstract

Fused‐ring electron acceptors have made significant progress in recent years, while the development of fully non‐fused ring acceptors has been unsatisfactory. Here, two fully non‐fused ring acceptors, o‐4TBC‐2F and m‐4TBC‐2F, were designed and synthesized. By regulating the location of the hexyloxy chains, o‐4TBC‐2F formed planar backbones, while m‐4TBC‐2F displayed a twisted backbone. Additionally, the o‐4TBC‐2F film showed a markedly red‐shifted absorption after thermal annealing, which indicated the formation of J‐aggregates. For fabrication of organic solar cells (OSCs), PBDB‐T was used as a donor and blended with the two acceptors. The o‐4TBC‐2F‐based blend films displayed higher charge mobilities, lower energy loss and a higher power conversion efficiency (PCE). The optimized devices based on o‐4TBC‐2F gave a PCE of 10.26 %, which was much higher than those based on m‐4TBC‐2F at 2.63 %, and it is one of the highest reported PCE values for fully non‐fused ring electron acceptors.

Publication date: January 2021

Source: Nano Energy, Volume 79

Author(s): Simon A. Svatek, Carlos Bueno-Blanco, Der-Yuh Lin, James Kerfoot, Carlos Macías, Marius H. Zehender, Ignacio Tobías, Pablo García-Linares, Takashi Taniguchi, Kenji Watanabe, Peter Beton, Elisa Antolín

Publication date: 18 November 2020

Source: Joule, Volume 4, Issue 11

Author(s): Xixiang Zhu, Guichuan Zhang, Jia Zhang, Hin-Lap Yip, Bin Hu

The excited state characteristics of organic hole transport materials in perovskite photovoltaics (PVs), such as transition dipole moment, is confirmed to be a critical factor in improving the built‐in potential of devices for efficient charge extraction along with reduced carrier recombination. Moreover, the aggregation property of the organic semiconductor can have a synergistic effect with its excited state property for high‐efficiency perovskite PVs.

Abstract

Intrinsic characteristics of organic semiconductor‐based hole transport materials (HTMs) such as facile synthesizability, energy level tunability, and charge transport capability have been highlighted as crucial factors determining the performances of perovskite photovoltaic (PV) cells. However, their properties in the excited state have not been actively studied, although PVs are operated under solar illumination. Here, the characteristics of organic HTMs in their excited state such as transition dipole moment can be a decisive factor that can improve built‐in potential of PVs, consequently enhancing their charge extraction property as well as reducing carrier recombination. Moreover, the aggregation property of organic semiconductors, which has been an essential factor for high‐performance organic HTMs to improve their carrier transport property, can induce a synergistic effect with their excited state property for the high‐efficiency perovskite PVs. Additionally, it is also confirmed that their optical bandgaps, manipulated to have their absorption in the UV region, are beneficial to block UV light that degrades the quality of perovskite, consequently improving the stability of perovskite PV in p–i–n configuration. As a proof‐of‐concept, a model system, composed of triarylamine and imidazole‐based organic HTMs, is designed, and it is believed that this strategy paves a way toward high‐performance and stable perovskite PV devices.

The excited state characteristics of organic hole transport materials in perovskite photovoltaics (PVs), such as transition dipole moment, is confirmed to be a critical factor in improving the built‐in potential of devices for efficient charge extraction along with reduced carrier recombination. Moreover, the aggregation property of the organic semiconductor can have a synergistic effect with its excited state property for high‐efficiency perovskite PVs.

Abstract

Intrinsic characteristics of organic semiconductor‐based hole transport materials (HTMs) such as facile synthesizability, energy level tunability, and charge transport capability have been highlighted as crucial factors determining the performances of perovskite photovoltaic (PV) cells. However, their properties in the excited state have not been actively studied, although PVs are operated under solar illumination. Here, the characteristics of organic HTMs in their excited state such as transition dipole moment can be a decisive factor that can improve built‐in potential of PVs, consequently enhancing their charge extraction property as well as reducing carrier recombination. Moreover, the aggregation property of organic semiconductors, which has been an essential factor for high‐performance organic HTMs to improve their carrier transport property, can induce a synergistic effect with their excited state property for the high‐efficiency perovskite PVs. Additionally, it is also confirmed that their optical bandgaps, manipulated to have their absorption in the UV region, are beneficial to block UV light that degrades the quality of perovskite, consequently improving the stability of perovskite PV in p–i–n configuration. As a proof‐of‐concept, a model system, composed of triarylamine and imidazole‐based organic HTMs, is designed, and it is believed that this strategy paves a way toward high‐performance and stable perovskite PV devices.

Lessons learned from the historical analysis of diverse solar cells provide a fundamental diagnosis of the relative and absolute development status of perovskite solar cells. The upscaling of perovskite solar cells and commercialization of various solar cells are comparatively analyzed and feasible technologies that can be applied to the perovskite upscaling process, both now and in the future, are suggested.

Abstract

The status and problems of upscaling research on perovskite solar cells, which must be addressed for commercialization efforts to be successful, are investigated. An 804 cm² perovskite solar module has been reported with 17.9% efficiency, which is significantly lower than the champion perovskite solar cell efficiency of 25.2% reported for a 0.09 cm² aperture area. For the realization of upscaling high‐quality perovskite solar cells, the upscaling and development history of conventional silicon, copper indium gallium sulfur/selenide and CdTe solar cells, which are already commercialized with modules of sizes up to ≈25 000 cm², are reviewed. GaAs, organic, dye‐sensitized solar cells and perovskite/silicon tandem solar cells are also reviewed. The similarities of the operating mechanisms between the various solar cells and the origin of different development pathway are investigated, and the ideal upscaling direction of perovskite solar cells is subsequently proposed. It is believed that lessons learned from the historical analysis of various solar cells provide a fundamental diagnosis of relative and absolute development status of perovskite solar cells. The unique perspective proposed here can pave the way toward the upscaling of perovskite solar cells.

A novel ternary active layer for polymer solar cells, PTQ10:4TIC‐4F:PC₆₁BM, is processed in semi‐industrial conditions. The devices show a promising performance–cost–photostability compromise. It is shown that PC₆₁BM is critical to balance the holes and electrons mobilities to increase significantly the fill factor due to an optimized bulk heterojunction morphology.

Bulk heterojunction polymer solar cells based on a novel combination of materials are fabricated using industry‐compliant conditions for large area manufacturing. The relatively low‐cost polymer PTQ10 is paired with the nonfullerene acceptor 4TIC‐4F. Devices are processed using a nonhalogenated solvent to comply with industrial usage in absence of any thermal treatment to minimize the energy footprint of the fabrication. No solvent additive is used. Adding the well‐known and low‐cost fullerene derivative PC₆₁BM acceptor to this binary blend to form a ternary blend, the power conversion efficiency (PCE) is improved from 8.4% to 9.9% due to increased fill factor (FF) and open‐circuit voltage (V _OC) while simultaneously improving the stability. The introduction of PC₆₁BM is able to balance the hole–electron mobility in the ternary blends, which is favourable for high FF. This charge transport behavior is correlated with the bulk heterojunction (BHJ) morphology deduced from grazing‐incidence wide‐angle X‐ray scattering (GIWAXS), atomic force microscopy (AFM), and surface energy analysis. In addition, the industrial figure of merit (i‐FOM) of this ternary blend is found to increase drastically upon addition of PC₆₁BM due to an increased performance–stability–cost balance.

A carbon‐encapsulated (Cs_0.15FA_0.85)Pb(I_0.9Br_0.1)₃ photocathode with a sandwich‐like structure is prepared and demonstrates state‐of‐the‐art performance for photo‐electrochemical (PEC) CO₂ reduction among organic–inorganic hybrid perovskite‐based PEC devices. A tandem device consisting of this photocathode and a Si photoanode further realizes unbiased PEC CO₂ reduction with an outstanding solar‐to‐CO energy conversion efficiency of 3.34%.

Abstract

Photo‐electrochemical (PEC) carbon dioxide reduction to chemicals or fuels has been regarded as an attractive strategy that can close the anthropogenic carbon cycle. However, identifying a PEC system capable of driving efficient and durable CO₂ conversion remains a critical challenge. Herein, the fabrication of a sandwich‐like organic–inorganic hybrid perovskite‐based photocathode with carbon encapsulation for PEC CO₂ reduction is reported. The carbon encapsulation not only affords protection to the perovskite, but also allows for efficient conductance of photogenerated electrons. When decorated with a cobalt phthalocyanine molecular catalyst, the photocathode shows an onset potential at 0.58 V versus reversible hydrogen electrode (RHE) and a high photocurrent density of −15.5 mA cm⁻² at −0.11 V versus RHE in CO₂‐saturated 0.5 m KHCO₃ under AM 1.5G illumination (100 mW cm⁻²), which represents state‐of‐the‐art performance in this field. Moreover, the photocathode remains stable during a continuous reaction that lasted for 25 h. Unbiased PEC CO₂ reduction is further realized by integrating the photocathode with an amorphous Si photoanode in tandem, delivering a solar‐to‐CO energy conversion efficiency of 3.34% and a total solar‐to‐fuel energy conversion efficiency of 3.85%.