JIANG ZHI XUAN

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.

Polymer additives in perovskite solar cells are found to act as barriers at the perovskite grain boundaries and hinder the ion migration, improving the device stability under both light irradiation and electrical stressing. The polymer incorporated perovskite solar cells have significantly increased electrical‐field tolerance with an increase in breakdown voltage from −0.4 to −2 V.

Abstract

Passivation of organometal halide perovskites with polar molecules has been recently demonstrated to improve the photovoltaic device efficiency and stability. However, the mechanism is still elusive. Here, it is found that both polymers with large and small dipole moment of 3.7 D and 0.6 D have negligible defect passivation effect on the MAPbI₃ perovskite films as evidenced by photothermal deflection spectroscopy. The photovoltaic devices with and without the polymer additives also have comparable power conversion efficiencies around 19%. However, devices with the additives have noticeable improvement in stability under continuous light irradiation. It is found that although the initial mobile ion concentrations are comparable in both devices with and without the additives, the additives can strongly suppress the ion migration during the device operation. This contributes to the significantly enhanced electrical‐field stress tolerance of the perovskite solar cells (PVSCs). The PVSCs with polymer additives can operate up to −2 V reverse voltage bias which is much larger than the breakdown voltage of −0.5 V that has been commonly observed. This study provides insight into the role of additives in perovskites and the corresponding device degradation mechanism.

Structural evolution of metal halide perovskites (MHPs) subjected to stress/strain induces the variation of optical and electronic properties. The in‐service performance and film stability of MHPs are deteriorated. By reviewing the evolution of structural related mechanical and opto‐electronic properties, strain engineering of MHPs coupling anisotropy to improve the performance of MHPs‐based devices is proposed.

Abstract

The power conversion efficiencies (PCEs) of the solar cells containing metal halide perovskites (MHPs) have rapidly increased and exceeded 25% during the past decade. The photovoltaic properties of these devices are extensively investigated in terms of their microstructures, environmental characteristics, and carrier dynamics, and the MHP structural evolution under high pressure is evaluated. In addition, the energy level structure, electron/hole dynamics, and optical/electronic properties of MHPs with anisotropic crystal structures are examined. However, the correlation between the structural anisotropy and material properties of these perovskites is rarely considered in the literature studies on their high‐pressure behavior. In this progress report, the optical/electronic properties of MHPs with anisotropic structures under thermal, mechanically imposed, and in‐service strains/stresses that have been previously neglected by researchers are summarized.

The complicated interactions between the guest and host components are studied to fabricate high‐performing ternary organic solar cells (TOSCs). Notably, the LA9 ternary devices yield the most competitive efficiency, up to 15.75%, in Y6‐absent TOSCs, owing to the superior charge transport networks originating from the appropriate interactions between the guest and host components.

Abstract

Ternary organic solar cells (TOSCs) offer a facile and efficient approach to increase the power conversion efficiencies (PCEs). However, the critical roles that guest components play in complicated ternary systems remain poorly understood. Herein, two acceptors named LA1 and LA9 with differing crystallinity are investigated. The overly crystalline LA9 induces large self‐aggregates in PM6:LA9 binary system, resulting in a lower PCE (13.12%) compared to PM6:LA1 device (13.89%). Encouragingly, both acceptors are verified as efficient guest candidates into the host binary PM6:NCBDT‐4Cl (PCE = 13.48%) and afford markedly improved PCEs up to 15.39% and 15.75% in LA1 and LA9 ternary devices, respectively. Interestingly, the higher crystallinity LA9 reveals smaller interaction energies with both the host acceptor and donor PM6. Compared to LA1, the appropriate mutual interactions in the LA9 ternary system not only induces the orderly crystallinity of PM6 but also better compatibility with the host acceptor, generating further optimized molecular orientations and ternary morphology. Therefore, enhanced charge transport and minimized recombination loss are detected in LA9 ternary devices, affording the most competitive performance among Y6‐sbsent TOSCs. This work suggests that complicated intermolecular interactions should be seriously considered when fabricating state‐of‐the‐art multiple components OSCs.

A new strategy for synergistically improving molecular doping and carrier mobility is proposed by copolymerizing donor–acceptor type and donor–donor type building blocks along polymer backbone. The copolymers show significantly improved mobilities of 1–2 cm² V⁻¹ s⁻¹ at a high doping level while the structural disorder endows a high Seebeck coefficient, indicating a great potential of random copolymer for thermoelectric application.

Abstract

In this work, it is demonstrated that random copolymerization is a simple but effective strategy to obtain new conductive copolymers as high‐performance thermoelectric materials. By using a polymerizing acceptor unit diketopyrropyrrole with donor units thienothiophene and oligo ethylene glycol substituted bithiophene (g₃2T), it is found that strong interchain donor–acceptor interactions ensure good film crystallinity for charge transport, while donor–donor type building blocks contribute to effective charge transfers. Hall effect measurements show that the high electrical conductivity results from increased free carriers with simultaneously improved mobility reaching over 1 cm² V⁻¹ s⁻¹. The synergistic effect of improved molecular doping and carrier mobility, as well as a high Seebeck coefficient ascribed to the structural disorder along polymer chains via random copolymerization, results in an impressive power factor up to 110 µW K⁻² m⁻¹ which is 10 times higher than that of solution‐processed polythiophenes.

The complicated interactions between the guest and host components are studied to fabricate high‐performing ternary organic solar cells (TOSCs). Notably, the LA9 ternary devices yield the most competitive efficiency, up to 15.75%, in Y6‐absent TOSCs, owing to the superior charge transport networks originating from the appropriate interactions between the guest and host components.

Abstract

Ternary organic solar cells (TOSCs) offer a facile and efficient approach to increase the power conversion efficiencies (PCEs). However, the critical roles that guest components play in complicated ternary systems remain poorly understood. Herein, two acceptors named LA1 and LA9 with differing crystallinity are investigated. The overly crystalline LA9 induces large self‐aggregates in PM6:LA9 binary system, resulting in a lower PCE (13.12%) compared to PM6:LA1 device (13.89%). Encouragingly, both acceptors are verified as efficient guest candidates into the host binary PM6:NCBDT‐4Cl (PCE = 13.48%) and afford markedly improved PCEs up to 15.39% and 15.75% in LA1 and LA9 ternary devices, respectively. Interestingly, the higher crystallinity LA9 reveals smaller interaction energies with both the host acceptor and donor PM6. Compared to LA1, the appropriate mutual interactions in the LA9 ternary system not only induces the orderly crystallinity of PM6 but also better compatibility with the host acceptor, generating further optimized molecular orientations and ternary morphology. Therefore, enhanced charge transport and minimized recombination loss are detected in LA9 ternary devices, affording the most competitive performance among Y6‐sbsent TOSCs. This work suggests that complicated intermolecular interactions should be seriously considered when fabricating state‐of‐the‐art multiple components OSCs.

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.

Nature Energy, Published online: 05 October 2020; doi:10.1038/s41560-020-00705-5

Ensuring both stability and efficiency in mixed lead–tin perovskite solar cells is crucial to the development of all-perovskite tandems. Xiao et al. use an antioxidant zwitterionic molecule to suppress tin oxidation thus enabling large-area tandem cells with 24.2% efficiency and operational stability over 500 hours.

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.

Polymer additives in perovskite solar cells are found to act as barriers at the perovskite grain boundaries and hinder the ion migration, improving the device stability under both light irradiation and electrical stressing. The polymer incorporated perovskite solar cells have significantly increased electrical‐field tolerance with an increase in breakdown voltage from −0.4 to −2 V.

Abstract

Passivation of organometal halide perovskites with polar molecules has been recently demonstrated to improve the photovoltaic device efficiency and stability. However, the mechanism is still elusive. Here, it is found that both polymers with large and small dipole moment of 3.7 D and 0.6 D have negligible defect passivation effect on the MAPbI₃ perovskite films as evidenced by photothermal deflection spectroscopy. The photovoltaic devices with and without the polymer additives also have comparable power conversion efficiencies around 19%. However, devices with the additives have noticeable improvement in stability under continuous light irradiation. It is found that although the initial mobile ion concentrations are comparable in both devices with and without the additives, the additives can strongly suppress the ion migration during the device operation. This contributes to the significantly enhanced electrical‐field stress tolerance of the perovskite solar cells (PVSCs). The PVSCs with polymer additives can operate up to −2 V reverse voltage bias which is much larger than the breakdown voltage of −0.5 V that has been commonly observed. This study provides insight into the role of additives in perovskites and the corresponding device degradation mechanism.

Hole‐Transporting Materials

In article number 2000327, Aung Ko Ko Kyaw, Haibo Ma, Gongqiang Li, and co‐workers synthesize and systemically characterize saddle‐shaped small molecules α, β‐COTh‐Ph‐OMeTAD and β, β‐COTh‐Ph‐OMeTAD as dopant‐free hole‐transporting materials (HTMs) in inverted perovskite solar cells (i‐PSCs). High power conversion efficiencies (PCEs) (17.59% and 18.53%) and stable‐enhanced PSCs devices are achieved, and more than 80% of the maximum PCE is retained after storing in glove box for 150 days.

Nature Communications, Published online: 13 October 2020; doi:10.1038/s41467-020-18940-5

Driven by recent improvement in efficiency and stability of perovskite solar cells, the next step toward commercialisation is upscaling. Here, the authors demonstrate pilot-scale fully roll-to-roll manufacturing of flexible perovskite solar cells through gravure-printing and antisolvent bathing.

In article number https://doi.org/10.1002/aenm.2020015672001567, Yu‐Ching Huang, Wei‐Fang Su, and co‐workers demonstrate a facile process for the mass‐production of perovskite solar cells. The process integrates slot‐die coating and near‐infrared irradiation heating techniques, combined with a new perovskite precursor formula to rapidly produce large‐area perovskite solar cells and modules in air.

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.

Mixtures of cations or halides with FAPbI₃ (where FA is formamidinium) lead to high efficiency in perovskite solar cells (PSCs) but also to blue-shifted absorption and long-term stability issues caused by loss of volatile methylammonium (MA) and phase segregation. We report a deposition method using MA thiocyanate (MASCN) or FASCN vapor treatment to convert yellow -FAPbI₃ perovskite films to the desired pure α-phase. NMR quantifies MA incorporation into the framework. Molecular dynamics simulations show that SCN^– anions promote the formation and stabilization of α-FAPbI₃ below the thermodynamic phase-transition temperature. We used these low-defect-density α-FAPbI₃ films to make PSCs with >23% power-conversion efficiency and long-term operational and thermal stability, as well as a low (330 millivolts) open-circuit voltage loss and a low (0.75 volt) turn-on voltage of electroluminescence.

High-efficiency lead halide perovskite solar cells (PSCs) have been fabricated with α-phase formamidinium lead iodide (FAPbI₃) stabilized with multiple cations. The alloyed cations greatly affect the bandgap, carrier dynamics, and stability, as well as lattice strain that creates unwanted carrier trap sites. We substituted cesium (Cs) and methylenediammonium (MDA) cations in FA sites of FAPbI₃ and found that 0.03 mol fraction of both MDA and Cs cations lowered lattice strain, which increased carrier lifetime and reduced Urbach energy and defect concentration. The best-performing PSC exhibited power conversion efficiency >25% under 100 milliwatt per square centimeter AM 1.5G illumination (24.4% certified efficiency). Unencapsulated devices maintained >80% of their initial efficiency after 1300 hours in the dark at 85°C.