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Nature Energy, Published online: 20 July 2020; doi:10.1038/s41560-020-0657-y

Nature Energy, Published online: 20 July 2020; doi:10.1038/s41560-020-0652-3

A series of copolymers via a random copolymerization approach are designed and synthesized. The well‐defined fibril interpenetrating morphology with appropriate phase separation in PT2‐based blends can efficiently suppress the unfavorable aggregation, resulting in excellent morphological stability and high efficiency. The work demonstrates the importance of optimization of fibril network morphology in realizing high‐efficiency and ambient‐stable polymer solar cells.

Abstract

Morphological stability is crucially important for the long‐term stability of polymer solar cells (PSCs). Many high‐efficiency PSCs suffer from metastable morphology, resulting in severe device degradation. Here, a series of copolymers is developed by manipulating the content of chlorinated benzodithiophene‐4,8‐dione (T1‐Cl) via a random copolymerization approach. It is found that all the copolymers can self‐assemble into a fibril nanostructure in films. By altering the T1‐Cl content, the polymer crystallinity and fibril width can be effectively controlled. When blended with several nonfullerene acceptors, such as TTPTT‐4F, O‐INIC3, EH‐INIC3, and Y6, the optimized fibril interpenetrating morphology can not only favor charge transport, but also inhibit the unfavorable molecular diffusion and aggregation in active layers, leading to excellent morphological stability. The work demonstrates the importance of optimization of fibril network morphology in realizing high‐efficiency and ambient‐stable PSCs, and also provides new insights into the effect of chemical structure on the fibril network morphology and photovoltaic performance of PSCs.

Solar Cell Fabrication

In article number 2000087, Gang Wu and co‐workers present a new additive‐assisted method, free of hot‐casting, to realize the fabrication of high‐quality two‐dimensional Dion–Jacobson perovskite films with preferred vertical orientation growth. This method avoids the hindrance of the insulating organic spacer cation layer to the charge transport and promotes the current along the vertical direction, resulting in improved power conversion efficiency of the photovoltaic device.

A low‐temperature crystallization strategy of CsPbIBr₂ perovskite solar cells is reported. The additive n‐butylammonium iodide (BAI) is incorporated into the perovskite precursor to improve crystallinity, optimize morphology, and passivate defects at 160 °C. As a result, a high‐level PCE of 10.78% with a high open‐circuit voltage (V _OC) of 1.25 V is achieved.

Inorganic cesium lead halide perovskite solar cells (PSCs) have been widely explored due to their outstanding thermal stability and photovoltaic performance. However, the application and development of CsPbIBr₂‐based PSCs is still hindered by major challenges such as high fabrication temperature and large voltage loss. To address these difficulties, additive engineering is conducted using n‐butylammonium iodide (BAI). It is found that it not only improves the crystallization and morphology of perovskite layers but also substantially decreases the annealing temperature. In addition, the BAI incorporation decreases trap state density and restrains nonradiative recombination. As such, a high power conversion efficiency (PCE) of 10.78% is achieved, 21% higher compared with that of the control sample (8.88%). It should be noted that this is particularly high for the CsPbIBr₂ PSCs fabricated at low temperatures (<200 °C) that are required for flexible devices based on polymeric substrates.

As one of the most promising hole transporting materials for perovskite solar cell, NiO is widely used in the inverted p‐i‐n cell structure due to its high stability, decent hole‐conductivity, and easy processability for hysteresis‐free cells. However, the efficiency of NiO‐based perovskite solar cell is still low, due largely to the poor perovskite/NiO interface. Herein, we introduce a sulfur‐doping strategy to modify NiO surface via ion exchange reaction by a simple and scalable chemical bath deposition technique, which greatly improves the photovoltaic performance of the derived devices. A systematic investigation has shown that sulfur doping leads to favorable interfacial energetics with reduced V_oc loss. Sulfur doping at the interface also improved the contact between NiO and perovksite and facilitated the formation of high‐quality perovskite films. Carrier dynamics studies demonstrate reduced defect states and trap‐assisted recombination with sulfur doping, which promote photovoltaic performance of the devices. These merits contribute concurrently to low‐loss charge transfer across the perovskite/NiO interface and facilitate charge transport through the perovskite films, leading to a high champion efficiency at 20.43% of the p‐i‐n structure solar cell devices.

This article is protected by copyright. All rights reserved.

Publication date: 19 August 2020

Source: Joule, Volume 4, Issue 8

Author(s): Lingeswaran Arunagiri, Zhengxing Peng, Xinhui Zou, Han Yu, Guangye Zhang, Zhen Wang, Joshua Yuk Lin Lai, Jianquan Zhang, Yan Zheng, Chaohua Cui, Fei Huang, Yingping Zou, Kam Sing Wong, Philip C.Y. Chow, Harald Ade, He Yan

Herein the morphology and exciton/charge carrier dynamics in bulk heterojunctions of the donor polymer PTQ10 and molecular acceptor IDIC are investigated. The results emphasize the potential for high material crystallinity to enhance charge separation and collection in organic solar cells, but also that long exciton diffusion lengths are likely to be essential for efficient exciton separation in such high crystallinity devices.

Abstract

Herein the morphology and exciton/charge carrier dynamics in bulk heterojunctions (BHJs) of the donor polymer PTQ10 and molecular acceptor IDIC are investigated. PTQ10:IDIC BHJs are shown to be particularly promising for low cost organic solar cells (OSCs). It is found that both PTQ10 and IDIC show remarkably high crystallinity in optimized BHJs, with GIWAXS data indicating pi‐pi stacking coherence lengths of up to 8 nm. Exciton‐exciton annihilation studies indicate long exciton diffusion lengths for both neat materials (19 nm for PTQ10 and 9.5 nm for IDIC), enabling efficient exciton separation with half lives of 1 and 3 ps, despite the high degree of phase segregation in this blend. Transient absorption data indicate exciton separation leads to the formation of two spectrally distinct species, assigned to interfacial charge transfer (CT) states and separated charges. CT state decay is correlated with the appearance of additional separate charges, indicating relatively efficient CT state dissociation, attributed to the high crystallinity of this blend. The results emphasize the potential for high material crystallinity to enhance charge separation and collection in OSCs, but also that long exciton diffusion lengths are likely to be essential for efficient exciton separation in such high crystallinity devices.

An industry compatible slot‐die coating process combined with near‐infrared irradiation heating enables rapid manufacture of large‐area and uniform perovskite solar cells in air. The highest power conversion efficiency for a device, which is fabricated using the slot‐die coated four layer, is nearly 11%.

Abstract

Currently, high‐efficiency perovskite solar cells are mainly fabricated by the spin‐coating process, which limits the possibility of commercial mass‐production of perovskite solar cells. In this work, the slot‐die coating process is combined with near‐infrared irradiation heating to quickly manufacture perovskite solar cells in air. The composition of the perovskite precursor solution is tuned by adding n‐butanol, with its low boiling point and low surface tension, to increase the near‐infrared energy absorption, facilitate the evaporation of the solvent system and film formation, and accelerate the crystallization of perovskite. High‐quality uniform perovskite film can be prepared within 18 s. Moreover, the all slot‐die coating process is demonstrated to prepare over an area of 12 cm × 12 cm, four layers of uniform film overlay on top of each other for the devices except electrode in ambient air. A power conversion efficiency of ≈11% is achieved when this all slot‐die coated film is used to fabricate device. This facile process can greatly reduce the cost, time and bypass post‐annealing to fabricate high‐efficiency large‐area perovskite solar cells in ambient air.

Tandem solar cells hold promise for breaking the second law of thermodynamics and Shockley–Queisser limits. So far, such devices have to be made via costly methods. The advent of perovskite‐based absorbers enables the fabrication of various tandem devices through low‐cost techniques by combination with different subcells.

Abstract

Tandem solar cells (TSCs) comprising stacked narrow‐bandgap and wide‐bandgap subcells are regarded as the most promising approach to break the Shockley–Queisser limit of single‐junction solar cells. As the game‐changer in the photovoltaic community, organic–inorganic hybrid perovskites became the front‐runner candidate for mating with other efficient photovoltaic technologies in the tandem configuration for higher power conversion efficiency, by virtue of their tunable and complementary bandgaps, excellent photoelectric properties, and solution processability. In this review, a perspective that critically dilates the progress of perovskite material selection and device design for perovskite‐based TSCs, including perovskite/silicon, perovskite/copper indium gallium selenide, perovskite/perovskite, perovskite/CdTe, and perovskite/GaAs are presented. Besides, all‐inorganic perovskite CsPbI₃ with high thermal stability is proposed as the top subcell in TSCs due to its suitable bandgap of ≈1.73 eV and rapidly increasing efficiency. To minimize the optical and electrical losses for high‐efficiency TSCs, the optimization of transparent electrodes, recombination layers, and the current‐matching principles are highlighted. Through big data analysis, wide‐bandgap perovskite solar cells with high open‐circuit voltage (V _oc) are in dire need in further study. In the end, opportunities and challenges to realize the commercialization of TSCs, including long‐term stability, area upscaling, and mitigation of toxicity, are also envisioned.

Layered hybrid perovskites based on (PDMA)FA _{n –1}Pb _n I_{3 n +1} (n = 1–3; PDMA = 1,4‐phenylenedimethanammonium) compositions are investigated by using combination of techniques, including X‐ray scattering measurements, molecular dynamics simulations, and density functional theory calculations, along with time‐resolved microwave conductivity measurements, to unravel unique structural and photophysical properties relevant to optoelectronic applications.

Abstract

Layered hybrid perovskites have emerged as a promising alternative to stabilizing hybrid organic–inorganic perovskite materials, which are predominantly based on Ruddlesden‐Popper structures. Formamidinium (FA)‐based Dion‐Jacobson perovskite analogs are developed that feature bifunctional organic spacers separating the hybrid perovskite slabs by introducing 1,4‐phenylenedimethanammonium (PDMA) organic moieties. While these materials demonstrate competitive performances as compared to other FA‐based low‐dimensional perovskite solar cells, the underlying mechanisms for this behavior remain elusive. Here, the structural complexity and optoelectronic properties of materials featuring (PDMA)FA _{n –1}Pb _n I_{3 n +1} (n = 1–3) formulations are unraveled using a combination of techniques, including X‐ray scattering measurements in conjunction with molecular dynamics simulations and density functional theory calculations. While theoretical calculations suggest that layered Dion‐Jacobson perovskite structures are more prominent with the increasing number of inorganic layers (n), this is accompanied with an increase in formation energies that render n > 2 compositions difficult to obtain, in accordance with the experimental evidence. Moreover, the underlying intermolecular interactions and their templating effects on the Dion‐Jacobson structure are elucidated, defining the optoelectronic properties. Consequently, despite the challenge to obtain phase‐pure n > 1 compositions, time‐resolved microwave conductivity measurements reveal high photoconductivities and long charge carrier lifetimes. This comprehensive analysis thereby reveals critical features for advancing layered hybrid perovskite optoelectronics.

Aiming at stable and efficient perovskite light‐emitting diodes (PeLEDs), this work proposes an all‐inorganic strategy involving an insulator–perovskite–insulator device structure and cascade ZnS‐ZnSe electron transport layers, which improve charge‐injection efficiency and suppress the electric‐field‐induced ion migration channels. The findings provide an addressable approach access to future commercialization of PeLEDs.

Abstract

Stability issue is one of the major concerns that limit emergent perovskite light‐emitting diodes (PeLEDs) techniques. Generally, ion migration is considered as the most important origin of PeLEDs degradation. In this work, an all‐inorganic device architecture, LiF/perovskite/LiF/ZnS/ZnSe, is proposed to address this imperative problem. The inorganic (Cs_{1− x} Rb _x )_{1− y} K _y PbBr₃ perovskite is optimized with achieving a photoluminescence quantum yield of 67%. Depth profile analysis of X‐ray photoelectron spectroscopy indicates that the LiF/perovskite/LiF structure and the ZnS/ZnSe cascade electron transport layers significantly suppress the electric‐field‐induced ion migrations of the perovskite layers, and impede the diffusion of metallic atoms from cathode into perovskites. The as‐prepared PeLEDs display excellent shelf stability (maintaining 90% of the initial external quantum efficiency [EQE] after 264 h) and operational stability (half‐lifetime of about 255 h at an initial luminance of 120 cd m⁻²). The devices also exhibit a maximum brightness of 15 6155 cd m⁻² and an EQE of 11.05%.

A highly efficient organic solar cell with a ternary architecture is successfully demonstrated by enhancing and balancing charge transport as well as matching integer charge transfer energy in a bulk heterojunction blend. As a result, a power conversion efficiency of 17.13% is obtained with the significantly improved fill factor of 0.813.

Abstract

Ternary architecture is one of the most effective strategies to boost the power conversion efficiency (PCE) of organic solar cells (OSCs). Here, an OSC with a ternary architecture featuring a highly crystalline molecular donor DRTB‐T‐C4 as a third component to the host binary system consisting of a polymer donor PM6 and a nonfullerene acceptor Y6 is reported. The third component is used to achieve enhanced and balanced charge transport, contributing to an improved fill factor (FF) of 0.813 and yielding an impressive PCE of 17.13%. The heterojunctions are designed using so‐called pinning energies to promote exciton separation and reduce recombination loss. In addition, the preferential location of DRTB‐T‐C4 at the interface between PM6 and Y6 plays an important role in optimizing the morphology of the active layer.

CsPbI₃ perovskite quantum dot (PQD) hybrid nonfullerene organic solar cells are fabricated. The devices based on a PTB7‐Th:FOIC blend with PQDs yield higher efficiency of 13.2% even at near‐zero driving force than that without PQDs (11.6%). Incorporation of PQDs also leads to efficiency enhancement from 15.4% to 16.6% for a PM6:Y6 blend.

Abstract

To take advantages of the intense absorption and fluorescence, high charge mobility, and high dielectric constant of CsPbI₃ perovskite quantum dots (PQDs), PQD hybrid nonfullerene organic solar cells (OSCs) are fabricated. Addition of PQDs leads to simultaneous enhancement of open‐circuit voltage (V _OC), short‐circuit current density (J _SC), and fill factor (FF); power conversion efficiencies are boosted from 11.6% to 13.2% for PTB7‐Th:FOIC blend and from 15.4% to 16.6% for PM6:Y6 blend. Incorporation of PQDs dramatically increases the energy of the charge transfer state, resulting in near‐zero driving force and improved V _OC. Interestingly, at near‐zero driving force, the PQD hybrid OSCs show more efficient charge generation than the control device without PQDs, contributing to enhanced J _SC, due to the formation of cascade band structure and increased molecular ordering. The strong fluorescence of the PQDs enhances the external quantum efficiency of the electroluminescence of the active layer, which can reduce nonradiative recombination voltage loss. The high dielectric constant of the PQDs screens the Coulombic interactions and reduces charge recombination, which is beneficial for increased FF. This work may open up wide applicability of perovskite quantum dots and an avenue toward high‐performance nonfullerene solar cells.

A general decomposition pathway from tetragonal CH₃NH₃PbI₃ and cubic CH₃NH₃PbBr₃ to lead halides is revealed, through the formation of an intermediate superstructure CH₃NH₃PbX_2.5 with ordered vacancies. A carbon coating is demonstrated to be effective in stabilizing the perovskite framework, and thus slowing down the decomposition.

Abstract

Organic–inorganic hybrid perovskites (OIHPs) have generated considerable excitement due to their promising photovoltaic performance. However, the commercialization of perovskite solar cells (PSCs) is still plagued by the structural degradation of the OIHPs. Here, the decomposition mechanism of OIHPs under electron beam irradiation is investigated via transmission electron microscopy, and a general decomposition pathway for both tetragonal CH₃NH₃PbI₃ and cubic CH₃NH₃PbBr₃ is uncovered through an intermediate superstructure state of CH₃NH₃PbX_2.5, X = I, Br, with ordered vacancies into final lead halides. Such decomposition can be suppressed via carbon coating by stabilization of the perovskite structure framework. These findings reveal the general degradation pathway of OIHPs and suggest an effective strategy to suppress it, and the atomistic insight learnt may be useful for improving the stability of PSCs.