JIANG ZHI XUAN

An anion/cation synergy strategy is proposed by the incorporation of ZnI₂ in CsPbI₃ quantum dots (QDs) to improve the stability and photoelectric properties. The obtained Zn:CsPbI₃ QDs show lower defect state density and enhanced structural stability. Perovskite quantum dot solar cells fabricated with Zn:CsPbI₃ QDs exhibit a champion power conversion efficiency over 16%.

Abstract

All‐inorganic CsPbI₃ quantum dots (QDs) have shown great potential in photovoltaic applications. However, their performance has been limited by defects and phase stability. Herein, an anion/cation synergy strategy to improve the structural stability of CsPbI₃ QDs and reduce the pivotal iodine vacancy (V _I) defect states is proposed. The Zn‐doped CsPbI₃ (Zn:CsPbI₃) QDs have been successfully synthesized employing ZnI₂ as the dopant to provide Zn²⁺ and extra I⁻. Theoretical calculations and experimental results demonstrate that the Zn:CsPbI₃ QDs show better thermodynamic stability and higher photoluminescence quantum yield (PLQY) compared to the pristine CsPbI₃ QDs. The doping of Zn in CsPbI₃ QDs increases the formation energy and Goldschmidt tolerance factor, thereby improving the thermodynamic stability. The additional I⁻ helps to reduce the V _I defects during the synthesis of CsPbI₃ QDs, resulting in the higher PLQY. More importantly, the synergistic effect of Zn²⁺ and I⁻ in CsPbI₃ QDs can prevent the iodine loss during the fabrication of CsPbI₃ QD film, inhibiting the formation of new V _I defect states in the construction of solar cells. Consequently, the anion/cation synergy strategy affords the CsPbI₃ quantum dot solar cells (QDSC) a power conversion efficiency over 16%, which is among the best efficiencies for perovskite QDSCs.

Two n‐type polymers based on dichlorodithienylethene (ClTVT) are synthesized. Using the two polymers doped with CoCp₂ and N‐DMBI, respectively, organic thermoelectric devices were prepared and compared. Doping of PClClTVT with N‐DMBI results in excellent air stability; the electrical conductivity and power factor is still maintained at 4.9 S m⁻¹ and 9.3 µW m⁻¹ K⁻² after 222 days.

Abstract

Two donor–acceptor (D–A) polymers are obtained by coupling difluoro‐ and dichloro‐substituted forms of the electron‐deficient unit BDOPV and the relatively weak donor moiety dichlorodithienylethene (ClTVT). The conductivity and power factors of doped devices are different for the chlorinated and fluorinated BDOPV polymers. A high electron conductivity of 38.3 and 16.1 S cm⁻¹ are obtained from the chlorinated and fluorinated polymers with N‐DMBI, respectively, and 12.4 and 2.4 S cm⁻¹ are obtained from the chlorinated and fluorinated polymers with CoCp₂, respectively, from drop‐cast devices. The corresponding power factors are 22.7, 7.6, 39.5, and 8.0 µW m⁻¹ K⁻², respectively. Doping of PClClTVT with N‐DMBI results in excellent air stability; the electron conductivity of devices with 50 mol% N‐DMBI as dopant remained up to 4.9 S m⁻¹ after 222 days in the air, the longest for an n‐doped polymer stored in air, with a thermoelectric power factor of 9.3 µW m⁻¹ K⁻². However, the conductivity of PFClTVT‐based devices can hardly be measured after 103 days. These observations are consistent with morphologies determined by grazing incidence wide angle X‐ray scattering and atomic force microscopy.

Highly efficient ternary all‐polymer solar cells (PSCs) based on an ultranarrow bandgap polymer acceptor are realized. The optimized ternary all‐PSCs achieve a full coverage of solar spectrum, yielding an excellent power conversion efficiency of 12.1% with a remarkable short‐circuit current density of 21.9 mA cm⁻².

Abstract

Developing organic solar cells (OSCs) based on a ternary active layer is one of the most effective approaches to maximize light harvesting and improve their photovoltaic performance. However, this strategy meets very limited success in all‐polymer solar cells (all‐PSCs) due to the scarcity of narrow bandgap polymer acceptors and the challenge of morphology optimization. In fact, the power conversion efficiencies (PCEs) of ternary all‐PSCs even lag behind binary all‐PSCs. Herein, highly efficient ternary all‐PSCs are realized based on an ultranarrow bandgap (ultra‐NBG) polymer acceptor DCNBT‐TPC, a medium bandgap polymer donor PTB7‐Th, and a wide bandgap polymer donor PBDB‐T. The optimized ternary all‐PSCs yield an excellent PCE of 12.1% with a remarkable short‐circuit current density of 21.9 mA cm⁻². In fact, this PCE is the highest value reported for ternary all‐PSCs and is much higher than those of the corresponding binary all‐PSCs. Moreover, the optimized ternary all‐PSCs show a photostability with ≈68% of the initial PCE retained after 400 h illumination, which is more stable than the binary all‐PSCs. This work demonstrates that the utilization of a ternary all‐polymer system based on ultra‐NBG polymer acceptor blended with compatible polymer donors is an effective strategy to advance the field of all‐PSCs.

Perpendicular crystal orientation and orderly n‐phase distribution in quasi‐2D perovskite films are simultaneously achieved by F‐substitution in phenethylammonium (PEA⁺), leading to an impressive 18.10%‐efficiency of perovskite solar cells with n = 4. Meanwhile, the horizontal crystal orientation and random n‐phase distribution are obtained in perovskite films based on PEA and (Cl/Br)‐substituted PEA, respectively.

Abstract

Halide substitution in phenethylammonium spacer cations (X‐PEA⁺, X = F, Cl, Br) is a facile strategy to improve the performance of PEA based perovskite solar cells (PSCs). However, the power conversion efficiency (PCE) of X‐PEA based quasi‐2D (Q‐2D) PSCs is still unsatisfactory and the underlying mechanisms are in debate. Here, the in‐depth study on the impact of halide substitution on the crystal orientation and multi‐phase distribution in PEA based perovskite films are reported. The halide substitution eliminates n = 1 2D perovskite and thus leads to the perpendicular crystal orientation. Furthermore, nucleation competition exists between small‐n and large‐n phases in PEA and X‐PEA based perovskites. This gives rise to the orderly distribution of different n‐phases in the PEA and F‐PEA based films, and random distribution in Cl‐PEA and Br‐PEA based films. As a result, (F‐PEA)₂MA₃Pb₄I₁₂ (MA = CH₃NH₃ ⁺, n = 4) based PSCs achieve a PCE of 18.10%, significantly higher than those of PEA (12.23%), Cl‐PEA (7.93%) and Br‐PEA (6.08%) based PSCs. Moreover, the F‐PEA based devices exhibit remarkably improved stability compared to their 3D counterparts.

Nature Materials, Published online: 23 October 2020; doi:10.1038/s41563-020-00835-x

A systematic analysis of a series of donor–acceptor organic blends shows that in solar cells based on low-bandgap non-fullerene acceptors an ionization energy offset of about 0.5 eV is required to ensure efficient charge separation.

Nature Materials, Published online: 26 October 2020; doi:10.1038/s41563-020-00836-w

Non-fullerene acceptors have successfully overcome energy losses that were thought to be unavoidable in organic solar cells based on fullerene derivatives. However, it is now shown that they have limits too.

Publication date: January 2021

Source: Nano Energy, Volume 79

Author(s): Ming-Hua Li, Tian-Ge Sun, Jiang-Yang Shao, Yu-Duan Wang, Jin-Song Hu, Yu-Wu Zhong

A low‐cost industrial organic pigment, quinacridone (QA), was applied as surface passivation agent for perovskite solar cells (PSCs) by solution processing of a soluble QA derivative followed by thermal annealing to convert it into insoluble QA. Passivation with strong interactions between QA molecules and metal halides, together with the hydrophobicity of QA coating, enabled highly efficient PSCs with remarkable stability.

Abstract

Surface passivation of perovskite solar cells (PSCs) using a low‐cost industrial organic pigment quinacridone (QA) is presented. The procedure involves solution processing a soluble derivative of QA, N,N‐bis(tert‐butyloxycarbonyl)‐quinacridone (TBOC‐QA), followed by thermal annealing to convert TBOC‐QA into insoluble QA. With halide perovskite thin films coated by QA, PSCs based on methylammonium lead iodide (MAPbI₃) showed significantly improved performance with remarkable stability. A PCE of 21.1 % was achieved, which is much higher than 18.9 % recorded for the unmodified devices. The QA coating with exceptional insolubility and hydrophobicity also led to greatly enhanced contact angle from 35.6° for the pristine MAPbI₃ thin films to 77.2° for QA coated MAPbI₃ thin films. The stability of QA passivated MAPbI₃ perovskite thin films and PSCs were significantly enhanced, retaining about 90 % of the initial efficiencies after more than 1000 hours storage under ambient conditions.

The two non‐fullerene acceptors 3,9‐bis(2‐methylene‐(3‐(1,1‐dicyanomethylene)‐indanone))‐5,5,11,11‐tetrakis(4‐hexylphenyl)‐dithieno[2,3‐d:2',3'‐d']‐s‐indaceno[1,2‐b:5,6‐b']dithiophene (ITIC)‐4F and ITIC‐4Cl co‐crystallize, a process that is suppressed when blended with the donor polymer PTB7‐Th. As a result, the corresponding ternary devices display stable photovoltaic performance up to 170 °C, in contrast to the binary devices that suffer acceptor crystallization. This indicates that acceptor mixtures allow to fabricate devices with excellent thermal stability.

Abstract

While photovoltaic blends based on non‐fullerene acceptors are touted for their thermal stability, this type of acceptor tends to crystallize, which can result in a gradual decrease in photovoltaic performance and affects the reproducibility of the devices. Two halogenated indacenodithienothiophene‐based acceptors that readily co‐crystallize upon mixing are studied, which indicates that the use of an acceptor mixture alone does not guarantee the formation of a disordered mixture. The addition of the donor polymer to the acceptor mixture readily suppresses the crystallization, which results in a fine‐grained ternary blend with nanometer‐sized domains that do not coarsen due to a high T _g ≈ 200 °C. As a result, annealing at temperatures of up to 170 °C does not markedly affect the photovoltaic performance of ternary devices, in contrast to binary devices that suffer from acceptor crystallization in the active layer. The results indicate that the ternary approach enables the use of high‐temperature processing protocols, which are needed for upscaling and high‐throughput fabrication of organic solar cells. Further, ternary devices display a stable photovoltaic performance at 130 °C for at least 205 h, which indicates that the use of acceptor mixtures allows to fabricate devices with excellent thermal stability.

2D stacked structures are being rapidly developed. However, the assembly and integration techniques of 2D material‐based devices are still subject to many restrictions, seriously hindering the design and development of new functional devices. As one of the most important aspects, 2D material stacking techniques are systematically summarized and analyzed in this review.

Abstract

2D layered materials have sparked great interest from the perspective of basic physics and applied science in the past few years. Extraordinarily, many novel stacked structures that bring versatile properties and applications can be artificially assembled, as exemplified by vertical van der Waals (vdW) heterostructures, twisted multilayer 2D materials, hybrid dimensional structures, etc. Compared with the ordinary synthesis process, the stacking technique is a powerful strategy to achieve high‐quality and freely controlled 2D material stacked structures with atomic accuracy. This review highlights the most advanced stacking techniques involving the preparation, transfer, and stacking of high‐quality single crystal 2D materials. Apart from the 2D–2D stacked structures, 2D–0D, 2D–1D, and 2D–3D structures offer a prospective platform for the increasing application of 2D materials. The assembly strategy and physical properties of these stacked structures strongly depend on the factors in the stacking process, including the surface quality, angle control, and sample size. In addition, comparative analysis tables on the techniques involved are also available. The summary of these strategies and techniques will hopefully provide a valuable reference for relevant work.

Large‐area prepatterned silver nanowire electrodes are prepared via gravure printing, which show high uniformity and balanced conductivity (10.8 Ω sq⁻¹) and transparency (95.4%). High power conversion efficiencies of 15.28% and 13.61% are achieved for 0.04 and 1 cm² cells, respectively.

Abstract

With the aim of developing high‐performance flexible polymer solar cells, the preparation of flexible transparent electrodes (FTEs) via a high‐throughput gravure printing process is reported. By varying the blend ratio of the mixture solvent and the concentration of the silver nanowire (AgNW) inks, the surface tension, volatilization rate, and viscosity of the AgNW ink can be tuned to meet the requirements of gravure printing process. Following this method, uniformly printed AgNW films are prepared. Highly conductive FTEs with a sheet resistance of 10.8 Ω sq⁻¹ and a high transparency of 95.4% (excluded substrate) are achieved, which are comparable to those of indium tin oxide electrode. In comparison with the spin‐coating process, the gravure printing process exhibits advantages of the ease of large‐area fabrication and improved uniformity, which are attributed to better ink droplet distribution over the substrate. 0.04 cm² polymer solar cells based on gravure‐printed AgNW electrodes with PM6:Y6 as the photoactive layer show the highest power conversion efficiency (PCE) of 15.28% with an average PCE of 14.75 ± 0.35%. Owing to the good uniformity of the gravure‐printed AgNW electrode, the highest PCE of 13.61% is achieved for 1 cm² polymer solar cells based on the gravure‐printed FTEs.

In situ grazing incidence wide angle X‐ray scattering during spin coating preparation of hybrid‐organic lead halide perovskite using gas quenching reveals that the intermediates formed using dimethyl sulfoxide depends on the composition. Otherwise, for 2‐methyl‐n‐pyrrolidone, a unique intermediate is formed. Such a formation mechanism has an impact on the device performance.

Abstract

The optoelectronic properties, morphology, and consequently the performance of metal halide perovskite solar cells are directly related to the crystalline phases and intermediates formed during film preparation. The gas quenching method is compatible with large‐area deposition, but an understanding of how this method influences these properties and performance is scarce in the literature. Here, in situ grazing incidence wide angle X‐ray scattering is employed during spin coating deposition to gain insights on the formation of MAPbI₃ and Cs _x FA_{1− x} Pb(I_0.83Br_0.17)₃ perovskites, comparing the use of dimethyl sulfoxide (DMSO) and 2‐methyl‐n‐pyrrolidone (NMP) as coordinative solvents. Intermediates formed using DMSO depend on the perovskite composition (e.g., Cs content), while for NMP the same intermediate [PbI₂(NMP)] is formed independently on the composition. For MAPbI₃ and Cs _x FA_{1− x} Pb(I_0.83Br_0.17)₃ with a small amount of Cs (10% and 20%), the best efficiencies are achieved using NMP, while the use of DMSO is preferred for higher (30% and 40%) amount of Cs. The inhibition of the 2H/4H hexagonal phase when using NMP is crucial for the final performance. These findings provide a deep understanding about the formation mechanism in multication perovskites and assist the community to choose the best solvent for the desired perovskite composition aiming to perovskite‐on‐silicon tandem solar cells.

An amphiphilic dendritic block copolymer is developed to serve as an efficient surface modifier of ZnO electron‐transporting layer in an organic photovoltaic device. When using an interlayer based on its hybridization with gold nanoparticles, the device can deliver improved performance and possess a lifetime of over 1.79 years when stored at room temperature in inert conditions.

Abstract

Herein, interfacial engineering is demonstrated to improve the thermal stability of non‐fullerene bulk‐heterojunction (BHJ) OPVs to a practical level. An amphiphilic dendritic block copolymer (DBC) is developed through a facile coupling method and employed as the surface modifier of ZnO electron‐transporting layer in inverted OPVs. Besides showing distinct self‐assembly behavior, the synthesized DBC possesses high compatibility with plasmonic gold nanoparticles (NPs) due to the constituent malonamide and ethylene oxide units. The hybrid DBC@AuNPs interlayer is shown to improve device's performance from 14.0% to 15.4% because it enables better energy‐level alignment and improves interfacial compatibility at the ZnO/BHJ interface. Moreover, the DBC@AuNPs interlayer not only improves the interfacial thermal stability at the ZnO/BHJ interface but also endows a more ideal BHJ morphology with an enhanced thermal robustness. The derived device reserves 77% of initial PCE after thermal aging at 65 °C for 3000 h and yields an extended T ₈₀ lifetime of >1100 h when stored at a constant thermal condition at 65 °C, outperforming the control device. Finally, the device is evaluated to possess a T ₈₀ lifetime of over 1.79 years at room temperature (298 K) when stored in an inert condition, showing great potential for commercialization.

Publication date: 18 November 2020

Source: Joule, Volume 4, Issue 11

Author(s): Kuan Liu, Qiong Liang, Minchao Qin, Dong Shen, Hang Yin, Zhiwei Ren, Yaokang Zhang, Hengkai Zhang, Patrick W.K. Fong, Zehan Wu, Jiaming Huang, Jianhua Hao, Zijian Zheng, Shu Kong So, Chun-Sing Lee, Xinhui Lu, Gang Li

Quinoxaline derivatives, featuring with rotatable and chemically fixed thienyl substitutes, are introduced as the core for constructing dopant‐free hole transporting materials (HTMs). The coplanar π‐extended quinoxaline‐based HTM TQ4 achieves the best photovoltaic performance (exceed 21 %) among planar n‐i‐p structured dopant‐free perovskite solar cells.

Abstract

Developing dopant‐free hole transporting materials (HTMs) is of vital importance for addressing the notorious stability issue of perovskite solar cells (PSCs). However, efficient dopant‐free HTMs are scarce. Herein, we improve the performance of dopant‐free HTMs featuring with a quinoxaline core via rational π‐extension. Upon incorporating rotatable or chemically fixed thienyl substitutes on the pyrazine ring, the resulting molecular HTMs TQ3 and TQ4 show completely different molecular arrangement as well as charge transporting capabilities. Comparing with TQ3, the coplanar π‐extended quinoxaline based TQ4 endows enriched intermolecular interactions and stronger π–π stacking, thus achieving a higher hole mobility of 2.08×10⁻⁴ cm² V⁻¹ s⁻¹. It also shows matched energy levels and high thermal stability for application in PSCs. Planar n‐i‐p structured PSCs employing dopant‐free TQ4 as HTM exhibits power conversion efficiency (PCE) over 21 % with excellent long‐term stability.

Publication date: 18 November 2020

Source: Joule, Volume 4, Issue 11

Author(s): Kuan Liu, Qiong Liang, Minchao Qin, Dong Shen, Hang Yin, Zhiwei Ren, Yaokang Zhang, Hengkai Zhang, Patrick W.K. Fong, Zehan Wu, Jiaming Huang, Jianhua Hao, Zijian Zheng, Shu Kong So, Chun-Sing Lee, Xinhui Lu, Gang Li

Apart from exciton–phonon coupling, octahedral distortion is revealed to significantly affect the exciton diffusion process. A simple fluorine substitution to phenethylammonium for the organic cations to tune the structural rigidity and octahedral distortion yields a record exciton diffusivity of 1.91 cm² s⁻¹ and a diffusion length of 405 nm along the in‐plane direction.

Abstract

Layered perovskites have been employed for various optoelectronic devices including solar cells and light‐emitting diodes for improved stability, which need exciton transport along both the in‐plane and the out‐of‐plane directions. However, it is not clear yet what determines the exciton transport along the in‐plane direction, which is important to understand its impact toward electronic devices. Here, by employing both steady‐state and transient photoluminescence mapping, it is found that in‐plane exciton diffusivities in layered perovskites are sensitive to both the number of layers and organic cations. Apart from exciton–phonon coupling, the octahedral distortion is revealed to significantly affect the exciton diffusion process, determined by temperature‐dependent photoluminescence, light‐intensity‐dependent time‐resolved photoluminescence, and density function theory calculations. A simple fluorine substitution to phenethylammonium for the organic cations to tune the structural rigidity and octahedral distortion yields a record exciton diffusivity of 1.91 cm² s⁻¹ and a diffusion length of 405 nm along the in‐plane direction. This study provides guidance to manipulate exciton diffusion by modifying organic cations in layered perovskites.

Nature Energy, Published online: 15 October 2020; doi:10.1038/s41560-020-00692-7

Low-bandgap tin–lead perovskites are key to all-perovskite tandem solar cells but simultaneous improvement in efficiency and stability has proven challenging. Now, Li et al. fabricate tin–lead perovskite cells with reduced methylammonium content that are 20.4% efficient and stable under illumination for 450 h.

Abstract

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.

B‐site doping provides a new approach to improve the optoelectronic properties and stability of CsPbX₃ inorganic perovskite. By judiciously selecting B‐site dopants and optimizing their concentration, B‐site doping strategy remarkably enhances the stability, tunes the bandgap, reduces the defects of CsPbX₃ inorganic perovskites, and thereby improves the photovoltaic performance of inorganic perovskite solar cells.

CsPbX₃ (X = I, Br) inorganic perovskite solar cells (PSCs) have been considered as one of the most appealing research topics in the fields of photovoltaic technologies in the past several years due to their excellent thermal stability and booming conversion efficiency. Nevertheless, there are still a large number of critical challenges and issues for inorganic PSCs, such as unstable phase structure of I‐rich inorganic perovskites at ambient condition, the wide bandgap of Br‐rich inorganic perovskites, and serious defect traps, hindering further development of inorganic PSCs. Recently, partially substituting Pb²⁺ with other metal ions has been shown to enhance the stability, tune the bandgap, reduce the defects of CsPbX₃ inorganic perovskites, and thereby improve the photovoltaic performance of inorganic PSCs. Herein, the recent progress in improving the photovoltaic performance of inorganic PSCs through the B‐site doping strategy is summarized, and the influence of the alternative metal ions on the stability and optoelectronic properties of inorganic perovskites and photovoltaic characteristics of CsPbX₃‐based PSCs is discussed. Finally, the issues that need to be understood in more detail are presented. It is believed that B‐site doping offers a practical strategy to gain high‐performance perovskite photovoltaic devices.