Chen Weijie

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.

A new type of methylammonium‐free formamidinium (FA) based perovskites is reported. The low‐dimensional perovskite films are obtained in the presence of the FACl additive, and the role of Cl is investigated through grazing‐incidence X‐ray diffraction. Solar cell devices based on (PDA)(FA)₃Pb₄I₁₃ films show extremely high thermal stability and a remarkable PCE of 13.8 %.

Abstract

Currently, most two‐dimensional (2D) metal halide perovskites are of the Ruddlesden–Popper type and contain the thermally unstable methylammonium (MA) molecules, which leads to inferior photovoltaic performance and mild stability. Here we report a new type of MA‐free formamidinium (FA) based low‐dimensional perovskites, featuring a general formula of (PDA)(FA) _n−1Pb_nI_3n+1 with propane‐1,3‐diammonium (PDA) as the organic spacer cation. The perovskite films with well‐oriented crystal grains are attained under the assistance of the FACl additive, where the role of Cl is investigated through the grazing‐incidence X‐ray diffraction technique. The photovoltaic device based on the optimized (PDA)(FA)₃Pb₄I₁₃ film demonstrates a remarkable power conversion efficiency of 13.8 %, the highest record for the FA‐based 2D perovskite solar cells. In addition, compared to (PDA)(MA)₃Pb₄I₁₃, the MA‐containing analogue and a renowned stable 2D perovskite, both the (PDA)(FA)₃Pb₄I₁₃ films and their derived devices exhibit exceedingly higher thermal stability.

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.

Herein, the origin of figures of merit for open‐circuit voltage, short‐circuit current density, and fill factor is discussed. Three design strategies of interface engineering, bandgap engineering, and process‐control engineering are proposed. The process‐control engineering is introduced, including fabrication atmosphere, synthesis routes, architecture optimization, physical deposition condition, and chemical process with multiple degrees of freedom.

Organic/inorganic lead‐halide perovskite and its solar cells (SCs) present a new research platform for the study of special photophysical and photovoltaic (PV) characteristics across materials science, chemistry, physics, and engineering disciplines. However, the current understanding of the crystal structures, origins of figures of merit, and design strategies of SCs is inadequate. These key parameters are critical for exploring further applications of organometallic‐halide perovskite films and their SCs. Therefore, herein, the material characteristics of lead‐halide perovskite are introduced, the origins of open‐circuit voltages, short‐circuit current densities, and fill factors are explored, and three design strategies using interface engineering, bandgap engineering, and process‐control engineering for high‐quality perovskite active‐layer fabrication, outstanding efficiency, and stable SCs are summarized. Herein, process‐control engineering is introduced for the first time in perovskite SCs. Based on favorable synergistic effects, these structural features, origins of crucial parameters, and design strategies all promote the development of new schemes to explore the underlying physics, optimize functional layers and cell architectures, and improve final PV performance and device stability.

Perovskite solar cells (PSCs) and mini‐modules (PSM) scaling‐up strategies based on the design of their active areas and geometrical shapes lead to relevant power conversion efficiency (PCE) improvements. Optimized thermally co‐evaporated PSM with 6.4 cm² active area and geometrical fill factor of ≈91% reach a PCE of 18.4% with just 0.7% absolute losses as compared to 40 times smaller PSCs.

Perovskite solar cells (PSCs) have emerged as a promising technology for next‐generation photovoltaics thanks to their high power‐conversion‐efficiency (PCE). Scaling up PSCs using industrially compatible processes is a key requirement to make them suitable for a variety of applications. Herein, large‐area PSCs and perovskite solar modules (PSMs) are developed based on co‐evaporated MAPbI₃ using optimized structures and active area designs to enhance PCEs and geometrical fill factors (GFFs). Small‐area co‐evaporated PSCs (0.16 cm²) achieve PCE over 19%. When the PSCs are scaled‐up, the thin films high quality allows them to maintain consistent V _oc and J _sc, while their fill factors (FF), which depend on the substrate sheet resistance, are substantially compromised. However, PSCs with active areas from 1.4 to 7 cm² show a substantially improved FF when rectangular designs with optimized length to width ratios are used. Reasoning these results in the PSM design with optimal subcell size and for specific dead areas, a 6.4 cm² PSM is demonstrated with a record 18.4% PCE and a GFF of ≈91%. Combining the high uniformity of the co‐evaporation deposition with active areas design, it is possible to scale up 40 times the PSCs with PCE losses smaller than 0.7% (absolute value).

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.

Organic photovoltaics (OPVs) demostrate certified cell efficiencies of over 17% and are expected to contribute to versatile applications powered by solar energy. By taking into consideration different critical and “soft” key performance indicators, this work demonstrates material strategies to accelerate the development of OPV technology toward a GW era.

Abstract

With the rise of the solar power century, photovoltaic applications and installations will go beyond the traditional green field power plants and enter any aspect of daily life. Organic photovoltaics (OPVs) demonstrate certified cell efficiencies of over 17% and are expected to contribute to versatile applications powered by solar energy, for instance, applications rely on flexibility, transparency, color management, or integrability. In this work, the progress of OPV technology is briefly reviewed and the material strategies to accelerate OPV technology toward a GW era are analyzed. In addition to the exciting efficiency values achieved for small area devices, there are many important criteria deciding the success of OPV technology. By taking into consideration the synthetic complexity of OPV materials and the operational stability of OPV devices, the industrial figure of merit (i‐FoM) is proposed as a fast and reliable method to verify the true potential of a novel material. Furthermore, “soft” key performance indicators are introduced, such as toxicity, flexibility, transparency, processing, which require different development strategies to reflect the potential of OPV technology for specific applications.

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.

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

Publication date: 18 November 2020

Source: Joule, Volume 4, Issue 11

Author(s): Fabrizio Gota, Malte Langenhorst, Raphael Schmager, Jonathan Lehr, Ulrich W. Paetzold