Chen Weijie

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.

The recent progress in potentially low‐cost polythiophene:nonfullerene‐based solar cells is reviewed from the viewpoints of molecular engineering and morphology control. The molecular design strategies of polythiophenes and nonfullerene acceptors are discussed, followed by the recent achievements in understanding and controlling the morphology of polythiophene:nonfullerene blends. Finally, the future challenges are delineated for advancing the commercial applications of polythiophenes in solar cells.

Abstract

With the advances in organic photovoltaics (OPVs), the development of low‐cost and easily accessible polymer donors is of vital importance for OPV commercialization. Polythiophene (PT) and its derivatives stand out as the most promising members of the photovoltaic material family for commercial applications, owing to their low cost and high scalability of synthesis. In recent years, PTs, paired with nonfullerene acceptors, have progressed rapidly in photovoltaic performance. This Review gives an overview of the strategies in designing PTs for nonfullerene OPVs from the perspective of energy level modulation. A survey of the typical classes of nonfullerene acceptors designed for pairing with the benchmark PT, i.e., poly(3‐hexylthiophene) (P3HT) is also presented. Furthermore, recent achievements in understanding and controlling the film morphology for PT:nonfullerene blends are discussed in depth. In addition to the effects of molecular weight and blend ratio on film morphology, the crucial roles of miscibility between PT and nonfullerene and processing solvent in determining film microstructure and morphology are highlighted, followed by a discussion on thermal annealing and ternary active layers. Finally, the remaining questions and the prospects of the low‐cost PT:nonfullerene systems are outlined. It is hoped that this review can guide the optimization of PT:nonfullerene blends and advance their commercial applications.

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.

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.

Publication date: January 2021

Source: Nano Energy, Volume 79

Author(s): Tahmineh Mahmoudi, Yousheng Wang, Yoon-Bong Hahn

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.

A terpolymer donor PM6‐Tz20 was developed by incorporating the third 5,5′‐dithienyl‐2,2′‐bithiazole (DTBTz) unit into the PM6 backbone. The introduction of DTBTz can tailor the molecular ordering, orientation, and aggregation properties, and then optimize the morphology and electrical properties of devices, ultimately improving fill factor (0.77) and thus device efficiency (17.6 %).

Abstract

Regulating molecular structure to optimize the active layer morphology is of considerable significance for improving the power conversion efficiencies (PCEs) in organic solar cells (OSCs). Herein, we demonstrated a simple ternary copolymerization approach to develop a terpolymer donor PM6‐Tz20 by incorporating the 5,5′‐dithienyl‐2,2′‐bithiazole (DTBTz, 20 mol%) unit into the backbone of PM6 (PM6‐Tz00). This method can effectively tailor the molecular orientation and aggregation of the polymer, and then optimize the active layer morphology and the corresponding physical processes of devices, ultimately boosting FF and then PCE. Hence, the PM6‐Tz20: Y6‐based OSCs achieved a PCE of up to 17.1% with a significantly enhanced FF of 0.77. Using Ag (220 nm) instead of Al (100 nm) as cathode, the champion PCE was further improved to 17.6%. This work provides a simple and effective molecular design strategy to optimize the active layer morphology of OSCs for improving photovoltaic performance.

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

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.

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 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 introduction of an intermediate layer to bridge different solar cell technologies, which have become mature for the optimization of single‐junction cells, is a critical technology for fabricating efficient hybrid tandem solar cells. The design and classification of an efficient interconnection layer that leverages the full potential of the hybrid tandems are discussed.

Abstract

Hybrid tandem solar cells offer the benefits of low cost and full solar spectrum utilization. Among the hybrid tandem structures explored to date, the most popular ones have four (simple stacking design) or two (terminal/tunneling layer addition design) terminal electrodes. Although the latter design is more cost‐effective than the former, its widespread application is hindered by the difficulty of preparing an interface between two solar cell materials. The oldest approach to the in‐series bonding of two or more bandgap solar cells relies on the introduction of a tunneling layer in multijunction III–V solar cells, but it has some limitations, e.g., the related materials/technologies are applicable only to III–V and certain other solar cells. Thus, alternative methods of realizing junction contacts based on the use of novel materials are highly sought after. Here, the strategies used to realize high‐performance tandem cells are described, focusing on interface control in terms of bonding two or more solar cells for tandem approaches. The presented information is expected to aid the establishment of ideal methods of connecting two or more solar cells to obtain the highest performance for different solar cell choices with minimized energy loss through the interface.

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