awn

Publication date: 18 November 2020

Source: Joule, Volume 4, Issue 11

Author(s): Xixiang Zhu, Guichuan Zhang, Jia Zhang, Hin-Lap Yip, Bin Hu

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

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.

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.

The precise composition of the intermixed phase in bulk heterojunction structures with device‐relevant size is determined via the analysis of the glass transition temperatures proven by fast scanning calorimetry. A relatively small fraction (<15 wt%) of an acceptor in the intermixed amorphous phase leads already to efficient charge generation. However, charge transport can only be sustained in blend morphologies with a significant amount of the acceptor in the intermixed phase (in this case: ≈58 wt%).

Abstract

The relation of phase morphology and solid‐state microstructure with organic photovoltaic (OPV) device performance has intensely been investigated over the last twenty years. While it has been established that a combination of donor:acceptor intermixing and presence of relatively phase‐pure donor and acceptor domains is needed to get an optimum compromise between charge generation and charge transport/charge extraction, a quantitative picture of how much intermixing is needed is still lacking. This is mainly due to the difficulty in quantitatively analyzing the intermixed phase, which generally is amorphous. Here, fast scanning calorimetry, which allows measurement of device‐relevant thin films (<200 nm thickness), is exploited to deduce the precise composition of the intermixed phase in bulk‐heterojunction structures. The power of fast scanning calorimetry is illustrated by considering two polymer:fullerene model systems. Somewhat surprisingly, it is found that a relatively small fraction (<15 wt%) of an acceptor in the intermixed amorphous phase leads to efficient charge generation. In contrast, charge transport can only be sustained in blends with a significant amount of the acceptor in the intermixed phase (in this case: ≈58 wt%). This example shows that fast scanning calorimetry is an important tool for establishing a complete compositional characterization of organic bulk heterojunctions. Hence, it will be critical in advancing quantitative morphology–function models that allow for the rational design of these devices, and in delivering insights in, for example, solar cell degradation mechanisms via phase separation, especially for more complex high‐performing systems such as nonfullerene acceptor:polymer bulk heterojunctions.

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.

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.

A high‐pressure nitrogen‐extraction strategy to drive the formation of a stable intermediate for uniform perovskite crystallization and an effective passivation strategy by utilizing an ionic liquid are reported. As such, the PCEs of a small‐area PSC and a large‐area PSC module are 22.7% and 19.6% respectively, representing a high level made using a large‐area fabrication process.

Abstract

Slot‐die coating holds advantages over other large‐scale technologies thanks to its potential for well‐controlled, high‐throughput, continuous roll‐to‐roll fabrication. Unfortunately, it is challenging to control thin.film uniformity over a large area while maintaining crystallization quality. Herein, by using a high‐pressure nitrogen‐extraction (HPNE) strategy to assist crystallization, a wide processing window in the well‐controlled printing process for preparing high‐quality perovskites is achieved. The yellow‐phase perovskite generated by the HPNE acts as a crucial intermediate phase to produce large‐area high‐quality perovskite film. Furthermore, an ionic liquid is developed to passivate the perovskite surface to reduce surface defect density and to suppress carrier recombination, resulting in significantly increased efficiency to 22.7%, the highest for large‐area fabrication. The strategies are successfully extended to large‐area device fabrication, making it possible to produce a 40 × 40 mm² module with stabilized PCE as high as 19.4%, the highest‐efficiency for a large‐area module to date.

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.

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.

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.

Molybdenum oxide thin films are successfully deposited using spatial atomic layer deposition (SALD), a tool designed for high‐throughput industrial film growth. The structural and optoelectronic properties of the film are evaluated for applications in silicon solar cell technologies. The results present a promising future for SALD in industrial settings such as photovoltaics manufacturing.

Abstract

Molybdenum oxide thin films are successfully deposited using spatial atomic layer deposition (SALD), a tool designed for high‐throughput industrial film growth. The structural and optical properties of the film are evaluated using ultraviolet photoelectron spectroscopy, high‐resolution transmission electron microscopy, and spectroscopic ellipsometry. To demonstrate the applicability of molybdenum oxide in industrial settings the films are applied as hole‐selective silicon heterojunction contacts for solar cells. When paired with intrinsic amorphous silicon passivation layers, implied open‐circuit voltages of 699 mV are achieved. The carrier transport is unaffected by low‐temperature contact anneals up to 300 °C with contact resistivities of ≈ 10 mΩ cm². Finally, the optical performance of silicon solar cells featuring different front hole‐selective heterojunction structures are simulated. It is shown that the generation current density of heterojunction solar cells can be significantly increased with the addition of SALD molybdenum oxide contacts.

Incorporation of a poly[N‐9′‐heptadecanyl‐2,7‐carbazole‐alt‐5,5‐(4′,7′‐di‐2‐thienyl‐2′,1′,3′‐benzothiadiazole)] (PCDTBT) fibrils as an efficient hole transfer layer (HTL) is demonstrated as an effective approach to significantly enhance air and mechanical stability of perovskite solar cells (PSCs) as an alternative to widely used doped HTLs, resulting from the interlocking effect of PCDTBT fibrils formed at the interface between perovskite and PDCTBT layers.

Abstract

Atmospheric and mechanical stability of perovskite solar cells (PSCs) must be guaranteed for successful commercialization. A fibrillar polymer, poly[N‐9′‐heptadecanyl‐2,7‐carbazole‐alt‐5,5‐(4′,7′‐di‐2‐thienyl‐2′,1′,3′‐benzothiadiazole)] (PCDTBT), is reported as an efficient hole transfer layer (HTL) which significantly improves air and mechanical stability of perovskite solar cells (PSCs). PCDTBT fibrils formed at the grain boundaries of perovskite layer induce the highest fracture energies in the PSCs, which provide extrinsic reinforcement and shielding for enhanced mechanical and chemical stability. Debonding energy increases by 30% for the PSCs with PCDTBT fibrils, which fractures at 2.66 J m⁻², compared to the devices without PCDTBT fibrils at 2.09 J m⁻²; more importantly, the threshold debonding driving force of the PCDTBT fibril‐based devices is greatly improved by twofold under ambient conditions.

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.