ZiQi Sun

The tunable bandgap of colloidal quantum dots (CQDs) makes them an attractive material for photovoltaics (PV). The best present-day CQD PV devices employ zinc oxide (ZnO) as an electron transport layer; however, it is found herein that ZnO's surface defect sites and unfavorable electrical band alignment prevent devices from realizing their full potential. Here, chloride (Cl)-passivated ZnO generated from a solution of presynthesized ZnO nanoparticles treated using an organic-solvent-soluble Cl salt is reported. These new ZnO electrodes exhibit decreased surface trap densities and a favorable electronic band alignment, improving charge extraction from the CQD layer and achieving the best-cell power conversion efficiency (PCE) of 11.6% and an average PCE of 11.4 ± 0.2%.

Surface-defect-passivated zinc oxide (ZnO) nanoparticles are achieved by a chloride (Cl)-passivation process, exhibiting decreased surface trap density and favorable band alignment, improving electron extraction. With these benefits, a colloidal quantum dot (CQD) photovoltaic device prepared using Cl-passivated ZnO shows a power conversion efficiency of 11.6%.

Organic solar cells that are free of burn-in, the commonly observed rapid performance loss under light, are presented. The solar cells are based on poly(3-hexylthiophene) (P3HT) with varying molecular weights and a nonfullerene acceptor (rhodanine-benzothiadiazole-coupled indacenodithiophene, IDTBR) and are fabricated in air. P3HT:IDTBR solar cells light-soaked over the course of 2000 h lose about 5% of power conversion efficiency (PCE), in stark contrast to [6,6]-Phenyl C61 butyric acid methyl ester (PCBM)-based solar cells whose PCE shows a burn-in that extends over several hundreds of hours and levels off at a loss of ≈34%. Replacing PCBM with IDTBR prevents short-circuit current losses due to fullerene dimerization and inhibits disorder-induced open-circuit voltage losses, indicating a very robust device operation that is insensitive to defect states. Small losses in fill factor over time are proposed to originate from polymer or interface defects. Finally, the combination of enhanced efficiency and stability in P3HT:IDTBR increases the lifetime energy yield by more than a factor of 10 when compared with the same type of devices using a fullerene-based acceptor instead.

Organic solar cells based on a nonfullerene acceptor are presented that are free of burn-in, the commonly observed rapid performance loss under light. The combination of enhanced efficiency and stability increases the lifetime energy yield by more than a factor of 10 when compared with the same type of devices using a fullerene-based acceptor instead.

Here, a facial and scalable method for efficient exfoliation of bulk transition metal dichalcogenides (TMD) and graphite in aqueous solution with poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) to prepare single- and few-layer nanosheets is demonstrated. Importantly, these TMD nanosheets retain the single crystalline characteristic, which is essential for application in organic solar cells (OSCs). The hybrid PEDOT:PSS/WS₂ ink prepared by a simple centrifugation is directly integrated as a hole extraction layer for high-performance OSCs. Compared with PEDOT:PSS, the PEDOT:PSS/WS₂-based devices provide a remarkable power conversion efficiency due to the “island” morphology and benzoid–quinoid transition. This study not only demonstrates a novel method for preparing single- and few-layer TMD and graphene nanosheets but also paves a way for their applications without further complicated processing.

A novel PEDOT:PSS/2D nanosheets ink as hole extraction layer is used to fabricate high-performance organic solar cells. 2D nanosheets with single- and few-layer structure exist stably in the ink because of PEDOT:PSS exfoliation and functionalization. The enhanced power conversion efficiency arises from the “island” morphology and benzoid–quinoid transition upon addition of 2D nanosheets.

Organolead halide perovskites have attracted a lot of attention over the recent years mostly due to their bright prospective application in photovoltaic devices. For further development, characterization of their physical properties plays a seminal role in order to gain an in-depth understanding of these mid-bandgap ionic semiconductors. Their unique optical and electronic properties are a result of their characteristic electronic structure. Temperature dependent optical spectroscopy, i.e., absorption and photoluminescence (PL) characterization, gives access to their electronic nature that draws strong correlations to their structural properties. Those properties include static and dynamic disorder, and defects or phase transitions, which will be demonstrated in the first part of this research view. The second part focuses on ion migration in these hybrid semiconductors, which can strongly affect the slow dynamics of optical properties. Light-activated ions result in a number of complex processes that can lead to an increase, but also a decrease of PL intensity, or induce PL intermittency. Parameters like light intensity, crystal quality, and defect density all influence these processes, and ultimately the electronic nature of the hybrid perovskites. We will briefly summarize current achievements and point out challenges for upcoming research.

The latest results on structural dynamics and their impact on selected optical properties in methylammonium lead iodide perovskites are highlighted. Temperature-dependent optical spectroscopy gives access to their electronic nature and enables strong correlations to respective structural properties. Ion migration and the presence of induced ions strongly affect the slow dynamics of optical properties, which are reviewed in particular herein.

Hybrid metal halide perovskites have become one of the hottest topics in optoelectronic materials research in recent years. Not only have they surpassed everyone's expectations and achieved similar performance as tried and true polycrystalline silicon photovoltaic devices, but they are also finding applications in a variety of different fields, including lighting. The main advantages of hybrid metal halide perovskites are simple processability, compatible with large-scale solution processing such as roll-to-roll printing, and abundance of ingredients, all coupled to materials properties reminiscent of GaAs. On the road to this remarkable success, a series of challenges have been overcome, while some still remain. In this review, some of these challenges and possible solutions are described. In particular, understanding of the perovskite crystallization process and how this knowledge can be harnessed to enable better performing devices, how to overcome reproducibility issues and mitigate hysteresis, and the long-term prospects of the technology in terms of stability and sustainability will all be discussed.

This review of perovskite solar cells discusses the current understanding of the perovskite crystallization process, and how this knowledge can be harnessed to enable better performing devices; how to overcome reproducibility issues and mitigate hysteresis; and the long-term prospects of perovskite solar cell technology in terms of stability, cost, and sustainability.

A comparison of the efficiency, stability, and photophysics of organic solar cells employing poly[(5,6-difluoro-2,1,3-benzothiadiazol-4,7-diyl)-alt-(3,3′″-di(2-octyldodecyl)-2,2′;5′,2″;5″,2′″-quaterthiophen-5,5′″-diyl)] (PffBT4T-2OD) as a donor polymer blended with either the nonfullerene acceptor EH-IDTBR or the fullerene derivative, [6,6]-phenyl C₇₁ butyric acid methyl ester (PC₇₁BM) as electron acceptors is reported. Inverted PffBT4T-2OD:EH-IDTBR blend solar cell fabricated without any processing additive achieves power conversion efficiencies (PCEs) of 9.5 ± 0.2%. The devices exhibit a high open circuit voltage of 1.08 ± 0.01 V, attributed to the high lowest unoccupied molecular orbital (LUMO) level of EH-IDTBR. Photoluminescence quenching and transient absorption data are employed to elucidate the ultrafast kinetics and efficiencies of charge separation in both blends, with PffBT4T-2OD exciton diffusion kinetics within polymer domains, and geminate recombination losses following exciton separation being identified as key factors determining the efficiency of photocurrent generation. Remarkably, while encapsulated PffBT4T-2OD:PC₇₁BM solar cells show significant efficiency loss under simulated solar irradiation (“burn in” degradation) due to the trap-assisted recombination through increased photoinduced trap states, PffBT4T-2OD:EH-IDTBR solar cell shows negligible burn in efficiency loss. Furthermore, PffBT4T-2OD:EH-IDTBR solar cells are found to be substantially more stable under 85 °C thermal stress than PffBT4T-2OD:PC₇₁BM devices.

A high efficiency, burn-in-free nonfullerene-based PffBT4T-2OD:EH-IDTBR solar cell is reported, fabricated without processing additives. Transient absorption and optoelectronic analyses elucidate the causes of this high efficiency and stability, with the superior stability compared to PC₇₁BM devices being correlated with increased crystallinity and reduced photogeneration of trap states.

The recent development of 2D monolayer lateral semiconductor has created new paradigm to develop p-n heterojunctions. Albeit, the growth methods of these heterostructures typically result in alloy structures at the interface, limiting the development for high-efficiency photovoltaic (PV) devices. Here, the PV properties of sequentially grown alloy-free 2D monolayer WSe₂-MoS₂ lateral p-n heterojunction are explores. The PV devices show an extraordinary power conversion efficiency of 2.56% under AM 1.5G illumination. The large surface active area enables the full exposure of the depletion region, leading to excellent omnidirectional light harvesting characteristic with only 5% reduction of efficiency at incident angles up to 75°. Modeling studies demonstrate the PV devices comply with typical principles, increasing the feasibility for further development. Furthermore, the appropriate electrode-spacing design can lead to environment-independent PV properties. These robust PV properties deriving from the atomically sharp lateral p-n interface can help develop the next-generation photovoltaics.

By sequential growth of alloy-free 2D monolayer WSe₂-MoS₂ lateral p-n heterojunction, photovoltaic devices show extraordinary power conversion efficiencies of 2.56%. The large surface active area of the devices enables the full exposure of the depletion region, leading to excellent omnidirectional light harvesting characteristic. Modeling studies demonstrate the devices comply with typical principles. The appropriate electrode-spacing design leads to environment-independent properties.

In article number 1601882, Keyou Yan, Jianbin Xu, and co-workers report a simple intermediate halide exchange reaction between nonstoichiometric FAI and unstable intermediate HPbI₂Br to produce high-quality FAPbI_3-xBr_x thin films with crystal domain up to 2-3 μm, eliminating the use of antisolvent dripping process. And it demonstrates outstanding opto-electronic performances as well as enhanced stability under moisture and heat stress.