ZiQi Sun

The challenge of continuous printing in high-efficiency large-area organic solar cells is a key limiting factor for their widespread adoption. A materials design concept for achieving large-area, solution-coated all-polymer bulk heterojunction solar cells with stable phase separation morphology between the donor and acceptor is presented. The key concept lies in inhibiting strong crystallization of donor and acceptor polymers, thus forming intermixed, low crystallinity, and mostly amorphous blends. Based on experiments using donors and acceptors with different degree of crystallinity, the results show that microphase separated donor and acceptor domain sizes are inversely proportional to the crystallinity of the conjugated polymers. This methodology of using low crystallinity donors and acceptors has the added benefit of forming a consistent and robust morphology that is insensitive to different processing conditions, allowing one to easily scale up the printing process from a small-scale solution shearing coater to a large-scale continuous roll-to-roll (R2R) printer. Large-area all-polymer solar cells are continuously roll-to-roll slot die printed with power conversion efficiencies of 5%, with combined cell area up to 10 cm². This is among the highest efficiencies realized with R2R-coated active layer organic materials on flexible substrate.

The morphology formation of different all-polymer solar cells during the coating process is investigated and that a low crystalline donor and acceptor polymer blend has stable morphology between the various coating methods is identified. Large-area all-polymer solar cells are continuously roll-to-roll slot die printed with power conversion efficiencies of 5%, with combined cell area up to 10 cm².

Hybrid and all-inorganic perovskite (PK) materials are a promising next generation of semiconducting materials due to their outstanding light-harvesting features, as well as their color-tunablility and efficient luminescent properties that lead to highly efficient photovoltaic and lighting devices. Bulk PK films are both ionic and electronic conductors under the presence of an externally applied electric field. In this work, the internal ion motion behavior is demonstrated within PK nanoparticles in thin-film devices by means of different long-time poling scheme assays and both static and dynamic electrochemical impedance spectroscopy measurements. In particular, the existence of a dynamic device behavior is related to the migration and rearrangement of different ionic species upon applying different driving schemes. The latter resembles the well-known signatures of the ionic motion in light-emitting electrochemical cells (LECs), that is, (i) the formation of electrical double layers due to the ionic distribution at the electrodes' interfaces, (ii) the growth of the doped regions once the charge injection is effective, and (iii) the subsequent formation of a non-doped region in the bulk of the device. Hence, this comprehensive study opens up an alternative route toward understanding the dynamics inside hybrid perovskite materials based on the large body of knowledge of LECs.

The effect of the ion motion on the mechanism of hybrid perovskite nanoparticles (PK NP) devices is investigated by static and dynamic electrochemical impedance spectroscopy and different poling schemes. Overall, these studies suggest that the electrical behavior of the PK NP devices resemble that of light-emitting electrochemical cells.

The long-term reliability of organic photovoltaic (OPV) devices is one of the urgent tasks to be solved for commercialization. In article number 1601289, Hyoung Jin Son, Sung Hyun Kim, and Dong Hwan Kim report the impact of the hole transport layer (HTL) and the back electrode on the stability of OPV devices. In particular, unknown solvents diffusing from the back electrode to the underlying layers and the photostability of PEDOT:PSS as a HTL are pointed out to be very important factors determining the lifetime of OPV.

Polymer solar cells (PSCs) possess the unique features of semitransparency and coloration, which make them potential candidates for applications in aesthetic windows. Here, the authors fabricate inverted semitransparent PSCs with high-quality hybrid Au/Ag transparent top electrodes and fine-tuned dielectric mirrors (DMs). It is demonstrated that the device color can be tailored and the light harvesting in the PSCs can be enhanced by matching the bandgap of the polymer donors in the active layer with the specifically designed maximum-reflection-center-wavelengths of the DMs. A detailed chromaticity analysis of the semitransparent PSCs from both bottom and top (mirror) views is also carried out. Furthermore, the inverted semitransparent PSCs based on PTB7-Th:PC₇₁BM with six pairs of DMs demonstrate a maximum power conversion efficiency (PCE) of 7.0% with an average visible transmittance (AVT) of 12.2%. This efficiency is one of the highest reported for semitransparent PSCs, corresponding to 81.4% of the PCE from opaque counterpart devices. The device design and processing method are also successfully adapted to a flexible substrate, resulting in a device with a competitive PCE of 6.4% with an AVT of 11.5%. To the best of our knowledge, this PCE value is the highest value reported for a flexible semitransparent PSC.

This study demonstrates that the performance, reproducibility, and colors of inverted semitransparent polymer solar cells (PSCs) can be significantly improved or tuned by combining hybrid Au/Ag, fine-tuned dielectric mirrors, and active layers with various bandgaps. The PTB7-Th:PC₇₁BM-based inverted semitransparent PSCs exhibit a maximum power conversion efficiency of 7.0% on rigid substrate and 6.4% on flexible substrate.

Perovskite solar cells (PSCs) based on organic monovalent cation (methylammonium or formamidinium) have shown excellent optoelectronic properties with high efficiencies above 22%, threatening the status of silicon solar cells. However, critical issues of long-term stability have to be solved for commercialization. The severe weakness of the state-of-the-art PSCs against moisture originates mainly from the hygroscopic organic cations. Here, rubidium (Rb) is suggested as a promising candidate for an inorganic–organic mixed cation system to enhance moisture-tolerance and photovoltaic performances of formamidinium lead iodide (FAPbI₃). Partial incorporation of Rb in FAPbI₃ tunes the tolerance factor and stabilizes the photoactive perovskite structure. Phase conversion from hexagonal yellow FAPbI₃ to trigonal black FAPbI₃ becomes favored when Rb is introduced. The authors find that the absorbance and fluorescence lifetime of 5% Rb-incorporated FAPbI₃ (Rb_0.05FA_0.95PbI₃) are enhanced than bare FAPbI₃. Rb_0.05FA_0.95PbI₃-based PSCs exhibit a best power conversion efficiency of 17.16%, which is much higher than that of the FAPbI₃ device (13.56%). Moreover, it is demonstrated that the Rb_0.05FA_0.95PbI₃ film shows superior stability against high humidity (85%) and the full device made with the mixed perovskite exhibits remarkable long-term stability under ambient condition without encapsulation, retaining the high performance for 1000 h.

Partial substitution of inorganic rubidium cation (Rb⁺) for formamidinium lead iodide (FAPbI₃) perovskite suppresses nonperovskite phase formation and increases fluorescence lifetime. Introduction of the smaller monovalent cation in FAPbI₃ renders the perovskite more tolerant to high humidity. These lead to enhanced photovoltaic performances and long-term stability of perovskite solar cells based on Rb-mixed FAPbI₃.

The performance of organic semiconductors in optoelectronic devices depends on the functional properties of the individual molecules and their mutual orientations when they are in the solid state. The effect of H- and J-aggregation on the photophysical properties and photovoltaic behavior of four electronically identical but structurally different thiophene–pyridine–diketopyrrolopyrrole molecules is studied. By introducing and changing the position of two hexyl side chains on the two peripheral thiophene units of these molecules, their aggregation in thin films between H-type and J-type is effectively tuned, as evidenced from the characteristics of optical absorption, fluorescence, and excited state lifetime. The two derivatives that assemble into J-type aggregates exhibit a significantly enhanced photovoltaic performance, up to an order of magnitude, compared to the two molecules that form H-type aggregates. The reasons for this remarkably different behavior are discussed.

The aggregation behavior of electronically identical thiophene–pyridine–diketopyrrolopyrrole molecules is modulated by introducing and modifying the position of two hexyl side chains on the peripheral thiophene rings. Improved photovoltaic performance is achieved for those molecules assembling into J-type aggregates in contrast to those forming H-aggregates as a result of a faster charge generation and a reduced bimolecular recombination.

Organic–inorganic hybrid perovskite materials with mixed cations have demonstrated tremendous advances in photovoltaics recently, by showing a significant enhancement of power conversion efficiency and improved perovskite stability. Inspired by this development, this study presents the facile synthesis of mixed-cation perovskite nanocrystals based on FA_(1−x)Cs_xPbBr₃ (FA = CH(NH₂)₂). By detailed characterization of their morphological, optical, and physicochemical properties, it is found that the emission property of the perovskite, FA_(1−x)Cs_xPbBr₃, is significantly dependent on the substitution content of the Cs cations in the perovskite composition. These mixed-cation perovskites are employed as light emitters in light-emitting diodes (LEDs). With an optimized composition of FA_0.8Cs_0.2PbBr₃, the LEDs exhibit encouraging performance with a highest reported luminance of 55 005 cd m⁻² and a current efficiency of 10.09 cd A⁻¹. This work provides important instructions on the future compositional optimization of mixed-cation perovskite for obtaining high-performance LEDs. The authors believe this work is a new milestone in the development of bright and efficient perovskite LEDs.

Organic–inorganic hybrid perovskite nanocrystals with mixed cations demonstrate tremendous advances in light-emitting diodes. The physicochemical properties of synthesized perovskite nanocrystals are significantly dependent on the substitution content of the caesium cations in the perovskite composition. This work provides important instructions on the future compositional optimization of mixed-cation perovskite for obtaining high-performance light-emitting diodes.

A new fluorinated nonfullerene acceptor, ITIC-Th1, has been designed and synthesized by introducing fluorine (F) atoms onto the end-capping group 1,1-dicyanomethylene-3-indanone (IC). On the one hand, incorporation of F would improve intramolecular interaction, enhance the push–pull effect between the donor unit indacenodithieno[3,2-b]thiophene and the acceptor unit IC due to electron-withdrawing effect of F, and finally adjust energy levels and reduce bandgap, which is beneficial to light harvesting and enhancing short-circuit current density (J_SC). On the other hand, incorporation of F would improve intermolecular interactions through CF···S, CF···H, and CF···π noncovalent interactions and enhance electron mobility, which is beneficial to enhancing J_SC and fill factor. Indeed, the results show that fluorinated ITIC-Th1 exhibits redshifted absorption, smaller optical bandgap, and higher electron mobility than the nonfluorinated ITIC-Th. Furthermore, nonfullerene organic solar cells (OSCs) based on fluorinated ITIC-Th1 electron acceptor and a wide-bandgap polymer donor FTAZ based on benzodithiophene and benzotriazole exhibit power conversion efficiency (PCE) as high as 12.1%, significantly higher than that of nonfluorinated ITIC-Th (8.88%). The PCE of 12.1% is the highest in fullerene and nonfullerene-based single-junction binary-blend OSCs. Moreover, the OSCs based on FTAZ:ITIC-Th1 show much better efficiency and better stability than the control devices based on FTAZ:PC₇₁BM (PCE = 5.22%).

Single-junction binary-blend nonfullerene polymer solar cells based on fluorinated acceptor ITIC-Th1 afford power conversion efficiency of 12.1%, which is much higher than those of nonfluorinated ITIC-Th (8.88%) and PC₇₁BM (5.22%) counterparts under the same condition. Moreover, the nonfullerene devices exhibit better thermal stability than the fullerene devices.

Owing to their high efficiency, low-cost solution-processability, and tunable bandgap, perovskite solar cells (PSCs) made of hybrid organic-inorganic perovskite (HOIP) thin films are promising top-cell candidates for integration with bottom-cells based on Si or other low-bandgap solar-cell materials to boost the power conversion efficiency (PCE) beyond the Shockley-Quiesser (S-Q) limit. In this review, recent progress in such tandem solar cells based on the emerging PSCs is summarized and reviewed critically. Notable achievements for different tandem solar cell configurations including mechanically-stacked, optical coupling, and monolithically-integrated with PSCs as top-cells are described in detail. Highly-efficient semitransparent PSC top-cells with high transmittance in near-infrared (NIR) region are critical for tandem solar cells. Different types of transparent electrodes with high transmittance and low sheet-resistance for PSCs are reviewed, which presents a grand challenge for PSCs. The strategies to obtain wide-bandgap PSCs with good photo-stability are discussed. The PCE reduction due to reflection loss, parasitic absorption, electrical loss, and current mismatch are analyzed to provide better understanding of the performance of PSC-based tandem solar cells.

Recent progress in the research and development PSCs-based tandem solar cells is reviewed. Different architectures, such as monolithically-integrated, mechanically-stacked, and optically-coupled tandems are discussed. The development of transparent electrodes and wide bandgap PSCs are presented. The power losses in the tandem solar cells are analyzed.

Hybrid photoconductive materials based on zinc oxide (ZnO) doped with perylene bisimide (PBI) dye molecules have emerged as new promising cathode interlayer materials for inverted polymer solar cells (i-PSCs). Such interlayers show increased conductivity under light irradiation (working condition of i-PSCs) due to enhanced electron mobility and carrier density, enabling high performance devices at interlayer thickness of up to 100 nm. Such increased interlayer thickness is desired for roll-to-roll processing techniques to facilitate mass production of photovoltaic modules.

Hybrid photoconductive cathode interlayers based on zinc oxide doped with perylene bisimide dye molecules show increased conductivity under light irradiation due to enhanced electron mobility and carrier density, enabling high performance polymer solar cells at interlayer thickness of up to 100 nm, which is desired for roll-to-roll processing techniques to facilitate mass production of photovoltaic modules.

Low-molecular-weight organic gelators are widely used to influence the solidification of polymers, with applications ranging from packaging items, food containers to organic electronic devices, including organic photovoltaics. Here, this concept is extended to hybrid halide perovskite-based materials. In situ time-resolved grazing incidence wide-angle X-ray scattering measurements performed during spin coating reveal that organic gelators beneficially influence the nucleation and growth of the perovskite precursor phase. This can be exploited for the fabrication of planar n-i-p heterojunction devices with MAPbI₃ (MA = CH₃NH₃⁺) that display a performance that not only is enhanced by ≈25% compared to solar cells where the active layer is produced without the use of a gelator but that also features a higher stability to moisture and a reduced hysteresis. Most importantly, the presented approach is straightforward and simple, and it provides a general method to render the film formation of hybrid perovskites more reliable and robust, analogous to the control that is afforded by these additives in the processing of commodity “plastics.”

Organic gelators, widely used in the processing of commodity “plastics,” are applied to hybrid halide perovskites solidification. They are shown to beneficially influence the nucleation and growth of the perovskite sol–gel precursor phase, leading to a material characterized by a higher stability to moisture and a reduced hysteresis in planar n-i-p heterojunction solar cells.