Ligang Yuan

Three vacuum-deposited donor–acceptor–acceptor (d–a–a') small molecule donors are studied with different side chains attached to an asymmetric heterotetracene donor block for use in high efficiency organic photovoltaics (OPVs). The donor with an isobutyl side chain yields the highest crystal packing density compared to molecules with 2-ethylhexyl or n-butyl chains, leading to the largest absorption coefficient and short circuit current in an OPV. It also exhibits a higher fill factor, consistent with its preferred out-of-plane molecular π–π stacking arrangement that facilitates charge transport in the direction perpendicular to the substrate. A power conversion efficiency of 9.3 ± 0.5% is achieved under 1 sun intensity, AM 1.5 G simulated solar illumination, which is significantly higher than 7.5 ± 0.4% of the other two molecules. These results indicate that side chain modification of d–a–a' small molecules offers an effective approach to control the crystal packing configuration, thereby improving the device performance.

Three vacuum-deposited donor–acceptor–acceptor's small molecule donors with different alkyl chain configurations (R₁–R₃) are synthesized and characterized to understand the side chain effect on organic photovoltaic (OPV) performance. The donor with an isobutyl (R₃) chain yields the highest crystal packing density and largest short circuit current among the three molecules. Its preferred face-on molecular stacking orientation on the substrate leads to the highest fill factor. The optimized OPV structure achieves a power conversion efficiency (PCE) = 9.3 ± 0.5%.

Hybrid organometal halide perovskites are known for their excellent optoelectronic functionality as well as their wide-ranging chemical flexibility. The composition of hybrid perovskite devices has trended toward increasing complexity as fine-tuned properties are pursued, including multielement mixing on the constituents A and B and halide sites. However, this tunability presents potential challenges for charge extraction in functional devices. Poor consistency and repeatability between devices may arise due to variations in composition and microstructure. Within a single device, spatial heterogeneity in composition and phase segregation may limit the device from achieving its performance potential. This review details how the nanoscale elemental distribution and charge collection in hybrid perovskite materials evolve as chemical complexity increases, highlighting recent results using nondestructive operando synchrotron-based X-ray nanoprobe techniques. The results reveal a strong link between local chemistry and charge collection that must be controlled to develop robust, high-performance hybrid perovskite materials for optoelectronic devices.

Hybrid halide perovskite thin films exhibit complex chemistry at the nanoscale that can affect their optoelectronic performance. In situ characterization of chemistry and functionality, such as by operando X-ray fluorescence and X-ray beam induced current measurements of perovskite solar cells, reveals insights into the relationship between local current collection, nonstoichiometry, and chemistry composition in hybrid lead perovskite absorbers.

Thermal degradation in perovskite solar cells is still an unsettled issue that limits its further development. In this study, 2-(1H-pyrazol-1-yl)pyridine is introduced into lead halide 3D perovskites, which allows 1D–3D hybrid perovskite materials to be obtained. The heterostructural 1D–3D perovskites are proved to be capable of remarkably prolonging the photoluminescence decay lifetime and suppressing charge carrier recombination in comparison to conventional 3D perovskites. The intrinsic properties of thermodynamically stable yet kinetically labile 1D materials allow the system to alleviate the lattice mismatch and passivate the interface traps of heterojunction region of 1D–3D hybrid perovskites that may occur during the crystal growth process. Importantly, the as-fabricated 1D–3D perovskite solar cells display a thermodynamic self-healing ability, which is induced through blocking the ion-migration channels of A-site ions by the flexible 1D perovskite with less densely close-packed structure. Particularly, the power conversion efficiency of as-fabricated unencapsulated 1D–3D perovskite solar cells is demonstrated to be reversible under temperature cycling (25–85 °C) at 55% relative humidity, which largely outperforms the pure 3D perovskite solar cell. The present study provides a facile approach to fabricate 1D–3D perovskite solar cells with high efficiency and long-term stability.

1D–3D hybrid perovskite is applied as absorber in a solar cell for the first time with remarkable thermodynamic self-healing capability. The power conversion efficiency of as-fabricated unencapsulated 1D–3D perovskite solar cells is demonstrated to be reversible under temperature cycling (25–85 °C) at 55% relative humidity, which largely outperforms the pure 3D perovskite solar cell.

CdS is investigated as low-cost, easily processable electron transport layer for perovskite solar cells. In contrast to TiO_x, CdS strongly suppresses hysteresis. This is caused on one hand by its much higher electron mobility. On the other hand, SIMS depth profiles reveal that iodide diffusion is suppressed when CdS (a) instead of TiO_x (b) is in contact to the perovskite layer.

Besides broadening of the absorption spectrum, modulating molecular energy levels, and other well-studied properties, a stronger intramolecular electron push–pull effect also affords other advantages in nonfullerene acceptors. A strong push–pull effect improves the dipole moment of the wings in IT-4F over IT-M and results in a lower miscibility than IT-M when blended with PBDB-TF. This feature leads to higher domain purity in the PBDB-TF:IT-4F blend and makes a contribution to the better photovoltaic performance. Moreover, the strong push–pull effect also decreases the vibrational relaxation, which makes IT-4F more promising than IT-M in reducing the energetic loss of organic solar cells. Above all, a power conversion efficiency of 13.7% is recorded in PBDB-TF:IT-4F-based devices.

Two critical factors (miscibility and vibrational relaxation) of nonfullerene molecular acceptors with the intramolecular electron push–pull effect are analyzed and related to their photovoltaic properties in organic solar cells (OSCs). A power conversion efficiency of 13.7% is recorded in OSCs by using a nonfullerene acceptor IT-4F, which shows a stronger intramolecular electron push–pull effect than its nonfluorinated counterpart.