Yingzhi Jin

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%.

Organic solar cells (OSCs) can directly convert the sunlight into electrical energy and present some advantages, such as low cost, light weight, flexibility, semitransparency, and roll-to-roll large-area fabrication. Due to the short diffusion length of exciton (≈10 nm) in organic semiconductor materials, the ideal nanoscale phase separation in the active layer is one of the crucial factors for achieving efficient exciton dissociation and charge transport. The morphology of the active layer is mainly determined by the nature of donors and acceptors (e.g., solubility, crystallinity, and miscibility), the film processing, the device configuration, and so on. In general, it is very hard to obtain ideal morphology in the as-cast films. Therefore, it is usually essential to take measures to achieve the active layer with good molecular stacking, proper domain size, high domain purity, and suitable vertical phase separation. In this review, recent developments in morphology control and morphology characterization are summarized and analyzed. This review might help the community to decipher active layer morphology at multiple length scales and to achieve ideal morphology toward high-performance OSCs.

Morphology of the bulk heterojunction organic solar cells is very important but very complicated. This review summarizes and discusses morphology characterization as well as morphology control.

Processing solvent additives in polymer:fullerene bulk heterojunction systems are known as a promising method to enhance photovoltaic performance. It is generally agreed that solvent additives enable polymers to have a high degree of molecular order which increases the device performance. However, the understanding of the efficiency enhancement is not complete. There is a lack of insight regarding the quantitative determination of the molecular miscibility between polymer and fullerene as well as the inner morphology changes induced by the additives. In this work, understanding of the influence of the solvent additive 1,8-octanedithiol (ODT) is provided on the classic system poly(3-hexylthiophene-2,5-diyl):[6,6]-phenyl-C61 butyric acid methyl ester (P3HT:PCBM) films. The impact on polymer crystallinity, surface structure, inner morphology, and quantitative molecular miscibility of P3HT and PCBM is studied as a function of ODT volume concentration. The crystallinity is probed with absorption spectroscopy and grazing incidence wide-angle X-ray scattering. The morphology and miscibility are characterized via atomic force microscopy and time-of-flight grazing incidence small angle neutron scattering. Besides an increased crystallinity and prominent phase separation, ODT increases the solubility of PCBM in P3HT and reduces the size of amorphous P3HT domains. Moreover, solvent processing with a high ODT concentration alters the vertical material composition of the active layer.

The function of 1,8-octanedithiol (ODT) on polymer:fullerene bulk heterojunction systems is comprehensively studied for the well-established model system poly(3-hexylthiophene-2,5-diyl):[6,6]-phenyl-C61 butyric acid methyl ester. Besides the positive influence of ODT on crystallinity and surface morphology, the influence on the molecular miscibility between polymer and fullerene is probed, providing a complete correlation between morphology and solar cell efficiency.

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.

Limited by the various inherent energy losses from multiple channels, organic solar cells show inferior device performance compared to traditional inorganic photovoltaic techniques, such as silicon and CuInGaSe. To alleviate these fundamental limitations, an integrated multiple strategy is implemented including molecular design, interfacial engineering, optical manipulation, and tandem device construction into one cell. Considering the close correlation among these loss channels, a sophisticated quantification of energy-loss reduction is tracked along with each strategy in a perspective to reach rational overall optimum. A novel nonfullerene acceptor, 6TBA, is synthesized to resolve the thermalization and V_OC loss, and another small bandgap nonfullerene acceptor, 4TIC, is used in the back sub-cell to alleviate transmission loss. Tandem architecture design significantly reduces the light absorption loss, and compensates carrier dynamics and thermalization loss. Interfacial engineering further reduces energy loss from carrier dynamics in the tandem architecture. As a result of this concerted effort, a very high power conversion efficiency (13.20%) is obtained. A detailed quantitative analysis on the energy losses confirms that the improved device performance stems from these multiple strategies. The results provide a rational way to explore the ultimate device performance through molecular design and device engineering.

Comprehensive optimization on organic solar cells is conducted, including molecular design, interfacial engineering, optical manipulation, and tandem architecture construction. Synergistical application of multiple strategies improves the balance of the energy losses from transmission, insufficient light trapping, thermalization, and carrier dynamic loss. An impressively high device performance up to 13.2% is achieved.

Tuning the blend composition is an essential step to optimize the power conversion efficiency (PCE) of organic bulk heterojunction (BHJ) solar cells. PCEs from devices of unoptimized donor:acceptor (D:A) weight ratio are generally significantly lower than optimized devices. Here, two high-performance organic nonfullerene BHJ blends PBDB-T:ITIC and PBDB-T:N2200 are adopted to investigate the effect of blend ratio on device performance. It is found that the PCEs of polymer-polymer (PBDB-T:N2200) blend are more tolerant to composition changes, relative to polymer-molecule (PBDB-T:ITIC) devices. In both systems, short-circuit current density (J_sc) is tracked closely with PCE, indicating that exciton dissociation and transport strongly influence PCEs. With dilute acceptor concentrations, polymer-polymer blends maintain high electron mobility relative to the polymer-molecule blends, which explains the dramatic difference in PCEs between them as a function of D:A blend ratio. In addition, polymer-polymer solar cells, especially at high D:A blend ratio, are stable (less than 5% relative loss) over 70 d under continuous heating at 80 °C in a glovebox without encapsulation. This work demonstrates that all-polymer solar cells show advantage in operational lifetime under thermal stress and blend-ratio resilience, which indicates their high potential for designing of stable and scalable solar cells.

Based on two representative and high performance organic nonfullerene bulk heterojunction blends, PBDB-T:ITIC and PBDB-T:N2200, the effect of blend ratio on device performance and the relevant device stability is investigated in-depth. Solar cell devices incorporating polymer–polymer blends exhibit stable device performance in a wide range of blend ratios and excellent stability under dark and thermal stress.

In article number 1705955, Wen-Yong Lai, Wenming Su, and co-workers develop screen-printed poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT: PSS) grids as ITO-free anodes for flexible organic light-emitting diodes (OLEDs). The great potential of the method is demonstrated by manufacturing printed large-area flexible grid electrodes. The image illustrates the screen printing process as well as the potential application of the flexible grid electrodes to construct flexible OLEDs.