H‑Aggregated Form II Spherulite of Poly(3-butylthiophene) Grown from Solution

New large-size poly­(3-butylthiophene) (P3BT) spherulites are obtained by solution aging. These P3BT spherulites are composed of nanoribbons, and the P3BT molecules arrange into Form II structure with a backbone π-stacking distance of 0.47 nm. P3BT lamellae adopt “flat-on” orientation at the edge of the spherulite, and the spherulite preferentially grows along the π–π stacking direction. These spherulites could be obtained in bulk solution or at the top or bottom of the solution, depending on the competition of gravity and Brownian motion. Temperature-dependent photoluminescence spectra demonstrate that the polymer chains are arranged in H-aggregation model. The 0–0 transition in UV–visible absorption spectra blue shifts from 2.03 eV (610 nm, Form I) to 2.11 eV (589 nm, Form II). These results provide a further understanding of the crystallization and photophysical properties of poly­(3-alkylthiophene) (P3AT), and the preparation method of large size and pure P3AT spherulites shows potential in applications of organic electronics

Design of Halogenated Donors for Efficient All-Small-Molecular Organic Solar Cells

Precise adjustment of the nanoscale morphology within the active layers is crucial for optimizing the photovoltaic performance of all-small-molecule organic solar cells (ASM-OSCs), and the halogen substituent strategy for photovoltaic materials plays a vital role in the development of the morphology evolution. In this work, we systematically study a series of acceptor–donor–acceptor (A-D-A) type small-molecule donors by incorporating halogenation at the thienyl benzo[1,2-b:4,5-b′]dithiophene (BDT-T) donor core unit named BSTR-F, BSTR-Cl, and BSTR-Br. Such halogenation is demonstrated to induce a significant increase in the ionization potential, i.e., deeper HOMO, and more ordered packing property. Using N3 as the acceptor, the BSTR-F-based devices achieve a power conversion efficiency (PCE) up to 15.93%, compared with the control nonhalogenated donor BSTR-H-based devices of 13.80%, indicating that the suitable halogenation strategy could effectively promote the high performance of ASM-OSCs

Terpolymer Containing a <i>meta</i>-Octyloxy-phenyl-Modified Dithieno[3,2‑<i>f</i>:2′,3′‑<i>h</i>]quinoxaline Unit Enabling Efficient Organic Solar Cells

With the rapid development of small-molecule electron acceptors, polymer electron donors are becoming more important than ever in organic photovoltaics, and there is still room for the currently available high-performance polymer donors. To further develop polymer donors with finely tunable structures to achieve better photovoltaic performances, random ternary copolymerization is a useful technique. Herein, by incorporating a new electron-withdrawing segment 2,3-bis(3-octyloxyphenyl)dithieno[3,2-f:2′,3′-h]quinoxaline derivative (C12T-TQ) to PM6, a series of terpolymers were synthesized. It is worth noting that the introduction of the C12T-TQ unit can deepen the highest occupied molecular orbital energy levels of the resultant polymers. In addition, the polymer Z6 with a 10% C12T-TQ ratio possesses the highest film absorption coefficient (9.86 × 104 cm–1) among the four polymers. When blended with Y6, it exhibited superior miscibility and mutual crystallinity enhancement between Z6 and Y6, suppressed recombination, better exciton separation and charge collection characteristics, and faster hole transfer in the D–A interface. Consequently, the device of Z6:Y6 successfully achieved enhanced photovoltaic parameters and yielded an efficiency of 17.01%, higher than the 16.18% of the PM6:Y6 device, demonstrating the effectiveness of the meta-octyloxy-phenyl-modified dithieno[3,2-f:2′,3′-h]quinoxaline moiety to build promising terpolymer donors for high-performance organic solar cells

Reconstructing Space- and Energy-Dependent Exciton Generation in Solution-Processed Inverted Organic Solar Cells

Photon absorption-induced exciton generation plays an important role in determining the photovoltaic properties of donor/acceptor organic solar cells with an inverted architecture. However, the reconstruction of light harvesting and thus exciton generation at different locations within organic inverted device are still not well resolved. Here, we investigate the film depth-dependent light absorption spectra in a small molecule donor/acceptor film. Including depth-dependent spectra into an optical transfer matrix method allows us to reconstruct both film depth- and energy-dependent exciton generation profiles, using which short-circuit current and external quantum efficiency of the inverted device are simulated and compared with the experimental measurements. The film depth-dependent spectroscopy, from which we are able to simultaneously reconstruct light harvesting profile, depth-dependent composition distribution, and vertical energy level variations, provides insights into photovoltaic process. In combination with appropriate material processing methods and device architecture, the method proposed in this work will help optimizing film depth-dependent optical/electronic properties for high-performance solar cells

Critical Role of Vertical Phase Separation in Small-Molecule Organic Solar Cells

An inverted device structure is a more stable configuration than a regular device structure for solution-processed organic solar cells (OSCs). However, most of the solution-processed small-molecule OSCs (SM-OSCs) reported in the literature used the regular device structure, and a regular device normally exhibits a higher efficiency than an inverted device. Herein, a representative small-molecule DR3TBDTT was selected to figure out the reason for photovoltaic performance differences between regular and inverted devices. The mechanisms for a reduced open-circuit voltage (<i>V</i>_oc) and fill factor (FF) in the inverted device were studied. The reduced <i>V</i>_oc and FF is due to the vertical phase separation with excess [6,6]-phenyl-C₇₁-butyric acid methyl ester near the air/blend surface, which leads to a reduction in build-in voltage and unbalanced charge transport in the inverted device. Another reason for the reduced FF is the unfavorable DR3TBDTT crystallite orientation distribution along the film thickness, which is preferential edge-on crystallites in the top layer of the blend film and the increased population of face-on crystallites in the bottom layer of the blend film. This study illustrates that the morphology plays a key role in photovoltaic performance difference between regular and inverted devices and provides useful guidelines for further optimization of the morphology of solution-processed SM-OSCs