Small-molecule donors for solar cells are usually end-capped with π-systems or aliphatic chains extending the π-conjugation of the molecules's backbone. Compared with alkyl terminals, π-systems can form π−π arrangements, for example, with an aligning spherical fullerene π-system. To study the effects of two kinds of terminals on the solar cell performance, the non-alkyl, branched aromatic and electron-donating diphenylamine (DPA) and the aliphatic n-butyl (n-Bu) unit are selected as end-capping groups on a diketopyrrolo­pyrrole-based linear backbone, affording two new solution-processable small-molecule donors. Photovoltaic data indicate that by changing the end-function from n-Bu to DPA, the photocurrent significantly increases from 8.35 to 15.64 mA cm⁻² and the efficiency from 3.2 to 5.8%. Characterization of absorption, morphology, recombination, and carrier transportation clearly demonstrates that the higher photocurrent can be attributed to a higher density of the mobile carriers (i.e., free holes, in this case). The DPA end-functions enhance the light-harvesting capacity, improve the charge dissociation, and reduce the recombination loss, all of which lead to more carriers being collected by the electrode. This work demonstrates that the choice of end-function along the molecular backbone is as important to improve the cell performance as the light-harvesting backbone and the side-chains.

We chose DPP-BDT-DPP {DPP=diketopyrrolopyrrole, BDT=4,8-di-[2-(2-ethylhexyl)-thienyl]benzo[1,2-b:4,5-b′]dithiophene} as a model backbone and varied the anchoring groups [C₅H₁₁, COOCH₃, and SiCH₃(OSiCH₃)₂] terminated on the N-substituted alkyl-chain spacer of the DPP units to study the effect of anchoring terminals on the morphology of blend film and on the device performances of bulk heterojunction solar cells. By replacing the nonpolar C₅H₁₁ anchoring terminal with the polar COOCH₃ anchoring terminal leads to an enhancement in the short-circuit current density (J_sc) (4.62 vs. 9.32 mA cm⁻²), whereas the value of J_sc sharply decreases to 0.45 mA cm⁻² if the C₅H₁₁ anchoring terminal is replaced by a SiCH₃(OSiCH₃)₂ group. The changes in J_sc are associated with changes in the π–π stacking distance (3.393.34 Å vs. 3.393.45 Å) and the phase size (5020 nm vs. 50>250 nm) through alteration of the anchoring group from C₅H₁₁ to COOCH₃ versus from C₅H₁₁ to SiCH₃(OSiCH₃)₂. Interestingly, the anchoring terminals bring about drastic changes in molecular orientations, which result in different out-of-plane hole transport. This is the first time this effect has been systemically demonstrated to improve photocurrent generation by manipulating the dipolar anchoring groups terminated on the alkyl-chain spacer.

Finding new molecular backbones is necessary for further advances in solution-processed small-molecule organic solar cells (SM-OSCs). Increasing molecular π conjugation generally enhances the light-harvesting ability, and the resulting strong π–π-stacking interactions improve the charge-carrier transport ability; both increase the efficiency. In this study, we focus on the phenyl-1,3,5-trithienyl (3T-P) backbone because of its C₃ symmetry, planarity, and particularly high conjugation between the three arms through the core phenyl unit. When the three arms were functionalized with diketopyrrolopyrrole (DPP) units to afford 3D-T-P, only modest efficiency was achieved (1.16 %). Introduction of 4,8-bis(2-(2-ethylhexylthienyl)) benzodithiophene (BDT) between the 3T-P and DPP units to give 3D-B-T-P enhanced the light-harvesting ability, and particularly improved the hole mobility by 1.5 orders of magnitude (5.91×10⁻² versus 1.05×10⁻³ cm² V⁻¹ s⁻¹). When using PC71BM as the acceptor material, 3D-B-T-P gave the best power conversion efficiency (PCE) of 2.27 %, which is about 1.9 times higher than the best efficiency from 3D-T-P (≈1.16 %). The efficiency can be improved up to 3.60 % with 3 % (v/v) of 1,8-diiodooctane (DIO) as the cosolvent and thermal annealing at 100 °C for 10 min. This PCE is, to the best of our knowledge, the highest efficiency reported to date among the phenyl-1,3,5-based C₃-symmetric molecules. Removing one DPP unit from 3D-T-P to form 2D-T-P, or from 3D-B-T-P to form 2D-B-T-P both decreased the light-harvesting ability and the hole mobility, thereby affording lower efficiency. Taken together, our results demonstrate that the planar phenyl-1,3,5-trithienyl-based C₃-symmetric structure can be a promising backbone, and enhancing the conjugation of the 3D-T-P backbone can effectively improve the device performance.

Supramolecular forces govern self-assembly and further determine the final morphologies of self-assemblies. However, how they control the morphology remains hitherto largely unknown. In this paper, we have discovered that the self-assembled nanostructures of rigid organic semiconductor chromophores can be finely controlled by the secondary forces by fine-tuning the surrounding environments. In particular, we used water/methanol/hydrochloric acid to tune the environment and observed five different phases that resulted from versatile molecular self-assemblies. The representative self-assembled nanostructures were nanotapes, nanoparticles and their 1D assemblies, rigid microplates, soft nanoplates, and hollow nanospheres and their 1D assemblies, respectively. The specific nanostructure formation is governed by the water fraction, R_w, and the concentration of hydrochloric acid, [HCl]. For instance, nanotapes formed at low [HCl] and R_w values, whereas hollow nanospheres formed when either the HCl concentration is high, or the water fraction is low, or both. The significance of this paper is that it provides a useful phase diagram by using R_w and [HCl] as two variables. Such a self-assembly phase diagram maps out the fine control that the secondary forces have on the self-assembled morphology, and thus allows one to guide the formation toward a desired nanostructure self-assembled from rigid organic semiconductor chromophores by simply adjusting the two key parameters of R_w and [HCl].