Understanding Structure-Property Relationships in All-Small-Molecule Solar Cells Incorporating a Fullerene or Nonfullerene Acceptor

Theoretical investigation by DFT and TDDFT the extension of π-conjugation of novel carbazole-based donor materials for bulk heterojunction organic solar cell applications

In this study, we have proposed seven designed symmetrical compounds (C₂-C₈) having a D-π-A-π-D structure based on derivative carbazole as a donor by introducing various π-spacer groups into the reference compound C₁-Ref having a D-A-D structure in order to understand the influence of different π-spacers on their efficiency in BHJ solar cells. Various parameters such as geometrical structures, frontier molecular orbitals (FMOs), molecular electrostatic potential (MEP), nonlinear optical properties (NLO), optical properties, light-harvesting efficiency (LHE), reorganization energy, chemical reactivity indices, exciton binding energy (E_b), open-circuit voltage (V_OC), and fill-factor (FF) have been investigated using the density functional theory (DFT) and time-dependent DFT (TD-DFT) methods. The results show that the extended π-conjugation of the designed compounds (C₂-C₈) produces a lower energy gap (E_g), a stronger and broader absorption spectrum, lower reorganization energies and exciton binding, and higher nonlinear optical properties compared to C₁-Ref, indicating that these designed compounds are promising as electron donors in BHJ-OSCs. Additionally, the calculated V_oc, FF, and LHE of all compounds showed that the C₂, C₃, C₄, C₅, and C₇ compounds have the best performance in BHJ solar cells compared to the others. In particular, C₄ and C₅ are excellent candidates for the effective donor materials of BHJ solar cells due to their large V_oc, FF, and LHE than the other compounds. This theoretical investigation is expected to provide new strategies to synthesize efficient donor materials for BHJ-OSCs.

Excimer evolution hampers symmetry-broken charge-separated states

Achieving long-lived symmetry-broken charge-separated states in chromophoric assemblies is quintessential for enhanced performance of artificial photosynthetic mimics. However, the occurrence of energy trap states hinders exciton and charge transport across photovoltaic devices, diminishing power conversion efficiency. Herein, we demonstrate unprecedented excimer formation in the relaxed excited-state geometry of bichromophoric systems impeding the lifetime of symmetry-broken charge-separated states. Core-annulated perylenediimide dimers (SC-SPDI2 and SC-NPDI2) prefer a near-orthogonal arrangement in the ground state and a π-stacked foldamer structure in the excited state. The prospect of an excimer-like state in the foldameric arrangement of SC-SPDI2 and SC-NPDI2 has been rationalized by fragment-based excited state analysis and temperature-dependent photoluminescence measurements. Effective electronic coupling matrix elements in the Franck-Condon geometry of SC-SPDI2 and SC-NPDI2 facilitate solvation-assisted ultrafast symmetry-breaking charge-separation (SB-CS) in a high dielectric environment, in contrast to unrelaxed excimer formation (Ex*) in a low dielectric environment. Subsequently, the SB-CS state dissociates into an undesired relaxed excimer state (Ex) due to configuration mixing of a Frenkel exciton (FE) and charge-separated state in the foldamer structure, downgrading the efficacy of the charge-separated state. The decay rate constant of the FE to SB-CS (k FE→SB-CS) in polar solvents is 8-17 fold faster than that of direct Ex* formation (k FE→Ex*) in non-polar solvent (k FE→SB-CS≫k FE→Ex*), characterized by femtosecond transient absorption (fsTA) spectroscopy. The present investigation establishes the impact of detrimental excimer formation on the persistence of the SB-CS state in chromophoric dimers and offers the requisite of conformational rigidity as one of the potential design principles for developing advanced molecular photovoltaics.

Fullerene/Non-fullerene Alloy for High-Performance All-Small-Molecule Organic Solar Cells

Organic solar cells (OSCs) that contain small molecules only were prepared with FG1 as the donor, a narrow band gap non-fullerene acceptor MPU4, and a wide band gap PC₇₁BM. The OSCs based on optimized FG1:MPU4 (1:1.2) and FG1:PC₇₁BM (1:1.5) active layers, respectively, gave power conversion efficiencies (PCEs) of 11.18% with a short circuit current (J_SC) of 19.54 mA/cm², open circuit voltage (V_OC) of 0.97 V, and fill factor (FF) of 0.59, and 6.62% with a J_SC of 12.50 mA/cm², V_OC of 0.84 V, and FF of 0.63%, respectively. A PCE of 13.26% was obtained from the optimized ternary FG1:PC₇₁BM:MPU4 (1:0.3:0.9) OSCs and this arises because of the boost in a J_SC of 21.91 mA/cm² and FF of 0.68. The V_OC of the ternary OSCs (0.89 V) lies between those for the OSCs based on FG1:MPU4 and FG1:PC₇₁BM, which indicates the formation of an alloy of the two acceptors. The increase in J_SC and FF in the ternary OSCs may result from the efficient energy transfer from PC₇₁BM to MPU4 as well as more charge-transfer donor/acceptor interfaces, enhanced charge carrier mobilities resulting in better adjusted charge transport, and lower bimolecular and trap-assisted recombination. The appropriate phase separation, increased crystallinity, and reduced π-π stacking distance in the ternary active layer are consistent with the enhancement in the FF for OSCs based on a ternary active layer. The results of this work suggest the merging of the fullerene acceptor into the non-fullerene acceptor to form a fullerene/non-fullerene acceptor alloy, and this may be a viable approach to obtain high-performance OSCs.

A–D–C–D–A type non-fullerene acceptors based on the benzotriazole (BTA) unfused core for organic solar cells

Increase of fluorine atoms to modulate the molecular orientation and thus enhanced the photovoltaic performances.

Modulation of Donor Alkyl Terminal Chains with the Shifting Branching Point Leads to the Optimized Morphology and Efficient All-Small-Molecule Organic Solar Cells

Terminal group modification is one of the most influential factors for small-molecular donors compared with their polymer counterparts, resulting in an opportunity to optimize the morphology of all-small-molecule organic solar cells (ASM-OSCs). In this article, we report three novel small-molecular donors with branching points at the 1-, 2-, and 3-positions in alkyl terminal chains, called BSCl-C1, BSCl-C2, and BSCl-C3, respectively. Using IDIC-4Cl as the acceptor, the subtle branching position shift achieves a dramatic disparity in photovoltaic parameters, as indicated by the short circuit current (J_sc) changing from 4.9 to 20.1 to 14.2 mA cm^-2 and the fill factor varying from 33.9 to 71.3 to 67.0% for BSCl-C1, BSCl-C2, and BSCl-C3, respectively. The best device performance of 12.40% is obtained by the BSCl-C2:IDIC-4Cl system, which not only ranks among the top values reported to date but also exhibits low energy loss in systems that use IDIC as acceptors. The notable device performance based on BSCl-C2 is attributed to the optimized phase morphology caused by the strong molecular crystallinity and suitable intermolecular interaction with IDIC-4Cl. These results demonstrate that suitably tuning the branching position of terminal groups could promote the high performance of ASM-OSCs.

Molecular Ordering and Performance of Ternary Nonfullerene Organic Solar Cells via Bar-Coating in Air with an Efficiency over 13

An in situ spectroscopy ellipsometry technique is utilized to probe the molecular ordering sequences of PBDB-T-2F/IT-4F/COi8DFIC ternary photovoltaic blends fabricated by bar-coating in air. The time-resolved dynamics show that the primary electron acceptor IT-4F aggregates ahead of the secondary acceptor COi8DFIC in the bar-coated photoactive layer, although the latter has much stronger crystallization ability. Wetting coefficient analysis supports that COi8DFIC locates at the interface between the host components PBDB-T-2F and IT-4F. We demonstrate that the suitable degree of phase separation with the presence of 20 wt % COi8DFIC facilitates exciton dissociation and charge transfer, leading to a remarkable power conversion efficiency of 13.2% as well as excellent stability of ternary organic solar cells (OSCs), which is among the highest reported efficiency for OSCs that were fabricated by scalable solution-casting in ambient conditions.

Side chain engineering in DTBDT-based small molecules for efficient organic photovoltaics

A new small-molecule donor with a dithieno[2,3-d:2',3'-d']-benzo[1,2-b:4,5-b']-dithiophene (DTBDT) core and both alkyl and alkylthio substituents is designed and synthesized to improve the miscibility between DTBDT-based small molecules and [6,6]-phenyl-C71-butyric acid methyl ester (PC₇₁BM). The alkyl substituent on the 4-position and the alkylthio substituent on the 5-position of the substituted thiophene are expected to improve intermolecular interactions and prevent severe aggregation of the small molecules. The new small molecule, DTBDT-S-C8-TTR, exhibits a homogenous blend morphology with small domains and edge-on-oriented crystalline structures in blends with PC₇₁BM, and give a maximum power conversion efficiency (PCE) of 8.43%. To recover the crystallinity of the DTBDT-S-C8-TTR small molecules weakened after being blended with PC₇₁BM, a solvent vapor annealing (SVA) treatment is performed. The SVA-treated blend films reveal well-developed crystalline domains with interconnected fibrillar structures. This blend morphology allows efficient charge carrier transport in blends and leads to increased PCEs. The maximum PCE of 9.18% achieved using DTBDT-S-C8-TTR suggests that substituting both alkylthio and alkyl groups into DTBDT can yield small-molecule-based organic photovoltaics (OPVs) displaying improved photovoltaic performances.