Modified fullerenes as acceptors in bulk heterojunction organic solar cells - a theoretical study

In the present study, electronic structure calculations were used to provide strategies for designing poly(3-hexylthiophene) (P3HT)-fullerene-derivative-based donor-acceptor materials for use in high-efficiency bulk heterojunction organic solar cells (BHJ OSCs). The work systematically analyses the impact of electron-donating and -withdrawing substituents on the opto-electronic properties of the fullerene structures. Parameters relating to the absorption spectra, orbital distributions, and energy ordering of the frontier molecular orbitals (FMO), the interactions between P3HT and the fullerene derivatives, and charge transfer across the interface were investigated. We found that substitution with the electron-withdrawing group NO₂ enhances the electronic coupling between the fullerene and P3HT; however, it reduces the open-circuit voltage (V_OC) of the OSC through lowering the LUMO energy level. Furthermore, the results show that substitution with an electron-withdrawing group (NO₂) and electron-donating group (OCH₃) can improve the power conversion efficiency (PCE) of the OSC, since this slightly improves the photon absorption abilities and charge transfer coupling at the interface without overly compromising V_OC relative to PC₆₁BM. Our study shows that alkyl chain modification in the PC₆₁BM acceptor is a promising strategy for improving the performances of OSCs.

Tuning of a A-A-D-A-A-Type Small Molecule with Benzodithiophene as a Central Core with Efficient Photovoltaic Properties for Organic Solar Cells

With the aim of upgrading the power conversion efficiency of organic solar cells (OSCs), four novel non-fullerene, A1-A2-D-A2-A1-type small molecules were designed that are derivatives of a recently synthesized molecule SBDT-BDD reported for its efficient properties in all-small-molecule OSCs (ASM-OSCs). Optoelectronic properties of the designed molecules were theoretically computed with a selected CAM-B3LYP functional accompanied by the 6-31G(d,p) basis set of density functional theory (DFT), and excited-state calculations were performed through the time-dependent self-consistent field. The parameters of all analyzed molecules describing the charge distribution (frontier molecular orbitals, density of states, molecular electrostatic potential), absorption properties (UV-vis absorption spectra), exciton dynamics (transition density matrix), electron-hole mobilities (reorganization energies), and exciton binding energies were computed and compared. All the designed molecules were found to be superior regarding the aforesaid properties to the reference molecule. Among all molecules, SBDT1 has the smallest band gap (3.88 eV) and the highest absorption maxima with broad absorption in the visible region. SBDT3 has the lowest binding energy (1.51 eV in chloroform solvent) ensuring easier and faster dissociation of excitons to produce free charge-carriers and has the highest open-circuit voltage (2.46 eV) with PC61BM as the acceptor. SBDT1 possesses the highest hole mobility because it has the lowest value of λ+ (0.0148 eV), and SBDT4 exhibits the highest electron mobility because it has the lowest value of λ- (0.0146 eV). All the designed molecules are good candidates for ASM-OSCs owing to their superior and optimized properties.

Roll coated large area ITO- and vacuum-free all organic solar cells from diketopyrrolopyrrole based non-fullerene acceptors with molecular geometry effects

We investigate three diketopyrrolopyrrole based small molecular non-fullerene acceptors with molecular geometry effect on mainly their PV performance of both spin-coated ITO based and roll coated large area, ITO- and vacuum-free organic solar cells.