Rational design of novel A-A-D-A-A type electron donors for small molecule organic solar cells

Low-bandgap push–pull molecules in polymer matrices for use in thin-film organic photovoltaic devices

Conjugated polymers are widely used in thin-film organic photovoltaic devices to absorb light and serve as electron donors or acceptors. Small molecular analogues are attractive substitutes because they have fully defined structures, can be purified rigorously, and are typically more soluble and volatile. However, producing active films composed primarily of small molecules remains challenging. We have devised bulk heterojunction solar cells in which poly(3-hexylthiophene-2,5-diyl) and poly[[9-(1-octylnonyl)-9H-carbazole-2,7-diyl]-2,5-thiophenediyl-2,1,3-benzothiadiazole-4,7-diyl-2,5-thiophenediyl] are used as matrices to prepare films containing low-bandgap push–pull molecules as electron donors and (6,6)-phenyl-C61-butyric acid methyl ester or (6,6)-phenyl-C71-butyric acid methyl ester as electron acceptors. Compared with reference devices devoid of push–pull molecular additives, increases in power conversion efficiencies up to 30.4% were measured.

A theoretical exploration of the effect of fluorine and cyano substitutions in diketopyrrolopyrrole-based polymer donor for organic solar cells

A series of polymer donor materials 1-5 based on diketopyrrolopyrrole and thiophene unit which have been widely used in organic solar cells (OSCs) were investigated based on quantum chemical calculations. The effect of fluorine and cyano substitutions in polymer donor materials was focused on. Based on the investigation on electronic structures and optical properties of the reported molecules 1 and 2 and the analysis on some parameters relevant to charge dissociation ability at donor/acceptor interface constituted by 1 and 2 with PC₆₁BM such as intermolecular charge transfer and recombination, driving force and Coulombic bound energy, we explained why fluorine substitution can improve OPV efficiency through strengthening eletron-withdrawing ability from a theoretical perspective. Then we designed cyano-substituted polymers 3-5 with the aim of obtaining better photovoltaic donor materials. The results reveal that our attempt to design donor materials which can balance large open-circuit voltage (V_oc) and high short-circuit current (J_sc) in OSCs has worked out. It is worth noting that the substitutions of fluorine and cyano groups synergistically reduce energy gap and HOMO energy level of polymers 3 and 4. Moreover, 3/PC₆₁BM and 4/PC₆₁BM heterojunctions show over 10⁷ and 10⁴ times higher than 1/PC₆₁BM on the ratios of intermolecular charge transfer and recombination rates (k_inter-CT/k_inter-CR). Thus, our work here may provide an efficient strategy to design promising donor materials in OPVs and we hope it could be useful in the future experimental synthesis.

DFT study of opto-electronic properties of benzothiazole derivatives for use in solar cells

A variety of organic donor–acceptor–donor materials based on thienylbenzothiadiazole (BTD[Formula: see text]–5) combined with different [Formula: see text]-conjugated systems are studied by density functional theory (DFT) and time-dependent DFT (TD-DFT) for the ground- and excited-state properties, respectively, using B3LYP and the 6-31G(d, p) basis set. The effect of different electron-donor groups on the structural, electronic and optoelectronic properties is studied. To provide for the bandgap and to guide the synthesis of novel low bandgap materials, we applied quantum chemistry techniques to calculate the difference in the highest occupied molecular orbital (HOMO) and lowest occupied molecular orbital LUMO energies. However, we have studied the effect of the reduction and oxidation properties on the electronic excitation transitions for all compounds. The emission energies have been obtained from TD-DFT calculations performed on the excited-state optimized S1 geometries. The theoretical results suggest that both the introduction of electron-donor groups and the doping process contribute significantly to the electronic and optoelectronic properties of the alternating donor–acceptor–donor conjugated systems studied.

Theoretical design and characterization of pyridalthiadiazole-based chromophores with fast charge transfer at donor/acceptor interface toward small molecule organic photovoltaics

The designed 2, 3, and 5 will be promising donor candidates for high-performance OPV device due to their fast intermolecular charge transfer and effective charge transport.

A time dependent DFT study of the efficiency of polymers for organic photovoltaics at the interface with PCBM

In this work an intuitive (TD-DFT) approach was developed to explain the experimental efficiencies seen for organic photovoltaic devices.