Intra- and Intermolecular Steric Hindrance Effects Induced Higher Open-Circuit Voltage and Power Conversion Efficiency

Abstract

A pair of donor−acceptor polymers PBDThDTBT and PBDTchDTBT are synthesized, which share the same conjugated backbone, but are designed with hexyl and cyclohexyl side chains, respectively. The stronger steric hindrance of cyclohexyl endows PBDTchDTBT a deeper lying HOMO energy level of −5.39 eV compared to −5.22 eV for PBDThDTBT. However, PBDThDTBT and PBDTchDTBT exhibit a similar optical bandgap around 1.72 eV and a hole mobility around 10 −5 cm 2 V −1 s −1 . Interestingly, the PBDTchDTBT/PC 71 BM blends exhibited higher hole mobility than PBDThDTBT/PC 71 BM after DIO was added. The higher hole mobility and fibrillar network in the active layer endows PBDTchDTBT higher power conversion efficiency of 7.9%, together with simultaneously improved open-circuit voltage of 0.80 V, short-circuit current density of 13.50 mA cm, and fill factor of 72.74% after a systemic study of their solar cell devices. S emiconducting polymer is considered as the ideal material for next generation large area optoelectronics application in virtue of its superior solution processing and film forming ability.1−5 When a semiconducting polymer is employed, research is mainly focused on its frontier molecular orbital energy levels and optical and optoelectronic characteristics, which have reliance on the polymer's planarity, interchain aggregation, alkyl substituents, and molecular weight.6−9 One simple and effective strategy employed to adjust the energy levels and optical property of a semiconducting polymer is intra-or intermolecular steric hindrance effect (SHE). 10−13However, SHE has been reported with negative effect on molecular self-organization in the donor−acceptor (D−A) oligmers.14 Intermolecular SHE introduced to the polymer backbone could prevent molecular packing and then result in weaker crystallinity.15 Intramolecular SHE may reduce the π-orbital overlap between two adjacent units and lead to undesirable hypochromatic shifted absorption spectra and wide bandgap (E g ), 10,16−20 however, as is well-known, wide absorption spectra, and narrow E g of the absorber are crucial for organic solar cells (OSCs) application. 21Although SHE has been considered as a negative effect for polymer's self-aggregation and effective conjugation, one advantage should be pointed out, that is, SHE can significantly decrease the highest occupied molecular orbital (HOMO) energy levels of the polymer. For example, poly(3-cyclohexylthiophene) (P3chT) exhibited more twisted conjugating backbone than poly(3-hexylthiophene) (P3hT) because of stronger SHE of cyclohexyl than hexyl, which resulted in a larger dihedral angle (θ) between the two adjacent thiophene rings together with a wider E g and a deeper lying HOMO of −5.72 eV, 0.72 eV lower than that of P3hT (−5.0 eV) experimentally.10 Theoretically, Bredas and co-workers reported that the ionic potential of the polythiophene increased gradually as the torsion angle increased from 0 to 90°.22 This

Influence of donor-acceptor conjugation between thiophene-phthalazinone structures containing Sp3 CN bond on the frontier orbital levels and optic-electronic properties

“…Such small backbone torsion is resulted from the small steric hindrance between two unalkyl substituents thiophene rings. This better backbone coplanarity is beneficial for improving the charge transport along the polymer chains, increasing the intermolecular interaction, decreasing the band gaps and expanding absorption . Additional, the LUMO level is mainly distributed on the electron‐drawing phthalazinone unit.…”

The effect of tuning chemical structure on the open‐circuit voltage and photovoltaic performance of narrow band‐gap polymers

“…Thus, a higher J sc should be expected for P1‐ and P3‐based PSCs due to the broad absorption. The larger optical band gap of P2 might be ascribed to the tortuous polymer backbone which was caused by the steric hindrance . In addition, all of these polymers showed small red shift from solution to thin film, and strong aggregation of polymer itself was supposed in the solution.…”

“…Along with more and more electron acceptor units for D–A polymers and buffer layer materials have been investigated, it is becoming very difficult to find and synthesize novel high‐performance donor materials. Compared with these methods as mentioned above, the fine tuning of the molecular structure is rather a simple way to lower the HOMO level and improve the V oc , such as introducing different kinds of side chains into the donor units or acceptor units, altering the atoms of the donor or acceptor units, and slightly changing the main chain on the basis of existing narrow band‐gap polymers.…”

Fine Control of Side Chains in Random π‐Conjugated Terpolymers for Organic Photovoltaics

“…Ternary donor–acceptor (D–A) copolymers, which comprise either one D and two A units or two D and one A moieties along the main chain, have recently attracted intense attention in the field of organic photovoltaics (OPVs) . Such terpolymer strategies can circumvent enormous synthetic work on developing novel yet efficient D and A moieties for traditional binary alternating D–A copolymers towards high‐performance OPV cells .…”