ABSTRACT
One of the best performing semiconducting polymers used in bulk heterojunction devices is PCDTBT, a polycarbazole derivative with solar-conversion efficiencies as high as 7.2%. Here we report the formation of bilayer ordering in PCDTBT, and postulate that this structural motif is a direct consequence of the polymer's molecular design. This bilayer motif is composed of a pair of backbones arranged side-to-side where the alkyl tails are on the outer side. This is in stark contrast to the monolayer ordering found in other conjugated polymers. The crystalline bilayer phase forms at elevated temperatures and persists after cooling to room temperature. The existence of bilayer ordering, along with its high-packing fraction of conjugated moieties, may guide the synthesis of new materials with improved optoelectronic properties.
ABSTRACT
A simple and effective strategy for optical band gap control is demonstrated through the use of a novel small acceptor/donor/acceptor molecule, 1, and group-13 Lewis acids. Chromophore 1 contains a dithienolesilole donor unit end-capped with benzo-2,1,3-thiadiazole (BT) acceptor units. Addition to 1 of stoichiometeric quantities of Lewis acids of varying strength resulted in the formation of Lewis adducts with progressively red-shifted primary charge-transfer absorption bands. These complexes have absorption spectra that approach those of known conjugated copolymers containing related units. NMR spectroscopy and X-ray diffraction studies revealed that 1 binds 2 equiv of Lewis acid via the nitrogen at the 3-position of BT.
ABSTRACT
So far, one of the fundamental limitations of organic photovoltaic (OPV) device power conversion efficiencies (PCEs) has been the low voltage output caused by a molecular orbital mismatch between the donor polymer and acceptor molecules. Here, we present a means of addressing the low voltage output by introducing novel trimetallic nitride endohedral fullerenes (TNEFs) as acceptor materials for use in photovoltaic devices. TNEFs were discovered in 1999 by Stevenson et al. ; for the first time derivatives of the TNEF acceptor, Lu(3)N@C(80), are synthesized and integrated into OPV devices. The reduced energy offset of the molecular orbitals of Lu(3)N@C(80) to the donor, poly(3-hexyl)thiophene (P3HT), reduces energy losses in the charge transfer process and increases the open circuit voltage (Voc) to 260 mV above reference devices made with [6,6]-phenyl-C(61)-butyric methyl ester (C(60)-PCBM) acceptor. PCEs >4% have been observed using P3HT as the donor material. This work clears a path towards higher PCEs in OPV devices by demonstrating that high-yield charge separation can occur with OPV systems that have a reduced donor/acceptor lowest unoccupied molecular orbital energy offset.
ABSTRACT
The most efficient plastic solar cells comprise a blend of conjugated polymer and a suitable electron acceptor, typically a fullerene derivative. Therefore narrow-bandgap conjugated polymers are currently sought for the fabrication of such devices. A significant challenge is being able to predict device function and performance from consideration of the molecular connectivity and dimensions of the partners within the active layer. Improved chemical syntheses are therefore required to make structurally varied polymers and enable the delineation of structure-function relationships with the aim of improving power conversion efficiencies. Here, we demonstrate that microwave heating in combination with the screening of comonomer reactant ratios can be used to obtain donor-acceptor copolymers with high average molecular weights and properties that make them suitable for solar cell incorporation. Furthermore, we highlight the importance of high molecular weight and the contribution of solubilizing side groups in determining the final device properties.
