Solar Cell Efficiency, Self-Assembly, and Dipole–Dipole Interactions of Isomorphic Narrow-Band-Gap Molecules

Abstract

We examine the correlations of the dipole moment and conformational stability to the self-assembly and solar cell performance within a series of isomorphic, solution-processable molecules. These charge-transfer chromophores are described by a D(1)-A-D-A-D(1) structure comprising electron-rich 2-hexylbithiophene and 3,3'-di-2-ethylhexylsilylene-2,2'-bithiophene moieties as the donor units D(1) and D, respectively. The building blocks 2,1,3-benzothiadiazole (BT) and [1,2,5]thiadiazolo[3,4-c]pyridine (PT) were used as the electron-deficient acceptor units A. Using a combination of UV-visible spectroscopy, field-effect transistors, solar cell devices, grazing incident wide-angle X-ray scattering, and transmission electron microscopy, three PT-containing compounds (1-3) with varying regiochemistry and symmetry, together with the BT-based compound 5,5'-bis{(4-(7-hexylthiophen-2-yl)thiophen-2-yl)-[1,2,5]thiadiazolobenzene}-3,3'-di-2-ethylhexylsilylene-2,2'-bithiophene (4), are compared and contrasted in solution, in thin films, and as blends with the electron acceptor [6,6]-phenyl-C(70)-butyric acid methyl ester. The molecules with symmetric orientations of the PT acceptor, 1 and 2, yield highly ordered blended thin films. The best films, processed with the solvent additive 1,8-diiodooctane, show donor "crystallite" length scales on the order of 15-35 nm and photovoltaic power conversion efficiencies (PCEs) of 7.0 and 5.6%, respectively. Compound 3, with an unsymmetrical orientation of PT heterocycles, shows subtle differences in the crystallization behavior and a best PCE of 3.2%. In contrast, blends of the BT-containing donor 4 are highly disordered and give PCEs below 0.2%. We speculate that the differences in self-assembly arise from the strong influence of the BT acceptor and its orientation on the net dipole moment and geometric description of the chromophore.

The effect of intermolecular interaction on excited states in p − DTS(FBTTH2)2

“…1 In the last few years, highly efficient organic BHJ solar cells have also been prepared that use soluble small molecules as the semiconducting donor component. [2][3][4][5][6][7][8][9] Power conversion efficiencies exceeding 10 % have even been reported when soluble small molecules are used in ternary blends. 10 Soluble small molecules are well-defined in terms of their molecular weight.…”

The structural evolution of an isoindigo-based non-fullerene acceptor for use in organic photovoltaics

“…[11][12][13][14][15] Early development of molecular active layer materials focused on replacing polymeric donor materials with small molecule donors. The invested interest in small molecule donors has led to PCEs above 9% when combined with fullerene derivatives in solution processable BHJ OPV devices.…”

Tuning the performance of the non-fullerene organic solar cells by the polarizability

“…Through the determination of the dipole moments, it can be understood that the distribution of the electron clouds and symmetry of the molecule in the molecular structure can be used to identify the geometric isomers and the stereostructure of the molecule. 34 The data from the dipole moment in Table 3 is comparable to the solar cell performance and shows that the low-dipole donor polymer has a higher performance than the bulk heterojunction with the acceptor polymer. In addition, in two different atoms, due to the different electron attraction of atomic nuclei, when they combine together, the electron cloud of the entire molecule will tend to move to one side, resulting in polarization, and the polarizability is the degree of polarization measurement.…”