Silicon atom substitution enhances interchain packing in a thiophene-based polymer system.

Silole-containing organic materials have been known to exhibit altered properties with respect to their carbon analogues. Recently, different categories of silole-containing polymers have been reported to show promising characteristics as materials for thin-film transistors and polymer solar cells. Most of the previous studies on silole derivatives mainly focus on the molecular orbital shapes and energies. How the replacement of a carbon atom by a silicon atom influences the interaction between molecules has not been addressed. Here we compare a silole-containing polymer with its carbon counterpart and show that the material properties, particular the packing of polymer chains, can be altered significantly. As a result, different processing conditions are needed to obtain high-efficiency solar cells. Packing simulations show that the geometrical effect caused by the replacement of the carbon atom with a silicon atom plays a dominant role effecting the polymer stacking property. Polymer solar cells have the advantages of having lower cost and lighter weight, as well as being fabricated via easier processing methods, when compared with commercial inorganic material-based solar cells. However, most of the current polymer solar cells show power conversion efficiencies (PCE) of less than 5%, mainly due to the low carrier mobilities. Recently an efficient solar cell based on a low bandgap polymer, poly{[4,4-bis(2ethylhexyl)-cyclopenta-(2,1-b;3,4-b’)dithiophene]-2,6-diyl-alt(2,1,3benzothiadiazole)-4,7-diyl} (PCPDTBT, Scheme 1a) was reported using additives to control the desired morphology. With the reports of the high carrier mobility of silole-containing polymers, attempts to further improve the efficiency of polymer solar cells by transforming PCPDTBT to the silole-containing polymer, poly{[4,40-bis(2-ethylhexyl)dithieno(3,2-b;20,30-d)silole]2,6-diyl-alt-(2,1,3-benzothidiazole)-4,7-diyl} (hereafter referred to as PSBTBT, Scheme 1b), has been recently realized. By replacing the 5-position carbon of PCPDTBT with a silicon atom, a PCE of as high as 5.6% [processed with chloroform, see the Supporting Information (SI), Fig. S1] can be achieved. While trying to optimize solar cells based on PCPDTBT and PSBTBT, we found that very different processing conditions are needed, indicating that the intrinsic material properties of these two polymers are rather different. In this manuscript, we explore experimentally how the silicon influences material properties and device performance, and quantum chemical calculations are used to identify the origin of changes from the carbon analogue. The polymer solar cells studied here are produced from blend solutions of polymer/PC70BM (see the Experimental section for details). For comparison, PCPDTBT is synthesized by the method reported in the literature and devices based on PCPDTBT/ PC70BM (1:3w/w) show a PCE comparable to the previously reported value ( 3.1%, without any additives). It has been reported that thermal annealing does not improve the device performance of solar cells based on PCPDTBT. Similarly, we did not observe any efficiency improvement after thermal annealing in our PCPDTBT-based devices. Our previous work showed that the efficiency of polymer solar cells based on the silole-containing polymer, PSBTBT, can be significantly improved from 3.8% to 5.6% by thermal annealing (SI, Fig. S1). To correlate the charge transport property of PSBTBTdevice with thermal annealing, the change of hole mobility is measured by thin-film transistors (TFT) before and after annealing. The mobility (extracted from the saturation regime; see SI, Fig. S2) obtained after annealing at 140 8C for 5min is 3 10 3 cm V 1 s , which is approximately five times higher than that before annealing ( 6 10 4 cm V 1 s ). A mobility higher than 10 3 cm V 1 s 1 is desired for high-efficiency polymer solar cells in order to have efficient charge transport. The increase in mobility after thermal annealing is likely one of the reasons for the increased