Page - 2 - in Photovoltaic Materials and Electronic Devices

A representative example involves the blends comprised of PTB7/PC71BM, which is one of the highest-performing systems, based on the addition of small quantities of a high boiling point additive such as diiodooctane (DIO), a kind of commonly used additive [15] to meliorate the morphology of the blend film [16]and promote the phase separation[17]. However, the functions of DIOare still a subjectof debate inbothpolymerandsmall moleculesystems[18],but in the case of PTB7/PC71BM it is known to improve the charge transporting by increasing the final crystalline content of the film and allowing the donor phase more than one polymorph during the film formation [19,20]. Nonetheless, adding DIO into the blend film cannot improve the light absorption of the blend film. Even worse, uptonow, there isstill little researchabout thenegativeeffectofDIOadditiveson performanceofOPVs[21]. Another effective strategy for PCEs improvement of OPVs is adding a high molecular polystyrene (PS) into the pristine active layer. PS can increase not only increase the pristine solution viscosity but also the film thickness without sacrificing desirablephase separation[7]and structuralorder anddecrease the recombination of electron-hole pairs in the blend film [22,23]. At the same time, PS can improve the light absorption of the blend film. Therefore, in this contribution, polystyrene (PS) was used to fabricate the BHJ polymer solar cell based on PTB7/PC71BM as the active layer. DIO and/or PS were varied with different ratios during the solution preparation of the organic active layer. The effect of the PS was investigated in PTB7:PC71BM blended films. The morphology of the active layer with different additiveratioshasbeenstudiedandtherelatedOPVsdeviceperformancealsohas beenreported. 2. ExperimentalSection 2.1. FabricationofSolarCells Devices used materials that were used as purchased. Poly (3,4-ethylenedioxythiophene): poly (styrenesulfonate) (PEDOT:PSS), poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro- 2-[(2-ethylhexyl) carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7) with a molecular weight of ~200 kg/mol and polydispersity of ~4, [6,6]-phenylC71-butyric acid methyl ester(PC71BM), and polystyrene(PS) with a molecular weight of ~370 kg/mol were purchased from Clevios P, 1-Material INC, Nano-C company andSigma-Aldrich Corporationrespectively. PTB7and PC71BMwere co-dissolved in chlorobenzene with a weight ratio of 1:1.5 to form the mixed solution with the concentration 20 mg/mL. All organic materials were weighed and dissolved in ambientair conditions. 2