Page - (000015) - in Photovoltaic Materials and Electronic Devices

IntegratedEffectsofTwoAdditivesonthe EnhancedPerformanceofPTB7:PC71BM PolymerSolarCells LinWang,SulingZhao,ZhengXu, JiaoZhao,DiHuangandLingZhao Abstract: Organic photovoltaics (OPVs) are fabricated with blended active layers of 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]]: [6,6]-phenylC71-butyricacid methylester (PTB7:PC71BM).Theactive layersarepreparedinchlorobenzene(CB) added different additives of 1, 8-Diiodooctane (DIO) and polystyrene (PS) with different concentrations by spin coating. A small addition, 0.5%–5% by weight relative to the BHJ components, of inert high molecular weight PS is used to increase the solution viscosity and film thickness without sacrificing desirable phase separation and structural order. The effects of the PS are studied with respect of photovoltaicparameterssuchasfill factor, shortcircuit currentdensity,andpower conversion efficiency. Together with DIO, the device with 3.0 v% DIO and 1 wt % PS shows a high power conversion efficiency (PCE) of 8.92% along with an open-circuit voltage (Voc)of0.76V,ashort-circuit current (Jsc)of16.37mA/cm2, andafill factor (FF) of 71.68%. The absorption and surface morphology of the active layers are investigatedbyUV-visiblespectroscopy,atomicforcemicroscopy(AFM)respectively. The positive effect of DIO and PS additives on the performance of the OPVs is attributed to the increased absorption and the charge carrier transport and collection. Reprinted fromMaterials. Cite as: Wang, L.; Zhao, S.; Xu, Z.; Zhao, J.; Huang, D.; Zhao, L. Integrated Effects of Two Additives on the Enhanced Performance of PTB7:PC71BMPolymerSolarCells.Materials2016,9, 171. 1. Introduction Molecular species with well-defined structures [1–3] are being considered as possible substitutions for conjugated polymer counterparts in the fabrication of bulk heterojunction (BHJ) organic photovoltaics (OPVs) [4–6]. High power conversion efficiencies (PCEs) have been achieved in solution-processed molecular solar cells through a combination of chemical design and deposition methods with optimized morphology [7]. Despite the advantages of the structural precision [8] and purity of materials [9–11], some challenges to control the thickness and morphology [12,13] of the active layer decided by the processing conditions are critical to the properties of solar cells, which reasonably influences the light absorption and recombination of carriers [14]. 1