Organic thin film solar cells (OTFSCs) were 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]] (PTB7)/[6,6]-phenyl-C71-butyric (PC71BM). The performances of active layers are prepared in chlorobenzene (CB) with different additives of 1-chloronaphthalene (CN) and 1,8-Diiodooctane (DIO) by a wet process with spin coating technique. The effects of different solvent additives on photovoltaic parameters such as fill factor, short circuit current density, and power conversion efficiency of active layers are reported. The absorption and surface morphology of the active layers are investigated using UV-visible spectroscopy and atomic force microscopy, respectively. The results indicate that structural and morphological changes were induced by the additives with solvent. The current density-voltage (J-V) characteristics of photovoltaic cells were measured under the illumination of simulated solar light with 100 mW/cm2(AM 1.5 G) by an Oriel 1000 W solar simulator. The OTFSCs of PTB7/PC71BM prepared with organic solvent additives of DIO+CN show more improved PCE of 4.96% by spin coating method.

We report the effect of a poly[(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctyl fluorene)] (PFN) interlayer on the performance of PTB7:PC 71 BM-based organic thin-film solar cells (OTFSCs) prepared using the spray-coating method. The active layer shows a broad absorption, but the effective contact area in the bulk hetrojunction (BHJ) is small because of the formation of aggregates on the surface. The surface aggregates dissolved by the addition of 1,8-diiodooctane (DIO) to the PTB7:PC 71 BM matrix, which results in an increase in the contact area between the donor and acceptor in the active layer and, thereby , an improvement in the device performance. The PFN interlayer enhances the charge-transport properties and suppresses exciton-recombination processes. This results in increases in the short-circuit current density (J sc = 13.87 mA/cm 2 ), open-circuit voltage (V oc = 0.747 V), and fill factor (FF = 57.97%), as a result of a high power conversion efficiency (PCE, η = 6.01%) compared to a device without a PFN layer (η = 4.84%). The thickness of the PFN interlayer was found to be of siginificant importance, as thicker layers exhibit an insulating nature instead of improving the electron transfer.

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