Impedance spectroscopy analysis of small molecule solution processed organic solar cell
In this paper we study the transport-recombination mechanisms using impedance spectroscopy of organic solar cells (OSC) based on a blend of a small molecule, 7,7-(4,4-bis(2ethylhexyl)-4H-silolo[3,2-b:4,5-b] dithiophene-2,6-diyl)bis(6-fluoro-4-(5-hexyl-[2,2-bithiophen] -5-yl)benzo[c][1,2,5] thiadiazole) (DTS(FBTTh2)2) and 1-(3-methoxycarbonyl)-propyl-1-1-phenyl-(6,6) C70 (PC70BM). We fabricate a cell with structure ITO/Poly(3,4-ethylenedioxythiophene)-poly(4- styrene sulfonate (PEDOT:PSS))/DTS(FBTTh2)2:PC 70BM/Ca/Al that exhibits Jsc=10.2 mA/cm2, Voc=0.816 V and FF=65% resulting in a PCE=5.4%. We model the impedance behavior using two circuital models, the parallel R-CPE and the transmission line model proposed by Belmonte et al. . We compared the results to those obtained for OSC based on a standard poly(3-hexylthiophene) (P3HT): 1-(3-methoxycarbonyl)-propyl-1-1-phenyl-(6,6) C61 (PC60BM) blend with structure ITO/PEDOT:PSS/P3HT:PC60BM/LiF/Al. We find that in the case of the small molecule based OSC diffusion dominates over recombination for this thickness, L=125 nm, even at high frequencies. We calculate the effective carrier lifetimes and mobilities for both structures using both models. Average electron mobility calculated for the small molecule cell is around 4-6.4×10-3 cm2/Vs, slightly higher than that obtained for the standard blend which is around 2×10-3 cm 2/Vs.