Impedance spectroscopy analysis of small molecule solution processed organic solar cell

• We have fabricated and characterized organic solar cells based on a blend of a small molecule, (DTS(FBTTh2)2): PC70BM.
• We have modeled dynamic behavior of the blend using impedance spectroscopy.
• We obtained a built-in potential and an acceptor density of Vbi=1.09 V and NA=4.1×1016 cm−3 respectively.
• Reconstructed the recombination current from impedance parameters agrees well with the measured short circuit current.
• We have calculated mobilities for the small molecule blend obtaining 4–6.4×10−3 cm2/Vs.

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:PC70BM/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. [1]. 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 cm2/Vs.