Formation of interfacial traps upon surface protonation in small molecule solution processed bulk heterojunctions probed by photoelectron spectroscopy

• Published in: Journal of materials chemistry. C, Materials for optical and electronic devices Materials for optical and electronic devices, 2013-01, Vol.1 (39), p.6223-6234
• Main Authors: Ratcliff, Erin L., Bakus II, Ronald C., Welch, Gregory C., van der Poll, Tom S., Garcia, Andres, Cowan, Sarah R., MacLeod, Bradley A., Ginley, David S., Bazan, Guillermo C., Olson, Dana C.
• Format: Article
• Language: English
• Subjects: Devices; Electrostatics; Extraction; Photoelectron spectroscopy; Pictures; Protonation; Ultraviolet
• ISSN: 2050-7526; 2050-7534
• DOI: 10.1039/c3tc31064g

This work expands on the recently reported protonation of the donor molecule 7,7'-(4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b']dith iophene-2,6-diy l)bis(4-(5'-hexyl-[2,2'-bithiophen]-5-yl)-[1,2,5]thiad iazolo[3,4-c]pyridine) (d-DTS(PTTh sub(2)) sub(2)) by the poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) interlayer to include an electrostatic picture of interfacial energetic states. Ultraviolet photoemission spectroscopy results initially suggested favorable band level alignment for hole extraction between d-DTS(PTTh sub(2)) sub(2) and PEDOT:PSS. However photovoltaic device performance yields a low fill factor and photovoltage, indicative of poor hole-extraction at the hole-collecting interface, relative to the nickel oxide device. Further investigation into the interfacial composition viatheory and X-ray photoelectron studies of both the interface and a control system of d-DTS(PTTh sub(2)) sub(2) reacted with p-toluenesulfonic acid verify the presence of a chemically unique species at the interface arising from protonation reaction with the residual acidic protons present in PEDOT:PSS that was masked in the UPS experiment. From these results, the energy band diagram is re-interpreted to account for the interfacial chemical reaction and modified interfacial density of states. Additionally, the detrimental protonation reaction is avoided when the pyridyl[1,2,5]thiadiazole acceptor unit was replaced with a 5-fluorobenzo[c][1,2,5]thiadiazole acceptor unit, which shows no such reaction with the PEDOT:PSS substrate. These results indicate the necessity of using a large analytical toolkit to elucidate the energetics and mechanisms of buried interfaces that will impact dynamics of hole collection.