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

Abstract

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′]dithiophene-2,6-diyl)bis(4-(5′-hexyl-[2,2′-bithiophen]-5-yl)-[1,2,5]thiadiazolo[3,4-c]pyridine) (d-DTS(PTTh₂)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(PTTh₂)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 via theory and X-ray photoelectron studies of both the interface and a control system of d-DTS(PTTh₂)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.