TY - JOUR
T1 - Formation of Interfacial Traps upon Surface Protonation in Small Molecule Solution Processed Bulk Heterojunctions Probed by Photoelectron Spectroscopy
AU - Ratcliff, Erin L.
AU - Bakus, Ronald C.
AU - Welch, Gregory C.
AU - Van Der Poll, Tom S.
AU - Garcia, Andres
AU - Cowan, Sarah R.
AU - MacLeod, Bradley A.
AU - Ginley, David S.
AU - Bazan, Guillermo C.
AU - Olson, Dana C.
PY - 2013/10/21
Y1 - 2013/10/21
N2 - 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.
AB - 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.
UR - http://www.scopus.com/inward/record.url?scp=84884508206&partnerID=8YFLogxK
U2 - 10.1039/C3TC31064G
DO - 10.1039/C3TC31064G
M3 - Article
AN - SCOPUS:84884508206
SN - 2050-7526
VL - 1
SP - 6223
EP - 6234
JO - Journal of Materials Chemistry C
JF - Journal of Materials Chemistry C
IS - 39
ER -
