Preparation, Optoelectronic Properties And Applications Of Star-shaped Oligothiophene Derivatives

As functional organic materials, oligothiophenes and their derivatives are getting more andmore attention because of their excellent optical and electrical properties. With the aim ofdeveloping novel oligothiophenes derivatives and their application, eight star-shapedoligothiophene derivatives were synthized, and their optical, electrical properties werediscussed. And then, the application of these star-shaped oligothiophene derivatives as organicelectrochromic and photovoltaic materials were studied.Two series of star-shaped oligothiophene derivatives with benzene or nitrogen as the"core" were synthized. These oligothiophene derivatives are:1,3,5-tri(2′-thienyl)benzene(3TB),1,3-bi(5′-formyl-2′-thienyl)-5-(2′-thienyl)benzene (3TB-2CHO),1,3,5-tri(5′-cyano-2′-thienyl) benzene (3TB-3CN),1,3,5-tri(5′-carboxyl-2′-thienyl)benzene (3TB-3COOH),1,3,5-tri(2′-(5′,2′′-bithienyl))benzene (6TB), tri(4-(2′-thienyl)) phenylamine (3TPA), tri(4-(5′-cyano-2′-thien-yl)) phenylamine (3TPA-3CN) and tri(4-(5′-carboxyl-2′-thienyl))phenylamine(3TPA-3COO-H). The eight compounds were characterized by MS,1HNMR,13CNMR and IR.Their optical and electrical properties were stutied. Their application as organicelectrochromic and photovoltaic materials were investigated.The photophysical and electrochemical properties of these star-shaped oligothiophene derivatives were measured by UV-vis absorption spectra, fluorescence spectra and cyclicvoltammetry. The optical energy gap, HOMO level, LUMO level and electrochemical energygap were determinded and calculated. The relationship between molecular structure andoptoelectronic properties were discussed. It was found that the optoelectronic properties ofoligothiophene derivatives could be adjusted by changing the core of the “star”, the length ofconjugated chain and introduction of different substituent groups. Based on these properties,the application of star-shaped oligothiophenes as organic electrochromic and photovoltaicmaterials were studied.Two kinds of electrochromic devices were fabricated. The devices structures are:(A)ITO/electrochromic active layer/electrolyte solution/Pt;(B) ITO/electrochromic active layer/gel electrolyte/ITO.Based on the device (A), the electrochromic properties of3TB,3TB-2CHO,3TB-3CN,6TB,3TPA and3TPA-3CN were investigated, and the electrochromic mechanism werediscussed.3TB-2CHO,3TB-3CN and3TPA-3CN solid films based on device (A) showedreversible, clear color change from colorless, colorless and yellow to brown, orange red andorange red on electrochemical doping and dedoping, respectively.The own colors of3TB,6TB and3TPA solid films were colourless, pale yellow and paleyellow, respectively. The3TB,6TB and3TPA solid films exhibited irreversible color changefrom own colors to light blue, dark blue and dark blue on electrochemical doping, respectively.These processes of color change were irreversible. That is to say, on electrochemical dedoping,the color couldn’t change from light blue, dark blue and dark blue to pale yellow, yellow andyellow, respectively. The reason may be that the3TB,6TB and3TPA didn’t have substituentgroup on-position of thiophene, they were electropolymerized on electrochemical dopingfirstly, and produced polymers, poly(3TB), poly(6TB) and poly(3TPA). The own colors ofpoly(3TB), poly(6TB) and poly(3TPA) solid films were pale yellow, yellow and yellow,respectively.3TB-3CN and3TPA-3CN solid films based on device (B) showed reversible, clear colorchange, which were similar to that of device(A). We also found that the driving voltage in thegel electrolyte were higher than that of in the electrolyte solution. It is possible that there aregood ionic conduction channel in the electrolyte solution. The photovoltaic properties of3TB,3TB-2CHO,3TB-3CN,3TB-2COOH,6TB and3TPAwere investigated. Using ITO glass and Al as electrode,3TB,3TB-2CHO,3TB-3CN,3TB-2COOH,6TB or3TPA as electron donor materials, PTCDA as electron acceptor material,the organic p-n heterojunction photovoltaic devices were fabricated. The structure of thesedevices was: ITO/star-shaped oligothiophene derivatives/PTCDA/Al. Introducing differentsubstituent group in position of thiophene could significantly affect the performance of thephotovoltaic device. Device ITO/3TB-2CHO/PTCDA/Al had the highest short-circuit currentin all devices, which may be due to the formation of intermolecular hydrogen bonds between-CHO group and PTCDA. The intermolecular hydrogen bonds could promot interfacialelectron transfer. Using3TPA or3TPA-3CN as electron donor materials, the devices hadhigher open circuit voltage and good photovoltaic properties. The introduction of cyano groupinto3TPA could enhance the photoelectric conversion efficiency greatly. The results showedthat the star-shaped oligothiophenes derivatives with nitrogen as the “core” was a kind ofpotential photovoltaic material.