Examining Structure–Property–Function Relationships in Thiophene, Selenophene, and Tellurophene Homopolymers

Abstract

Heavy atom main group element-containing conjugated polymers have attracted increasing attention in recent years. The synthesis of these compounds is generally involved, and little is known about their optoelectronic device performance. Here we examine the relationship between polymer structure and optoelectronic behavior in a series of chalcogenophene homopolymers of thiophene, selenophene, and tellurophene with well-matched molecular weights, dispersity, and regioregularity. We employ fast and slow drying device preparations to study the effect of polymer–fullerene separation on charge separation and collection in canonical bulk heterojunction photovoltaic cells. In both preparations, increasing heteroatom size leads to larger proportions of finely mixed polymer–fullerene domains. Differences in polymer–fullerene separation between preparations result in the formation of optimal morphologies in selenophene and tellurophene devices with little impact on thiophene devices. We then use planar heterojunction devices to directly examine the effects of heteroatom substitution on charge transport and charge generation and find that in the absence of polymer–fullerene mixing, devices exhibit similar diode behavior. We further demonstrate that ultrafast decay pathways unique to heavy heteroatom-containing polymers are apparent in both planar and bulk heterojunctions and thus not dependent on polymer–fullerene mixing or polymer assembly. This work directly examines the role of heteroatom substitution in defining the photovoltaic performance of conjugated homopolymers. Through single-atom substitution we are able to significantly modify polymer assembly, mixing, and optoelectronic properties. Specific emphasis on tellurophene polymers reveals relationships between polymer structure and properties that are not apparent in more traditional light-atom chalcogenophenes such as thiophene and selenophene.

Applying Heteroatom Substitution in Organic Photovoltaics

“…Recent work from our group resulted in the successful preparation of poly(3‐alkyltellurophenes) with both high molecular weights and narrow dispersities . This has enabled a comprehensive preliminary assessment of the differences in optoelectronic properties between group 16 homopolymers and has revealed unique behavior specific to the heaviest analogue . Ongoing research comparing the S, Se and Te homopolymers in transistors, thermoelectrics and OPVs reveals that the heaviest heteroatom dramatically improves charge transport, alters polymer assembly and results in ultrafast behavior less apparent in polythiophenes and selenophenes .…”

“…Heteroatom substitution has been successfully used to tune absorbance, carrier mobility, and fluorescence intensity and has been demonstrated to dramatically alter packing and mixing . While polythiophene and selenophene derivatives have been a central focus of OPV research polytellurophenes have only recently emerged as a promising new analogue …”

Redox chemistry of π-extended tellurophenes

“…Most of these reactions involve transition metal complexes, however, recent emphasis has been placed on main group inorganic compounds due to their lightweight and more abundant nature [1][2][3][4][5][6] . In this context, tellurophene, a telluriumcontaining five-membered aromatic ring 7,8 , is of particular interest given that its π-delocalized nature resembles more traditional aromatic heterocycles and that it incorporates a metalloid that is capable of several stable oxidation states 9-11 . In the last decade, the incorporation of the tellurophene-motif into π-extended molecules and polymers has been recognized as a promising design for electronic devices, such as thin film transistors, light emitters, photovoltaics, and thermoelectrics [12][13][14][15][16][17][18][19][20][21][22][23][24][25] . In comparison with thiophene-containing or selenophenecontaining counterparts, characteristic properties derived from Te atom have been reported.…”

Self‐Organization and Charge Transport Properties of Selenium and Tellurium Analogues of Polythiophene

“…We first calculated ionization potential and electron affinities of corresponding tetramers. For all polymers, the variation of IPs and EAs is small (Table S3, Supporting Information) as confirmed experimentally by cyclic voltammetry . Hence the change in mobility cannot be attributed to a drastically different HOMO level or different injection barrier (Scheme S2, Supporting Information).…”