Abstract
Single‐material organic solar cells (SMOSCs) promise several advantages with respect to prospective applications in printed large‐area solar foils. Only one photoactive material has to be processed and the impressive thermal and photochemical long‐term stability of the devices is achieved. Herein, a novel structural design of oligomeric donor–acceptor (D–A) dyads 1–3 is established, in which an oligothiophene donor and fullerene acceptor are covalently linked by a flexible spacer of variable length. Favorable optoelectronic, charge transport, and self‐organization properties of the D–A dyads are the basis for reaching power conversion efficiencies up to 4.26% in SMOSCs. The dependence of photovoltaic and charge transport parameters in these ambipolar semiconductors on the specific molecular structure is investigated before and after post‐treatment by solvent vapor annealing. The inner nanomorphology of the photoactive films of the dyads is analyzed with transmission electron microscopy (TEM) and grazing‐incidence wide‐angle X‐ray scattering (GIWAXS). Combined theoretical calculations result in a lamellar supramolecular order of the dyads with a D–A phase separation smaller than 2 nm. The molecular design and the precise distance between donor and acceptor moieties ensure the fundamental physical processes operative in organic solar cells and provide stabilization of D–A interfaces.
Highlights
Introduction properties in DA systems is an important aspect in the overall structural design for tuning and controlling distance-dependentCovalently linked donor–acceptor (D–A) dyads, triads, and multi- charge transfer (CT), whereby intermolecular interactions and self-assembly deterads have been extensively investigated in the context of mine charge separation in thin films
The general synthetic route to the targeted D–A dyads with varying alkyl ester chain lengths, which was already established for dyad 2,[14] is shown in Scheme 1 and detailed synthetic procedures are given in Supporting Information (SI)
The solvent vapor annealing (SVA) treatment of the dyads led to different morphological changes: in the case of dyad 3 slightly bigger structures of 10–20 nm diameter and some few crystallites of about 60 nm are visible in the topography image and a strong phase contrast, ΔPh 1⁄4 115, concomitant with a bimodal histogram are shown in the corresponding phase image (Figure 12f )
Summary
The general synthetic route to the targeted D–A dyads with varying alkyl ester chain lengths, which was already established for dyad 2,[14] is shown in Scheme 1 and detailed synthetic procedures are given in Supporting Information (SI). Hydroxylalkylated DTPs are rarely described in the literature, and Lutkenhaus et al recently published the synthesis of DTP-propan-1-ol.[18] We reacted 3,3’-dibromo-2,2’-bithiophene 1 with α,ω-(triisopropylsilyloxy) alkylamines 2 and 3 in twofold Pd-catalyzed BuchwaldHartwig aminations to provide N-functionalized DTPs 4–5 in yields of 87–96% (Scheme 1). Twofold lithiation of DTPs 4–5 with n-BuLi and subsequent stannylation with trimethyltinchloride gave metallated intermediates 6–7 in nearly quantitative yields. These were coupled with brominated bithiophene aldehyde 8[19] in Stille-type cross-coupling reactions to furnish oligomeric dialdehydes 9–10 in yields of 72–86%. Alcohols 13–14 were esterified with fullerene carboxylic acid PC61BA 15 to targeted D–A dyads 1 and 3 in yields between 62% and 69% by in situ activation with 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) under basic conditions. The structures of all novel precursors and dyads were confirmed by 1H- and 13C-NMR spectroscopy (Figure S2–S14, Supporting Information) and high-resolution mass spectra (HRMS) (Figure S15–S21, Supporting Information)
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