PCDTBT: From Polymer Photovoltaics to Light-Emitting Diodes by Side-Chain-Controlled Luminescence

Poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole) (PCDTBT) is a copolymer composed of alternating thiophene-benzothiadiazole-thiophene (TBT) and carbazole (Cbz) repeat units widely used for stable organic photovoltaics. However, the solubility of PCDTBT is limited, which decreases polymer yield and makes synthesis and purification tedious. Here, we introduce a strategy to increase both solubility and luminescence by the statistical incorporation of additional hexyl side chains at the TBT unit (hex-TBT). An increasing amount of hex-TBT as comonomer from 0 to 100% enhances solubility, leads to backbone torsion, and causes a blue-shift in the absorption and emission spectra. While photovoltaic performance of both PCDTBT:P3HT blends and PCDTBT:PC$_{71}$BM blends decreases with increasing content of hex-TBT due to weaker and blue-shifted absorption, the luminescence properties can be systematically improved. Both photo- and electroluminescence (PL and EL) quantum efficiencies increase with increasing hex-TBT content. We further demonstrate solution-processed red polymer light-emitting diodes based on fully hexylated PCDTBT showing an EL quantum efficiency enhancement of up to 7 times and 2 orders of magnitude enhancement of brightness compared to standard PCDTBT. Fully hexylated PCDTBT shows a peak external quantum efficiency of 1.1% and a peak brightness of 2500 cd/m2Financial support from the Fonds der Chemischen Industrie (FCI), the Research Innovation Fund of the University of Freiburg and the DFG (SPP1355) is greatly acknowledged. F.L. greatly acknowledges the EPSRC for funding. D.D. acknowledges the Department of Physics (University of Cambridge) and the KACST-Cambridge University Joint Centre of Excellence for support

On the Effect of Prevalent Carbazole Homocoupling Defects on the Photovoltaic Performance of PCDTBT:PC17_{17}BM Solar Cells

The photophysical properties and solar cell performance of the classical donor–acceptor copolymer PCDTBT (poly($\textit{N}$-9′-heptadecanyl-2,7-carbazole-$\textit{alt}$ -5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole))) in relation to unintentionally formed main chain defects are investigated. Carbazole–carbazole homocouplings (Cbz hc) are found to significant extent in PCDTBT made with a variety of Suzuki polycondensation conditions. Cbz hc vary between 0 and 8 mol% depending on the synthetic protocol used, and are quantified by detailed nuclear magnetic resonance spectroscopy including model compounds, which allows to establish a calibration curve from optical spectroscopy. The results are corroborated by extended time-dependent density functional theory investigations on the structural, electronic, and optical properties of regularly alternating and homocoupled chains. The photovoltaic properties of PCDTBT:fullerene blend solar cells significantly depend on the Cbz hc content for constant molecular weight, whereby an increasing amount of Cbz hc leads to strongly decreased short circuit currents J$_{sc}$. With increasing Cbz hc content, J$_{sc}$decreases more strongly than the intensity of the low energy absorption band, suggesting that small losses in absorption cannot explain the decrease in J$_{sc}$ alone, rather than combined effects of a more localized LUMO level on the TBT unit and lower hole mobilities found in highly defective samples. Homocoupling-free PCDTBT with optimized molecular weight yields the highest efficiency up to 7.2% without extensive optimization.F.L., M.S., and R.F. gratefully acknowledge the EPSRC for funding. M.S. thanks the University of Freiburg (Innovationsfond Forschung) and the DFG for funding (SPP 1355). D.F. acknowledges the Alexander von Humboldt foundation for a postdoctoral research fellowship. A.D.Z.M. and C.M. thank the Swedish Research Council for funding

Hexylation Stabilises Twisted Backbone Configurations in the Prototypical Low-Bandgap Copolymer PCDTBT.

Publication status: PublishedConjugated donor-acceptor copolymers hold great potential as materials for high-performance organic photovoltaics, organic transistors and organic thermoelectric devices. Their low optical bandgap is achieved by alternation of donor and acceptor moieties along the polymer chain, leading to a pronounced charge-transfer character of electronic excitations. However, the influence of appended side chains and of chemical defects of the backbone on their photophysical and conformational properties remains largely unexplored on the level of individual chains. Here, we employ room temperature single-molecule photoluminescence spectroscopy on four compounds based on the prototypical copolymer PCDTBT with systematically changed chemical structure. Our results show that an increasing density of statistically added hexyl chains to the TBT comonomer distorts the molecular conformation, likely through the increase of average dihedral angles along the backbone. We find that, although the conformation becomes more twisted with high hexyl density, the side chains appear to stabilize the backbone in this twisted conformation. In addition, we demonstrate that homocoupling defects along the backbone barely influence the PL spectra of single chains, and thus intra-chain electronic properties

Benzoyl side-chains push the open-circuit voltage of PCDTBT/PCBM solar cells beyond 1 V

The synthesis, characterization and solar cell performance of PCDTBT and its highly soluble analogue hexyl-PCDTBT with cross-conjugated benzoyl moieties at the carbazole comonomer are presented. Through the use of both model reactions and time-controlled microwave-assisted Suzuki polycondensation, the base-induced cleavage of the benzoyl group from the polymer backbone has been successfully suppressed. Compared to the commonly used symmetrically branched alkyl motif, the benzoyl substituent lowers the energy levels of PCDTBT as well as the band gap, and consequently increases energy of the charge transfer state in blends with PC71BM. As a result, photovoltaic diodes with high-open circuit voltage of above 1 V are realized

High molecular weight mechanochromic spiropyran main chain copolymers via reproducible microwave-assisted Suzuki polycondensation

Suzuki-Miyaura polycondensation (SPC) is widely used to prepare a variety of copolymers for a broad range of applications. Although SPC protocols are often used in many instances, the limits of this method and issues of molecular weight reproducibility are not often looked at in detail. By using a spiropyran-based (SP) mechanochromic copolymer, we present an optimized protocol for the microwave-assisted synthesis of a mechanochromic, alternating copolymer P(SP-alt-C-10) via SPC that allows the reproduction of molecular weight distributions. Several parameters such as microwave power, temperature, stoichiometry, and ligand are screened, leading to molecular weights up to M-w similar to 174 kg mol(-1). The process of optimization is guided by NMR end group analysis which shows that dehalogenation, oxidative deborylation and SP cleavage are the limiting factors that impede further increase of molar mass, while other classical side reactions such as protiodeborylation are not observed. Embossing films of P(SP-alt-C-10) yields the colored merocyanine (MC) copolymer P(MC-alt-C-10) that undergoes a thermally facilitated back reaction to P(SP-alt-C-10). DFT suggests that the barrier of the SP -> MC transition has two contributions, with the first one being related to the color change and the second one to internal bond reorganizations. The barrier height is 1.5 eV, which suggests that the ease of the thermally facilitated back reaction is either due to residual energy stored in the deformed polymer matrix, or arises from an MC isomer that is not in the thermodynamically most stable state