Charge separation in low-bandgap polymer/fullerene blends

This thesis addresses charge separation and charge recombination in different, mainly low bandgap, polymer/fullerene blend films and their relation to device performance. Charge separation and recombination was studied as a function of variables including the difference in the LUMO levels of the polymer and fullerene, the polymer/fullerene blend ratio, the presence of a fluorine atom on the polymer backbone and the use of a bulky fullerene acceptor (Indene-C60-trisadduct, ICTA). A key focus of the thesis is on the impact of film microstructural differences upon charge generation and recombination kinetics. Charge generation and recombination was studied via transient absorption spectroscopy (TAS) with time resolutions from femtoseconds to microseconds. In Chapter 1, an introduction to the field is presented. Basic concepts of polymer solar cells and the steps of light-to-electrical energy conversion are included. The chapter focuses on the current discussions on charge generation, separation and recombination and their relationships with other parameters such as material energetics and morphology. In Chapter 2, the experimental methodologies are presented. A description of the materials used, the techniques used to prepare the samples, and the mainly optical techniques used to study them: steady state photoluminescence (PL), TAS (fs and microsecond), X-ray diffraction (XRD) and device characterization. Chapter 3 to 6 present the results of each project. In Chapter 3, the role of the driving energy for charge separation (ECS) for low bandgap DPP-based polymers is addressed. A s-TAS characterization of DPP-based polymer/fullerene blends is presented, and the yield of charges correlated with the experimentally obtained ECS. The correlation was then extended to other low-bandgap polymers and the trend compared with that obtained for larger bandgap polymers. Chapter 4 deals with the effect of DPPTT-T/PC70BM blend ratio upon the film photophysics. With PL quenching and fs-TAS studies, it is demonstrated that the limitations in the performance of DPPTT-T polymer mainly come from an incomplete exciton quenching for all the compositions studied. The study is in agreement with morphology probes including transmission electron and atomic force microscopies, as well as with the changes in crystallinity, as observed by XRD. Chapter 5 deals with the effect of polymer backbone fluorination on a low-bandgap polymer (PGeDTBT). PL quenching and fs to s TAS data is presented and correlated with structural analyses and theoretical calculations to compare the properties of non-fluorinated and fluorinated version of the polymers. It was found that charge generation seems to be equally efficient, despite the lower driving energy for charge separation (ECS) in the fluorinated polymer. A four-fold slowing down in non-geminate recombination was also observed upon fluorination, correlated with a larger polymer tendency to aggregate, thus demonstrating its multiple effects on material properties and photovoltaic behaviour. Chapter 6 deals with the effect of mixed and “flatter” interfaces upon charge separation. XRD data are presented to show the contrast in intercalation between the polymer and the acceptors (PC70BM and ICTA). These results are correlated with fs-TAS data to show the change in the regime of recombination: while the highly intercalated blends present a high predominance of geminate recombination, the blends with ICTA predominantly present non-geminate recombination. Finally in Chapter 7, the conclusions of every chapter are summarized. A general discussion on the relationship of the conclusions is given and the areas where further research is needed are discussed.Open Acces

Charge Separation in Intermixed Polymer:PC₇₀BM Photovoltaic Blends: Correlating Structural and Photophysical Length Scales as a Function of Blend Composition

A key challenge in achieving control over photocurrent generation by bulk-heterojunction organic solar cells is understanding how the morphology of the active layer impacts charge separation and in particular the separation dynamics <i>within</i> molecularly intermixed donor–acceptor domains versus the dynamics <i>between</i> phase-segregated domains. This paper addresses this issue by studying blends and devices of the amorphous silicon–indacenodithiophene polymer SiIDT-DTBT and the acceptor PC₇₀BM. By changing the blend composition, we modulate the size and density of the pure and intermixed domains on the nanometer length scale. Laser spectroscopic studies show that these changes in morphology correlate quantitatively with the changes in charge separation dynamics on the nanosecond time scale and with device photocurrent densities. At low fullerene compositions, where only a single, molecularly intermixed polymer–fullerene phase is observed, photoexcitation results in a ∼ 30% charge loss from geminate polaron pair recombination, which is further studied via light intensity experiments showing that the radius of the polaron pairs in the intermixed phase is 3–5 nm. At high fullerene compositions (≥67%), where the intermixed domains are 1–3 nm and the pure fullerene phases reach ∼4 nm, the geminate recombination is suppressed by the reduction of the intermixed phase, making the fullerene domains accessible for electron escape

Fused Dithienogermolodithiophene Low Band Gap Polymers for High-Performance Organic Solar Cells without Processing Additives

We report the synthesis of a novel ladder-type fused ring donor, dithienogermolodithiophene, in which two thieno­[3,2-<i>b</i>]­thiophene units are held coplanar by a bridging dialkyl germanium. Polymerization of this extended monomer with <i>N</i>-octylthienopyrrolodione by Stille polycondensation afforded a polymer, <b>pDTTG-TPD</b>, with an optical band gap of 1.75 eV combined with a high ionization potential. Bulk heterojunction solar cells based upon <b>pDTTG-TPD</b>:PC₇₁BM blends afforded efficiencies up to 7.2% without the need for thermal annealing or processing additives

Isostructural, Deeper Highest Occupied Molecular Orbital Analogues of Poly(3-hexylthiophene) for High-Open Circuit Voltage Organic Solar Cells

We present the synthesis and characterization of two novel thiazole-containing conjugated polymers (<b>PTTTz</b> and <b>PTTz</b>) that are isostructural to poly­(3-hexylthiophene) (P3HT). The novel materials demonstrate optical and morphological properties almost identical to those of P3HT but with HOMO and LUMO levels that are up to 0.45 eV deeper. An intramolecular planarizing nitrogen–sulfur nonbonding interaction is observed, and its magnitude and origin are discussed. Both materials demonstrate significantly greater open circuit voltages than P3HT in bulk heterojunction solar cells. <b>PTTTz</b> is shown to be an extremely versatile donor polymer that can be used with a wide variety of fullerene acceptors with device efficiencies of up to 4.5%. It is anticipated that this material could be used as a high-open circuit voltage alternative to P3HT in organic solar cells