Marina Ide

Thienoisoindigo-based low-band gap polymers for organic electronic devices

We synthesized a series of new low-band gap donor–acceptor copolymers containing 4,4′-bis(alkyl)-[6,6′-bithieno[3,2-b]pyrrolylidene]-5,5′(4H,4′H)-dione. This acceptor unit, so-called dithienoketopyrrole (DTKP), is an analogue of isoindigo, the phenyl rings of which are replaced by thiophenes. Donor moieties such as benzodithiophene, cyclopentadithiophene, fluorene, and dithienothiophene are polymerized with DTKP in an alternating fashion by Stille or Suzuki–Miyaura coupling methods. Exceedingly low-band gaps (Eg = 1.0–1.6 eV) were achieved in these copolymers through internal charge transfer interactions between the donor and acceptor moieties. The structural, photophysical, and electrochemical properties of the resultant copolymers were characterized, and field-effect transistor (FET) mobilities were measured. The copolymers showed electronic absorption spectra extending to the near infrared region (600–1400 nm) with absorption maxima at 745–971 nm, along with a low-lying LUMO of −3.8 eV. Density functional theory (DFT) calculation indicated high planarity for the copolymer backbone when compared to that of its phenyl-isoindigo counterparts. FET hole mobilities on the order of 10−4 to 10−3 cm2 V−1 s−1 were obtained, demonstrating a feasibility to use them in organic photovoltaic cells.

A Versatile Approach to Organic Photovoltaics Evaluation Using White Light Pulse and Microwave Conductivity

State-of-the-art low band gap conjugated polymers have been investigated for application in organic photovoltaic cells (OPVs) to achieve efficient conversion of the wide spectrum of sunlight into electricity. A remarkable improvement in power conversion efficiency (PCE) has been achieved through the use of innovative materials and device structures. However, a reliable technique for the rapid screening of the materials and processes is a prerequisite toward faster development in this area. Here we report the realization of such a versatile evaluation technique for bulk heterojunction OPVs by the combination of time-resolved microwave conductivity (TRMC) and submicrosecond white light pulse from a Xe-flash lamp. Xe-flash TRMC allows examination of the OPV active layer without requiring fabrication of the actual device. The transient photoconductivity maxima, involving information on generation efficiency, mobility, and lifetime of charge carriers in four well-known low band gap polymers blended with phenyl-C(61)-butyric acid methyl ester (PCBM), were confirmed to universally correlate with the PCE divided by the open circuit voltage (PCE/V(oc)), offering a facile way to predict photovoltaic performance without device fabrication.

Molecular engineering of benzothienoisoindigo copolymers allowing highly preferential face-on orientations

Orientation of conjugated polymers is increasingly important in organic photovoltaics (OPV) to achieve high power conversion efficiency (PCE). The optimized orientation of conjugated backbones for photo-generated charge carriers in OPV devices is in contrast to organic semiconductor devices, demanding new strategies to control and realize face-on orientation of conjugated systems onto substrates. Here we report new conjugated polymers composed of electron-accepting benzothienoisoindigo (BTIDG), an asymmetric unit of isoindigo and thienoisoindigo. BTIDG was coupled with weakly electron-donating thiazolothiazole or benzobisthiazole, concurrently leading to moderate optical band gaps (1.41–1.52 eV) and the highest occupied molecular orbital (−5.35 to −5.50 eV). The alkylthiophene spacer between BTIDG and the donor unit provided a marked control over the orientation of polymers, among which the degree of face-on orientation as high as 95% was revealed by grazing incidence X-ray diffraction. The maximum PCE was improved up to 4.2% using the system with [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM). We present a useful basis on the structure (orientation)–property (OPV output) relationship to lay down new guidelines for the design of efficient solar cell materials.

A ternary blend of a polymer, fullerene, and insulating self-assembling triptycene molecules for organic photovolatics

Interest in ternary blend organic photovoltaics (OPVs) has been triggered by improved power conversion efficiency (PCE), simple fabrication, and a variety of material combinations. They typically consist of a p-type donor, an n-type fullerene, and a secondary p-type donor, where the two donors undergo complementary photoabsorption to increase the photocurrent. In contrast to this general strategy, we have incorporated propeller-shaped, high-bandgap triptycene (TP) into a bulk heterojunction (BHJ) composed of a conjugated polymer (P3HT, PTB7, and PffBT4T) and methano[60]fullerene (PCBM). The TP molecules self-organize into two-dimensional (2D) layers. Based on their energy levels, these layers are not expected to trap charge carriers. The performance of these OPV devices is discussed in conjunction with film morphology, transient photoconductivity, mobility, crystalline structure, and surface free energy. Herein, we demonstrate that the PCE of the crystalline polymer (PffBT4T):PCBM with 5 wt% TP is slightly improved (maximum: 9.41%, average: 8.96 ± 0.23%) without the use of a solvent additive, along with much enhanced long-term stability. This is presumably due to the presence of the TP sheet that acts as a nucleation agent and facilitates the crystallization of the polymer. The features of the TP scaffold give rise to a high-performance, additive-free ternary BHJ. Thus, further exploration through chemical functionalization of TP is warranted.

Blackening of aza-BODIPY analogues by simple dimerization: panchromatic absorption of a pyrrolopyrrole aza-BODIPY dimer

Dimerization of the so-called pyrrolopyrrole aza-BODIPY, which is a new class of aza-BODIPY analogues exhibiting intense absorption and emission in the visible (Vis) and near infrared (NIR) region, via a bithienyl linkage led to the creation of a novel black dye with dual emission and panchromatic absorption properties in the Vis/NIR region. The role of the linkage in the dye was unambiguously elucidated by comparison of the optical properties with those of its biphenyl-linked counterpart.

Exploring Alkyl Chains in Benzobisthiazole-Naphthobisthiadiazole Polymers: Impact on Solar-Cell Performance, Crystalline Structures, and Optoelectronics

The shapes and lengths of the alkyl chains of conjugated polymers greatly affect the efficiencies of organic photovoltaic devices. This often results in a trade-off between solubility and self-organizing behavior; however, each material has specific optimal chains. Here we report on the effect of alkyl side chains on the film morphologies, crystallinities, and optoelectronic properties of new benzobisthiazole-naphthobisthiadiazole (PBBT-NTz) polymers. The power conversion efficiencies (PCEs) of linear-branched and all-branched polymers range from 2.5% to 6.6%; the variations in these PCEs are investigated by atomic force microscopy, two-dimensional X-ray diffraction (2D-GIXRD), and transient photoconductivity techniques. The best-performing linear-branched polymer, bearing dodecyl and decyltetradecyl chains (C12-DT), exhibits nanometer-scale fibers along with the highest crystallinity, comprising predominant edge-on and partial face-on orientations. This morphology leads to the highest photoconductivity and the longest carrier lifetime. These results highlight the importance of long alkyl chains for inducing intermolecular stacking, which is in contrast to observations made for analogous previously reported polymers.

Insight into the energy loss in organic solar cells based on benzotrithiophene copolymers: A dark current analysis at low temperature

Owing to the formation of the charge transfer (CT) state, the open-circuit voltage (Voc) of organic photovoltaic (OPV) devices commonly suffers an energy loss of 0.8–1.3 eV from the effective bandgap. Benzotrithiophene (BTT)-based low-bandgap polymers that we have recently reported showed deep HOMO levels (−5.4 to −5.6 eV) and moderate optical bandgaps of 1.7–1.8 eV, which resulted in high Vocs of 0.78–0.98 V and relatively low energy losses when blended with methano[60]fullerene (PCBM). Here, we report the temperature-dependent dark current analysis of organic solar cells of BTT copolymers:PCBM blends. Shockley diode analyses revealed the dominant contribution of CT energy and concomitant pre-exponential factor of dark saturation current density associated with charge recombination. The findings could establish a fundamental aspect to draw a design rule in BTT-based polymers towards their evolutions in OPV devices.

Organic photovoltaics of diketopyrrolopyrrole copolymers with unsymmetric and regiorandom configuration of the side units

Diketopyrrolopyrrole (DPP) is a representative electron acceptor incorporated into narrow-bandgap polymers for organic photovoltaic cells (OPV). Commonly, identical aromatic units are attached to the sides of the DPP unit, forming symmetric DPP polymers. Herein we report the synthesis and characterization of DPP copolymers consisting of unsymmetric configurations of the side aromatics. The unsymmetric DPP copolymer with thienothiophene and benzene side moieties exhibits good solubility owing to the twisted dihedral angle at benzene and regiorandom configuration. A significant shallowing of the highest occupied molecular orbital level is observed in accordance with the electron-donating nature of the side units (benzene, thiophene, and thienothiophene). The overall power conversion efficiencies of the unsymmetric DPPs (2.3–2.4%) are greater than that of the centrosymmetric analogue (0.45%), which is discussed in view of bulk heterojunction morphology, polymer crystallinity, and space-charge-limited current mobilities. This comparative study highlights the effect of unsymmetric design on the molecular stacking and OPV performance of DPP copolymers.