3,6-Carbazole Incorporated into Poly[9,9-dioctylfluorene-<i>alt</i>-(bisthienyl)benzothiadiazole]s Improving the Power Conversion Efficiency

A novel concept of D–A-type copolymers based on fluorene polymer incorporated with 3,6-carbazole unit enhances the device performance for organic photovoltaic cells. <b>P­(F</b>_<b>45</b><b>C</b>_<b>5</b><b>-DTBT)</b>, incorporating 5 mol % 3,6-carbazole into <b>P­(2,7F-DTBT)</b>, shows an almost 2-fold improvement (5.1%) in power conversion efficiency relative to <b>P­(2,7F-DTBT)</b> (2.6%). This results is ascribed to the good balance between electron and hole mobilities in the devices (μ_e/μ_h ∼ 1.8 for <b>P­(F</b>_<b>45</b><b>C</b>_<b>5</b><b>-DTBT)</b> vs 152 for <b>P­(2,7F-DTBT)</b>), and the formation of a nanoscale morphology in the blend of the copolymer and [6,6]-phenyl C71-butyric acid methyl ester (PC₇₁BM)

Amorphous Thieno[3,2‑<i>b</i>]thiophene and Benzothiadiazole Based Copolymers for Organic Photovoltaics

Three types of amorphous thienothiophene (TT)-benzothiadiazole (BT) based copolymers (<b>PFTTBT</b>) were synthesized by incorporating alkyl-substituted fluorene moieties as a third component in the polymer backbone. Their optical, electrochemical, morphological, and photovoltaic properties were examined by a comparison with those of a crystalline TT-BT derivative (<b>PTTBT14</b>). <b>PTTBT14</b> was reported to have a high hole mobility (0.26 cm²/(V s)) due to the pronounced interchain ordering but poor photovoltaic power conversion efficiency (PCE) of 2.4–2.6% was reported due to excessively strong self-interactions with poor miscibility with fullerene structures. By incorporating fluorene units, the UV–vis spectra showed an increased bandgap (∼1.9 eV) with the disappearance of the packing-originated shoulder peak, and the valence band decreased compared to crystalline <b>PTTBT14</b>. The amorphous <b>PFTTBT</b> polymers showed substantially improved photovoltaic properties compared to <b>PTTBT14</b>, even though they showed poor hole mobility (∼10^–6 cm²/(V s)) and fill factor. The optimal devices were achieved by blending with excess PC₇₁BM (polymer:PC₇₁BM = 1:4 by weight), showing little improvement in the thermal and additive treatments. Under simulated solar illumination of AM 1.5 G, the best PCE of 6.6% was achieved for a <b>PFehTTBT</b>:PC₇₁BM device with an open-circuit voltage of 0.92 V, a short-circuit current of 15.1 mA/cm², and a fill factor of 0.48. These results suggest that it is useful to disrupt partially the interchain organizations of excessively crystalline polymers, enabling fine-control of intermolecular ordering and the morphological properties (i.e., miscibility with fullerene derivatives, etc.) to utilize the advantages of both crystalline and amorphous materials for further improving PCE of polymer solar cells

Effects of Cyano-Substituents on the Molecular Packing Structures of Conjugated Polymers for Bulk-Heterojunction Solar Cells

The molecular packing structures of two conjugated polymers based on alkoxy naphthalene, one with cyano-substituents and one without, have been investigated to determine the effects of electron-withdrawing cyano-groups on the performance of bulk-heterojunction solar cells. The substituted cyano-groups facilitate the self-assembly of the polymer chains, and the cyano-substituted polymer:PC₇₁BM blend exhibits enhanced exciton dissociation to PC₇₁BM. Moreover, the electron-withdrawing cyano-groups lower the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels of the conjugated polymer, which leads to a higher open circuit voltage (<i>V</i>_OC) and a lower energy loss during electron transfer from the donor to the acceptor. A bulk-heterojunction device fabricated with the cyano-substituted polymer:PC₇₁BM blend has a higher <i>V</i>_OC (0.89 V), a higher fill factor (FF) (51.4%), and a lower short circuit current (<i>J</i>_SC) (7.4 mA/cm²) than that of the noncyano-substituted polymer:PC₇₁BM blend under AM 1.5G illumination with an intensity of 100 mW cm^–2. Thus, the cyano-substitution of conjugated polymers may be an effective strategy for optimizing the domain size and crystallinity of the polymer:PC₇₁BM blend, and for increasing <i>V</i>_OC by tuning the HOMO and LUMO energy levels of the conjugated polymer

All-Small-Molecule Solar Cells Incorporating NDI-Based Acceptors: Synthesis and Full Characterization

A series of naphthalene diimide (NDI)-based small molecules were synthesized as nonfullerene acceptors and incorporated in all-small-molecule solar cells. Three NDI-based small molecules, NDICN-T, NDICN-BT, and NDICN-TVT, were designed with different linkers between two NDI units to induce the different conjugation length and modulate the geometric structures of the NDI dimers. The small NDI-based dimer electron acceptors with slip-stacked structures that facilitate π–π stacking interactions and/or hinder excessive aggregation exhibited different morphological behaviors, such as miscibility or crystallinity in bulk heterojunction blends with 7,7′-(4,4-bis­(2-ethylhexyl)-4<i>H</i>-silolo­[3,2-<i>b</i>:4,5-<i>b</i>′]­dithiophene-2,6-diyl)­bis­(6-fluoro-4-(5′-hexyl-[2,2′-bithiophen]-5-yl)­benzo­[<i>c</i>]­[1,2,5]­thiadiazole) (DTS-F) electron donors. The photovoltaic devices prepared with NDICN-TVT gave the highest power conversion efficiency (PCE) of 3.01%, with an open-circuit voltage (<i>V</i>_oc) of 0.75 V, a short-circuit current density (<i>J</i>_sc) of 7.10 mA cm^–2, and a fill factor of 56.2%, whereas the DTS-F:NDICN-T and DTS-F:NDICN-BT devices provided PCEs of 1.81 and 0.13%, respectively. Studies of the charge-generation properties, charge-transfer dynamics, and charge-transport properties for understanding the structure–property relations revealed that DTS-F:NDICN-TVT blend films with well-developed domains and well-ordered crystalline structures performed well, whereas an excessive miscibility between DTS-F and NDICN-BT disrupted the crystallinity of the material and yielded a poor device performance

Alkyl Chain Length Dependence of the Field-Effect Mobility in Novel Anthracene Derivatives

We report six asymmetric alkylated anthracene-based molecules with different alkyl side chain lengths for use in organic field-effect transistors (OFETs). Alkyl side chains can potentially improve the solubility and processability of anthracene derivatives. The crystallinity and charge mobility of the anthracene derivatives may be improved by optimizing the side chain length. The highest field-effect mobility of the devices prepared here was 0.55 cm²/(V s), for 2-(<i>p</i>-pentylphenylethynyl)­anthracene (PPEA). The moderate side chain length appeared to be optimal for promoting self-organization among asymmetric anthracene derivatives in OFETs, and was certainly better than the short or long alkyl side chain lengths, as confirmed by X-ray diffraction measurements