Cross-Linkable Molecular Hole-Transporting Semiconductor for Solid-State Dye-Sensitized Solar Cells

In this study, we investigate the use of a cross-linkable organosilane semiconductor, 4,4′-bis­[(<i>p</i>-trichlorosilylpropylphenyl)­phenylamino]­biphenyl (TPDSi₂), as a hole-transporting material (HTM) for solid-state dye-sensitized solar cells (ssDSSCs) using the standard amphiphilic Z907 dye which is compatible with organic HTM deposition. The properties and performance of the resulting cells are then compared and contrasted with the ones based on poly­(3-hexylthiophene) (P3HT), a conventional polymeric HTM, but with rather limited pore-filling capacity. When processed under N₂, TPDSi₂ exhibits excellent infiltration into the mesoporous TiO₂ layer and thus enables the fabrication of relatively thick devices (∼5 μm) for efficient photon harvesting. When exposed to ambient atmosphere (RH_amb ∼ 20%), TPDSi₂ readily undergoes cross-linking to afford a rigid, thermally stable hole-transporting layer. In addition, the effect of <i>tert</i>-butylpyridine (TBP) and lithium bis­(trifluoromethylsulfonyl)­imide salt (Li-TFSI) additives on the electrochemical properties of these HTMs is studied via a combination of cyclic voltammetry (CV) and ultraviolet photoemission spectroscopy (UPS) measurements. The results demonstrate that the additives significantly enhance the space charge limited current (SCLC) mobilities for both the P3HT and TPDSi₂ HTMs and induce a shift in the TPDSi₂ Fermi level, likely a p-doping effect. These combined effects of improved charge transport characteristics for the TPDSi₂ devices enhance the power conversion efficiency (PCE) by more than 2-fold for ssDSSCs

Buta-1,3-diyne-Based π‑Conjugated Polymers for Organic Transistors and Solar Cells

We report the synthesis and characterization of new alkyl-substituted 1,4-di­(thiophen-2-yl)­buta-1,3-diyne (R-DTB) donor building blocks, based on the −CC–CC– conjugative pathway, and their incorporation with thienyl-diketopyrrolo­pyrrole (R′-TDPP) acceptor units into π-conjugated PTDPP-DTB polymers (<b>P1</b>–<b>P4</b>). The solubility of the new polymers strongly depends on the DTB and DPP solubilizing (R and R′, respectively) substituents. Thus, solution processable and high molecular weight PDPP-DTB polymers are achieved for <b>P3</b> (R = <i>n</i>-C₁₂H₂₅, R′ = 2-butyloctyl) and <b>P4</b> (R = 2-ethylhexyl, R′ = 2-butyloctyl). Systematic studies of <b>P3</b> and <b>P4</b> physicochemical properties are carried using optical spectroscopy, cyclic voltammetry, and thermal analysis, revealing characteristic features of the dialkynyl motif. For the first time, optoelectronic devices (OFETs, OPVs) are fabricated with 1,3-butadiyne containing organic semiconductors. OFET hole mobilities and record OPV power conversion efficiencies for acetylenic organic materials approach 0.1 cm²/(V s) and 4%, respectively, which can be understood from detailed thin-film morphology and microstructural characterization using AFM, TEM, XRD, and GIWAXS methodologies. Importantly, DTB-based polymers (<b>P3</b> and <b>P4</b>) exhibit, in addition to stabilization of frontier molecular orbitals and to −CC–CC– relief of steric torsions, discrete morphological pliability through thermal annealing and processing additives. The advantageous materials properties and preliminary device performance reported here demonstrate the promise of 1,3-butadiyne-based semiconducting polymers

Slip-Stacked Perylenediimides as an Alternative Strategy for High Efficiency Nonfullerene Acceptors in Organic Photovoltaics

Perylenediimide (PDI)-based acceptors offer a potential replacement for fullerenes in bulk-heterojunction (BHJ) organic photovoltaic cells (OPVs). The most promising efforts have focused on creating twisted PDI dimers to disrupt aggregation and thereby suppress excimer formation. Here, we present an alternative strategy for developing high-performance OPVs based on PDI acceptors that promote slip-stacking in the solid state, thus preventing the coupling necessary for rapid excimer formation. This packing structure is accomplished by substitution at the PDI 2,5,8,11-positions (“headland positions”). Using this design principle, three PDI acceptors, <i>N</i>,<i>N</i>-bis­(n-octyl)-2,5,8,11-tetra­(<i>n</i>-hexyl)-PDI (<b>Hexyl-PDI</b>), <i>N</i>,<i>N</i>-bis­(n-octyl)-2,5,8,11-tetraphenethyl-PDI (<b>Phenethyl-PDI</b>), and <i>N</i>,<i>N</i>-bis­(n-octyl)-2,5,8,11-tetraphenyl-PDI (<b>Phenyl-PDI</b>), were synthesized, and their molecular and electronic structures were characterized. They were then blended with the donor polymer <b>PBTI3T</b>, and inverted OPVs of the structure ITO/ZnO/Active Layer/MoO₃/Ag were fabricated and characterized. Of these, 1:1 <b>PBTI3T</b>:<b>Phenyl-PDI</b> proved to have the best performance with <i>J</i>_sc = 6.56 mA/cm², <i>V</i>_oc = 1.024 V, FF = 54.59%, and power conversion efficiency (PCE) = 3.67%. Devices fabricated with <b>Phenethyl-PDI</b> and <b>Hexyl-PDI</b> have significantly lower performance. The thin film morphology and the electronic and photophysical properties of the three materials are examined, and although all three materials undergo efficient charge separation, <b>PBTI3T</b>:<b>Phenyl-PDI</b> is found to have the deepest LUMO, intermediate crystallinity, and the most well-mixed domains. This minimizes geminate recombination in <b>Phenyl-PDI</b> OPVs and affords the highest PCE. Thus, slip-stacked PDI strategies represent a promising approach to fullerene replacements in BHJ OPVs

Systematic Investigation of Organic Photovoltaic Cell Charge Injection/Performance Modulation by Dipolar Organosilane Interfacial Layers

With the goal of investigating and enhancing anode performance in bulk-heterojunction (BHJ) organic photovoltaic (OPV) cells, the glass/tin-doped indium oxide (ITO) anodes are modified with a series of robust silane-tethered bis­(fluoroaryl)­amines to form self-assembled interfacial layers (IFLs). The modified ITO anodes are characterized by contact angle measurements, X-ray reflectivity, ultraviolet photoelectron spectroscopy, X-ray photoelectron spectroscopy, grazing incidence X-ray diffraction, atomic force microscopy, and cyclic voltammetry. These techniques reveal the presence of hydrophobic amorphous monolayers of 6.68 to 9.76 Å thickness, and modified anode work functions ranging from 4.66 to 5.27 eV. Two series of glass/ITO/IFL/active layer/LiF/Al BHJ OPVs are fabricated with the active layer = poly­(3-hexylthiophene):phenyl-C₇₁-butyric acid methyl ester (P3HT:PC₇₁BM) or poly­[[4,8-bis­[(2-ethylhexyl)­oxy]­benzo­[1,2-b:4,5-b’]­dithiophene-2,6-diyl]­[3-fluoro-2-[(2-ethylhexyl)-carbonyl]­thi-eno­[3,4-b]­thiophenediyl]]:phenyl-C₇₁-butyric acid methyl ester (PTB7:PC₇₁BM). OPV analysis under AM 1.5G conditions reveals significant performance enhancement versus unmodified glass/ITO anodes. Strong positive correlations between the electrochemically derived heterogeneous electron transport rate constants (<i>k</i>_s) and the device open circuit voltage (<i>V</i>_oc), short circuit current (<i>J</i>_sc), hence OPV power conversion efficiency (PCE), are observed for these modified anodes. Furthermore, the strong functional dependence of the device response on <i>k</i>_s increases as greater densities of charge carriers are generated in the BHJ OPV active layer, and is attributable to enhanced anode carrier extraction in the case of high-<i>k</i>_s IFLs

Metal-Free Tetrathienoacene Sensitizers for High-Performance Dye-Sensitized Solar Cells

A new series of metal-free organic chromophores (TPA-TTAR-A (<b>1</b>), TPA-T-TTAR-A (<b>2</b>), TPA-TTAR-T-A (<b>3</b>), and TPA-T-TTAR-T-A (<b>4</b>)) are synthesized for application in dye-sensitized solar cells (DSSC) based on a donor-π-bridge-acceptor (D−π–A) design. Here a simple triphenylamine (TPA) moiety serves as the electron donor, a cyanoacrylic acid as the electron acceptor and anchoring group, and a novel tetrathienoacene (TTA) as the π-bridge unit. Because of the extensively conjugated TTA π-bridge, these dyes exhibit high extinction coefficients (4.5–5.2 × 10⁴ M^–1 cm^–1). By strategically inserting a thiophene spacer on the donor or acceptor side of the molecules, the electronic structures of these TTA-based dyes can be readily tuned. Furthermore, addition of a thiophene spacer has a significant influence on the dye orientation and self-assembly modality on TiO₂ surfaces. The insertion of a thiophene between the π-bridge and the cyanoacrylic acid anchoring group in TPA-TTAR-T-A (dye <b>3</b>) promotes more vertical dye orientation and denser packing on TiO₂ (molecular footprint = 79 Ų), thus enabling optimal dye loading. Using dye <b>3</b>, a DSSC power conversion efficiency (PCE) of 10.1% with <i>V</i>_oc = 0.833 V, <i>J</i>_sc = 16.5 mA/cm², and FF = 70.0% is achieved, among the highest reported to date for metal-free organic DSSC sensitizers using an I^–/I₃^– redox shuttle. Photophysical measurements on dye-grafted TiO₂ films reveal that the additional thiophene unit in dye <b>3</b> enhances the electron injection efficiency, in agreement with the high quantum efficiency