One-Step Synthesis of Precursor Oligomers for Organic Photovoltaics: A Comparative Study between Polymers and Small Molecules

Highly Efficient Mixed Conduction in a Fused Oligomer n-Type Organic Semiconductor Enabled by 3D Transport Pathways

Tailoring organic semiconductors to facilitate mixed conduction of ionic and electronic charges when interfaced with an aqueous media has spurred many recent advances in organic bioelectronics. The field is still restricted, however, by very few n-type (electron-transporting) organic semiconductors with adequate performance metrics. Here, a new electron-deficient, fused polycyclic aromatic system, TNR, is reported with excellent n-type mixed conduction properties including a µC* figure-of-merit value exceeding 30 F cm^-1 V^-1 s^-1 for the best performing derivative. Comprising three naphthalene bis-isatin moieties, this new molecular design builds on successful small-molecule mixed conductors; by extending the molecular scaffold into the oligomer domain, good film-forming properties, strong π-π interactions, and consequently excellent charge-transport properties are obtained. Through judicious optimization of the side chains, the linear oligoether and branched alkyl chain derivative bgTNR is obtained which shows superior mixed conduction in an organic electrochemical transistor configuration including an electron mobility around 0.3 cm² V^-1 s^-1 . By optimizing the side chains, the dominant molecular packing can be changed from a preferential edge-on orientation (with high charge-transport anisotropy) to an oblique orientation that can support 3D transport pathways which in turn ensure highly efficient mixed conduction properties across the bulk semiconductor film.

Novel Oligomer Enables Green Solvent Processed 17.5% Ternary Organic Solar Cells: Synergistic Energy Loss Reduction and Morphology Fine-Tuning

The large non-radiative recombination is the main factor that limits state-of-the-art organic solar cells (OSCs). In this work, two novel structurally similar oligomers (named 5BDTBDD and 5BDDBDT) with D-A-D-A-D and A-D-A-D-A configuration are synthesized for high-performance ternary OSCs with low energy loss. As third components, these PM6 analogue oligomers effectively suppress the non-radiative recombination in OSCs. Although the highest occupied molecular orbital (HOMO) levels of 5BDTBDD and 5BDDBDT are higher than that of PM6, the oligomers enabled ultra-high electroluminescence quantum efficiency (EQE_EL ) of 0.05% and improved V_OC , indicating suppressing non-radiative recombination overweighs the common belief of deeper HOMO requirement in third component selection. Moreover, the different compatibility of 5BDTBDD and 5BDDBDT with PM6 and BTP-BO4Cl fine-tunes the active layer morphology with synergistic effects. The ternary devices based on PM6:5BDTBDD:BTPBO4Cl and PM6:5BDDBDT:BTP-BO4Cl achieve a significantly improved PCEs of 17.54% and 17.32%, representing the state-of-the art OSCs processed by green solvent of o-xylene. The strategy using novel oligomer as third component also has very wide composition tolerance in ternary OSCs. This is the first work that demonstrates novel structurally compatible D-A type oligomers are effective third components, and provides new understanding of synergetic energy loss mechanisms towards high performance OSCs.

Designing of near-infrared sensitive asymmetric small molecular donors for high-efficiency organic solar cells

Herein, we have designed four small molecular donors (SMDs) with Donor–Acceptor–Acceptor (D–Á–A) backbone having different acceptor units for highly efficient organic solar cells (OSCs). The specific molecular modeling has been made by replacing the additional acceptor unit (A) of recently synthesized TPA-DAA-MDN molecule (R) by employing different highly efficient acceptor units in order to improve the photovoltaic performances of the molecules. A theoretical approach (DFT and TD-DFT) has been applied to investigate the photophysical, opto-electronic and photovoltaic parameters of the designed molecules (DAA1–DAA4) and compared with the reference molecule (R). The red-shifting absorption of SMDs is the most important factor for highly efficient OSCs. Our all formulated molecules showed a red shifted absorption spectrum and also exhibit near IR sensitivity. Acceptor unit modification of R molecule causes reduction in HOMO-LUMO energy gap; therefore, all designed molecules offer better opto-electronic properties as compared to R molecule. A variety of certain critical factors essential for efficient SMDs like frontier molecular orbitals (FMOs), absorption maxima, dipole moment, exciton binding energy along with transition density matrix, excitation energy, open circuit voltages and charge mobilities of (DAA1–DAA4) and R have also been investigated. Generally, low values of reorganizational energy (hole and electron) offer high charge mobility and our all designed molecules are enriched in this aspect. High open circuit voltage values, low excitation energies, large dipole moment values indicate that our designed SMDs are suitable candidates for high-efficiency OSCs. Furthermore, conceptualized molecules are superior and thus are suggested to experimentalist for out-looking future progresses of highly efficient OSCs devices.

Design, Synthesis and Computational Study of Fluorinated Quinoxaline-Oligothiophene-based Conjugated Polymers with Broad Spectral Coverage

Donor-acceptor (D-A) copolymers typically show two absorption peaks in the visible region, flanking a valley region of limited absorptivity. One strategy for more panchromatic light harvesting is to incorporate side-groups orthogonal to the polymer backbone, which enable 2D π conjugation and can give rise to additional absorption peaks. Here we design and synthesize two D-A polymers which both carry a fluorinated quinoxaline acceptor unit, but while P1 includes a benzodithiophene donor moiety with thiophene side-groups (2D-BDT), the P2 polymer lacks 2D conjugation in its simpler pentathiophene donor segment. The P1 polymer consequently shows an atypical absorption profile with more panchromatic absorption with no apparent valley in the spectrum. In order to understand the structure-electronic relations, the optical and electrochemical properties were predicted using a previously developed computational approach. The predicted optical properties show very good agreement with the experimental results. Solar cells made from P1 show a short-circuit current more than twice as large as P2, attributed to its enhanced spectral coverage. However, poor fill factors limit the preliminary power conversion efficiencies to 3.3 % for P1 and 1.0 % for P2 as blended with PCBM[70] in a 1 : 1.5 (w/w) ratio.

Revisiting the Optimal Nano-Morphology: Towards Amorphous Organic Photovoltaics

Organic photovoltaic cells commonly use an active layer with a polycrystalline bulk heterojunction. However, for simplifying the fabrication process, it may be worthwhile to use an amorphous active layer to lessen the burden on processing to achieve optimal performance. While polymers can adopt amorphous phases, molecular glasses, small molecules that can readily form glassy phases and do not crystallize over time, offer an appealing alternative, being monodisperse species. Our group has developed a series of reactive molecular glasses that can be covalently bonded to chromophores to form glass-forming adducts, and this strategy has been used to synthesize glass-forming donor and acceptor materials. Herein, the results of devices incorporating these materials in either partially or fully amorphous active layers are summarized. Additionally, these molecular glasses can be used as ternary components in crystalline systems to enhance efficiency without perturbing the morphology.

Quinoxaline-Containing Nonfullerene Small-Molecule Acceptors with a Linear A2-A1-D-A1-A2 Skeleton for Poly(3-hexylthiophene)-Based Organic Solar Cells

We used the quinoxaline (Qx) unit to design and synthesize two nonfullerene small-molecule acceptors of Qx1 and Qx1b with an A₂-A₁-D-A₁-A₂ skeleton, where indacenodithiophene (IDT), Qx, and rhodanine (R) were adopted as the central donor (D), bridge acceptors (A₁), and terminal acceptors (A₂), respectively. Qx1 and Qx1b contain different side chains of 4-hexylphenyl and octyl in the central IDT segment to modulate the properties of final small molecules. Both small molecules show good thermal stability, high solubility, and strong and broad absorption spectra with optical band gaps of 1.74 and 1.68 eV, respectively. Qx1 and Qx1b exhibit the complementary absorption spectra with the classic poly(3-hexylthiophene) (P3HT) and the high-lying lowest unoccupied molecular orbital energy levels of -3.60 and -3.66 eV, respectively. Polymer solar cells based on P3HT:Qx1 showed a high open-circuit voltage ( V_oc) of 1.00 V and a power conversion efficiency (PCE) of 4.03%, whereas P3HT:Qx1b achieved a V_oc of 0.95 V and a PCE of 4.81%. These results demonstrate that the Qx unit is also an effective building block to construct promising n-type nonfullerene small molecules to realize a relatively high V_oc and PCE for P3HT-based solar cells.

Efficient small molecule photovoltaic donor based on 2,3-diphenyl-substituted quinoxaline core for solution-processed organic solar cells

An excellent PCE of 6.25% was achieved based on a novel OSM (TPACN)2Qx containing 2,3-diphenyl-substituted quinoxaline (Qx) as electrophilic core.

Exploring the origin of high optical absorption in conjugated polymers

The specific optical absorption of an organic semiconductor is critical to the performance of organic optoelectronic devices. For example, higher light-harvesting efficiency can lead to higher photocurrent in solar cells that are limited by sub-optimal electrical transport. Here, we compare over 40 conjugated polymers, and find that many different chemical structures share an apparent maximum in their extinction coefficients. However, a diketopyrrolopyrrole-thienothiophene copolymer shows remarkably high optical absorption at relatively low photon energies. By investigating its backbone structure and conformation with measurements and quantum chemical calculations, we find that the high optical absorption can be explained by the high persistence length of the polymer. Accordingly, we demonstrate high absorption in other polymers with high theoretical persistence length. Visible light harvesting may be enhanced in other conjugated polymers through judicious design of the structure.