Dithienopicenocarbazole-Based Acceptors for Efficient Organic Solar Cells with Optoelectronic Response Over 1000 nm and an Extremely Low Energy Loss

Star-Shaped and Fused Electron Acceptors based on C3h -Symmetric Coplanar Trindeno[1, 2-b: 4, 5-b': 7, 8-b'']trithiophene Core for Non-Fullerene Solar Cells

Two new star-shaped and fused electron acceptors, TITT-3IC and TITT-3ICF have been designed and synthesized, which consist of a C_3h -symmetric coplanar trindeno[1, 2-b: 4, 5-b': 7, 8-b'']trithiophene (TITT) as the central core and 3-(dicyanomethylidene)indan-1-one and 2-(5,6-difluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene) malononitrile as the peripheral electron-withdrawing groups, respectively. With the large coplanar configuration and electron-rich nature of π-conjugated backbone, these two acceptors exhibit strong intermolecular charge transfer absorption in the region of 500-650 nm with the optical band gaps around 1.9 eV. Relative to TITT-3IC, TITT-3ICF shows the downshifted LUMO level and the slightly redshifted absorption with the higher molar extinction coefficient due to the stronger electron-withdrawing effect of fluorination. When blending with PTB7-Th, the TITT-3ICF-based device displays a higher power conversion efficiency (PCE) of 4.26 % than the TITT-3IC-based device (PCE=3.87 %). Comparing with the TITT-3IC-based device, the increased short circuit current (J_SC ) and fill factor (FF) are responsible for the higher PCE value of the TITT-3ICF-based device, which benefits from its strong and redshifted absorption for light harvesting and proper phase separation morphology for effective exciton dissociation and charge transport. This work demonstrates that as an alternative electron-donating core, TITT will be promising in designing star-shaped non-fullerene materials.

Fused selenophene-thieno[3,2-b]thiophene–selenophene (ST)-based narrow-bandgap electron acceptor for efficient organic solar cells with small voltage loss

A novel NIR-light-absorbing fused-ring electron acceptor, STIC, was successfully developed for organic solar cells and achieved a PCE of 9.68%.

Double acceptor block-based copolymers for efficient organic solar cells: side-chain and π-bridge engineered high open-circuit voltage and small driving force

Different from the traditional concept of donor–acceptor (D–A) structures, the newly reported acceptor₁–acceptor₂ (A₁–A₂) type copolymers has been regarded as an efficient approach to boost the open circuit voltage (V_oc).

Efficient as-cast semi-transparent organic solar cells with efficiency over 9% and a high average visible transmittance of 27.6%

Efficient as-cast ST-OSCs based on a trifluorinated polymer donor PBFTT and a tetrachlorinated NF-acceptor IT-4Cl were fabricated and a high efficiency of 9.1% was achieved under a high AVT of 27.6% in the photopic region.

Chlorinated Wide-Bandgap Donor Polymer Enabling Annealing Free Nonfullerene Solar Cells with the Efficiency of 11.5

Substituting the hydrogen atoms on the conjugated side chain of a wide-bandgap polymer J52 with chlorine atoms can simultaneously increase the J_sc, V_oc, and FF of nonfullerene OSCs, leading to an efficiency boost from 3.78 to 11.53%, which is among the highest efficiencies for as-cast OSCs reported to date. To illustrate the impressive 3-fold PCE enhancement, the chlorination effect on the optical properties and energy levels of polymers, film morphology, and underlying charge dynamics is systematically investigated. Grazing incidence wide-angle X-ray scattering studies show that chlorinated J52-2Cl exhibits strong molecule aggregation, the preferred face-on orientation, and enhanced intermolecular π-π interactions, hence increasing the charge carrier mobility by 1 order of magnitude. Moreover, chlorination modifies the miscibility between the donor and acceptor and consequently optimizes the phase separation morphology of J52-2Cl:ITIC blend films. These results highlight chlorination as a promising approach to achieve highly efficient as-cast OSCs without any extra treatment.

Near-Infrared Electron Acceptors with Fluorinated Regioisomeric Backbone for Highly Efficient Polymer Solar Cells

Solar photon-to-electron conversion with polymer solar cells (PSCs) has experienced rapid development in the recent few years. Even so, the exploration of molecules and devices in efficiently converting near-infrared (NIR) photons into electrons remains critical, yet challenging. Herein presented is a family of near-infrared nonfullerene acceptors (NIR NFAs, T1-T4) with fluorinated regioisomeric A-Aπ-D-Aπ-A backbones for constructing efficient single-junction and tandem PSCs with photon response up to 1000 nm. It is found that the tuning of the regioisomeric bridge (Aπ) and fluoro (F)-substituents on a molecular skeleton strongly influences the backbone conformation and conjugation, leading to the optimized optoelectronic and stable stacking of resultant NFAs, which eventually impacts the performance of derived PSCs. In PSCs, the proximal NFAs with varied F-atoms (T1-T3) mostly outperform than that of distal NFA (T4). Notably, single-junction PSC with PTB7-Th:T2 blend can reach 10.87% power conversion efficiency (PCE), after implementing a solvent additive to improve blend morphology. Moreover, efficient tandem PSCs are fabricated through integrating such NIR cells with mediate bandgap nonfullerene-based subcells, to achieve a PCE of 14.64%. The results reveal the structural design of organic semiconductor and device with improved photovoltaic performance.

Chlorine Atom-Induced Molecular Interlocked Network in a Non-Fullerene Acceptor

The differences between the introduction of chlorine and fluorine atoms to small-molecule acceptors were deeply investigated. From the single-crystal structures of three molecules, the Cl-substitution intervention into the molecular configuration and packing mainly lies in three aspects as follows: single molecule configuration, one direction of the intermolecular arrangement, and three-dimensional (3D) molecular packing. First, the introduction of the chlorine atom in IDIC-4Cl leads to a more planar molecular configuration than IDIC-4H and IDIC-4F because of the formation of a molecular interlocked network induced by the strong Cl···S intermolecular interactions. Second, IDIC-4Cl shows the closest π-π stacking distance and the smallest dihedral angle (0°) between adjacent molecules to form ideal J-aggregation, which should be beneficial for charge transportation between different connected molecules in this direction. Finally, the interlocked interactions between Cl and S atoms lead to a highly ordered 3D molecular packing, in which the end groups will form an ideal overlapped packing among different molecules, whereas the other two analogues with H or F show less ordered packing of their 1,1-dicyanomethylene-3-indanone ending groups. Organic solar cells based on IDIC-4Cl show the highest power conversion efficiency (PCE) of 9.24%, whereas the PCEs of IDIC-4H- and IDIC-4F-based devices are 4.57 and 7.10%, respectively.

Near-Infrared Ternary Tandem Solar Cells

The paucity of near-infrared (NIR) organic materials with high absorption at long wavelengths, combined with large diffusion lengths and charge mobilities, is an impediment to progress in achieving high-efficiency organic tandem solar cells. Here a subcell is employed within a series tandem stack that comprises a solution-processed ternary blend of two NIR-absorbing nonfullerene acceptors and a polymer donor combined with a small-molecular-weight, short-wavelength fullerene-based subcell grown by vacuum thermal evaporation. The ternary cell achieves a power conversion efficiency of 12.6 ± 0.3% with a short-circuit current of 25.5 ± 0.3 mA cm^-2 , an open-circuit voltage of 0.69 ± 0.01 V, and a fill factor of 0.71 ± 0.01 under 1 sun, AM 1.5G spectral illumination. The success of this device is a result of the nearly identical offset energies between the lowest unoccupied molecular orbitals (LUMOs) of the donors with the highest occupied molecular orbital (HOMO) of the acceptor, resulting in a high open-circuit voltage. A tandem structure with an antireflection coating combining these subcells demonstrates a power conversion efficiency of 15.4 ± 0.3%.

Fluorinated Thieno[2',3':4,5]benzo[1,2-d][1,2,3]triazole: New Acceptor Unit To Construct Polymer Donors

A new acceptor unit, fluorinated thieno[2',3':4,5]benzo[1,2-d][1,2,3]triazole (fBTAZT), has been designed and synthesized to build two donor-acceptor (D-A) copolymers with the none/fluorinated benzodithiophene (BDT) unit, which have been applied as the electron-donating material with ITIC as an electron-accepting material to fabricate the nonfullerene polymer solar cells (PSCs). It is found that fluorination at the BTAZT unit and BDT unit exerts a significant influence on photophysical properties and photovoltaic performances of the PSCs. As a result, when the fluorine atom is introduced both into the BTAZT unit and the side-chain thiophene ring of the BDT unit, the corresponding polymer PfBTAZT-fBDT exhibits deeper highest occupied molecular orbital-lowest unoccupied molecular orbital energy level and shows stronger interchain interaction with a little broad and red-shift absorption and high charge mobilities as well as good phase-separated morphologies, thus leading to higher power conversion efficiency of 6.59% in nonfullerene PSCs compared with another polymer PfBTAZT-BDT without F atom at the BDT unit, indicating that fBTAZT can be acted as a medium strong organic acceptor to build D-A polymer donor for high efficient PSCs.

Small-Molecule Electron Acceptors for Efficient Non-fullerene Organic Solar Cells

The development of organic electron acceptor materials is one of the key factors for realizing high performance organic solar cells. Compared to traditional fullerene acceptor materials, non-fullerene electron acceptors have attracted much attention due to their better optoelectronic tunabilities and lower cost as well as higher stability. Non-fullerene organic solar cells have recently experienced a rapid increase with power conversion efficiency of single-junction devices over 14% and a bit higher than 15% for tandem solar cells. In this review, two types of promising small-molecule electron acceptors are discussed: perylene diimide based acceptors and acceptor(A)-donor(D)-acceptor(A) fused-ring electron acceptors, focusing on the effects of structural modification on absorption, energy levels, aggregation and performances. We strongly believe that further development of non-fullerene electron acceptors will hold bright future for organic solar cells.

Efficient Non-fullerene Organic Solar Cells Enabled by Sequential Fluorination of Small-Molecule Electron Acceptors

Three small-molecule non-fullerene electron acceptors containing different numbers of fluorine atoms in their end groups were designed and synthesized. All three acceptors were found to exhibit relatively narrow band gaps with absorption profiles extending into the near-infrared region. The fluorinated analog exhibited enhanced light-harvesting capabilities, which led to improved short-circuit current densities. Moreover, fluorination improved the blend film morphology and led to desirable phase separation that facilitated exciton dissociation and charge transport. As a result of these advantages, organic solar cells based on the non-fullerene acceptors exhibited clearly improved short-circuit current densities and power conversion efficiencies compared with the device based on the non-fluorinated acceptor. These results suggest that fluorination can be an effective approach for the molecular design of non-fullerene acceptors with near-infrared absorption for organic solar cells.

Organic and solution-processed tandem solar cells with 17.3% efficiency

Although organic photovoltaic (OPV) cells have many advantages, their performance still lags far behind that of other photovoltaic platforms. A fundamental reason for their low performance is the low charge mobility of organic materials, leading to a limit on the active-layer thickness and efficient light absorption. In this work, guided by a semi-empirical model analysis and using the tandem cell strategy to overcome such issues, and taking advantage of the high diversity and easily tunable band structure of organic materials, a record and certified 17.29% power conversion efficiency for a two-terminal monolithic solution-processed tandem OPV is achieved.

Long-Lived, Non-Geminate, Radiative Recombination of Photogenerated Charges in a Polymer/Small-Molecule Acceptor Photovoltaic Blend

Minimization of open-circuit-voltage ( V_OC) loss is required to transcend the efficiency limitations on the performance of organic photovoltaics (OPV). We study charge recombination in an OPV blend comprising a polymer donor with a small molecule nonfullerene acceptor that exhibits both high photovoltaic internal quantum efficiency and relatively high external electroluminescence quantum efficiency. Notably, this donor/acceptor blend, consisting of the donor polymer commonly referred to as PCE10 with a pseudoplanar small molecule acceptor (referred to as FIDTT-2PDI) exhibits relatively bright delayed photoluminescence on the microsecond time scale beyond that observed in the neat material. We study the photoluminescence decay kinetics of the blend in detail and conclude that this long-lived photoluminescence arises from radiative nongeminate recombination of charge carriers, which we propose occurs via a donor/acceptor CT state located close in energy to the singlet state of the polymer donor. Additionally, crystallographic and spectroscopic studies point toward low subgap disorder, which could be beneficial for low radiative and nonradiative losses. These results provide an important demonstration of photoluminescence due to nongeminate charge recombination in an efficient OPV blend, a key step in identifying new OPV materials and materials-screening criteria if OPV is to approach the theoretical limits to efficiency.

Sulfanilic Acid Pending on a Graphene Scaffold: Novel, Efficient Synthesis and Much Enhanced Polymer Solar Cell Efficiency and Stability Using It as a Hole Extraction Layer

In this contribution, we describe a novel, facile, and scalable methodology for high degree functionalization toward graphene by the reaction between bulk graphite fluoride and in situ generated amine anion. Using this, the rationally designed sulfanilic acid pending on a graphene scaffold (G-SO₃H), a two-dimensional (2D) π-conjugated counterpart of poly(styrenesulfonate), is available. Combined reliable characterizations demonstrate that a very large quantity of sulfanilic blocks are linked to graphene through the foreseen substitution of carbon-fluorine units and an unexpected reductive defluorination simultaneously proceeds during the one-step reaction, endowing the resultant G-SO₃H with splendid dispersity in various solvents and film-forming property via the former, and with recovered 2D π-conjugation via the latter. Besides, the work function of G-SO₃H lies at -4.8 eV, well matched with the P3HT donor. Awarded with these fantastic merits, G-SO₃H behaves capable in hole collection and transport, indicated by the enhanced device efficiency and stability of polymer solar cells (PSCs) based on intensively studied P3HT:PCBM blends as an active layer. In particular, comparison with conventional poly(3,4-ethylenedioxythiophene) doped with poly(styrenesulfonate) and recently rising and shining graphene oxide, G-SO₃H outperforms above 17 and 24%, respectively, in efficiency. More impressively, when these three unencapsulated devices are placed in a N₂-filled glovebox at around 25 °C for 7 weeks, or subject to thermal treatment at 150 °C for 6 h also in N₂ atmosphere, or even rudely exposed to indoor air, G-SO₃H-based PSCs exhibit the best stability. These findings enable G-SO₃H to be a strongly competitive alternative of the existing hole extraction materials for PSC real-life applications.

A Wide Band Gap Polymer with a Deep Highest Occupied Molecular Orbital Level Enables 14.2% Efficiency in Polymer Solar Cells

To simultaneously achieve low photon energy loss ( E_loss) and broad spectral response, the molecular design of the wide band gap (WBG) donor polymer with a deep HOMO level is of critical importance in fullerene-free polymer solar cells (PSCs). Herein, we developed a new benzodithiophene unit, i.e., DTBDT-EF, and conducted systematic investigations on a WBG DTBDT-EF-based donor polymer, namely, PDTB-EF-T. Due to the synergistic electron-withdrawing effect of the fluorine atom and ester group, PDTB-EF-T exhibits a higher oxidation potential, i.e., a deeper HOMO level (ca. -5.5 eV) than most well-known donor polymers. Hence, a high open-circuit voltage of 0.90 V was obtained when paired with a fluorinated small molecule acceptor (IT-4F), corresponding to a low E_loss of 0.62 eV. Furthermore, side-chain engineering demonstrated that subtle side-chain modulation of the ester greatly influences the aggregation effects and molecular packing of polymer PDTB-EF-T. With the benefits of the stronger interchain π-π interaction, the improved ordering structure, and thus the highest hole mobility, the most symmetric charge transport and reduced recombination are achieved for the linear decyl-substituted PDTB-EF-T (P2)-based PSCs, leading to the highest short-circuit current density and fill factor (FF). Due to the high Flory-Huggins interaction parameter (χ), surface-directed phase separation occurs in the P2:IT-4F blend, which is supported by X-ray photoemission spectroscopy results and cross-sectional transmission electron microscope images. By taking advantage of the vertical phase distribution of the P2:IT-4F blend, a high power conversion efficiency (PCE) of 14.2% with an outstanding FF of 0.76 was recorded for inverted devices. These results demonstrate the great potential of the DTBDT-EF unit for future organic photovoltaic applications.