Research progress and application of high efficiency organic solar cells based on benzodithiophene donor materials

In recent decades, the demand for clean and renewable energy has grown increasingly urgent due to the irreversible alteration of the global climate change. As a result, organic solar cells (OSCs) have emerged as a promising alternative to address this issue. In this review, we summarize the recent progress in the molecular design strategies of benzodithiophene (BDT)-based polymer and small molecule donor materials since their birth, focusing on the development of main-chain engineering, side-chain engineering and other unique molecular design paths. Up to now, the state-of-the-art power conversion efficiency (PCE) of binary OSCs prepared by BDT-based donor materials has approached 20%. This work discusses the potential relationship between the molecular changes of donor materials and photoelectric performance in corresponding OSC devices in detail, thereby presenting a rational molecular design guidance for stable and efficient donor materials in future.

Progress and Future Potential of All-Small-Molecule Organic Solar Cells Based on the Benzodithiophene Donor Material

Organic solar cells have obtained a prodigious amount of attention in photovoltaic research due to their unique features of light weight, low cost, eco-friendliness, and semitransparency. A rising trend in this field is the development of all-small-molecules organic solar cells (ASM-OSCs) due to their merits of excellent batch-to-batch reproducibility, well-defined structures, and simple purification. Among the numerous organic photovoltaic (OPV) materials, benzodithiophene (BDT)-based small molecules have come to the fore in achieving outstanding power conversion efficiency (PCE) and breaking the 17% efficiency barrier in single-junction OPV devices, indicating the significant potential of this class of materials in commercial photovoltaic applications. This review specially focuses on up-to-date information about improvements in BDT-based ASM-OSCs since 2011 and provides an outlook on the most significant challenges that remain in the field. We believe there will be more exciting BDT-based photovoltaic materials and devices developed in the near future.

Efficient carbazole-based small-molecule organic solar cells with an improved fill factor

In this study, a new acceptor–donor–acceptor (A–D–A) small molecule, DI3TCz, with carbazole as the central unit and 1,3-indanedione as the end group, was designed and synthesized for application in organic solar cells. In contrast to the molecule DR3TCz with rhodanine as end groups, DI3TCz exhibited deep a LUMO energy level and a nearly unchanged HOMO energy level with a narrow optical band gap of 1.75 eV and red shifted absorption. Compared with DR3TCz, the DI3TCz device showed a PCE of 6.46% with a remaining high V_oc value of 0.97 V, improved J_sc of 10.40 mA cm⁻¹ and a notable FF of 0.65, which is the highest PCE value reported to data for carbazole-based small molecules OPVs.

In this study, a new acceptor–donor–acceptor (A–D–A) small molecule, DI3TCz, with carbazole as the central unit and 1,3-indanedione as the end group, was designed and synthesized for application in organic solar cells.

Ternary Organic Solar Cells with Coumarin7 as the Donor Exhibiting Greater Than 10% Power Conversion Efficiency and a High Fill Factor of 75

Ternary bulk heterojunction (BHJ) is a brilliant photovoltaic technology for improving the performance of organic solar cells (OSCs), because the light absorption range can be significantly extended by using multiple donors or acceptor materials. In this paper, coumarin7 (C7), a small organic molecule typical led used in organic light-emitting diodes, was initially exploited as second electron-donor component in ternary bulk heterojunction OSCs along with conventional blend system spolythieno[3,4-b]-thiophene/benzodithiophene(PTB7) and [6,6]-phenyl-C71 -butyric acid methyl(PC₇₁ BM). A champion PCE value of 10.28% was realized in the ternary OSCs when incorporated with 10 wt % C7 doping ratio in the donors, corresponding to about 35% enhancement compared with the PTB7:PC₇₁BM-based OSCs, a high fill factor (FF) of 75.03%, a short-circuit currentdensity (J_sc) of 18.72 mA cm^-2 and an open-circuit voltage (V_oc) of 0.73 V. The enhanced performance of the ternary OSCs can be attributed to the simultaneous improvement of the FF and the J_sc. In addition to extended light absorption, a perfect nanofiber filament active layer morphology is obtained due to the good compatibility between C7 and PTB7, which facilitates the balance of charge transportation and the suppression of charge recombination. This investigation suggests that coumarin derivatives, which have completely different structure with polymer donors, can also be used to fabricate ternary solar cells and have the potential applications to obtain amazing performance after further device engineering and optimization.

High photovoltaic performance of as-cast devices based on new quinoxaline-based donor–acceptor copolymers

Two new quinoxaline-based copolymers with different alkylthio side chains show high photovoltaic performance in as-cast devices.

Developing high-performance small molecule organic solar cells via a large planar structure and an electron-withdrawing central unit

We designed and synthesized a new small molecule donor material named DR3TBDD using an electron-withdrawing unit BDD as the central building block. A PCE of 9.53% with a highV_ocof around 1 V was achieved.

A D–π–A1–π–A2 push–pull small molecule donor for solution processed bulk heterojunction organic solar cells

Herein, benzothiadiazole (BTD), as an acceptor A1, has been used as a backbone to link triphenylamine (TPA) as donor and naphthalimide (NPI) as acceptor (A2) moieties through ethylene linkers to design a small molecule.

An oligothiophene chromophore with a macrocyclic side chain: synthesis, morphology, charge transport, and photovoltaic performance

Molecular chromophores tend to form crystals beyond nanometer sizes upon thermal aging. A novel ring-protection structure has shown promise to solve morphological stability problem of solution-processed small molecule solar cell devices.

D–A–D–π–D–A–D type diketopyrrolopyrrole based small molecule electron donors for bulk heterojunction organic solar cells

Two organic small molecules based on diketopyrrolopyrrole (DPP) units having a D–A–D–π–D–A–D structure denoted as DPP-DPP and DPPTDPP were synthesized.

Dicyanoquinodimethane-substituted benzothiadiazole for efficient small-molecule solar cells

Dicyanoquinodimethane and tetracyanobutadiene functionalized benzothiadiazoles were synthesized and investigated for small molecule solar cells.

Subtle Balance Between Length Scale of Phase Separation and Domain Purification in Small-Molecule Bulk-Heterojunction Blends under Solvent Vapor Treatment

A series of solvents with different solubilities for DR3TBDTT and PC71 BM, and different boiling points, is used for solvent vapor annealing (SVA) treatment to systematically investigate the solvent-morphology-performance relationship. The presence of solvent molecules inside bulk-heterojunction (BHJ) thin films promotes the mobility of both donor and acceptor molecules, leading to crystallization and aggregation, which are important in modulating morphology.

Efficient bulk heterojunction solar cells based on solution processed small molecules based on the same benzo[1,2-b:4, 5-b']thiophene unit as core donor and different terminal units

We report the synthesis, characterization, and optical and electrochemical of properties of two novel molecules DRT3-BDT (1) and DTT3-BDT (2), comprising the same BDT central core (donor) and different end capped acceptor units, i.e. rhodanine with ethyl hexyl and thiazolidione with ethylhexyl, respectively, linked via an alkyl-substituted terthiophene (3 T) π-conjugation bridge. The electrochemical properties of these small molecules indicate that their energy levels are compatible with energy levels of PC71BM for efficient exciton dissociation. These molecules have been used as electron donors along with PC71BM as an electron acceptor, for the fabrication of solution processed "small molecule" bulk heterojunction (BHJ) solar cells (smOPV). The device prepared from optimized 1:PC71BM(1:1) processed cast from DIO (3%v)/CF solvent exhibited a power conversion efficiency of 6.76% with Jsc = 11.92 mA cm(-2), Voc = 0.90 and FF = 0.63. The device with 2:PC71BM under the same conditions showed a lower PCE of 5.25% with Jsc = 10.52 mA cm(-2), Voc = 0.86 and FF = 0.56. The AFM, TEM and PL quenching measurements revealed that the high Jsc is a result of the appropriate morphology and exciton dissociation. The performances were compared for the devices based on two small molecules. The higher Jsc for device 1 was attributed to its better nanoscale phase separation, smooth surface and higher carrier mobility in the 1:PC71BM blend film. Moreover, the higher value of FF for the 1:PC71BM based device was ascribed to a good balance between the electron and hole mobilities.

A–D–A small molecules for solution-processed organic photovoltaic cells

The recent representative progress in the design and synthesis of A–D–A small molecules for organic solar cells is summarized.

Novel solution-processible small molecules based on benzo[1,2-b:3,4-b′:5,6-b′′]trithiophene for effective organic photovoltaics with high open-circuit voltage

Two novel A–D–A small molecules D1 and D2, containing benzo[1,2-b:3,4-b′:5,6-b′′]trithiophene as the central electron-donating unit and two thiophenes or three thiophenes as conjugated π-bridges, were designed and synthesized.

Effect of dye end groups in non-fullerene fluorene- and carbazole-based small molecule acceptors on photovoltaic performance

Fluorene- and carbazole-based small molecules with dye end groups were synthesized for use as non-fullerene acceptors in organic photovoltaic cells.

Utilizing alkoxyphenyl substituents for side-chain engineering of efficient benzo[1,2-b:4,5-b′]dithiophene-based small molecule organic solar cells

A new alkoxyphenyl substituted benzo[1,2-b:4,5-b′]dithiophene-based small molecule was designed and synthesized for solution-processed organic solar cells.

Side chain structure affects the molecular packing and photovoltaic performance of oligothiophene-based solution-processable small molecules

Small molecules with alkyl side chains of different lengths were prepared with 2,2′-bithiophene, terthiophene and thiobarbituric acid as the central core, spacer and end-cap. Uniform, shorter chain lengths gave stronger intermolecular interactions, favoring crystallization.

N,N-Diarylamino end-capping as a new strategy for simultaneously enhancing open-circuit voltage, short-circuit current density and fill factor in small molecule organic solar cells

N,N-Diarylamino end-capping strategy for asymmetrical squaraines with simultaneously enhanced V_oc, J_sc and FF in solution-processed small molecule organic solar cells.

S,N-Heteropentacene based small molecules with A–D–A structure for solution processed organic bulk heterojunction solar cells

SN(BTTh₂)₂ and SN(BTAOTh₂)₂, containing an electron rich planar S,N-heteropentacene flanked with alkoxy substituted and unsubstituted benzothiadiazole and end capped with hexyl-substituted bi-thiophene units, were designed and synthesized.

Bulk heterojunction organic solar cells based on carbazole–BODIPY conjugate small molecules as donors with high open circuit voltage

The power conversion efficiency of an optimized3a:PC71BM active layer based device is 5.05%.

Synthesis and optical properties of a new triphenylamine-p-phenylenevinylene-small molecule with applications in high open-circuit voltage organic solar cells

Fluorescence quenching studies enabled us to preview applications of a new small molecule as electron-donor in OSCs. Planar heterojunction cells are also shown.

Influence of thermal and solvent annealing on the morphology and photovoltaic performance of solution processed, D–A–D type small molecule-based bulk heterojunction solar cells

Four new small molecules CSDPP9–CSDPP12 were obtained with appended electron donating units in the molecular terminals of a DPP core. On solvent and thermal annealing, for the CSDPP11:PC₇₁BM blend, the BHJ device displayed a PCE of 5.47%.