Small molecules based on benzo[1,2-b:4,5-b']dithiophene unit for high-performance solution-processed organic solar cells

The effect of branched versus linear alkyl side chains on the bulk heterojunction photovoltaic performance of small molecules containing both benzodithiophene and thienopyrroledione

Compared toBDTO-TTPD,BDTEH-TTPDshowed stronger light absorption and longer-range ordering, which results in higher power conversion efficiency.

Low band gap disk-shaped donors for solution-processed organic solar cells

Performance of pyrene-cored donors in BHJ solar cells is enhanced by introduction of rhodanine due to expansion of the light-harvesting area.

A low bandgap asymmetrical squaraine for high-performance solution-processed small molecule organic solar cells

A low-bandgap asymmetrical squaraine ASQ-5 bearing an indoline substituent for solution-processed organic solar cells with high J_sc and PCE was synthesized.

Photovoltaic performance of novel push–pull–push thienyl–Bodipy dyes in solution-processed BHJ-solar cells

Original flat push–pull–push dyes have been engineered from thienyl–Bodipy–triphenylamine frameworks for solar cell application.

Enhancing the performance of solution-processed bulk-heterojunction solar cells using hydrogen-bonding-induced self-organization of small molecules

Small-molecule solar-cell performance is highly sensitive to the crystallinity and intermolecular connectivity of the molecules. In order to enhance the crystallinity for the solution-processed small molecule, it is possible to make use of carboxylic acid end-functional groups to drive hydrogen-bonding-induced π-π stacking of conjugated molecules. Herein, we report the synthesis and characterization of quarterthiophenes with carboxylic acid as end groups. The formation of hydrogen bonds between neighboring acid groups gives rise to a pseudo-polymeric structure in the molecules, which leads to substantial improvement in the organization and crystallinity of the active layers. This resulted in a four-fold increase in the hole mobility and a two-fold improvement in the performance of the solar cell device for the acid-functionalized molecule, compared to its ester analogue. More importantly, optimal device performance for the acid-functionalized molecule was achieved for the as-cast film, thereby reducing the reliance on thermal annealing and solvent additives.

25th anniversary article: a decade of organic/polymeric photovoltaic research

Organic photovoltaic (OPV) technology has been developed and improved from a fancy concept with less than 1% power conversion efficiency (PCE) to over 10% PCE, particularly through the efforts in the last decade. The significant progress is the result of multidisciplinary research ranging from chemistry, material science, physics, and engineering. These efforts include the design and synthesis of novel compounds, understanding and controlling the film morphology, elucidating the device mechanisms, developing new device architectures, and improving large-scale manufacture. All of these achievements catalyzed the rapid growth of the OPV technology. This review article takes a retrospective look at the research and development of OPV, and focuses on recent advances of solution-processed materials and devices during the last decade, particular the polymer version of the materials and devices. The work in this field is exciting and OPV technology is a promising candidate for future thin film solar cells.

A DMF-assisted solution process boosts the efficiency in P3HT:PCBM solar cells up to 5.31%

To achieve ideal phase separation in P3HT:PCBM (1:1, w/w) blend films, a selective second solvent, DMF, which cannot dissolve P3HT but slightly dissolves PCBM, was added into 1,2-dichlorobenzene. Addition of 10% DMF led to better charge transport, better morphology and average power conversion efficiency (PCE) enhancement from 3.75% to 4.29%. To form an acceptor rich layer near the cathode and achieve effective vertical phase separation, PCBM solution in DMF was spin-coated on the top surface of the P3HT:PCBM active layer. The PCBM rich layer enhanced hole blocking and electron transport, leading to an average PCE improvement from 4.29% to 4.83% (for the PCBM rich layer formed by a 2 mg ml(-1) PCBM solution). Finally, to obtain more uniform and smooth films with better contact of Ca/PCBM and BHJ/PCBM interfaces, the films were thermally annealed at 120 ° C for 10 min after spin coating the PCBM rich layer, leading to an average PCE enhancement from 4.83% to 5.17% (the best PCE was 5.31%).

The role of additive in diketopyrrolopyrrole-based small molecular bulk heterojunction solar cells

A new diketopyrrolopyrrole-based small molecule for solution-processed OPVs is synthesized. The chemical additive has a profound effect in refining the morphology and, thus, significantly increases the efficiencies of the devices. The role of the additive is characterized by various techniques and the additive-driven structure-property relationship is established, which reveals that the additive affects the crystallization, PCBM aggregation, and phase separation.

Solution-processed small-molecule solar cells: breaking the 10% power conversion efficiency

A two-dimensional conjugated small molecule (SMPV1) was designed and synthesized for high performance solution-processed organic solar cells. This study explores the photovoltaic properties of this molecule as a donor, with a fullerene derivative as an acceptor, using solution processing in single junction and double junction tandem solar cells. The single junction solar cells based on SMPV1 exhibited a certified power conversion efficiency of 8.02% under AM 1.5 G irradiation (100 mW cm(-2)). A homo-tandem solar cell based on SMPV1 was constructed with a novel interlayer (or tunnel junction) consisting of bilayer conjugated polyelectrolyte, demonstrating an unprecedented PCE of 10.1%. These results strongly suggest solution-processed small molecular materials are excellent candidates for organic solar cells.

Solvent additive effects on small molecule crystallization in bulk heterojunction solar cells probed during spin casting

Solvent additive processing can lead to drastic improvements in the power conversion efficiency (PCE) in solution processable small molecule (SPSM) bulk heterojunction solar cells. In situ grazing incidence wide-angle X-ray scattering is used to investigate the kinetics of crystallite formation during and shortly after spin casting. The additive is shown to have a complex effect on structural evolution invoking polymorphism and enhanced crystalline quality of the donor SPSM.

High performance photovoltaic applications using solution-processed small molecules

Energy remains a critical issue for the survival and prosperity of humancivilization. Many experts believe that the eventual solution for sustainable energy is the use of direct solar energy as the main energy source. Among the options for renewable energy, photovoltaic technologies that harness solar energy offer a way to harness an unlimited resource and minimum environment impact in contrast with other alternatives such as water, nuclear, and wind energy. Currently, almost all commercial photovoltaic technologies use Si-based technology, which has a number of disadvantages including high cost, lack of flexibility, and the serious environmental impact of the Si industry. Other technologies, such as organic photovoltaic (OPV) cells, can overcome some of these issues. Today, polymer-based OPV (P-OPV) devices have achieved power conversion efficiencies (PCEs) that exceed 9%. Compared with P-OPV, small molecules based OPV (SM-OPV) offers further advantages, including a defined structure for more reproducible performance, higher mobility and open circuit voltage, and easier synthetic control that leads to more diversified structures. Therefore, while largely undeveloped, SM-OPV is an important emerging technology with performance comparable to P-OPV. In this Account, we summarize our recent results on solution-processed SM-OPV. We believe that solution processing is essential for taking full advantage of OPV technologies. Our work started with the synthesis of oligothiophene derivatives with an acceptor-donor-acceptor (A-D-A) structure. Both the backbone conjugation length and electron withdrawing terminal groups play an important role in the light absorption, energy levels and performance of the devices. Among those molecules, devices using a 7-thiophene-unit backbone and a 3-ethylrhodanine (RD) terminal unit produced a 6.1% PCE. With the optimized conjugation length and terminal unit, we borrowed from the results with P-OPV devices to optimize the backbone. Thus we selected BDT (benzo[1,2-b:4,5-b']dithiophene) and DTS (dithienosilole) to replace the central thiophene unit, leading to a PCE of 8.12%. In addition to our molecules, Bazan and co-workers have developed another excellent system using DTS as the core unit that has also achieved a PCE greater than 8%.

Efficient small-molecule photovoltaic cells using a crystalline diindenoperylene film as a nanostructured template

A cascade-type small-molecule organic photovoltaic cell using a crystalline diindenoperylene film as a nanostructured template is demonstrated. This cell architecture simultaneously realizes organic nanostructure and cascade energy concepts, which significantly improves the photocurrent generation and fill factor, leading to a power conversion efficiency of 5.2±0.3%.

Tri-diketopyrrolopyrrole molecular donor materials for high-performance solution-processed bulk heterojunction solar cells

Two new high-performance DPP-containing donor molecules employing a molecular architecture with three DPP chromorphores (tri-DPP) in conjugated backbones are synthesized and characterized. The two tri-DPP molecules with only a structural difference on alkyl substitutions, when blended with PC71 BM, lead to power conversion efficiencies up to 4.8 and 5.5%, respectively.

A potential perylene diimide dimer-based acceptor material for highly efficient solution-processed non-fullerene organic solar cells with 4.03% efficiency

A highly efficient acceptor material for organic solar cells (OSCs)--based on perylene diimide (PDI) dimers--shows significantly reduced aggregation compared to monomeric PDI. The dimeric PDI shows a best power conversion efficiency (PCE) approximately 300 times that of the monomeric PDI when blended with a conjugate polymer (BDTTTT-C-T) and with 1,8-diiodooctane as co-solvent (5%). This shows that non-fullerene materials also hold promise for efficient OSCs.

Optimization of solution-processed oligothiophene:fullerene based organic solar cells by using solvent additives

The optimization of solution-processed organic bulk-heterojunction solar cells with the acceptor-substituted quinquethiophene DCV5T-Bu 4 as donor in conjunction with PC61BM as acceptor is described. Power conversion efficiencies up to 3.0% and external quantum efficiencies up to 40% were obtained through the use of 1-chloronaphthalene as solvent additive in the fabrication of the photovoltaic devices. Furthermore, atomic force microscopy investigations of the photoactive layer gave insight into the distribution of donor and acceptor within the blend. The unique combination of solubility and thermal stability of DCV5T-Bu 4 also allows for fabrication of organic solar cells by vacuum deposition. Thus, we were able to perform a rare comparison of the device characteristics of the solution-processed DCV5T-Bu 4 :PC61BM solar cell with its vacuum-processed DCV5T-Bu 4 :C60 counterpart. Interestingly in this case, the efficiencies of the small-molecule organic solar cells prepared by using solution techniques are approaching those fabricated by using vacuum technology. This result is significant as vacuum-processed devices typically display much better performances in photovoltaic cells.

Directed metalation cascade to access highly functionalized thieno[2,3-f]benzofuran and exploration as building blocks for organic electronics

A tandem directed metalation has been successfully applied to the preparation of thieno[2,3-f]benzofuran-4,8-dione, providing an efficient and facile approach to symmetrically and unsymmetrically functionalize the thieno[2,3-f]benzofuran core at the 2,6 positions as well as to introduce the electron-withdrawing or -donating groups (EWG or EDG) at its 4,8 positions. The presence of various functional groups makes late-stage derivatization attainable.

Solution-processed small molecules using different electron linkers for high-performance solar cells

Conjugated acceptor-donor-acceptor small molecules using different electron linkers are designed, synthesized, and used in organic solar cells. All of these small molecules show high photoconversion efficiencies (PCEs), ranging from 3.18-6.15% under simulated AM 1.5G condition. A maximum PCE of 6.15% combined with a high Voc of 0.85 V, a Jsc of 10.79 mA cm(-2) and a notable FF of 67.1% are achieved with T3/PC71 BM blend based devices using polydimethylsiloxane as additive.

A high-performing solution-processed small molecule:perylene diimide bulk heterojunction solar cell

By combining the molecular donor p-DTS(FBTTh2 )2 with a readily produced perylene diimide acceptor we are able to achieve a power conversion efficiency of 3.0%, making this one of the most efficient non-fullerene organic solar cells to date. The reduced power conversion efficiency of the present system compared to the use of phenyl-C71 -butyric acid methyl ester as an electron acceptor is shown to primarily be related to a significant reduction in the internal quantum efficiency. These results indicate the potential of small-molecule:non-fullerene bulk-heterojunction organic photovoltaics.

Effects of stereoisomerism on the crystallization behavior and optoelectrical properties of conjugated molecules

Three stereoisomers of DPP(TBFu)2 are separated and identified to investigate the effects of stereoisomerism on crystal structures and the optoelectrical properties. The crystal structures and FET mobility are sensitive to stereoisomers, in which the mesomer possesses the highest carrier mobility and the greatest crystallization tendency to dominate the crystallization in spin-cast films of the as-synthesized stereoisomeric mixture.

Monodisperse Low-Bandgap Macromolecule-Based 5,5'-Bibenzo[c][1,2,5]thiadiazole Swivel Cruciform for Organic Solar Cells

A novel low bandgap star-like macromolecule was synthesized and applied as electron donor material in the bulk heterojunction solar cells, in which the 5,5'-bibenzo[c][1,2,5]thiadiazole was used as the central node, in conjunction with four conjugated donor-acceptor arms. Compared with the corresponding small molecule with first generation arms, the macromolecule with second generation branches exhibited significantly enhanced photovoltaic device performances (blended with PC71BM as the active layer) due to dramatically improved short-circuit current density (J_sc) and fill factor (FF). The improvement in J_sc and FF can be attributed to the more broad absorption and the more favorable phase separation when comparing a monodisperse macromolecule with the second generation arms (SFTBT) with a small molecule with first generation branches (DFTBT).

Solution-processed DPP-based small molecule that gives high photovoltaic efficiency with judicious device optimization

A solution-processed diketopyrrolopyrrole (DPP)-based small molecule, namely BDT-DPP, with broad absorption and suitable energy levels has been synthesized. The widely used solvents of chloroform (CF) and o-dichlorobenzene (o-DCB) were used as the spin-coating solvent, respectively, and 1,8-diiodooctane (DIO) was used as additive to fabricate efficient photovoltaic devices with BDT-DPP as the donor material and PC71BM as the acceptor material. Devices fabricated from CF exhibit poor fill factor (FF) of 43%, low short-circuit current density (Jsc) of 6.86 mA/cm(2), and moderate power conversion efficiency (PCE) of 2.4%, due to rapid evaporation of CF, leading to poor morphology of the active layer. When 0.3% DIO was added, the FF and Jsc were improved to 60% and 8.49 mA/cm(2), respectively, because of the better film morphology. Active layer spin-coated from the high-boiling-point solvent of o-DCB shows better phase separation than that from CF, because of the slow drying nature of o-DCB, offering sufficient time for the self-organization of active-layer. Finally, using o-DCB as the parent solvent and 0.7% DIO as the cosolvent, we obtained optimized devices with continuous interpenetrating network films, affording a Jsc of 11.86 mA/cm(2), an open-circuit voltage (Voc) of 0.72 V, an FF of 62%, and a PCE of 5.29%. This PCE is, to the best of our knowledge, the highest efficiency reported to date for devices prepared from the solution-processed DPP-based small molecules.

Star-shaped D-A small molecules based on diketopyrrolopyrrole and triphenylamine for efficient solution-processed organic solar cells

Three star-shaped D-A small molecules, (P-DPP)(3)TPA, (4-FP-DPP)(3)TPA, and (4-BuP-DPP)(3)TPA were designed and synthesized with triphenylamine (TPA) as the core, diketopyrrolopyrrole (DPP) as the arm, and unsubstituted or substituted benzene rings (phenyl, P; 4-fluoro-phenyl, 4-FP; 4-n-butyl-phenyl, 4-BuP) as the end-group. All the three small molecules show relatively narrow optical band gaps (1.68-1.72 eV) and low-lying highest occupied molecular orbital (HOMO) energy levels (-5.09∼-5.13 eV), implying that they are potentially good electron donors for organic solar cells (OSCs). Then, photovoltaic properties of the small molecules blended with [6,6]-phenyl-C(61)-butyric acid methyl ester (PC(61)BM) as electron acceptor were investigated. Among three small molecules, the OSC based on (P-DPP)(3)TPA:PCBM blend exhibits a best power conversion efficiency (PCE) of 2.98% with an open-circuit voltage (V(oc)) of 0.72 V, a short-circuit current density (J(sc)) of 7.94 mA/cm(2), and a fill factor (FF) of 52.2%, which may be ascribed to the highest hole mobility of (P-DPP)(3)TPA.