Solution-processed Bulk Heterojunction Solar Cells Research Articles

High open-circuit voltage solution-processed organic solar cells based on a star-shaped small molecule end-capped with a new rhodanine derivative

A new star-shaped small molecule named TCNR3TTPA, with a triphenylamine (TPA) unit as the central building block and 2-(1,1-dicyanomethylene)-3-octyl rhodanine (CNR) as the end-capped group, has been designed and synthesized. TCNR3TTPA showed a deep highest occupied molecular orbital (HOMO) energy level (−5.60 eV) and broad absorption. The solution-processed bulk heterojunction (BHJ) solar cells based on TCNR3TTPA:PC61BM (1:1, w/w) exhibited a high open-circuit voltage (V oc) of 0.99 V, a short-circuit current density (J sc) of 5.76 mA/cm2, and a power conversion efficiency (PCE) of 2.50% under the illumination of AM 1.5 G, 100 mW/cm2. The high V oc is ascribed to the strong electron-with- drawing ability of the end-capped 2-(1,1-dicyanomethylene)-3-octyl rhodanine group. These results demonstrated that the V oc of small-molecule organic solar cells could be increased by introducing a strong electron-withdrawing end-capped block, and that this is an effective strategy to design high-performance small molecules for organic solar cells.

Improvement of optoelectronic and photovoltaic properties through the insertion of a naphthalenediimide unit in donor–acceptor oligothiophenes

Herein, we report optoelectronic and photovoltaic properties of conjugated naphthalenediimide functionality in a given donor–acceptor small molecular oligothiophene, when used in solution-processable bulk heterojunction solar cells.

Corannulene derivatives as non-fullerene acceptors in solution-processed bulk heterojunction solar cells

Corannulene derivatives were used in organic solar cells for the first time. Using Cor-PI and Cor-NI as acceptors, we achieved power conversion efficiencies up to 0.32% and 1.03%, suggesting potential applications of these fullerene segments as non-fullerene acceptors.

Solution-processed organic solar cells based on dialkylthiol-substituted benzodithiophene unit with efficiency near 10%.

A small molecule named DR3TSBDT with dialkylthiol-substituted benzo[1,2-b:4,5-b']dithiophene (BDT) as the central unit was designed and synthesized for solution-processed bulk-heterojunction solar cells. A notable power conversion efficiency of 9.95% (certified 9.938%) has been achieved under AM 1.5G irradiation (100 mW cm(-2)), with an average PCE of 9.60% based on 50 devices.

Supramolecular engineering of oligothiophene nanorods without insulators: hierarchical association of rosettes and photovoltaic properties.

Supramolecular rosettes of oligothiophenes that do not bear long aliphatic tails have been designed as semiconducting nanomaterials for solution-processable bulk heterojunction solar cells. The rosettes consist of six barbiturated thienyl[oligo(hexylthiophene)] units (Bar-T-hTn ; n=3,4,5) aggregated by multiple hydrogen bonds, which have been directly visualized by scanning tunneling microscopy (STM) at a solid-liquid interface. (1) H NMR spectroscopy in [D8 ]toluene showed that Bar-T-hTn exists as a mixture of monomers and small hydrogen-bonded aggregates. Hierarchical organization of the hydrogen-bonded aggregates took place through π-π stacking interactions upon casting their toluene solutions, resulting in the growth of highly ordered nanorods whose widths are consistent with the diameters of the rosettes. The nanorods could be generated in the presence of soluble fullerene derivatives via solution casting or the annealing of the resulting thin films. The solar cells fabricated based on these bulk heterojunction films showed power conversion efficiencies of 1-3 %, which are far higher than those of the non-hydrogen-bonded reference oligothiophene and the derivative that possesses long aliphatic tails.

High open circuit voltage organic solar cells based upon fullerene free bulk heterojunction active layers

We have recently reported on a small organic molecule containing a bithiophene core with end-capping phthalimide units (PthTh2Pth) that exhibited a H-aggregation tendency in the solid state and high electron mobility in organic field effect transistors. In this contribution, we have studied both the physical and electrical properties of poly(3-hexylthiophene) (P3HT) and PthTh2Pth thin films by measuring the optical absorption, Frontier molecular orbital energy levels, photoluminescence quenching, thermal properties, and photovoltaic response. Our results have provided a useful insight into the use of PthTh2Pth as an electron acceptor material for organic photovoltaic applications. In comparison with high-performance, fullerene-based, solution-processed bulk heterojunction solar cells reported in the literature, a relatively high open circuit voltage (∼0.94 V) was obtained for various donor–acceptor blend ratios. These results highlight the potential for PthTh2Pth to act as an alternative to fullerenes as acceptors in organic solar cell devices.

Development of new two-dimensional small molecules based on benzodifuran for efficient organic solar cells.

A new organic small molecule, DCA3TBDF, with a 2D benzo[1,2-b:4,5-b']difuran (BDF) moiety as the central core and octyl cyanoacetate units as the end-capped blocks, was designed and synthesized for solution-processed bulk heterojunction solar cells. DCA3TBDF possesses good solubility in common organic solvents such as toluene, CH2Cl2, chlorobenzene, and CHCl3 and good thermal stability with an onset decomposition temperature with 5% weight-loss occurring at 361 °C. The DCA3TBDF thin film showed a broad absorption at λ=320-700 nm and high crystallinity. Small-molecule organic solar cells based on DCA3TBDF and [6,6]-phenyl-C61-butyric acid methyl ester demonstrated promising power conversion efficiency with a high fill factor under the illumination of AM 1.5G (100 mW cm(-2)).

Solution-processed bulk-heterojunction organic solar cells employing Ir complexes as electron donors

To explore enhancing photocurrent in organic solar cells (OSCs) via harvesting triplet excitons, two novel bicycloiridium complexes (R1 and R2) are designed and synthesized. Conventional bulk-heterojunction triplet OSCs are solution processed using R1 or R2 as sole electron donors and phenyl-C71-butyric acid methyl ester (PC71BM) as the electron acceptor. A decent short circuit current (Jsc) of 6.5 mA cm−2 is achieved though the overlap between the absorption spectrum (with ∼550 nm absorption onset) of R2 and the solar flux is relatively small. With an open circuit voltage of 0.74 V and a fill factor of 0.42, an encouraging power conversion efficiency of 2.0% is achieved in the OSCs based on R2 and PC71BM without any processing additives and post-treatments. Our preliminary result demonstrates the possibility of utilizing Ir complexes as sole electron donors in OSCs, which extends available soluble small molecules for OSCs.

Synthesis, characterization and optoelectronic properties of a new perylene diimide–benzimidazole type solar light harvesting dye

A perylene diimide type small molecule (BI-PDI) has been synthesized through Suzuki coupling reaction between N,N′-bis(2,6-diisopropylphenyl)-1,7-dibromoperylene-3,4,9,10-tetracarboxylic diimide and 2-(2-hydroxyphenyl)-7-phenyl-1H-benzimidazole-4-boronic acid. BI-PDI small molecule has showed an absorption band between 350 and 750nm on thin films. HOMO and LUMO energy levels of BI-PDI dye have been calculated to be about −5.92eV and −3.82eV, respectively. Solution-processed bulk heterojunction (BHJ) solar cells have been constructed using BI-PDI as donor and [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) as acceptor or poly(3-hexylthiophene) (P3HT) as donor and BI-PDI as acceptor. The external quantum efficiencies (EQE) of the devices cover the most of the visible region between 400 and 700nm for both configurations. Photovoltaic performances of BI-PDI-based organic solar cells are limited by the aggregation tendency of PDI structure and poor hole/electron mobilities of the active layer.

Thieno[3,4-c]pyrrole-4,6-dione-Based Small Molecules for Highly Efficient Solution-Processed Organic Solar Cells

Two small molecules named BT-TPD and TBDT-TTPD with a thieno[3,4-c]pyrrole-4,6-dione (TPD) unit were designed and synthesized for solution-processed bulk-heterojunction solar cells. Their thermal, electrochemical, optical, charge-transport, and photovoltaic characteristics were investigated. These compounds exhibit strong absorption at 460-560 nm and low highest occupied molecular orbital levels (-5.36 eV). Field-effect hole mobilities of these compounds are 1.7-7.7×10(-3) cm(2) V(-1) s(-1). Small-molecule organic solar cells based on blends of these donor molecules and a acceptor display power conversion efficiencies as high as 4.62% under the illumination of AM 1.5G, 100 mW cm(-2).

Solution-processed bulk heterojunction solar cells based on a porphyrin small molecule with 7% power conversion efficiency

Solution-processed BHJ solar cells based on a porphyrin small molecule as the donor material exhibit a PCE of up to 7.23%.

Synthesis, optical and electrochemical properties of small molecules DMM-TPA[DTS(FBTTh3)3] and TPA[DTS(FBTTh3)3], and their application as donors for bulk heterojunction solar cells

Two organic small molecules were synthesized, characterized and used as donors along with PC71BM for use in solution-processed bulk heterojunction solar cells. The PCE of the solution-processed SMBHJ solar cells was improved up to 5.16% and 4.70%.

Indacenodithiophene core-based small molecules with tunable side chains for solution-processed bulk heterojunction solar cells

Two indacenodithiophene (IDT) core-based small molecules with different side chains of bulky 4-hexylphenyl and flexible n-dodecyl with the same number of carbon atom, namely SM1 and SM2, respectively, were designed and synthesized as the donor materials in organic solar cells (OSCs). The impacts of the different side chains combined with the IDT core on the optical absorption, electrochemical property, hole mobility, film morphology, and solar cell performance were studied thoroughly. The two compounds possess a broad absorption covering the wavelength range of 450–700 nm and relatively low HOMO energy levels of −5.46 and −5.52 eV. The power conversion efficiency (PCE) of the OSCs based on SM2 as the donor material and PC61BM as the acceptor material (1 : 2, w/w) is 2.33%. In contrast, a PCE of 4.72% was achieved for the device based on SM1 as the donor and PC71BM as the acceptor (1 : 2, w/w) without any treatment such as thermal annealing or the utilization of a solvent additive.

Development of Efficient and Stable Inverted Bulk Heterojunction (BHJ) Solar Cells Using Different Metal Oxide Interfaces.

Solution-processed inverted bulk heterojunction (BHJ) solar cells have gained much more attention during the last decade, because of their significantly better environmental stability compared to the normal architecture BHJ solar cells. Transparent metal oxides (MeOx) play an important role as the dominant class for solution-processed interface materials in this development, due to their excellent optical transparency, their relatively high electrical conductivity and their tunable work function. This article reviews the advantages and disadvantages of the most common synthesis methods used for the wet chemical preparation of the most relevant n-type- and p-type-like MeOx interface materials consisting of binary compounds AxBy. Their performance for applications as electron transport/extraction layers (ETL/EEL) and as hole transport/extraction layers (HTL/HEL) in inverted BHJ solar cells will be reviewed and discussed.

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.

Rational molecular engineering towards efficient non-fullerene small molecule acceptors for inverted bulk heterojunction organic solar cells

Two non-fullerene small molecules based on fluoranthene-fused imide were developed as acceptors for solution-processed inverted organic bulk heterojunction (BHJ) solar cells, which showed good power conversion efficiency and high open-circuit voltage.

Near-infrared response thienoisoindigo-based small molecule for solution-processed bulk-heterojunction solar cells

5′-Hexyl-2,2′-bithiophene end-capped thienoisoindigo-based small molecule (TIDO2T) is synthesized by Stille coupling reaction. The TIDO2T exhibits good solubility in common organic solvents such as THF, toluene, dichlorobenzene and chloroform. The UV–vis absorption spectrum of TIDO2T shows wide absorption between 427nm and 871nm in thin solid film, and the optical band-gap calculated from absorption onset was 1.42eV. Bulk heterojunctions (BHJ) solar cell was fabricated with TIDO2T as the donor and 6,6-phenyl-C61-butyric acid methyl ester (PC61BM) as the acceptor by simple solution spin-coating processing. The device obtained a power conversion efficiency of 1.14% with VOC 0.44V, JSC 7.76mAcm−2 and FF 33% under AM 1.5G solar simulator. The external quantum efficiency (EQE) curve extends to near-infrared region (around 900–1000nm).

Facile Synthesis of [6,6]-Phenyl-C61/71-Butyric Acid Methyl Esters via Sulfur Ylides for Bulk-Heterojunction Solar Cell

A one-step, mild synthesis of methanofullerenes as electron acceptors for solution-processed bulk-heterojunction solar cells was developed. [6,6]-Phenyl-C₆₁-butyric acid methyl ester {[60]PCBM} was directly synthesized in good yields, by the reaction of fullerene with sulfur ylide derivatives in the presence of 1,8-diazabicyclo-[5.4.0]undec-7-ene (DBU) at room temperature. This method was also successfully applied to the preparation of [6,6]-phenyl-C₇₁-butyric acid methyl ester {[70]PCBM}.

Solution-Processed and High-Performance Organic Solar Cells Using Small Molecules with a Benzodithiophene Unit

Three small molecules named DR3TBDTT, DR3TBDTT-HD, and DR3TBD2T with a benzo[1,2-b:4,5-b']dithiophene (BDT) unit as the central building block have been designed and synthesized for solution-processed bulk-heterojunction solar cells. Power conversion efficiencies (PCEs) of 8.12% (certified 7.61%) and 8.02% under AM 1.5G irradiation (100 mW cm(-2)) have been achieved for DR3TBDTT- and DR3TBDT2T-based organic photovoltaic devices (OPVs) with PC71BM as the acceptor, respectively. The better PCEs were achieved by improving the short-circuit current density without sacrificing the high open-circuit voltage and fill factor through the strategy of incorporating the advantages of both conventional small molecules and polymers for OPVs.

Oligothiophene‐Bridged Bis(arylene ethynylene) Small Molecules for Solution‐Processible Organic Solar Cells with High Open‐Circuit Voltage

A new series of conjugated oligothiophene-bridged bis(arylene ethynylene) small molecules have been designed, synthesized, and characterized by photophysical, electrochemical and computational methods. These compounds were found to have optimal LUMO levels that ensure effective charge transfer from these compounds to [6,6]-phenyl-C71-butyric acid methyl ester (PC70BM). They were utilized as good electron-donor materials that can be blended with electron-acceptor PC70BM in the fabrication of solution-processed molecular bulk heterojunction (BHJ) solar cells. All of these BHJ devices showed very high open-circuit voltage (V(oc)) of 0.90-0.97 V, and the best power conversion efficiency achieved was 3.68%. The high V(oc) is consistent with the deeper low-lying HOMO level and is relatively insensitive to the donor : acceptor blend ratio. The spin-coated thin films of these small molecules showed p-channel field-effect charge transport with the hole mobilities of up to 2.04×10(-4) cm(2) V(-1) s(-1). These compounds illuminate the potential of solution-processible small-molecular aryl acetylide compounds for efficient power generation in photovoltaic implementation.