Solution-processed Small Molecule Bulk Heterojunction Research Articles

Carbazole-based green and blue-BODIPY dyads and triads as donors for bulk heterojunction organic solar cells.

Two BODIPY derivatives with one (B2) and two (B3) carbazole moieties were synthesized and applied as electron-donor materials in organic photovoltaic cells (OPV). Their optical and electrochemical properties were systematically investigated. These BODIPY dyes exhibit excellent solubility in organic solvents and present high molar extinction coefficients (1.37-1.48 × 105 M-1 cm-1) in solutions with absorption maxima at 586 nm for mono-styryl groups and at 672 nm for di-styryl groups. The introduction of the styryl moieties results in a large bathochromic shift and a significant decrease in the HOMO-LUMO energy-gaps. The BODIPY dyes show relatively low HOMO energies ranging from -4.99 to -5.16 eV as determined from cyclic voltammetry measurements. Cyclic voltammetry measurements and theoretical calculations demonstrate that the frontier molecular orbital levels of these compounds match with those of PC71BM as the acceptor, supporting their application as donor materials in solution-processed small molecule bulk heterojunction (BHJ) organic solar cells. After the optimization of the active layer, B2:PC71BM and B3:PC71BM based organic solar cells showed an overall power conversion efficiency of 6.41% and 7.47%, respectively. The higher PCE of the B3-based OSC is ascribed to the more balanced charge transport and exciton dissociation, better crystallinity and molecular packing.

Understanding the loss mechanisms in high-performance solution-processed small molecule bulk heterojunction solar cells doped with a PFN impurity.

Contamination of the active layer with an impurity could result in significant degradation in the performance of bulk heterojunction (BHJ) solar cells as a result of enhancing the loss of the charge carriers via a trap-assisted recombination. In this study, PFN as an impurity was intentionally introduced to a BHJ solar cell composed of a high-performance solution-processed small molecule (p-DTS(FBTTh2)2 as a donor and PC60BM as an acceptor. The power conversion efficiency (PCE) of PFN doped devices degrades owing to the reduction of short-circuit current (Jsc) and fill factor (FF). At a low concentration, PFN mostly reduces the generation of charge carriers, whereas doubling the PFN concentration conversely affects both generation and collection of charge carriers. Charge carrier dynamics of devices has also been probed using photovoltage decay, time-resolved charge extraction (TRCE) and photoinduced charge extraction by linearly increasing voltage (photo-CELIV) before and after incorporation of PFN. The results reveal that traps introduced by PFN reduce the decay of charge carriers via bimolecular recombination, leading to a higher charge carrier density and photovoltage at long times under an open-circuit potential (Voc). However, under short-circuit (Jsc) conditions, traps considerably impede the collection of charge carriers causing the appearance of an S-shaped current density-voltage curve.

Investigation of S-shaped current-voltage characteristics in high-performance solution-processed small molecule bulk heterojunction solar cells

The appearance of an S-shaped current-voltage curve and its impact on the performance of solution-processed small molecule p-DTS(FBTTh2)2:PC70BM bulk heterojunction solar cell devices were investigated. The power conversion efficiency of the devices showing S-curve characteristics was almost half of what was expected for this combination of materials, which was due to a low fill factor (FF) and 20% lower short circuit current density. A normal diode-like current-voltage curve was retrieved when the PCBM content of the active layer was increased, with power conversion efficiency reaching 6%. Analysis of the current-voltage characteristics of the S-curve solar cell devices showed that the effective voltage of the devices throughout the active layer was diminished due to an energy barrier. As a result, the charge collection efficiency was significantly affected. In addition, smaller external quantum efficiency (EQE) values in the absorption range of the small molecule donor in the S-curve devices suggested that charge generation was also affected. Recombination dynamics of both the normal and S-curve devices under steady-state conditions were quite similar. However, photovoltage decay results showed a distinguished pattern of recombination at open circuit conditions in the S-curve devices. This suggested different recombination rates at the cathode and anode electrodes, which is possibly due to the different composition of the donor-acceptor film near the contacts.

BODIPY-based panchromatic chromophore for efficient organic solar cell

A novel donor-acceptor-donor (D-A-D) type chromophore, named BDP-dBDT, with a boron dipyrromethene (BODIPY) linked through alkynyl with two benzo [1,2-b:4,5-b’]dithiophene (BDT) terminal donors was designed and synthesized for solution-processed small molecule bulk heterojunction (BHJ) solar cell. The thermal and optochemical properties, BHJ morphology and solar cell performance were investigated. The new molecule shows panchromatic absorption with narrow optical band gap and high molar extinction coefficient. Moreover, the molecule displays deep highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels. The BHJ morphology was systematically optimized by additives, thermal annealing (TA) and solvent vapor annealing (SVA). Processed by a combination of TA and SVA treatments, the BDP-dBDT:PC71BM device exhibits a short-circuit current density (Jsc) of 12.98 mA cm−2, and a power conversion efficiency (PCE) of 5.61%, which is one of the best values among the ever reported BODIPY based organic solar cells.

Vinazene end-capped acceptor-donor-acceptor type small molecule for solution-processed organic solar cells

An acceptor-donor-acceptor (A-D-A) type molecule based on dioctyltertthiophene-benzo[1,2-b:4,5-b′]dithiophene-dioctyltertthiophene central donor and vinazene terminal acceptor was designed and synthesized for solution-processed small molecule bulk-heterojunction (BHJ) solar cells. The thermal and optochemical properties, BHJ morphology and solar cell performance were investigated. The BHJ morphology was systematically optimized by thermal annealing, solvent vapor annealing, and the use of solvent additives. Processed by a combination of thermal annealing and solvent vapor annealing treatments, V-BDT:PC71BM device showed an optimized PCE of 3.73% with a VOC of 0.89 V, an JSC of 6.88 mA cm−2 and a FF of 0.61.

Asymmetrical Squaraines Bearing Fluorine-Substituted Indoline Moieties for High-Performance Solution-Processed Small-Molecule Organic Solar Cells.

Two novel asymmetrical squaraines based on the indoline unit, ASQ-5-F and ASQ-5-DF, with one and two fluorine substituents, have been developed to investigate the effect of fluorine substituted on small-molecule bulk-heterojunction (BHJ) organic solar cells (OSCs). In comparison with non-fluorine-substituted ASQ-5, both fluorine-substituted ASQ-5-F and ASQ-5-DF possess analogous absorption band gaps but 0.05 and 0.10 eV lowered highest occupied molecular orbital (HOMO) energy levels, respectively. Single-crystal analysis exhibits that ASQ-5-DF shows more desirable intermolecular packing patterns for the hole-carrier collection than ASQ-5 does; hence, higher hole mobility could be acquired. Therefore, solution-processed small-molecule BHJ OSCs fabricated with ASQ-5-F/PC71BM and ASQ-5-DF/PC71BM blends exhibit extremely higher power conversion efficiency (PCE; 5.0% and 6.0%, respectively) than that of ASQ-5/PC71BM (4.5%). The much improved PCE could be attributed to the simultaneously enhanced Voc, Jsc, and FF relative to those of the ASQ-5-based device. To our knowledge, this is the highest PCE (6.0%) among squaraine-based solution-processed BHJ OSCs and the highest PCE in OSCs based on the fluorinated donor segment of small molecules.

Use of a commercially available nucleating agent to control the morphological development of solution-processed small molecule bulk heterojunction organic solar cells

The nucleating agent DMDBS is used to modulate the crystallization of solution-processed small molecule donor molecules in bulk heterojunction organic photovoltaic (BHJ OPV) devices. This control over donor molecule crystallization leads to a reduction in optimized thermal annealing times as well as smaller donor molecule crystallites, and therefore more efficient devices, when using an excessive amount of solvent additive. We therefore demonstrate the use of nucleating agents as a powerful and versatile processing strategy for solution-processed, small molecule BHJ OPVs.

Structural modification of thieno[3,4-c]pyrrole-4,6-dione: structure–property relationships and application in solution-processed small-molecule organic solar cells

We present a method for the modification of the structure of thieno[3,4-c]pyrrole-4,6-dione (TPD) to gain two novel acceptors TPN and TIN. And a series of donor–acceptor–donor-type oligomers 3TTPD3T, 3TTPN3T, and 3TTIN3T, based on TPD, TPN and TIN, have been designed and synthesized for solution-processed small-molecule bulk-heterojunction (BHJ) solar cells. The impact of these different central acceptor moieties on their optical, electrochemical properties, morphology, and solar cell performance were studied. Optical and electrochemical data showed that structural modification of TPD exhibited a slight impact on the band gap and HOMO/LUMO energy levels. Atomic force microscopy and grazing incidence wide-angle X-ray scattering studies revealed that the central acceptors of these oligomers played an important role in determine the morphology of solid-state packing. Oligomer 3TTPD3T showed weak crystallinity in a thin film. The larger conjugation plane counterpart 3TTPN3T showed an improved structure order. Surprisingly, the structurally twisted compound 3TTIN3T showed a strong crystalline nature with large crystalline domains of several hundred nanometers. The photovoltaic properties of these oligomers were evaluated by fabricating BHJ devices with PC71BM. 3TTPD3T/PC71BM blends with a weight ratio of 2 : 1 showed a best power conversion efficiency of 1.87%, with a Jsc of 4.93 mA cm−2, a Voc of 0.87 V, and a fill factor of 44%.

Soluble porphyrin donors for small molecule bulk heterojunction solar cells

This paper describes the rational design of porphyrin donors for application in solution-processed small molecule bulk heterojunction (BHJ) solar cells. The porphyrin donors were designed to possess high solubility and strong intermolecular interactions, via the appropriate selection of peripheral substituents: these molecules bear tetraethynyl porphyrin cores with two aromatic and two aliphatic groups in a trans manner. We have also demonstrated the substitution effect on the aromatic groups: energy levels, solubility and film morphology were significantly modified by the substituents, which consequently influenced the efficiencies of the solar cells as well. After optimization, the device based on [5,15-bis(phenylethynyl)-10,20-bis[(triisopropylsilyl)ethynyl]porphyrinato] magnesium(II) (2):PC60BM showed a PCE of 2.5%.

Solution-processed small-molecule solar cells with 6.7% efficiency

Organic photovoltaic devices that can be fabricated by simple processing techniques are under intense investigation in academic and industrial laboratories because of their potential to enable mass production of flexible and cost-effective devices. Most of the attention has been focused on solution-processed polymer bulk-heterojunction (BHJ) solar cells. A combination of polymer design, morphology control, structural insight and device engineering has led to power conversion efficiencies (PCEs) reaching the 6-8% range for conjugated polymer/fullerene blends. Solution-processed small-molecule BHJ (SM BHJ) solar cells have received less attention, and their efficiencies have remained below those of their polymeric counterparts. Here, we report efficient solution-processed SM BHJ solar cells based on a new molecular donor, DTS(PTTh(2))(2). A record PCE of 6.7% under AM 1.5 G irradiation (100 mW cm(-2)) is achieved for small-molecule BHJ devices from DTS(PTTh(2))(2):PC(70)BM (donor to acceptor ratio of 7:3). This high efficiency was obtained by using remarkably small percentages of solvent additive (0.25% v/v of 1,8-diiodooctane, DIO) during the film-forming process, which leads to reduced domain sizes in the BHJ layer. These results provide important progress for solution-processed organic photovoltaics and demonstrate that solar cells fabricated from small donor molecules can compete with their polymeric counterparts.

A modular molecular framework for utility in small-molecule solution-processed organic photovoltaic devices

We report on the design, synthesis and characterization of light harvesting small molecules for use in solution-processed small molecule bulk heterojunction (SM-BHJ) solar cell devices. These molecular materials are based upon an acceptor/donor/acceptor (A/D/A) core with donor endcapping units. Utilization of a dithieno(3,2-b;2′,3′-d)silole (DTS) donor and pyridal[2,1,3]thiadiazole (PT) acceptor leads to strong charge transfer characteristics, resulting in broad optical absorption spectra extending well beyond 700 nm. SM-BHJ solar cell devices fabricated with the specific example 5,5′-bis{7-(4-(5-hexylthiophen-2-yl)thiophen-2-yl)-[1,2,5]thiadiazolo[3,4-c]pyridine}-3,3′-di-2-ethylhexylsilylene-2,2′-bithiophene (6) as the donor and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as the acceptor component showed short circuit currents above −10 mA cm−2 and power conversion efficiencies (PCEs) over 3%. Thermal processing is a critical factor in obtaining favorable active layer morphologies and high PCE values. A combination of UV-visible spectroscopy, conductive and photo-conductive atomic force microscopies, dynamic secondary mass ion spectrometry (DSIMS), and grazing incident wide angle X-ray scattering (GIWAXS) experiments were carried out to characterize how thermal treatment influences the active layer structure and organization.

Efficiency enhancement in small molecule bulk heterojunction organic solar cells via additive

Improved performance of small molecule bulk heterojunction (BHJ) organic solar cells based on 4,7-bis(2-triphenylamino-3-dodecyl-thiophene-5-yl)-benzo[c][1,2,5]thiadiazole (TTBTT)/PC71BM (1:3, w/w) blend has been obtained by using 1,8-octanedithiol (ODT) as a processing additive. Based on the analysis of absorption spectrum, charge transport, and film morphology, the 30% enhancement in power conversion efficiency (PCE) via 1% ODT addition is attributed to aggregated domain formation, enhanced absorption, improved hole mobility, and more balanced charge transport. Finally, a PCE as high as 2.86% was achieved, which is among the top reported for solution-processed small molecule BHJ organic solar cells.

Solution‐Processed Organic Photovoltaic Cells with Anthracene Derivatives

Solution-processed small-molecule bulk heterojunction photovoltaic cells are fabricated by using [6,6]-phenyl-C(61)-butyric acid methyl ester (PCBM) as electron acceptor and triisopropylsilylethynyl anthracene (TIPSAnt) derivatives substituted with naphthalene (TIPSAntNa) and bithiophene (TIPSAntBT) as electron donors. In contrast to TIPS-pentacene, the TIPSAnt derivatives are not susceptible to Diels-Alder reactions with PCBM when processed in solution, as confirmed by UV/Vis measurements. Photoluminescence quenching measurements show exciton diffusion lengths of 5 and 3 nm for TIPSAntBT and TIPSAntNa, respectively. Blending TIPSAntBT and TIPSAntNa with PCBM (1:1, 1:2, 1:3, and 1:4 weight ratios) produces films that possess adequate hole and electron mobilities. The morphological changes that result from varying the blending ratio range from obvious phase-segregated crystalline domains at a 1:1 ratio to homogeneous, nearly amorphous phases at a 1:4 ratio. Bulk heterojunction solar cells prepared by using a TIPSAntBT:PCBM blend reach power conversion efficiencies as high as 1.4 %.