Silaindacenodithiophene-based molecular donor: morphological features and use in the fabrication of compositionally tolerant, high-efficiency bulk heterojunction solar cells.

A novel solution-processable small molecule, namely, benzo[1,2-b:4,5-b]bis(4,4'-dihexyl-4H-silolo[3,2-b]thiophene-2,2'-diyl)bis(6-fluoro-4-(5'-hexyl-[2,2'-bithiophene]-5-yl)benzo[c][1,2,5]thiadiazole (p-SIDT(FBTTh2)2), was designed and synthesized by utilizing the silaindacenodithiophene (SIDT) framework as the central D(2) donor unit within the D(1)AD(2)AD(1) chromophore configuration. Relative to the widely studied 7,7'-[4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b']dithiophene-2,6-diyl]bis[6-fluoro-4-(5'-hexyl-[2,2'-bithiophene]-5-yl)benzo[c][1,2,5]thiadiazole] (p-DTS(FBTTh2)2), which contains the stronger donor fragment dithienosilole (DTS) as D(2), one finds that p-SIDT(FBTTh2)2 exhibits a wider band gap and can be used to fabricate bulk heterojunction solar cells with higher open circuit voltage (0.91 V). Most remarkably, thin films comprising p-SIDT(FBTTh2)2 can achieve exceptional levels of self-organization directly via solution deposition. For example, high-resolution transmission electron microscopy analysis shows that p-SIDT(FBTTh2)2 spin-cast from chlorobenzene organizes into crystalline domains with lattice planes that extend over length scales on the order of hundreds of nanometers. Such features suggest liquid crystalline properties during the evolution of the film. Moreover, grazing incidence wide-angle X-ray scattering analysis shows a strong tendency for the molecules to exist with a strong "face-on" orientation relative to the substrate plane. Similar structural features, albeit of more restricted dimensions, can be observed within p-SIDT(FBTTh2)2:PC71BM bulk heterojunction thin films when the films are processed with 0.4% diiodooctane (DIO) solvent additive. DIO use also increases the solar cell power conversion efficiencies (PCEs) from 1.7% to 6.4%. Of significance from a practical device fabrication perspective is that, for p-SIDT(FBTTh2)2:PC71BM blends, there is a wide range of compositions (from 20:80 to 70:30 p-SIDT(FBTTh2)2:PC71BM) that provide good photovoltaic response, i.e., PCE = 4-6%, indicating a robust tendency to form the necessary continuous phases for charge carrier collection. Light intensity photocurrent measurements, charge selective diode fabrication, and internal quantum efficiency determinations were carried out to obtain insight into the mechanism of device operation. Inclusion of DIO in the casting solution results in films that exhibit much lower photocurrent dependence on voltage and a concomitant increase in fill factor. At the optimum blend ratio, devices show high charge carrier mobilities, while mismatched hole and electron mobilities in blends with high or low donor content result in reduced fill factors and device performance.

Understanding the charge-transfer state and singlet exciton emission from solution-processed small-molecule organic solar cells.

Electroluminescence (EL) from the charge-transfer state and singlet excitons is observed at low applied voltages from high-performing small-molecule bulk-heterojunction solar cells. Singlet emission from the blends emerges upon altering the processing conditions, such as thermal annealing and processing with a solvent additive, and correlates with improved photovoltaic performance. Low-temperature EL measurements are utilized to access the physics behind the singlet emission.

Interplay of solvent additive concentration and active layer thickness on the performance of small molecule solar cells.

A relationship between solvent additive concentration and active layer thickness in small-molecule solar cells is investigated. Specifically, the additive concentration must scale with the amount of semiconductor material and not as absolute concentration in solution. Devices with a wide range of active layers with thickness up to 200 nm can readily achieve efficiencies close to 6% when the right concentration of additive is used.

Ultrafast Charge Generation in an Organic Bilayer Film.

The dynamics of charge generation in a high performing molecular photovoltaic system, p-SIDT(FBTTh2)2 (see Figure 1 ) is studied with transient absorption. The optimized bulk heterojunction material shows behavior observed in many other systems; the majority of charges are generated at short time scales (<150 fs), and a slower contribution from incoherently diffusing excitons is observed at low pump fluence. In a separate experiment, the role of bulk heterojunction material morphology on the process of ultrafast charge generation was investigated with bilayers made with solution processed donor molecules on a photopolymerized C60 layer. The majority of carriers are again produced at short time scales, ruling out the idea that subpicosecond charge generation can be understood wholly in terms of localized excitons. We evaluate possible causes of this behavior and propose that the excited state is highly delocalized on short time scales, providing ample probability density at the charge generating interface.

Palladium-catalyzed annulation of 1,2-diborylalkenes and -arenes with 1-bromo-2-[(Z)-2-bromoethenyl]arenes: a modular approach to multisubstituted naphthalenes and fused phenanthrenes.

(Z)-1,2-Diaryl-1,2-bis(pinacolatoboryl)ethenes underwent double-cross-coupling reactions with 1-bromo-2-[(Z)-2-bromoethenyl]arenes in the presence of [Pd(PPh(3))(4)] as a catalyst and 3 M aqueous Cs(2)CO(3) as a base in THF at 80 °C. The double-coupling reaction gave multisubstituted naphthalenes in good to high yields. Annulation of 1,2-bis(pinacolatoboryl)arenes with bromo(bromoethenyl)arenes in the presence of a catalyst system that consisted of [Pd(2)(dba)(3)] (dba=dibenzylideneacetone) and 2-dicyclohexylphosphino-2',6'-dimethoxybiphenyl (SPhos) under the same conditions produced fused phenanthrenes in good to high yields. The first annulation coupling occurred regiospecifically at the bromoethenyl moiety. This procedure is applicable to the facile synthesis of polysubstituted anthracenes, benzothiophenes, and dibenzoanthracenes through a double annulation pathway by using the corresponding dibromobis[(Z)-2-bromoethenyl]benzenes as diboryl coupling partners.

Facile synthesis and palladium-catalyzed cross-coupling reactions of 2,3-bis(pinacolatoboryl)-1,3-butadiene.

Treatment of 1,1-bis(pinacolatoboryl)ethene with an excess of 1-bromo-1-lithioethene gave 2,3-bis(pinacolatoboryl)-1,3-butadiene in high yield. Palladium-catalyzed cross-coupling of the resulting diborylbutadiene with aryl iodides took place smoothly in the presence of a catalytic amount of Pd(OAc)2/PPh3 and aqueous KOH to give 2,3-diaryl-1,3-butadienes in good yields. The coupling reaction with commercially available 4-acetoxyphenylmethyl chloride under the same conditions followed by hydrolysis of the acetyl groups gave anolignan B in a one-pot manner. A variety of [3]- to [6]dendralenes were synthesized by palladium-catalyzed coupling of the diene or 1,1-bis(pinacolato)borylethene with alkenyl or dienyl halides, respectively, in good yields.