Molecular Bulk Heterojunction Solar Cells Research Articles

Dithieno[3,2-b:2′,3′-d]pyrrole-benzo[c][1,2,5]thiadiazole conjugate small molecule donors: effect of fluorine content on their photovoltaic properties

Two new small molecule donors, namely ICT4 and ICT6 with D1-A-D2-A-D1 architecture having 2,4-bis(2-ethylhexyl)-4H-dithieno[3,2-b:2',3'-d]pyrrole (EHDTP, D1) and 4,8-bis((2-ethylhexyl)oxy)benzo[1,2-b:4,5-b']dithiophene (OBDT, D2) as the terminal and central donor, and benzo[c][1,2,5]thiadiazole (BT for ICT4) and 5,6-difluorobenzo[c][1,2,5]thiadiazole (F2BT for ICT6) as the acceptor (A) moieties, are synthesized and their optical, electronic and photovoltaic properties are investigated. Both ICT4 and ICT6 have considerable solubility in various solvents and possess efficient light absorption ability [ε (×105 mol-1 cm-1) is 0.99 and 1.06, respectively for ICT4 and ICT6] and appropriate frontier molecular orbital energy offsets with [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM). Bulk heterojunction solar cells (BHJSCs) are fabricated using ICT4/ICT6 and PC71BM as donors and acceptors, respectively and BHJSCs with two-step annealed (thermal followed by solvent vapor annealing) active layers of ICT4 and ICT6 show overall power conversion efficiencies (PCEs) of 5.46% and 7.91%, respectively. The superior photovoltaic performance of the ICT6 based BHJSCs is due to the favourable morphology with a nanoscale interpenetrating network in the ICT6:PC71BM active layer induced by the fluorine atoms on the BT acceptor, which significantly enhances the dissociation of excitons, charge transport and the charge collection efficiency, and suppresses bimolecular recombination in the BHJ. The observed higher PCE of 7.91% indicates that ICT6 is one of the best BT based donor material for small molecular BHJSCs.

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.

Self-Assembled Amphiphilic Diketopyrrolopyrrole-Based Oligothiophenes for Field-Effect Transistors and Solar Cells

An amphiphilic diketopyrrolopyrrole-based oligothiophene (DPPamphi) has been prepared that displays robust self-assembly behavior in the bulk, in solution, and on surfaces. The material has been used as the active component in organic field-effect transistors and molecular bulk-heterojunction solar cells.

Polydimethylsiloxane as a Macromolecular Additive for Enhanced Performance of Molecular Bulk Heterojunction Organic Solar Cells

The effect of the macromolecular additive, polydimethylsiloxane (PDMS), on the performance of solution processed molecular bulk heterojunction solar cells is investigated, and the addition of PDMS is shown to improve device power conversion efficiency by ∼70% and significantly reduce cell-to-cell variation, from a power conversion efficiency of 1.25 ± 0.37% with no PDMS to 2.16 ± 0.09% upon the addition of 0.1 mg/mL PDMS to the casting solution. The cells are based on a thiophene and isoindigo containing oligomer as the electron donor and [6,6]-phenyl-C61 butyric acid methyl ester (PC(61)BM) as the electron acceptor. PDMS is shown to have a strong influence on film morphology, with a significant decrease in film roughness and feature size observed. The morphology change leads to improved performance parameters, most notably an increase in the short circuit current density from 4.3 to 6.8 mA/cm(2) upon addition of 0.1 mg/mL PDMS. The use of PDMS is of particular interest, as this additive appears frequently as a lubricant in plastic syringes commonly used in device fabrication; therefore, PDMS may unintentionally be incorporated into device active layers.

Oligoselenophene Derivatives Functionalized with a Diketopyrrolopyrrole Core for Molecular Bulk Heterojunction Solar Cells

Solution-processable oligoselenophenes functionalized with diketopyrrolopyrrole cores have been synthesized for use as the donor material in bulk heterojunction solar cells. The optical absorption of these materials extends to the edge of the visible spectrum. Power conversion efficiencies of 1.53 ± 0.04% for DPPS and 0.84 ± 0.04% for DPPDS were obtained under simulated 100 mW/cm(2) AM 1.5G irradiation for devices when PC(61)BM was used as an acceptor. DPPS showed hole mobilities of 4 × 10(-5) cm(2)/(V s) and a peak external quantum efficiency (EQE) of 25%, while DPPDS showed hole mobilities of 2 × 10(-5) cm(2)/(V s) and a peak EQE of 19%. To the best of our knowledge, these are the first oligoselenophenes that have been reported in molecular bulk heterojunction solar cells and this study could serve as a springboard for the design and optimization of high-performance selenophene-containing photovoltaics.

ChemInform Abstract: A Tailored Hybrid BODIPY—Oligothiophene Donor for Molecular Bulk Heterojunction Solar Cells with Improved Performances.

AbstractFixation of a hexyl‐2,2′‐bithienyl unit on a conjugated BODIPY donor increases the conversion efficiency of the resulting molecular bulk heterojunction solar cell significantly.

Detection of trap charge in small molecular organic bulk heterojunction solar cells

We detect and measure the trap charges in a small molecular bulk heterojunction solar cell under operating conditions. The trap-charge density is estimated from capacitance measurements with light incident on the sample. At high intensities ($\ensuremath{\sim}1\text{ }\text{sun}$, $100\text{ }\text{mW}/{\text{cm}}^{2}$), the trapped charge concentration leads to a spatial distortion of the electric field in the device. The lower limit of the trap-charge density is estimated to be $6\ifmmode\times\else\texttimes\fi{}{10}^{16}\text{ }{\text{cm}}^{\ensuremath{-}3}$. The frequency dependence of the capacitance suggests that the charges are trapped in a manifold of deep states present in the energy gap of the semiconductors. The distortion of the electric field by this trap charge affects the charge-carrier collection efficiency.

Quaterthiophene-based multipods as promising materials for solution-processible organic solar cells and field effect transistors

The preparation and characterization of novel electron donor compounds with two ( D1) and four ( D2) quaterthiophene-containing arms attached to the flexible carbosilane–siloxane core are described. These materials were investigated in organic field effect transistors and photovoltaic cells. A power conversion efficiency of ∼0.9% obtained for D1:[70]PCBM blends is one of the highest values achieved up to now for solution-processible small molecular bulk heterojunction solar cells with large band-gap oligothiophene-based donor materials comprising no other chromophore units.

A tailored hybrid BODIPY–oligothiophene donor for molecular bulk heterojunction solar cells with improved performances

Fixation of a 5-hexyl-2,2'-bithienyl unit on a conjugated BODIPY donor increases the conversion efficiency of the resulting molecular bulk heterojunction solar cells from 1.30 to 2.20%.

A “Cyanine−Cyanine” Salt Exhibiting Photovoltaic Properties

Association between cationic and anionic heptamethine dyes led to the formation of a new organic salt (3) displaying exceptional light-harvesting properties in the NIR spectral range and having amphoteric redox character. Preliminary results of molecular bulk heterojunction solar cells based on the title compound 3 and [60]PCBM as the only active layer reveal this new dye as a promising light-harvesting material for photovoltaics.

Multi-donor molecular bulk heterojunction solar cells: improving conversion efficiency by synergistic dye combinations

Molecular bulk heterojunction solar cells with a power conversion efficiency of 1.70% have been fabricated using as donor BODIPY dyes with complementary light-harvesting properties.