Molecular Bulk Heterojunction Research Articles

Regioregularity and Electron Deficiency Control of Unsymmetric Diketopyrrolopyrrole Copolymers for Organic Photovoltaics.

Manipulating the electron deficiency and controlling the regioregularity of π-conjugated polymers are important for the fine-tuning of their electronic and electrochemical properties to make them suitable for an organic solar cell. Here, we report such a molecular design of unsymmetric diketopyrrolopyrrole (DPP) based copolymers with different aromatic side units of either thiophene (Th), pyridine (Py), or fluorobenzene (FBz). The unsymmetric electron acceptors of Th–DPP–Py and Th–DPP–FBz were polymerized with the electron donor of two-dimensional benzobisthiophene (BDT-Th), affording two regiorandom DPP copolymers. They exhibited contrasting molecular orbital levels and bulk heterojunction morphology in methanofullerene-blended films, leading to power conversion efficiencies of 3.75 and 0.18%, respectively. We further synthesized a regioregular DPP copolymer via sandwiching the centrosymmetric BDT-Th unit by two Th–DPP–Py units in an axisymmetric manner. The extensive characterization through morphology observation, X-ray diffraction, and space-charge-limited current mobilities highlight the case-dependent positive/negative effects of regioregularity and electron deficiency control.

Solution-processable all-small molecular bulk heterojunction films for stable organic photodetectors: near UV and visible light sensing

Organic photodetectors with solution-processed all small-molecular bulk heterojunction layers detect near UV light and are stable under high-intensity lights stronger than sun light.

Direct arylation as a versatile tool towards thiazolo[5,4-d]thiazole-based semiconducting materials.

A series of thiazolo[5,4-d]thiazole-based small molecule organic optoelectronic materials is synthesized via a straightforward microwave-activated Pd-catalyzed C-H arylation protocol. The procedure allows us to obtain extended 2,5-dithienylthiazolo[5,4-d]thiazole chromophores with tailor-made energy levels and absorption patterns, depending on the introduced (het)aryl moieties and the molecular (a)symmetry, by shortened sequences without organometallic intermediates. The synthesized materials can be applied as either electron donor or electron acceptor light-harvesting materials in molecular bulk heterojunction organic solar cells.

Role of Localized States on Carrier Transport in Bulk Heterojunction Materials Comprised of Organic Small Molecule Donors

The photocarrier transport properties in two leading molecular bulk-heterojunction systems, p-DTS(PTTh2)2:PC71BM and p-DTS(FBTTh2)2:PC71BM, are studied using transient photoconductivity measurements. We find that in these systems shallow trap states are distributed at relatively narrow energy range, giving rise to time-independent carrier mobility in the nanosecond time regime that evidently contributes to the high performance of molecular-based OPV devices.

Surface Plasmon Enhanced Organic Solar Cells with a MoO3 Buffer Layer

High-efficiency surface plasmon enhanced 1,1-bis-(4-bis(4-methyl-phenyl)-amino-phenyl)-cyclohexane:C70 small molecular bulk heterojunction organic solar cells with a MoO3 anode buffer layer have been demonstrated. The optimized device based on thermal evaporated Ag nanoparticles (NPs) shows a power conversion efficiency of 5.42%, which is 17% higher than the reference device. The improvement is attributed to both the enhanced conductivity and increased absorption due to the near-field enhancement of the localized surface plasmon resonance of Ag NPs.

A dual-functional additive improves the performance of molecular bulk heterojunction photovoltaic cells

In this paper we report a simple and effective approach towards improving the performance of molecular bulk heterojunction (BHJ) photovoltaics through incorporation of a new, nonvolatile processing additive; (3-chloropropyl)trimethoxysilane (CP3MS). A small amount of the additive CP3MS combined with post-annealing treatment significantly enhanced the power conversion efficiency (PCE) of dialkylated diketopyrrolopyrrole chromophore (SMDPPEH)-based molecular BHJ solar cells. The PCE increased from 2.75% for a device prepared without the additive or annealing to 4.55% for a device containing 0.1% CP3MS that had been subjected to post-annealing treatment at 100 °C for 10 min. CP3MS performed an interesting dual function when incorporated as an additive in molecular BHJ devices. The first was that it controlled the morphology: the addition of 0.1% CP3MS to the SMDPPEH:PC61BM blend was sufficient to improve the film's crystallinity and morphology. The second function was the spontaneous migration of the CP3MS molecules from the bulk to the interface between the active layer and the Al cathode, forming an ultrathin interlayer that acted as a buffer layer suppressing charge recombination and enhancing charge transport at the interface. Analyses using atomic force microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy, together with examinations of device performance, confirmed the dual-functional nature of CP3MS as an additive. CP3MS also showed its promising ability to enhance the performance of different blend systems. The performance of benzodithiophene (BDT)-based molecular solar cells enhanced from 3.05% without additive to 3.8% with the incorporation of 0.1% CP3MS.

Driving Energy for the Formation of Molecular Bulk Heterojunction in Small Molecule Based Organic Photodiode

not Available.

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.

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.

Tailor-Made Additives for Morphology Control in Molecular Bulk-Heterojunction Photovoltaics

Tailor-made additives, which are molecules that share the same molecular structure as a parent molecule with only slight structural variations, have previously been demonstrated as a useful means to control crystallization dynamics in solution. For example, tailor-made additives can be added to solutions of a crystallizing parent molecule to alter the crystal growth rate, size, and shape. We apply this strategy as a means to predictably control morphology in molecular bulk-heterojunction (BHJ) photovoltaic cells. Through the use of an asymmetric oligomer substituted with a bulky triisobutylsilyl end group, the morphology of BHJ blends can be controlled resulting in a near doubling (from 1.3 to 2.2%) in power conversion efficiency. The use of tailor-made additives provides promising opportunities for controlling crystallization dynamics, and thereby film morphologies, for many organic electronic devices such as photovoltaics and field-effect transistors.

Photoinduced Charge Generation in a Molecular Bulk Heterojunction Material

Understanding the charge generation dynamics in organic photovoltaic bulk heterojunction (BHJ) blends is important for providing the necessary guidelines to improve overall device efficiency. Despite more than 15 years of experimental and theoretical studies, a universal picture describing the generation and recombination processes operating in organic photovoltaic devices is still being forged. We report here the results of ultrafast transient absorption spectroscopy measurements of charge photogeneration and recombination processes in a high-performing solution-processed molecular BHJ. For comparison, we also studied a high-performing polymer-based BHJ material. We find that the majority of charge carriers in both systems are generated on <100 fs time scales and posit that excited state delocalization is responsible for the ultrafast charge transfer. This initial delocalization is consistent with the fundamental uncertainty associated with the photon absorption process (in the visible, λ/4π > 30 nm) and is comparable with the phase-separated domain size. In addition, exciton diffusion to charge-separating heterojunctions is observed at longer times (1-500 ps). Finally, charge generation in pure films of the solution processed molecule was studied. Polarization anisotropy measurements clearly demonstrate that the optical properties are dominated by molecular (Frenkel) exictons and delocalized charges are promptly produced (t < 100 fs).

Pyridalthiadiazole-Based Narrow Band Gap Chromophores

π-Conjugated materials containing pyridal[2,1,3]thiadiazole (PT) units have recently achieved record power conversion efficiencies of 6.7% in solution-processed, molecular bulk-heterojunction (BHJ) organic photovoltaics. Recognizing the importance of this new class of molecular systems and with the aim of establishing a more concrete path forward to predict improvements in desirable solid-state properties, we set out to systematically alter the molecular framework and evaluate structure-property relationships. Thus, the synthesis and properties of 13 structurally related D(1)-PT-D(2)-PT-D(1) compounds, where D represents a relatively electron-rich aromatic segment compared to PT, are provided. Physical properties were examined using a combination of absorption spectroscopy, cyclic voltammetry, thermal gravimetric analysis, differential scanning calorimetry, and solubility analysis. Changes to end-capping D(1) units allowed for fine control over electronic energy levels both in solution and in the bulk. Substitution of different alkyl chains on D(2) gives rise to controllable melting and crystallization temperatures and tailored solubility. Alterations to the core donor D(2) lead to readily identifiable changes in all properties studied. Finally, the regiochemistry of the pyridal N-atom in the PT heterocycle was investigated. The tailoring of structures via subtle structural modifications in the presented molecular series highlights the simplicity of accessing this chromophore architecture. Examination of the resulting materials properties relevant for device fabrication sets forth which can be readily predicted by consideration of molecular structure and which lack a systematic understanding. Guidelines can be proposed for the design of new molecular frameworks with the possibility of outperforming the current state of the art OPV performance.

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.

A Systematic Approach to Solvent Selection Based on Cohesive Energy Densities in a Molecular Bulk Heterojunction System

AbstractThe solubilities of 3,6‐bis(5‐(benzofuran‐2‐yl)thiophen‐2‐yl)‐2,5‐bis(2‐ethylhexyl)pyrrolo[3,4‐c]pyrrole‐1,4‐dione (DPP(TBFu)2) and [6,6]‐phenyl‐C71‐butyric acid methyl ester (PC71BM) in a series of solvents are measured, and this data is used to calculate the Hansen solubility parameters of the two materials. The dispersion, polar, and H‐bonding parameters of DPP(TBFu)2 and PC71BM were found to be (19.3, 4.8, 6.3) and (20.2, 5.4, 4.5) MPa1/2, respectively, with an error of ± 0.8 MPa1/2. Based on the solubility properties of the two materials, three new solvents (thiophene, trichloroethylene and carbon disulfide) were utilized for the DPP(TBFu)2:PC71BM system which, after device optimization, led to power conversion efficiencies up to 4.3%.

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%.