Relation between the Electronic Properties of Regioregular Donor-Acceptor Terpolymers and Their Binary Copolymers.

By analyzing the optical band gap and energy levels of seven different regioregular terpolymers in which two different electron-rich donor moieties are alternating with a common electron-deficient acceptor unit along the backbone, we establish a direct correlation with the properties of the corresponding binary copolymers in which one donor and one acceptor are combined. For this study, we use diketopyrrolopyrrole as the common acceptor and different π-conjugated aromatic oligomers as donors. We find that the optical band gap and frontier orbital energies of the terpolymers are the arithmetic average of those of the parent copolymers with remarkable accuracy. The same relationship is also found for the open-circuit voltage of the bulk heterojunction solar cells made with the ter- and copolymers in combination with [6,6]-phenyl-C71-butyric acid methyl ester. Comparison of these findings with data in the literature suggests that this is a universal rule that can be used as a tool when designing new π-conjugated polymers. The experimental results are supported by a semiempirical quantum chemical model that accurately describes the energy levels of the terpolymers after parametrization on the energy levels of the copolymers and also provides a theoretical explanation for the observed arithmetic relations.

Impact of polymorphism on the optoelectronic properties of a low-bandgap semiconducting polymer.

Polymorphism of organic semiconducting materials exerts critical effects on their physical properties such as optical absorption, emission and electrical conductivity, and provides an excellent platform for investigating structure-property relations. It is, however, challenging to efficiently tune the polymorphism of conjugated polymers in aggregated, semi-crystalline phases due to their conformational freedom and anisotropic nature. Here, two distinctly different semi-crystalline polymorphs (ß1 and ß2) of a low-bandgap diketopyrrolopyrrole polymer are formed through controlling the solvent quality, as evidenced by spectroscopic, structural, thermal and charge transport studies. Compared to ß1, the ß2 polymorph exhibits a lower optical band gap, an enhanced photoluminescence, a reduced π-stacking distance, a higher hole mobility in field-effect transistors and improved photocurrent generation in polymer solar cells. The ß1 and ß2 polymorphs provide insights into the control of polymer self-organization for plastic electronics and hold potential for developing programmable ink formulations for next-generation electronic devices.

The influence of siloxane side-chains on the photovoltaic performance of a conjugated polymer.

The effect of gradually replacing the branched alkyl side chains of a diketopyrrolopyrrole (DPP) conjugated polymer by linear side chains containing branched siloxane end groups on the photovoltaic performance of blends of these polymers with a common fullerene acceptor is investigated. With an increasing proportion of siloxane side chains, the molecular weight and solubility of the polymers decreases. While the siloxane containing polymers exhibit a higher hole mobility in field-effect transistors, their performance in solar cells is less than the polymer with only alkyl sides chains. Using grazing-incidence wide-angle X-ray scattering, transmission electron microscopy, and fluorescence spectroscopy we identify two main reasons for the reduced performance of siloxane containing polymers in solar cells. The first one is a somewhat coarser phase-separated morphology with slightly wider polymer fibers. This is unexpected as often the fiber width is inversely correlated with polymer solubility. The second one is stronger non-radiative decay of the pristine polymers containing siloxane side chains.

On the homocoupling of trialkylstannyl monomers in the synthesis of diketopyrrolopyrrole polymers and its effect on the performance of polymer-fullerene photovoltaic cells.

Homocoupling of monomers in a palladium-catalyzed copolymerization of donor-acceptor polymers affects the perfect alternating structure and may deteriorate the performance of such materials in solar cells. Here we investigate the effect of homocoupling bis(trialkylstannyl)-thiophene and -bithiophene monomers in two low band gap poly(diketopyrrolopyrrole-alt-oligothiophene) polymers by deliberately introducing extended oligothiophene defects in a controlled fashion. We find that extension of the oligothiophene by one or two thiophenes and creating defects up to at least 10% does not significantly affect the opto-electronic properties of the polymers or their photovoltaic performance as electron donor in solar cells in combination with [6,6]-phenyl C71 butytic acid methyl ester as acceptor. By using model reactions, we further demonstrate that for the optimized synthetic protocol and palladium-catalyst system the naturally occurring defect concentration in the polymers is expected to be less than 0.5%.

Near-Infrared Tandem Organic Photodiodes for Future Application in Artificial Retinal Implants.

Photovoltaic retinal prostheses show great potential to restore sight in patients suffering from degenerative eye diseases by electrical stimulation of the surviving neurons in the retinal network. Herein, organic photodiodes (OPDs) sensitive to near-infrared (NIR) light are evaluated as photovoltaic pixels for future application in retinal prostheses. Single-junction and tandem OPDs are compared. In the latter, two nominally identical single-junction cells are processed on top of each other, effectively doubling the open-circuit voltage (V OC ). Both single-junction and tandem OPD micropixels can deliver the required charge to stimulate neurons under pulsed NIR light at physiologically safe intensities when connected to stimulating microelectrodes in a physiological saline solution. However, only tandem OPD pixels can cover the entire charge per pulse neural stimulation window due to their higher V OC (≈1.4 V). This demonstrates the viability of high-resolution retinal prostheses based on flexible OPD arrays.

The effect of side-chain substitution and hot processing on diketopyrrolopyrrole-based polymers for organic solar cells.

The effects of cold and hot processing on the performance of polymer-fullerene solar cells are investigated for diketopyrrolopyrrole (DPP) based polymers that were specifically designed and synthesized to exhibit a strong temperature-dependent aggregation in solution. The polymers, consisting of alternating DPP and oligothiophene units, are substituted with linear and second position branched alkyl side chains. For the polymer-fullerene blends that can be processed at room temperature, hot processing does not enhance the power conversion efficiencies compared to cold processing because the increased solubility at elevated temperatures results in the formation of wider polymer fibres that reduce charge generation. Instead, hot processing seems to be advantageous when cold processing is not possible due to a limited solubility at room temperature. The resulting morphologies are consistent with a nucleation-growth mechanism for polymer fibres during drying of the films.

The effect of branching in a semiconducting polymer on the efficiency of organic photovoltaic cells.

The impact of branching in a diketopyrrolopyrrole polymer on the performance of polymer-fullerene photovoltaic cells is investigated. Compared to the linear polymer, the branched polymer affords a more finely dispersed fibrillar network in the photoactive layer and as a result a large enhancement of the photocurrent and power conversion efficiency.

Polymer Solar Cells: Solubility Controls Fiber Network Formation.

The photoactive layer of polymer solar cells is commonly processed from a four-component solution, containing a semiconducting polymer and a fullerene derivative dissolved in a solvent-cosolvent mixture. The nanoscale dimensions of the polymer-fullerene morphology that is formed upon drying determines the solar cell performance, but the fundamental processes that govern the size of the phase-separated polymer and fullerene domains are poorly understood. Here, we investigate morphology formation of an alternating copolymer of diketopyrrolopyrrole and a thiophene-phenyl-thiophene oligomer (PDPPTPT) with relatively long 2-decyltetradecyl (DT) side chains blended with [6,6]-phenyl-C71-butyric acid methyl ester. During solvent evaporation the polymer crystallizes into a fibrous network. The typical width of these fibers is analyzed by quantification of transmission electron microscopic images, and is mainly determined by the solubility of the polymer in the cosolvent and the molecular weight of the polymer. A higher molecular weight corresponds to a lower solubility and film processing results in a smaller fiber width. Surprisingly, the fiber width is not related to the drying rate or the amount of cosolvent. We have made solar cells with fiber widths ranging from 28 to 68 nm and found an inverse relation between fiber width and photocurrent. Finally, by mixing two cosolvents, we develop a ternary solvent system to tune the fiber width. We propose a model based on nucleation-and-growth which can explain these measurements. Our results show that the width of the semicrystalline polymer fibers is not the result of a frozen dynamical state, but determined by the nucleation induced by the polymer solubility.

Homocoupling defects in diketopyrrolopyrrole-based copolymers and their effect on photovoltaic performance.

We study the occurrence and effect of intrachain homocoupling defects in alternating push-pull semiconducting PDPPTPT polymers based on dithienyl-diketopyrrolopyrrole (TDPPT) and phenylene (P) synthesized via a palladium-catalyzed cross-coupling polymerization. Homocoupled TDPPT-TDPPT segments are readily identified by the presence of a low-energy shoulder in the UV/vis/NIR absorption spectrum. Remarkably, the signatures of these defects are found in many diketopyrrolopyrrole (DPP)-based copolymers reported in the literature. The defects cause a reduction of the band gap, a higher highest occupied molecular orbital (HOMO) level, a lower lowest unoccupied molecular orbital (LUMO) level, and a localization of these molecular orbitals. By synthesizing copolymers with a predefined defect concentration, we demonstrate that their presence reduces the short-circuit current and open-circuit voltage of solar cells based on blends of PDPPTPT with [70]PCBM. In virtually defect-free PDPPTPT, the power conversion efficiency is as high as 7.5%, compared to 4.5-5.6% for polymers containing 20% to 5% defects.