Polymers For Optoelectronic Devices Research Articles

Meticulous Molecular Engineering of Crystal Orientation and Morphology in Conjugated Polymer Thin Films for Field-Effect Transistors.

The ability to precisely tailor molecular packing and film morphology in conjugated polymers offers a robust means to control their optoelectronic properties. This, however, remains a grand challenge. Herein, we report the dependency of molecular packing of an important conjugated polymer, poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT), on a set of intrinsic parameters and unveil the correlation between their crystalline structures and charge transport characteristics. Specifically, a family of PBTTT with varying side chains (i.e., hexyl, octyl, decyl, dodecyl, tetradecyl, and hexadecyl referred to as C6, C8, C10, C12, C14, and C16, respectively) and molecular weights (MWs) with a focus on C14 are judiciously designed and synthesized. Various crystalline structures are yielded by tuning the alkyl chain and MW of PBTTT together with thermal annealing. It reveals that extending the alkyl chain length of PBTTT to C14, along with a larger MW and heating at 180 °C, promotes the formation of edge-on crystallites with significantly improved orientation and ordering. Furthermore, these distinct crystalline structures greatly impact their charge mobilities. This study sheds light on the tailored design of crystalline structures in PBTTT through a synergetic approach, which paves the way for potential applications of PBTTT and other conjugated polymers in optoelectronic devices with enhanced performance.

Interchain Charge-Transfer States Mediate Triplet Formation in Purified Conjugated Polymer Aggregates

The pathways and dynamics of converting spin-allowed (S = 0) singlet excitons to spin-forbidden (S = 1) triplets have significant implications in determining performance metrics of conjugated polymers in optoelectronic devices. We study the effect of structural ordering factors on triplet formation in self-assembled aggregate π-stacked chains of poly(3-hexylthiophene) (P3HT) using single-molecule time-resolved intensity modulation and electric-field-dependent photoluminescence (PL) spectroscopy. Triplet generation is only efficient in P3HT aggregates of high purity, and formation yields are found to increase with nanofiber size. We propose that the high intrachain order in purified aggregates that extends exciton coherence lengths, leading to J-aggregate spectral signatures, is also important for populating interchain charge transfer (CT) states that, at longer times, recombine preferentially to triplets according to spin statistics. Electric-field-dependent PL decays of isolated P3HT aggregates show larg...

Material Crystallinity as a Determinant of Triplet Dynamics and Oxygen Quenching in Donor Polymers for Organic Photovoltaic Devices

This paper is concerned with the photophysics of triplet excitons in conjugated donor polymers, and their quenching by molecular oxygen. These photophysics are assayed by transient absorption spectroscopy, and correlated with X‐ray diffraction measurements of relative material crystallinity. Eleven different donor polymers are considered, including representatives from several classes of donor polymers recently developed for organic solar cell applications. Triplet lifetimes in an inert (nitrogen) environment range from <100 ns to 5 μs. A remarkably quantitative correlation is observed between these triplet lifetimes and polymer XRD strength, with more crystalline polymers exhibiting shorter triplet lifetimes. Given the broad range of polymers considered, this correlation indicates that material crystallinity is the dominant factor determining triplet lifetime for the polymers studied herein. The rate constant for oxygen quenching of these triplet states, determined from a comparison of transient absorption data under inert and oxygen environments, also show a correlation with material crystallinity. Overall these dependencies result in the yield of oxygen quenching of polymer triplet states increasing strongly as the crystallinity of the polymer is reduced. These photophysical data are compared with photochemical stability of these donor polymers, assayed by photobleaching studies of polymer films under continuous light exposure in an oxygen environment. A partial correlation is observed, with the most stable polymers being the most crystalline, exhibiting negligible oxygen quenching yields. These results are discussed in terms of the likely origins of the correlations between material crystallinity and photophysics, and in terms of their implications for the environmental stability of such donor polymers in optoelectronic devices.

Optimization of the side-chain density to improve the charge transport and photovoltaic performances of a low band gap copolymer

Poor processability and low molecular weights are often hindering the efficient utilization of novel conjugated polymers in optoelectronic devices. Increasing the alkyl side-chain density generally enhances the polymer solubility but may affect as well its optoelectronic properties. In this work, we use density functional theory to identify ways to increase the side-chain density of donor–acceptor alternate copolymers based on 2,1,3-benzothiadiazole, thiophene and thieno[3,2- b]thiophene units, without modifying there otherwise promising frontier orbital energy levels. Following the theoretical results, a new polymer could be synthesized, exhibiting good processability and improved charge transport. As a consequence, the photovoltaic device performances of this polymer family could be enhanced, reaching a 3.7% power conversion efficiency in a standard device configuration and without any post-deposition treatment.

Multicolor emission and thin film transistor properties of 1,8-diethynylcarbazole-based conjugated copolymers

1,8-Carbazole-based conjugated copolymers are synthesized by the Sonogashira polycondensation between the 1,8-diethynylcarbazole derivative and dibromo comonomers. The resulting polymers are fully characterized by GPC, and 1H NMR and IR spectroscopies, and their high thermal stability with the 5% decomposition temperature exceeding 350 °C by thermogravimetric analyses. The optical properties of the carbazole polymers reveal that the comonomer structures significantly affect the absorption and emission spectra. For example, a bathochromic shift of the spectra is achieved when electron-accepting comonomers are selected, finally leading to the production of additive primaries. The mixtures of three carbazole polymers with red, green, and blue emission colors provide a white light emission both in solutions and in thin solid films. In addition, the thin film transistor properties of the carbazole polymers are investigated. The 1,8-carbazole-based polymers display an intermediate mobility between the 2,7-carbazole and 3,6-carbazole-based counter polymers. All these results suggest the potential application possibilities of the 1,8-carbazole-based conjugated polymers in optoelectronic devices.

Wavelength dependence of nanoscale photodegradation in poly(3-octylthiophene) thin films

The use of conducting polymers in optoelectronic devices is subject to the understanding of the electro-optical processes that take place at the nanoscale. One of the photo-induced processes that limit their application is the photodegradation, which reduces the device working life. In this work the photodegradation of poly(3-octylthiophene) thin films was studied by combining Kelvin probe microscopy and optical microscopy. In this way, a direct correlation between morphological and contact potential changes with optical density changes as a function of the irradiation wavelength and intensity can be made. These results, complemented with Raman spectra help to clarify the degradation processes that are taking place. We find that the photodegradation strongly depends on the irradiation wavelength blue light being much more aggressive than UV. In addition, the optical properties change abruptly before any substantial change in the morphology is observed.

Charge Transport along Coiled Conjugated Polymer Chains

The motion of charges on coiled polymer chains was studied using a combination of experimental and theoretical methods. The conductive properties of dilute solutions of polyfluorene and fluorene−binaphthyl copolymers were studied by the pulse-radiolysis time-resolved microwave conductivity technique. This technique enables measurement of the (high frequency, 34 GHz) mobility of charges on isolated polymer chains. The motion of positive charges on the coiled polymer chains was studied theoretically by charge transport simulations with parameters from density functional theory calculations. This combined experimental and theoretical study shows that the mobility of charges decreases with increasing degree of chain coiling. The mobility along (infinitely long) stretched polyfluorene is calculated to be as high as tens of centimeters2/volt·second. Our results imply that the performance of conjugated polymers in optoelectronic devices can be significantly improved by optimization of the organization on a molecular scale.

Effect of intermolecular disorder on the intrachain charge transport in ladder-type poly(p-phenylenes)

We have studied the real and imaginary parts of the complex intrachain mobility of charge carriers on solid samples of ladder-type polymers using time-resolved microwave conductivity measurements. Experiments on samples with a different average polymer chain length show that the motion of charge carriers is limited by the chain ends. The experimental results can be described by one-dimensional diffusive motion along the polymer backbone. The intrachain mobility deduced for an infinitely long ladder-type polymer chain (with no barriers to charge transport such as defects or conjugation breaks) subject to interchain interactions is $30\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{2}∕\mathrm{V}\phantom{\rule{0.2em}{0ex}}\mathrm{s}$. This value is 20 times lower than the intrachain mobility found for charges along infinitely long isolated ladder-type polymer chains in dilute solution. Thus we find that interchain interactions in solid samples severely decrease the intrachain charge carrier mobility. However, the intrachain mobility of $30\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{2}∕\mathrm{V}\phantom{\rule{0.2em}{0ex}}\mathrm{s}$ is more than four orders of magnitude higher than mobility values obtained from time of flight measurements reported in the literature. Hence, the performance of ladder-type polymers in optoelectronic devices can be significantly improved.

Electrochemical and Photophysical Properties of a Novel Polythiophene with Pendant Fulleropyrrolidine Moieties: Toward “Double Cable” Polymers for Optoelectronic Devices

We have prepared a novel bithiophene carrying a fulleropyrrolidine substituent. Its electrochemical polymerization affords an electron donor−acceptor material that has been studied by means of electrochemical and spectroscopic techniques. The results from photoinduced absorption (PIA) and light-induced electron spin resonance (LESR) experiments demonstrate a photoinduced electron transfer from the polythiophene backbone to the tethered acceptor moieties.

Interchain and intrachain exciton transport in conjugated polymers: ultrafast studies of energy migration in aligned MEH-PPV/mesoporous silica composites

In this paper, we show how composite samples consisting of chains of the semiconducting polymer MEH-PPV embedded into the channels of oriented, hexagonal nanoporous silica glass allow control over energy transfer and exciton migration in the polymer. The composite samples are characterized by two polymer environments: randomly oriented and film-like segments with short conjugation-length outside the channels, and well aligned, long conjugation segments that are isolated by encapsulation within the porous glass. Ultrafast emission anisotropy measurements show that excitons migrate unidirectionally from the polymer segments outside the pores to the oriented chains within the pores, leading to a spontaneous increase in emission polarization with time. Because the chains in the pores are isolated, the observed increase in polarization can take place only by exciton migration along the polymer backbone. The anisotropy measurements show that energy migration along the backbone occurs more slowly than Förster energy transfer between polymer chains; transfer along the chain likely takes place by a thermally-activated hopping mechanism. Similar time scales for intra- and interchain energy transfer are also observed for MEH-PPV chains in solution. All the results provide new insights for optimizing the use of conjugated polymers in optoelectronic devices.