The Influence of Excited State Physics in Conjugated Polymer Devices

Lead Research Organisation: Heriot-Watt University
Department Name: Sch of Engineering and Physical Science

Abstract

There has been remarkable progress over the last decade in making displays, lighting panels, solar cells and lasers out of flexible, plastic materials. This has a wide range of potential applications, such as roll up TV displays or having power generation, sensors and data communications systems woven into your clothing. The technology of organic LEDs has now matured to the degree that OLED displays are mass produced in consumer products such as mobile phones.

The next generations of plastic electronics products will include OLED lighting, solar cells and lasers. It is now clear however that to deliver the technology for these demanding applications it is necessary to develop a deeper understanding of the basic materials physics. In all of these devices the physics of the excited states of molecules plays a crucial role in performance. In OLEDs the efficiency at high brightness is limited by the absorption due to charge carriers and various interactions that quench the light emission from excited states. In lasers there is a delicate interplay of the excited state physics and laser losses, and so far little is known about how the chemical and structural properties of the materials may be used to control this.

This proposal seeks to develop this understanding by bringing together the expertise of two groups: one who are experts in measuring the optoelectronic performance of these polymers and in their application for photonics, and the other who are experts in the quantum theory of organic materials. Through a combination of theory and experiment we will aim to understand the complex excited state interactions of organic semiconductors, and uncover new design strategies to control these processes. This would help us to optimise the performance (e.g. efficiency and brightness) of current devices; and enable new generations of photonic devices based on these materials.

We will make optical measurements of the fundamental excited-state processes in the materials and their behaviour under device conditions. Using state-of-the-art techniques in quantum mechanics we can also simulate the microscopic physics which gives rise to these effects. Measuring these interactions in working devices is particularly demanding and to achieve this we will also draw on specific complementary expertise from our project partners at Cambridge Display Technologies and the University of Alicante. We will then apply our new knowledge of excited states to the operation of a range of organic devices including OLEDs, lasers, solar cells and optical amplifiers. We will quantify the significance of the different excited state interactions and develop design strategies that can minimise parasitic processes and optimise operation.
 
Description Excited state absorption (ESA) is studied using time-dependent density functional theory and compared with experiments performed in dilute solutions. The molecules investigated are a fluorene pentamer, polyfluorene F8, the alternating F8 copolymer with benzothiadiazole F8BT, and two blue-emitting random copolymers F8PFB and F8TFB. Calculated and measured spectra show qualitatively comparable results. The ESA cross-section of co-polymers at its maximum is about three times lower than that of F8. The ESA spectra are found to change little upon structural relaxation of the excited state, or change in the order of sub-units in a co-polymer, for all studied molecules. In all these molecules, the strongest ESA transition is found to arise from the same electronic process, exhibiting a reversal of the charge parity. In addition, F8PFB and F8TFB are found to possess almost identical electronic behaviour.

Electronic energy transfer (EET) in organic materials is a key mechanism that controls the efficiency of many processes, including light harvesting antennas in natural and artificial photosynthesis, organic solar cells, and biological systems. In this paper we have examined EET in solid-state thin-films of polyfluorene, a prototypical conjugated polymer, with ultrafast photoluminescence experiments and theoretical modeling. We observe EET occurring on a 680 ± 300 fs time scale by looking at the depolarisation of photoluminescence. An independent, predictive microscopic theoretical model is built by defining 125 000 chromophores containing both spatial and energetic disorder appropriate for a spin-coated thin film. The model predicts time-dependent exciton dynamics, without any fitting parameters, using the incoherent Förster-type hopping model. Electronic coupling between the chromophores is calculated by an improved version of the usual line-dipole model for resonant energy transfer. Without the need for higher level interactions, we find that the model is in general agreement with the experimentally observed 680 ± 300 fs depolarisation caused by EET. This leads us to conclude that femtosecond EET in polyfluorene can be described well by conventional resonant energy transfer, as long as the relevant microscopic parameters are well captured. The implications of this finding are that dipole-dipole resonant energy transfer can in some circumstances be fully adequate to describe ultrafast EET without needing to invoke strong or intermediate coupling mechanisms.

We describe a general scheme to obtain force-field parameters for classical molecular dynamics simulations of conjugated polymers. We identify a computationally inexpensive methodology for calculation of accurate intermonomer dihedral potentials and partial charges. Our findings indicate that the use of a two-step methodology of geometry optimization and single-point energy calculations using DFT methods produces potentials which compare favorably to high level theory calculation. We also report the effects of varying the conjugated backbone length and alkyl side-chain lengths on the dihedral profiles and partial charge distributions and determine the existence of converged lengths above which convergence is achieved in the force-field parameter sets. We thus determine which calculations are required for accurate parametrization and the scope of a given parameter set for variations to a given molecule. We perform simulations of long oligomers of dioctylfluorene and hexylthiophene in explicit solvent and find peristence lengths and end-length distributions consistent with experimental values.

Excited-state properties of conjugated polymers play a central role in applications ranging from organics-based photovoltaics to nonlinear photonics. From a theoretical and computational point of view, however, an accurate first-principles description poses a formidable task. Typical molecule sizes go well beyond the size limits for which highly reliable wave function based electronic-structure methods can be applied. In the present work, we demonstrate that nonlinear-response density functional theory can be used to accurately model the excited state absorption process in an important class of conjugated materials. We compute transitions between up to 100 excited states for fluorene oligomers containing up to about 100 conjugated atoms. Furthermore, we demonstrate that this approach can explain the nature of absorption bands in the ESA in near-infrared and visible spectral range. These systems are large enough that we approach the polymer limit in terms of electronic properties of excited states. The results obtained are in good agreement with available experimental data.
Exploitation Route These finding could be build on in the modelling of organic PV cells. For example our feely available force-field got fluorenes and thiophenes could be used to perform other molecular dynamics simulations for mechanical or optical properties.
Sectors Aerospace, Defence and Marine,Digital/Communication/Information Technologies (including Software),Energy

 
Title OPLS Force fields for Fluorene and Thiophene polymers 
Description OPLS Force fields for Fluorene and Thiophene polymers 
Type Of Material Database/Collection of data 
Year Produced 2016 
Provided To Others? Yes  
Impact Unknown, Force-fields are now Open access 
URL http://pubs.acs.org/doi/suppl/10.1021/acs.jctc.5b01195
 
Description Cambridge Display Technologies 
Organisation Cambridge Display Technology
Country United Kingdom 
Sector Private 
PI Contribution We are performing the basic science in the project
Collaborator Contribution CDT staff are helping us with insights from their commercial activities and the provision of specific device samples.
Impact None yet.
Start Year 2013
 
Description Dassault Systeme Biovia 
Organisation Dassault Systemes UK Ltd
Country United Kingdom 
Sector Private 
PI Contribution New collaboration for future work borne out of Molecular dynamics force fields developed under this proposal. We have developed a protocol for finding such force fields in conjugated semiconductors which is a class of material not widely covered in the MD literature.
Collaborator Contribution DS-Biovia are major software developers in this field and are interested in all works associated with force-field development which can broaden the coverage of their application suites.
Impact EPSRC proposal with DS biota as a supporting partner about to be submitted ( March 2017)
Start Year 2017