Characterization of the charge carriers in molecularly doped organic semiconductors using optical spectroscopy and Raman spectroscopy

​Motivation
Organic semiconductors (OSC) have emerged as an important class of materials for electronic applications. Their inherent chemical versatility, mechanical flexibility and ability to be processed from solutions have put them as a front-runner candidate towards applications where flexible devices and cost-effective large scale production are desired. However, due to the intrinsic low charge carrier density in OSCs, additional strategies to improve their electrical transport properties were investigated and has been the focus of the research community for decades.
A successful strategy that has been developed and intensely researched is molecular doping of OSC. Molecular doping is defined as the process of adding a small number of molecules (dopants) into a matrix of an OSC (host). Such a process – when carried out with a proper choice of dopant and host – results in increasing the charge carrier density which in turn enhances the electrical conductivity of OSC, as well as, tuning their energy levels which improves the charge injection and collections at interfaces in electronic devices.
A feature of electrical charges in OSC is that they couple with the molecular entity of the OSC. This means that the introduced charges by doping are accompanied by a geometrical distortion in OSC, which localized the charges over a certain part of the OSC (a number of monomeric units in the polymer). These charges are known as either polarons or bipolarons depending on the number of charges coupled with the geometric distortion (polymer segment).
The prevalent importance of identifying, quantifying and understanding the characteristics of the charge carriers in OSC has led to the reliance on several spectroscopic techniques to uncover and optimize the doping process, such as electron paramagnetic resonance spectroscopy (EPR), optical spectroscopy and vibrational spectroscopy. While EPR is a conclusive technique to differentiate polarons and bipolarons, it is a specialized technique the requires relatively cumbersome sample preparation and sophisticated data measurements and analysis, as compared to optical spectroscopy and Raman spectroscopy. Utilizing the versatility and simplicity of optical and vibrational spectroscopy to study the type and characteristics of charge carriers in OSC is highly beneficial for the scientific community towards the investigation of molecularly doped polymers.
Approach
During my postdoctoral research assignment at Humboldt University in Berlin, I focused on investigating the nature of doping introduced charge carriers in polymers using UV-Vis-NIR optical spectroscopy and Raman spectroscopy. Optical spectroscopy is a highly suitable technique to study polarons and bipolarons in polymers since their formation is accompanied by the introduction of new localized electronic states in the bandgap of the polymer, which are involved in the electronic transitions directly detected by optical spectroscopy. On the other hand, Raman spectroscopy, which detects the vibrational modes along the polymer chain, is a facile and sensitive tool to probe the changes in the geometry of the polymer upon the introduction of changes due to the strong electron-vibron coupling in OSC.
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Identification of the optical signatures of polarons on isolated chains and aggregates in molecularly doped P3HT:
  • Polymers processed from solutions tend to form semi-crystalline thin films, meaning that ordered domains are embedded in disordered amorphous domains. This is due to pre-aggregation in solutions as well as the drying process during film formation.
  • Molecular doping is known to result in the formation of charge carriers (polarons) in both the ordered domains, resembling the aggregates in solution, and amorphous domains, resembling isolated chains in solution.
  • The separation of the doped aggregates and doped single chains in solution can be achieved by a filtration process (filters with opening sizes of 450 nm or 200 nm).
  • Such separation allows for unique identification of the shape of the polarons related feature in the optical absorption spectra. Such identification allows for characterization of the characteristic of charge carriers and their location in doped polymer thin films. 
Identification of the Raman signature of polarons and bipolarons in molecularly doped P3HT:
  • A sufficiently strong dopant is capable of introducing both polarons and bipolarons in molecularly doped polymers depending on the dopant concentration.
  • Vibrational signatures in the Raman spectra of polymers change their positions depending on the effective conjugation length (length over which the π-electrons cloud delocalizes on the polymer chain without interruption).
  • Accordingly, the introduction of charges on polymers, being either polarons or bipolarons, are known to shift the key Raman signature of polymers. This is due to the expected planarization in the doped polymer chains, and the counter-effect of disorder introduction in the polymers due to the presence of the dopant counter ions in the polymer matrix.
  • A concentration-dependent measurement of the Raman spectra of molecularly doped P3HT, using a strong molecular dopant (capable of introducing polarons and subsequently bipolarons), reveals distinct differences of the Raman spectra of polaron carrying polymer segments on the one hand, and bipolaron carrying polymer segments at higher concentration on the other hand.
  • Including first-principles DFT calculations to model single and doubly charged oligomer systems is essential to understand the shifts in the key vibrational modes in the experimental Raman spectra on doped P3HT.
  • The combination of experimental data analysis and theoretical calculations allows for direct identification of the Raman signatures of polarons and bipolarons in molecularly doped P3HT as well as the degree of order in each charged state. 

Modulation of the electronic, optical and transport properties of graphene via chemical doping: Towards application as a transparent conductive electrodes

Motivation:
​Graphene has emerged as a strong candidate as a transparent conductive electrodes (TCE) as a potential replacement for the popular Indium Tin oxide (ITO). This is due to the unique fundamental electronic, optical, mechanical and chemical properties of graphene. However, large scale production routes of graphene (currently most popular it the Chemical Vapor Deposition "CVD") is  poly-crystalline and defective, resulting in a lower conductivity as compared to higher quality graphene. 
Hence to capture the advantage of the large-scale processability of CVD graphene, chemical doping can significantly reduce its sheet resistance, in addition to largely modifying its work function which enables universality of using graphene either as a low work function cathode or a high work function anode in optoelectronic applications. 
Approach: 
  • In my research I focus on non-covalent chemical doping routs, which would interact with the graphene surface without inducing damage to it basal plane. This way, the expected reduction in the carrier mobility would only result from charged scatterer rather a damaged structure, which would be overcome by the increase in the a carriers density and generally result in an increased conductivity.
  • My research specially focus on optically transparent CVD few layers graphene (FLG), due to its higher resilience and robustness as compared to single layer graphene (SLG), in addition to the possibility of intercalating dopants in between the sheets in a similar manner to graphite intercalation compounds (GIC). The intercalation of FLG can be imagined as a bulk doping route to graphene, which allows to higher uptake of the dopant and major reduction in the sheet resistances as compared to the minor drops in the optical transmittance.  
  • Large, high molecular weight molecules with a large redox potential are strong and stable dopant to graphene. I use these molecules to act as surface dopant that modulate the transport properties of the materials in addition to largely modulating the work function of graphene.

Chemical Vapor Deposition (CVD) of Graphene - optimization and clean transfer

Motivation:
Chemical vapor deposition (CVD) is currently the most promising route towards large-scale production of graphene, especially with very recent reports on the roll-to-roll adaptation of both the synthesis and transfer process. The properties and uniformity of CVD graphene are largely influence by the conditions using during it preparation and to a large extent by the transfer process. A proper optimization of both of these processes is an essential step for any laboratory working on graphene. 
Approach:
  • Proper cleaning and characterization of the copper catalyst foil (Solvent cleaning, Electro-polishing, vacuum annealing or sputtering)
  • Control the evaporation of the copper foil during the annealing and the growth steps of graphene by encapsulation of the foils in a small closure. 
  • Optimization of the polymer support layer during the transfer process, via minimizing the thickness and controlling the adhesion. 
  • Proper design of CVD furnace to avoid leakage and contamination. 

Structural and Morphological changes to CVD graphene upon exposure to Oxygen plasma

Motivation:
Graphene oxide has emerged as a solution-processed route to produce graphene thin films. However, to be the conductivity of graphene oxide films are significantly higher than those of synthesized from CVD, and hence a reduction process is required to restore the properties of pristine graphene. However, it has been challenging to restore the structure of pristine graphene and achieve full removal of the oxide species. A better understanding of the structural, morphological and chemical evolution of the oxide species on graphene can aid in developing better strategies for reducing graphene oxide towards pristine graphene.
Approach:
  • Controllable oxidation of an initially high quality pristine graphene (CVD Single Layer Graphene).
  • Chemical and Structural analysis at each oxidation stage via XPS and Raman spectroscopy
  • Monitoring the morphological changes via AFM.