Charge modulation spectroscopy

Charge modulation spectroscopy is an electro-optical spectroscopy technique tool.{{cite journal|last1=Caironi|first1=Mario|last2=Bird|first2=Matt|last3=Fazzi|first3=Daniele|last4=Chen|first4=Zhihua|last5=Di Pietro|first5=Riccardo|last6=Newman|first6=Christopher|last7=Facchetti|first7=Antonio|last8=Sirringhaus|first8=Henning|date=9 September 2011|title=Very Low Degree of Energetic Disorder as the Origin of High Mobility in an n-channel Polymer Semiconductor|journal=Advanced Functional Materials|volume=21|issue=17|pages=3371–3381|doi=10.1002/adfm.201100592|s2cid=95644182 }} It is used to study the charge carrier behavior of organic field-effect transistors. It measures the charge introduced optical transmission variation{{cite journal|last1=Sirringhaus|first1=H.|last2=Brown|first2=P. J.|last3=Friend|first3=R. H.|last4=Nielsen|first4=M. M.|last5=Bechgaard|first5=K.|last6=Langeveld-Voss|first6=B. M. W.|last7=Spiering|first7=A. J. H.|last8=Janssen|first8=R. A. J.|last9=Meijer|first9=E. W.|date=October 1999|title=Two-dimensional charge transport in self-organized, high-mobility conjugated polymers|journal=Nature|volume=401|issue=6754|pages=685–688|doi=10.1038/44359|last10=Herwig|first10=P.|last11=de Leeuw|first11=D. M.|bibcode=1999Natur.401..685S |s2cid=4387286}}{{cite journal|last1=Brown|first1=Peter J.|last2=Sirringhaus|first2=Henning|last3=Harrison|first3=Mark|last4=Shkunov|first4=Maxim|last5=Friend|first5=Richard H.|date=12 March 2001|title=Optical spectroscopy of field-induced charge in self-organized high mobility poly(3-hexylthiophene)|journal=Physical Review B|volume=63|issue=12|page=125204 |doi=10.1103/physrevb.63.125204|bibcode=2001PhRvB..63l5204B }} by directly probing the accumulation charge at the burning interface of semiconductor and dielectric layer{{cite book|title=Large area and flexible electronics|publisher=Wiley-VCH|isbn=9783527336395|date=2015-05-04}} where the conduction channel forms.

File:CMS_setup.png

Principles

Unlike ultraviolet–visible spectroscopy which measures absorbance, charge modulation spectroscopy measures the charge introduced optical transmission variation. In other words, it reveals the new features in optical transmission introduced by charges. In this setup, there are mainly four components: lamp, monochromator, photodetector and lock-in amplifier. Lamp and monochromator are used for generating and selecting the wavelength. The selected wavelength passes through the transistor, and the transmitted light is recorded by the Photodiode. When the signal to noise ratio is very low, the signal can be modulated and recovered with a Lock-in amplifier.

In the experiment, a direct current plus an alternating current bias are applied to the organic field-effect transistor. Charge carriers accumulate at the interface between the dielectric and the semiconductor (usually a few nanometers{{cite journal|last1=Bässler|first1=H.|date=1 January 1993|title=Charge Transport in Disordered Organic Photoconductors a Monte Carlo Simulation Study|journal=Physica Status Solidi B|volume=175|issue=1|pages=15–56|doi=10.1002/pssb.2221750102|bibcode=1993PSSBR.175...15B }}). With the appearance of the accumulation charge, the intensity of the transmitted light changes. The variation of the light intensity ({{slink||Bleaching and Charge absorption signal}}) is then collected though the photodetector and lock-in amplifier. The charge modulation frequency is given to Lock-in amplifier as the reference.

Modulate charge at the organic field-effect transistor

File:Organic_field_effect_transistor.png

There are four typically Organic field-effect transistor architectures:{{cite book|title=The WSPC reference on organic electronics : organic semiconductors (in 2 volumes)|isbn=9789814699228|last1=Marder|first1=Seth R.|last2=Bredas|first2=Jean-Luc|date=2016-01-29}} Top-gate, bottom-contacts; bottom-gate, top-contacts; bottom-gate, bottom-contacts; top-gate, top-contact.

In order to create the accumulation charge layer, a positive/negative direct current voltage is applied to the gate of the organic field-effect transistor (positive for the P type transistor, negative for the N type transistor).{{cite book|title=Organic thin film transistor integration : a hybrid approach|publisher=Wiley-VCH|isbn=978-3527634453|date=2011-03-21}} In order to modulate the charge, an AC voltage is given between the gate and source. It is important to notice that only mobile charge can follow the modulation and that the modulation frequency given to lock-in amplifier has to be synchronous.

Charge modulation spectra

The charge modulation spectroscopy signal can be defined as the differential transmission $\bigtriangleup T$ divided by the total transmission $T$ . By modulating the mobile carriers, an increase transmission $\bigtriangleup T/T>0$ and decrease transmission $\bigtriangleup T/T<0$ features could be both observed.{{cite journal|last1=Zhao|first1=N.|last2=Noh|first2=Y.-Y.|last3=Chang|first3=J.-F.|last4=Heeney|first4=M.|last5=McCulloch|first5=I.|last6=Sirringhaus|first6=H.|date=5 October 2009|title=Polaron Localization at Interfaces in High-Mobility Microcrystalline Conjugated Polymers|journal=Advanced Materials|volume=21|issue=37|pages=3759–3763|doi=10.1002/adma.200900326|bibcode=2009AdM....21.3759Z |s2cid=95098433 }} The former relates to the bleaching and the latter to the charge absorption and electrically induced absorption (electro-absorption). The charge modulation spectroscopy spectra is an overlap of charge-induced and electro-absorption features. In transistors, the electro-absorption is more significant during the high voltage drop.{{cite journal|last1=Chin|first1=Xin Yu|last2=Pace|first2=Giuseppina|last3=Soci|first3=Cesare|last4=Caironi|first4=Mario|date=2017|title=Ambipolar charge distribution in donor–acceptor polymer field-effect transistors|journal=Journal of Materials Chemistry C|volume=5|issue=3|pages=754–762|doi=10.1039/c6tc05033f|url=https://zenodo.org/record/3451199}} There are several ways to identify the electro-absorption contribution, such as get the second harmonic $\bigtriangleup T/T$ , or probe it at the depletion region.

= Bleaching and charge absorption =

When the accumulation charge carrier removes the ground state of the neutral polymer, there is more transmission in the ground state. This is called bleaching $\bigtriangleup T > 0$ . With the excess hole or electrons at the polymer, there will be new transitions at low energy levels, therefore the transmission intensity is reduced $\bigtriangleup T < 0$ , this is related to charge absorption.

= Electro-absorption =

The electro-absorption is a type of Stark effect in the neutral polymer,{{cite journal|last1=Chemla|first1=D. S.|last2=Damen|first2=T. C.|last3=Miller|first3=D. A. B.|last4=Gossard|first4=A. C.|last5=Wiegmann|first5=W.|date=15 May 1983|title=Electroabsorption by Stark effect on room‐temperature excitons in GaAs/GaAlAs multiple quantum well structures|journal=Applied Physics Letters|volume=42|issue=10|pages=864–866|doi=10.1063/1.93794|bibcode=1983ApPhL..42..864C }} it is predominant at the electrode edge since there is a strong voltage drop. Electro-absorption can be observed from the second harmonic charge modulation spectroscopy spectra.

Charge modulation microscopy

Charge modulation microscopy is a new technology which combines the confocal microscopy with charge modulation spectroscopy.{{cite journal|last1=Sciascia|first1=Calogero|last2=Martino|first2=Nicola|last3=Schuettfort|first3=Torben|last4=Watts|first4=Benjamin|last5=Grancini|first5=Giulia|last6=Antognazza|first6=Maria Rosa|last7=Zavelani-Rossi|first7=Margherita|last8=McNeill|first8=Christopher R.|last9=Caironi|first9=Mario|date=16 November 2011|title=Sub-Micrometer Charge Modulation Microscopy of a High Mobility Polymeric n-Channel Field-Effect Transistor|journal=Advanced Materials|volume=23|issue=43|pages=5086–5090|doi=10.1002/adma.201102410|pmid=21989683|bibcode=2011AdM....23.5086S |s2cid=205241797 }} Unlike the charge modulation spectroscopy which is focused on the whole transistor, the charge modulation microscopy give us the local spectra and map. Thanks for this technology, the channel spectra and electrode spectra can be obtained individually. A more local dimension of charge modulation spectra (around submicrometer) can be observed without a significant Electro-absorption feature. Of course, this depends on the resolution of the optical microscopy.

The high resolution of charge modulation microscopy allows mapping of the charge carrier distribution at the active channel of the organic field-effect transistor. In other words, a functional carrier morphology can be observed. It is well known that the local carrier density can be related to the polymer microstructure. Based on Density functional theory calculations, a polarized charge modulation microscopy can selectively map the charge transport associated with a relative direction of the transition dipole moment.{{cite journal|last1=Fazzi|first1=Daniele|last2=Caironi|first2=Mario|date=2015|title=Multi-length-scale relationships between the polymer molecular structure and charge transport: the case of poly-naphthalene diimide bithiophene|journal=Physical Chemistry Chemical Physics|volume=17|issue=14|pages=8573–8590|doi=10.1039/c5cp00523j|pmid=25740386|bibcode=2015PCCP...17.8573F }} The local direction can be correlated to the orientational order of polymer domains.{{cite journal|last1=Martino|first1=Nicola|last2=Fazzi|first2=Daniele|last3=Sciascia|first3=Calogero|last4=Luzio|first4=Alessandro|last5=Antognazza|first5=Maria Rosa|last6=Caironi|first6=Mario|date=13 May 2014|title=Mapping Orientational Order of Charge-Probed Domains in a Semiconducting Polymer|journal=ACS Nano|volume=8|issue=6|pages=5968–5978|doi=10.1021/nn5011182|pmid=24815931|hdl=11858/00-001M-0000-0024-A80B-8|hdl-access=free}} More ordered domains show a high carrier mobility of the organic field-effect transistor device.