Doping of organic semiconductors¶
Doping of organic semiconductors is of crucial importance particularly for organic field-effect transistors (OFETs) and thermoelectric generators, the first being already in widespread use and the second of increasing importance, particularly in light of the current climate crisis and the need for making much better use of energy sources available. While doping of organic semiconductors is widely applied, fundamental questions regarding the mechanisms involved are still to be answered.
Due to its inherent sensitivity to paramagnetic species and doped molecules usually possessing unpaired electron spins, EPR spectroscopy is a perfect match here.
Some questions that can be addressed using EPR spectroscopy:
What is the doping efficiency for a given system?
What is the relative orientation of dopant and host with respect to each other?
What is the doping stability?
Additionally, one question arises that needs to be addressed to apply EPR spectroscopy successfully to the above questions:
How can we reliably discriminate between the signals of dopant and host?
Here, conventional cw-EPR spectroscopy at X-band frequencies and room temperature is highly valuable, with samples usually consisting of films coated on flat substrates. Both, quantitative as well as angular-dependent cw-EPR measurements are performed, and analysis requires, inter alia, the global fitting of EPR spectra including partial orientation and multiple species.
For discriminating between the signals of dopant and host, additionally EPR spectroscopy at higher fields/frequencies may be used. Another approach will be in-situ electrochemistry within the EPR spectrometer to electrochemically create charged species of either dopant or host. Quantum-chemical calculations (DFT, …) for calculating g-tensors give additional insight and help with assigning EPR signals to spectral species.
Furthermore, usually there are quite many samples involved in these projects. Hence, automating the standard procedure for cw-EPR spectroscopy, including data processing and analysis, is of high value.
What is the doping efficiency for a given system?¶
method: quantitative cw-EPR @ X-band, room-temperature
preparation: mostly films on flat substrates
analysis: global fitting of EPR spectra (including multiple species)
What is the doping stability?¶
method: quantitative cw-EPR @ X-band, room-temperature
preparation: mostly films on flat substrates
analysis: global fitting of EPR spectra (including multiple species)
What is the relative orientation of dopant and host with respect to each other?¶
method: angular-dependent cw-EPR @ X-band, room-temperature
preparation: (oriented) films on flat substrates
analysis: global fitting of EPR spectra (including partial orientation and multiple species)
Additionally, there are questions that need to be addressed to apply EPR spectroscopy successfully to the above questions:
How can we reliably discriminate between the signals of dopant and host?¶
method: cw-EPR @ X-band, room-temperature (and probably higher fields/frequencies)
method: quantum-chemical calculations (DFT, …) for calculating g-tensors
method: in-situ electrochemistry within the EPR spectrometer
analysis: global fitting of EPR spectra (including partial orientation and multiple species)
Furthermore, usually there are quite many samples involved in these projects. Hence, automating the standard procedure for cw-EPR spectroscopy, including data processing and analysis, is of high value.