Organic semiconductors, in particular, are
carbon-based materials that combine the excellent mechanical
characteristics of organic materials, being robust, flexible and
lightweight, with the optoelectronic properties of semiconducting
systems. However, any electronics used in space or high in the
atmosphere are subject to bombardment by cosmic rays, which can generate
neutrons that cause damage through neutron-nucleus collisions.
There has been no detailed study of neutron radiation effects on some established organic semiconductors, so we don’t yet know how they will perform under cosmic ray bombardment. Irradiation of two poly(thiophene)-based organic semiconductors using the VESUVIO beamline at the ISIS spallation neutron source and various spectroscopic techniques showed different tolerances, correlating to differences in crystalline order, and significant and irreversible changes in both polymers. These results provide a first indication of the material design criteria needed to increase radiation hardness in conjugated polymers.
Organic avionics and cosmic rays
Thiophene, originally discovered as a contaminant in benzene, can be used to produce organic semiconductors and optoelectronic components, including light-emitting diodes, thin film transistors and photovoltaic diodes. These lightweight polymeric semiconductors are an ideal choice for use in aviation and space applications, where it is imperative to minimise payloads.
Cosmic rays pose a variety of hazards to such materials, through atomic displacements following neutron-nucleus collisions. However, there is no detailed study of the effect of neutron radiation on even some of the most established organic semiconductors, and there is a need for more study of the effects of the radiation environments such devices will encounter. The International Space Station for example, is estimated to receive an annual dose of around 2.8 ⤫ 1011 neutrons/cm2, with energies ranging from 10-1 to 1011 eV, generated by the interaction of cosmic rays with the Station.
One of the most disruptive components of cosmic ray particle cascades, highly energetic neutrons cause atomic displacements and the generation of irreversible defects in materials, along with reversible ‘soft errors’. Whilst these effects are more intense in space, outside the protection of Earth’s atmosphere, they occur on Earth as well, and the neutron flux generated by thunderstorms can pose significant hazards for civil aviation.
ISIS neutron and muon source
A spallation neutron source, such as ISIS neutron and muon source, is the ideal tool for shedding light on the effects of neutron radiation, due to the broad energy spectrum of neutrons it produces. ISIS is also significantly more intense than the flux of cosmic rays in space, allowing researchers to mimic several years of space irradiation in just a few hours.
ISIS’s VESUVIO beamline is a unique neutron spectrometer, using a high intensity of neutrons in the eV energy range (epi thermal neutrons) to mass-separate spectra into a collection of nuclear momentum distributions. It is used for everything from fundamental physics through to chemistry and materials science. In this particular case, VESUVIO was used to demonstrate the capability of spallation sources this kind of work during the development of ChipIr. ChipIr is one of the first dedicated facilities outside of the US to look at how silicon microchips respond to cosmic neutron radiation.
Neutron irradiation of poly(thiophene)-based organic semiconductors
This research focused on two notable examples of conjugated polymers - poly(3-hexylthiophene), P3HT and poly(2,5-bis(3-hexadecylthiophen-2-yl)thieno[3,2 b]thiophene), PBTTT. The researchers studied the effect of neutron irradiation on the optical, chemical and vibrational features of both polymers, using ultraviolet-visible absorption (UV-Vis), XPS and Raman spectroscopy, correlating these with the electrical properties revealed by characterisation of FETs. Thermal annealing was used to investigate the reversibility of any defects caused by the irradiation.
The results showed significant and irreversible changes in both polymers, consistent with a neutron-induced doping process being the main degradation pathway for these materials. However, the two materials responded differently to neutron irradiation, with PBTTT showing greater radiation tolerance to P3HT. This correlates with the higher crystalline order and the more rigid and structurally stable backbone present in PBTTT.
These findings offer the first molecular design guidelines that can be used to develop more neutron-tolerant polymeric semiconductors, paving the way for the use of organic semiconductors in avionics and space applications.
Paternò GM et al. Neutron Radiation Tolerance of Two Benchmark Thiophene-Based Conjugated Polymers: the Importance of Crystallinity for Organic Avionics. Nature Scientific Reports 7, Article number 41013 (2017). DOI:10.1038/srep41013