# Nuclear Security Detection Workshop

Apr 15 – 16, 2019
University of Surrey
Europe/London timezone

## Developing organic semiconducting sensors for thermal neutrons

Apr 15, 2019, 5:30 PM
1h
University of Surrey

#### University of Surrey

Guildford, UK
Submitted Poster Detector Materials

### Speaker

Mr Prodromos Chatzispyroglou

### Description

The discovery of semiconducting polymers was followed by a rapidly growing commercial interest for many applications in electronic devices (polymer light-emitting-diodes, field-effect-transistors, polymer solar cells etc.) but not as much in the field of ionising radiation detection. The aim of the research presented here is the fabrication of an organic, semiconducting sensor for neutron detection.

Organic semiconductor-based sensors, although outperformed by their inorganic counterparts, benefit from low manufacturing cost and simple fabrication techniques.

Polytriarylamine (PTAA) is a p-type semiconducting polymer that belongs to the group of π-conjugated polymers and serves as the main active component of the sensor. This type of polymer exhibits interesting electrical properties due to the structure of the backbone that consists of an alternate sequence of single (σ) and double (σ and π) bonds enabling conductivity. In a radiation detector, the polymer is sandwiched between a transparent indium-tin-oxide (ITO) electrode and an aluminium electrode in a Metal-Semiconductor-Metal (MSM) configuration, that was found elsewhere [1]. Voltage is applied to the sensor electrodes in order to collect charge carriers after their excitation in the polymer.

Films thicker than 50 $\mu$m have been fabricated by drop-casting. Typical current-voltage (IV) analysis shows a diode-like behaviour of the sensor that is dependent on the particular work function of the metal in the top contact [2]. Visible light excitation results in photocurrent of up to two orders of magnitude larger than the dark current. The sensors are stable over time when stored in a dry, oxygen-free, light-tight environment.

In a novel material designed for thermal neutron detection, boron-10 (due to its high cross section) is blended in the polymer matrix in the form of natural boron nanopowder (19.8 % $^{10}$B). At the presence of thermal neutrons, boron-10 undergoes the capture reaction $^{10}$B(n,a) $^{7}$Li. When the nanopowder is not well dispersed in the PTAA, so that there is a percolating path of the conducting boron phase, the composite has a high conductivity and Ohmic IV characteristics. However, the boron is dispersed homogeneously in the PTAA at concentrations below the percolation threshold [3], allowing the sensors to retain their rectifying electrical characteristics, despite boron loading reaching up to 10 vol. %. Boron-loaded PTAA films as thick as 80 $\mu$m have been achieved, which are capable of stopping the products of the neutron capture reaction. Quantum efficiencies of up to 2.8 % are expected. Plans are underway to test the neutron detectors at the ISIS thermal neutron source.

$\textbf{References}$
1. A. Intaniwet et al., (2009) ‘Characterization of thick film poly(triarylamine) semiconductor diodes for direct x-ray detection’, Journal of Applied Physics, 106(6), 064513.
2. A. Intaniwet et al., (2010) ‘Achieving a stable time response in polymeric radiation sensors under charge injection by X-rays’, ACS Applied Materials & Interfaces, 2(6), pp. 1692–1699.
3. M. D. Rintoul and S. Torquato (1997) ‘Precise determination of the critical threshold and exponents in a three-dimensional continuum percolation model’, Journal of Physics A: Mathematical and General, 30, no. 16, L585.

### Co-authors

Dr Sion Richards (Science & Technology Facilities Council) Dr Mark Baker (University of Surrey) Paul Seller (Science & Technology Facilities Council) Prof. Joseph L. Keddie (University of Surrey) Prof. Paul Sellin (University of Surrey)