(Invited) Dynamic Modeling of Photocurrents in Organic Thin-Film Transistors
Authors: Valletta Antonio , Calvi Sabrina , Basirico' Laura , Carturan Sara , Fratelli Ilaria , De Rosa Stefania , Pino Felix , Ciavatti Andrea , Tortora Luca , Scagliotti Mattia , Rapisarda Matteo , Moretto Sandra , Verdi Matteo , Bertoldo Stefano , Chiari Massimo , Tommasino Francesco , Sarnelli Ettore , Mariucci Luigi , Branchini Paolo , Quaranta Alberto , Fraboni Beatrice 
Published: August 2024

The development of high performance, thin and flexible light and radiation sensors for real-time detection of ionizing radiation at affordable cost is of interest in many application areas, ranging from medical diagnostics and therapy to astrophysics, high energy physics and industrial testing, including civil radiation protection.
The use of organic semiconductor technology could meet the requirements of large area, conformability and portability, light weight and low power operation.
In the field of organic photodetectors, phototransistors (OPT) have gained significant attention due to their ability to combine high sensitivity, switching function, and intrinsic amplification without noise problems in a single device, thus allowing significant simplification of the external signal conditioning circuitry [1].
The operation of an OPT under light illumination is governed by the trapping of minority carriers, resulting in a reduction of the threshold voltage which determines an increase in the majority carrier density and, consequently, an increase of the photocurrent [2].
The increase of photocurrent in organic semiconductors has been attributed by several authors to an increase in the trapping of minority carriers induced by light exposure. In fact, the light-induced creation of deep traps for carriers has already been experimentally demonstrated in the analysis of several organic semiconductors used in different optoelectronic applications [3]. Moreover, TCAD simulations show that the photocurrent can be assumed linearly dependent upon the total amount of generated traps, at least for the range of radiation exposures used in this work.
In this work, we present a dedicated kinetic model capable of correctly describing the photocurrent behavior in dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT) based OPTs operating in subthreshold condition. The OPTs have been used as direct light detectors and in an indirect flexible proton detector that has been fabricated by coupling a 500 μm thick scintillating plastic film (a polysiloxane-based matrix carrying a primary dye and a wavelength shifter) directly on top of the OPT’s passivation layer.
By analyzing the response of the OPTs in light and in the proton detectors we note that:
1 - the relaxation in dark of the photocurrent (fig. A) is well described by a stretched exponential with an exponent β almost constant (around 0.5) and a characteristic time τs that strongly depends on the exposure conditions (fig. B).
2 - dynamic photocurrent measurements under repeated exposures show a systematic drift due to the build-up of a persistent photocurrent component that decays with a characteristic times of the order of 105 s, in addition to the swift component that has recovery times in the range of a few seconds (fig. C).
To model these effects, we propose a set of rate equations similar to the one already presented by Street et al [3], suitably modified to reproduce the dynamics of the creation and recovery of two different types of defects with distributed recovery activation energies. The consideration of two different types of defects allows the reproduction of the swift and persistent photocurrent components, while the stretched exponential behavior of the photocurrent decay is reproduced by introducing a distribution in the recovery activation energies.
The proposed new model is able to quantitatively reproduce the dynamic photoresponse for a wide range of exposure intensities under both photon and proton fluxes [4]. As can be seen in fig. C, the curves computed by the model with the fitted parameters superimpose well on the experimental ones, correctly reproducing the rise and fall dynamics of the exposure response and the progressive build-up of a persistent photocurrent.
The model also provides some valuable physical insights to explain some features of the dynamic photoresponse.
This work has been funded by the Italian National Institute of Nuclear Physics – INFN -5th commission, under the “FIRE” project (2019-2023).

Lucas, T. Trigaud, C. Videlot-Ackermann, Organic transistors and phototransistors based on small molecules: Organic transistors and phototransistors, Polym. Int. 61 (2012) 374–389. https://doi.org/10.1002/pi.3213

Basiricò, L. et al. Direct X-ray photoconversion in flexible organic thin film devices operated below 1 V. Nat Commun 7, 13063 (2016).

Street, R. A., Yang, Y., Thompson, B. C. & McCulloch, I. Capacitance Spectroscopy of Light Induced Trap States in Organic Solar Cells. J. Phys. Chem. C 120, 22169–22178 (2016).

S. Calvi, L. Basiricò, S. M. Carturan, I. Fratelli, A. Valletta, A. Aloisio, S. De Rosa, F. Pino, M. Campajola, A. Ciavatti, L. Tortora, M. Rapisarda, S. Moretto, M. Verdi, S. Bertoldo, O. Cesarini, P. Di Meo, M. Chiari, F. Tommasino, E. Sarnelli, L. Mariucci, P. Branchini, A. Quaranta, B. Fraboni, “Flexible fully organic indirect detector for megaelectronvolts proton beams”, npj Flexible Electronics (2023) 7:5; https://doi.org/10.1038/s41528-022-00229-w.

Figure 1