Abstract

1. Introduction

Many flexible electronics devices have recently employed organic materials as the active component¹1 Seo HK, Kim H, Lee J, Park M, Jeong S, Kim Y et al. Efficient flexible organic/inorganic hybrid perovskite light-emitting diodes based on graphene anode. Adv Mater. 2017;29(12):1-6. http://dx.doi.org/10.1002/adma.201605587.
https://doi.org/ http://dx.doi.org/10.10... . Hence studies on new semiconductor materials such as II-VI compound materials, two-dimensional materials, complex and simple oxides, small molecules and polymers, biological molecules, colloidal quantum dots, and perovskites have gained wide attention. These semiconductors are less expensive than their conventional inorganic counterparts that can be applied to many forms such as flexible electronics, particularly thin film transistors (TFTs) on polymer substrates, which are crucial for space exploration applications, such as space-borne telescopes, balloons solar sails and synthetic aperture radar systems²2 Indluru A, Holbert KE, Alford TL. Gamma radiation effects on indium-zinc oxide thin-film transistors. Thin Solid Films. 2013;539:342-4. http://dx.doi.org/10.1016/j.tsf.2013.04.148.
http://dx.doi.org/10.1016/j.tsf.2013.04.... . These devices require distributed sensing and electronic health to monitor the thin, low mass and large area deployable structures, which cannot be implemented using conventional engineering materials such as metals and alloys³3 Zhou L, Jackson T, Brandon E, West W. Flexible substrate a-Si:H TFTs for space applications. In: Conference Digest [Includes 'Late News Papers' volume] Device Research Conference. Proceedings. New York: IEEE; 2004. p. 123-124.. However, the operating environment of these devices during storage and deployment is far from stable and has often been disturbed, especially in outer space. Hence, it becomes a major factor contributing to the components' ability to function properly in harsh radiation environments. Interaction of ionizing radiation with matter, especially gamma radiation, is crucial in both theoretical and practical⁴4 Mivolil DS, Chee FP, Rasmidi R, Alias A, Salleh S, Salleh KAM. Gamma ray and neutron radiation effects on the electrical and structural properties of n-ZnO/p-CuGaO 2 schottky diode. ECS J Solid State Sci Technol. 2020;9(4):045019. http://dx.doi.org/10.1149/2162-8777/ab8f19.
http://dx.doi.org/10.1149/2162-8777/ab8f...

5 Rashad M, Tekin HO, Zakaly HM, Pyshkina M, Issa SAM, Susoy G. Physical and nuclear shielding properties of newly synthesized magnesium oxide and zinc oxide nanoparticles. Nucl Eng Technol. 2020;52(9):2078-84. http://dx.doi.org/10.1016/j.net.2020.02.013.
http://dx.doi.org/10.1016/j.net.2020.02....

6 Kim J, Pearton SJ, Fares C, Yang J, Ren F, Kim S et al. Radiation damage effects in Ga2O3 materials and devices. J Mater Chem C Mater Opt Electron Devices. 2019;7(1):10-24. http://dx.doi.org/10.1039/C8TC04193H.
http://dx.doi.org/10.1039/C8TC04193H...

7 Oryema B, Jurua E, Madiba IG, Nkosi M, Sackey J, Maaza M. Effects of low-dose γ-irradiation on the structural, morphological, and optical properties of fluorine-doped tin oxide thin films. Radiat Phys Chem. 2020;176(July):109077. http://dx.doi.org/10.1016/j.radphyschem.2020.109077.
http://dx.doi.org/10.1016/j.radphyschem....

8 Park S, Choi S, Lee H, Lee J, Woo Y, Jung Y et al. Impact of gamma-ray irradiation on the electronic structures of PCBM and P3HT organic semiconductor films. Polym Degrad Stabil. 2021;186:109518. http://dx.doi.org/10.1016/j.polymdegradstab.2021.109518.
http://dx.doi.org/10.1016/j.polymdegrads... ^-99 Alyamani A, Mustapha N. Effects of high dose gamma irradiation on ITO thin film properties. Thin Solid Films. 2016;611:27-32. http://dx.doi.org/10.1016/j.tsf.2016.05.022.
http://dx.doi.org/10.1016/j.tsf.2016.05.... . Hence, the effect of gamma radiation on the structural, optical and electrical properties of various organic thin films and semiconductor electronics was widely investigated. Previous studies have found that when the samples are exposed to ionizing radiation, their structural, optical and electrical properties are altered¹⁰10 Lorenz K, Peres M, Franco N, Marques JG, Miranda SMC, Magalhães S et al. Radiation damage formation and annealing in GaN and ZnO. Oxide-based Mater Devices II. 2011;7940:79400O. http://dx.doi.org/10.1117/12.879402.
http://dx.doi.org/10.1117/12.879402...

11 Parida MK, Tripura Sundari S, Sathiamoorthy V, Sivakumar S. Current-voltage characteristics of silicon PIN diodes irradiated in KAMINI nuclear reactor. Nucl Instruments Methods Phys Res Sect A Accel Spectrometers Detect Assoc Equip. 2018;905:129-37. http://dx.doi.org/10.1016/j.nima.2018.07.014.
http://dx.doi.org/10.1016/j.nima.2018.07...

12 Pearton SJ, Ren F, Patrick E, Law ME, Polyakov AY. Review: ionizing radiation damage effects on GaN devices. ECS J Solid State Sci Technol. 2016;5(2):Q35-60. http://dx.doi.org/10.1149/2.0251602jss.
http://dx.doi.org/10.1149/2.0251602jss...

13 Ramirez JI, Li YV, Basantani H, Leedy K, Bayraktaroglu B, Jessen GH et al. Radiation-hard ZnO thin film transistors. IEEE Trans Nucl Sci. 2015;62(3):1399-404. http://dx.doi.org/10.1109/TNS.2015.2417831.
http://dx.doi.org/10.1109/TNS.2015.24178...

14 Palatnikov MN, Sidorov NV, Makarova OV, Panasyuk SL, Kurkamgulova ER, Yudin IV. Relationship between the optical damage resistance and radiation hardness and the influence of threshold effects on the radiation hardness of ZnO-Doped LiNbO3 crystals. Inorg Mater. 2018;54(1):55-9. http://dx.doi.org/10.1134/S0020168518010120.
http://dx.doi.org/10.1134/S0020168518010...

15 Tashiro J, Torita Y, Nishimura T, Kuriyama K, Kushida K, Xu Q et al. Gamma-ray irradiation effect on ZnO bulk single crystal: origin of low resistivity. Solid State Commun. 2019;292:24-6. http://dx.doi.org/10.1016/j.ssc.2019.01.019.
http://dx.doi.org/10.1016/j.ssc.2019.01....

16 Park CH, Park J, Kim FS. Gamma-ray irradiation effects on the electrical properties of organic field-effect transistors. Mol Cryst Liq Cryst. 2019;687(1):1-6. http://dx.doi.org/10.1080/15421406.2019.1648047.
http://dx.doi.org/10.1080/15421406.2019....

17 Rasmidi R, Duinong M, Chee FP. Radiation damage effects on zinc oxide (ZnO) based semiconductor devices- a review. Radiat Phys Chem. 2021;184:109455. http://dx.doi.org/10.1016/j.radphyschem.2021.109455.
http://dx.doi.org/10.1016/j.radphyschem.... ^-1818 Shin H, Baang S, Hong C, Kim D, Park J, Choi JS. Electrical characteristics of a 6,13-Bis(triisopropylsilylethynyl)pentacene thin-film transistor under light absorption. Mol Cryst Liq Cryst. 2019;679(1):1-7. http://dx.doi.org/10.1080/15421406.2019.1597539.
http://dx.doi.org/10.1080/15421406.2019.... . Thus, the radiation response of the material must be thoroughly investigated before the material is exposed to a harsh radiation environment, specifically for flexible space-borne electronic applications.

TIPS Pentacene represents one of the most promising organic semiconductor materials for low-cost, flexible electronic devices due to its high solubility in common organic solvents and good environmental stability²2 Indluru A, Holbert KE, Alford TL. Gamma radiation effects on indium-zinc oxide thin-film transistors. Thin Solid Films. 2013;539:342-4. http://dx.doi.org/10.1016/j.tsf.2013.04.148.
http://dx.doi.org/10.1016/j.tsf.2013.04....

3 Zhou L, Jackson T, Brandon E, West W. Flexible substrate a-Si:H TFTs for space applications. In: Conference Digest [Includes 'Late News Papers' volume] Device Research Conference. Proceedings. New York: IEEE; 2004. p. 123-124.

4 Mivolil DS, Chee FP, Rasmidi R, Alias A, Salleh S, Salleh KAM. Gamma ray and neutron radiation effects on the electrical and structural properties of n-ZnO/p-CuGaO 2 schottky diode. ECS J Solid State Sci Technol. 2020;9(4):045019. http://dx.doi.org/10.1149/2162-8777/ab8f19.
http://dx.doi.org/10.1149/2162-8777/ab8f... ^-55 Rashad M, Tekin HO, Zakaly HM, Pyshkina M, Issa SAM, Susoy G. Physical and nuclear shielding properties of newly synthesized magnesium oxide and zinc oxide nanoparticles. Nucl Eng Technol. 2020;52(9):2078-84. http://dx.doi.org/10.1016/j.net.2020.02.013.
http://dx.doi.org/10.1016/j.net.2020.02.... . TIPS Pentacene is a conjugated form of pentacene commonly used for small molecule organic semiconductors due to the bulky functionalized group that helps increase solubilities⁶6 Kim J, Pearton SJ, Fares C, Yang J, Ren F, Kim S et al. Radiation damage effects in Ga2O3 materials and devices. J Mater Chem C Mater Opt Electron Devices. 2019;7(1):10-24. http://dx.doi.org/10.1039/C8TC04193H.
http://dx.doi.org/10.1039/C8TC04193H...

7 Oryema B, Jurua E, Madiba IG, Nkosi M, Sackey J, Maaza M. Effects of low-dose γ-irradiation on the structural, morphological, and optical properties of fluorine-doped tin oxide thin films. Radiat Phys Chem. 2020;176(July):109077. http://dx.doi.org/10.1016/j.radphyschem.2020.109077.
http://dx.doi.org/10.1016/j.radphyschem.... ^-88 Park S, Choi S, Lee H, Lee J, Woo Y, Jung Y et al. Impact of gamma-ray irradiation on the electronic structures of PCBM and P3HT organic semiconductor films. Polym Degrad Stabil. 2021;186:109518. http://dx.doi.org/10.1016/j.polymdegradstab.2021.109518.
http://dx.doi.org/10.1016/j.polymdegrads... . TIPS Pentacene has a two-dimension π-stacking molecular structure that helps improve mobility¹⁸18 Shin H, Baang S, Hong C, Kim D, Park J, Choi JS. Electrical characteristics of a 6,13-Bis(triisopropylsilylethynyl)pentacene thin-film transistor under light absorption. Mol Cryst Liq Cryst. 2019;679(1):1-7. http://dx.doi.org/10.1080/15421406.2019.1597539.
http://dx.doi.org/10.1080/15421406.2019.... . These bulky functionalized groups assist molecular ordering in the face-to-face interface, overlapping the pi-orbital and increasing device performances¹⁹19 Choi D, Ahn B, Kim SH, Hong K, Ree M, Park CE. High-performance triisopropylsilylethynyl pentacene transistors via spin coating with a crystallization-assisting layer. ACS Appl Mater Interfaces. 2012;4(1):117-22. http://dx.doi.org/10.1021/am201074n.
http://dx.doi.org/10.1021/am201074n... . Although several technological applications of TIPS Pentacene exist in microelectronics¹⁹19 Choi D, Ahn B, Kim SH, Hong K, Ree M, Park CE. High-performance triisopropylsilylethynyl pentacene transistors via spin coating with a crystallization-assisting layer. ACS Appl Mater Interfaces. 2012;4(1):117-22. http://dx.doi.org/10.1021/am201074n.
http://dx.doi.org/10.1021/am201074n...

20 Basiricò L, Basile AF, Cosseddu P, Gerardin S, Cramer T, Bagatin M et al. Space environment effects on flexible, low-voltage organic thin-film transistors. ACS Appl Mater Interfaces. 2017;9(40):35150-8. http://dx.doi.org/10.1021/acsami.7b08440.
http://dx.doi.org/10.1021/acsami.7b08440...

21 Vaklev NL, Müller R, Muir BVO, James DT, Pretot R, van der Schaaf P et al. High-performance flexible bottom-gate organic field-effect transistors with gravure printed thin organic dielectric. Adv Mater Interfaces. 2014;1(3):1-6. http://dx.doi.org/10.1002/admi.201300123.
http://dx.doi.org/10.1002/admi.201300123... ^-2222 Choi MH, Kim BS, Jang J. High-performance flexible TFT circuits using TIPS pentacene and polymer blend on plastic. IEEE Electron Device Lett. 2012;33(11):1571-3. http://dx.doi.org/10.1109/LED.2012.2213294.
http://dx.doi.org/10.1109/LED.2012.22132... , it is quite unreliable under harsh radiation environments. In particular, the current knowledge of the radiation response of TIPS Pentacene-based devices is unclear, and the physical effects of the radiation in harsh environments are unknown⁴4 Mivolil DS, Chee FP, Rasmidi R, Alias A, Salleh S, Salleh KAM. Gamma ray and neutron radiation effects on the electrical and structural properties of n-ZnO/p-CuGaO 2 schottky diode. ECS J Solid State Sci Technol. 2020;9(4):045019. http://dx.doi.org/10.1149/2162-8777/ab8f19.
http://dx.doi.org/10.1149/2162-8777/ab8f... ^,99 Alyamani A, Mustapha N. Effects of high dose gamma irradiation on ITO thin film properties. Thin Solid Films. 2016;611:27-32. http://dx.doi.org/10.1016/j.tsf.2016.05.022.
http://dx.doi.org/10.1016/j.tsf.2016.05.... . Hence, this study investigates the physical and optical characteristics of TIPS Pentacene organic thin film upon exposure to gamma irradiation.

2. Experimental Details

The Indium Tin Oxide (ITO) substrate was rinsed using distilled water, ethanol, and acetone for 10 minutes in an ultrasonic bath. The substrate was then rinsed in distilled water and dried with nitrogen gas. The powdered TIPS Pentacene (purity > 99.5%, Sigma-Aldrich) was dissolved in toluene (purity 99.9%, Sigma-Aldrich) and stirred at room temperature for 24 hours to form a 1.0% wt solution. The organic layer of TIPS Pentacene was deposited using the spin coating method. 1 mL of TIPS Pentacene solution was dispensed and spread onto all substrate samples with the sintering of 500 rpm for 5 s. The sample was spun at a speed of 1000 rpm for 20 s.

The fabricated samples were irradiated at the MINTec-Sinagama facility, Malaysia Nuclear Agency. Using the tote irradiator model JS8900, the gamma irradiation total ionizing dose was 10 kGy, 50 kGy, 100 kGy, 150 kGy, 200 kGy, 250 kGy and 300 kGy. The activity of Co-60 was 352191 Curie with a dose rate of 1.58 kGy/hr measured by a Ceric Cerous dosimeter. As illustrated in Figure 1, lead containers accommodate samples during irradiation by gamma rays from the Co-60 source.

Figure 1
Experimental setup inside the gamma radiation facility.

Using XRD, structural analysis of both the post-irradiated and unirradiated films was performed at room temperature. A MultiMode 8 atomic force microscope (AFM) was used to examine the surface properties of the samples. The samples' optical transmittance and band gap were investigated using a Lambda EZ210 UV-Vis spectrometer.

3. Results and Discussion

3.1. Structural properties of TIPS pentacene thin film

As shown in Figure 2, the structural properties of pre-and post-irradiation of TIPS Pentacene indicate the thin film maintains its crystalline properties despite rising total ionizing dose. The XRD analysis for un-irradiated thin film shows a good c-axis orientation perpendicular to the substrate surface. Two XRD spectra peaks at 5.33° and 10.25° correspond to (001) and (002) planes, respectively.

Figure 2
XRD spectra of TIPS Pentacene before and after irradiation with different total ionizing doses.

However, full-width half maxima (FWHM) exhibit a change in structure with the increase in ionizing dose. The length of peak width influences the FWHM of thin films. As the intensity becomes wider, the crystal quality of thin films deteriorates. This intensity degradation might be due to the irradiation-induced defects created in the sample²³23 Kumar Anbalagan A, Jao CY, Syabriyana M, Fan CL, Gupta S, Chaudhary M et al. Influence of gamma-ray irradiation and post-annealing studies on pentacene films: the anisotropic effects on structural and electronic properties. RSC Advances. 2020;10(36):21092-9. http://dx.doi.org/10.1039/D0RA04522E.
http://dx.doi.org/10.1039/D0RA04522E... ^,2424 El-Nahass MM, Khalifa BA, Soliman IM. Gamma radiation-induced changes on the structural and optical properties of aluminum phthalocyanine chloride thin films. Opt Mater. 2015;46:115-21. http://dx.doi.org/10.1016/j.optmat.2015.04.010.
http://dx.doi.org/10.1016/j.optmat.2015.... .

Based on the XRD analysis, the FWHM of the thin film increases with the rising total ionizing dose, indicating the rising grain size diminished with a higher dose²⁵25 Chikaoui K. Gamma rays irradiation effects in thin film polyethylene terephthalate polymer. Radiat Phys Chem. 2019;162:18-22. http://dx.doi.org/10.1016/j.radphyschem.2019.04.034.
http://dx.doi.org/10.1016/j.radphyschem.... . At the highest ionizing dose of 300 kGy, the FWHM of the irradiated TIPS Pentacene thin film is 1.08, corresponding to a grain size of 7.70 nm. The unirradiated thin film shows an FWHM of 0.11 with a grain size of 71.7 nm. The grain size can be calculated from the FWHM obtained from the XRD result by using Scherrer's equation⁴4 Mivolil DS, Chee FP, Rasmidi R, Alias A, Salleh S, Salleh KAM. Gamma ray and neutron radiation effects on the electrical and structural properties of n-ZnO/p-CuGaO 2 schottky diode. ECS J Solid State Sci Technol. 2020;9(4):045019. http://dx.doi.org/10.1149/2162-8777/ab8f19.
http://dx.doi.org/10.1149/2162-8777/ab8f... ^,2626 Soni A, Mulchandani K, Mavani KR. Crystallographically oriented porous ZnO nanostructures with visible-blind photoresponse: controlling the growth and optical properties. Materialia. 2019;6:100326. http://dx.doi.org/10.1016/j.mtla.2019.100326.
http://dx.doi.org/10.1016/j.mtla.2019.10... ^,2727 Kariper IA. Optical and structural properties and surface tension of uranium oxide thin film. Int J Surface Sci Eng. 2016;10(5):432-43. http://dx.doi.org/10.1504/IJSURFSE.2016.079041.
http://dx.doi.org/10.1504/IJSURFSE.2016.... :

Where K, $\lambda$, $\theta$, and $\beta$ are the shape factor, X-ray wavelength, diffraction angle, and the full width half maximum (FWHM) of the diffraction peak of the thin film, respectively.

For the optical transmittance of Tips Pentacene thin film, the values are calculated using reflectivity ® and absobance(A) spectra using the following equation²⁸28 Kariper IA. A new inorganic azo dye and its thin film: MoO4N 4H6. Int J Miner Metall Mater. 2014;21(5):510-4. http://dx.doi.org/10.1007/s12613-014-0936-3.
http://dx.doi.org/10.1007/s12613-014-093...

29 Kariper A. Crystalline TeO 2 thin film with chemical bath deposition. Indian J Pure Appl Phy. 2019;57(3):175-9.^-3030 Kariper IA. Pb-Ag/I thin film by co-precipitation method. Iran J Sci Technol Trans A Sci. 2016;40(2):137-43. http://dx.doi.org/10.1007/s40995-016-0017-8.
http://dx.doi.org/10.1007/s40995-016-001... :

While the determination of its optical band gap adapts the utilization of Beer Lambert's Law, as in Equation 3 ³¹31 Miandal K, Tak HH, Mohamad KA, Chee FP, Alias A. The structural and optical properties of poly(Triarylamine) (PTAA) thin films prepared at different spin rate using spin coating method. Adv Sci Lett. 2017;23(2):1337-9. http://dx.doi.org/10.1166/asl.2017.8363.
http://dx.doi.org/10.1166/asl.2017.8363... :

Where α, h, υ, and E_g represent the absorption coefficient, Planck's constant, frequency, and energy band gap, respectively, with values β as the absorption edge with parameter and n held constant as 2 for direct allowed transition.

Gamma irradiation's strong effect caused TIPS Pentacene to alter its structure, such as the widening of the FWHM, which has an inverse proportionality to the grain size. As the samples were exposed to gamma rays, the Compton interaction produced electrons that set the primary knock-on atoms in motion. The phenomenon occurs when the incident radiation gains enough energy to subsequently displace atoms within the secondary lattice structure, causing displacement cascade, structural distortion and swelling of the grain size³²32 Pervez MF, Mia MNH, Hossain S, Saha SMK, Ali MH, Sarker P et al. Influence of total absorbed dose of gamma radiation on optical bandgap and structural properties of Mg-doped zinc oxide. Optik (Stuttg). 2018;162:140-50. http://dx.doi.org/10.1016/j.ijleo.2018.02.063.
http://dx.doi.org/10.1016/j.ijleo.2018.0... ^,3333 Al-Hamdani NA, Al-Alawy RD, Hassan SJ. Effect of gamma irradiation on the structural and optical properties of ZnO thin film. Comput Eng. 2014;16(1):11-6. http://dx.doi.org/10.9790/0661-16191116.
http://dx.doi.org/10.9790/0661-16191116... . Figure 3 illustrates the relationship between FWHM data obtained from the XRD analysis and the sample's grain size.

Figure 3
Variation of grain size and FWHM of TIPS Pentacene thin films with different total ionizing doses.

AFM was used to examine the morphological variations and surface roughness of the un-irradiated and gamma-irradiated TIPS Pentacene thin films. The results of the AFM images of the TIPS Pentacene thin film are shown in Figure 4. It is observed that the grain size decreased with a higher total ionizing dose. The reduced grain size can be attributed to the ionization of the thin films caused by gamma irradiation. The shrinking grain size increases its electrical conductivity. Thus, exposure to ionizing radiation increased the turn-on-voltage. The decrease of majority charges resulted in generating vacancies under irradiation and charge capture on defects³⁴34 Arshak K, Korostynka O. Thin film pn-junctions based on oxide materials as γ-radiation sensors. Sens Actuators A Phys. 2004;113(3):307-11. http://dx.doi.org/10.1016/j.sna.2004.01.026.
http://dx.doi.org/10.1016/j.sna.2004.01.... .

Figure 4
2D AFM images of un-irradiated and gamma irradiated TIPS Pentacene thin films.

Surface roughness significantly influences carrier mobility and carrier scattering. High surface roughness increases carrier scattering and acts as a trap state in the channel, declining the electronic device's performance³⁵35 Lee JH, Kim YH, Ahn SJ, Ha TH, Kim HS. Grain-size effect on the electrical properties of nanocrystalline indium tin oxide thin films. Mater Sci Eng B Solid-State Mater Adv Technol. 2015;199:37-41. http://dx.doi.org/10.1016/j.mseb.2015.04.011.
http://dx.doi.org/10.1016/j.mseb.2015.04... . Referring to Figure 4, the surface roughness decreased from 77.03 nm for an unirradiated thin film to 29.65 nm for samples exposed at the highest ionizing dose of 300 kGy. Concerning the modification of the surface topology, the degradation with a higher total ionizing dose is proven through particle analysis, where the average density of TIPS Pentacene decreases from 0.55 µm^-2 to 0.215 µm^-2with a higher dose of radiation. The results justify the decrease in grain size drops with a higher total ionizing dose attributed to the reduction in the overall density. However, the results may very well be attributed to homogeneity differences within the surface of the scanned area.

As indicated in Table 1, it is observed that a higher ionizing dose causes a decrease in grain size, RMS roughness and particle density of all films. The degradation of TIPS Pentacene shows a strong breakdown in its material properties with higher radiation exposure³⁶36 Neuhold A, Novak J, Flesch HG, Moser A, Djuric T, Grodd L et al. X-ray radiation damage of organic semiconductor thin films during grazing incidence diffraction experiments. Nucl Instrum Methods Phys Res B. 2012;284:64-8. http://dx.doi.org/10.1016/j.nimb.2011.07.105.
http://dx.doi.org/10.1016/j.nimb.2011.07... ^,3737 Ahmed Ali AM, Ahmed NM, Mohammad SM, Sabah FA, Kabaa E, Alsadig A et al. Effect of gamma irradiation dose on the structure and pH sensitivity of ITO thin films in extended gate field effect transistor. Results Phys. 2019;12:615-22. http://dx.doi.org/10.1016/j.rinp.2018.10.066.
http://dx.doi.org/10.1016/j.rinp.2018.10... . Despite the grain quality degradation, organic-based semiconductors' performance cannot be applied directly to their electrical performance due to its molecular properties³⁸38 Andrew H-S, Len A. Handbook of radiation effects. 2nd ed. New York: Oxford University Press; 1993..

3.2. Optical properties of TIPS pentacene thin film

Figure 5 shows the absorbance and transmittance of irradiated and un-irradiated TIPS Pentacene thin films for wavelengths from 300 to 1100 nm. Overall, the transmittance of all thin films decreased after gamma irradiation. It decreased from 70% to 55% in the wavelength region of 380 to 700 nm³⁹39 Huang K, Yang K, Li H, Zheng S, Wang J, Guo H et al. γ-ray radiation on flexible perovskite solar cells. ACS Applied Energy Materials. 2020;3(8):7318-24. http://dx.doi.org/10.1021/acsaem.0c00540.
http://dx.doi.org/10.1021/acsaem.0c00540... . The reduction in transmittance with a higher dose is due to the surface and lattice defects produced via radiation (voids and pores)⁹9 Alyamani A, Mustapha N. Effects of high dose gamma irradiation on ITO thin film properties. Thin Solid Films. 2016;611:27-32. http://dx.doi.org/10.1016/j.tsf.2016.05.022.
http://dx.doi.org/10.1016/j.tsf.2016.05.... . The higher vacancy defects simultaneously increase its absorbance.

Figure 5
Transmittance and absorbance spectra of different gamma-absorbed doses of TIPS Pentacene.

Table 2 shows the optical bandgap values of TIPS Pentacene for different gamma-absorbed doses. The decrease in bandgap is due to the lattice defects produced by radiation exposure, causing structural and morphological changes⁴⁰40 Najar FA, Mir FA, Vakil GB, Dar SA, Ghayas B. Effect of γ-radiations on the optoelectrical parameters of coumarin-poly vinyl alcohol composite thin films. Radiat Phys Chem. 2022;193(January):109973. http://dx.doi.org/10.1016/j.radphyschem.2022.109973.
http://dx.doi.org/10.1016/j.radphyschem.... , as shown in Figure 3. Furthermore, the decrease in the band gap energy is due to the increase in crystallinity, as validated by the XRD measurements. The irradiation could initiate new energy levels in the forbidden gap, causing radical changes in the carrier concentration in the material.

Figure 6 shows the refractive index dispersion for un-irradiated and irradiated TIPS Pentacene thin films. It is observed that the refractive index of the investigated films decreases with higher gamma radiation dose. The results are due to the decreased density of the investigated films with irradiation due to ionization from gamma rays' collision with the thin film, altering its structural properties⁴¹41 El-Hagary M, Emam-Ismail M, Shaaban ER, El-Taher A. Effect of γ-irradiation exposure on optical properties of chalcogenide glasses Se 70S 30-xSb x thin films. Radiat Phys Chem. 2012;81(10):1572-7. http://dx.doi.org/10.1016/j.radphyschem.2012.05.012.
http://dx.doi.org/10.1016/j.radphyschem.... . Figure 7 shows several optical conductivity spectra for differing gamma-absorbed doses. It is observed that the optical conductivity of the thin films is shifted to the lower energy.

Figure 6
Refractive index of different gamma-absorbed doses of TIPS Pentacene.

Figure 7
Optical conductivity of different gamma-absorbed doses of TIPS Pentacene.

Based on the optical transmittance, the optical band gap of TIPS Pentacene thin films was calculated using the intercept of the Tauc plot⁴²42 Duinong M, Chee FP, Salleh S, Alias A, Mohd Salleh KA, Ibrahim S. Structural and optical properties of gamma irradiated CuGaO2 thin film deposited by Radio Frequency (RF) sputtering. J Phys Conf Ser. 2019;1358(1). http://dx.doi.org/10.1088/1742-6596/1358/1/012047.
http://dx.doi.org/10.1088/1742-6596/1358...

43 Zaki MF. Gamma-induced modification on optical band gap of CR-39 SSNTD. J Phys D Appl Phys. 2008;41(17). http://dx.doi.org/10.1088/0022-3727/41/17/175404.
http://dx.doi.org/10.1088/0022-3727/41/1... ^-4444 Yabagi JA, Kimpa MI, Muhammad MN, Bin Rashid S, Zaidi E, Agam MA. The effect of gamma irradiation on chemical, morphology and optical properties of polystyrene nanosphere at various exposure time. IOP Conf Ser Mater Sci Eng. 2018;298:012004. http://dx.doi.org/10.1088/1757-899X/298/1/012004
http://dx.doi.org/10.1088/1757-899X/298/... . TIPS Pentacene thin film's common band gap value is 1.72 eV⁴⁵45 Kadri DA, Karim DA, Seck M, Diouma K, Marcel P. Optimization of 6,13Bis(triisopropylsilylethynyl)pentacene (TIPS-Pentacene) organic field effect transistor: annealing temperature and solvent effects. Mater Sci Appl. 2018;09(11):900-12. http://dx.doi.org/10.4236/msa.2018.911065.
http://dx.doi.org/10.4236/msa.2018.91106... ^,4646 Griffith OL, Anthony JE, Jones AG, Lichtenberger DL. Electronic properties of pentacene versus triisopropylsilylethynyl- substituted pentacene: environment-dependent effects of the silyl substituent. J Am Chem Soc. 2010;132(2):580-6. http://dx.doi.org/10.1021/ja906917r.
http://dx.doi.org/10.1021/ja906917r... . The optical band gap observed for an unirradiated sample and at maximum ionizing dose is 1.87 eV and 1.50 eV, respectively. The decrease in band gap at the irradiated film is caused by a slight 'red shift' in the optical spectra and the rising energy width of the band tails of localized states. Structural defects due to radiation effects also influence polarization and spontaneous polarization along the c-axis and a-axis of the local electric fields. The defects can lead to band bending at the crystallite boundaries, affecting energy band gap, increasing the conductivity of irradiated thin films⁴⁷47 Nikolić D, Stanković K, Timotijević L, Rajović Z, Vujisić M. Comparative study of gamma radiation effects on solar cells, photodiodes, and phototransistors. Int J Photoenergy. 2013;2013. http://dx.doi.org/10.1155/2013/843174.
http://dx.doi.org/10.1155/2013/843174... . This finding is in agreement with other previous studies⁹9 Alyamani A, Mustapha N. Effects of high dose gamma irradiation on ITO thin film properties. Thin Solid Films. 2016;611:27-32. http://dx.doi.org/10.1016/j.tsf.2016.05.022.
http://dx.doi.org/10.1016/j.tsf.2016.05.... ^,2424 El-Nahass MM, Khalifa BA, Soliman IM. Gamma radiation-induced changes on the structural and optical properties of aluminum phthalocyanine chloride thin films. Opt Mater. 2015;46:115-21. http://dx.doi.org/10.1016/j.optmat.2015.04.010.
http://dx.doi.org/10.1016/j.optmat.2015.... ^,3434 Arshak K, Korostynka O. Thin film pn-junctions based on oxide materials as γ-radiation sensors. Sens Actuators A Phys. 2004;113(3):307-11. http://dx.doi.org/10.1016/j.sna.2004.01.026.
http://dx.doi.org/10.1016/j.sna.2004.01.... ^,4242 Duinong M, Chee FP, Salleh S, Alias A, Mohd Salleh KA, Ibrahim S. Structural and optical properties of gamma irradiated CuGaO2 thin film deposited by Radio Frequency (RF) sputtering. J Phys Conf Ser. 2019;1358(1). http://dx.doi.org/10.1088/1742-6596/1358/1/012047.
http://dx.doi.org/10.1088/1742-6596/1358... .

4. Conclusion

The results of the irradiated samples demonstrate the benefits of modifications in their structural and optical properties compared to the unirradiated sample. XRD analysis showed that unirradiated and irradiated TIPS Pentacene thin films have crystalline structures, maintaining their semiconductor properties. The gamma-irradiated film exhibit higher aggregation on its surface morphology than the unirradiated film. The surface analysis by AFM also indicated that the grain size decreased after gamma-irradiation, which was validated using Scherrer's equation. The transmittance also decreased with the higher dose. The optical band gap decreases from 1.87 eV for the un-irradiated samples to 1.50 eV at the maximum total ionizing dose of 300 kGy. These structural and optical parameters alterations by gamma-irradiation make these thin films a promising material to replace conventional silicon-based semiconductor materials for the integrated optical device in terrestrial applications.

5. Acknowledgments

The authors wish to acknowledge the support of the Ministry of Higher Education for providing funding assistance based on the Fundamental Research Grant Scheme FRG0534-2020 titled "Explication On The Damage Mechanism Induced By High Energy Radiation On ZnO Based Photoconductive Radiation Detector For Space Borne Application", and GUG0387-2/2019, with the title "Investigation of Damage Mechanism of Hybrid ZnO based Devices due to Ionizing Radiation for Flexible Electronic in Space Application".

6. References

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