I. INTRODUCTION

Section:

ChooseTop of pageABSTRACTI. INTRODUCTION <<II. EXPERIMENTIII. RESULTS AND DISCUSSI...IV. SUMMARY AND CONCLUSIO...REFERENCESPhototransistors are well known,11. H. Yoo, I. S. Lee, S. Jung, S. M. Rho, B. H. Kang, and H. J. Kim, Adv. Mater. 33, 2006091 (2021). https://doi.org/10.1002/adma.202006091 but not organic phototransistors, in part because of the difficulties of making an organic electronic transistor. Even less common are phototransistors that have no photocurrent enhancement but rather a decrease in the source drain current, when exposed to light. Indeed, this sort of “inverse” phototransistor is unusual. In recent years, there has been an increasing appreciation for organic photo-sensing devices because of the low processing costs and device flexibility and applicability to a wide range of applications including organic photodetectors, organic photodiodes, organic photosensors, and organic photo transistors.2–62. J. Huang et al., Adv. Mater. 32, 1906027 (2020). https://doi.org/10.1002/adma.2019060273. Y. L. Wu, K. Fukuda, T. Yokota, and T. Someya, Adv. Mater. 31, 1903687 (2019). https://doi.org/10.1002/adma.2019036874. C. Wang, X. Zhang, and W. Hu, Chem. Soc. Rev. 49, 653 (2020). https://doi.org/10.1039/C9CS00431A5. H. Wang et al., Adv. Mater. 29, 1701772 (2017). https://doi.org/10.1002/adma.2017017726. Z. Lin, Q. Chen, Y. Yan, Y. Liu, E. Li, W. Yu, H. Chen, and T. Guo, IEEE Electron Device Lett. 42, 1358 (2021). https://doi.org/10.1109/LED.2021.3097327 The low-cost fabrication method and ability to make changes in the properties of the material by just varying the chemical structure make organic electronics very appealing.77. S. R. Forrest and M. E. Thompson, Chem. Rev. 107, 923 (2007). https://doi.org/10.1021/cr0501590 Polypeptides, i.e., polymer chains of amino acids connected via the peptide bond, are one class of organic materials widely studied for organic electronics.8–108. B. D. Wall, A. E. Zacca, A. M. Sanders, W. L. Wilson, A. L. Ferguson, and J. D. Tovar, Langmuir 30, 5946 (2014). https://doi.org/10.1021/la500222y9. S. Khanra, T. Cipriano, T. Lam, T. A. White, E. E. Fileti, W. A. Alves, and S. Guha, Adv. Mater. Interfaces 2, 1500265 (2015). https://doi.org/10.1002/admi.20150026510. H. A. M. Ardona and J. D. Tovar, Bioconjug. Chem. 26, 2290 (2015). https://doi.org/10.1021/acs.bioconjchem.5b00497Organic ferroelectrics have a dipole moment that can be aligned under an applied electric field giving rise to a spontaneous nonvolatile polarization.1111. K. Asadi, D. M. de Leeuw, B. de Boer, and P. W. M. Blom, Nat. Mater. 7, 547 (2008). https://doi.org/10.1038/nmat2207 In an organic ferroelectric material, the reorientation of the dipoles, in the presence of an external electric field, generates a polarization hysteresis loop with voltage,1212. V. S. Bystrov, E. Paramonova, I. Bdikin, S. Kopyl, A. Heredia, R. C. Pullar, and A. L. Kholkin, Ferroelectrics 440, 3 (2012). https://doi.org/10.1080/00150193.2012.741923 first observed in Rochelle salt.1313. J. Valasek, Phys. Rev. 17, 475 (1921). https://doi.org/10.1103/PhysRev.17.475 The phenomenon of ferroelectricity has also been observed in some amino acids and peptide nanotubes.14–1614. P. Hu, S. Hu, Y. Huang, J. R. Reimers, A. M. Rappe, Y. Li, A. Stroppa, and W. Ren, J. Phys. Chem. Lett. 10, 1319 (2019). https://doi.org/10.1021/acs.jpclett.8b0383715. I. Bdikin, V. Bystrov, S. Kopyl, R. P. G. Lopes, I. Delgadillo, J. Gracio, E. Mishina, A. Sigov, and A. L. Kholkin, Appl. Phys. Lett. 100, 043702 (2012). https://doi.org/10.1063/1.367641716. A. Heredia et al., Adv. Funct. Mater. 22, 2996 (2012). https://doi.org/10.1002/adfm.201103011 The phenomena of ferroelectricity require static nonvolatile alignment, but while polar molecules can be ferroelectric, antiferroelectric, pyroelectric, and polar liquid phenomena are also possible. Polar molecules may sometimes behave like a ferroelectric, but if there is no remnant polarization due to the free flow of charges or if the conductivity is too high for retention of a zero bias electric field, then the polar molecules are not conventionally ferroelectric. Additionally, polar molecules having low resistance and voltage switchable property can be used to lock the spin state of spin crossover molecules,1717. T. K. Ekanayaka et al., Magnetochemistry 7, 37 (2021). https://doi.org/10.3390/magnetochemistry7030037 while decreasing the overall device impedance, although at the cost of higher spin state “write” currents.As shown here, poly-d-lysine, whose schematic structure is shown in Fig. 1, can be added to the growing class of polar organic semiconductors and can be a useful component in a voltage controlled organic phototransistor structure, thus making it a potential candidate for a competitive memory or optical molecular device. Poly-lysine is an amino acid biopolymer that has attracted attention as a component for biosensors18–2118. N. P. Huang, J. Vörös, S. M. De Paul, M. Textor, and N. D. Spencer, Langmuir 18, 220 (2002). https://doi.org/10.1021/la010913m19. J. Movilli, A. Rozzi, R. Ricciardi, R. Corradini, and J. Huskens, Bioconjugate Chem. 29, 4110 (2018). https://doi.org/10.1021/acs.bioconjchem.8b0073320. Y. Chen et al., ACS Appl. Mater. Interfaces 7, 2919 (2015). https://doi.org/10.1021/am508399w21. X. Wen, H. He, and L. J. Lee, J. Immunol. Methods 350, 97 (2009). https://doi.org/10.1016/j.jim.2009.07.011 but also can serve in organic electronics.For optoelectronic applications, increasing the light response is important, and the addition of a dye can be a valuable route to improve the optical response. Improvements to the optical-electrical response are more likely if the dye dopes the host polymer and leads to decreased device impedance without a loss in the device on/off ratio. In the past, various organic dyes, such as methylene blue,2222. Y. S. Ocak, M. Kulakci, T. Kılıçoğlu, R. Turan, and K. Akkılıç, Synth. Met. 159, 1603 (2009). https://doi.org/10.1016/j.synthmet.2009.04.024 rhodamine 6G,2323. T. Fukuda, S. Kimura, and Z. Honda, Mol. Cryst. Liq. Cryst. 566, 67 (2012). https://doi.org/10.1080/15421406.2012.701830 safranin-T,2424. S. Sen, P. K. Das, and N. B. Manik, J. Phys. Commun. 5, 045004 (2021). https://doi.org/10.1088/2399-6528/abf2cf fuschin,2525. Ö Güllü, S. Asubay, Ş Aydoğan, and A. Türüt, Physica E 42, 1411 (2010). https://doi.org/10.1016/j.physe.2009.11.079 etc., have been used as possible additives to improve molecular device performance. Owing to the presence of 14 π electrons2222. Y. S. Ocak, M. Kulakci, T. Kılıçoğlu, R. Turan, and K. Akkılıç, Synth. Met. 159, 1603 (2009). https://doi.org/10.1016/j.synthmet.2009.04.024 and the ability to promote enhancement in the capacitance,2626. S. Roldán, M. Granda, R. Menéndez, R. Santamaría, and C. Blanco, Electrochim. Acta 83, 241 (2012). https://doi.org/10.1016/j.electacta.2012.08.026 methylene blue is a particularly promising combination with poly-d-lysine and has been used here to enhance the poly-d-lysine thin film organic device photoresponse.

II. EXPERIMENT

Section:

ChooseTop of pageABSTRACTI. INTRODUCTIONII. EXPERIMENT <<III. RESULTS AND DISCUSSI...IV. SUMMARY AND CONCLUSIO...REFERENCESA solution of (a) poly-d-lysine (Sigma-Aldrich) and (b) poly-d-lysine plus methylene blue (Sigma-Aldrich), in distilled water, was drop casted to make a film on several different interdigitated electrode system (MicruX Technologies and Metrohm DropSens) and on organic field effect transistors (OFETs) (Fraunhofer Institute for Photonic Microsystems IPMS). This demonstrated the reproducibility of the results as the measurements were similar and independent of the prepatterned electrodes used. The current-voltage and capacitance-voltage characteristics presented in Fig. 2, and Figs. 5 and 6 were studied on an interdigitated electrode system with a spacing of 5 μm (Metrohm DropSens). The data presented in Fig. 4 were studied on interdigitated electrode system with a spacing of 5 μm (MicruX Technologies) and that in Fig. 7 were studied on both interdigitated electrode systems with a spacing of 5 μm (MicruX Technologies and Metrohm DropSens) as specified in the Fig. 7 caption. The transistor characteristics presented in Fig. 3 were studied on an OFET by Fraunhofer Institute for Photonic Microsystems IPMS with a channel length of 5 μm and a channel width of 10 mm. 5 mg of poly-d-lysine hydrobromide was dissolved in 6.25 ml of distilled water to make a poly-d-lysine solution. The poly-d-lysine solution was deposited on the interdigitated electrodes by the drop cast method to form a thin film for the current-voltage and capacitance-voltage measurements. For the transistor measurement, the poly-d-lysine solution was drop casted on the OFET (Fraunhofer Institute for Photonic Microsystems IPMS) to make a thin film organic transistor.

To make films of poly-d-lysine with methylene blue as an additive, 1 ml of the poly-d-lysine solution (made of 5 mg poly-d-lysine hydrobromide dissolved in 6.25 ml distilled water) was mixed with 20 μl of methylene blue solution (made of 100 ± 2 mg methylene blue hydrate in 10 ml of distilled water).

The drop cast films were allowed to dry before the measurements were taken. As noted above, the current-voltage and capacitance-voltage (4200A-SCS parameter analyzer) and the transistor (Cryogenic Lakeshore Probe Station) characteristics were measured. All the capacitance-voltage measurements were done at a frequency of 10 kHz. The measurements in the light were taken after the films were illuminated by a 425 nm wavelength 26 W Hg lamp for about 60–120 min. The films were left in dark and exposed to light for about 60–120 min intervals in alternate dark and light cycles. All the measurements were done at room temperature. The voltage sweep rates were roughly 150 mV/s.

We also performed piezoelectric force microscopy (PFM) on both the thin films of poly-d-lysine and poly-d-lysine plus methylene blue, but the conductance of the poly-d-lysine thin films was too high to obtain a reliable PFM signal.

III. RESULTS AND DISCUSSION

Section:

ChooseTop of pageABSTRACTI. INTRODUCTIONII. EXPERIMENTIII. RESULTS AND DISCUSSI... <<IV. SUMMARY AND CONCLUSIO...REFERENCESPoly-d-lysine displays a conductance hysteresis loop, as a function of applied voltage, in both dark (black curve) and light (red curve), as seen in Fig. 2. Noteworthy is the fact that the current does not go to zero, at zero bias, which indicates the presence of trapped charges, or a net polarization. The implication of the observed zero bias current is that there are trapped charges present or a net polarization as there is a small conductance at zero voltage for both dark and light. If we compare Fig. 2(a) to the I(V) characteristics of the poly-d-lysine thin film with methylene blue as an additive, i.e., Fig. 2(b), the current in the latter case is smaller and decreases significantly with illumination.As shown in Fig. 3, for poly-d-lysine as the semiconductor channel in the transistor geometry, both with and without methylene blue as an additive, the drain current in the absence of illumination (i.e., dark, black curve) is higher than the drain current with illumination (red curve). For poly-d-lysine with methylene blue as an additive, as seen in Fig. 3(b), the drain current is lower with illumination. This trend is consistent with the current versus voltage curves plotted in Fig. 2, except that there is significant hysteresis with gate voltage. As a result of the significant hysteresis, Fig. 3 indicates that the thin film transistors of poly-d-lysine and poly-d-lysine plus methylene blue behave both as p-type and n-type transistors, i.e., bistable transistors, as it appears that the majority carrier can be controlled by the applied gate voltage.Figure 4 shows the capacitance-voltage characteristics of the poly-d-lysine thin film at various frequencies. While there is clearly a frequency dependence to the capacitance, it is noteworthy that the C(V) curves, taken at different frequencies, remain comparable. The photoresponse of the capacitance is evident from Fig. 5 and comparing Fig. 4(a) with Fig. 4(b), it is evident that the capacitance is much lower at each frequency, under illumination.The capacitance versus voltage, C(V) measurements at 10 kHz for the poly-d-lysine thin films and poly-d-lysine with methylene blue as an additive are shown in Figs. 5(a) and 5(b), respectively. While the current flow, with an applied voltage, is greater without illumination than under illumination, as seen in Figs. 2 and 3, it is evident that low conductance is actually associated with a smaller capacitance, as seen in Figs. 4 and 5. Both capacitance versus voltage and current versus voltage can be combined to estimate the drift carrier lifetime.Figure 6 shows the charge carrier lifetime versus voltage for both (a) the poly-d-lysine thin film and (b) poly-d-lysine thin film with methylene blue as an additive and the capacitance in both cases were measured at a frequency of 10 kHz. The charge carrier lifetime was calculated via27,2827. Z. G. Marzouk, A. Dhingra, Y. Burak, V. Adamiv, I. Teslyuk, and P. A. Dowben, Mater. Lett. 297, 129978 (2021). https://doi.org/10.1016/j.matlet.2021.12997828. C. C. Ilie et al., J. Phys. Condens. Matter 30, 18LT02 (2018). https://doi.org/10.1088/1361-648X/aab986 τ=(((CDω2G0)2+1)2−1)ω1/2.Here, “τ” is the charge carrier lifetime, “CD” is the capacitance having frequency dependency, “ω” is the angular frequency defined as ω = 2πf with frequency “f”, and “G0” is the conductance defined as G0=dIdV with “V” being the voltage applied and “I” being the corresponding current.27,2827. Z. G. Marzouk, A. Dhingra, Y. Burak, V. Adamiv, I. Teslyuk, and P. A. Dowben, Mater. Lett. 297, 129978 (2021). https://doi.org/10.1016/j.matlet.2021.12997828. C. C. Ilie et al., J. Phys. Condens. Matter 30, 18LT02 (2018). https://doi.org/10.1088/1361-648X/aab986As seen in Fig. 6, for the poly-d-lysine thin films with and without methylene blue, the charge carrier lifetime is higher under no illumination than in the presence of illumination. The photoexcitons are unable to move and are, thus, more likely to recombine than unbind.The current at zero bias is nonzero (Fig. 2), yet the poly-d-lysine thin films are not solar cell materials as this nonzero current occurs in both light and dark conditions. The presence of methylene blue in the poly-d-lysine thin films results in higher currents in the dark than with illumination. This too is not characteristic of a solar cell material. As the current is less under illumination, especially in the transistor geometry as seen in Fig. 3, this further indicates that poly-d-lysine with and without methylene blue as an additive is not intrinsically a solar cell material. It is clear that the photoexcitons that are bound charges low mobility and the electron hole pair does not unbind and are responsible for the decrease in current with illumination as seen in Figs. 2 and 3.The capacitance exhibits hysteresis with voltage in the absence of illumination as seen in Figs. 4 and 5. The butterfly loop hysteresis of the C(V) characteristics is also clearly an indication of the material being polar as the extent of polarization would change the dielectric capacitance. The occurrence of polarization switching corresponding to the applied positive and negative biases is observed in the transistor characteristics, as a function of gate voltage in both Figs. 3(a) and 3(b). This type of current hysteresis is also common in ferroelectric materials.29–3229. M. H. Amiri, J. Heidler, K. Müllen, and K. Asadi, ACS Appl. Electron. Mater. 2, 2 (2020). https://doi.org/10.1021/acsaelm.9b0053230. M. M. Hasan, C. W. Ahn, T. H. Kim, and J. Jang, Appl. Phys. Lett. 118, 152901 (2021). https://doi.org/10.1063/5.003565331. S. Moon, J. Shin, and C. Shin, Electronics 9, 704 (2020). https://doi.org/10.3390/electronics905070432. S. Das and J. Appenzeller, Nano Lett. 11, 4003 (2011). https://doi.org/10.1021/nl2023993Clearly, the poly-d-lysine films with and without methylene blue as an additive are p-type with one polarization, and then, after applying sufficient voltage, the films become n-type, associated with the opposite polarization as seen in the transistor characteristics of Fig. 3. The majority carriers can change from holes to electrons as a result of polarization change, controlled by the gate voltage. The maximum capacitance value at positive and negative bias corresponds to the coercive voltage (+Vc) and (−Vc) in the butterfly C(V) hysteresis loop (Figs. 4 and 5) and in the transistor characteristics seen in Fig. 3. The flat shape of the C(V) loop around zero voltage has also been observed in thin ferroelectric films and was attributed to the trap states due to the electrode interface effect.3333. M. Vehkamäki, T. Hatanpää, M. Kemell, M. Ritala, and M. Leskelä, Chem. Mater. 18, 3883 (2006). https://doi.org/10.1021/cm060966v Clearly polar, and while not proven because of the changing majority carrier, ferroelectric materials seem implicated, but antiferroelectric is not completely excluded by the data presented for both the poly-d-lysine films, with and without methylene blue as an additive.The results presented in Fig. 3 illustrate that poly-d-lysine can be used to make a phototransistor, that is in the “off state” when illuminated by light and in the “on state” when in dark. The fact that poly-d-lysine can be used to make a photoactive transistor with lower conductance under illumination than in the dark is unusual. This photoactive response is true not only of transistor devices but also of the capacitive device structure. As shown in Figs. 4 and 5, for a given range of voltage, the value of capacitance is higher when the light is turned off, i.e., in dark in comparison to when the light is turned on, even though the capacitance is frequency dependent. High capacitance is consistent with the significant dielectric properties of a polar material.The decrease in capacitance under illumination for the poly-d-lysine film with and without methylene blue (Fig. 5) is difficult to reconcile with the decrease in conductance (Figs. 2 and 3), but photo excitations may decrease the net polarization. When the light is turned on, although the presence of light increases the number of photoexcitons, they lead to depolarization; hence, the capacitance is reduced (Fig. 5). Thus, poly-d-lysine, with and without methylene blue as an additive, remains dielectric under illumination but a carrier for lifetime, and possibly, the carrier concentration is suppressed. Similar trends are seen for the poly-d-lysine films with and without methylene blue although the drift carrier lifetime is higher in the poly-d-lysine film without methylene blue than with methylene blue as an additive.Figure 7 shows the capacitance values measured at alternate dark and light cycles. The films were left in dark and exposed to light for about 60–120 min intervals in alternate cycles and the capacitance value for the range of −3 V to +3 V was recorded. Each point in Fig. 7 corresponds to the capacitance value at 3 V; however, the capacitance value in Fig. 7(b) on Metrohm DropSens electrodes was scaled with respect to the capacitance value on MicruX electrodes. It is evident from Fig. 7(a) that the capacitance is higher for dark than light. As seen in Fig. 7(b), we performed the capacitance measurements on two different electrode surfaces and scaled it as mentioned in the caption of Fig. 7, the results of which also demonstrate that the capacitance is higher in dark than in light. This behavior of poly-d-lysine provides a valid reason for it to be used as an inverse phototransistor.

IV. SUMMARY AND CONCLUSIONS

Section:

ChooseTop of pageABSTRACTI. INTRODUCTIONII. EXPERIMENTIII. RESULTS AND DISCUSSI...IV. SUMMARY AND CONCLUSIO... <<REFERENCESThere are sufficient examples of organic ferroelectric materials such as the variations of polyvinylidene fluoride (including polyvinylidene fluoride trifluroethylene: PVDF-TrFE),34,3534. L. M. Blinov, V. M. Fridkin, S. P. Palto, A. V. Bune, P. A. Dowben, and S. Ducharme, Phys. USP. 43, 243 (2000). https://doi.org/10.1070/PU2000v043n03ABEH00063935. S. Horiuchi and Y. Tokura, Nat. Mater. 7, 357 (2008). https://doi.org/10.1038/nmat2137 croconic acid,3636. X. Jiang et al., Appl. Phys. Lett. 109, 102902 (2016). https://doi.org/10.1063/1.4962278 thiourea,37,3837. G. J. Goldsmith and J. G. White, J. Chem. Phys. 31, 1175 (1959). https://doi.org/10.1063/1.173056838. A. L. Solomon, Phys. Rev. 104, 1191 (1956). https://doi.org/10.1103/PhysRev.104.1191 tricyclohexylmethanol (TCHM),3939. Y. Yamamura, H. Saitoh, M. Sumita, and K. Saito, J. Phys. Condens. Matter 19, 176219 (2007). https://doi.org/10.1088/0953-8984/19/17/176219 diazabicyclo[2.2.2]octane (dabco) salts,4040. D. W. Fu, J. X. Gao, W. H. He, X. Q. Huang, Y. H. Liu, and Y. Ai, Angew. Chem. Int. Ed. 59, 17477 (2020). https://doi.org/10.1002/anie.202007660 hydrogen-bonding chains of 3-hydroxyphenalenone (3-HPLN),4141. S. Horiuchi, R. Kumai, and Y. Tokura, Adv. Mater. 23, 2098 (2011). https://doi.org/10.1002/adma.201100359 1,6-bis(2,4-dinitrophenoxy)-2,4-hexadiyne,42–4442. P. Gruner-Bauer and E. Dormann, J. Phys. Condens. Matter 4, 5599 (1992). https://doi.org/10.1088/0953-8984/4/25/01343. H. Winter, E. Dormann, M. Bertault, and L. Toupet, Phys. Rev. B 46, 8057 (1992). https://doi.org/10.1103/PhysRevB.46.805744. G. Nemec and E. Dormann, J. Phys. Condens. Matter 6, 1417 (1994). https://doi.org/10.1088/0953-8984/6/7/013 and amino acids like glycine.14,1614. P. Hu, S. Hu, Y. Huang, J. R. Reimers, A. M. Rappe, Y. Li, A. Stroppa, and W. Ren, J. Phys. Chem. Lett. 10, 1319 (2019). https://doi.org/10.1021/acs.jpclett.8b0383716. A. Heredia et al., Adv. Funct. Mater. 22, 2996 (2012). https://doi.org/10.1002/adfm.201103011 Information storage is one of the most important aspects of any memory device, and the polarization response observed in ferroelectric materials is suitable for memory applications.4545. R. C. G. Naber, K. Asadi, P. W. M. Blom, D. M. De Leeuw, and B. De Boer, Adv. Mater. 22, 933 (2010). https://doi.org/10.1002/adma.200900759 The common examples of memory devices based on organic ferroelectricity are ferroelectric capacitors4646. T. Lenz et al., Phys. Status Solidi Appl. Mater. Sci. 212, 2124 (2015). https://doi.org/10.1002/pssa.201532267 and ferroelectric field effect transistors.4747. T. Kanashima, Y. Katsura, and M. Okuyama, Jpn. J. Appl. Phys. 53, 04ED11 (2014). https://doi.org/10.7567/JJAP.53.04ED11 Because of the higher conductivity, the poly-d-lysine thin films with and without methylene blue cannot be ferroelectric based on the evidence presented here because a remanent polarization is not demonstrated. Although there appears to be a transport behavior indicative of the switchable dipoles, the conductance is high, and thus, the retention of uncompensated charge at an interface may not be possible.

Poly-d-lysine in combination with methylene blue can be used to fabricate a phototransistor which is a combination of both p-type and n-type and is off state when illuminated and on state when not illuminated—in other words, an inverse phototransistor. The presence of methylene blue in the poly-d-lysine thin film enhances its inverse phototransistor effect, improving the on/off ratio at the cost of only a small increase in device impedance. The poly-d-lysine thin film could be an ideal sensor because it operates in the reverse sense of other photodiodes and phototransistors. The low cost of materials and trivial deposition method also makes it very attractive, as this potentially reduces fabrication costs. Because these organic thin films are easily fabricated, and compatible with flexible substrates, there is a potential here for a low-cost organic inverse phototransistor. This study provides a route for creating an organic inverse phototransistor that is cost effective and easy to fabricate.