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Materials Research Express PAPER Facile and low-cost synthesis of PEDOT:PSS/FTO polymeric counter electrode for DSSC photosensor with negative capacitance phenomenon To cite this article: S S Shenouda et al 2019 Mater. Res. Express 6 065004 View the article online for updates and enhancements. This content was downloaded from IP address 195.43.22.140 on 10/05/2019 at 03:50 Mater. Res. Express 6 (2019) 065004 https://doi.org/10.1088/2053-1591/ab0861 PAPER RECEIVED 10 November 2018 REVISED 3 February 2019 Facile and low-cost synthesis of PEDOT:PSS/FTO polymeric counter electrode for DSSC photosensor with negative capacitance phenomenon ACCEPTED FOR PUBLICATION 19 February 2019 PUBLISHED S S Shenouda1 6 March 2019 1 2 3 4 5 , I S Yahia2,3 , Hoda S Hafez4 and F Yakuphanoglu5 Semiconductors Lab. and Nanoscience Lab. for Environmental and Bio-medical Applications (NLEBA), Department of Physics, Faculty of Education, Ain Shams University, Roxy, 11757 Cairo, Egypt Research Center for Advanced Materials Science (RCAMS), King Khalid University, Abha 61413, PO Box 9004, Saudi Arabia Advanced Functional Materials & Optoelectronic Laboratory, Department of Physics, Faculty of Science, King Khalid University, PO Box 9004, Abha, Saudi Arabia Nano-Photochemistry and its Environmental Applications Laboratory, Environmental Studies and Research Institute (ESRI), University of Sadat City, Sadat City, 23897 Menofia, Egypt Department of Physics, Faculty of Science, Firat University, Elazig, Turkey E-mail: shenouda.fam@edu.asu.edu.eg Keywords: DSSCs, PEDOT:PSS, photosensor, photovoltaic, polymeric counter electrode, conduction mechanism Abstract In this research, PEDOT:PSS film is employed as a counter electrode in the dye-sensitized solar cell (DSSC) for low-cost photo-sensing applications. This DSSC is based on nanostructured TiO2 as photoanode. The morphology of the counter electrode was examined using atomic force microscope showing well distributed spherical clusters with average size 214.04 nm. This DSSC behaves like Schottky diode in the dark condition with rectification ratio 40 at 0.5 V. The photovoltaic behavior was studied in the light intensity range 10–130 mW cm−2. This solar cell has a stable fill factor (about 0.5) for most of the studied illumination intensity range. The photo-response of the current suggests that the new designed DSSC with the polymeric counter electrode is a promising device for low-cost photosensor. The cell shows capacitance inversion from positive to negative with increasing the applied frequency at about 100 kHz which recommends this cell for further applications. Also, the effect of frequency on the built-in potential and the series resistance has been investigated. 1. Introduction Dye-sensitized solar cells have attracted much attention of many researchers due to their easy preparation, low cost, non-toxic, environmental friendly and relatively high efficiency [1–3]. In general, DSSC consists of four main components; TiO2 photoanode, iodide electrolyte, Ruthenium complex as photosensitizer and Platinum (Pt) counter electrode [3]. Platinum has high-cost production for the industrial scale technology. Thus, to decrease the production cost, conducting polymers have been used as counter electrodes. PEDOT:PSS (Poly(3,4-ethylenedioxythiophene):polystyrene sulfonate) is a conducting polymer which has several advantages such as easy fabrication, high stability, low cost, good catalytic activity, lightweight, environmental friendly, flexible, hole transporting characteristics and good conductivities up to 1000 S cm−1 [4–7]. Since photosensors could be designed using devices with linear photo-response of current [8], DSSC based on PEDOT:PSS counter electrode would be promising for designing cheap photosensors. Tamburri et al [9] could determine the best experimental conditions to improve the redox performance of PEDOT:PSS films as a counter electrode in the DSSC. Also, Heo et al [10] demonstrated that 3D PEDOT:PSS films could be an efficient Pt-free catalyst in DSSC. Moolsarn et al [11] used PEDOT:PSS with carbon derived from Human Hair as a counter electrode for the DSSC. They have found that C0.6P gives the best efficiency (6.54±0.11%). This efficiency is lower than Pt’s (7.29±0.01%) but it is promising due to the low cost. Also, Yue et al [12] applied PEDOT:PSS with polypyrrole © 2019 IOP Publishing Ltd Mater. Res. Express 6 (2019) 065004 S S Shenouda et al Figure 1. 2D AFM micrographs of PEDOT:PSS/FTO polymeric counter electrode. (PPy) as composite counter electrode in DSSC and achieved efficiency comparable to that of the DSSC based on Pt. In our study, PEDOT:PSS polymeric counter electrode was used instead of Pt to reduce the production cost. We present the conduction mechanism and the photovoltaic response (current-voltage) of the DSSC based on PEDOT:PSS in details. From which the dependence of the photocurrent on the light intensity has been analyzed for the photosensing application. The effect of frequency on the capacitance-voltage (C–V) characterization has also been studied. This C–V characterization is a non-destructive technique which determines the distribution of the charges and the interface states in the semiconducting materials when the biasing voltage participates in the emission and capture of charges at the interface traps [13, 14]. Such traps are charged leading to production of a dipole layer. Any small modulation of the AC bias will modify the barrier height leading to a shift in the Fermi level position. This will change the properties of the device [14, 15]. 2. Experimental techniques TiO2 was painted three times on FTO substrate by a doctor blade to form a (3–5 μm) thick-film. This film was sintered at 450 °C for 30 min in air. The TiO2 film was immersed in ‘K30 (RuII (4,5-diazafluoren-9-one) (4,40dicarboxy-2,20-bipyridyl) di (thiocyanate), [RuII (L1) (H2dcbpy) (NCS)2])’ dye with 25×10−5 M soluble in absolute ethanol for 1 day. Then, this sensitized film was washed using pure ethanol followed by drying in air. The cell has area about 0.25 cm2. The used electrolyte is a mixture of iodine (0.05 M) and lithium iodide (0.5 M) in pure acetonitrile. PEDOT:PSS polymer was deposited on FTO using spin coater (1000 rpm at room temperature for 30 s) and annealed for 20 min at 120 °C. A drop of the liquid electrolyte is placed between the PEDOT:PSS polymer electrode and the dye adsorbed TiO2 electrode then, the edges of the DSSC were sealed. Three cells have been fabricated for each measurement to take the average. The morphology of PEDOT:PSS/FTO was examined using atomic force microscope (AFM, Park-system, XE-100). The pictures were analyzed using PARK-XEI software. The current-voltage and conductance-voltage were measured by a KEITHLEY(4200) characterizationsystem at room temperature using Class-BBA solar simulator (re-calibrated by TM-206 solar power meter). 3. Results and discussions 3.1. Structure identification Structure of the TiO2 photoanode electrode is examined using x-ray diffraction and scanning electron microscope in our previous work showing the presence of rutile and anatase phases (with average crystal sizes 38 and 14 nm, respectively) and uniform morphology without any cracks [16]. The morphology of PEDOT:PSS/ FTO film was studied and analyzed using 2D-AFM micrographs (5×5 μm2 and 1×1 μm2) as shown in figure 1. It shows well distributed spherical clusters with average size 214.04 nm. Average of the roughness is 75.237 nm. 2 Mater. Res. Express 6 (2019) 065004 S S Shenouda et al Figure 2. Current-voltage characteristic of the DSSC in dark; inset, ln I f versus voltage. Figure 3. Current-voltage characteristic of the DSSC under different illumination intensities. 3.2. Current-voltage characteristics The current-voltage (I–V) in dark condition of the DSSC with PEDOT:PSS counter electrode is shown in figure 2. Clearly, the cell shows rectification behavior with rectification ratio (RR) about 40 at 0.5 V. This means that the cell seems to be a diode-like device. At lower forward voltages 0.5 V, the current increases exponentially with the biasing voltage. This means that this DSSC behaves as a Schottky diode [17, 18]. Thus, the I–V can be analyzed according to [18]: ⎛ eV ⎞ I = Is exp ⎜ ⎟ ⎝ nkB T ⎠ (1) where Is is the reverse saturation current, kB is Boltzmann’s constant n is the cell ideality factor and e is the electron’s charge. Plotting of ln If versus the biasing voltage is presented in the inset of figure 2 showing good straight line fitting. n was determined from its slope (about 7.6). This high value (more than one) indicates the non-ideality behavior of the diode-like device [18]. I–V characteristics under different illumination intensities (5–130 mW cm−2) are shown in figure 3. This cell shows photovoltaic behavior i.e. there are short circuit current (Isc ) and open circuit voltage (Voc ) at zero bias and zero current, respectively. Figure 4(a) presents the effect of illumination intensity on Isc and Voc . Clearly, both Voc and Isc increase with increasing the illumination intensity. Dependence of Isc on the illumination intensity suggests that this cell could be used as photosensor [8] obeying the following equation [19, 20]: 3 Mater. Res. Express 6 (2019) 065004 S S Shenouda et al Figure 4. (a) Variation of photocurrent and open circuit voltage with light intensity, (b) ln Isc versus ln L. Isc = const. Lg (2) The exponent g can be calculated from the relation ln g versus ln L as seen in figure 4(b). g has values between 0.5 and 1.0 in case of the continuous distribution of the trapping centers and has higher values in case of presence of lower density of the unoccupied trap states [19]. The obtained value in our cell is 1.5 indicating the existence of low unoccupied trap states. Although there are many devices used as photosensors such as Al/p-Si/C3N4/Au Schottky diode, CdO/p-Si diode, ITO/CdS/PbS/C heterostructure and DSSC based on Pt counter electrode [8, 21–23], our DSSC based on PEDOT:PSS counter electrode is cheaper, less toxic and easy to fabricate without any complicated technology than other devices. Figure 5 shows the output power (P = IV ) versus the biasing voltage. It has a peak denoting a maximum power PM at certain current IM and voltage VM . The fill factor (FF ) of the cell is calculated according to [24]: FF = VM IM Voc Isc (3) Values of IM , VM and FF are listed in table 1. These values show the stability of the DSSC over the investigated range of illumination intensity. 3.3. Capacitance-voltage and negative capacitance For further characterization of the cell, C–V is measured under different applied frequencies ranges from 10 kHz to 1 MHz as shown in figures 6(a), (b). C–V plots have three peaks and their positions are approximately frequency independent. The peak’s value decreases with increasing the frequency. These peaks result from the interface states and the series resistance (Rs ) effect [25]. Value and position of the peak depend on many parameters like Rs , the doping concentration (Nd ) and surface state density (Nss ) [25, 26]. Figure 7 shows the dependence of the capacitance on the frequency. Clearly, in all the biasing voltage range, the capacitance decreases fast at the beginning then slows down with increasing the frequency. At around 4 Mater. Res. Express 6 (2019) 065004 S S Shenouda et al Figure 5. Output power-voltage characteristics of the DSSC under different illumination intensities. Table 1. Photovoltaic parameters of the DSSC photosensor with PEDOT:PSS polymeric counter electrode. Pin, 10 20 30 40 50 60 70 80 90 100 110 120 130 ( ) mW cm2 Isc , (m A) Voc , (volt) IM , (m A) VM , (volt) PM , (m W) FF 2.4 4.02 8.46 14.37 20.71 26.86 34.93 43.74 52.62 64.46 72.71 85.79 90.48 0.135 0.235 0.225 0.28 0.29 0.31 0.315 0.325 0.335 0.34 0.345 0.355 0.36 0.67 3.885 4.60 8.88 14.04 19.77 26.36 31.07 37.37 45.33 51.21 61.28 65.09 0.11 0.19 0.195 0.205 0.205 0.205 0.21 0.23 0.235 0.235 0.235 0.24 0.245 0.08 0.738 0.90 1.82 2.88 4.05 5.54 7.15 8.78 10.65 12.03 14.71 15.95 0.25 0.78 0.47 0.45 0.48 0.49 0.50 0.50 0.50 0.49 0.48 0.48 0.49 100 kHz, the capacitance starts to be inverted to a negative value which increases gradually with increasing the frequency; then it becomes nearly negative constant value (−1 nF) in the high frequencies (400–1000 kHz). The high capacitance at the low frequency is owing to the interface states following the applied AC signal within the semiconductor. The capacitance decreases at higher frequencies since most of the interface states become unable to follow the applied AC frequency [26]. In fact, the negative capacitance has been detected in a lot of optoelectronic and electronic devices [14]. The devices with such behavior are popular in providing voltage amplification in the devices with low power nanoscale [14, 27]. This phenomenon was illustrated by analyzing the transient current [28] and ascribed to the kinetic reactivity of the delay in the flowing current change with changing the applied voltage [29]. This inductance-like behavior leads to an unprecedented change in the profile of the material intrinsic energy. This leads to completely new applications such as developing coil-free resonators and oscillators, boosting voltages at various part of a circuit, the negative capacitance in an antenna, and so on [30]. For further analysis, the capacitance is related to the biasing voltage in some regions according to the following equation [19, 26]: 1 2 (V - V ) = 2 bi 2 C A es eo eNd (4) where A, Vbi, eo , es and Nd are the effective area, built-in potential, dielectric permittivity of space, the dielectric 1 constant of TiO2 and doping concentration, respectively. According to this equation, C 2 versus V yields to linear fitting in certain range as seen in figure 8. Values of Vbi are calculated from the best fitting and shown in table 2. 5 Mater. Res. Express 6 (2019) 065004 S S Shenouda et al Figure 6. Capacitance-Voltage characterization at different frequencies: (a) 10–90 kHz; (b) 100 kHz–1 MHz. Figure 7. Frequency dependence of the capacitance at different biasing voltages. 6 Mater. Res. Express 6 (2019) 065004 S S Shenouda et al Figure 8. 1/C 2 versus the biasing voltage. Table 2. Frequency dependence of the built-in potential. Frequency, [kHz] 10 20 30 40 50 60 70 80 90 Vbi, [volt] 0.276 0.377 0.533 0.724 0.861 0.929 1.13 1.24 1.42 Figure 9. Series resistance—voltage at different frequencies. 7 Mater. Res. Express 6 (2019) 065004 S S Shenouda et al Clearly, the built-in potential increases with increasing the applied frequency. This built-in potential is produced as a result of charge carriers’ diffusion through the interfaces [26]. 3.4. Series resistance—voltage Series resistance (Rs) profile of the cell plays a great effect on the cell performance. Rs for the cell is calculated using the G–V and C–V measurements according to the equation [31]: Rs = G2 G - C2 (5) Figure 9 presents the series resistance Rs versus the applied voltage at different applied frequencies. As the frequency increases, Rs decreases and then becomes almost constant. At the beginning (lower frequencies), the charge carriers possess the required energy to escape from the trap levels. Then, the charges become unable to follow the AC signal with increasing the frequency [26]. This behavior is similar to other cells and diodes [26, 32]. 4. Conclusion PEDOT:PSS polymer can be used as a counter electrode for the DSSC. This cell behaves as a non-ideal Schottky diode with rectification ratio 40 at 0.5 V. It shows promising photovoltaic and photoresponse characteristics. It has a stable fill factor of about 0.5 under different illumination intensities. This DSSC shows a good response of the photocurrent with the illumination intensity. Thus, this cell could be used as a low-cost photosensor. The charge carriers’ diffusion through the interfaces of this cell produces a built-in potential which increases with increasing the applied frequency. Also, the cell shows capacitance inversion from positive to negative at higher frequencies. Thus, it has inductive behavior. This opens the way for using this PEDOT:PSS-based DSSC for many other low-cost applications. Acknowledgments The work was supported by the Research Center for Advanced Material Science at King Khalid University - with grant number (RCAMS/KKU/008-18). ORCID iDs S S Shenouda https://orcid.org/0000-0002-2907-1502 I S Yahia https://orcid.org/0000-0002-9855-5033 References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] Ghayoor R, Keshavarz A, Rad M N S and Mashreghi A 2019 Mater. Res. Express 6 025505 Noor S, Sajjad S, Leghari S A K, Shaheen S and Iqbal A 2018 Mater. Res. Express 5 095905 Yue G, Li F, Yang G and Zhang W 2016 Nanoscale Res. Lett. 11 239 Gemeiner P, Peřinka N, Švorc L, Hatala M, Gál L, Belovičová M, Syrový T and Mikula M 2017 Mat. Sci. Semicon. Proc. 66 162–9 Lee C-P, Lai K-Y, Lin C-A, Li C-T, Ho K-C, Wu C-I, Lau S-P and He J-H 2017 Nano Energy 36 260–7 Sun Y, Yang Z, Gao P, He J, Yang X, Sheng J, Wu S, Xiang Y and Ye J 2016 Nanoscale Res. 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