Accepted Manuscript Investigation of optical and electrical properties of p-nitro-benzylidenemalononitrile thin films for optoelectronic applications Hamza Saidi, Nejeh Dhahri, Walid Aloui, Abdelaziz Bouazizi, Taoufik Boubaker, Regis Goumont PII:
S0749-6036(18)31018-8
DOI:
10.1016/j.spmi.2018.05.038
Reference:
YSPMI 5705
To appear in:
Superlattices and Microstructures
Received Date: 14 May 2018 Revised Date:
20 May 2018
Accepted Date: 21 May 2018
Please cite this article as: H. Saidi, N. Dhahri, W. Aloui, A. Bouazizi, T. Boubaker, R. Goumont, Investigation of optical and electrical properties of p-nitro-benzylidenemalononitrile thin films for optoelectronic applications, Superlattices and Microstructures (2018), doi: 10.1016/j.spmi.2018.05.038. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Investigation of optical and electrical properties of p-nitro-benzylidenemalononitrile thin films for optoelectronic applications Hamza Saidia*, Nejeh Dhahrib, Walid Alouia, Abdelaziz Bouazizia, Taoufik Boubakerb, Regis Goumontc Laboratoire de la Matière Condensée et des Nanosciences, Université de Monastir, Avenue
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a
de l’Environnement, 5019, Monastir, Tunisie. b
Laboratoire C.H.P.N.R, Université de Monastir, Avenue de l’Environnement, 5019 Monastir,
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Tunisie.
Institut Lavoisier, UMR 8180, Université de Versailles, 45, Avenue des Etats-Unis, 78035
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Versailles Cedex, France. *
Corresponding authors:
[email protected]
Abstract
We have successfully synthesized and characterized a donor-bridge-acceptor type conjugated
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molecule. The absorption and photoluminescence spectra of NO2-benzylidenemalononitrile (NO2-BMN) thin films have been investigated. The NO2-BMN film shows an emission in the visible range, which makes it possible for optoelectronic application. The optical band gap
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was obtained by the Tauc method from the absorption spectra to be equal to 2.2eV. However
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the electrochemical gap calculated from cyclic voltammetry is 2.42 eV. Electrical properties of the ITO/NO2-BMN/Al structure was investigated by I-V characteristics. Diode parameters, such as the barrier height ϕb and the ideality factor n were calculated. The conduction is governed by space-charge-limited current (SCLC) mechanism. The effective hole mobility calculated is comparable to that of C60 and 2,7-distyrylcarbazole new molecule (≈10−5 cm2/Vs). The obtained results of the materials have promising to be applicable for various optoelectronic applications. Keywords: Photoluminescence; Absorption; Electrical properties.
ACCEPTED MANUSCRIPT Introduction Small organic π-conjugated molecules have attracted a great attention to the development of high-performance organic optoelectronics due to their optical emission properties and the ability to control energy level. In fact, several research groups have carried out many
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theoretical and experimental studies that highlighted the importance of small organic molecules such as carbon nanotubes, fullerenes and graphene on the optoelectronic and photonic device development [1–3]. The common feature for this kind of materials is the
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conjugation system, thus they had good ability to accept an electron. Moreover, these semiconductors emerged as reinforcement in polymeric matrix that made them a potential
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application in various areas such as Photonics, Optoelectronics and Nanomedicine [4–6]. Particularly, many works focus on donor-bridge-acceptor small organic molecules due to their charge transfer mechanism and their simple charge trapping/detrapping mechanism [7–9]. In addition, the most characteristic justifying the interest in those semiconductor types is the
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richness of the organic synthesis and therefore from an adjustable molecular structure [10, 11]. The superelectrophile molecules are among the most extensively investigated donorbridge-acceptor skeletons; they constitute a useful class of materials due to their importance in
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biological processes, as well as synthetic organic chemical applications [12, 13]. Recently, we have studied the optical and electrical proprieties of p-cyano-benzylidenemalononitrile thin
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films [14]. The obtained results of the materials have promised to be applicable for various optoelectronic applications such as memory devices. Our current interest is to study the optical and electrical proprieties of p-nitrobenzylidenemalononitrile thin films (NO2-BMN). Indeed, some preliminary results of these materials will be given and possible optoelectronic applications will be described.
ACCEPTED MANUSCRIPT 2. Experimental details We prepare 50mL of a mixture of solution containing one equivalent of malononitrile and one equivalent of 4-nitrobenzaldehyde in toluene. Then, we add 0.1mL of glacial acetic acid and 0.05mL of a catalytic amount of piperidine. The prepared mixture was refluxed during one
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night with a Dean-Stark water trap, until no further water separated. After that, the solvent was removed under reduced pressure and the residue was taken up in Et2O which was washed with 50mL of HCl. Then, washed several times with water and dried (MgSO4). In this step,
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the solvent was evaporated and the residue was further purified by recrystallization from a
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pentane/ethyl acetate mixture [15]. In recent work, the kinetics of these materials was studied and we have confirmed that they present a superelectrophile character that is to say that they are deficient in electron [16]. In order to prepare the thin film, ITO substrates were cleaned in an ultrasonic bath of acetone and in isopropyl alcohol bath. Then, clean substrates were dried with an Azote gas flow. We dissolved 20mg of NO2-BMN in 1ml of chloroform solvent.
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Then, the solution was deposited using a spin coating at 1500rpm during 20s onto Indium Tin Oxide (ITO) glass substrates. Finally, aluminum (Al) top electrodes were deposited by
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thermal evaporation through a shadow mask. 3. Results and Discussion
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3.1 Optical proprieties
In order to clarify the origin of the absorption band of the nanocomposite and to extract the optical band gap, the UV-visible absorption spectra of the NO2-BMN thin film was displayed in Figure 1.a from the experimental measurement. The absorption spectra exhibits two strong absorption bands located at 292 nm and 450 nm. The first band is linked to the π-π* transition of the benzene ring, however, the second one is related to n-π* transition of the nitro group (NO2) conjugated with aromatic nuclei.
ACCEPTED MANUSCRIPT In the aim to extract the gap energy value, tauc equation was used : (αhν)2 = c(hν - Eg). It allows to describe the band gap energy as a function of the incident energy αhν. The optical gap is defined as occurring at the intercept of the linear extrapolation with the energy axis (Figure.1 b). Where c is a constant, Eg is the band gap and hν is the photonic energy. The
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energy band gap was obtained by plotting (αhν)2 as a function of photon energy (hν). The results show that the optical band gap of the NO2-BMN is equal to 2.2eV.
Figure 2 shows the photoluminescence spectra of NO2-BMN thin film excited using 488nm
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at room temperature and a fixed power of 40 W/cm2 . It shows a strong PL emission and
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exhibits two characteristic peaks at 611nm and 664nm. It has been reported that p-substitutedbenzylidenemalononitrile (BMN) exhibits only one pic at 644mn [14]. This molecule differs from the our investigated molecule by the substitution of a CN group by a nitro group. (Figure3.b). In this case, The pic located at 644nm is attributed to the interchain transition [17] and the pic located at 611nm is linked to the nitro group. Noting that NO2-BMN exhibits
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a higher PL intensity as compared to that of BMN molecule[14]. This mainly due to the photoinduced electron-transfer (PET) process. In fact, The nitrogen atom and the electrondonating groups usually contain a relatively high-energy non-bonding electron pair. After
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laser excitation, an electron of the highest occupied molecular orbital (HOMO) is excited to
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the lowest unoccupied molecular orbital (LUMO). Then, it leaves a hole in the former orbital. It can be filled via a fast intramolecular electron transfer from the non-bonding electron pair originated from the nitrogen atom to the HOMO of the excited fluorophore. This phenomenon provides a mechanism for non-radiative deactivation of the excited state. Priori, NO2-BMN can be used in MEH-PPV-based OLED or PCDTBT-based OPV structures because it increases the absorption of the PCDTBT and the emission of the MEH-PPV. Thus, it replaces the carbon derivative materials since it is an acceptor molecule.
ACCEPTED MANUSCRIPT 3.2 Electrical properties Based on cyclic-Voltammetry results (Fig.3), NO2 BMN EOx= 1.46V and ERed = -0.96V. The estimations were done using the empirical relation in order to calculate the corresponding HOMO and LUMO levels [18]: EHOMO = [(Eox- Efero) +4.8] eV and ELUMO = [(Ered- Efero) +
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4.8] eV .Taking into account the external standard Ferrocene which used to calculate the EHOMO and ELUMO (Efero = 0.98V). EHOMO = 5.28eV, Egap = 2.42eV and ELUMO =2.86eV. This result is normal if we compare this value the optical band gap (2.2eV). In fact, the
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electrochemical reduction requires higher energy than LUMO and for oxidation; the energy
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should be lower than HOMO. These extra energy requirements for redox makes the value of the electrochemical band gap higher than the optical band gap one [19]. The I-V characteristic of the ITO/ NO2-BMN /Al structure (Figure. 4a) shows an organic diode behaviour with an approximately low threshold voltage VS of 1.5V. It exhibits a clearly moderate rectifying behaviour due to the injection of the charge carriers from the Al
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electrodes to the NO2-BMN layer . Indeed, the I-V curve indicates a symmetric behaviour for low-voltages. It was explained by the localized state theory with defects which provides the localized gap states [20]. At higher voltages, we observe an asymmetric I-V curve linked to
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the difference between the injection barriers to electrons and holes from the anode ITO (4.7
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eV) and the Al cathode (4.3 eV). In recent work The I-V behaviour of BMN shows a current hysteresis behavior useful for memory devices [14]. By replacing the cyano group by a nitro group, we have eliminated the hysteresis behavior and we have changed completely the application of our molecule.
In order to well analyse the I-V curve, we present the
characteristic in a semi-logarithmic plot (Figure.4b). The curve shows two regions: at low voltages, a linear region in which the current is limited by the shunt resistance Rsh and at higher voltages a second region where the current begins to be saturated due to the series resistance Rs and interface states [21, 22]. Both the saturation density current value Is and the
ACCEPTED MANUSCRIPT values of the barrier height ϕB were extracted from experimental I-V data using the following equation:
−φ Is = A* T2 exp B (Eq.1) kT
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The values found are Is= 2.78.10-5A, ϕB=0.61V.
The ideality factor n presents the difference between our diode structure and the ideal one. It
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represents the slope of the exponential range of the I-V curve on a semi logarithmic representation. The ideality factor can be calculated through the following relation:
(
)
(Eq.2)
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=
(
)
The calculated value of n is in the order of 2 and it is higher than the unity which indicates a deviation from the ideal I–V behaviour. This result is due to the high interface density
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localized at the electrodes and the inhomogeneity and the tunnelling effect [23, 24]. In order to understand the conduction mechanisms through the junction, we present the I-V characteristics in double logarithmic scale. As shown in figure 5, three different linear regions
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identifying three distinct conduction mechanisms. In each region, the current depends on the
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voltage (I α Vm) where different m (m presents the slope for each region) values provide information about the different conduction mechanisms. [25, 26]. The slope of the first region is close to unity defining the ohmic region. This Indicates the existence of a small amount of interface barrier which hinders the charge injection. Otherwise, the number of thermally excited load carriers is insufficient and the trap levels are almost empty [27]. The current density varies followings this equation:
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= .
. .
(Eq.3)
ACCEPTED MANUSCRIPT where q is the electronic charge, µ is the charge mobility, p0 is the free carrier density and d is the film thickness. The second region has a slope of 1.89, which shows space charge limited current (SCLC) mechanism. Moreover, we also recorded a fast increase on the density of carriers injected from electrodes. Since the applied voltage passes through the transition
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voltage V= 1.7V, the density of the injected carriers dominate the transport capacity of the NO2-BMN layer [28]. The SCLC model has been widely used to describe the behavior of
=
.
.
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organic diode [29]. Here, the current density varies following this equation: (Eq.4)
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Where, Ɛ is the permittivity of the material (assumed to 4Ɛ0 (Ɛ0 is the vacuum permittivity)). The µeff is the effective carrier mobility and is equal to θµ, where θ is the fraction of free charge (θ= p/(p + pt )) and p and pt are free and trapped charge-carriers densities, respectively). d is the film thickness and V is the voltage.
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The hole effective mobility (µeff) in the NO2-BMN film was calculated and we have found 1,10 (10-5cm2/Vs) by fitting the J-V characteristics to the SCLC model (Eq.4). The estimated charge-carrier mobility is comparable to the achieved hole mobility of 2,7-distyrylcarbazole
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new molecule[30] and fullerene C60 (≈10−5 cm2/Vs) [31]. With increasing voltage (V= 2.2V),
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we observe a third linear region with a slope m=4,2 which represents trapped charge limiting current (TCLC) region. It is the case of space charge limited conduction with an exponential trap distribution. Otherwise, the distribution of trapping levels affects the transition from a SCLC region to a TCLC region, which occurs when the density of the injected carrier greatly exceeds the free carrier density [32].
ACCEPTED MANUSCRIPT Conclusion To sum up, we have succeeded in synthesizing a donor-bridge-acceptor molecule and studying the optical and the electrical properties. The NO2-BMN film shows two absorption bands in the range of 292-500nm and a strong PL emission. Based on the absorbance, the
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optical band gap energy was estimated to be 2.2eV. We conclude that NO2-BMN can be used as an acceptor in OPV and OLED structures. The electrical investigation of ITO/ NO2-BMN /Al structure was performed for the first time. The I-V curves shows a diode behavior and the
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effective hole mobility found is comparable to that of fullerene molecule and 2,7-
benzylidenemalononitrile
constitutes
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distyrylcarbazole new molecule. This result provides convincing evidence that our p-NO2an
interesting
materials
for
high-performance
optoelectronic devices. We aim to investigate the optical and the electrical properties of NO2-
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BMN integrated in different polymer matrix.
ACCEPTED MANUSCRIPT References [1] W. Aloui, H. Saidi, A. Bouazizi, Journal of Nanomaterials. (2016) 8795374. [2] A.K. Pandey, M. Aljada, A. Pivrikas, M. Velusamy, P.L. Burn, P. Meredith, E. Namdas,
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ACS Photon. 1 (2014) 114-120. [3] W. Aloui, T. Adhikari, J.-M. Nunzi, A. Bouazizi, K. Khirouni, Mater. Sci. Semicond.
[4] D. Tuncel, H.V. Demir, Nanoscale 2 (2010) 484–494.
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Process. 39 (2015) 575-581.
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[5] J. Pecher, S. Mecking, Chem. Rev. 110 (2010) 6260–6279.
[6] J. Yoon, J. Kwag, T.J. Shin, J. Park, Y.M. Lee, Y. Lee, J. Park, J. Heo, C. Joo, T.J. Park, P.J. Yoo, S. Kim, J. Park, Adv. Mater. 26 (2014) 4559–4564.
[7] S.G. Hahm, S. Choi, S.H. Hong, T.J. Lee, S. Park, D.M. Kim, W.S. Kwon, K. Kim, O.
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Kim, M. Ree, Adv. Funct. Mater. 18 (2008) 3276-3282.
[8] K.L. Wang, Y.L. Liu, J.W. Lee, K.G. Neoh, E.T. Kang, Macromolecules. 43 (2010) 7159-
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7164.
[9] Y.K. Fang, C.L. Liu, W.C. Chen, J. Mater. Chem. 21 (2011) 4778-4786.
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[10] Y.K. Fang, C.L. Liu, G.Y. Yang, P.C. Chen, W.C. Chen, Macromolecules. 44 (2011) 2604-2612.
[11] N.G. Kang, B. Cho, B.G. Kang, S. Song, T. Lee, J.S. Lee, Adv. Mater. 24 (2012) 385390. [12] N. Streidl, B. Denegri, O. Kronja, H. Mayr, Acc. Chem. Res. 43 (2010) 1537-1549. [13] H. K. Oh, J. H. Yang, H.W. Lee, I. Lee, J. Org. Chem. 65 (2000) 2188-2191.
ACCEPTED MANUSCRIPT [14] W. Aloui, N. Dhahri, A. Bouazizi, T. Boubaker, R. Goumont, Superlattices Microstruct. 91 (2016) 302-305. [15] B. B. Corson, R. W. Stoughton, J. Am. Chem. Soc. 50 (1928) 2825-2837.
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[16] N. Dhahri, B. Taoufik, R. Goumont, J. Phys. Org. Chem. 27 (2014) 484-489. [17] G. He, Y. Li, J. Liu, and Y. Yang, Appl. Phys. Lett. 80, 4247 (2002).
[18] Ashkan Shafiee, Muhamad Mat Salleh & Muhammad Yahaya, Sains Malaysiana, 40
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(2011) 173–176.
[19] S. N. Inamdar, P. P. Ingole, S. K. Haram,ChemPhysChem, 9 (2008) 2574 – 2579.
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[20] S.A. Jeglinski, M.E. Hollier, J. Gold, Z.V. Vardeny, Y. Ding, T. Barton, Mol. Cryst. Liq. Cryst. 256 (1994) 555.
[21] S. Sanyal, P. Chattopadhyay, Solid State Electron. 45 (2001) 315–324.
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[22] Hamza Saidi, Aloui Walid, Abdelaziz Bouazizi, Beatriz Romero Herrero and Faouzi Saidi, Mater. Res. Express, 4 (2017) 035007.
Oxford, 1988.
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[23] E.H. Rhoderick, R.H. Williams, Metal-Semiconductor Contacts, Clarendon Press,
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[24] S.M. Sze, Physics of Semiconductor Devices, second ed. Wiley, New York, 1981. [25] S. Demirezen, Z. Sonmez, U. Aydemir̈ , Ş. Altındal, Curr. Appl. Phys. 12 (2012) 266– 272.
[26] F. Yakuphanoglu, M. Shah, W. Aslan Farooq, Acta Phys. Pol. 120 (2011) 558–562. [27] Z. Çaldıran, A.R. Deniz, Ş. Aydoğan, A. Yesildag, D. Ekinci, Superlattices Microstruct. 56 (2013) 45–54. [28] S.M. Khan, M.H. Sayyad, World Acad. Sci. Eng. Tech. 60 (2011) 1582–1586.
ACCEPTED MANUSCRIPT [29] Z.B. Wang, M.G. Helander, M.T. Greiner, J. Qiu, Z.H. Lu, J. Appl. Phys. 107 (2010) 034506–034510. [30] A. Jebnouni, M. Chemli, P. Lévêque, S. Fall, M. Majdoub, N. Leclerc, Organic Electronics 56 (2018) 96–110.
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[31] C.A. Amorim, M.R. Cavallari, G. Santos, F.J. Fonseca, A.M. Andrade, S. Mergulhão, J. Non-Cryst. Solids 358 (2012) 484–491.
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[32] P.K. Kalita, B.K. Sarma, H.L. Das, Bull. Mater. Sci. 26 (2003) 613–617.
ACCEPTED MANUSCRIPT Figure caption Figure 1: (a) UV-visible absorption spectra of NO2-BMN thin film versus wavelength, (b) UV-visible absorption spectra of NO2-BMN thin film versus energy.
Figure 3: Cyclic voltammetry of NO2-BMN.
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Figure 2: Photoluminescence spectra of NO2-BMN thin film.
Figure. 4: (a) I-V characteristic of the ITO/ NO2-BMN /Al structure, (b) I-V characteristic of
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the ITO/ NO2-BMN /Al structure in semi-logarithmic scale.
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Figure. 5: LnI-LnV characteristic of the ITO/ NO2-BMN /Al structure.
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ACCEPTED MANUSCRIPT Highlights: Optical properties of p-nitro-benzylidenemalononitrile are investigated.
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Electrical properties of p-nitro-benzylidenemalononitrile are studied.
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Electrochemical band gap was calculated.
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