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Electrical properties of ZnO / p-Si heterojunction for solar cells

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https://www.eduzhai.net International Journal of M aterials Engineering 2013, 3(4): 59-65 DOI: 10.5923/j.ijme.20130304.01 Electrical Properties of ZnO/p-Si Heterojunction for Solar Cell Application F. Z. Bedia*, A. Bedia, D kherbouche, B. Benyoucef URM ER, Abou-Bakr Belkaid University, 13000, Tlemcen Algeria Abstract in this study, we report the ZnO thin films prepared by spray pyrolysis method at 550C° with good transparency in the v isible region. The Zn O film was deposited on Si substrates to fro mthe n -Zn0/ p -Si heterojuction. The morphology and electrical properties of the films have been carried out by means of scanning electron microscopy (SEM) and I-V measurements. The current-voltage characteristic of the n-Zn 0/ p-Si heterojunction device measured at roo m temperature in the dark and under illu mination (lamp/160 W). Keywords ZnO thin film, ZnO/p-Si heterojunction, Electrical Properties 1. Introduction ZnO is a pro mising semiconductor material for various optoelectronic applications such as thin film solar cell[1], transparent conducting electrodes[2], light-emitting diodes (LEDs)[3], due to its large direct bandgap (3.37 eV) and exciton binding energy (60 meV) at roo m temperature. Many techniques, such as reactive evaporation[4], chemical vapor deposition (HVP-CVD)[5], sol-gel spin coating method[6] spray pyrolysis[7] magnetron sputtering[8], have been developed and used to grow ZnO on a variety of substrates for fabrication heterojunction structure. Heterojunction solar cells consisting of a wide band gap transparent conductive oxide (TCO) on a single crystal silicon wafer have a number of potential advantages such as an excellent blue light response, simple processing steps, and low processing temperatures[9]. One pro mising type of TCO/Si solar cells uses undoped ZnO on Si wafer. ZnO/Si heterojunctions are of particular interest in the integration of optoelectronic devices utilizing the hybrid advantages of the large exaction binding energy of the ZnO thin film and the cheapness of Si substrates. However, there are a few reports on the n-ZnO/p-Si heterojunction where the ZnO film is grown by different techniques. For instance, Ajimsha and al[10] reported the electrical characteristics of n-ZnO/p-Si hetero junction diodes grown by pulsed laser deposition at different o xygen pressures. Sun[11] achieved UV electrolu minescence (EL) emission fro m Zn O n ano rods with n -Zn O/p -Si h etero jun ct ion structure fabricated by the hydrothermal method. Baik et al * Corresponding author: f_bedia@yahoo.fr (F. Z. Bedia) Published online at https://www.eduzhai.net Copyright © 2013 Scientific & Academic Publishing. All Rights Reserved [12] have prepared ZnO/n-Si junction solar cells with conversion efficiency of 5.3% by sol-gel method, and studied the effect of surface-doping concentration on the ZnO/n-Si solar cells, by using the phosphor silicate glass films. Moreover, zinc o xide/n-Si junction solar cells produced by spray pyrolysis method with relat ively high conversion efficiency 6.9% to 8.5% have been achieved by Kobayashi et al.[13]. The conversion efficiency of ZnO/Si solar cell depends greatly on the properties of ZnO films, depending on the growth conditions including deposition temperature, growth pressure and deposition time. Many researchers have found that the electrica l properties strongly depend on the thickness of ZnO films.[14],[15] for application in optoelectronic devices, the optimu m film thickness should be chosen for the best device performance. In this study, we report n-type behavior of ZnO thin film based on ZnO/p-Si heterojunction solar cells, where the type n ZnO film is prepared by spray pyrolysis. We describe the electrical properties of ZnO/Si heterojunction solar cells. 2. Experimental ZnO thin films were grown on p-type silicon (111) and glass substrates by spray pyrolysis method. The spray solution is prepared by dissolving 0.085 mo l zinc acetate dehydrated (Zn (CH3COO)2· 2H2O) in methanol (CH3OH). Co mpressed ambient air is used to atomize the solution. The flow rate is kept about 5ml/1min during preparation of samples. The nozzle-substrate distance is maintained at 27 cm and the substrate temperature is fixed at 550°C and controlled with in ±5°C by using an electronic temperature controller 38XR-A Dig ital Multimeter kept on the metallic hot plate surface. Then the substrates are regularly heated up to the required temperature, before being sprayed on. 60 F. Z. Bedia et al.: Electrical Properties of ZnO/p-Si Heterojunction for Solar Cell Application Then the films are annealing at 550°C fo r 1 h in order to eliminate organic products. The film was found to be n-type by using a hot-probe method. The Cu metal contact was deposited on the ZnO layer and the backside of the Si substrate to form electrodes of the p-n diode. The distance of the Cu contacts on ZnO film is 1 mm. The schematic structure of the ZnO/p-Si heterojunction is shown in figure 1. A pair of contacts was made on the backside of a separate piece of the sample to check for Oh mic contact formation. The morphology of the fabricated ZnO thin film was observed in a scanning electron microscope (SEM). The surface morphologies of ZnO thin film grow on p-Si wafer by spray pyrolysis are shown in Figure 2. was constructed based on Anderson’s model[16]. The band gap and electron affinity values for ZnO and Si are assumed to be EgZnO = 3.37 eV, χ ZnO = 4.35 eV[17] and EgSi = 1.12 eV, χSi =4.05 eV[18], respectively. The left side region represents n-type ZnO thin film and the right side is p-type Si substrate. 3. Results and Discussion The transmittance spectra of the ZnO film was measured in the wavelength range 300–800 n m at roo m temperature, as shown in Fig. 3. At short wavelengths, it observed low transmission values because of its high absorbing properties. At long wavelengths the transmission values are high due to non-existence of absorption. The transmittance in this region varies between 70 and 90%. In addition, the ZnO film is opaque material in Ult ra-vio let region and transparent in visible region. Similar behaviour has also reported in literature[19]. The refract ive index n is an impo rtant parameter for optical materials and applications. The expression for refract ive index is given by Figure 1. Band gap structure of the n-ZnO/p-Si heterojunction p-Si ZnO Fi gure 2. SEM micrographs of ZnO film grown on p-Si wafer The electrical p roperties of ZnO thin film was characterized by two point probe at roo m temperature. The current–voltage characteristic of the device was measured by 38XR-A Dig ital Mult imeter semiconductor parameter analyzer. The figure 1 shows the energy band diagram of the heterojunction at equilibriu m. The energy band diagram n1,2 = N+ N 2 − n 2 s (1) where; N = 2ns Tmax − Tmin TmaxTmin + ns2 + 1 2 (2) and ns is the refractive index of the glass substrate. The refractive inde x n ca lculated using equation 1 equal to 2.03. This value is in a good agreement with to theoretical refractive index of ZnO film in the visible region (n = 2). The thickness of the films was calculated using the equation by d = λ1λ2 2(λ1n2 − λ2n1 ) (3) where n1 and n2 are the refractive indices corresponding to wavelengths λ1 and λ2, respectively. The thickness of ZnO film is calcu lated about 1.3µm. The optical energy gap of the ZnO film grow on glass substrate was determined by Tauc law in a direct transition between valence and conduction bands from the e xpres s ion[20] : (αhν )2 = A(hν − Eg )m (4) Where A is a constant, hν is the photon energy , Eg is the optical energy band and m is an inde x that characterizes the optical absorption process. The exponent m depends on the nature of the transition, m = 1/2, 2, 3/2, or 3 for allowed direct, allo wed nondirect, forb idden direct, or forb idden nondirect transitions, respectively[21]. It is well known that ZnO has an allowed direct band gap and m = 1/2 was used for the band gap calculation. International Journal of M aterials Engineering 2013, 3(4): 59-65 61 Transmittance(%) 100 80 60 40 20 0 300 400 500 600 700 800 900 1000 1100 wavelength(nm) Figure 3. T ransmission spectra of ZnOthin film grow on glass 6x109 5x109 (αhν)2 (eV cm-1)2 4x109 3x109 2x109 1x109 Eg = 3,29 eV 0 2,50 2,75 3,00 3,25 3,50 hν (eV) Figure 4. The plots of (αhν)2 as function of photon energy of the ZnO Fig.4 shows plot of (αhv)2 versus (hv) where the optical band gap of the film was determined by extrapolat ing the linear region to (αhν)2 = 0. Optical energy gap of ZnO film was calculated as 3.29 eV. This value was smaller to the band gap of intrinsic ZnO which is 3.37 eV[19]. The optical band gap is in good agreement with that of ZnO thin film prepared by spray pyrolysis[19]. Figure 5 shows the current–voltage characteristic of the ZnO/ p-Si heterojunction measured in the dark and under illu mination at room temperature. The current values 62 F. Z. Bedia et al.: Electrical Properties of ZnO/p-Si Heterojunction for Solar Cell Application increase exponentially with increasing in the forward bias voltage. The turn-on voltage is around 11.5 V for the forward bias and a reverse bias breakdown voltage of 12.5V as seen from the I-V curve in the dark. Moreover, it is seen fro m the figure the device has high forward current that reverse current. The rectification ratio IF/IR (IF and IR stand for forward and reverse current, respectively) of the structure at 20 V is found to 4. According to the p–n junction theory, the standard diodes I– V relation[22] I = IS exp qV ηkBT − 1 (1) Where V is voltage bias and Is is saturation current wich derived fro m the straight line intercept of ln I at V = 0 and is given by: I S= A*ST2  exp   qϕb kBT    (2) With q electronic charge, kB Bolt zman constant, A* the effective Richardson constant taken as 32 A cm−2 K−2 for ZnO[23], S the area of the diode and qΦb the barrier height (eV). Here, n is the quality factor that measures the conformity of the diode to pure thermionic emission, which is given by: η = q kBT dV d (ln I ) (3) The value of the ideality factor of the heterojunction is determined fro m the slop of the straight line region of the forward bias ln (I) -V characteristics. The typical values of the ideality factors and the reverse saturation current are 5.12 and 8.01×10-8 A, respectively. The saturation current Is is co mparable to P.Klason et al.[24] value 6.53×10-8A and to the value 1.78×10-7 A reported by N. Zebbar et al.[25] for an-ZnO/p-Si heterojunction. The ideality factor is larger than the latter value (2). This indicates that the diode exhibits a non-ideal behavior due to the oxide layer and the presence of surface states[26]. Th is is rather close to the previous reported values of 5.47[26] and 5.1[27]. Values of the saturation current I0 were obtained by extrapolation of the linear reg ion of the semi-logarith mic forward I– V curves to zero applied voltage and were used to calculate the apparent barrier height by the follo wing function: qϕb =  kT ln   AA*T2 IS    (4) In the study, the calculated potential barrier height value at room temperature is 0.57 eV. This value is according with 0.6 eV[28] reported by other authors. In reality, the I-V behavior of the diode is affected by parasitic resistances such as series resistance (Rs) related to the interfaces between two semiconductors and shunt resistance (Rsh) due to semiconductor electrode interface properties[29]. Thus, it is important to determine these parameters. The junction resistance Rj for the diode is e xpressed as Rj = ∂V ∂I (5) Fi gure 5. I–V characterist ics curves ZnO/p-Si het erojunct ion in dark and in light (light 160W white lamp) International Journal of M aterials Engineering 2013, 3(4): 59-65 63 Fi gure 6. (a) junct ion resist ance RJ as a funct ion of forward bias. (b) junction resist ance RJ as a funct ion of reverse bias Therefore, Rs and Rsh values can be easily determined fro m the p lot of Rj vs. V At h igher forward voltages, Rj value approaches to a constant value corresponding to Rs. Whereas, at higher reverse voltages, the Rj value becomes nearly Rsh[30]. It is worth noting that series and shunt resistances are important parameters for solar cells characterizat ion. In Figure 6 we have drawn the calculated Rs and Rsh of diode junction, fo r fo rward bias and reverse bias, respectively. Figure 6-(a) shows roughly three regions correspondents to the previous (I), (II) and (III) regions. In the region (I) (V < 1V) the RJ value decreases abruptly with the applied bias, whereas, in the region (II) (1V < V< 3V), RJ decreases more g radually, and in reg ion (III) (V > 3 V) it reaches a saturation value Rs equal to 6.63 k.Ω. This value corresponds to the series resistance of the heterojunction, which is mainly due to the ZnO thin film resistance at room temperature. However, in the case of reverse voltage region, the RJ represents the shunt resistance Rsh of the 64 F. Z. Bedia et al.: Electrical Properties of ZnO/p-Si Heterojunction for Solar Cell Application heterojunction. This value decreases gradually with the applied bias and it saturates to 2.27x10-5 Ω. The shunt resistance originates from the surface, bulk and grain boundaries carriers reco mbination[25]. The I– V characteristics were measured under illu mination by power white light (160 W ) lamp, as shown in figure 5. Typical good rectifying and photoelectric behavior were observed for the device. The dark leakage current is small, whereas its photocurrent generated under illu mination is higher. It is observed that the heterojunction exhibits a rectifying behavior in the presence of light too. Under reverse b ias conditions photocurrent caused by the n-ZnO /p-Si hetero junction irradiated under illu mination by white light lamp was evidently much larger than the dark current. For examp le, when the reverse bias is -18 V, the dark current is only 2.17×10-3 A. While the reverse-bias photocurrent reach to 3.08×10-3 A under lamp illu mination. Figure 5 shows the n-ZnO /p-Si heterojunction under white lamp illu mination has great photovoltaic effect. The photovoltage of the heterojunction is 6 mV and the short circuit current is 4 µA. Majority carriers are blocked fro m tunneling by the band gap of n-ZnO. Tunneling can also occur via defects states at the interface, we regard JST to be the dominant tunnel transition via defects in ZnO -Si, where JST is the tunnel transition current. The photoelectric effect in the structure is because of the light-induced electron generation at the depletion region of the structure [2]. 4. Conclusions The current–voltage (I– V) characteristics of the p–n heterostructure show nonlinear d iode like behavior. We calculated the ideality factor and the saturation current are 5.12 and 8.01×10-8A, respectively. The ideality factor is higher than 2, indicat ing that the diode exh ibits a non-ideal behavior due to the oxide layer and the presence of surface states. The heterojunction shows great photoelectric effect under power (160W) lamp illu minate. The photocurrent responses were detected for the solar cell. The solar cell exhibited a short-circuit current density of 4×10-3 mA, an open-circuit voltage of 6mV. Doping studies and fine tuning of the junction morphology will be necessary to further imp rove the performance of Zn O/Si heterojunction solar cells. Aluminum-doped ZnO films for transparent electrode and antireflection coating of β-FeSi2 optoelectronic devices, Thin Solid Films, 476, 30-34. 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