n-Si heterojunctions for solar cells application

n-Si heterojunctions for solar cells application

Current Applied Physics 11 (2011) 1265e1268 Contents lists available at ScienceDirect Current Applied Physics journal homepage: www.elsevier.com/loc...

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Current Applied Physics 11 (2011) 1265e1268

Contents lists available at ScienceDirect

Current Applied Physics journal homepage: www.elsevier.com/locate/cap

Fabrication and electrical characterization of p-Sb2S3/n-Si heterojunctions for solar cells application K.F. Abd-El-Rahman a, b, A.A.A. Darwish c, * a

Physics Department, Faculty of Education, Ain Shams University, Rorxy 11757, Cairo, Egypt Basic Science Department, Faculty of Engineering, The British University in Egypt, El-Sherouk City, Egypt c Physics Department, Faculty of Education at Al-Mahweet, Sana’a University, Al-Mahweet, Yemen b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 3 September 2010 Received in revised form 15 November 2010 Accepted 8 December 2010 Available online 15 December 2010

Antimony trisulphide (Sb2S3) films were prepared by thermal evaporation technique on n-type single crystal Si substrates to fabricate p-Sb2S3/n-Si heterojunctions. The electrical transport properties of the peSb2S3/n-Si heterojunctions were investigated by currentevoltage (IeV) and capacitanceevoltage (CeV) measurements. The temperature-dependent IeV characteristics revealed that the forward conduction was determined by multi-step tunnelling current and the activation energy of saturation current was about 0.54 eV. The 1/C2eV plots indicated the junction was abrupt and the junction built-in potential was 0.6 V at room temperature and decreased with increasing temperature. The solar cell parameters have been calculated for the fabricated cell as Voc ¼ 0.50 V, Jsc ¼ 14.53 mA cm2, FF ¼ 0.32 and h ¼ 4.65% under an illumination of 50 mW cm2. Ó 2011 Elsevier B.V. All rights reserved.

Keywords: Heterojunctions Photovoltaic IeV characteristics

1. Introduction Photovoltaic is, universally, recognized to be one of the alternative renewable energy sources to supplement power generation using conventional fuels. Currently, efforts are being made at various laboratories to develop new solar cell materials to improve the conversion. The factors that should be considered in developing new semiconductor materials include: (i) a suitable energy band gap that matches the solar spectrum to maximize absorption of the incident solar radiation, (ii) the ability to deposit the material with an acceptable efficiency using a low-cost deposition method (iii) abundance of the elements, and (iv) there are low environmental costs. Antimony trisulphide (Sb2S3) is one of the promising materials since it exhibits p-type conductivity [1] and shows strong absorption of light in the wavelength shorter than 900 nm [2e4]. Thin films of Sb2S3 were prepared by several methods, namely spray pyrolysis [5,6], chemical deposition [7,8], electrodeposition [8], dip try method [9] and thermal evaporation [10,11]. The studies of Sb2S3 thin films are attracting wide attention, due to their special applications to the target material of television cameras [12], microwave device [13], switching devices [14] and various optoelectronic devices [11,15]. * Corresponding author. E-mail address: [email protected] (A.A.A. Darwish). 1567-1739/$ e see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2010.12.006

Thin film of Sb2S3 has good photoconducting properties and high thermoelectric power, which allows possible applications for optical devices and thermoelectric cooling devices [16]. These properties make Sb2S3 suitable for photovoltaic application. Messina et al. [17] prepared Sb2S3 thin films for photovoltaic cell. The obtained cells have open circuit voltage (Voc) about 450e670 mV, and short circuit current density (Jsc) about 0.02e1.4 mA/cm2. X-ray diffraction studies on thermally evaporated Sb2Se3 thin films [18] showed that the annealing at 423 K convert films from an amorphous to crystalline phase. Recently, a photovoltaic structure Ag/Sb2S3:C/CdS/ITO were obtained by Arato et al. [19], which showed Voc ¼ 500 mV and Jsc ¼ 0.5 mA/cm2 under illumination by a tungsten halogen lamp. A cell with CdS/Sb2S3/PbS structures were prepared by Messina et al. [20], which showed Voc ¼ 640 mV, Jsc ¼ 3.73 mA/cm2, fill factor 0.29, and conversion efficiency 0.7% under 1000 W/m2 sunlight. From the survey of literature, it can be seen that no attempt has been made to study the electrical properties of the p-Sb2S3/n-Si heterojunctions. Therefore, it was thought that it would be of important to fabricate p-Sb2S3/n-Si heterojunctions and investigate their optoelectronic properties. The dependence of the currentevoltage (IeV) characteristics on the temperature was studied. In addition, capacitanceevoltage (CeV) measurements were applied for characterization of these heterojunctions. Also, this article reports a study of the resulting optoelectronic parameters of these heterojunctions. This enables us to discuss possible physical

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mechanisms, which might be responsible for the observed temperature dependence.

a

0.20

0.15

2. Experimental details

3. Results and discussion

I (mA)

0.05

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dark illumination

-0.05

-0.10 -2.5

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b 10

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V (volts) Fig. 1. Dark and illumination IeV characteristics for p-Sb2S3/n-Si heterojunction at room temperature.

(1)

where I is the current subject to an applied voltage V, Io is the saturation current, q is the electronic charge, k is Boltzmann’s constant, T is the absolute temperature and n is the diode quality factor. So, graphical representations of ln(I) versus V reveal parallel straight lines in the temperature range studied. This result indicates that the diode quality factor n decreases from about 3.06 to 2.47 as

the temperature increase from 300 to 373 K. This behaviour suggests that the dominant current transport mechanism through the p-Sb2S3/n-Si heterojunction is a tunnelling mechanism [22], which the forward currents can be expressed by [23]:

-8

300 K 328 K 348 K 373 K

-10

ln [ If (A) ]

Fig. 1(a) shows IeV characteristics of p-Sb2S3/n-Si heterojunction in the dark and under white light illumination. It can be seen from this figure that the current value at a given voltage for p-Sb2S3/n-Si heterojunction under illumination is higher than that in the dark. This indicates that the light generates carrier-contributing photocurrent due to the production of electronehole pairs as a result of the light absorption. Also, it is obvious from Fig. 1(a) that the heterojunction is rectifying in nature with a turn-on voltage of w0.6 V. Fig. 1(b) shows the dark forward current in logarithmic scale. The forward currents can be classified into two regions according to the applied voltages. In region above 0.8 V, the forward current deviates from linearity due to the effect of a series resistance on the system. The typical dark forward IeV characteristics of p-Sb2S3/n-Si heterojunction measured over the temperature range 300e373 K is shown in Fig. 2. This figure indicates an exponential dependence of currentevoltage in the voltage range up to about 0.6 V. The forward IeV characteristics in this exponential region is first described by the standard diode relationship [21]:

I ¼ Io exp½qV=nkT

0.10

I (A)

The single crystal silicon (n-Si) wafer was thoroughly degreased and boiled in (H2O þ H2SO4) solution for 30 min, followed by a deionised-water rinse and dilute hydrofluoric acid treatment for only 10e15 s. The wafer was then rinsed for about 5 min in running hot deionised-water and dried. It was then ion-cleaned in the vacuum chamber to remove the natural oxide impurities and then one side of Si was coated with indium layer as a bottom electrode. The Si wafers were coated from the other side by thin films of Sb2S3 using a high vacuum coating unit (Edwards, E306A). The Sb2S3 layer was deposited in a vacuum better than 104 Pa. During deposition, the temperature of n-Si substrate was kept at 310 K. The deposition rate was about 5 nm/min and the thickness of Sb2S3 was 450 nm. The obtained devices were annealed at 373 K for 2 h to enhance the performance of the devices. The other Ohmic contacts were obtained by evaporating indium fingers on Sb2S3 sides. The thickness of the semitransparent indium fingers was approximately 10 nm. All devices obtained were tested on a curve tracer and good ones were selected for detailed IeV measurements in dark and light. The dark CeV characteristics were measured at 1 MHz using Model 410 CeV Meter. Measurements of dark currentevoltage characteristics were made in dark within the temperature range 300e380 K under atmospheric pressure by conventional methods. The current flowing through the cell was determined using a stabilized power supply and a Keithley 617 electrometer. The temperature of samples was measured during electrical measurements by NiCreNiAl thermocouple with accuracy 1 K. The cells were exposed to light coming from a white light source (halogen lamp) to get an intensity of incident power of about 50 mW/cm2.

-12 -14 -16 -18 -20

0.0

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V (volts) Fig. 2. Plots of ln(If) versus V at different temperature.

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K.F. Abd-El-Rahman, A.A.A. Darwish / Current Applied Physics 11 (2011) 1265e1268

0V 0.1 V 0.2 V 0.3 V 0.4 V

-10

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-3

10

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300 K 328 K 348 K 373 K

If (A)

ln [ If (A) ]

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10

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300

320

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360

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1

T (K)

V (volts)

Fig. 3. Plots of ln(If) versus T at different voltages.

Fig. 5. Variation of log J with log V at higher forward voltage bias.

I ¼ Io expðaVÞ

(2)

where a is a coefficient. The temperature dependence of the parameter a depends on the dominant current transport mechanism. If the current is controlled by tunnelling, a is a constant independent of temperature. If the current is controlled by some other mechanisms, a is generally dependent on temperature. It is obvious from Fig. 2 that a is independent of the measurement temperature T, which indicates that the current in this region is dominated by a tunnelling mechanism. The parameter a deduced from the plots of ln(I) versus V is about 12.6 V1. The forward current can be explained by a multi-step tunnelling model, which is attributed to the recombination of holes, tunnelling from Sb2S3 into the gap states in n-Si, and electrons tunnelling across the heterojunction barrier from n-Si to peSb2S3, where they hop between localized states through a multi-step tunnelling process. In the multi-step tunnelling mechanism the saturation current density is given by [24]:

Io ¼ Ioo expðgTÞ

(3)

where Ioo is a constant proportional to the density of traps of appropriate energy in the silicon depletion region and g is a constant independent of applied voltage. Fig. 3 is a semi-logarithmic

graphical representation of forward current versus temperature at constant potential bias, approximately parallel straight lines are obtained in temperature range 300e373 K. Such a behavior indicates that g is a constant and equal to 0.056 K1. The ratio of g/a can be used to determine the change of built-in voltage (Vb) with respect to the absolute temperature (T) that is given by [24]:

g dVb ¼  a dT

(4)

The calculated value of dVb/dT is equal to 4.44  103 V/K. The model for a multi-step tunnelling process across a semiconductor junction shows the dependence of the currents on both the temperatures and the voltages. This model has successfully explained the voltage and temperature dependencies observed for an amorphous n-Si or crystalline p-Si heterojunction [25]. It is believed that it is also applicable to the case of p-Sb2S3/n-Si heterojunctions. The temperature dependence of the saturation current Io can be obtained by extrapolating the forward current curves and the relation can be expressed as:

 Io wexp

DEa

 (5)

kT

where DEa is the activation energy of carrier conduction. Fig. 4 shows the Arrhenius plot of ln(Io) versus 1000/T. From this figure,

-15 0.6

VD (volts)

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C (pF)

ln [ Io (A) ]

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1000 / T (K ) Fig. 4. Plots of ln(Io) versus 1000/T.

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140 290

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370

T (K) Fig. 6. Capacitance and the built-in voltage as functions of temperature.

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16

To estimate the efficiency of the cell, an illuminated cell of area 0.78 cm2 was connected to a load resistance variable from zero to infinity. Fig. 7 shows the JeV graph of p-Sb2S3/n-Si heterojunction under illumination of 50 mW. The device parameters estimated for the best cell are Voc ¼ 500 mV, Jsc ¼ 14.53 mA cm2, FF ¼ 0.32, and efficiency h ¼ 4.65%. The photovoltaic properties may be explained on basis of absorption of light through the cell. The generation of photoelectrons may be followed by electrons transfer from p-Sb2S3 into n-Si through the potential barrier at the interface. The obtained poor fill factor could be attributed to surface effects on the junction.

14 12

J (mAcm-2)

10 8 6 4 2 0 0.0

Voc = 0.50 V Vm = 0.28 V Jsc = 14.53 mAcm Jm = 8.32 mAcm

4. Conclusion

-2

-2

FF = 0.32 η = 4.65%

0.1

0.2

0.3

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0.5

V (volts) Fig. 7. Load JV characteristics for p-Sb2S3/n-Si heterojunction under illumination of 50 mW cm2.

Io varies exponentially with 1/T indicating a multi-step tunnelling model [26]. Ea determined from this plot is given about 0.54 eV. Under relatively high forward voltages (0.8  V  2) the conduction is operated by another mechanism where the current density shows a power dependence on voltage, obeying the relation JaVm, where m was obtained to be w2.08. This is clearly seen in Fig. 5 which represent the relation between log I and log V. This power dependence shows that I is a space-charge-limited current density controlled by a single dominant trap level [27]. Fig. 6 illustrates the capacitance C as a function of the junction temperature. As shown, the capacitance increases linearly with increasing temperature. This behavior can be explained on the basis of variation of the width of the depletion region with temperature. The 1/C2 versus V is a straight line indicating an abrupt junction and the built-in potential VD was obtained by extrapolating the 1/C2 curve at low reverse bias to zero. It was found that the built-in voltage VD decreases linearly with increasing temperature as shown in Fig. 6. This indicates that the interface density of charges increases on increasing the temperature. The interface density of charges has a considerable effect on the apparent built-in voltage. As expected, increasing the interface density of charges decreases the built-in voltage. The high interface density of states also behaves as effective tunnelling centers. So, the predicted conduction mechanism is multi-step tunnelling mechanism. This operating mechanism may depend on the method of the sample preparation, the quality of materials and the type of the two components composing the junction under test. From the data of Fig. 6, we find that the rate of changed dVb/dT is about - 4.4  103 V/K, which is close to the ratio g/a determined from the IeV measurements.

In summary, the p-Sb2S3/n-Si heterostructures have been successfully fabricated on Si(100) substrates using a thermal evaporation technique. The IeV characteristics of p-Sb2S3/n-Si heterostructures showed rectification clearly. Temperature-dependent IeV measurements revealed that at forward bias, the multitunnelling model was applicable to the heterostructures and the activation energy of saturation current was about 0.54 eV. CeV measurements confirmed the existence of an abrupt junction. The junction properties have been studied and the current transport across the junction modelled as a composite of tunnelling and recombination mechanisms. Photovoltaic action was observed for these devices which had solar conversion efficiencies of 4.65%. References [1] M.J. Chockalingam, K.N. Rao, N. Rangarajan, C.V. Supyanarayana, J. Phys. D: Appl. Phys. 3 (1970) 1641. [2] R.N. Bhattacharya, P. Pramanik, J. Electrochem. Soc. 129 (1982) 332. [3] R.S. Mane, B.R. Sankapal, C.D. Lokhande, Mater. Chem. Phys. 60 (1999) 158. [4] R.S. Mane, C.D. Lokhande, Mater. Chem. Phys. 59 (1999) 1. [5] V.V. Killedar, C.D. Lokhande, C.H. Bhosale, Indian J. Pure Appl. Phys. 36 (1998) 33. [6] V.V. Killedar, C.D. Lokhande, C.H. Bhosale, Mater. Chem. Phys. 47 (1997) 104. [7] J.D. Desai, C.D. Lokhande, Thin Solid Films 237 (1994) 29. [8] K.C. Mandal, A. Mondal, J. Phys. Chem. Solids 51 (1990) 1339. [9] B.B. Nayak, H.N. Acharya, Thin Solid Films 122 (1984) 93. [10] K.H.A. Mady, M.M. El-Nahas, A.M. Farid, H.S. Soliman, J. Mater. Sci. 23 (1988) 3636. [11] J. George, M.K. Radhakrishnan, SolidState Commun. 33 (1980) 987. [12] D. Cope, US Patent 2,875,359 (1959). [13] J. Grigas, J. Meshkanskas, A. Orliakas, Phys. Status Solidi A 37 (1976) K39. [14] M.S. Ablowa, A.A. Andreev, T.T. Dedagkaev, B.T. Melekh, A.B. Penstsov, N.S. Sheridel, L.N. Shumilova, Sov. Phys. Semicond 10 (1976) 629. [15] M.J. Chockalingam, K. Nagaraja Rao, N. Rangarajan, C.V. Suryanarayana, J. Phys. D: Appl. Phys. 3 (1970) 1641. [16] J.D. Desai, C.D. Lokhande, J. Non-Cryst. Solids 181 (1995) 70. [17] S. Messina, M.T.S. Nair, P.K. Nair, Thin Solid Films 515 (2007) 5777. [18] E.A. El-Sayad, A.M. Moustafa, S.Y. Marzouk, Physica B 404 (2009) 1119. [19] A. Arato, E. Cárdenas, S. Shaji, J.J. O’Brien, J. Liu, G. Alan Castillo, T.K. Das Roy, B. Krishnan, Thin Solid Films 517 (2009) 2493. [20] S. Messina, M.T.S. Nair, P.K. Nair, Thin Solid Films 517 (2009) 2503. [21] C.A. Dimitriadis, J. Appl. Phys. 70 (1991) 5423. [22] A.R. Riben, D.L. Fecucht, Solid State Electron 9 (1966) 1055. [23] S. Martinuzzi, O. Mallen, Phys. Status Solidi A 16 (1973) 339. [24] D. Song, D. Neuhaus, J. Xia, A.G. Aberle, Thin Solid Films 422 (2002) 180. [25] H. Matsuura, T. Okuno, H. Okushi, K. Tanaka, J. Appl. Phys. 55 (1984) 1012. [26] M. Niraula, T. Aoki, Y. Nakanishi, Y. Hatanaka, J. Appl. Phys. 83 (1998) 2656. [27] M.A. Lampert, Rep. Prog. Phys. 27 (1964) 329.