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Studies on ZnS0.5Se0.5 buffer based thin film solar cells
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Thin Solid Films 516 (2008) 7060 – 7064 www.elsevier.com/locate/tsf
Studies on ZnS0.5Se0.5 buffer based thin film solar cells Y.P.Venkata Subbaiah a , P. Prathap b , K.T.Ramakrishna Reddy b , R.W. Miles c , J. Yi d,⁎ a
Department of Physics, Yogi Vemana University, Kadapa 516 003, India Thin Film Laboratory, Department of Physics, S.V. University, Tirupati 517-502, India c School of Engineering and Technology, Northumbria University, Newcastle, NE1 8ST, UK School of Information and Communication Engineering, Sungkyunkwan University, Suwon 440-746, South Korea b
d
Available online 23 December 2007
Abstract ZnS0.5Se0.5 has been proposed as a novel buffer layer, alternative to CdS for solar cell application. ZnS0.5Se0.5 layers were deposited at 300 °C using close-spaced evaporation. The layers exhibited cubic structure with a sharp (111) reflection. The films showed an optical transmittance N 80% with an energy band gap of 3.01 eV and the electrical resistivity of the films was ~ 105 Ωcm. CuInS2 (CIS) films synthesized by sulphurisation process of rf sputtered Cu/In precursor layers formed on Mo coated glass were used as absorber. Structural, optical and electrical studies were performed to characterize the synthesized CuInS2 films. Thin film solar cells were fabricated by depositing a thin ZnS0.5Se0.5 buffer layer onto the Glass/Mo/CuInS2 with sprayed ZnO:Ga as a window layer. The interfacial properties of the resulting heterojunction were studied using current–voltage (I–V), capacitance–voltage (C–V) and spectral response measurements and the results were discussed. © 2007 Elsevier B.V. All rights reserved.
1. Introduction The thin film photovoltaic devices that have been fabricated at laboratory-scale to date exhibited record efficiencies of ~ 19.5% with chemical bath deposited (CBD) CdS buffer [1,2] and CBD Cd1 − xZnxS (x ~ 0.2) buffer [3] in CuInGaSe2 (CIGS) absorber and around 12.3% with CBD CdS in case of CuInS2 (CIS) [4]. Highly efficient thin film solar cells fabricated to date include generally ‘Cd’ containing compounds like CdS buffer layer and CdZnS (less Zn). Since, Cd is a heavy metal associated with high toxicity, these compounds must be excluded in the ultimate solar cell devices in view of the ecological and economic point of concern. Further more, the less band gap of the CdS (2.4 eV) increases the photo absorption in shorter wavelength region that makes no useful photocurrent contribution to the final device. The attractive alternate layers investigated by to date includes many Zn-based buffer layers of ZnSe [5], Zn1 − xMgxO [6], Zn(O,S) [7], Zn(O,OH) [8],
⁎ Corresponding author. Tel.: +82 31 290 7139; fax: +82 31 290 7159. E-mail addresses: [email protected] (Y.P.V. Subbaiah), [email protected] (J. Yi). 0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2007.12.115
ZnSxOyHz [9] and other non Zn-compounds like InxSy [10] and In2Se3 [11]. Out of these various kinds of buffers, Zn-based compounds are promising and reached the efficiencies close to those achieved with the conventional buffer, CdS. Again, all these alternative Zn-based buffers, involved in high efficiency solar cells were prepared mostly by CBD process. However, from ecological point of view this technique must be replaced by an industry friendly process because of greater public apprehension with the disposal of the solution and waste recycling for large-scale manufacturing. Therefore, keeping in view the importance of alternate buffer layers and the process of their synthesis for heterojunction solar cells, in the present study, we investigated a novel buffer material, ZnSxSe1 − x prepared by close-spaced evaporation for the first time and examined its potentiality and suitability as a buffer layer by fabricating solar cell device with CIS absorber. This material is promising because it can be lattice matched to CuInS2, reducing strain at the interface and minimizing the density of misfit dislocations present at the interface [12]. It also has a wider energy band gap than CdS resulting in an enhanced transmission of the blue part of the solar spectrum. Also, the technique used in the present investigation is simple, inexpensive and amenable for industrial production.
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2. Experimental In the present investigation, CuInS2 absorber films were produced using the sulphurisation of rf sputtered Cu/In multistacks. The Cu/In stacks of 0.6 μm deposited on Mo-coated glass substrates at room temperature were used as starting layers for preparing CuInS2 films. These starting layers were procured from the Northumbria Photovoltaics Applications Center, Northumbria University, UK. The sulphurisation was carried out using the system fabricated in our laboratory by annealing Cu/In multilayers in excess elemental sulphur at 400 °C. Sulphurisation of the Glass/Mo/Cu/In precursors at 400 °C for 30 min. resulted in the complete conversion of Cu/In layers into CuInS2. The final layers formed were ~1.0 μm thick. Later on, a thin buffer layer of ZnS0.5Se0.5 (thickness ~60 nm) was deposited using close-spaced evaporation onto the Glass/Mo/CuInS2 substrates at a temperature of 300 °C. The detailed experimental procedure for preparation of ZnSxSe1 − x films was reported earlier [13]. Gallium (5 at.%) doped ZnO films were then deposited onto ZnS0.5Se0.5 using spray pyrolysis with zinc acetate precursor of 0.1 M concentration at a substrate temperature of 350 °C. The other deposition conditions for spray pyrolysis like solution flow rate, airflow rate and nozzle-substrate distance were fixed as 6 ml/min, 8 l/min and 25 cm respectively. The thickness of these layers was 0.6 μm. Finally a 0.4 μm thick gold film was vacuum evaporated ZnO:Ga as the grid for external connection. The active area of the device is 0.5 cm2 . The individual layers were characterized using appropriate techniques to evaluate the physical and chemical properties. The fabricated solar cell devices were characterized by studying the I–V measurements in dark and under illumination, C–V and spectral response measurements. 3. Results and discussion 3.1. CuInS2 Films The X-ray diffraction data obtained for the as-procured Cu/In precursor layers indicated the presence of Cu11In9 as the predominant phase without any secondary or elemental Cu or In phases present. This is the most commonly observed phase
Fig. 2. EDAX spectrum of Cu/In layers sulphurised at 400 °C for 30 min.
among the different Cu-In phases [14] and its existence in the present study was attributed to the well mixing of the large number of alternate ultra thin Cu and In layers, deposited at room temperature. The surface topological studies indicated that the layers were uniformly flat and dense without any inhomogenities. The RBS studies showed that the as-deposited Cu/In film has a Cu/In ratio of 0.95 and confirmed the highly uniform variation of composition with depth throughout the layer. Fig. 1 shows the Xray diffraction (XRD) spectrum of the sulpherised Cu/In layers at 400 °C (CuInS2) for a time period of 30 min. The spectrum showed the different peaks that correspond to (103), (211), (220)/ (204) and (116)/(312) planes of CuInS2 in addition to the (112) peak. Although, the (112) reflection is very intense, indicating that the crystallites were strongly oriented along this plane, the intensities of the other peaks were also prominent and in agreement with that of the standard powder pattern. This suggests that the grown film was polycrystalline. The tetragonal splitting of the (220)/(204) and (116)/(312) diffraction planes was also observed in the figure, which is a characteristic of the chalcopyrite structure. The single phase CIS layers showed a “cauliflower” like topology and the evaluated grain size was ~410 nm. Fig. 2 shows the energy dispersive analysis of x-rays (EDAX) spectrum of Cu/ In layers sulphurised at 400 °C for 30 min. The EDAX analysis indicated that the composition was nearly stoichiometric for the grown CIS films. The evaluated composition of CuInS2 layer was Cu = 23.5 at.%, In = 23.7 at.% and S = 52.8 at.%. There was no evidence that elemental sulphur was present on the samples as investigated using EDAX and XRD. The films had electrical resistivities of 5.2 × 101 Ωcm, free carrier density of 7.5 × 1015 cm− 3 and Hall mobility of 16.7 cm2 V- 1 s- 1. The optical measurements indicated an absorption coefficients, α N 105 cm− 1 for photon energies higher than the energy band gap. The energy band gap determined from the (αhν)2 versus incident photon energy, hν plot was about 1.48 eV. 3.2. ZnS0.5Se0.5 films
Fig. 1. XRD spectrum of the sulpherised Cu/In layers at 400 °C (CuInS2) for a period of 30 min.
The preliminary studies on ZnSxSe1 − x films [13,15,16] indicated that the films deposited at 300 °C were well crystalline
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Fig. 3. EDAX spectrum of ZnS0.5Se0.5 layers deposited at 300 °C. Fig. 5. (αhν)2 versus hν plot for ZnS0.5Se0.5 films.
with better optical properties and found to be stoichiometric in composition when compared with those deposited at other substrate temperatures and deposition parameters. Fig. 3 depicts the EDAX spectrum of ZnS0.5Se0.5 layers deposited at 300 °C. The counts of Zn and (S + Se) peaks had relatively the same heights for ZnS0.5Se0.5 layers, indicating that their compositions were nearly stoichiometric with an average (S + Se)/ Zn ratio of 1.03. The calculated composition of Zn, S and Se for x = 0.5 in ZnSxSe1 − x film is 49.18 at.%, 26.35 at.% and 24.47 at.% respectively. Fig. 4 shows the XRD profile of ZnS0.5Se0.5 films deposited at 300 °C. The spectra clearly revealed that the layers were homogeneous with only ZnSSe phase without any secondary phases such as ZnS or ZnSe in the layers. The presence of (111) peak at 27.89° indicates that the layers were crystallized in cubic crystal structure. The evaluated inter-planar spacing and lattice constant were 3.196 Å and 5.536 Å respectively. The films had an optical transmittances N 80% for photons with energy greater than the fundamental energy band gap. The optical band gap of the films was determined from (αhν)2 versus hν plot (Fig. 5) and found to be 3.04 eV. The electrical
resistivity of the as-grown films was found to be very high, 107 Ωcm. The observed high resistivity might be due to the voids, defects and impurities present in the layers that were incorporated at the initial stages of the film growth. These defects in the as-deposited layers can be eliminated to some extent when the layers were subjected to heat treatment. Stuitius et al. [17] reported that both the ZnS and ZnSe thin films generally exhibit high resistivity irrespective of the deposition technique. The resistivity of ZnS0.5Se0.5 films was decreased to ∼105 Ωcm when the layers were annealed in vacuum for 30 min. at 350 °C. One of the pre-requisites for the materials that are useful in solar cells as a buffer layer is that it should have high resistivity (~ 105 Ωcm) [18], so that it can effectively act as a barrier layer to the diffusion of unwanted impurities from the window to the absorber layer and to provide interface passivation [19]. Therefore, these annealed layers were used as a buffer layer in solar photovoltaic cells. The dependence of electrical conductivity, σ on the annealing temperature, T (35– 350 °C) for ZnS0.5Se0.5 films deposited at 300 °C was studied
Fig. 4. XRD profile of ZnS0.5Se0.5 films deposited at 300 °C.
Fig. 6. XRD spectrum of 600 nm thick ZnO:Ga films.
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3.4. CuInS2/ZnS0.5Se0.5/ZnO:Ga solar cell
Fig. 7. Transmittance versus wavelength for ZnO:Ga film of 600 nm thickness.
that showed Arrhenius type of behavior. The ln σ versus 1/T plots indicated two different linear portions with different slopes in the low and high temperature regions. The activation energy for the low temperature region (35–130 °C) was 9 meV, whereas in the high temperature region (130–350 °C) it was 0.32 eV. The activation energy values obtained in this study are in good agreement with the values reported by Ganguly et al. for nanocrystalline ZnSxSe1 − x films deposited by high pressure sputtering [20]. 3.3. ZnO:Ga films Fig. 6 shows the XRD spectrum for ZnO:Ga films deposited on glass substrates with a thickness of 600 nm. The spectra showed only the (002) diffraction peak at 2θ ~ 34.31o. This indicated that the films were polycrystalline in nature with hexagonal wurtzite structure and had (002) orientation along the c-axis perpendicular to the plane of the substrate. The observed 2θ value is very close to the reported value of 34.41o for single crystal ZnO. Reflections corresponding to the other hexagonal structure of ZnO were not found in the XRD spectra. Also the spectra does not exhibit any peak related to elemental gallium, which indicated that the doped ‘Ga’ atoms perfectly replaced Zn atoms in the hexagonal lattice and/or segregated into the noncrystalline region such as grain boundary. It was observed that the film surface had ordered crystals that are densely packed with uniform distribution. The average grain size of the film, estimated using the scanning electron microscope (SEM) was ~ 98 nm which is comparable with the value calculated from the XRD data. The transmittance versus wavelength measurements on ZnO:Ga films indicated highest transmittance N 85%. Fig. 7 shows the change of transmittance with wavelength in ZnO:Ga film of 600 nm thickness formed at 350 °C. The optical energy band gap of the films was evaluated using the (αhν)2 versus hν plots was 3.35 eV. Hall effect measurements showed that the films had electrical resistivity of 7.5 × 10− 4 Ωcm with a free carrier concentration of 2.5 × 1020 cm− 3 and an electron mobility of 33.2 cm2/VS.
Thin film Mo/CuInS2/ZnS0.5Se0.5/ZnO:Ga device had a reverse saturation current density, Jo of 4.5 × 10− 5 A/cm2 with a diode quality factor, A of 1.86. The illuminated current–voltage characteristics of the device with an active area of 0.5 cm2 were studied at room temperature using a light source of 100 mW/ cm−2 under AM1.5 condition. The device showed an electrical conversion efficiency of 1.6% with an open circuit voltage, Voc = 387 mV, short circuit current, Jsc = 8.9 mA/cm2 and fill factor, FF = 0.46 when illuminated. Though the efficiency value obtained in this study was very lower than the reported values with other alternative buffer layers such as ZnS, ZnSe, In2S3 and ZnInS2 [21–23], a successful debut attempt was made to introduce the novel material, ZnSSe for application as a buffer layer in heterojunction solar cells. The observed low efficiency in this initial and un-optimized device could be attributed to the (i) lower thickness of absorber layer (1 μm) lowering the photon absorption, (ii) high series resistance, limiting the short circuit current, (iii) lower thickness of the window layer causing to an increase of the lateral series resistance of the junction and (iv) large difference of electrical resistivity (three orders in magnitude) and carrier density (five orders in magnitude) between absorber and window layer that will significantly affect the final efficiency [24]. The capacitance of the cell was measured at room temperature using digital LCR meter (Model HP4271B) fixed at a frequency of 100 KHz and an external voltage source was used to provide the required bias voltage to the cell. The capacitance value (1/C2) decreased linearly with the decrease of reverse voltage and reached a minimum value at zero bias. The acceptor concentration (NA) determined from the slope of the 1/ C2–V plot was 6.3 × 1015 cm− 3. The built-in voltage (Vb) calculated from C–V studies was ~ 0.62 V, while the depletion layer width, Wd [25] was 0.65 μm for the junction. The spectral response of the CuInS2/ZnS0.5Se0.5/ZnO:Ga device at room temperature is shown in Fig. 8. The device showed an average quantum efficiency of 65% in the wavelength region of 300– 900 nm. The fall-off in the spectral response at lower (~ 370 nm) and higher (~ 840 nm) wavelengths indicated the ‘heterojunction window effect’. It is clear from the spectral response that
Fig. 8. The spectral response of the CuInS2/ZnS0.5Se0.5/ZnO:Ga junction.
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the photons with the energies greater than the band gap of ZnO: Ga and less than the band gap of CuInS2 do not contribute to the final junction photo current. Hence, only the photons with wavelength λZnO b λ b λCIS produce photocurrent. The lower response at longer wavelengths reflects the longer minority carrier diffusion length, L as generally expected in polycrystalline materials. The value of L determined using the optical absorption coefficient and quantum efficiency data [26] was found to be 0.40 μm. 4. Conclusions The potentiality and applicability of a novel buffer layer, ZnSxSe1 − x in CuInS2–based polycrystalline thin film heterojunction solar cell with the structure, CuInS2/ZnS0.5Se0.5/ZnO: Ga has been investigated for the first time. The device showed an electrical conversion efficiency of 1.6% with an open circuit voltage, Voc = 387 mV, short circuit current density, Jsc = 8.9 mA/ cm2 and fill factor, FF = 0.46. Despite the limited efficiencies of these first devices fabricated, it is anticipated that the modified device design would lead to substantially better efficiencies. The built-in voltage and the depletion layer width for the device calculated from the C–V studies were ~ 0.62 V and 0.65 μm respectively. The spectral response measurements indicated an average quantum efficiency of 65%. The lower response at longer wavelengths reflects the longer minority carrier diffusion length, which is found to be 0.40 μm for the present device. References [1] K. Ramanathan, M.A. Contreras, C.L. Perkins, S. Asher, F.S. Hasoon, J. Keane, D. Young, M. Romero, W. Metzger, R. Noufi, J. Ward, A. Duda, Prog. Photovolt. 11 (2003) 225. [2] M.A. Contreras, K. Ramanathan, J. Abushama, F. Hasoon, D.L. Young, B. Egaas, R. Noufi, Prog. Photovolt. 13 (2005) 209. [3] R.N. Bhattacharya, M.A. Contreras, B. Egaas, R. Noufi, A. Kanevce, J.R. Sites, Appl. Phys. Lett. 89 (2006) 253503. [4] R. Kaigawa, A. Neisser, R. Klenk, M.-Ch. Lux-steiner, Thin Solid Films 415 (2005) 266.
[5] A.E. Delahoy, M. Akthar, J. Cambridge, L. Chen, R. Govindarajan, S. Guo, M.J. Romero, 29th IEEE Photovoltaic Specialists Conference, New Orleans, USA, 2002, p. 640. [6] T. Torndahl, C. Platzer-Bjorkman, J. Kessler, M. Edoff, Prog. Photovolt.: Res. Appl. 15 (2007) 225. [7] C. Platzer-Bjorkman, T. Torndahl, D. Abou-Ras, J. Malmstorm, J. Kesler, L. Stolt, J. Appl. Phys. 100 (2006) 044506. [8] M. Bar, J. Reichardt, A. Grimm, I. Kotschau, I. Lauermann, S. Sokoll, M.C. Lux-Steiner, Ch-H. Fischer, L. Weinhardt, U. Umbach, C. Heske, Ch. Jung, T.P. Niesen, S. Visbeck, J. Appl. Phys. 98 (2005) 053702. [9] S. Neve, W. Bohne, J. Klaer, R. Klenk, R. Scheer, Proc. 17th European Photovoltaic Solar Energy Conference, Munich, Germany, 2001, p. 1102. [10] D. Abou-Ras, G. Kostorz, A. Strohm, H.W. Schock, A.N. Tiwari, J. Appl. Phys. 98 (2005) 123512. [11] N. Naghavi, S. Spiering, M. Powalla, B. Canava, D. Lincot, Prog. Photovolt. 11 (2003) 437. [12] H.R. Moutinho, R.G. Dhere, M.M. Al-Jassim, A.B. Swartzlander, M.R. Young, L.L. Kazmerski, Proc. 28th IEEE Photovoltaic Specialists Conf., Anchorage, USA, 2000, p. 646. [13] Y.P. Venkata Subbaiah, P. Prathap, M. Devika, K.T. Ramakrishna Reddy, Physica B: Cond. Matter 365 (2005) 240. [14] J. Kessler, H. Dittrich, F. Grundwald, H.W. Schock, Proc. 10th European Photovoltaic Solar Energy Conf., Lisbon, Portugal, 1991, p. 891. [15] Y.P. Venkata Subbaiah, K.T. Ramakrishna Reddy, Mater. Chem. Phys. 92 (2005) 448. [16] Y.P. Venkata Subbaiah, P. Prathap, K.T. Ramakrishna Reddy, Appl. Surf. Sci. 253 (2006) 2409. [17] W. Stutius, J. Cryst. Growth 59 (1982) 1. [18] L.M. Caicedo, G. Cediel, A. Dussasn, J.W. Sandino, C. Calderon, G. Gordillo, Physica Status Solidi B, Basic Res. 220 (2000) 249. [19] G. Gordillo, C. Caldron, H. Infante, Proc. 29th IEEE Photovoltaics Specialists Conf., New Orleans, USA, 2002, p. 644. [20] A. Ganguly, S. Chaudhuri, A.K. Pal, J. Phys. D: Appl. Phys. 34 (2001) 506. [21] A.E. Delhoy, M. Akhtar, J. Cambridge, L. Chen, R. Govindarajan, S. Guo, M.J. Romero, Proc. 29th IEEE Photovoltaic Specialists Conf., New Orleans, USA, 2002, p. 644. [22] T.T. John, M. Mathew, C. Sudha Kartha, K.P. Vijayakumar, T. Abe, Y. Kashiwaba, Sol. Energy Mater. Sol. Cells 89 (2005) 27. [23] D. Hariskos, S. Spiering, M. Powalla, Thin Solid Films 480- 481 (2005) 99. [24] L.L. Kazmerski, Inst. Phys. Conf. 35 (1977) 217. [25] R.K. Kotnala, N.P. Singh, “Essentials of Solar Cells”, Allied pub. Pvt. Ltd., New Delhi, 1986. [26] K.W. Mitchell, A.L. Fahrenbruch, R.H. Bube, J. Appl. Phys. 48 (1977) 4365.
Thin Solid Films 516 (2008) 7060 – 7064 www.elsevier.com/locate/tsf
Studies on ZnS0.5Se0.5 buffer based thin film solar cells Y.P.Venkata Subbaiah a , P. Prathap b , K.T.Ramakrishna Reddy b , R.W. Miles c , J. Yi d,⁎ a
Department of Physics, Yogi Vemana University, Kadapa 516 003, India Thin Film Laboratory, Department of Physics, S.V. University, Tirupati 517-502, India c School of Engineering and Technology, Northumbria University, Newcastle, NE1 8ST, UK School of Information and Communication Engineering, Sungkyunkwan University, Suwon 440-746, South Korea b
d
Available online 23 December 2007
Abstract ZnS0.5Se0.5 has been proposed as a novel buffer layer, alternative to CdS for solar cell application. ZnS0.5Se0.5 layers were deposited at 300 °C using close-spaced evaporation. The layers exhibited cubic structure with a sharp (111) reflection. The films showed an optical transmittance N 80% with an energy band gap of 3.01 eV and the electrical resistivity of the films was ~ 105 Ωcm. CuInS2 (CIS) films synthesized by sulphurisation process of rf sputtered Cu/In precursor layers formed on Mo coated glass were used as absorber. Structural, optical and electrical studies were performed to characterize the synthesized CuInS2 films. Thin film solar cells were fabricated by depositing a thin ZnS0.5Se0.5 buffer layer onto the Glass/Mo/CuInS2 with sprayed ZnO:Ga as a window layer. The interfacial properties of the resulting heterojunction were studied using current–voltage (I–V), capacitance–voltage (C–V) and spectral response measurements and the results were discussed. © 2007 Elsevier B.V. All rights reserved.
1. Introduction The thin film photovoltaic devices that have been fabricated at laboratory-scale to date exhibited record efficiencies of ~ 19.5% with chemical bath deposited (CBD) CdS buffer [1,2] and CBD Cd1 − xZnxS (x ~ 0.2) buffer [3] in CuInGaSe2 (CIGS) absorber and around 12.3% with CBD CdS in case of CuInS2 (CIS) [4]. Highly efficient thin film solar cells fabricated to date include generally ‘Cd’ containing compounds like CdS buffer layer and CdZnS (less Zn). Since, Cd is a heavy metal associated with high toxicity, these compounds must be excluded in the ultimate solar cell devices in view of the ecological and economic point of concern. Further more, the less band gap of the CdS (2.4 eV) increases the photo absorption in shorter wavelength region that makes no useful photocurrent contribution to the final device. The attractive alternate layers investigated by to date includes many Zn-based buffer layers of ZnSe [5], Zn1 − xMgxO [6], Zn(O,S) [7], Zn(O,OH) [8],
⁎ Corresponding author. Tel.: +82 31 290 7139; fax: +82 31 290 7159. E-mail addresses: [email protected] (Y.P.V. Subbaiah), [email protected] (J. Yi). 0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2007.12.115
ZnSxOyHz [9] and other non Zn-compounds like InxSy [10] and In2Se3 [11]. Out of these various kinds of buffers, Zn-based compounds are promising and reached the efficiencies close to those achieved with the conventional buffer, CdS. Again, all these alternative Zn-based buffers, involved in high efficiency solar cells were prepared mostly by CBD process. However, from ecological point of view this technique must be replaced by an industry friendly process because of greater public apprehension with the disposal of the solution and waste recycling for large-scale manufacturing. Therefore, keeping in view the importance of alternate buffer layers and the process of their synthesis for heterojunction solar cells, in the present study, we investigated a novel buffer material, ZnSxSe1 − x prepared by close-spaced evaporation for the first time and examined its potentiality and suitability as a buffer layer by fabricating solar cell device with CIS absorber. This material is promising because it can be lattice matched to CuInS2, reducing strain at the interface and minimizing the density of misfit dislocations present at the interface [12]. It also has a wider energy band gap than CdS resulting in an enhanced transmission of the blue part of the solar spectrum. Also, the technique used in the present investigation is simple, inexpensive and amenable for industrial production.
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2. Experimental In the present investigation, CuInS2 absorber films were produced using the sulphurisation of rf sputtered Cu/In multistacks. The Cu/In stacks of 0.6 μm deposited on Mo-coated glass substrates at room temperature were used as starting layers for preparing CuInS2 films. These starting layers were procured from the Northumbria Photovoltaics Applications Center, Northumbria University, UK. The sulphurisation was carried out using the system fabricated in our laboratory by annealing Cu/In multilayers in excess elemental sulphur at 400 °C. Sulphurisation of the Glass/Mo/Cu/In precursors at 400 °C for 30 min. resulted in the complete conversion of Cu/In layers into CuInS2. The final layers formed were ~1.0 μm thick. Later on, a thin buffer layer of ZnS0.5Se0.5 (thickness ~60 nm) was deposited using close-spaced evaporation onto the Glass/Mo/CuInS2 substrates at a temperature of 300 °C. The detailed experimental procedure for preparation of ZnSxSe1 − x films was reported earlier [13]. Gallium (5 at.%) doped ZnO films were then deposited onto ZnS0.5Se0.5 using spray pyrolysis with zinc acetate precursor of 0.1 M concentration at a substrate temperature of 350 °C. The other deposition conditions for spray pyrolysis like solution flow rate, airflow rate and nozzle-substrate distance were fixed as 6 ml/min, 8 l/min and 25 cm respectively. The thickness of these layers was 0.6 μm. Finally a 0.4 μm thick gold film was vacuum evaporated ZnO:Ga as the grid for external connection. The active area of the device is 0.5 cm2 . The individual layers were characterized using appropriate techniques to evaluate the physical and chemical properties. The fabricated solar cell devices were characterized by studying the I–V measurements in dark and under illumination, C–V and spectral response measurements. 3. Results and discussion 3.1. CuInS2 Films The X-ray diffraction data obtained for the as-procured Cu/In precursor layers indicated the presence of Cu11In9 as the predominant phase without any secondary or elemental Cu or In phases present. This is the most commonly observed phase
Fig. 2. EDAX spectrum of Cu/In layers sulphurised at 400 °C for 30 min.
among the different Cu-In phases [14] and its existence in the present study was attributed to the well mixing of the large number of alternate ultra thin Cu and In layers, deposited at room temperature. The surface topological studies indicated that the layers were uniformly flat and dense without any inhomogenities. The RBS studies showed that the as-deposited Cu/In film has a Cu/In ratio of 0.95 and confirmed the highly uniform variation of composition with depth throughout the layer. Fig. 1 shows the Xray diffraction (XRD) spectrum of the sulpherised Cu/In layers at 400 °C (CuInS2) for a time period of 30 min. The spectrum showed the different peaks that correspond to (103), (211), (220)/ (204) and (116)/(312) planes of CuInS2 in addition to the (112) peak. Although, the (112) reflection is very intense, indicating that the crystallites were strongly oriented along this plane, the intensities of the other peaks were also prominent and in agreement with that of the standard powder pattern. This suggests that the grown film was polycrystalline. The tetragonal splitting of the (220)/(204) and (116)/(312) diffraction planes was also observed in the figure, which is a characteristic of the chalcopyrite structure. The single phase CIS layers showed a “cauliflower” like topology and the evaluated grain size was ~410 nm. Fig. 2 shows the energy dispersive analysis of x-rays (EDAX) spectrum of Cu/ In layers sulphurised at 400 °C for 30 min. The EDAX analysis indicated that the composition was nearly stoichiometric for the grown CIS films. The evaluated composition of CuInS2 layer was Cu = 23.5 at.%, In = 23.7 at.% and S = 52.8 at.%. There was no evidence that elemental sulphur was present on the samples as investigated using EDAX and XRD. The films had electrical resistivities of 5.2 × 101 Ωcm, free carrier density of 7.5 × 1015 cm− 3 and Hall mobility of 16.7 cm2 V- 1 s- 1. The optical measurements indicated an absorption coefficients, α N 105 cm− 1 for photon energies higher than the energy band gap. The energy band gap determined from the (αhν)2 versus incident photon energy, hν plot was about 1.48 eV. 3.2. ZnS0.5Se0.5 films
Fig. 1. XRD spectrum of the sulpherised Cu/In layers at 400 °C (CuInS2) for a period of 30 min.
The preliminary studies on ZnSxSe1 − x films [13,15,16] indicated that the films deposited at 300 °C were well crystalline
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Fig. 3. EDAX spectrum of ZnS0.5Se0.5 layers deposited at 300 °C. Fig. 5. (αhν)2 versus hν plot for ZnS0.5Se0.5 films.
with better optical properties and found to be stoichiometric in composition when compared with those deposited at other substrate temperatures and deposition parameters. Fig. 3 depicts the EDAX spectrum of ZnS0.5Se0.5 layers deposited at 300 °C. The counts of Zn and (S + Se) peaks had relatively the same heights for ZnS0.5Se0.5 layers, indicating that their compositions were nearly stoichiometric with an average (S + Se)/ Zn ratio of 1.03. The calculated composition of Zn, S and Se for x = 0.5 in ZnSxSe1 − x film is 49.18 at.%, 26.35 at.% and 24.47 at.% respectively. Fig. 4 shows the XRD profile of ZnS0.5Se0.5 films deposited at 300 °C. The spectra clearly revealed that the layers were homogeneous with only ZnSSe phase without any secondary phases such as ZnS or ZnSe in the layers. The presence of (111) peak at 27.89° indicates that the layers were crystallized in cubic crystal structure. The evaluated inter-planar spacing and lattice constant were 3.196 Å and 5.536 Å respectively. The films had an optical transmittances N 80% for photons with energy greater than the fundamental energy band gap. The optical band gap of the films was determined from (αhν)2 versus hν plot (Fig. 5) and found to be 3.04 eV. The electrical
resistivity of the as-grown films was found to be very high, 107 Ωcm. The observed high resistivity might be due to the voids, defects and impurities present in the layers that were incorporated at the initial stages of the film growth. These defects in the as-deposited layers can be eliminated to some extent when the layers were subjected to heat treatment. Stuitius et al. [17] reported that both the ZnS and ZnSe thin films generally exhibit high resistivity irrespective of the deposition technique. The resistivity of ZnS0.5Se0.5 films was decreased to ∼105 Ωcm when the layers were annealed in vacuum for 30 min. at 350 °C. One of the pre-requisites for the materials that are useful in solar cells as a buffer layer is that it should have high resistivity (~ 105 Ωcm) [18], so that it can effectively act as a barrier layer to the diffusion of unwanted impurities from the window to the absorber layer and to provide interface passivation [19]. Therefore, these annealed layers were used as a buffer layer in solar photovoltaic cells. The dependence of electrical conductivity, σ on the annealing temperature, T (35– 350 °C) for ZnS0.5Se0.5 films deposited at 300 °C was studied
Fig. 4. XRD profile of ZnS0.5Se0.5 films deposited at 300 °C.
Fig. 6. XRD spectrum of 600 nm thick ZnO:Ga films.
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Fig. 7. Transmittance versus wavelength for ZnO:Ga film of 600 nm thickness.
that showed Arrhenius type of behavior. The ln σ versus 1/T plots indicated two different linear portions with different slopes in the low and high temperature regions. The activation energy for the low temperature region (35–130 °C) was 9 meV, whereas in the high temperature region (130–350 °C) it was 0.32 eV. The activation energy values obtained in this study are in good agreement with the values reported by Ganguly et al. for nanocrystalline ZnSxSe1 − x films deposited by high pressure sputtering [20]. 3.3. ZnO:Ga films Fig. 6 shows the XRD spectrum for ZnO:Ga films deposited on glass substrates with a thickness of 600 nm. The spectra showed only the (002) diffraction peak at 2θ ~ 34.31o. This indicated that the films were polycrystalline in nature with hexagonal wurtzite structure and had (002) orientation along the c-axis perpendicular to the plane of the substrate. The observed 2θ value is very close to the reported value of 34.41o for single crystal ZnO. Reflections corresponding to the other hexagonal structure of ZnO were not found in the XRD spectra. Also the spectra does not exhibit any peak related to elemental gallium, which indicated that the doped ‘Ga’ atoms perfectly replaced Zn atoms in the hexagonal lattice and/or segregated into the noncrystalline region such as grain boundary. It was observed that the film surface had ordered crystals that are densely packed with uniform distribution. The average grain size of the film, estimated using the scanning electron microscope (SEM) was ~ 98 nm which is comparable with the value calculated from the XRD data. The transmittance versus wavelength measurements on ZnO:Ga films indicated highest transmittance N 85%. Fig. 7 shows the change of transmittance with wavelength in ZnO:Ga film of 600 nm thickness formed at 350 °C. The optical energy band gap of the films was evaluated using the (αhν)2 versus hν plots was 3.35 eV. Hall effect measurements showed that the films had electrical resistivity of 7.5 × 10− 4 Ωcm with a free carrier concentration of 2.5 × 1020 cm− 3 and an electron mobility of 33.2 cm2/VS.
Thin film Mo/CuInS2/ZnS0.5Se0.5/ZnO:Ga device had a reverse saturation current density, Jo of 4.5 × 10− 5 A/cm2 with a diode quality factor, A of 1.86. The illuminated current–voltage characteristics of the device with an active area of 0.5 cm2 were studied at room temperature using a light source of 100 mW/ cm−2 under AM1.5 condition. The device showed an electrical conversion efficiency of 1.6% with an open circuit voltage, Voc = 387 mV, short circuit current, Jsc = 8.9 mA/cm2 and fill factor, FF = 0.46 when illuminated. Though the efficiency value obtained in this study was very lower than the reported values with other alternative buffer layers such as ZnS, ZnSe, In2S3 and ZnInS2 [21–23], a successful debut attempt was made to introduce the novel material, ZnSSe for application as a buffer layer in heterojunction solar cells. The observed low efficiency in this initial and un-optimized device could be attributed to the (i) lower thickness of absorber layer (1 μm) lowering the photon absorption, (ii) high series resistance, limiting the short circuit current, (iii) lower thickness of the window layer causing to an increase of the lateral series resistance of the junction and (iv) large difference of electrical resistivity (three orders in magnitude) and carrier density (five orders in magnitude) between absorber and window layer that will significantly affect the final efficiency [24]. The capacitance of the cell was measured at room temperature using digital LCR meter (Model HP4271B) fixed at a frequency of 100 KHz and an external voltage source was used to provide the required bias voltage to the cell. The capacitance value (1/C2) decreased linearly with the decrease of reverse voltage and reached a minimum value at zero bias. The acceptor concentration (NA) determined from the slope of the 1/ C2–V plot was 6.3 × 1015 cm− 3. The built-in voltage (Vb) calculated from C–V studies was ~ 0.62 V, while the depletion layer width, Wd [25] was 0.65 μm for the junction. The spectral response of the CuInS2/ZnS0.5Se0.5/ZnO:Ga device at room temperature is shown in Fig. 8. The device showed an average quantum efficiency of 65% in the wavelength region of 300– 900 nm. The fall-off in the spectral response at lower (~ 370 nm) and higher (~ 840 nm) wavelengths indicated the ‘heterojunction window effect’. It is clear from the spectral response that
Fig. 8. The spectral response of the CuInS2/ZnS0.5Se0.5/ZnO:Ga junction.
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the photons with the energies greater than the band gap of ZnO: Ga and less than the band gap of CuInS2 do not contribute to the final junction photo current. Hence, only the photons with wavelength λZnO b λ b λCIS produce photocurrent. The lower response at longer wavelengths reflects the longer minority carrier diffusion length, L as generally expected in polycrystalline materials. The value of L determined using the optical absorption coefficient and quantum efficiency data [26] was found to be 0.40 μm. 4. Conclusions The potentiality and applicability of a novel buffer layer, ZnSxSe1 − x in CuInS2–based polycrystalline thin film heterojunction solar cell with the structure, CuInS2/ZnS0.5Se0.5/ZnO: Ga has been investigated for the first time. The device showed an electrical conversion efficiency of 1.6% with an open circuit voltage, Voc = 387 mV, short circuit current density, Jsc = 8.9 mA/ cm2 and fill factor, FF = 0.46. Despite the limited efficiencies of these first devices fabricated, it is anticipated that the modified device design would lead to substantially better efficiencies. The built-in voltage and the depletion layer width for the device calculated from the C–V studies were ~ 0.62 V and 0.65 μm respectively. The spectral response measurements indicated an average quantum efficiency of 65%. The lower response at longer wavelengths reflects the longer minority carrier diffusion length, which is found to be 0.40 μm for the present device. References [1] K. Ramanathan, M.A. Contreras, C.L. Perkins, S. Asher, F.S. Hasoon, J. Keane, D. Young, M. Romero, W. Metzger, R. Noufi, J. Ward, A. Duda, Prog. Photovolt. 11 (2003) 225. [2] M.A. Contreras, K. Ramanathan, J. Abushama, F. Hasoon, D.L. Young, B. Egaas, R. Noufi, Prog. Photovolt. 13 (2005) 209. [3] R.N. Bhattacharya, M.A. Contreras, B. Egaas, R. Noufi, A. Kanevce, J.R. Sites, Appl. Phys. Lett. 89 (2006) 253503. [4] R. Kaigawa, A. Neisser, R. Klenk, M.-Ch. Lux-steiner, Thin Solid Films 415 (2005) 266.
[5] A.E. Delahoy, M. Akthar, J. Cambridge, L. Chen, R. Govindarajan, S. Guo, M.J. Romero, 29th IEEE Photovoltaic Specialists Conference, New Orleans, USA, 2002, p. 640. [6] T. Torndahl, C. Platzer-Bjorkman, J. Kessler, M. Edoff, Prog. Photovolt.: Res. Appl. 15 (2007) 225. [7] C. Platzer-Bjorkman, T. Torndahl, D. Abou-Ras, J. Malmstorm, J. Kesler, L. Stolt, J. Appl. Phys. 100 (2006) 044506. [8] M. Bar, J. Reichardt, A. Grimm, I. Kotschau, I. Lauermann, S. Sokoll, M.C. Lux-Steiner, Ch-H. Fischer, L. Weinhardt, U. Umbach, C. Heske, Ch. Jung, T.P. Niesen, S. Visbeck, J. Appl. Phys. 98 (2005) 053702. [9] S. Neve, W. Bohne, J. Klaer, R. Klenk, R. Scheer, Proc. 17th European Photovoltaic Solar Energy Conference, Munich, Germany, 2001, p. 1102. [10] D. Abou-Ras, G. Kostorz, A. Strohm, H.W. Schock, A.N. Tiwari, J. Appl. Phys. 98 (2005) 123512. [11] N. Naghavi, S. Spiering, M. Powalla, B. Canava, D. Lincot, Prog. Photovolt. 11 (2003) 437. [12] H.R. Moutinho, R.G. Dhere, M.M. Al-Jassim, A.B. Swartzlander, M.R. Young, L.L. Kazmerski, Proc. 28th IEEE Photovoltaic Specialists Conf., Anchorage, USA, 2000, p. 646. [13] Y.P. Venkata Subbaiah, P. Prathap, M. Devika, K.T. Ramakrishna Reddy, Physica B: Cond. Matter 365 (2005) 240. [14] J. Kessler, H. Dittrich, F. Grundwald, H.W. Schock, Proc. 10th European Photovoltaic Solar Energy Conf., Lisbon, Portugal, 1991, p. 891. [15] Y.P. Venkata Subbaiah, K.T. Ramakrishna Reddy, Mater. Chem. Phys. 92 (2005) 448. [16] Y.P. Venkata Subbaiah, P. Prathap, K.T. Ramakrishna Reddy, Appl. Surf. Sci. 253 (2006) 2409. [17] W. Stutius, J. Cryst. Growth 59 (1982) 1. [18] L.M. Caicedo, G. Cediel, A. Dussasn, J.W. Sandino, C. Calderon, G. Gordillo, Physica Status Solidi B, Basic Res. 220 (2000) 249. [19] G. Gordillo, C. Caldron, H. Infante, Proc. 29th IEEE Photovoltaics Specialists Conf., New Orleans, USA, 2002, p. 644. [20] A. Ganguly, S. Chaudhuri, A.K. Pal, J. Phys. D: Appl. Phys. 34 (2001) 506. [21] A.E. Delhoy, M. Akhtar, J. Cambridge, L. Chen, R. Govindarajan, S. Guo, M.J. Romero, Proc. 29th IEEE Photovoltaic Specialists Conf., New Orleans, USA, 2002, p. 644. [22] T.T. John, M. Mathew, C. Sudha Kartha, K.P. Vijayakumar, T. Abe, Y. Kashiwaba, Sol. Energy Mater. Sol. Cells 89 (2005) 27. [23] D. Hariskos, S. Spiering, M. Powalla, Thin Solid Films 480- 481 (2005) 99. [24] L.L. Kazmerski, Inst. Phys. Conf. 35 (1977) 217. [25] R.K. Kotnala, N.P. Singh, “Essentials of Solar Cells”, Allied pub. Pvt. Ltd., New Delhi, 1986. [26] K.W. Mitchell, A.L. Fahrenbruch, R.H. Bube, J. Appl. Phys. 48 (1977) 4365.