DEVELOPMENT OF In x Se y BUFFER LAYERS FOR APPLICATION IN CdTe BASED THIN FILM SOLAR CELLS M. L. Madugu*, P. A. Bingham, H. I. Salim, O. I Olusola and I. M. Dharmadasa Materials and Engineering Research Institute, Sheffield Hallam University, Sheffield, S1 1WB, UK. *Email: maduguu@yahoo.com; Tel: +44 114 225 691 Fax: +44 114 225 693 ABSTRACT Indium selenide (In x Se y ) thin films have been grown and characterised for incorporation as buffer layers in the /CdS interface of CdS/CdTe solar cells. This is to test its suitability in avoiding pinholes by covering the glass/ surface to remove shorting of devices. Buffer layers are generally integrated as inter-layers to improve thin film solar cell device parameters for efficient solar energy conversion. The films were grown using electrodeposition and were characterised using a wide range of analytical techniques. The prepared films show amorphous behaviour even after heat-treatment and have good adhesion to the glass/ substrates. The films show p-type in electrical conduction in both as-deposited and heat-treated forms with bandaps in the range of 1.8-1.9 ev. Preliminary devices integrating In x Se y as a buffer layer in glass//in x Se y /CdS/CdTe/Au structure show encouraging device parameters (V oc, J sc, and FF). The other two layers used, CdS and CdTe are also grown using electrodeposition in an aqueous medium. The results of characterisation and device fabrication will be discussed in this paper. Keywords: Buffer layer, Indium selenide, CdTe, Electrodeposition, solar cells. 1. INTRODUCTION Indium selenide (In x Se y ) is an important direct bandgap III-VI semiconductor material. It has been researched in the areas of solar cells [1], Li-batteries [2] and microelectronics [3]. It can be n- or p-type in electrical conduction. It is known that InSe can exist in five different phases; these include,,, and depending on the stoichiometry [4]. A number of methods have been identified for growing In x Se y ; these include flash evaporation [5], Spray Pyrolysis [6], Molecular Beam Epitaxy (MBE) [7] and Electrodeposition [4]. Buffer layers are generally integrated as inter-layers to increase thin film solar cell device parameters for optimum solar energy conversion efficiency. CdS has been the champion material to use as a window layer in both CdTe and CIGS based solar cells. The generation of pinholes means that there is the need to improve the device parameters by improving the /CdS interface in CdTe based solar cells to achieve higher solar cells efficiency. The incorporation of buffer layers is one such way to enhance device performance. Buffer layers which are now under intense research are ZnO, ZnSe, InS, ZnS and In x Se y. Among these, ZnSe and In x Se y give a good performance due to their reasonable optical bandgap and property of wetting the surface of substrates to give the next layer to grow in layer-by-layer mode [4]. Observations from SEM and AFM of electrodeposited CdS and CdTe show nano-and micro-rod type growth modes, which affect our device parameters hence the idea to incorporate In x Se y which has a smoothening effect avoiding pinholes due to its amorphous nature. The main aim of this paper is to report some preliminary results obtained when In x Se y is incorporated as a buffer layer in CdS/CdTe solar cell. It has been experimentally proven by Godillo et al [8] and Calderon et al [9] that the incorporation of In x Se y as a buffer layer in CIS based solar cells shows promising results with efficiencies of over 8.. Our preliminary devices incorporating In x Se y as a buffer layer at the TCO/CdS interface show encouraging device parameters (V oc, J sc and FF). In the authors group, electrodeposition is the technique used for electroplating of materials employing a 2-electrode configuration. The use of a 2-electrode configuration in the authors group is to simplify the system, reduce cost and to avoid leakage of ions like Na + and K + from the reference electrode which are known to be detrimental in n-type CdTe based solar cells [1]. Full characterisation using XRD, SEM, optical absorption and thickness measurement are presented. 2. EXPERIMENTAL 2.1 Electroplating of In x Se y Indium selenide (In x Se y ) thin films were electrodeposited on glass/fluorine doped tin oxide (glass/) substrate using a potentiostatic technique and the counter electrode was a high purity graphite rod. High purity 5N (99.999 ) InCl 3 and SeO 2 were used as In and Se source respectively. Details on the substrate preparation and deposition procedure can be found in our recent publication [4]. The source of power supply was a Gill AC computerised potentiostat. The obtained films are uniform with good coverage of the substrate, and they have good adhesion to the glass/ substrate and are transparent. The thicknesses of the films were estimated using both UBM Microfocus optical measurement system 1847
Intensity (arb. unit) (-) Se (311)C Intensity (arb. unit) 29th European Photovoltaic Solar Energy Conference and Exhibition and SEM cross sections. Thicknesses produced are in the range (3-4) nm. 3. SUMMARY OF EXPERIMENTAL RESULTS 3.1 X-ray Diffraction (XRD) The structural properties of In x Se y layers grown on glass/ substrates were studied using a Philips X pert Pro X-ray diffractometer in the range 2θ = 2-7 using Cu-Kα (λ =1.545 Å) radiation. Initial characterisation of both as-deposited and heat-treated films show that the material has a property of amorphous nature even after heat-treatment as shown in figure 1. 12 offset by the advantageous wetting property of this layer possibly due to selenium, as was also observed for ZnSe [4]. 3.2 Scanning Electron Microscopy (SEM) The morphology of In x Se y surface was examined using FEI 2 Nova Nano SEM. This was conducted on both as-deposited and heat-treated films (figure 2(a and b)) which show good coverage of the glass/ substrate with a dense and smooth surface with difficulty to measure grain size. These results further support the amorphous nature of this film as shown by the XRD. Film thickness, which is crucial in device fabrication and specially in optimising the In x Se y buffer layer for optimum device parameters, is considered very important. This technique was also used to estimate the film thicknesses of these layers using cross sections such as that shown in figure 3. 8 15 mv a 4 145 mv 2 3 4 5 6 7 (a) 2 Theta ( ) 12 8 4 15 mv 14mV b (b) 2 3 4 5 6 7 2 Theta ( ) Figure 1: X-ray diffraction pattern of In x Se y layers grown on glass/ substrate at different cathodic voltages; (a) as deposited and (b) heat-treated at 25 for 1 minutes in air. Figure 1a and 1b show the X-ray diffraction pattern of asdeposited and heat-treated In x Se y films. It can be observed that the material is amorphous in nature (figure 1a) as all peaks shown are due to substrate. But after heat-treatment (figure 1b), the films show the existence of Se at 2 =23.5 and another peak of cubic In 2 Se 3 at 2 =29.6 which is due to reflection along the (311) plane. This could be due to material re-crystallisation where-by small grains coalesce to form larger ones resulting in increased peak intensities or it could be due to further reaction between elemental In and Se to form more In x Se y materials. The weak intensities of the peaks indicates the low crystallinity of the material but this is Figure 2: InSe layer (a) as-deposited and (b) heat treated for at 25 for 15 min. 1848
A 2 (arb. unit) A 2 (arb.unit) 29th European Photovoltaic Solar Energy Conference and Exhibition Platinum layer 4 (a) E g =1.96 ev 3 In x Se y 2 1 glass + SiO 2 1.5 2 2.5 3 Photon energy (ev) Figure 3: A typical SEM image of a cross-section of glass//in x Se y structure indicating the smoothening effect of In x Se y on the rough surface. Figure 3: shows a cross sectional morphology of In x Se y layer with a 44 nm thickness. The smoothening and coverage of In x Se y is clear and this is among the reasons for its recent incorporation as a buffer layer in our devices, the layer smoothens the glass/ surface removing much of its roughness, thus avoiding direct conduction pathways via any pinholes which may occur in subsequently-applied CdS/CdTe layers and thereby improving device efficiencies. 3.3 Optical Absorption: The study of the optical absorption spectra of the films was carried out using Cary 5 Scan UV-visible spectrophotometer. The knowledge of the optical bandgap of a semiconducting material especially in solar cells application is a crucial one. In this study, optical bandgap studies of In x Se y in both as-deposited and heattreated condition were carried out in the wavelength range (2-8) nm at room temperature. The optical bandgaps (Eg) of the samples were obtained by extrapolating the linear portion of the square of absorbance, A 2 versus photon energy (hν) to A 2 = [4]. The bandgap of these films are in the range of about 1.7-1.9 ev in both as-deposited and heat-treated conditions which suggest that the material grew as In 2 Se 3 phase. A decrease in the bandgap can be observed after heattreatment which could be due to loss of selenium and grain recrystallization [4]. 4 3 2 1 (b) Figure 4: Plots of typical optical absorption spectra for (a) as-deposited and (b) heat- treated In x Se y layers. In the as-deposited film (figure 4a), a very strong absorption edge is observed. After heat-treatment, sharp optical absorption edge is noticed. The optical bandgap for as-deposited is 1.96 ev while after heat-treatment, the bandgap shifted to 1.8 ev (figure 4b). As a buffer layer, the films must have high transmittance coefficients. 3.4 DISCUSSION E g =1.8 ev 1.5 2 2.5 3 Photon energy (ev) Phase diagrams are widely employed to show clearly how material crystallisation processes takes place as a function of composition and temperature. Using this tool it is possible to predict the equilibrium phase at a particular composition and temperature [11] at constant pressure. 1849
Name Title T\ o C 8 7 6 5 4 3 2 1 L1+L2 In-solidus 155 o 66 o 645 o 6 o 55 o In4Se3 InSe In6Se7 In5Se6 195 o In2Se3 L1+L2 Se-solidus 22 o ɤ β α α' 793 473 148 used in this device were grown using electrodeposition technique employing 2-electrode system. In 2 Se 3 film of 3 nm was deposited on glass/ substrate at a growth temperature, T g of 4 5 then CdS window layer of 2 nm was deposited on the In x Se y layer and the sample was heat-treated with CdCl 2 at 4 for 2 minutes. Then a CdTe absorber layer of 1.5 μm thickness was deposited and the sample heat-treated at 38 for 15 minutes in air with CdCl 2 + CdF 2. Both CdS and CdTe were deposited at a temperature of 85±5. The structure was then etched in both acidic and basic solution and finally the metal (Au) back contact of.31 cm 2 was evaporated using Auto 36 evaporation system with tungsten boat at a vacuum pressure of 1.5 1-7 mbar. In 1 2 3 4 5 6 7 8 9 Se In2Se3 atomic % of Se Au Figure 5: Phase diagram of the In-Se system (redrawn from ref. 11). The In-Se binary phase diagram is a complicated system due to its multi- phase nature. As can be seen from figure 5, the formation of any In-Se phase depends strongly on the material composition and temperature and different phases form either side of 5:5 stoichiometry. The solidus temperature are 155 and 22 for In and Se - rich compounds, respectively, and the liquidus surface changes markedly as a function of composition. Figure 5 shows that -In 2 Se 3 phase has the highest melting point at about 89 with stoichiometric In:Se ratio (4:6). Indium monoselenide (InSe) is the only phase that has In:Se (5:5) stoichiometry. All other phases are formed with either In-rich or Se-rich stoichiometric composition. A miscibility gap occurs for In-rich compositions above 52 two liquids occur for compositions of 32 at. Se. Another miscibility gap occurs for Se-rich compositions at temperatures of 72 at approximately 65 to 95 at.% Se. Miscibility gap refer to a region of a particular composition and temperature where at least two different phases co-exist [12]. An In-Se phase diagram structure on the phase transition of In-Se was put in place by Likforman et al [13] as shown below. 623K 823K 473K Metastable Asabe et al [14] and Gopal et al [15] have shown the possibility of In 2 Se 3 formation at room temperature using CBD and electrodeposition respectively. It has also been shown the possibility of formation of -In 2 Se 3 using a combination of CBD and resistive heating and heattreating the films at 1 for 1hr [16]. 4. DEVICES INCORPORATING In x Se y Stable Figure 6 shows a schematic diagram of device structure fabricated incorporating In 2 Se 3 as a buffer layer at the /CdS interface. All the materials except glass/ Figure 6: Schematic diagram of glass//in x Se y /CdS/CdTe/Au. Table 1: Comparison of the device parameters of a CdS/CdTe based solar cell fabricated with and without InxSey buffer layer. Device structure V oc J sc FF η(%) (mv) (macm -2 ) (%) g//in xse y/cds/cdte/au g//inxsey/cds/cdte/au g//cds/cdte/au g//cds/cdte/au Table 1 show the devices parameters for CdS/CdTe solar cells fabricated with and without In x Se y buffer layer. It can be observed that the devices without CdS have better solar energy conversion parameters. The results show that, the parameters obtained for devices with CdS only is better than with InSe buffer layers. CdS is a known and established material but experimentation is on-going for the optimisation of In x Se y buffer layers to achieved highest possible efficiency. 5. CONCLUSION CdTe ( 1.5 m) CdS ( 2 nm) In xse y ( 3 nm) ( 12 nm) glass.43 19.5.29 2.4.46 6.9.3 2.3.565 26..47 6.9.564 26.2.48 7.1 Electrodeposited In 2 S 3 buffer layer has been successfully incorporated in /CdS interface in CdS/CdTe based solar cell. The preliminary devices with In x Se y buffer layers show promising solar cell parameters (V oc, J sc and FF) though they show lower devices parameters than the well- 185
established CdS. The optimisation of the In x Se y layers is in progress to achieve highest possible efficiency. Future effort will focus on the optimisation of all the three semiconductor layers, to achieve higher solar energy conversion parameters. 6. ACKNOWLEDGEMENT The authors wish to acknowledge the contributions made by Fijay Fauzi, N. A. AbdulManaf and Ayotunde Ojo. The main author wish to thank the (PTDF) Nigeria for financial support. 7. REFERENCE 1. Altsufumi Hirohata, J. S. Moodera and G. P. Berera "Structural and electrical properties of InSe polycrystalline films and diode fabrication" Thin Solid Films 51 (26) 247-25. 2. A. Chaiken, K. Nauka, G. A. Gibson, H. Lee, C. C. Young, J. Wu, J. W. Ager, K. M. Yu and W. Walukiewicz "Structural and electronic properties of Amorphous and polycrystalline In 2 Se 3 films. 3. J. Weszka, Ph. Daniel, A. M. Burian and M. Zelechower "Resonance Raman Scattering in In.45 Se.55 Amorphous films" Solid State Communication 118 (21) 97-12. 4. M. L. Madugu, L. Bowen, O. K. Echendu and I. M. Dharmadasa "Preparation of Indium selenide Thin Film by Electrochemical Technique " J Mater Sci: Mater Electron (214) 25:3977 3983. 5. M. Persin, A. Persin, B. Celustka and B. Etlinger "Preparation of flash evaporated thin films of In 2 Se 3 " Thin Solid Films 11 (1972) 153-16. 6. H. Bouzounta, N. Bouguila, S. Duchemin, S. Fiechter, A. Dhouib "Preparation and characterisation of In 2 Se 3 thin films" Renewable energy 25 (22) 131-138. 7. Christian Chatillon "Critical analysis of the thermodynamic properties of the In-Se gaseous and solid phases "Journal of crystal growth 129 (1993) 297-311 Holland 8. G. Gordillo and C. Calderon "CIS thin film solar cells with evaporated InSe buffer layers" Solar Energy Materials and Solar Cells 77 (23) 163-173. 9. C. Calderon, P. Bortolo-perez and G. Gordillo "Development of CIS Solar Cells with evaporated In x Se y buffer layer "Phys. Stat. Sol.(c) S1, S92-S95 (24). 1. Stephen Dennison "Dopant and impurity Effects in Electrodeposited CdS/CdTe Thin Films for Photovoltaic Application "Journal of Mater. Chem. 1994, 4(1) 41-46. 11. M. Yudasaka, T. Matsuoka and K. Nakanishi "Indium Selenium "Thin Solid Films" 146 (1987) 65-73. 12. Jacob Greenberg "Thermodynamic Basis of Crystal Growth- Phase P-T-X Equilibrium and Non-Stoichiometry. Springer publication series, 21. 13. A. Likforman, P.H.Fourcroy, M. Guittard, J. Flahaut, R. Poirier and N. Szydlo, J. Solid Stat. Chem. 33 (198) 91. 14. S. Marsillac, A. M. Combot-Marie, J. C. Bernede and A. Conan "Experimental evidence of the low-temperature formation of -In 2 Se 3 thin films obtained by a solid-state reaction" Thin solid films 288 (1996) 14-46. 15. M. R. Asabe, P. A. Chate, S. D. Delekar, K. M. Garadkar, I. S. Mulla, P. P. Hankare "Journal of Physics and Chemistry of Solids 69 (28) 249-254 16. S. Gopal, C. Viswanathan, B. Karunagaran, Sa. K. Narayandass, D. Mangalaraj and Junsin Yi "Preparation and Characterisation of Electrodeposited indium selenide thin films" Crystal Res. Technol. 4, No. 6, 557-562 (25). 1851