Gallium Doped ZnO for Thin Film Solar Cells

Transcription

1 Gallium Doped ZnO for Thin Film Solar Cells A. Jäger-Waldau, H.-J. Muffler, R. Klenk, M. Kirsch, C. Kelch and M. Ch. Lux-Steiner Hahn-Meitner-Institut Berlin, Glienicker Straße 100, Berlin, Germany Abstract: Gallium doped ZnO films with resistivities as low as cm were grown by rf-magnetron sputtering. For a given thickness the sputter time to deposit gallium doped ZnO was in average only two thirds of that needed to deposit aluminium doped ZnO. Despite the higher sputter rates for ZnO:Ga the electrical transport properties were almost the same for both type of films with mobilities around 10cm 2 /Vs and net carrier concentrations of n = cm -3. The band gap was determined to be E g = 3.24 ev. Solar cells prepared with Cu(In,Ga)(S,Se) 2 absorber layers exhibited efficiencies of up to 13.6%. 1. Introduction ZnO transparent conductive oxide (TCO) films are standard window layers for Cu(In,Ga)(S,Se) 2 (CIGS) thin film solar cells [1]. In addition, a growing interest to develop ZnO as a replacement for the highly priced ITO in displays could be observed during the last few years [2]. For these applications the ZnO acts as window material and the requirements for that purpose are high transmittance and low reflectivity as well as a low resistivity in order to minimize the series resistance and enhance the fill factor. Recently it was also reported, that CIGS solar cells with gallium doped ZnO have a better stability against environmental influences. This led to a smaller decrease of the efficiency in not encapsulated cells after a few weeks of outside testing [3]. The presented investigations were performed to study whether rf-sputtered Ga 2 O 3 doped ZnO (GZO), which had been reported to exhibit resistivities as low as cm [4], is a good window material for thin film solar cells. 2. Experiments The GZO films were rf-sputtered in an argon atmosphere using an Ardenne LS520 sputtering machine from a Ga 2 O 3 (5.7 wt%) doped ZnO target. To investigate the influence of the sputter parameters, the sputter pressure was varied between and mbar and the sputter power was varied between 50 and 400 W. In addition the sputter time was varied between 5 and 10 minutes. For the investigation of the electrical and optical properties the GZO films were deposited on chemically cleaned floatglass substrates with no additional substrate heating. The optical properties were determined by transmission and reflection measurements using a double beam spectral photometer with an Ulbricht sphere to include the diffuse reflection. To

2 calculate the optical constants the following model system was used: air/film/substrate/air. Under the assumption of multi-reflection and phase shift the calculation was done using the equations given by Z. Hecht [5]. This led to a two dimensional equation system which was solved numerically by Newton-Raphson-Iteration. The electrical transport properties were measured by Hall measurements at room temperature. The resistivity of the films was determined from the sheet resistance and the film thickness. The final optimization of the sputter parameters for the GZO films was performed with the preparation of photovoltaic devices. The used Cu(In,Ga)(S,Se) 2 absorbers were samples supplied by Siemens Solar Industries and are standard samples from their pilot plant production. A standard CdS buffer layer has been deposited by chemical bath deposition. On top of this structure a ZnO bi-layer was rf-sputter deposited. The first ZnO layer is an undoped high resistivity ZnO layer with thickness t = 110 nm sputtered at 200 W and mbar in 180 s. Then highly conductive gallium doped ZnO (3N, Asahi Glass Co., 5.7 wt% Ga 2 O 3, relative density above 95%) or aluminium doped ZnO (5N, CERAC, 2 wt% Al 2 O 3, relative density approx. 70%) was deposited. As front contact a Ni/In grid covering approximately 7 to 10% of the total area was deposited. 3. Results and discussion The variation of sputter power, pressure and time revealed, that for different sputter powers the corresponding pressure had to be adjusted to an optimum value. This relation between power and pressure was slightly different for gallium and aluminium doped films. The standard parameters for gallium doped ZnO are listed in Table 1. Table 1: Standard parameters for gallium doped ZnO sputter power residual Ar pressure sputter rate 400 W mbar 28 1 Å/s 300 W mbar 20 1 Å/s 200 W mbar 12 1 Å/s These tuning experiments were performed by depositing single layers of gallium or aluminium doped ZnO films. The thickness of the films was between 400 and 1000 nm with a sheet resistance between 5 and 23. In average the values varied between 0.5 and cm with some results as low as cm. Compared to the aluminium doped ZnO the GZO deposition rate was about 50% higher which led to a reduction of one third in the deposition time for the same thickness. The reason for this behaviour is probably the different relative densities of the used targets. According to the manufacturer of the ZnO:Ga target, the very high relative density of this target is of advantage for higher deposition rates during sputtering. The transmission and reflection behaviour of gallium and aluminium doped films are shown in Fig. 1. The transmission of both types of films in the visible light range are comparable. The roughness of the films on glass substrates was in average about 2% for ZnO:Al and 6% for ZnO:Ga. The higher roughness for the ZnO:Ga films is mainly caused by the larger film thickness. The small difference in the behaviour of the transmission and reflection measurements for wavelength larger 850 nm can be attributed to the different thickness of the films as well as a somewhat higher free carrier absorption of the ZnO:Ga films. However, no significant difference in the electric transport properties could be observed. The calcula-

3 [cm -1 ] transmission / reflection [%] Preprint ISCS98, October 1998, Nara, Japan tions of the absorption coefficient revealed below 3 ev no significant difference in the behaviour as shown in Fig Transmittance Al-doped (200W) Ga-doped (300W) Ga-doped (200W) Reflectance [nm] Fig. 1: Spectral characteristica of different doped ZnO films on glass substrates (ZnO:Al t 200W = 380 nm, ZnO:Ga t 300W = 500 nm, t 200W = 660 nm) 8,0x10 4 ZnO:Ga ZnO:Al 6,0x10 4 4,0x10 4 2,0x10 4 0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 4,0 h [ev] Fig. 2: Absorption coefficient of aluminium (t = 380 nm) and gallium (t = 580 nm) doped ZnO films sputtered with 200W The similar increase of the absorption in the infrared region corresponds well with the Hall measurements of these films which revealed no difference in the net carrier concentration of both types of films. The values for the measurements of the transport properties for those films at room temperature are given in Table. 2.

4 Table 2: Electrical transport properties at room temperature sample resistivity [ cm] mobility [cm 2 /Vs] net carrier concentration [cm -3 ] ZnO:Ga n = ZnO:Al n = The difference in the absorption behaviour of the different film types above 3 ev is most likely an artefact caused by some interference of the transmission and reflection data on the calculation and no effect of a Burnstein-Moss shift. If a Burnstein-Moss shift would occur, a difference in the behaviour of the transmittance of the films should be observed (Fig. 1). However, this is not the case as well as no difference in the net carrier concentration of the Hall measurements could be observed. In order to determine the direct band gap of the ZnO:Ga film ( h ) 2 was plotted versus h [6]. The intersection of the linear fit with the x-axis led to a value of E g =3.24 ev which is in good accordance with other values given for thin films E g =3.2 ev [7] and somewhat lower than those of bulk material E g =3.35 ev [8]. 4,0x10 10 ( h ) 2 [ev cm -1 ] 2 3,5x ,0x ,5x ,0x ,5x ,0x ,0x10 9 ZnO:Ga 0,0 2,8 3,0 3,2 3,4 3,6 3,8 4,0 h [ev] Fig. 3: ( h ) 2 vs. h for the determination of the band gap As depicted in Fig. 4, the reflectivity of the different ZnO films on the CIGS solar cells is comparable and reduces the power conversion efficiency by approx. 5%. The slightly higher reflectivity of the aluminium doped ZnO in the range between 1000 and 1300 nm has no significant influence on the efficiency, as the band gap of the used absorbers is above 1.1 ev. The illuminated I(V) characteristic under a 1000 W/m 2 solar simulator for one of the higher efficiency devices is shown in Fig. 5. The corresponding GZO film on the glass sample had a sheet resistance in the range of and a thickness of 600 nm resulting in a resistivity of cm. The achieved power conversion efficiencies of up to = 13.6% are in good agreement with the efficiencies reached with aluminium doped ZnO on these substrates. This clearly indicates that GZO has an interesting potential as window layers for chalcopyrite thin film solar cells.

5 current densnity [ma/cm 2 ] reflection [%] Preprint ISCS98, October 1998, Nara, Japan 22,5 20,0 17,5 15,0 12,5 10,0 7,5 5,0 2,5 0,0 Al-doped (200W) Ga-doped (300W) Ga-doped (200W) [nm] Fig. 4: Reflectance of CIGS solar cells with different window layers -0,25 0,00 0,25 0,50 0, solar cell properties I sc = 36.1 ma/cm 2 U oc = 541 mv ff = 69,6 % = 13.6 % ,25 0,00 0,25 0, ,75 voltage U [V] Fig. 5: I-V characteristics of a GZO/CdS/CIGS solar cell (A = 0.5 cm 2 ) 4. Conclusion Highly conductive gallium doped ZnO films were deposited by rf-sputtering from a Ga 2 O 3 (5.7wt%) doped ZnO target on glass substrates. The obtained films had a very low absorption in the visible region of the light and a direct band gap of 3.24 ev. The films exhibited average resistivities between 0.5 and cm in a broad process window for sputter powers varying from 200 to 400 W. The best results were achieved at 200 W with resistivities as low as

6 cm. The sputter rates achieved with the GZO target were around 50% higher than for the ZnO:Al. It is assumed, that the higher relative density of the GZO target compared to the ZnO:Al target is the main reason for the drastic improvement of the sputter rates. The efficiencies of Cu(In,Ga)(S,Se) 2 (CIGS) solar cells prepared with GZO were comparable with results obtained with Al 2 O 3 doped ZnO and led to power conversion efficiencies up to = 13.6%. The main advantage for the use of gallium doped ZnO is the reduction of the sputter time by one third compared to aluminium doped ZnO. These findings make ZnO:Ga an interesting material for solar cell applications. Further investigations have now to be focused on the question whether ZnO:Ga indeed exhibits a better stability versus environmental impacts and whether it can be deposited at a competitive price per W p. Acknowledgement The authors wish to thank Asahi Glass Co. Ltd. and Mr. Kunihiko Adachi for supplying the Ga 2 O 3 doped ZnO target. Special thanks to Dr. Robert R. Gay from Siemens Solar Industries for the supply of the CIGS substrates. This work was supported by the German Ministry of Science and Education BMBF under the contract No which is gratefully acknowledged. References [1] J.R. Tuttle, M.A. Contreras, T.J. Gillespie, K.R. Ramanathan, A.L. Tennant, J. Keane, A.M. Gabor, R. Noufi, Progress in Photovoltaics: Research and Application, Vol 3 (1995) 235 [2] A.E. Delahoy and M. Cherny, Mat. Res. Soc. Symp. Proc. 426 (1996) 467 [3] K. Kushiya, B. Sang, D. Okumura, O. Yamase, Proc. 2 nd Korean-Japan Joint Seminar on Photovoltaics and 7 th Workshop on High-Efficiency Solar Cells and PV Systems, September 22 23, 1998, Cheju, Korea [4] M.Miyazaki, K,Sato, A,Mitsui and N.Nishimura, Journal of Non-Crystalline Solids, 218 (1997) [5] Z. Hecht, 1974, Optics, (New York, Addison-Wesley) [6] J.I. Pankove, 1975, Optical Processes in Semiconductors (New York, Dover Publications) [7] M. Vögt, 1992, Konstanzer Dissertationen (Konstanz, Hartung-Gorre Verlag) [8] S.M. Sze, 1985, Semiconductor devices (New York, John Wiley and Sons)