Fabrication of Copper Indium Sulfide Thin Film Solar

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Stainless Steel Foil: Effect of Aluminum Oxide Barrier Layer Thickness ... novel copper indium sulfide (CIS) indium sulfide (InS) heterojunction thin film solar cells.
Fabrication of Copper Indium Sulfide Thin Film Solar Cells on Flexible Stainless Steel Foil: Effect of Aluminum Oxide Barrier Layer Thickness E. Ugur, E. Aydin, N.D. Sankir Department of Materials Science and Nanotechnology, TOBB University of Economics and Technology, Ankara, Turkey Abstract In this study, novel copper indium sulfide (CIS) indium sulfide (InS) heterojunction thin film solar cells have been fabricated on AISI 430 flexible stainless steel foils by spray pyrolysis technique. In order to prevent the iron (Fe) diffusion into the absorber layer (CIS) from the bulk of stainless steel foils, aluminum oxide (Al2O3) thin films have been deposited on foils by RF sputtering technique. Effects of the diffusion barrier layer thickness on the electrical, structural and the photovoltaic properties have been studied systematically. Electrical resistivity of the films, measured by physical probe, was in the range of 1010 Ω.cm and was independent from the thickness. Amorphous structure of Al2O3 films has been confirmed by XRD technique. CIS absorber layer deposited onto stainless steel foils had about 62 nm crystallite size. To obtain complete solar cell device structure ultrathin novel sandwich structure TCO layers sputtered and Ni/Al grids were evaporated as a top contact. As a general trend, when the alumina thickness increased, catastrophic effects due to Fe diffusion diminished and finally 0.32 % power conversion efficiency was obtained for 1.2 μm-thick Al2O3 barrier layer. The obtained primary results are promising for low cost fabrication of thin film solar cells by wet chemical methods on flexible steel foils. With this study, for the first time in literature CIS thin film solar cells have been fabricated on stainless steel flexible foils by a facile spray pyrolysis technique. Introduction For unconventional applications i.e. space and/or buildings, CIS/CIGS solar cells have been adapted on the flexible substrates. Polyimide and metal foils have been used as flexible substrates and several groups reported the thin film solar cell performance on flexible substrates [1-4]. Stainless steel foils have been gaining attention in thin film solar cell technology due to their thermal and mechanical strength [5]. Moreover, its lightweight, low cost, flexibility and compatibility to roll-to-roll manufacturing makes these foils notable candidates for flexible solar cells. Apart from these advantages, it is well known that Fe and/or other metal ion diffusion through the absorber layer have affected solar cell performance adversely. To prevent the Fe diffusion and for electrical isolation in monolithic integration, dielectric barrier layer deposition before back contact formation becomes essential for steel substrates [5, 6]. Herein, alumina (Al 2O3) is prevalent material since its coefficient of thermal expansion bridges the stainless steel foil (AISI 430) and the molybdenum (Mo) back contact. Materials and Methods 100 μm-thick stainless steel foils (AISI 430) were used as a substrate. Dielectric barrier layer of Al2O3 films were deposited with various thicknesses using VAKSIS Midas Magnetron Sputtering PVD MT/2M2T system. Thickness of Al2O3 films was controlled by adjusting the sputter duration. Mo back contact was deposited onto alumina films via RF sputtering. Sono-Tek FlexiCoat ultrasonic spray pyrolysis system has been used to build CIS absorber layer and indium sulfide (InS) layer with 1% silver doping. It should be noted that before InS deposition, rapid thermal annealing (RTA) was applied to absorber layers under nitrogen atmosphere at 600 °C for 2 min. Transparent conductive oxide layers (i:ZnO and very thin AZO/Ag/AZO sandwich structure) were coated via RF magnetron sputter at room temperature. Finally, to complete the solar cell sturucture, Ni/Al contacts were thermally evaporated. Dektak profilometer has been used to measure the thickness of Al 2O3 films. Structural properties of CIS absorber layers, deposited on steel foils, were determined using Pananalytical X'pert Pro MPD X-ray diffractometer. To investigate the resistivity of alumina films and J-V characteristic of spray pyrolyzed solar celIs, I-V measurements have been done using Keitley 2400 Sourcemeter. For illumination measurements, solar cells were illuminated using Lot-Oriel solar simulator with 150W Xenon short arc lamb mounted. Metal ion diffusion to absorber layer has been determined using ON-TOF ToF-SIMS 5 secondary ion mass spectrometer.

Figure 1. Photograph of CIS absorber layer on steel foil after RTA process

Results/Discussion Electrical properties of RF sputtered Al2O3 films on stainless steel foils were characterized using I-V measurements. Bulk resistivity of Al 2O3 films did not vary drastically with the film thicknesses (Table 1). Hence, with lower thicknesses, Al2O3 barrier layers could be suitable for monolithic integration since all Al2O3 films were highly resistive [6]. Sample

Al2 O3 Thickness (nm)

ρ (Ω.cm)

S1 S2 S3

430 770 1200

5.85 x 10 10 6.61 x 10 10 9.50 x 10 10

Table 1. Thickness and volume resis tivity values of RF sputtered Al2O3 films

It is well known that room temperature Al2O3 fabrication results with amorphous alumina structure. Additionally, amorphous characteristic improves the performance of diffusion barriers [7]. XRD measurements were performed to clarify the crystallographic structure of the barrier layer and absorber layers. As shown in Fig. 2., the Al2O3 barrier layer had an amorphous structure. CIS layer showed preferential crystal directions with two major peaks around 27.9° (2θ) and 46.3° (2θ). Crystallite size has been calculated as 62 nm for CIS absorber layer deposited onto steel foils. J-V measurements have been done to indentify the role of the barrier layers in the spray pyrolyzed solar cell performances. No cell performance has been obtained for the configuration having 430 nm-thick alumina film probably due to the severe Fe diffusion and/or the stress based formed cracks. On the other hand, increasing the barrier layer thickness reduced impurities in absorber and leads to increase in efficiency. Short circuit current density also (Jsc ) increased from 1.65 to 6.50 mA/cm 2 with increasing the thickness of Al2O3 barrier layer.

Figure 2. XRD peaks of CuInS2 absorbers on steel foil after RTA process

Table 2. Cell parameters of CIS solar cells with various barrier layer thicknesses

Sample

Jsc (mA/cm 2)

Voc (V)

Fill Factor

Efficienc y (%)

S1 S2 S3

1.65 6.50

0.30 0.20

0.26 0.33

0.10 0.32

Figure 3. Photograph of CIS solar cells on stainless steel foil

References [1] Otte K., L. Makhova L., Braun A., Konovalov I., Flexible Cu(In,Ga)Se2 thin-film solar cells for space application, Thin Solid Films 511 – 512 (2006) 613 – 622. [2] Chirila A., Buecheler A., Pianezzi F., Bloesch P., Gretener C., Uhl A. R., Fella C., L. Kranz L., Perrenoud J., Seyrling S., Verma R., Nishiw aki S., Romanyuk Y. E., Bilger G., Tiw ari A. N., Highly efficient Cu(In,Ga)Se2 solar cells grown on flexible polymer films, Nature Materails 10 (2011) 857-861. [3] Brémaud D., Rudmann D., Kaelin M., Ernits K., Bilger G., Döbeli M., Zogg H., Tiw ari A.N., Flexible Cu(In,Ga)Se2 on Al foils and the effects of Al during chemical bath deposition, Thin Solid Films 515 (2007) 5857–5861. [4] Wuerz R., Eicke A., Frankenfeld M., Kessler F., Pow alla M., Rogin P., Yazdani-Assl O., CIGS thin-film solar cells on steel substrates, Thin Solid Films 517 (2009) 2415–2418. [5] Dow on B., Sehan K., Joonjae O., Joow on L., Wookyoung K., Fabrication of High Efficiency Flexible CIGS Solar Cell with ZnO Diffusion Barrier on Stainless Steel Substrate, Mater. Res. Soc. Symp. Proc. 1324 (2011) 115-120. [6] Moriw aki K., Nomoto M., Yuuya S., Murakami N., Ohgoh T., Yamane K., Ishizuka S., Niki S., Monolithically integrated flexible Cu(In,Ga)Se2 solar cells and submodules using newly developed structure metal foil substrate with a dielectric layer , Solar Energy Materials & Solar Cells 112 (2013) 106–111.

Corre sponding Author: N. Demirci Sankır, Department of Mat erials Science and Nanot echnology, TOBB University of Economics and Technology, Sogutozu Caddesi, No:43, 06560 Ank ara, Turk ey, +90 312 292 4332, [email protected]