Reaktives Magnetron Sputtern von CuInS 2 -Schichten: Untersuchung des Schichtwachstums. Stefan Seeger, Klaus Ellmer

Transkript

1 Reaktives Magnetron Sputtern von CuInS 2 Stefan Seeger, Klaus Ellmer

2 Introduction: Thin film solar cells based on CuInS 2 sunlight.6 µm front contact (TCO).1 µm buffer layer CdS 2.5 µm absorber layer CuInS 2 Molybdenum 2 µm float glass 2 µm Float-Glass Mo-Target dcmagnetronsputtering laser scribing sequential process Cu -Target In-target RF/DC - magnetronsputtering magnetronsputtering In Cu magnetronsputtering H 2 S 5 C heater S sulfurization KCN etching step CdS chemical bath deposition mechanical scribing ZnO-Target mechanical scribing Heater -5 C reactive magnetron sputtering 2

3 Why reactive magnetron sputtering of CuInS 2 absorber layers? -large area depostion technique -lower substrate temperature -completly sputtered solar cell -avoiding of KCN etching 3

4 Prospects V OC (V) η (% ) RMS reactive magnetron sputtering RTP rapid thermal process EV evaporation single crystall modules solar cell parameters (at AM1.5) efficiency (η) and open circuit voltage (V oc ) are indepently of the deposition methode in principle reactive sputtering is suited to prepare high quality CuInS 2 layers calendar year 4

5 Reactive sputtering of CuInS 2 Films: Prospects and Problems System parameters: target dimension 4 x 54 x mm³ base pressure: p < -4 Pa gas flow: H 2 S 17 sccm Ar 34 sccm deposition area: 5 x 5 mm² target-to-substrate distance: 18 mm heater: boron nitrid encapsulated graphit meander up to 7 C 5

6 soalr cell efficiency (%) Reproducibility mean efficiency 11.4% Sequential Process max. efficiency macroscopic defects scratches flakes from copper target dust particles Cu In 2 material defects crystal structure (CuAu-ordering) intrinsic material quality 1 nm batch number 1 µm 12 1 mm microscopic defects pin holes morphology (change of Cu/In ratio) 6

7 cause for microscopic defects CuS x Molybdenum microscopic 2 µm defects depend on the copper-to-indium ratio 7

8 Microscopic defects: SEM and IR-Thermography Solar Cell Efficiency: % Solar Cell Efficiency: 1 % good intrinsic material quality but solar cells are shunted due to microscopic defects 8

9 Aim: Preparation of CuInS 2 films with a compact morphology (1). ion assistance: applying a negative substrate bias rf-plasma excitation (27.12 MHz) - lower discharge voltage compared to dc-excitation - charge built-up is avoided (sulfidic target surface) - higher ionization probability (low H 2 S/Ar ratio (5%)) - target-to-substrate distance 9 cm variation of the substrate bias - floating potential (+ V) up to a substrate voltage of -15 V

10 schematic potential distribution potential 2 target voltage target S - 1 e - Ar + Ar (secondary electrons) plasma potential V t =-45 V V pl =+ V 3 S + e - Ar + substrate V fl =- V distance floating potential 1 neutrals Cu, In (< ev) 2 negative ions S -, InS -, InS - 2 (up to 45 ev) 3 positive ions S +, Cu +, In +, In 2+, InS +, InS + 2 (15- ev)

11 Substrate bias: film composition and morphology Cu/In ratio atomic concentration (atoms/cm²) x 17 x 17 x 17 RBS-Spectrometry (Rutherford Backscattering) sulphur indium copper 8 -V sub (V) 1 16 V s =-15 V V s =-25 V V s =-5 V 2 µm 2 µm 2 µm Cu: 1W, In:18W, p=1pa, T=45 C, H 2 S:Ar=5% high substrate bias (more than -5 V) leads to: loss of sulfur loss of indium Cu/In ratio increases up to V s =-5 V mainly CuInS 2 phase (XRD) size of the crystallites increases 11

12 Motivation: Preparation of CuInS 2 films with a compact morphology (2). Cu/In ratio: in situ controll of the deposition rate - deposition monitor - optical methodes - EDXRD (energy dispersive X-ray diffraction)

13 In situ energy dispersive X-ray diffraction (EDXRD) [1,2] Cu In [1] K. Ellmer, R. Mientus, V. Weiß, H. Rossner, Meas. Sci. Technol. 14 (3) 336.

14 in situ energy dispersive X-ray diffraction (EDXRD) fluorescence signal (In Kαβ Cu Kαβ ) diffraction pattern counts 8 6 In 6 S 7 esc esc In 6 S 7 In K a In K b In 6 S 7 3 time resolution: 5 s deposition time (s) photon energy (kev)

15 intensity reactive sputtering of indium sulfide: temperature variation T=375 C InKa InKb T=325 C In 2 S 3 peak area (InKa) rate (a.u.) T ( C) T=325 C 3 5 T=225 C T=25 C T=225 C 15 3 T=25 C 5 deposition time (s) 5 3 photon energy (kev) InS 5 deposition rate depends on the substrate temperature

16 reactive sputtering of copper sulfide: temperature variation intensity CuKa CuKb 3 3 Cu 2 S (14) 3 3 photon energy (kev) T=375 C 5 T=325 C 5 T=225 C 5 T=25 C 5 peak area (Cu Ka) deposition time (s) 5 6 deposition rate depends not on the substrate temperature at a temperature of about 375 C X-ray amorph (liquid)

17 summary reactive Magnetron Sputtering is suitable to prepare thin film solar cells based on CuInS 2 absorber layers to establish a stable CuInS 2 deposition process an in situ-control of the Cu/In ratio and the film morphology seems to be mandatory ion assistance with ion energies up to 5 ev during the films growth effects the film composition 17

18 Acknowledgements The work was financially supported by the AiF Köln / EFDS Dresden We gratefully thank P. Voelz for technical support and J. Klaer, C. Kelch and M. Kirsch for their assistance in preparing solar cells. Thank you for your attention! 18