Light Trapping by Porous Silicon

Transcription

1 Light Trapping by Porous Silicon Sean Erik Foss Josefine Helene Selj, Arve Holt, Erik Stensrud Marstein IFE 2nd Nordic Workshop on Crystalline Silicon Solar Cells March 5.-6., Narvik, Norway

2 Overview Why the need for light trapping? What are the state-of-the-art technologies? Why porous silicon (PSi)? What is PSi? How can PSi help? Challenges Passivation of PSi Incorporating electrochemical processes in fabrication The future IFE

3 The need for light trapping Bare, polished silicon: >30% reflectance loss of current Back-side: Keep light in Front-side: Let light pass & keep light in Thin film solar cells: light trapping critical avoid light lost by transmission & reflection 25 µm thick wafer 200 µm thick wafer Effect of absorption in silicon wafer on different wavelengths

4 The need for light trapping Texturing reduces reflectance by introducing multiple reflections increases absorption by increasing the effective thickness

5 The need for light trapping Antireflection coating (ARC)...uses interference effect to minimize reflected light Single-, double-, or simple multilayer difficult/expensive ~λ/(n 4) impractical Graded index: best performance for a given thickness

6 Status on light trapping Alkaline etch texturing: NaOH or KOH gives anisotropic etch not good for multicrystalline Si Acidic etch texturing: HF/HNO3/dilutant gives isotropic etch currently being used in industry SiN ARC: optimal refractive index, low absorption, passivating properties Thin Conducting Oxide ARC: conductive, low absorption Photolithographically defined inverse pyramids (best texture) A. Holt, E. Marstein, IFE Parretta et al., Optics Communications 172, 1-6, (1995)

7 Why porous silicon? Texturization & damage removal Concurrent etching of saw-damage and texturization Cheap: room temperature and no vacuum Antireflection coating & light diffusion Pore size << λ n eff porosity single or multilayer interference filters/arcs can be fabricated PSi/substrate interface have an intrinsic roughness + scattering in PSi layer diffusion of light into the solar cell Cheap: room temperature and no vacuum PSi is unique, no (or few) alternatives for: grading, light diffusion...

8 What is porous silicon? Porosified single crystal silicon by electrochemical etching in HF electrolyte Pores most often columnar and parallell, but other morphologies can be obtained Pores & walls 2 nm (microporous) 10 µm (macroporous), dependent on formation parameters J.P.Zheng et al., Electrochem. and Solid-State Lett., 3, (2000)

9 What is porous silicon? Variable parameters: Electrolyte contents Current density Temperature Crystal orientation Doping type and concentration Time Chemical etching w/o current possible: HF/HNO 3 HNO 3 oxidizes, HF removes oxide Stain etching

10 How can Porous Silicon be used? Refractive index control conventional ARC graded ARC backside reflector Texturization Macropores for macro scattering/trapping of light similar to KOH, acidic or plasma texture Light diffusor Micropores may give efficient scattering PS/substrate interface roughness Within certain pore structures Advanced ideas

11 PSi thin film concept Incorporating PSi into fabrication process of thin cell: Layer transfer with PSi Epitaxial growth of Si thin film on top of PSi etched into surface of low quality Si substrate w/annealing in H containing atmosphere PSi used for easy separation of epi-thin film from substrate PSi can have light trapping function also A.Wolf et al., Prog. Photovolt: Res. Appl. 15, (2007)

12 Anti-Reflection Coating Conventional ARC & selective emitter Etching done after contact formation Degrading of contacts avoidable Some absorption + nonoptimal surface passivation Efficiency on multicrystalline cells: ~14%

13 Anti-Reflection Coating Graded index ARC Example of refractive index profile of graded index ARC W.H.Southwell, Optics Letters 8, 584 (1983) Experimental results of SiN ARC and porous silicon ARC made at IFE/UiO

14 Anti-Reflection Coating Back-side reflector Multilayer possible As good reflection as possible over a wide spectrum I. Kuzma-Filipek et al., Phys. Stat. Sol. (a) 204, (2007) Foss, UiO

15 Texturization Isotropic texture especially beneficial for multi-crystalline silicon Texture independent of grain orientation 8.5 % weighed average reflectivity nm w/o ARC R. Tena-Zaera, Phys. Stat. Sol. (a) 204, (2007)

16 Light diffusor Mesoporous silicon: pore size roughly 2-50 nm Lambertian diffusor: equal scattering to all angles Depends on thickness of layer 160 nm gives good scattering Total randomization of entrance angle effective angle of 60 inside Si Scattering also by macro pores (100 nm 10 µm) and by voids in H-annealed PSi L. Stalmans et al., Prog. Photovolt. Res. Appl. 6, (1998)

17 Advanced PSi 2D & 3D Photonic crystals Optical band gaps in IR shown Use for improving cell More on reflectance Photonic Crystals back-side & scattering Need to make similar structures with pore sizes ~λ Not tested for solar cells later by Jo Gjessing V. Kochergin, H. Föll, Mat. Sci. and Eng. R 52, (2006)

18 Advanced PSi Diffraction gratings Post etching of PSi with interfering laser beams Control of angle of incident light Need to make grating period smaller for visible light Not been used for solar cells (yet) G. Lérondel et al., Appl. Phys. Lett. 71, (1997) S.Setzu et al., Appl. Surf. Sci. 186, (2002)

19 The passivation challenge Hard to passivate PSi due to very large surface area: <600 m 2 /cm 3 On the clean silicon surface, dangling bonds from Si atoms at the surface will attract charge carriers and function as recombination centers, covering the surface with Oxygen results in a electronically passivated surface, surface states, ie dangling bonds are removed. Dry, high temperature oxidation considered best electronic surface passivation on polished samples. Partly or fully high temp. oxidation of PSi also possible giving good results loose advantage of PSi as low temp. process + changes optical properties of layer

20 The passivation challenge Rapid thermal oxidation may be used, still high temp. but fast low thermal budget, may change optical properties less Other surface covering techniques may be used to deposit/form: SiO, SiN, SiC, and other materials, although good passivation has not been shown Ozone passivation at room temperature has shown indications of good passivation, only measured by photoluminescence stability Light induced passivation: charge generated during irradiation of UV light has resulted in good passivation with a PSi layer, dependent on PSi morphology As prepared mesoporous PSi gives a certain passivation, not enough for highly efficent solar cells. This is probably due to difference in bandgap (quantum confinement) and/or fixed charges in the structure Pore filling with material containing fixed charges resulting in field passivation

21 The fabrication challenge Anodic etching requires single (or parallel) wafer setup + potentially large currents Alternative solutions: Stain-etched PSi (acidic texture) is compatible with standard batch processing, and is used today, but does not give all the benfits of anodically etched PSi Light may be used more creatively to control the etching process, removing the need for externally applied voltage Pulsed current etching

22 The Future Within the newly started program (ISP): Thin and highly efficient silicon-based solar cells incorporating nanostructures at IFE, one PhD student (Josefine Helene Selj) + researchers will work on integration of PSi technology in thin Si solar cells New equipment!

23 References Scattering enhanced absorption: T.J. Rinke, R.B. Bergmann, J.H. Werner, Quasi-monocrystalline silicon for thinfilmdevices, Appl. Phys. A 68, (1999) First use of PSi (oxidised) for ARC: Prasad A., Balakrishnan S., Jain S.K., Jain G.C., Porous Silicon-Oxide Antireflection Coating for Solar-Cells, J. Electrochem. Soc. 129, 3, (1982) Perfect cell with 0-refl., totally randomizing front + totally reflecting back: T. Tiedje, E. Yablonovitch, G. D. Cody and B. G. Brooks, Limiting Efficiency of Si Solar Cells, IEEE Trans. Electron. Devices ED-31, (1984) Experimental indication and discussion of scattering from meso-ps/substrate interface: L. Stalmans, J. Poortmans, H. Bender, M. Caymax, K. Said, E. Vazsonyi, J. Nijs and R. Mertens, Porous Silicon in Crystalline Silicon Solar Cells: A Review and the Effect on the Internal Quantum Efficiency, Prog. Photovolt. Res. Appl. 6, (1998) Passivating by Ozone treatment: S.Y. Chen, Y.H. Huang, H.K. Lai, C. Li, J.Y. Wang, Investigation of passivation of porous silicon at room temperature, Solid State Communications 142, (2007) Light induced negative charge field passivation in PSi: O. Nichiporuk, A. Kaminski, M. Lemiti, A. Fave, S. Litvinenko, V. Skryshevsky, Passivation of the surface of rear contact solar cells by porous silicon, Thin Solid Films , (2006) Diffraction grating in PSi: G. Lérondel, R. Romestain, J. C. Vial, and M. Thönissen, Porous silicon lateral superlattices, Appl. Phys. Lett. 71, (1997) 3D Photonic crystals in macroporous Si: V. Kochergin, H. Föll, Novel optical elements made from porous Si, Mat. Sci. and Eng. R 52, (2006) 2D Diffraction grating in PSi: S.Setzu, P.Ferrand, G.Lérondel, R.Romestain, Photo-lithography for 2D optical microstructures in porous silicon: application to nucleation of macropores, Appl. Surf. Sci. 186, (2002) PSi Epi-thin film w/backreflector: I. Kuzma-Filipek, F. Duerinckx, K. Van Nieuwenhuysen, G. Beaucarne, J. Poortmans, and R. Mertens, Porous silicon as an internal reflector in thin epitaxial solar cells, Phys. Stat. Sol. (a) 204, (2007) Thin film PSi cell review: L. Stalmans, J. Poortmans, H. Bender, M. Caymax, K. Said, E. Vazsonyi, J. Nijs, and R. Mertens, Porous Silicon in Crystalline Silicon Solar Cells: A Review and the Effect on the Internal Quantum Efficiency, Prog. Photovolt. Res. Appl. 6, (1998)

24 More information? -> Solenergi Thank you for your attention!