Advanced characterization of solar energy materials and novel solar cell concepts

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1 Advanced characterization of solar energy materials and novel solar cell concepts Klaus Magnus Johansen Head of the Micro - and Nanofabrication lab at University of Oslo Norwegian Micro and Nanofabrication Facility - NorFab

2 Outline Introduction to the open access national clean room infrastructure NorFab Processing and characterization possibilities at UiO An overview and examples from the solar cell activity at the UiO Micro and Nanotechnology Laboratory

3 Norwegian Micro- and Nanofabrication Facility NorFab in numbers: Project period: NorFab I: (7 M RCN support) NorFab II: (14 M RCN-support) Ca. 90M total investment cost (60M buildings /30M equipment) 10 M running cost/year 22 engineers 535 users/ user hours 257 instruments 2300m 2 cleanroom area 80 industrial users (41 companies) NTNU NanoLab Norwegian University of Science and Technology SINTEF MiNaLab Horten MST-lab UC Buskerud Vestfold Norway Trondheim Oslo Denmark MiNaLab University of Oslo Sweden Finland

4 Nordic NanoLab Network (NNN) Cooperation of the national cleanroom infrastructures in the Nordic countries on the management level on the expert level on the user level University of Iceland, Reykjavik Finland NTNU NanoLab Norway MiNaLab (SINTEF/UoO) MST-lab HBV Denmark Sweden Ångström MSL Uppsala MC2 NFL Chalmers Electrumlab KTH/Acreo Lund Nano Lab, Lund University Key numbers (2014): Open access to10 cleanrooms in 4 national infrastructures serve over users >1500 tools for micro- and nanofabrication in over m 2 cleanroom area Almost user hours Slide 4

5 Application submitted for ESFRI European Strategy Forum on Research Infrastructures

6 UiO MiNaLab Novel semiconductors Location: Oslo node Area: 440 m 2 Type: R&D, education Staff: ~50 researchers, 3 engineers and 1 Administrator 2nd floor The UiO MiNaLab facility is operated by the LENS center of excellence at UiO. Main competence: Semiconductor physics Ion beam modification and analysis Thin film manufacturing Electrical/chemical/optical defect characterization Electronic devices

7 Material systems Wide bandgap materials: Metal oxides Metal oxynitrides GaN- ZnO Ga 2 O 3 SiC Materials for LEDs, Solar cells, thermoelectrics and power electronics TiO 2 ZnO Si Defect characterization Optical Electrical In 2 O 3 NiO Cu 2 O ZnSb

8 UiO MiNaLab Available equipment Deposition - Magnetron sputtering - E-beam deposition - Thermal deposition - Metal organic vapor phase epitaxy(movpe) - Atomic layer deposition (ALD) - Plasma-enhanced chemical vapor deposition (PECVD) - Epitaxial sputter deposition Processing - Ion implantation/ modification - Optical lithography - Chemical processing - Thermal processing; RTP and furnaces up to 1800 C - Reactive ion etching (RIE) - Cross sectional polisher Characterization - Electrical char. - Temperature dep. Hall effect - Temperature dep. scanning probe microscopy (AFM/SPM) - Spectrophotometer - Solar simulator - Ellipsometry - Fourier transform infrared absorption (FTIR) - High resolution x-ray diffraction (HRXRD) - Chemical characterization - Rutherford backscattering spectroscopy (RBS/C) - Cathodoluminesence

9 Solar cell UiO MiNaLab The Norwegian Research Centre for Solar Cell Technology (Solar United) Research Center for Sustainable Solar Cell Technology (SUSOLTECH) Development of a Hetero-Junction Oxide-Based Solar Cell Device (HeteroSolar) Longer lifetime and higher efficiency of CZTS thin-film solar cells (PV-Life) Novel Semiconducting Alloys in Energy Technology (SALIENT)

10 How does a solar cell work Sunlight shines on a pn-diode (i.e., with an internal electric field) Electron-hole pairs are generated and separated by the field Can be used to set up a current through an external circuit Key limiting factors The fraction of sunlight captured Reduce reflection Reduce transmission Lifetime of the electron/hole Avoid defect recombination Heat loss

11 Solar Cell MiNaLab UiO Commercial solar cell AR-coating Emitter (Si) Absorber (Si)

12 Solar Cell MiNaLab UiO Commercial solar cell AR-coating Emitter (Si) Absorber (Si) Silicon properties Defects

13 Solar Cell MiNaLab UiO Flash lamp annealing, shallow B emittter formation Commercial solar cell AR-coating Emitter (Si) Absorber (Si) Riise et al. Appl. Phys. Lett. 107, (2015) Shallow junction formation Diffusion Silicon properties Defects

14 Solar Cell MiNaLab UiO Commercial solar cell Next generation solar cells AR-coating Emitter (Si) Absorber (Si) TCO Emitter (Si) Absorber (Si) TCO = Transparent conductive oxide TCO Absorber (Si) Replacing AR with TCO TCO as active emitter Shallow junction formation Diffusion Silicon properties Defects

15 Cu 2 O on glass ZnO/Cu 2 O on glass Solar Cell MiNaLab UiO Commercial solar cell Next generation solar cells ZnMgO:Al/Cu 2 O/Au on quartz AR-coating Emitter (Si) Absorber (Si) Tandem TCO cell Emitter (Si) Absorber (Si) TCO Absorber (Si) Tandem cell concepts Replacing AR with TCO TCO as active emitter Shallow junction formation Diffusion Silicon properties Defects

16 Solar Cell MiNaLab UiO Commercial solar cell Next generation solar cells Light conversion/ Multi-carrier generation Novel concepts AR-coating Emitter (Si) Absorber (Si) Tandem TCO cell Emitter (Si) Absorber (Si) Nano TCO Absorber (Si) Tandem cell concepts Replacing AR with TCO TCO as active emitter Shallow junction formation Diffusion Silicon properties Defects

17 «Why bother with solar cells beyond Silicon?» Si has been extremely successful: - Cheap - Reliable - Well studied - Optimized efficiencies mage from: photography by OhWeh

18 There are two main drawbacks with Si Small bandgap Light with energy above ~1 ev is converted to heat 1.34 ev would be optimal for a single pn-junction according to the Shockley Queisser limit The limit for Si based cells is approximately 29 % Efficiency for biofuel < 1% Low absorption Requires relatively thick solar cells to optimize the energy harvest. Typically µm No flexibility

19 Challenges for thin film solar cell materials Cost CIGS (CuInGaS) InGaP / GaAs / Ge / InGaAs Toxicity CdTe Perovskite (Often contains Pb) Reliability Perovskite Organic Dye-sensitized Band gap Efficiency Image from First Solar

20 CZTS (Copper Zinc Tin Sulfide) CIGS current record device is 22.6 % (ZSW) CZTS current record is 13.7% with band gap grading (KIST) Cu(In,Ga)Se 2 (CIGS) Good absorption 1-2 μm thick Cu 2 ZnSnS 4 (CZTS) In- free gives lower cost Indium is a rather expensive metal More research before industrial production Liquid phase non-vacuum deposition methods are very successful cheap production Potential for tandem solar cell with a wide band gap solar cell on top Wide band gap ZnO:Al i-zno CdS Cu 2 ZnSnS 4 Mo Sigbjørn Grini Substrate

21 CdS Cu 2 ZnSnS 4 Band gap ~ 1.5 ev Cu 2 ZnSnS 4 Exchange sulfur with selenium CdS Band gap ~ 2.4 ev Band misalignment CdS Cu 2 ZnSn(S 1-x,Se x ) 4 x 0 Cu 2 ZnSn(S 1-x,Se x ) 4 E g ~ ev Band gap grading CdS E g ~ 2.4 ev x 0

22 Cu 2 ZnSnS 4 Se/S gradient? Se atmosphere Cu 2 ZnSnSe C minutes Cu 2 ZnSnS 4 32 S 80 Se 80 Se N. Ross, J. Larsen, S. Grini, L. Vines, C. Platzer-Björkman, Practical limitations to selenium annealing of compound co-sputtered Cu2ZnSnS4 as a route to achieving sulfur-selenium graded solar cell absorbers, Thin Solid Films, Volume 623, 1 February 2017, Pages , ISSN ,

23 ~1 ev Adapting the solar spectrum Heat Conduction band Photons ca 3 ev Blue/UV-light Valence band The Si-cell We get a current, however, a lot of the energy is lost as heat

24 Per-Anders Hansen, TiO2 and europium (Eu) ~3 ev ~1 ev Photonsplitting is one approach Conduction band Photons ca 3 ev Valence band - Photon splitting material The solar cell material

25 The tandem concept is another approach Cu 2 O ZnO heterojunction Cu 2 O Cheap and reliable Direct band gap of 2.1 ev Well studied ZnO Cheap and reliable Direct band gap of 3.4 ev Well studied Difficult to make n-type Cu 2 O Heterojunction necessary Lattice mismatch/defects at interface p - Cu 2 O n - ZnO Kristin Bergum

26 Literature advances Until 2010, the highest efficiency was ~2% 8.1% Cu 2 O:Na / Zn 0.38 Ge 0.62 O / AZO/MgF 2 Oxidized Cu-metal sheet Reactive magnetron sputtering Minami et al., Appl Phys Express, 9, , 2016

27 Reactive sputter deposition of Cu 2 O Annealing of Cu 2 O films Best solar cells (8%) are from thermally oxidized Cu sheets High crystallinity (mmsized grains) Us: ~ nm Mobility of ~100 cm 2 /Vs Us: <20 cm 2 /Vs Carrier concentration of ~10 13 cm -3 Us: ~10 15 cm Biccari, Francesco, «Defects and doping in Cu 2 O», PhD thesis

28 Annealing of Cu 2 O films Mobility and optical properties Mobility, (cm 2 /Vs) as-dep Annealing temperature, ( C) Combination of a change in both crystallinity and defects Temperature dependent Hall reveals (at least) two electrically active defects v Cu and A still unknown defect

29 GaN ZnO alloys Both have 3.4 ev direct bandgap The alloys are predicted to have a smaller bandgap Of interest for, e.g. Water splitting Solar cells

30 Summary: Solar activity at UiO A modern open access cleanroom lab for materials processing and characterization Activity related to several aspects of the the Si-cell and beyond Transparent conductive oxides, ITO, AZO, GZO Oxide heterojunction cells CZTS-thin film cells Shallow emitter formation Defect characterization of solar grade Si

31 LENS Light and electricity from novel semiconductors

32 The energy bandgap Sunlight can provide the energy to overcome the gap Different semiconductor materials can have different gaps