Dr. Joan Haysom Solar Project Manager SUNLAB Solar Research Group School of Electrical Eng. and Comp. Science University of Ottawa 1
1. Semiconductor Materials 2. Single junction and 3. Multi- junction solar cells 4. Epitaxial Growth 5. Market Applications of III/Vs 6. Advanced III/V cells 1 st Canadian PV Grad School May 16-18 th , 2012, University of Ottawa, Slide 2
1 st Canadian PV Grad School May 16-18 th , 2012, University of Ottawa, Slide 3
VI III/V II/VI 1 st Canadian PV Grad School May 16-18 th , 2012, University of Ottawa, Slide 4
e- h+ Ga Si As Ga Si Si fermi level E g As Ga As Si Si Si � Covalent bonding between group IV (e.g. Si) atoms or group III+V atoms (e.g. Ga + As) results in octets of shared electrons and a filled valence band � bonds can be broken without too much energy, promoting a free electron into the empty conduction energy band and leaving a free hole in the valence band � Temperature, dopants, photon absorption all ↑ ↑ # free carriers ↑ ↑ Figures adapted from PVDCROM 1 st Canadian PV Grad School May 16-18 th , 2012, University of Ottawa, Slide 5
n-type doping shown Ga Si As Ga Si Si fermi level E g e- As Si As Si Si Si � Dopants – provide an extra electron or hole carrier, shifts fermi level � n-dopants � Si (IV) replacing Ga (III) resulting in extra valence electron � Sn (IV) also used � p-dopants � C (IV) replacing As (IV) resulting in missing valence electron � Zn or Be (II) replacing Ga(III) also used Figures adapted from PVDCROM 1 st Canadian PV Grad School May 16-18 th , 2012, University of Ottawa, Slide 6
1 st Canadian PV Grad School May 16-18 th , 2012, University of Ottawa, Slide 7
1 st Canadian PV Grad School May 16-18 th , 2012, University of Ottawa, Slide 8
Energy Band Diagram of a Semiconductor thermalization P out = I x V Conduction Band (empty excited state) photon E g Valence Band (full thermalization of electrons) I is set by # photons absorbed (can only absorb above the bandgap) V max is set by bandgap of the material (E g /q) 1 st Canadian PV Grad School May 16-18 th , 2012, University of Ottawa, Slide 9
Device requires a p-n junction for separating electron from hole, and thus extracting current Energy Band Diagram at equilibrium photons J sc E ip J sc e- Energy � e- E f h+ E in h+ Distance from bottom � http://en.wikipedia.org/wiki/File:Pn_junction_equilibrium.svg 1 st Canadian PV Grad School May 16-18 th , 2012, University of Ottawa, Slide 10
Decreasing Eg -----> Source: Renewable and Efficient Electric Power Systems, Gilbert Masters, Wiley Interscience, 2004. 1 st Canadian PV Grad School May 16-18 th , 2012, University of Ottawa, Slide 11
� Spectrum losses of 49% � 20% below bandgap of Si � above-bandgap photons lose on average 30% of their energy as heat � Recombination � Need high quality crystals for long carrier diffusion lengths � Lower hole mobility than electron mobility in Si, 10% ↓ ↓ from ↓ ↓ � Max theoretical efficiency theoretical of Si is 33% � Blackbody Radiation � Warm cell re-radiates ~ 7% of total incoming http://en.wikipedia.org/wiki/Shockley-Queisser_limit 1 st Canadian PV Grad School May 16-18 th , 2012, University of Ottawa, Slide 12
V ∝ E g • The maximum efficiency single junction cell is with a material I ∝ 1/E g with a band gap of ~1.4 eV (=GaAs) • Theory 33.7% efficient 1.4eV • 1.1eV Record 28% achieved by Alta. GaAs Si • Production 23%, thin cell with substrate reuse • Additionally, GaAs has the following advantages over Si: • Si is cheap and abundant with a • Direct bandgap band gap in a good range (1.1 Included in η eV), and with efficiencies of: • high carrier mobilities • Theory 30% • Radiation hardness • Record 24% • Lower temperature coefficient • Commercial ~ 15-20% of bandgap shift http://en.wikipedia.org/wiki/Shockley-Queisser_limit 1 st Canadian PV Grad School May 16-18 th , 2012, University of Ottawa, Slide 13
E g =1.1 eV E g =1.4 eV � In energy (E) versus crystal momentum (k) space, the lowest point in the conduction band does not align with the top point of the valence band � To conserve momentum, a lattice vibration (phonon) is required, resulting it a three particle process, which is less efficient and blurs the turn-on around the bandgap http://physicsarchives.com/index.php/courses/967 & J. Nelson Physics of Solar Cells, Imperial College Press, 2003 1 st Canadian PV Grad School May 16-18 th , 2012, University of Ottawa, Slide 14
1 st Canadian PV Grad School May 16-18 th , 2012, University of Ottawa, Slide 15
Decreasing Eg -----> Source: Renewable and Efficient Electric Power Systems, Gilbert Masters, Wiley Interscience, 2004. 1 st Canadian PV Grad School May 16-18 th , 2012, University of Ottawa, Slide 16
1.6 1.4 AM1.5D, 900 W/m 2 Radiance (a.u.) 1.2 1.0 0.8 0.6 0.4 0.2 0.0 280 780 1280 1780 2280 2780 Wavelength (nm) • Bandgaps of each subcell’s III-V semiconductors can be selected for improved absorption achieving a wider spectral response for improved η 1 st Canadian PV Grad School May 16-18 th , 2012, University of Ottawa, Slide 17
( I 1 , V 1 ) = + + V V V V 1 2 3 tot ( I 2 , V 2 ) = = I I I 1 2 3 ( I 3 , V 3 ) � Three subcells in series => voltages add, which is great! � Each subcell generates different current determined by its bandgap and the solar spectrum, BUT current through series connected stack is limited by weakest performer => design challenge 1 st Canadian PV Grad School May 16-18 th , 2012, University of Ottawa, Slide 18
Band Structure including TJ Band Structure including TJ n+ n J sc p p+ n+ n p p+ n n p Because the length of the depletion Because the length of the depletion region is narrow and very high region is narrow and very high doping is used, carriers can tunnel. doping is used, carriers can tunnel. 1 st Canadian PV Grad School May 16-18 th , 2012, University of Ottawa, Slide 19
1 st Canadian PV Grad School May 16-18 th , 2012, University of Ottawa, Slide 20
Epitaxy: deposition of a crystalline overlayer on a crystalline substrate. The overlayer is called an epitaxial film or epitaxial layer. comes from the Greek roots epi , meaning "above", and taxis , meaning "in ordered manner“ Substrate acts as a seed crystal – it locks down the lattice constant and lattice structure Homoepitaxy – same alloy as substrate Heteroepitaxy – different alloy grown above 1 st Canadian PV Grad School May 16-18 th , 2012, University of Ottawa, Slide 21
(Exaggerated step heights) Substrates are polished to smooth finish, and must be clean and oxide free – “epi ready”. Yet, some impurites at initial interface => buffer • Layer by layer growth on perfectly layer smooth surface might be preferred, but Typical Ge substrates small imperfections in crystal lead to used are miscut ~6 o => preferred locations for nucleation. Provide sufficient # • Which growth mode depends on steps for Step-flow • Substrate Temperature growth • Substrate surface morphology • Saturation of incoming elements 1 st Canadian PV Grad School May 16-18 th , 2012, University of Ottawa, Slide 22
Zinc blende crystal structure Ge and GaAs have same lattice constant, Ge substrates are much cheaper and provide low λ absorption 1 st Canadian PV Grad School May 16-18 th , 2012, University of Ottawa, Slide 23
A. Metal Organic Chemical Vapour Deposition (MOCVD) B. Molecular Beam Epitaxy (MBE) C. Chemical Beam Epitaxy (CBE) � Also called MO-MBE…a blend of the two � (not covered here) Veeco, 1 st Canadian PV Grad School May 16-18 th , 2012, University of Ottawa, Slide 24
� High vacuum 10 -6 torr � Solid sources heated in Knudsen cell deliver direct beams of elemental molecules/atoms sent towards substrate � High vac allows in-situ monitoring: RHEED (reflection high energy electron diffraction) � Can also have Gas Source MBE for group-Vs (in particular P) 1 st Canadian PV Grad School May 16-18 th , 2012, University of Ottawa, Slide 25
� substrate ~ 400 o C (lower than MOCVD for good sticking of atoms) � Atoms adsorb and diffuse along surface � Find lowest energy positions along step edges � Atoms naturally arrange themselves into ordered crystal structure Veeco, http://www.veeco.com/markets/materials-science/molecular-beam-epitaxy.aspx 1 st Canadian PV Grad School May 16-18 th , 2012, University of Ottawa, Slide 26
� Very high purity possible at low growth rates � Great for Al containing materials (low O environment) � Very good control, monitoring, and modeling, of growth � Qdots developed on MBE � Great for research � Has some in-roads in production too Gen III Veeco MBE at U. Delaware 1 st Canadian PV Grad School May 16-18 th , 2012, University of Ottawa, Slide 27
� Low vacuum 100-10 -2 torr, substrate heated to ~600-750 o C to enable chemical reactions � III and V sources fed as gases (mixed with carrier gas H 2 or N 2 ) into chamber, react on or in vicinity of substrate’s surface � High gas flows, high growth rates, dynamic gas mixing effects � MO by-products pumped to scrubbers and exhaust http://www.tu-ilmenau.de/pv/forschung/ 1 st Canadian PV Grad School May 16-18 th , 2012, University of Ottawa, Slide 28
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