Physics of monocrystalline solar cells

1. MONOCRYSTALLINE
SOLAR CELLS
Ms.S.Gomathy M.E.,M.B.A
AP(SrG)
KEC/EEE
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S.Gomathy M.E.,M.B.A

2. PRINCIPLES OF CELL DESIGN
• Most common solar cell design is a monocrystalline p-n junction
• p-n homojunction have advantage over heterojunction designs – there is
no material interface at the junction, so losses due to interface state can
be avoided
• Efficient PV energy conversion requires – efficient light absorption,
efficient charge separation, efficient charge transport
• Good optical absorption – optical depth of device = high – reflectivity of
the surface = small
• Good charge separation - Vbi = large – charge recombination in junction
= slow
• Efficient carrier transport - Rs, Rsh = high – to avoid leakage of carriers
back across the junction – minority carrier lifetime (Zn,Zp) and diffusion
lengths (Ln,Lp) = long – surface recombination velocities (Sn,Sp) = small
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S.Gomathy M.E.,M.B.A

3. MATERIALAND DESIGN ISSUES
• For theoretical conversion efficiency in AM 1.5 – efficiency >= 30% =
band gap lie in the range 1-1.6 eV - efficiency > 20% = range 0.7-2 eV
• Diffusion lengths – long compared to absorption depth – this elements
the range of material can be used
• Among other materials, only silicon has a suitable band gap
• Germanium and Selenium – band gaps too small : microcrystalline –
grain size - smaller
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S.Gomathy M.E.,M.B.A

4. DESIGN FACTORS
• Junction polarity
• Junction depth
• Doping levels
• Doping gradients
• Cell thickness
• Surface treatments
• Contact design
• Cell design depend – optical properties of the material
• Weakly absorbing material like silicon – maximise the absorption =
important – recombination at rear surface = avoided
• Highly absorbing Gallium arsenide – minimize recombinaton near the
front surface and around the junction
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S.Gomathy M.E.,M.B.A

5. GENERAL DESIGN FEATURES OF
p-n JUNCTION CELLS
• Design features apply to p-n homojunction cells independent of the
material
• Efficient light absorption – thickness high
• Junction should be shallow compared to – diffusion length and
absorption length – to avoid dead layer
• Emitter - doped heavily – to improve conductivity on the front of the
cell
Base – doped lightly – improves collection in the neutral base region
• Reflection of light = minimised
▪ crystalline semiconductor – materials treated with anti-reflection
(AR) coat
▪ AR coat material and thickness are chosen to maximise photocurrent
generation for the relevant incident spectrum.
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S.Gomathy M.E.,M.B.A

6. SILICON MATERIAL
PROPERTIES
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S.Gomathy M.E.,M.B.A

7. BAND STRUCTURE AND OPTICAL
ABSORPTION
• Silicon
group IV element
tetrahedral crystal structure
fundamental band gap = 1.1eV – reduces optical absorption
optical absorption occurs when photon energies above 3eV – not
useful for photovoltaics
Refractive index = 3.4
Natural reflectivity = 40%
Single or multilayer AR coat reduces this to less than 5%
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S.Gomathy M.E.,M.B.A

8. DOPING
• n type – pentavalent phosphorus impurity atoms
• p type – trivalent boron
• Addition of impurity atoms – degrades the material quality
• Higher doping level needed in emitter – to reduce series resistance,
increase Vbi, increases Voc
• In silicon, doping is limited by the shrinkage of band gap
• Voc cannot exceed 81% of Eg
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S.Gomathy M.E.,M.B.A

9. RECOMBINATION
• Effect of recombination in the depletion and emitter layer is low – so
neglected
• This leaves e- recombination in the p region as the dominant volume
recombination process
• This includes radiative, Auger, trap-assisted mechanisms
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S.Gomathy M.E.,M.B.A

10. Net recombination rate U
U = Urad + UAug + USRH
Urad = (n-n0)/(τn,rad)
τn,rad = radiative lifetime
UAuger = An . Na
2 (n-n0)
For Shockley Read Hall recombination,
USRH = (n-n0)/(τn,SRH)
τn,SRH = minority carrier lifetime
τn,SRH = 1/(Bn . Nt)
τn,SRH decreases, doping increases
U = (n-n0)/ τn
τn = e
- lifetime
1/ τn = (1/ τSRH) + (1/ τrad) + (1/ τAug)
Lightly doped n type material = shorter = 1 μs
Heavily doped = Auger dominates
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S.Gomathy M.E.,M.B.A

11. CARRIER TRANSPORT
• e- mobility in p type silicon – higher than the hole mobility in n type
silicon and e- diffusion length – longer
• Fundamental impurities in silicon are of acceptor type and important in n
type than p type material
• e- and hole mobilities are determined by the frequency of scattering events
within the conduction or valence band.
• Minority carrier diffusion lengths – L = square root(τ D)
Z = net recombination
Zn = in μs
Ln = 100 μm
• Resistivity – depends upon the level of doping
ɸ = (1/ σ ) = 1/ q(μn . n + μp . p)
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S.Gomathy M.E.,M.B.A

12. • Emitter is highly doped relative to base?
Series resistance effects can be more serious in emitter, where the
funnelling of current from a large semiconductor area into a very small
contact area creates high current densities
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S.Gomathy M.E.,M.B.A