Photovoltaic effect and cell principles

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Photovoltaic effect and cell principles
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1. Light absorption in materials and excess
carrier generation
Photon energy h? hc/? (h is the Planck
constant) photon momentum ? 0
Light is absorbed in the material.
?(x) is the light intensity
? ?(?) is the absorption coefficient
Absorption is due to interactions with material
particles (electrons and nucleus). If particle
energy before interaction was W1, after photon
absorption is W1 h?

• interactions with the lattice low energy
  photons, results in an
• 
  increase of temperature
• interactions with free electrons - important
  when the carrier
• concentration is high, results also in
  temperature increase
• interactions with bonded electrons- the
  incident light may generate
• some excess carriers (electron/hole pairs)

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Light intensity decreases with the distance x
form the surface
F0 Fin (1 R)
R is the surface reflexivity
is the so-called absorption length
xxL F(xL) 0.38 F0
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Photovoltaic Quantum generator
This process can be realised in different
materials
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Semiconductors
Before interaction with photon (in thermodynamic
equilibrium)
After interaction with photons h? gt Wg
n  n0 ?n , p p0 ?p
np gt ni2
?n, ?p excess carrier concentration
(no thermodynamic equilibrium)
(?n ?p, because electron-hole pairs are
generated )
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Excess carrier generation
conduction band
thermalisation
Wc
bandgap
Wg
Wv
valence band
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Silicon
crystalline
amorphous
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hn (eV)
l (nm)
Carrier generation with respect to solar spectrum
Total generation
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Efficiency of excess carrier generation by solar
energy depens on the semiconductor band gap
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Carrier recombination
t is carrier lifetime
irradiative recombination
Auger recombination
recombination via loca1 centres
Resulting carrier lifetime
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Excess carrier concentration
Diffusion current is connected with carrier
concentration gradient
Dn kTµn/q
Dp kTµp/q
Continuity equations
usually tn tp t
In the dynamic equilibrium
electron diffusion length
hole diffusion length
Excess carrier concentration can be found solving
continuity equations under proper boundary
conditions
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Electrical neutrality is in homogeneous
semiconductor no potencial
difference
To separate excess carrier generated, an
inhomogeneity with a strong internal electric
field must be created
WFn
W
WFp
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Photovoltaic effect and basic solar cell
parameters
To obtain a potential difference that may be used
as a source of electrical energy, an
inhomogeneous structure with internal electric
field is necessary.

• Suitable structures may be
• PN junction

• heterojunction (contact of different materials).

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Principles of solar cell function
In the illuminated area generated excess carriers
diffuse towards the PN junction. The density JFV
is created by carriers collected by the junction
space charge region

• in the N-type region

• in the P-type region

• in the PN junction space charge region

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Illuminated PN junction A
supperposition of photo-generated current andPN
junction (dark) I-V characteristic
I
I
in dark
VOC
irradiation
V
V
IPV
illuminated
ISC
Solar cell I-V chacteristic and its importan
points
VOC
Vmp
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Modelling I-V characteristics of a solar cell
PN junction I-V characteristics
Parallel resistance Rp
Series resistance RS
Aill illuminated cell area A - total cell
area
Output cell voltage V Vj- RsI
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Influence of parasitic resistances (Rs and Rp)
If Rp is high
If
V
V
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Influence of temperature
I (A)
I01
Consequently
V(mV)
For silicon cells the decrease of VOC is about
0.4/K
Pm (W)
Rs increases with increasing temperature Rp
decreases with increasing temperature
Both fill factor and efficiency decrease with
temperature
K-1
At silicon cells
temperature (C)
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Organic semiconductors
Hoping mechanism A1-  A2 -gt  A1  e-  A2 
-gt  A1  A2- 
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P N materials and cells
Perylen pigment (n)
Cu Phtalocyanin (p)
Technological advantages of OSCs

• Wet processing (Ink pad printing)
• Soft cells
• Large surfaces
• Low cost
• Molecular materials

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Photochemical cells
TCO coating
glass
Pt
electrolyte
dye on TiO2 nanocrystals
TCO coating
glass
light
Iodine/iodide redox system 3I- I3-
2e-
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• To maximise current density JPV
• it is necessary
• maximise generation rate G
• minimise losses

losses
electrical
optical
recombination

• reflection
• shadowing
• not absorbed radiation

• emitter region
• base region
• surface

• series resistance
• parallel resistance