THIRD GENERATION PV: TANDEM JUNCTION

THIRD GENERATION PV: TANDEM JUNCTION
MSE COURSE: MATERIALS FOR ENERGY AND STORAGE
©2015 Onwukwe E.A, Cooper K.L, Numfor, L.B
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© 2010 Pearson Education South Asia Pte Ltd
Physical Chemistry 2nd Edition
Chapter 16: The Particle in the Box and the Real WorldTHIRD GENERATION PV: TANDEM JUNCTION
“The fuel in the earth will be exhausted in a thousand or more years, and
its mineral wealth, but man will find substitutes for these in the winds, the
waves, the sun's heat, and so forth.” – John Burroughs (1916)
ONWUKWE, EMMANUELA. ID#: 40399 STREAM: MSc. PHY
COOPER, KOLLIE LACKAY ID#: 40447 STREAM: MSc. MSE
NUMFOR, LINDA BIH ID#: 40416 STREAM: MSc. MSE
COURSE INSTRUCTOR: PROF. ESIDOR NTSOENZOK
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OUTLINE
INTRODUCTION VARIOUS APPROACHES
DESIGN OF EFFICIENT
TANDEM
PROSPECTS AND
CONCLUSION
 Motivation
 Aim and Objectives
 3rd Generation PV : Goal
Fundamentals, description
 Multi junction (Tandems)
 Multiple carrier excitation
 Hot Carriers
 Other Approaches
 Efficiency evaluation
 Design Consideration
Design constraints
 Material Selection
 Final design and evaluation
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INTRODUCTION
MOTIVATION
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The energy system at the beginning of the 21st century is
characterized by over six billion people that live on the
Earth with a total energy consumption of approximately 13
TW.
Electrical energy which represents about 12% of all energy
consumed worldwide is widely used for industrial and
household applications though some 2 billion people do not
have access to electricity.

Fig 1.0 An overview of today’s energy sources
Source: Horazak, D.A , Brushwood, J.S (2010)
Fig 1.1 Electricity generation and consumption in
Today’s energy system
Source: Horazak, D.A , Brushwood, J.S (2010)
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The direct conversion of solar radiation into electricity is often described as
a photovoltaic (PV) energy conversion because it is based on the
photovoltaic effect - the generation of a potential difference at the junction
of two different materials in response to visible or other radiation.

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WHY THE LOW PERCENTAGE OF 0.01% IN PV CONSUMPTION?
• Relative High Efficiency though less than Shockley-
Quiesser limit of 33.7%
• Extremely high cost dominated by material costs
of silicon wafer, glass cover sheet and encapsulant
• significantly decreased material cost but
compromised efficiency badly <15%
Calculations based on thermodynamics demonstrate that the limit on the
conversion efficiency of sunlight to electricity is 93% as opposed to the
upper limit of 33% for a single junction solar cell, such as a silicon wafer
and most present thin-film solar cells – hence need for 3rd generation.
Conversion efficiency has to be
increased substantially in order to
progress further.

AIM AND OBJECTIVES
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AIM: To design and present an efficient third generation
PV namely a tandem junction
THROUGH THE FOLLOWING OBJECTIVES
(1) A detailed discussion of the goal, fundamental and
description of the third generation photovoltaics
(2) A detailed consideration of the various approaches
used to achieve third generation PV while paying
attention to their description, and advantages,
disadvantages .
(3) Achieve an optimal design by relevant design
considerations and implementation.

3RD GENERATION PV
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The main goal of the third generation materials is the
improvement of poor electrical performances of second
generation materials while keeping low production costs
with clear targets of 30-60% efficiency.
Statistics indicate that costs are targeted to decrease to
well below the $1/W level of second-generation PVs to
$0.50/W, potentially to $0.20/W or better, by significantly
increasing efficiencies but maintaining the economic and
environmental cost advantages of thin-film deposition
techniques

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Increasing efficiency strongly
leverages lower costs because
the smaller area required for a
given power also reduces
balance-of-systems costs, such
that efficiency values well above
30% could dramatically decrease
these costs per Watt.
The fundamental process is to
circumvent the Shockley-Queisser
limit for single-bandgap devices
through multiple energy
thresholds
Fig. 1.3 Efficiency and cost projections for first-
(I), second- (II), and third- generation (III) PV
technologies
Source: Conibeer, G (2007)

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The fundamental description of the third generation is built by
identifying the loss mechanisms in a standard solar cell and seeking
out ways to reduce them
(1) Non absorption of below
band gap photons
(2) Lattice thermalization loss
(3) Junction and contact voltage
loss
(4) Junction and contact voltage
loss
(5) Recombination loss
Fig. 1.3 Loss Mechanism in standard Solar cells

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VARIOUS APPROACHES
MULTIJUNCTIONS (TANDEMS)
Decreasing
Bandgap
Suggested by Jackson in 1955 approach
increases energy level by stacking of P-
N junctions in different semiconductor
materials
Fig. 2.1 Schematic of Tandem Cells
Highest tandem efficiency is
by optimizing power in each
cell through appropriate
choice of:
MATERIAL SYSTEM
• bandgap
•Thickness
•Junction depth
•Doping characteristics
CONFIGURATION

Fig. 2.2 Configurations for tandems Source: Arizona State university series (2009)
MATERIAL SYSTEM
III-V ALLOYS
THIN FILMS
a-Si
Si Nanostructure
CONFIGURATION
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CONCENTRATED PV
ADVANTAGES
(1) Ultra-high efficiencies are often in concentrators
(2) the voltage-to-current ratio increases, thus decreasing resistive
losses in the high current densities of concentrator cells
DISADVANTAGES
(1) They make the overall more complex and less modular.
(2) They require direct sunlight, hence do not work with overcast sky.

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MULTIPLE CARRIER EXCITATION
Carriers generated from high-energy photons (at least twice the
bandgap energy) absorbed in a semiconductor can undergo
impact ionization events resulting in two or more carriers close to
the bandgap energy
A device based on this approach requires a means of allowing the
multiple electron-hole pairs to be separated, transported, and
collected in a bulk structure. This is the subject of ongoing research

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HOT CARRIERS
The final option for increasing efficiencies is to allow absorption of a
wide range of photon energies but then to collect the
photogenerated carriers before they have a chance to thermalize
The concept underlying hot carrier solar cells is to slow the rate of
photoexcited carrier cooling, which is caused by phonon
interaction in the lattice, to allow time for carriers to be collected
while they are still at elevated energies (‘hot’).

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OTHER APPROACHES
QUANTUM ANTENNA
The idea of a quantum antenna is to use the wave nature of light
rather than its particle nature. In-coming light waves oscillate
electrons in an antenna tuned to the wavelength of light – hence
the devices have to be of the order of a few hundred nanometers.
Maximum achievable efficiency is 48%
THERMOPHOTONIC
Thermophotonics is a variation in which the thermal source heats a
luminescent diode similar to the TPV cell, this then illuminates the
cell with a spectrum strongly peaked just above their common
bandgap. These approaches would normally use waste heat from
an industrial process or similar and hence not be PV, but they can
be coupled to an emitter heated by solar thermal energy

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DESIGN OF AN EFFICIENT TANDEM
EFFICIENCY EVALUATION
Fig. 4.1 Junction and Efficiencies (Arizona State university series (2009)

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DESIGN CONSIDERATION
High efficiency requires the realization of solar cells with
specified band gaps, and each solar cell requires nearly
ideal properties.
– Means that must be able to make each solar cell
with high minority carrier lifetime
THUS MAJOR DESIGN CONSTRAINTS ARE:
(1) Series connection
(2) Band Gap and lattice Matching
DESIGN CONSTRAINTS
High efficiency requires the realization of solar cells with
specified band gaps, and each solar cell requires nearly
ideal properties.
– Means that must be able to make each solar cell
with high minority carrier lifetime
THUS MAJOR DESIGN CONSTRAINTS AFFECTING
MATERIAL SELECTION:
(1) Series connection
(2) Band Gap and lattice Matching

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CONCENTRATED VS UNCONCENTRATED
(Major consideration cost)
OPTICAL CHOICE

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FINAL DESIGN AND EVALUATION

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PERSPECTIVE
Higher junction tandems ranging from 5 junctions and
above are theoretical said not to exceed the maximum
of 86.8 % under concentration. The verification of this is the
perspective for future studies.
Multicarrier excitation and Hot carrier cells as two
different approaches to 3rd generation realization has
not been fully realized and is a perspective for future
consideration.

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CONCLUSION
Third generation tandem cells is the technology to
improve efficiency and maintain low production cost of
photovoltaic cells as the efficiency is increased the cost is
further reduced.
We have discussed several approaches to achieve them
with an efficient design of a 4 Junction tandem
presented.

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REFERENCES
ACKNOWLEDGEMENT
Lecture series Prof Esidor
Galvin Conibeer (2007) Third Generation Photovoltaics. Materials today,
Volume 10, Issue 11, page 42-50
Advanced Solar Cells lecture series, Arizona state University (2009)
Special thanks to our lecturer, Prof. Esidor and my colleagues too
numerous to mention.

THIRD GENERATION PV: TANDEM JUNCTION

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