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statementdizzyeyedSemiconductor

Nov 1, 2013 (3 years and 10 months ago)

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Organic Photovoltaics

S. Ismat Shah


Department
of Materials Science and
Engineering
Department
of Physics and
Astronomy

University
of Delaware,
USA


Baki

Dovlet

Universiteti
, May 2013

Earth’s atmosphere hits highest level of
CO
2

in human
history

(New York Times, May 11, 2013)






1

Price of Natural Gas

2

2008

09

10

11

12

13

14

15

02

20

$/1000
cf

Year

3

Dust Storm, Australia, Jan 5

Winter Storm, USA Mar 5

Sandy, USA Oct 31

Oklahoma, USA May 20

Hashimoto Predictions

Hashimoto Solution

There is no unique/complete solution!

-
There is very little hope in new technology but they have to be
pursued because there is no other option. (look for small scale
regional solutions)


There is no unique/complete solution!

-
There is very little hope in new technology but they have to be
pursued because there is no other option. (look for regional
solutions)

-

The only
partial

solution comes from increased efficiencies, new
materials and designs, and most importantly ……………


There is no unique/complete solution!

-
There is very little hope in new technology but they have to be
pursued because there is no other option. (look for regional
solutions)

-

The only
partial

solution comes from increased efficiencies, new
materials and designs, and most importantly
reduction in
consumption.

Harvesting Solar Energy

Radiant Facts

Diameter:

About 100 times that of earth

Mass:

99.8% of the Solar System (Jupiter has most of the rest)

Core Temperature:

15.6 x 10
6

K

Surface Temperature: 5800K

Energy Production:

386 billion
billion

megawatts

Insolation
:

1000
-

250 Watts per square meter


Age:

4.5 billion Years (5 billion years more to go)

Current PV Status


2011: Cumulative installed PV electricity generation capacity in the
world was around
20 GW
, with Europe accounting for more than 60%
of this (11.5 GW, Germany


9 GW)


China as the new leading producer of solar cells, with an annual
production of about 2.4 GW, followed by Europe with 1.9 GW, Japan
with 1.2 GW and Taiwan with 0.8 GW.

World Energy Production

BP 2006 statistical review

World Power Production

The Disconnect

1. Materials Issues

2. Device Issues

Nate Lewis (CIT) calculations

How does the area required changes with
high efficiency solar cells?

How does the area required changes with
high efficiency solar cells?


20
TWatt

model


With 10% cells, we needed 5 x 10
11

square meters of solar
cells.


With 50% cells, we will still need about 10
11

square meters of
solar cells.


We currently produce about
1
million sq. meters of solar
panels.


We need to increase production by 5 orders of magnitude.

How much material do we need?


For 1 x 10
11
m
2
, we will need



(10
11

x 10
4
x 0.01 cm
3
)/(2.33 g/cm
3
)




=
5 x 10
9

Kg of Silicon

How much material do we need?


For 1 x 10
11
m
2
, we will need




(
10
11

x 10
4
x 0.01 cm
3
)/(2.33 g/cm
3
)




=
5 x 10
9

kg of Silicon


Each kg of Si requires
15
-

20
kg of carbon
to produce electronic grade
Si.


(Availability, Toxicity)

How much material do we need?


For 1 x 10
11
m
2
, we will need




(
10
11

x 10
4
x 0.01 cm
3
)/(2.33 g/cm
3
)




=
5 x 10
9

kg of Silicon


Each kg of Si requires
15
-

20
kg of carbon
to produce electronic grade
Si.

To obtain a kg of refined grade of (poly)Si, we use up about 200 kWh of
energy emitting 40 kg of CO
2
, using
1000
gallons of water.

(Availability, Toxicity)

Device Issues

Shockley
-
Queisser

Limit

Three types of losses are
described:

1.
Sub
-
band radiation

2.
Radiative

recombination

3.
Thermalization


Loss Mechanisms

Shockley
-
Queisser

Limit


33% for single junction

Breaking S
-
Q limit

Approaches to High Efficiency

Assumption in
Shockley
-
Queisser

Approach which circumvents assumption

Examples

Input is solar
spectrum

Multiple spectrum solar cells
: transform the
input spectrum to one with same energy but
narrower wavelength range

Up/down conversion

Thermophotonics


One photon = one
electron
-
hole pair

Multiple absorption path solar cells
: any
absorption path in which one photon


ne
-
elec瑲n hle pair

Impac琠inia瑩n

Tw
-
ph瑯n absrp瑩n

One quasi
-
Fermi level
separa瑩n

Mul瑩ple energy level slar cells
㨠䕸is瑥nce 映
mul瑩ple me瑡
-
s瑡ble light
-
genera瑥d carrier
ppula瑩ns wi瑨in a single device

In瑥rmedia瑥 band

Quan瑵m well slar
cells

Constant temperature
= cell temperature =
carrier temperature

Multiple temperature solar cells
. Any device in
which energy is extracted from a difference in
carrier or lattice temperatures

Hot carrier solar cells

Steady state

(


equilibrium)

AC slar cells
㨠Rec瑩晩ca瑩n 映elec瑲magne瑩c
wave.

Rec瑥nna slar cells

Multiple Junction (Tandem)

Solar Cells

Tandem Solar Cells

Tandem Solar Cells

Dimroth and Kurtz MRS Bulletin
Multiple Junction (Tandem)
Solar

Cells


Multiple junction (tandems)
are first class of approaches to
exceed single junction
efficiency.


To reach >50% efficiency,
need ideal
E
g

6
-
stack tandem
or equivalent, can reach ~75%
of detailed balance limit.


Key issue in tandem is to
identify materials which can
be used to implement ideal
tandem stack.


# junctions in
solar cell

1 junction

2 junction

3 junction

1 sun
h

30.8%

42.9%

49.3%



junctin

68.2%

Max
cn.
h

40.8%

55.7%

63.8%

86.8%

UD
-

DARPA
: Very High Efficiency Solar Cell


Goal 50% Efficient Solar Module


Prototype: 0.5W 10 cm
2


Reduce weight of batteries carried
by soldier


Initial application: charge batteries
for flashlight


Less sensitive to spectral variation


Need for tracking reduced


Best efficiency 42.7
% (individual cells: ~ 20 suns)



Multiple Spectrum Solar Cells

Multiple Spectrum Solar Cells

Multiple spectrum devices: take the input solar spectrum, and change it to a new
spectrum with the same power density

Does not need to be incorporated into solar cell


can use existing solar cells, and
add additional optical coatings

Does not require electrical

transport of generated

carriers


no contacts,

collection, resistivity,

mobility issues.

Efficient optical processes

desired for applications

other than solar



development effort is

shared.

Requires efficient optical

conversion over broad

spectrum.


Multiple Spectrum Solar Cells

Approaches for multiple spectrum solar cells.

Thermophotonics
: Use thermally
-
excited LED to generate a narrow solar spectrum.

Assuming efficient spectrum conversion and max concentration, efficiency can
be >80%

Requires demonstration of efficient thermally
-
excited LED and cooling from light
emission

Using known materials and biases, efficiency is 50%.




Biased

Nanocomposite Solar Cells

Two conditions to become conductive:

The first condition
for this is that the polymer consists of
alternating single and double bonds, called
conjugated
double bonds
.


Every bond contains a localized “sigma” (σ) bond which
forms a strong chemical bond. In addition, every double
bond also contains a less strongly localized “pi” (π) bond
which is weaker.



Conducting

Polymers

The second condition
is that the polymer has to be altered
-

either by
removing electrons from (oxidation),
or

inserting

them

into

(
reduction
), the material.




1
-
oxidation
with

halogen

(
or

p
-
doping).








2
-

Reduction

with

alkali

metal (
n
-
doping).



Conducting Polymer Inventors

Conjugated Polymers


The most important aspect of conjugated polymers from an
electrochemical perspective is their
ability to act as electronic
conductors


Material which could combine the processibility, environmental
stability, and weight advantages of a fully organic polymer with the
useful electrical properties of a metal


All conjugated polymers have quasi
-
infinite system extending over a
large number of recurring monomer units. This gives the materials a
directional conductivity,
strongest along the axis of the chain. The
simplest possible form polyacetylene (CH)
x



While polyacetylene itself is too unstable to be of any practical
value, its structure constitutes the core of all conjugated polymers.

Organic Semiconductors

Bonding:


The semiconducting nature of organic semiconductors


arises from the π electron bonds that exist when molecules are fully
conjugated (i.e., have alternating single and double bonds).


The weakly held π electrons are responsible for all interesting
optical and electronic transitions in organic semiconductors.


The π to π * transitions in organic semiconductors are typically in
the range of 1.4

2.5 eV, which overlaps well with the solar spectrum


Conjugated Polymers: Semiconducting behavior


Double and single bonds provide the semiconducting
properties

2
s
2
p
3

sp
3

hybrid

2s

2s

sp
2

hybrid

2s 2p
2

p
z

unhybridized

2p


x y z


x y z

+


π
-
bond

σ
-
bond

Organic semiconductor

LUMO: lowest unoccupied molecular orbital

HOMO: highest occupied molecular orbital


Inorganic semiconductor & metal



CB: conduction band VB: valance band

F
. Gutmann, “Organic semiconductors”, Wiley, New York, 1967.


N =
1 2 4
…. Polymer
Si,Ge… Metal

Energy

LUMO


CB

VB

VB

CB

HOMO

HOMO

LUMO

E
G

(Number of

molecules)

π
*

π


What is
an organic semiconductor?

Selected Materials for organic solar cells

Molecular structure and conjugated π
-
electron system

Ref. McMurry, J.,
Organic Chemistry. 3rd ed.; Brooks/Cole Publishing Company:
1992. and
Angew. Chem. Int. Ed., 47, 58 (2008)

(b)