Dye Sensitized Solar cells: synthesis and applications of a novel sensitizer and pentacene as hole conductor material

 13
Prefazione 
 
Questo lavoro di tesi di Dottorato è stato condotto nei laboratori dell’Università di Lecce, 
Dipartimento di Ingegneria dell’Innovazione e nei laboratori dell’Università UVA di Amsterdam 
(Olanda) e del Centro Nazionale dell’Energie ECN (Petten-Olanda). La tesi è stata finanziata 
dall’Università degli Studi di Lecce e dal Progetto olandese Zenit e dalla Comunità Europea col 
programma FP5 della borsa di studio Maria Curie. 
Vorrei ringraziare il Prof. Giuseppe Vasapollo ed il Dott. Giuseppe Ciccarella per avermi 
suggerito questo interessante argomento di ricerca e supervisionato la tesi e per la loro guida 
durante il lavoro.  
Vorrei anche ringraziare la Prof. Luisa De Cola del gruppo di Materiali Fotonici dell’Università 
di Amsterdam per avermi dato molto di più di alcune nozioni di sintesi innovative e di 
fotochimica utili per condurre questo lavoro, e specialmente la gente di ambedue i laboratori 
(Lecce ed Amsterdam) per il piacevole ambiente di lavoro ed il loro aiuto in problemi pratici. 
Vorrei anche ringraziare: Alex Mayer dell’Università di Cornell (USA) del gruppo del Prof. G. 
G. Malliaras, il Dr. Brian O’Regan e Sjoerd Veenstra dell’ECN, il Prof. Renè Janssen 
dell’Università Tecnologica di Eindhoven, Ali Afzali del gruppo di ricerca IBM di NY e la gente 
dell’Istituto dei Polimeri Olandese, per l’interesse e la cooperazione per la parte sperimentale di 
questo lavoro. 
Specialmente vorrei ringraziare la mia famiglia ed i miei amici per il loro incoraggiante supporto 
durante i miei studi. 
 
Lecce, Luglio 2004, Vito Sgobba 
 14
1 Introduction 
 
 
1.1 The need for renewable energy 
 
It is assumed that the world energy demand and CO
2
 emissions will both increase by about 70% 
between 2000 and 2030. [1] Moreover fossil fuels supplying 80% of all energy consumed 
worldwide, are facing rapid resource depletion. [2] The resource of fossil fuels in the whole 
world in 2002 were projected to last 40 years for oil, 60 years for natural gas and 200 years for 
coal. [3] Because of a growing demand for energy, combined with the depletion of fossil 
resources, global warming and its associated climate change caused by the CO
2
 emissions [4], 
there is an urgent need for environmentally sustainable energy technologies. Renewable energy 
is the production of electricity, transport fuel or process heat from sources that, for all practical 
purposes, do not run out. Renewable energy technologies, which include photovoltaic, solar 
thermal, wind turbines, hydropower, wave and tidal power, biomass-derived liquid fuels and 
biomass-fired electricity generation, supply to date only 14% of all energy consumed worldwide. 
[5] 
 
Recent detailed analyses indicate that from 10 to 30 Tw/yr of annual carbon-free energy (1 TW 
= 1 x 10
12
 watts) will be required globally by the year 2050 to accommodate the world’s 
expected population of 10-11 billion people, combined with a modest annual global economic 
growth rate of about 2 % [6]. The amount of carbon-free energy that will be needed depends 
upon the level of atmospheric CO
2
 that can be tolerated with respect to its impact on global 
climate change. The present concentration of CO
2
 is 275 ppm, up from 175 ppm before the 
industrial revolution. 
If CO
2
 is to be stabilized at 750 ppm by 2050 (a level considered extremely dangerous and 
seriously disruptive by many climatologists), and if carbon sequestration is not considered, then 
about 10 TW/yr of annual carbon-free energy will be required by 2050. If the CO
2
 level needs to 
be stabilized at 400 ppm, then 30 TW/yr of carbon-free energy will be required annually. The 
current global population is about 6 billion people and the total annual global energy 
consumption is about 13 TW/yr. Thus, enormous levels of carbon-free energy will have to be 
introduced in the coming decades, levels that are from 1 to 3 times the total level of energy 
consumed today from all sources (fossil fuel, nuclear power, and renewable energy). 
 15
All major climate models agree on the scenario of a global earth warming up due to the 
greenhouse effect mainly induced by the consumption of fossil energies. In this context, the 
question of sustainable development needs to be seriously considered. 
World leaders gathered in Kyoto, Japan, in December 1997 to consider a world treaty restricting 
emissions of greenhouse gases, mainly of carbon dioxide, that are thought to cause global 
warming. Renewable energy will capture a meaningful share of  the Global Energy Market in 
the next 25 years. Key drivers will be: falling costs for renewable energy, declining fossil fuel 
production, increasing energy demand worldwide, environmental concerns. 
 16
2 Basics Of Photovoltaic Energy Conversion 
 
 
2.1 Solar Radiation 
 
The solar radiation is emitted from the sun's photosphere at 6000 K temperature, which gives it a 
spectral distribution resembling closely to that of a black body at the corresponding temperature. 
The average energy flux incident on a unit area perpendicular to the beam outside the Earth's 
atmosphere is known as the solar constant: 
 
S=1367 W/m
2
 
 
In general, the total power from a radiant source falling on a unit area is called irradiance. 
The black-body temperature of solar radiation which agrees with this value of S can be obtained 
from the Stefan-Boltzmann law and a simple geometrical argument involving the Sun-Earth 
distance RSE and the radius of the Sun RS: 
 
4
S
TfS σ
ω
=    (1) 
 
where the geometrical factor fω is given by: 
 
2
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
=
SE
S
R
R
f
ω
  (2) 
 
The solid angle ω
S
 subtended by the Sun, related to f
ω
 by f
ω
 = ω
S
/π, is also sometimes used. 
The concept of black body radiation makes it possible to use the Planck’s radiation formula for 
the photon flux received by a planar surface of unit area in a frequency interval δυυυ +→ : 
 
δνν
π
ν
δννπ
δφ
ν
ωω
SB
kT
h
S
ef
c
kT
h
e
f
c
−
⋅≅
−
⋅=
2
2
2
2
2
1
2
  (3) 
 
where h is the Planck constant, c is the speed of light in vacuum, and k is the Boltzmann 
constant. 
 17
When the solar radiation enters the Earth's atmosphere, a part of the incident energy (about 10 up 
to 45 %) is removed by scattering or absorption by air molecules (in particular oxygen, ozone, 
water vapor, and carbon dioxide), clouds, dust and particulate matter usually referred to as 
aerosols. The radiation that is not reflected or scattered and reaches the surface directly in line 
from the solar disc is called direct or beam radiation. The scattered radiation which reaches the 
ground is called diffuse radiation. Some of the radiation may reach a receiver after reflection 
from the ground, and is called the albedo. The total radiation consisting of these three 
components is called global. 
The amount of radiation that reaches the ground is, of course, extremely variable. In addition to 
the regular daily and yearly variation due to the apparent motion of the Sun, irregular variations 
are caused by the climatic conditions (cloud cover), as well as by the general composition of the 
atmosphere. For this reason, the design of a photovoltaic system relies on the input of measured 
data close to the site of the installation. 
The total yearly solar irradiation on horizontal surface is 700-1000 kWh/m
2
 in North Europe, 
900-1300 kWh/m
2
 in Middle Europe, 1300-1800 kWh/m
2
 in South Europe, 1800-2300 kWh/m
2
 
in the equator, and 2000-2500 kWh/m
2
 in the so called "solar belt" i.e. between 20° and 
30° latitude. 
The attenuation of the sunlight by the atmosphere depends on the optical path length to 
observation point. The ratio between ‘any path lengths through the air on earth’ and ‘this 
minimum above’ is called the optical air mass or air mass. In case the sun is directly overhead, 
the air mass is unity. 
This path length is shortest at the moment that the sun is directly overhead. Therefore, on a clear 
summer day at sea level, the sun light flux at zenith corresponds to air mass 1 (abbreviated to 
AM1) irradiance; at other times, the air mass is approximately equal to 1/cos θ, where θ is the 
zenith angle. 
 18
 
 
 
 
 
Fig1 and 2: Solar spectral photon flux for AM 0 and AM 1.5 sunlight.[7] 
 
 
The extraterrestrial spectrum, denoted by AM0, is important for satellite applications of solar 
cells. AM1.5 is a typical solar spectrum on the Earth's surface in a clear day which, with total 
irradiance of 1 kW/m
2
, is used for the calibration of solar cells and modules. 
Instead of irradiance, the design of photovoltaic systems (particularly stand-alone ones) is 
usually based on daily solar radiation: the energy received by a unit area in one day. 
An idea of how large quantity this is can be obtained as follows. The total energy flux incident 
on the Earth is equal to S multiplied by the area of the disk presented to the Sun’s radiation by 
the Earth. The average flux incident on a unit surface area is then obtained by dividing this 
number by the total surface area of the Earth.  
 19
Making allowance for 30% of the incident radiation being scattered and reflected into space, the 
average daily solar radiation G on the ground is equal to: 
 
kWhS
R
R
G
E
E
74.5
4
7.024
2
2
=⋅⋅⋅=
π
π
   (4) 
 
where R
E
 is the radius of the Earth. [8] 
 
 
2.2 Semiconductors 
 
At present the majority of the solar cells is made from semi-conductor materials therefore is 
useful to describe some basic physical properties. 
In a solid material, N atomic orbitals combine to N molecular orbital with a spacing between the 
discrete energy levels, which decreases with increasing number of atoms, i.e. increasing the 
number of molecular orbitals. When the number of atom is large enough, the energy levels will 
form a continuum, since the spacing between the discrete energy levels becomes infinitely small. 
The ensembles of energy levels are called energy bands. The electronic structure may have a 
band gap, a range of energies in which no orbital states exist for the electrons. Due to differences 
in the electronic structure, solid materials can be divided into conductors (metals) and dielectrics 
(semiconductors and insulators). For a semiconductor or insulator, as opposed to a metal, there is 
a gap (bandgap, E
g
) between the highest filled energy band (valence band, VB) and the lowest 
empty band (conduction band CB). 
 
 
 
Fig.3: energy level diagram of intrinsic semiconductors in vacuum. 
 20
The valence band is occupied by valence electrons and the conduction band is associated with 
excited state energy levels. The highest filled energy level and the lowest empty energy level are 
called the valence band edge (E
v
) and the conduction band edge (E
c
), respectively. In the ground 
state at zero Kelvin, the ½N orbitals in the valence band are completely filled with N electrons. 
Electrons in a completely filled band cannot carry current. At temperatures above zero Kelvin, 
electrons can be excited by the thermal motion of the atoms, resulting in electrons populating 
orbitals in the conduction band.  
The electron are now mobile, and the solid is an electric conductor. There will now be a vacant 
position in the valence band (a hole) and this vacancy makes motion in the valence band 
possible. Hence, the current flow in a semiconductor can be regarded as being due to the sum of 
the motion of electrons in the conduction band and holes in the valence band. Thus, electronic 
conductivity in semiconductors requires electrons to be excited to the conduction band or holes 
present in the valence band.  
This may be done thermally or optically. If the gap is large, however, very few electrons will be 
promoted thermally at ordinary temperatures and the conductivity will remain close to zero, 
giving an insulator. Materials with band gap energies between 0 to 3-4 eV, between a conductor 
and an insulator, are called semiconductors. 
The number of electrons per unit volume occupying levels in the conduction band is given by 
integration: 
 
∫
=
max
)()(
c
c
E
E
C
dEEfENn    (5) 
 
N(E) is the number of allowed states per unit volume at energy E. At an energy near the 
conduction band edge, N(E) is given by: 
 
2/1
3
2/3
)(
28
)(
c
e
EE
h
m
EN −=
∗
π
   (6) 
 
with an effective mass of one electron, 
∗
e
m  and Planck constant, h. An effective mass need to be 
introduced since the mass of an electron in a crystal lattice differs from that of an electron in free 
space.  
 21
The Fermi-Dirac distribution f(E), is the fractional occupation of the available orbitals at energy 
E and temperature T: 
 
kTEE
F
e
Ef
/)(
1
1
)(
−
+
=   (7) 
 
where k is the Boltzmann constant. The Fermi energy E
f
, is the energy level where one half of 
the orbitals are occupied (f(E)=1/2). The fractional occupation of the available orbitals decreases 
rapidly for levels below E
f
 under standard conditions, and consequently most of the electrons in 
the conduction band are clustered near the conduction band edge. 
For energies well above E
f
, the exponential term 
kTEE
F
e
/)( −
 is very much larger than unity, and a 
Boltzmann-like distribution may replace the Fermi-Dirac equation: 
 
kTEE
F
eEf
/)(
)(
−−
≅    (8) 
 
At the energy of the conduction band edge this is valid (E
c
-E
f
>>kT) and the concentration of 
electrons in the conduction band is obtained by: 
 
kTEE
c
kTEE
e
c
FcFc
eNe
h
kTm
n
/)(/)(
2/3
2
2
2
−−−−
∗
=
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
=
π
   (9) 
 
where N
c
 is a constant at fixed T, known as the effective density of states within few kT above 
the conduction band edge. As a rough order of magnitude, N
c
 at room temperature is around 10
20
 
per cm
3
, or one state per hundred atoms. 
The concentration of holes in the valence band (p
v
) can be calculated from a similar treatment. 
For an intrinsic semiconductor the concentration of electrons is equal to the concentration of 
holes and E
f
 is located in the middle of the band gap. 
A perturbation of the electronic structure of semiconductors is often caused by defects in their 
crystal structure. These perturbations can give rise to new energy levels, which change the Fermi 
level and alter the electrical properties of the semiconductor. The relative positions of the 
additional energy levels depend on the type of defects. 
 22
The introduction of impurities into the lattice in order to create charge carrier is called doping. A 
semiconductor can be either n-doped by the introduction of donor states: 
 
 
 
Fig.4: energy level diagram of n-doped semiconductors in vacuum. 
 
or p-doped by the introduction of acceptor states: 
 
 
 
Fig.5: energy level diagram of p-doped semiconductors in vacuum. 
 
As an example of a localized donor impurity state, we may consider a phosphorous atom (with 
five valence electrons), which replaces a silicon atom in an otherwise perfect silicon crystal. The 
fifth electron of phosphorous has no valence band to occupy. The electron is still weakly, 
bonded to the phosphorous nucleus, so it is not a free conduction band electron. The fifth 
electron has a localized orbital with energy E
D
 in the band gap. The energy required to excite the 
electron from the impurity state into the conduction band is known as the donor ionization 
energy. The donor energy level should be close to the conduction band so the donor electrons 
can easily be thermally excited into the conduction band, increasing the conductivity of the 
semiconductor.  
 23
The electron concentration in the conduction band and the Fermi level for an n-type 
semiconductor can be approximated according to the following equations: 
 
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
+=
==
−−
C
D
CF
kTEE
CDc
N
N
kTEE
eNNn
FC
ln
/)(
   (10, 11) 
 
where N
D
 is the effective donor intensity. With typical values for N
C
=10
20
 cm
-3
 and N
D
=10
17
cm
-
3
, the difference between E
C
 and E
F
 can be calculated to 0.2 eV. 
The hole concentration in the valence band and the Fermi level for a p-type semiconductor can 
be approximated in a similar way. 
Crystal surfaces and grain boundaries are two other examples of defects that cause energy levels 
in the band gap. Energy levels in the band gap are often referred to as traps or recombination 
centres, depending on the electron lifetime in the state. The band gap states can play an 
important role in the charge transport and recombination dynamics. [9] Depending on the energy 
distance from the conduction band, traps are divided into shallow and deep traps. The probability 
for an electron in a shallow trap to be thermally excited to the conduction band is relatively large 
whereas the probability is low for an electron in a deep trap. 
In a semiconductor, electrons in the conduction band and holes in the valence band (electron 
vacancies) are the charge carriers. In a n-type semiconductor, electrons dominate the carrier 
transport (majority carrier). In a p-type semiconductor, holes are the majority carrier. 
Under the influence af an applied electric field, a randomly moving free electron would 
accelerate in a direction opposite to the field. Due to collisions with atoms an electron in a 
crystal lattice would not continue to accelerate for very long. The velocity of electrons between 
collision caused by the electric field is called the drift velocity. The electron mobility µ
n
 is 
defined as this electron drift velocity in an electric field of unit strenght and is inversely 
proportional to the effective mass of the electron. The mobility is a material constant. Electron 
mobilities are tipically in the range of 100-1000 cm
2
V
-1
s
-1
 for semiconductors. The conductivity 
κ of electrons is proportional to the electron mobility and electron concentration in the 
conduction band. The conductivity κ of holes is proportional to the holes mobility and holes 
concentration in the valence band. Hole mobilities are typically in the range 1-1000 cm
2
V
-1
s
-1
 for 
semiconductors. 
 24
For semiconductors with both electrons and holes as carriers, the conductivity is determined by: 
 
κ=q
e
(µ
n
n
c
+µ
p
p
V
)   (12) 
 
in which q
e
 is the elementary charge. 
Apart from motion by drift, carriers in semiconductor can also flow down a concentration 
gradient, i.e. by diffusion. Drift and diffusion processes are related. The diffusion coefficient of 
electrons, D
n
 and holes D
p
, can be calculated from the electron and hole mobilities, respectively, 
according to the Einstein equation: 
 
n
e
n
q
kT
D µ=    (13) 
p
e
p
q
kT
D µ=   (14) 
 
If for instance the electrons dominate the carrier transport, the variation of conductivity with 
temperature can be interpreted using the Arrhenius equation: 
 
)/( RTE
a
Be
κ
κ
−
=    (15) 
 
where R is the gas constant, the parameter B is called a pre-exponential factor (related to the 
frequency of electron movement) and E
aκ
 is the activation energy (the minimum kinetic energy) 
for electron conduction. The fraction of electron movement with a kinetic energy in excess of 
energy E
aκ
 hence leading to conduction, is given by the Boltzmann distribution 
)/( RTE
a
e
κ
−
. The 
variation of electron mobility with temperature can be interpreted in a similar treatment of the 
Arrhenius equation. The overall activation energy for electron conduction may be a sum of 
terms: 
 
agaa
EEE
C
+=
µκ
   (16) 
 
where 
C
a
E
µ
 and 
ag
E  are the energies associated with electron mobility in the conduction band 
and free electron generation, respectively. The mobility is usually not or only weakly activated 
in a semiconductor. [10] 
 25
2.3 The discovery of photovoltaic effect and first application of solar cells 
 
Historically, solar cell operation was first discovered in a photochemical cell. In 1839, the french 
scientist Becquerel observed that an electric current was produced when he illuminated one of 
two similar platinum, gold, brass, or silver-silver halide electrodes immersed in dilute acid. [11] 
Therefore, Becquerel’s solar cell was an electrolytic cell made up of two electrodes placed in an 
electrolyte, in which the current increased when the cell was exposed to light. [12] Forty years 
later, in 1877, Adams and Day observed the photovoltaic effect in the solid material selenium. 
The first silicon solar cell was developed by Chapin, Fuller and Pearson at the Bell Telephone 
Laboratories (also the birthplace of the transistor) in the 1954, and it had already about 6% 
efficiency [13]. Solar cells were used primarily in space applications until the mid-70s [13]. 
 
 
 
Fig.6: PV panel, developed by TRW for a communications satellite in 1966 
 
 
2.4 Photovoltaic cell performance 
 
Generation of electrical power under illumination is achieved by the capability of the 
photovoltaic device to produce voltage over an external load and current through the load at the 
same time.  
The Photovoltaic power conversion efficiency η (P
out
/P
in
) of a solar cell is defined as the ratio of 
the maximum electric power extracted to the illumination G times the surface S of the module: 
 
GS
P
⋅
=
max
η   (17) 
(it is often expressed as a percentage), and depends on: 
• Temperature 
• Illumination power 
 26
• Spectral distribution light source. 
For a meaningful comparison of results, measured efficiencies should be independent of 
measuring institute and technique. Therefore, a set of Standard Test Conditions are defined 
following IEC and ASTM E1021-84 norms [14]: 
Radiant intensity: 1000 W/m
2
, 
Spectral distribution: AM1.5 global (IEC 904-3), 
Cell temperature: 25 
o
C. 
Recording the current–voltage characteristics of a cell in the dark and under illumination permits 
an evaluation of most of its photovoltaic performances as well as its electric behaviour. When an 
external load is connected to the illuminated solar cell, the total current (I), is the result of two 
counteracting components: 
 
I=I
P
-I
D
  (18) 
 
Where I
P
 is the photogenerated current and I
D 
is the light indipendent recombination or “dark” 
current. 
When the cell is short circuited under illumination, the maximum current, the short circuit 
current (ISC), is generated, while under open circuit conditions no current can flow and the 
voltage is at its maximum, called the open circuit voltage (V
OC
). The point in the IV-curve 
yielding maximum product of current and voltage, i.e. power, is called the maximum power 
point (max). 
 
  
 
Fig.7: I–V characteristics of an ideal solar cell in the (a) dark, and (b) under illumination.