BARIUM OXIDE AS AN INTERMEDIATE LAYER FOR
POLYMER TANDEM SOLAR CELL
A Thesis
Presented to
The Graduate Faculty of The University of Akron
In Partial Fulfillment
of the Requirements for the Degree
Master of Science
Zhehui Li
May, 2013
BARIUM OXIDE AS AN INTERMEDIATE LAYER FOR
POLYMER TANDEM SOLAR CELL
Zhehui Li
Thesis
Approved
Accepted
________________________
________________________
Advisor
Dr. Xiong Gong
Department Chair
Dr. Robert Weiss
________________________
________________________
Committee Member
Dr. Alamgir Karim
Dean of the College
Dr. Stephen Z. D. Cheng
_________________________
_________________________
Committee Member
Dr. Yu Zhu
Dean of the Graduate School
Dr. George R. Newkome
_________________________
Date
ii
ACKNOWLEDGEMENTS
I would like to thank first and foremost my research advisor Dr. Xiong
Gong for his patience, encouragement, and guidance throughout the course of this
research. Also, my gratefulness is given to all the group members: Mr. Tingbin
Yang, Mr. Hangxing Wang, Ms. Xilan Liu, Mr. He Ren, Mr. Wei Zhang, Mr. Chao
Yi, Mr. Bohao Li, Mr. Kai Wang, Ms. Chang Liu for their warm caring about my
life and helpful comments and suggestions on research.
Finally, I would like to express my deepest gratitude to my parents, Mr. Jun
Li and Ms. Xiaoyan Xu for their love and support.
iii
ABSTRACT
Polymer solar cells (PSCs), a member of organic solar cell family, have
attracted increasing research interest. PSCs possess significant advantages over their
inorganic solar cell counter parts: mechanical flexibility, light weight, low expense,
and the potential to achieve roll-to-roll large-scale production. Tandem solar cells,
in which two solar cells are linked to take more use of the solar energy, were
fabricated with each solution processed layer using Bulk Heterojunction (BHJ)
materials comprising semiconducting polymers and fullerene derivatives. For years,
tremendous efforts have been put in seeking for an efficient intermediate layer to
successfully connect two sub-cells together in the tandem structure. Even though
many kinds of intermediate layers such as ZnO/MoO3 etc. have been explored, most
of them suffer from the low conductivity or complicated manipulation
disadvantages. Barium oxide (BaO) is both high conductive and wide bandgap
n-type semiconductor. We successfully fabricated the polymer tandem solar cell
using all thermal vacuum deposition fashion with BaO/Ag/MoO3 as an intermediate
layer. VOC of the tandem structure is the sum of the component cells demonstrating
our proposed intermediated layer can efficiently connect sub-cells with no
potential/energy loss.
iv
TABLE OF CONTENTS
Page
CHAPTER
I.
INTRODUCTION
1
1.1.
General Background
1
1.2.
Polymer Solar Cells
4
1.3.
II..
1.2.1. Working Principle
4
1.2.2. Device Geometry
8
1.2.3. High Performance: Three Essential Parameters
10
Polymer Tandem Solar Cells
12
1.3.1. General Background
13
1.3.2. Efficient Single Cells
15
1.3.3. Efficient Intermediate Layer
17
1.3.4. Tandem Polymer Solar Cell Characterization
21
1.3.5. Processing Issues of the Tandem Structure.
22
EXPERIMENT
23
2.1.
Materials Preparation
23
2.2.
Device Fabrication Procedures
24
2.3.
Calibration and Characterization
26
v
III.
2.4.
UV-Vis Absorption Spectrum
26
2.5.
Atomic Force Microscopy (AFM) Observation
26
2.6.
Sol-gel ZnO Nanoparticles Preparation
26
RESULTS
28
3.1.
Energy Levels
28
3.2.
Performance Investigation
29
3.2.1. Current Density-Voltage (J-V) Characteristics
29
3.2.2. UV-Vis Absorption
33
3.2.3.Atomic Force Micrometer (AFM) Images Observation
35
3.3.
IV.
Comparison of the Intermediate Layers
DISCUSSION, CONCLUSIONS AND OUTLOOK
REFERENCES
36
40
44
v
LIST OF FIGURES
Figure
Page
1.1
Number of scientific publications contributing to the subject of
‘polymer solar cell(s)
1.2
(a) The unit cell of silicon; (b) Simplified energy band diagram for a
semiconductor
5
1.3
(a) Bulk Heterojunction (BHJ) Structure of the active layer in polymer
7
solar cell, (b) working principle of polymer solar cells
1.4
Photoinduced process in the D-A system. (a) photoinduced charge transfer
in a forward direction;(b) exciton recombination happens on a time scale of
nm; (c) charge transfer in a back direction
8
1.5
(a) The conventional device structure; (b) Bulk heterjunction configuration
9
in organic solar cells
1.6
The organic solar cells with (a) conventional geometry and (b) inverted
geometry
9
1.7
Current-Voltage Characteristics of a polymer solar cell under illumination
(red line) and in the dark (black line)
10
1.8
(a) Typical tandem solar cell device geometry and (b) simplified procedure
14
of the stacking process of two sub-cells
1.9
The current-voltage characteristics of two sub-cells under illumination. The
front cell delivered more photocurrent than the bottom cell
16
1.10
Basic principle of an organic tandem solar cell using an intermediate layer.
The arrows indicate the hole currents and the electron currents. ETL
denotes the electron transport layer and HTL indicates the hole transport
layer.
18
1.11
Simplified energy level diagram of the metal and n-type semiconductor (a)
before contact and (b) band bending in the Ohmic contact
18
vi
3
1.12
Schematic energy level diagram at open circuit of a double heterojunction
19
solar cell with highly doped layers as recombination contact
1.13
Dark Current Density verse Voltage (J-V) characteristics of a tandem cell
before and after light illumination.
20
2.1
Molecular structures of PCPDTBT, P3HT and PCBM respectively
2.2
Polymer tandem solar cell geometries with (a) PCPDTBT:PCBM as an
upper layer and (b) P3HT:PCBM as an upper layer
25
24
3.1
Energy levels of the single cell composed of bulk heterojunction polymer
blends (a) P3HT: PCBM and (b) PCPDTBT: PCBM.
28
3.2
The energy levels of the tandem solar cells composing the upper polymer
29
layer of (a) P3HT : PCBM and (b) PCPDTBT:PCBM.
3.3
The current density-voltage (J-V) characteristics of single reference cells
using identical P3HT:PCBM polymer systems and tandem cell. (a) J-V
curves under illumination and (b) in dark.
31
3.4
The current density-voltage (J-V) characteristics of single reference cells
using P3HT:PCBM and PCPDTBT:PCBM and the tandem cell. J-V
33
curves (a)under illumination and (b) in dark.
3.5
UV-Vis absorption spectra of a PCPDTBT:PCBM bulk heterojunction
composite film, a P3HT:PCBM bulk heterojunction composite film, and a
bilayer of the two as relevant to the tandem device structure. a.u. optical
density.
34
3.6
AFM images of (a) MoO3 surface morphology of BaO/Ag/MoO3
intermediate layer and (b) PEDOT:PSS on ITO coated glass substrate. The
islands observed are due to surface roughness. Note that the islands
distribution is more intensive for (b), indicating the roughness is higher
for PEDOT:PSS.
35
3.7
AFM height profiles of ZnO nanoparticles under the condition of (a) fast
annealing and (b) slow annealing
36
3.8
The intermediate layer composed of ZnO/MoO3. (a) Current
Density-Voltage (J-V) Characteristics and (b) Sol-gel preparation of ZnO
nanoparticles
37
3.9
The electronic performance of BaO/Ag/MoO3 intermediate layer. (a)
Diode property of BaO/MoO3 P-N junction; (b) Conductive property of
BaO/Ag/MoO3 intermediate layer
38
viii
3.10
AFM images of two types of intermediate layers. (a) ZnO/MoO3 and (b)
BaO/Ag/MoO3 on ITO coated glass substrate. The islands observed are
39
due to surface roughness
ix
LIST OF TABLES
Table
Page
1. Photovoltaic performance of the single reference cell P3HT : PCBM and the
corresponding tandem cell.
31
2. Photovoltaic performance of the single reference cells P3HT :
PCBM/PCPDTBT : PCBM and the corresponding tandem cell.
x
33
CHAPTER I
INTRODUCTION
1.1.
General Background
Nowadays, environmental pollution and resource depletion are the problems
that need to be solved urgently. Due to heavily environmental pollution brought by
the widely used traditional energy sources such as oil and gasoline, people are seeking
for an environmentally-friendly alternative energy source. Harvesting nature energy to
generate power is regarded as one of the promising methods. In this spirit, solar
energy is one of the best available alternatives, for its embedded nature of both clean
and unlimited.
The photovoltaic effect in Silicon (Si) was first proposed in 1954 in Bell
Laboratory and the power conversion efficiency (PCE) was reported reaching 6%.1
Since then, the inorganic-based solar cells including but not limited to Si such as
GaAs, CdTe, CIFGS have been intensively explored. In the past decade, the
technology of PV is mushrooming at an annual rate of 48% and gradually
commercialized.2 Despite the fact that the inorganic photovoltaic has been booming
quite fast, it only takes account for less than 0.1% of the energy demand world widely.
Unfortunately, there are many embedded disadvantages of inorganic PV responsible
for its bottleneck of development. On the one hand, the Silicon processing consumes
large quantities of acid and much poisonous waste is disposed into the environment;
1
On the other hand, the installation of Silicon based photovoltaic (PV) expense
is as high as 1500usd/m2, which inevitably hinders its wide application. To
circumvent those issues, researchers are looking for a better solar cell in regards of
pollution-free and low-cost.
No doubt inspired by discovering the ultra-fast photo induced charge transfer
in 19923, collaborative efforts by interdisciplinary researchers in the fields of
synthetic chemistry and optical physics have been put in the study of organic solar
cells. Based on organic materials, this new member of PV is regarded as a promising
alternative because it is low processing expense, light weight, and could be fabricated
in a continuous fashion. In this way, the organic solar cell can be implemented on
flexible substrate carrying the possibility of achieving roll-to-roll printing technique.4
Polymer photovoltaic is not a precise definition but typically considered as a
generation of OPV, and it means applying semiconducting conjugated polymers5 as
active materials within the thin film PVs. The first highly conductive polymer was
reported in 1977.6 The highly chemically doped polyacetylene can form a new class of
conducting polymers, and the electrical conductivity property can be systematically
and continuous varied over a large magnitude. Another notable event in the polymer
solar revolution occurs at 2000 when Heeger, MacDiarmid and Shirakawa were
nominated Nobel Laureates in recognition of their outstanding contribution in
‘discovery and development of conducting polymers’. Figure1.1 displays an overview
of the current development tendency of polymer solar cells.7
2
Figure 1.1 Number of scientific publications contributing to the subject of
‘polymer solar cell(s)’. Search done through ISI, Web of Science, 2007
The principal working mechanism of polymer solar cells is: First, the
conjugated polymer with localized π electrons can absorb sunlight and forms a
coulombically bound pair of electron-hole named exciton; Second, this pair of
electron-hole is dissociated at bulk hetrojucntion interface into free charge carriers
and those carriers transport through active layer and finally reach the electrodes. The
detailed working principle and the device geometry will be disclosed later.
To date, thanks to the tremendous efforts by interdisciplinary researchers, the
power conversion efficiency of a single polymer solar cell has been pushed to a value
which is competent with their inorganic counterparts. Furthermore, the theoretic value
of OPV is around 20% and pushing the single cell to 10% has become a reality
through thoughtful design of electron-donor polymer and careful device fabrication.
Besides using better materials to fabricate the solar cell, another logical thinking is to
modifying the device structure. In this spirit, the newly created tandem solar cell was
proposed.
3
The tandem solar cell where two sub-cells are connected in series through an
intermediate layer is one of the most commonly employed tandem structure. The first
reported two terminal organic tandem solar cell was proposed by Hiramoto et al.8
Previously, people are focusing on the small molecular to make sub-cells, because the
applied dry coating fashion is an easy way to stack different sub-cells together.
Nowadays, thanks to the development of polymer chemistry, tremendous types of
polymers with good electrical conductivity have been successfully synthesized. This
breakthrough has overcome the choice limitation of available material and more work
was emphasized on the polymer based tandem solar cells afterwards. It was Kawano
et al.9 that demonstrated the first polymer based tandem solar cells.
The intermediate layer in the tandem solar cells has been very attractive to
numerous investigators since it is key point to connect sub-cells successfully. Several types
of intermediate layers in either evaporation or solution processed fashion were
explored to connect two sub-cells, and its modification is never overstated. I would
disclose the deep-in knowledge about the intermediate layer later.
1.2.
1.2.1.
Polymer Solar Cells
Working Principle
Polymer solar cells (PSCs) is an important member of the OPV family,
distinguished by utilizing the π-conjugated polymer as an active component. To better
understand the working principle in the PSC, we can simply compare it with the
inorganic solar cell, Si based photovoltaic, to be specific. Crystal Si possesses a
diamond lattice structure, with each silicon covalent bonded to another four silicon
4
atoms. The pure crystal Si is commonly regarded as a semiconductor material, and the
Fermi level is located at the middle of the valance band and conduction band. Figure
1.2 shows the one silicon lattice and the simplified band structure of pure silicon.10
Figure 1.2 (a) The unit cell of silicon; (b) Simplified energy band diagram for a
semiconductor.10
As we know, electric conductivity is proportional to the concentration of
mobile charge carriers and therefore the electric conductivity for pure silicon is
comparatively low. To overcome this issue, one commonly used method is to add
dopants, also called impurities, into the pure Si. The electric conductivity of the
semiconductor is considerable increased after being doped. The element chosen to be
a dopant usually possesses or lacks an extra electron compared to silicon. Undoped
silicon carries the equal number of electrons and holes and it is called ‘intrinsic’
silicon, and dopant will generate an excess of either electron or hole. Hence, there are
two types of doped silicon: n-type silicon, and ‘n’ refers to negatively charged carriers
(electron); p-type silicon, and ‘p’ refers to positively charged carriers (hole). When
the n and p-type silicon is connected together, the ‘p-n junction’ is formed. This ‘p-n’
junction is a key point and a platform for energy conversion and exciton generation in
the inorganic PV. Same case with organic solar cells, the importance of
donor-acceptor interface can never be overstated. Similarly, the organic solar cells
5
also possess a donor-acceptor (D-A) interface like ‘p-n junction’ in inorganic
photovoltaic. The state-of-art active layer structure is called Bulk Heterojunction
(BHJ) structure which was first reported in 199511. This active layer is commonly
consisted of two materials, namely: an electron donor material and an electron
acceptor material. Usually, the conjugated polymer serves as an electron donor and a
fullerene derivative as an electron acceptor. Blending the donor materials with the
acceptor materials together prepared by dissolving them in the common solvent and
spin cast to form a BHJ structure is a good way to enhance the interfacial area and to
break photoexcited excitons into free charge carriers. Poly (3-hexylthiophene) (P3HT)
is one of the commercially available donor materials and 1-(3-methoxycarbonyl)
propyl-1-phenyl[6,6]C61 (PCBM) is acceptor material .
When shining the light to the active layer, an electron of the donor material
will absorb a photon and be excited from the Highest Occupied Molecular Orbital
(HOMO) level to the Lowest Unoccupied Molecular Orbital (LUMO) level, leaving a
hole in the HOMO level. Because of the small dielectric constant of organic materials,
this pair of electron and hole (called exciton) is tightly coulombically bound. At the
interface of donor and acceptor, driven by the difference of electron affinity, this pair
of electron and hole is dissociated. Afterwards, the free electron and hole transport
though the bulk and reach the respective electrodes. To be concluded, the process of
conversion of light into electricity by PSC can be described by the following steps12: 1.
Absorption of photon leads to the formation of an exciton; 2. Excion is dissociated at
the ‘D-A’ interface; 3. Free charge carriers transport through the bulk volume; 4. Free
6
charge carriers accumulate at electrodes, respectively. The bulk heterojunction (BHJ)
configuration of the active layer and working principle of OPV are schematically
shown in Figure 1.3
Figure 1.3 (a) Bulk Heterojunction (BHJ) Structure of the active layer in
polymer solar cell, (b) working principle of polymer solar cells. Copyright © 2010
Elsevier Ltd.
There are two critical steps determining how efficiently the device can
convert solar energy to electrical energy, one is the efficient excition dissociation and
the other is the efficient charge transport through bulk active layer. The ultrafast
photophysical studies demonstrate that the photoinduced charge transfer in the D-A
blends happens on a time scale of 50fs.13 For efficient photovoltaic devices, the
created charges need to be transported to the appropriate electrodes within the exciton
life time. Figure 1.4 schematically shows the comparison between photoinduced
charge transfer and its competing processes like photoluminescence and back transfer
which usually happen on the time scale larger than ns.7
7
Figure 1.4 Photoinduced process in the D-A system. (a) photoinduced charge
transfer in a forward direction;(b) exciton recombination happens on a time scale of
nm; (c) charge transfer in a back direction. Copyright © Springer-Verlag Berlin
Heidelberg.
1.2.2.
Device Geometry
The bilayer structure of solar cell was first proposed by C. W. Tang in
1986.14 In the first reported bilayer structure, two layers of small molecules are
layer-by-layer vacuum deposited in the vertical direction on the indium tin oxide( ITO)
coated glass substrate. The device is finalized by the thermal deposition of back
electrode, namely Ag. In such a device, only the exciton created within the distance of
10-20nm from the interface can be efficiently dissociated, and the thicknesses of two
active layers are heavily limited. To circumvent those issues, the Bulk Heterojunction
(BHJ) structure was proposed.11 As mentioned before, Bulk Heterojunction is a blend
of the donor and acceptor components in a bulk volume. The advantages of BHJ over
bilayer structure is twofold: First, in this nano-scale interpenetrating network, each
donor-acceptor (D-A) interface is within 10-20nm length scale to guarantee efficient
exciton dissociation; Second, BHJ can tremendously increase orders of magnitude of
the interfacial area favoring more exciton generation. The idea of the conventional
8
device structure and BHJ configuration of active layer are schematically displayed in
Figure 1.5.
Figure 1.5 (a) the conventional device structure; (b) Bulk Heterojunction
configuration in polymer active layer. Copyright© 2001, Kirchberg in Tirol,
Österreich
In general, there are two geometries existing for a single cell in terms of the
functions of electrodes. The conventional structure and the inverted structures are
schematically shown in the Figure 1.6. In the normal geometry, the device is built up
by stacking the buffer layers and the active layer in sequence on top of ITO ( a high
work-function metal ) and finally covered with a vacuum deposited layer of Al ( a
typical low work-function metal ). This conventional structure suffers from the poor
stability issue because of the easily oxidized Al. Also, Al is hard to achieve roll-to-roll
large-scale printing mass production. To overcome those shortcomings, the alternative
inverted device was proposed 15,16, where the two electrodes are in the opposite
positions.
Figure 1.6. The organic solar cells with (a) conventional geometry and (b)
inverted geometry. Copyright © 2006, American Institute of Physics
9
The two electrodes can be further modified by introducing buffer layers on
the ITO side and the back metal side. The Hole Transportation Layer (HTL) and
Electron Transportation Layer (ETL) are two kinds of buffer layers commonly
employed to selectively transport charge carriers. HTL plays a role of selectively
transporting holes and blocking electrons and ETL selectively transporting electrons
and blocking holes. Poly (3,4-ethylene dioxythiophene): (polystyrene sulfonic acid)
PEDOT:PSS17 and MoO318 are two types of HTL that widely used to improve the
charge extraction in the anode side.
1.2.3.
High Performance: Three Essential Parameters
The current-voltage characteristics of a solar cell under illumination (red line)
and in the dark (black line) are shown in Figure 1.7.19
Figure 1.7 Current-voltage characteristics of a polymer solar cell under
illumination (red line) and in the dark (black line). Copyright © 2007, American
Chemical Society
There are three critical parameters for solar cell efficiency: Open Circuit
Voltage (Voc), Short Circuit Current Density (Jsc), and Fill Factor (FF). The
10
photovoltaic power conversion efficiency of a solar cell is determined in the following
formula.19
Here, Pin is the incident light power density standardized at 1000W/m2, and
Impp and Vmpp are the current and voltage at the maximum power point.
Open Circuit Voltage: The open circuit voltage is the point where the
current-voltage characteristics under illumination intersect with the vertical
coordinates. As mentioned before, one of the two important steps towards efficient
solar cells is that the created charges need to be efficiently transported to the
electrodes. The driving force for this process is the gradient in the chemical potentials
of electrons and holes built up in the donor-acceptor junction, namely built-in
potential. This gradient is determined by the difference between the HOMO level of
the donor and LUMO of the LUMO level of the acceptor. An agreement has been met
that the open circuit voltage is given by this built-in potential.20 However, Voc is not
only related to energy levels of the used materials but also their interfaces, and
therefore the Voc of the real device would vary case by case. The open-circuit voltage
of a conjugated polymer–PCBM solar cell is estimated in the following formula.21
Voc = (1/e)(EDonorHOMO – EPCBMLUMO) – 0.3 V
Short Circuit Current Density: The short circuit current (Isc) is the point
where the current-voltage characteristics under illumination intersect with the
horizontal coordinates, and Short Circuit Current Density ( Jsc) is Isc divided by
11
electrode area. It has been demonstrated that Jsc is correlated to the absorption of the
active layer and exciton dissociation efficiency, which requires that the materials have
a better matched absorption with the solar spectrum and high dielectric constant,
respectively.
In the ideal case, Isc is determined by the product of photoinduced charge
carrier density and the charge carrier mobility within the organic semiconductor.
Isc =neµE
n; density of charge carriers
e; elementary charge
µ; charge carrier mobility
E; electric field
Fill Factor: Telling from the current-voltage characteristics in Figure 1.7, Fill
Factor (FF) is determined by the ratio between the area of the yellow rectangle and
the area of rectangle with grey border. That is to say, the ratio between VMPP *IMPP (or
the maximum power) and Voc*Isc is called the fill factor. In the respect of device
physics, charge carriers reaching the electrodes can determine the fill factor, when the
built-in potential is lowered towards the open circuit voltage. Fill factor can also be
calculated in the following formula:
FF
=
PMAX
VMPP × J MPP
=
Voc × J sc
Voc × J sc
Fill factor is a very important parameter to achieve solar cell high performance, and it is
considerably influenced by the series resistances and finite conductivity of the ITO covered
substrate of solar cell.22
12
1.3.
Polymer Tandem Solar Cells
1.3.1. General Background
In general, the tandem solar cells can be classified into three categories: 1.
Small molecular tandem solar cell; 2. Hybrid tandem solar cell; 3. Fully-solution
processed tandem solar cell.
Since the discovery of ultrafast photoinduced charge transfer, researchers
have been intensively explored how to push upwards the efficiency of solar cells. It
was predicted in 2009 that it is possible the PCE can be over 10% analyzed from the
theoretically standing point, however, the PCE was staying around 5% at that time.23
Nowadays, the PCE for a single cell has been reported as high as 13%.
One bottleneck of further increasing PCE generates from the inherent nature
of a single cell: limited by the band-gap, a semiconductor can’t make the best use of
every energetic photon in the solar spectrum. People have known a while that the way
to broaden the solar spectrum absorption is to make a tandem solar cell. In a tandem
solar cell, two or more single cells are absorbing in a complementary wavelength
range are stacked together. The most commonly employed device structure is a two
terminal tandem cell where two sub-cells are connected in series by an
interconnecting layer. Figure 1.8 shows the typical tandem solar cell and the
simplified procedure of the stacking process.
13
Figure 1. 8. (a) Typical tandem solar cell device geometry and (b) simplified
procedure of the stacking process of two sub-cells. Copyright © 2009, Royal Society
of Chemistry
In 1990, the tandem solar cell was first proposed where the small molecules are
employed as an active layer in the two sub-cells.8 In this spirit, the construction of a
tandem structure can be easily manipulated by dry-coating method. However, due to
the limited choice of small molecular donor materials which carry considerable
different absorption profiles, the hybrid tandem solar cells are further explored.24 In
the hybrid tandem solar cell, the bottom sub-cell is processed from polymers from
solution process, while the top cell is still through the thermal deposition of small
molecules. Driven by the prospect that it is possible to achieve roll-to-roll large scale
production through printing technology, the fully solution processed tandem solar cell
is further explored. Not until in 2006 did Kawano et al.9 developed the first fully
solution processed tandem cells using two identical polymer Bulk Heterojunction
(BHJ) sub-cells. Generally, an ideal tandem structure would utilize a large band-gap
cell as the front cell and the low band-gap cell as the bottom cell. It was in the same
year, Hadipour et al. first fabricated a tandem solar cell employing a low band-gap
polymer and a large band-gap polymer at the same time, and PCE reported 0.57%.25
A milestone of the PCE improvement in the tandem solar cell occurs at the year of
14
2007 when Kim et al. demonstrated the efficiency of all solution processed polymer
tandem solar cell can be over 6%.26 The further calculation suggests that tandem solar
cell with more than 15% power conversion efficiency is feasible.27 However, due to
several issue such as the still inefficient utilization of solar spectrum, large series
resistance, and thermal energy loss, the highest efficiency reported so far is slight
larger than 7%.28
To circumvent those issues and achieve high performance, several criteria
should be considered, for instance, sub-cells with minimum absorption overlap, an
efficient intermediate layer, and compatible fabrication process. Many research efforts
so far have been put into the new polymer design for active layer, and one big
problem remaining is how to make a good intermediate layer that can successfully
connect two sub-cells with the minimum energy loss. In addition, it is more attractive
if the intermediate layer can be both efficient and cheap, so that the fabrication
expense would be low down.
1.3.2.
Efficient Single Cells
In the case that two sub-cells are connected, the three critical parameters of
the whole device, namely Open Circuit Voltage (VOC), Short Current Density (JSC) ,
Fill Factor( FF) would inevitably differ from the single cells. It has been demonstrated
that in the ideal case assuming no potential loss in the intermediate layer or device
fabrication derivation from the standard procedure, the VOC of the tandem solar cell is
the sum of the two sub-cells: VOC=VFront + VBottom. On the other hand, the total
generated photocurrent will be constant throughout the device. it is also believed that
15
JSC of the tandem cell is limited by the smallest JSC going through the component cell
on the condition that the fill factor of the two sub-cells are the same.29 When in a
more realistic case where FF of the two component cells are not identical, JSC of the
tandem solar cell is more dominated by the cell having higher FF. To be concluded,
the general relationship between VOC and JSC of the tandem solar cell and the
component sub-cells is displayed in the following formulas.
JTandem = JBottom + JTop
VTandem = VBottom + VTop
Because of the narrow solar spectrum absorption of a single cell limits PCE,
it is critical to employ two sub-cells with complementary solar spectrum absorption in
a tandem solar cell. Due to the embedded nature of the intermediate layer and its
non-conductive nature in a 2-terminal tandem cell, characterization of two sub-cells
independently in a tandem structure is impossible. However, the simulation statistics
demonstrates that the bottom cell (Figure 1.9) 29 generates much more photocurrent
than the top cell under short circuit condition.
Figure 1.9. The current-voltage characteristics of two sub-cells under
illumination. The front cell delivered more photocurrent than the bottom cell.
Copyright © 2008, Elsevier
16
Besides materials’ selection for active layer, the thickness of the thin film
need to be well tuned in a way that more solar photon can be absorbed in the bottom
cell as much as possible to balance the JSC in both top and bottom cells.
1.3.3.
Efficient Intermediate Layer
The importance of employing an intermediate layer is never overstated.
Fabrication of the sub-cells in series without a separation layer in between them will
cause the formation of an inverse bulk heterojunction (BHJ) between the donor layer
of the top cell and the acceptor of the bottom layer. Hence, the critical step in making
a good tandem solar cell is to make an efficient intermediate layer. An inefficient
intermediate layer brings about potential loss leading to the VOC of the tandem solar
cell is not equal to the sum of the VOC of the component cells. There are three main
requirements for such a recombination contact: First, it has to ensure that the electrons
from the first sub cell and the holes from the second sub cell meet at the same energy
level. Therefore a splitting of the electron and hole quasi-Fermi levels has to be
avoided. Second, the recombination contact has to be highly transparent to avoid
absorption and reflection reducing the power conversion efficiency and disturbing the
current matching in the tandem solar cell. Third, it should be compatible to future
mass production processes.30 Figure 1.10 shows the basic principle of an organic
tandem solar cell using an intermediate layer.30
17
Figure 1.10 Basic principle of an organic tandem solar cell using an intermediate
layer. The arrows indicate the hole currents and the electron currents. ETL denotes the
electron transport layer and HTL indicates the hole transport layer. ITO is the
conductive transparent indium tin oxide bottom contact. Copyright © 2010, American
Institute of Physics
In solid-state physics, the work function is determined by the minimum energy
needed to remove an electron from a solid to a point outside the solid surface (or
energy which is needed to move an electron from the Fermi level into
vacuum).31When the work function of a metal is smaller than the fermi level of a
semiconductor, then after these two materials contact, the electron will flow from the
metal into the semiconductor leading to the metal positively charged (Figure 1. 11).
Under this condition, there is no barrier for the charge transfer, and we call this
contact Ohmic contact.
18
Figure 1.11． Simplified energy level diagram of the metal and n-type
semiconductor (a) before contact and (b) band bending in the Ohmic contact.
A possible approach to the desired intermediate layer is to make use of thick
metal layers. If the layers are thick enough, a closed metal layer is formed which acts
as an Ohmic contact. This approach has been introduced in polymer solar cells,
because the mental can prevent the underlying layer from dissolving during the
spin-casting of the second solar cell.32,33 The disadvantage of this approach is the high
absorbance and reflectance of the metal layer resulting in the losses and unbalanced
absorption in the sub-cells. An alternative approach is to use the highly doped
semiconducting layers (n-type and p-type) as an intermediate layer (Figure 1.12)30.
Figure 1.12 Schematic energy level diagram at open circuit of a double heterojunction solar cell with highly doped layers as recombination contact. Copyright
© 2010, American Institute of Physics
From this energy level diagram, we can observe that the doped
semiconductor layers form Ohmic contact with back and front cells. This ohmic
contact can efficiently extract electrons and holes from the two sub-cells. Typical
materials for n-type semiconductor are solution processed TiOX and ZnO2, and
p-type semiconductors are poly (ethylenediox-ythiophene) doped with poly
(styrenesulfonate) (PEDOT: PSS), MOO3, and WO3.34 However, these materials
still need to be modified to serve as an efficient part of intermediate layer. One big
19
issue of the olution-processed ZnO in as an n-type intermediate layer lies in the
inherent nature of its solution process that can’t withstand the top polymer sub-cell.
Besides efficiently extracting electrons and holes from the two component
cells, the intermediate layer should also act as an efficient recombination zone for the
charge carriers. A thin layer of mental material is a promising candidate to serve this
purpose given the good conductivity and suitable working function.
The rectification of the current-voltage characteristics in dark has also been
studied recently. Rectification concept originally comes from rectifier device. A
rectifier can convert alternating current (AC) to direct current (DC), and this process is
known as rectification. In the polymer solar cell, rectification represents how many
orders of magnitude of the current at the most positive bias voltage higher than the
current at the most negative bias voltage for the current-voltage characteristics in the
dark.
To date, more and more work is focusing on how to improve the rectification
of the current-voltage characteristics, but merely no breakthrough has been make in
this area. Some reports have demonstrated that the UV illumination can help to
increase the rectification of the current-voltage characteristics in dark (Figure 1.13).35
20
Figure 1.13 Dark Current Density verse Voltage (J–V) characteristics of a tandem
cell before and after light illumination. Copyright © 2010 WILEY-VCH Verlag
GmbH & Co. KGaA, Weinheim
1.3.4.
Tandem Polymer Solar Cell Characterization.
Due to the embedded nature of the interlayer and its non-conductive nature
of a two-terminal tandem cell, characterization of sub-cells independently in a tandem
structure is impossible. Therefore, the way to characterize tandem cells is most based
on the knowledge of a single cell. For a single cell, poly (3-hexylthiophene) (P3HT) is
one of the most studied polymer BHJ solar cell applications. Important qualities of the
thin film include the surface morphology and the degree of crystallinity domain are
very necessary in the study of power conversion efficiency. The film morphology can
be further modified through thermal annealing leading to better order within the P3HT
and mixing of the blend.36 As a critical parameter, the degree of phase separation can
be studied by Atomic Force Microscopy (AFM) and Transmission Electron
Microscopy (TEM).37 In addition, the high-resolution cross-section TEM can reveal
the layer by layer feature in the tandem solar cell. The Power Conversion Efficiency
(PCE), Open Circuit Voltage (VOC), Short Current Density (JSC), and Fill Factor (FF)
21
can be detected in a way that the device is test under the solar simulator and analyzed
by Keithley 236 source measurement.
1.3.5.
Processing Issues of the Tandem Structure
One of the major challenges in fabricating polymer tandem solar cells is
layer-by-layer solution procedure without washing away the layer underneath. Initial
efforts on the hybrid tandem solar cell structure fabrication utilized a thermal
deposition method to build up the bottom cell, and the fully solution processed
tandem cells are even more challenging. One approach is to use orthogonal solvents
so that solution processing of one layer does not affect the underlying layer. Most
polymers for solar cell application are soluble in chlorinated organic solvents such as
dichlorobenzene (DCB), chloroform (CB) etc. Therefore, finding a solvent for the top
polymer layer that does not dissolve the underlying polymer layer is very difficult.
One concern raised by Sista et al is that the use of low boiling point solvent (the
solvent drying process is fast) for the upper polymer layer can largely prevent damage
to the underlying polymer layer.
22
CHAPTER II
EXPERIMENT
2.1.
Materials Preparation
2.1.1 Substrate: Conductive and transparent indium tin oxide (ITO) coated glass
substrates with a sheet resistance of 15 V and surface roughness ~2nm have been
purchased. Rectangular pieces of the correct size for the experiments will be cut
especially for solar cell device fabrication.
2.1.2. PEDOT:PSS: the doped p-type soluble semiconductor PEDOT:PSS (VP AI
4083 from H. C. Stark, PEDOT4083) has been purchased from Heraeus Precious
Metals GmbH & Co.. The material has been stored in the refrigerator.
2.1.3. Barium oxide: The barium oxide (BaO) has been purchased from
Sigma-Aldrich. It is a white powder with the density of 5.72 g/mL at 25 °C(lit.).
23
2.1.4. Solvent: For achieving optimum performance, we used chlorobenzene (CB) as
the solvent for upper polymer layer and dichlorobenzene (DCB) as the solvent for
underlying polymer layer.
2.1.5. PCPDTBT: PCBM and P3HT:PCBM ratio and concentration: The best device
performance is achieved when the mixed solution has PCPDTBT:PCBM ratio of
1.0:3.0 with a concentration of 0.7wt % PCPDTBT plus PCBM 2.5wt% in
Chlorobenzene (CB), and P3HT:PCBM ratio of 1.0 : 0.8 with a concentration of 1wt%
P3HT plus PCBM 0.8wt% in a mixed solvent composed of 97% Dichorobenzene
(DCB) and 3% 1,8-diiodooctane (DIO). The molecular structures of the active
materials: PCPDTBT, P3HT, PCBM are schematically shown in figure 2.1.38
O
N
S
S
S
O
N
n
S
n
n
Figure 2.1 Molecular structures of PCPDTBT, P3HT, PCBM respectively
2.2.
Device Fabrication Procedures
2.2.1. Substrate clearance: The ITO-coated glass substrate was cleaned with
detergent, then ultrasonicated in acetone and isopropyl, and subsequently dried in an
oven overnight.
2.2.2. Device Fabrication Procedure: Polymer tandem cells were prepared
according to the following procedure: The initially cleaned ITO-coated glass substrate
24
was UV-Ozone treated for 20 mins and hold for another 20 mins. Conducting
poly(3,4-ethylenedioxylenethiophene)-polystylene sulfonic acid (PEDOT:PSS) was
spin-cast (4000 rpm, 30s) with thickness ~40 nm from aqueous solution (after passing
a 0.45 µm filter). The substrate was dried for 10 minutes at 150˚C in air, and then
after cooling down to room temperature, moved into a glove box for spin-casting of
the photoactive layer. The dichlorobenzene (DCB) solution comprised of P3HT
(1wt%) plus PCBM (0.8wt%) was then spin-cast at 1200 rpm with thickness ~150 nm
on top of the PEDOT layer to become the first charge separation layer of the tandem
cell. Then the substrate was placed on a hot plate, thermal annealed at 80˚C for
15mins to further increase the degree of crystallinity. Afterwards, the substrate was
pumped down in vacuum (~10-6 torr), and a thickness of 5nm barium oxide (BaO),
2.5nm Ag, 5nm molybdenum oxide (MOO3) was thermal deposited at the rate of
0.1A/s in sequence. After finishing the deposition of the intermediate layer, the
substrate was transferred into the glove-box to spin coating another polymer system of
PCPDTBT: PCBM at the spin rate of 1600rpm/s with thickness~100nm. Then the
substrate was thermal annealing at 80˚C for 5 mins. Finally, the device was pumped
down in vacuum (~10-6 torr) again, and a ~100 nm Al electrode was deposited on top.
The deposited Al electrode area defines an active area of the devices as 4.5 mm2.
Therefore, the structure of the polymer tandem solar cell is ITO/40 nm PEDOT/150
nm P3HT: PCBM /5 nm BaO / 2.5nm Ag / 5 nm MOO3 / 100nm PCPDTBT:PSS/5nm
Ca/100 nm Al.
25
Another tandem solar cell device was fabricated with the upper polymer
system using P3HT:PCBM, and the fabrication procedure is as in a similar fashion as
the former stated one. The device geometry with P3HT:PCBM and PCPDTBT:
PCBM as upper polymer system are schematically shown in Figure 2.2.
Figure 2.2. Polymer tandem solar cell geometry with ( a) PCPDTBT:PCBM as
an upper layer and (b) P3HT:PCBM as an upper layer.
2.3.
Calibration and Characterization
For calibration of the solar simulator, the solar spectrum was carefully
minimized using an AM 1.5G filter, and then the light intensity was calibrated using
calibrated standard silicon solar cells. Current density-voltage characteristics were
measured with a Keithley 236 source measurement unit.
2.4.
UV-Vis Absorption Spectrum
The absorption spectrum of the reference single cells and the corresponding
tandem cells were recorded by using a spectrometer (Hitachi U-3900 PC).
2.5.
Atomic Force Microscopy (AFM) Observation
26
The quality of the buffer layer and the intermediate layer including the
surface roughness were checked by Atomic Force Microscopy (AFM). The AFM was
used in tapping model. The AFM images were taken on a surface area of 5.0µm *
5.0µm. The instrument settings were a scan rate of 0.996 Hertz with a set-point of
330mV. At least two images weretaken from separate locations on each sample to
ensure that they are representative. The AFM images will be analyzed using the
Nanoscope Analysis software.
2.6.
Sol-gel ZnO Nanoparticals Preparation
The fabrication of sol-gel processed ZnO nanoparticle films with different
surface morphologies were made from spin coating the same precursor solution but
annealing under different conditions. The precursor solution, consisting of 0.75M zinc
acetate dihydrate and 0.75M monoethanolamine in 2-methoxyethanol was first
spun-coated onto indium tin oxide (ITO) substrates at 4000rpm for 40s. For fast
annealing treatment, the substrate was immediately placed onto a hotplate that was
preheated at 250 oC for 5min. For the slow annealing treatment, the spin-coated
substrate was first placed onto a hotplate that was initially at room temperature while
it was still not completely dry. The temperature was then raised at a ramping rate of
50 oC/min to 250 oC and the substrates were subsequently removed from the hot plate
when the final temperature was reached.
27
CHAPTER III
RESULTS
3.1. Energy Levels
Because of the embedded nature of the tandem solar cells, it’s impossible to
investigate the two sub-cells in the tandem structure independently. Hence, we
fabricated the two reference cells independently with the geometries: (a) ITO/PEDOT:
PSS/P3HT: PCBM/Ca/Al. (b) ITO/PEDOT: PSS/PCPDTBT: PCBM /Ca/Al. The
28
energy levels of the single reference cells are schematically shown in Figure 3.1.
Figure 3.1 Energy levels of single cells composed of bulk polymer blends (a)
P3HT:PCBM and (b) PCPDTBT:PCBM.
As for the tandem solar cell, both two types of tandem solar cells use
polymer system of P3HT: PCBM as the bottom cell while different materials for
upper cell, the proposed tandem solar cell device geometries are:
(a) ITO/PEDOT:PSS/P3HT:PCBM/BaO/Ag/MOO3/P3HT:PCBM/Ca/Al.
(b) ITO/PEDOT:PSS/P3HT:PCBM/BaO/Ag/MOO3/PCPDTBT:PCBM/Ca/Al.
Figure 3.2 is the energy-level diagram showing the HOMO and LUMO
energies of each of the component materials in a tandem structure.
29
Figure 3.2
The energy levels of the tandem solar cells composing the upper
polymer layer of (a) P3HT:PCBM and (b) PCPDTBT:PCBM.
3.2.
Performance Investigation
3.2.1.
Current Density-Voltage (J-V) Characteristics
(a) ITO/PEDOT: PSS/P3HT:PCBM/BaO/Ag/MOO3/P3HT:PCBM/Ca/Al.
The Current Density- Voltage (J-V) characteristics under Air Mass 1.5
Global (AM1.5G) illumination and in dark of the reference cell P3HT:PCBM and the
tandem structure with two identical sub-cells P3HT:PCBM are shown in Figure 3.3.
The photovoltaic performances are summarized in Table 2.
30
a
31
b
Figure 3.3 The Current Density-Voltage (J-V) characteristics of single reference
cells using P3HT:PCBM and tandem cell fabricated using the identical reference cells.
(a) J-V curves under illumination and (b) in dark.
Table 1.
cell.
Photovoltaic performance of the single cells and the corresponding tandem
Device
P3HT : PCBM
Tandem
PCE (%)
2.31
1.00
VOC (V)
0.60
1.20
Jsc ( mA /cm2)
6.93
1.78
FF (%)
55.4
47.0
(b) ITO/PEDOT: PSS/P3HT:PCBM/BaO/Ag/MOO3/PCPDTBT : PCBM/Ca/Al.
The Current Density –Voltage (J-V) Characteristics of single solar cells and
the tandem solar cell with P3HT: PCBM and PCPDTBT: PCBM polymer systems are
shown in Figure 3.4. The characterization was under Air Mass 1.5 global (AM1.5G)
32
illumination. The photovoltaic performance of the single cells and tandem cells are
summarized in Table 2.
a
33
ab
1
Figure 3.4 The current density-voltage (J-V) characteristics of single reference cells
using P3HT:PCBM and PCPDTBT:PCBM and tandem cell fabricated using the same
polymer system. (a) J-V curves under illumination and (b) in dark.
Table 2. Photovoltaic performance of the single reference cells and the
corresponding tandem cell.
Device
PCE (%)
VOC (V)
JSC (mA
FF (%)
/cm2)
P3HT : PCBM
2.31
0.60
6.93
55.4
PCPDTBT :PCBM
2.38
0.65
7.61
48.2
Tandem
1.06
0.95
2.70
41.6
34
3.2.2.
UV-Vis Absorption
The absorption spectrum of a film of the bulk heterojunction composite of each sub
cells containing P3HT:PCBM and PCPDTBT:PCBM polymer systems, respectively,
and the bilayer tandem cell composed of P3HT:PCBM/PCPDTBT:PCBM is shown in
Figure 3.5.
Absorbance (a.u.)
1.5
P3HT:PCBM
PCPDTBT:PCBM
P3HT:PCBM/PCPDTBT:PCBM
1.0
0.5
0.0
300 350 400 450 500 550 600 650 700 750 800 850
Wavelength (nm)
Figure 3.5 Absorption spectra of a PCPDTBT:PCBM bulk heterojunction composite
film, a P3HT:PCBM bulk heterojunction composite film, and a bilayer of the two as
relevant to the tandem device structure. a.u., optical density.
The PCPDTBT:PCBM polymer system has weak absorption in the visible
spectral range but has two strong bands: one in the near-infrared (near-IR) between
700 and 850 nm resulted from the interband p-p* transition of the PCPDTBT and one
in the ultraviolet (UV) arising primarily from the HOMO-LUMO transition of the
PCBM. The absorption of the P3HT:PCBM film falls in the complementary apart of
the PCPDTBT:PCBM spectrum and covers the visible spectral range. The electronic
absorption spectrum of the tandem structure can be just described as a superposition
of the two complementary composites absorption spectra. In addition, the
35
PEDOT:PSS and intermediate layers have negligible absorption in the tandem device
structure.
3.2.3.
Atomic Force Micrometer (AFM) Images Observation
Figure 3.6 illustrates the Atomic Force Micrometer (AFM) tapping mode
height images of the intermediate layer BaO/Ag/MOO3 and Hole Transportation
Layer PEDOT: PSS coated on the ITO substrate. The islands observed are due to
surface roughness and it is clear to see that the distribution of the up-and-down trend
is more intense on the PEDOT:PSS surface. The calculated room-mean-square
roughness of the intermediate layer and PEDOT:PSS are 0.99nm and 1.72nm,
respectively.
Figure 3.6 AFM images of (a) MOO3 surface morphology of BaO/Ag/MOO3
intermediate layer and (b) PEDOT:PSS on ITO coated glass substrate. The islands
observed are due to surface roughness. Note that the islands distribution is more
intensive for (b), indicating the roughness is higher for PEDOT:PSS.
36
3.3.
Comparison of the Intermediate layers
Figure 3.7 shows AFM height images of sol-gel ZnO nanoparticles fabricated on a
ITO coated substrate. It can be seen that the size of ZnO is significantly greater under
the fast annealing condition. The calculated Root-Mean-Square roughnesses of the
ZnO nanoparticles under fast-annealing and slow-annealing are 3.82nm and 1.39nm,
respectively.
Figure 3.7 AFM height profiles of ZnO nanoparticles under the condition of (a) fast
annealing and (b) slow annealing
We fabricated the intermediate layer containing an electron transportation
layer sol-gel ZnO nanoparticles and a hole transportation layer thermal deposited
MoO3. The ZnO thin films are prepared in two annealing fashions namely, fast
annealing and slow annealing. The electronic performance of the intermediate layer
and the procedures are illustrated in Figure 3.8.
37
Figure 3.8 The intermediate layer composed of ZnO/MoO3 (a) Current
Density-Voltage (J-V) Characteristics and (b) Sol-gel preparation of ZnO
nanoparticles.
As we can tell from the Figure 3.8, the current under the slow annealing
condition shows a better diode rectification within the sweep voltage from -1V to 1V.
However, when the applied bias was extended as large as 2V, the current under
negative bias is just slightly smaller than the current in the forward bias. The obvious
diode rectification performance different in the low sweep voltage may tentatively
due to the different surface roughness of the ZnO thin film. As the surface get rougher,
there are more defects on the surface leading to the electron traps. In that case, under a
low bias voltage, the current density versus voltage performance is quite different.
When comes to a larger bias, the ZnO and MoO3 intrinsic property would dominate
the diode performance and annealing method does not have much influence.
Figure 3.9 illustrates the BaO/MoO3 P-N junction current density versus
voltage performance and the conductivity property of the intermediate layer
BaO/Ag/MoO3. The diode performance of BaO/MoO3 is comparable to the
ZnO/MoO3 with a slightly decrease in the negative bias voltage. After inserting an
38
electron hole recombination layer Ag in between BaO and MoO3, the intermediate
layer shows a very good electronic conductivity as illustrated in Figure 3.9 (b).
Figure 3.9 The electronic performance of BaO/Ag/MoO3 intermediate layer. (a)
Diode property of BaO/MoO3 P-N junction; (b) Conductive property of
BaO/Ag/MoO3 intermediate layer.
39
To further investigate the quality of these two intermediate layers, we
checked the surface morphology of them and make comparison (Figure 3.10).The
calculated root-mean-square roughness ZnO/MoO3 is 1.16nm which is slightly
rougher than the surface of BaO/Ag/MoO3.
Figure 3.10 AFM images of two types of intermediate layers (a) ZnO/MoO3 and (b)
BaO/Ag/MoO3 on ITO coated glass substrate. The islands observed are due to surface
roughness.
40
CHAPTER IV
DISCUSSION, CONCLUSIONS AND OUTLOOK
From Figure 3.3 and Table 1, we can clearly observe that VOC of the tandem
solar cell double the VOC of the sub cells. However, the Jsc is comparatively low
compared to the single reference cell. This is tentatively attributed to the non-fully
absorption of the solar spectrum due to the identical sub cells.
The PCPDTBT:PCBM polymer system has weak absorption in the visible
spectral range but has two strong bands: one in the near-infrared (near-IR) between
700 and 850 nm one in the ultraviolet (UV) . The absorption of the P3HT:PCBM film
falls in the “hole” in the PCPDTBT:PCBM spectrum and covers the visible spectral
range. From Table 2 we can see that the VOC of the tandem cell is 0.95, which is
almost 85% of the sum of VOC of the two component cells. The 15% of VOC loss
indicates that there is some potential loss in this layer-by-layer construction. Some
factors can be responsible to the potential loss/energy loss such as the thermalization
of carriers and electron traps at the interface. From the J-V curves under illumination
of the tandem cell shown in Figure 3.4, we noted that there is a significant hump,
so-called‘S-shape’, near VOC.
It has been demonstrated by researchers that this ‘S-shape’ should be
related to an interfacial barrier for charge transport.39 In the theoretical standing point
of energy levels of photoelectronic materials: when shining the light, the exciton
41
generated and then set apart at donor-acceptor (D-A) interface in the front
cell. The electron will transport from the D-A interface through the bulk of the
acceptor material PCBM and reaches the interface between bulk hetrojunction (BHJ)
polymer layer and BaO. LUMO levels offset of P3HT and BaO is approximately 3eV.
Regarding Ag/MoO3, since MoO3 is hole-transport semiconductor, when the hole
transports in a direction from MoO3 to Ag, this is merely no contact barrier for hole.
Finally, the electron and hole combines in the recombination area where Ag is serving
at this purpose.
To be concluded, one possible reason of the energy loss may locate at the
interface between the polymer blends and intermediate layer. The electron may traps
at the interfacial defects and could not be wiped out by the sweep voltage, then part of
the solar energy may be converted to thermal energy leading to a significant potential/
energy loss. We noted that despite the fact the PCPDTBT has almost identical
photovoltaic performance in terms of PCE, JSC, FF is severally lower than P3HT. Fill
factor is determined by charge carriers reaching the electrode, and it denotes the
competition process between charge carrier recombination and transportation. That is
to say, in a single cell, the lower FF, the higher series resistance the cell would have.
Furthermore, in the tandem cell, the overall photovoltaic performance is limited by
the component cell possessing lower FF.
From the J-V curves in Figure 3.3 and Figure 3.4, Jsc of the tandem structure
is always lower than that of the reference cells. The reason is tentatively attributed to
41
the enhanced series resistance and contact barrier of the tandem cell compared to the
reference single cells.
Interfacial quality is another critical point in deciding the performance of the
solar cell and we demonstrated the importance by AFM observation (Figure 3.6). For
many commonly employed single cells, active polymer layers are spin casted directly
on the surface of PEDOT :PSS and acceptable performance is reached. Our results
proved that our proposed intermediate layer at least has the compatible surface quality
with PEDOT :PSS. The smoother surface can favor the spread of polymer solution
and form a uniform thin firm after dying.
To be concluded, the key feature of the polymer solar cells is an efficient
recombination contact at the interface between the solar cells in stack, and our
proposed structure BaO/Ag/MoO3 proves to be an efficient intermediate layer.
(1) BaO is a high conductive, wide band-gap n-type semiconductor.
Compared to the sol-gel processed ZnO as part of the intermediate layer,
the fully thermal evaporation fashion of our proposed layer is both
continuous and convenient.
(2) The concept of metal clusters sandwiched between n-type and p-type
semiconductors is promising in the intermediate layer construction.
(3) This proposed intermediate layer can efficient connect two sub cells
together with diminished potential loss.
The present laboratory scale effort will conclude with completion of certain
tasks and collection of certain data and that is not presently available, including:
42
(1) Trying to use other polymer systems associated with BaO/Ag/MoO3
intermediate layer to fabricate tandem solar cell.
(2) Fabricating the tandem solar cell with an inverted structure with opposite
electrodes compared to the conventional geometry.
(3) The interfacial modification of the intermediate layer, including
diminishing electron traps and better energy alignment.
43
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