This document discusses research into developing monolithically integrated cadmium telluride (CdTe) solar cell devices deposited via atmospheric pressure metal-organic chemical vapor deposition (AP-MOCVD). The research aims to improve the fabrication process and efficiency of CdTe modules. Key steps studied include AP-MOCVD deposition of CdZnS/CdTe layers, addition of back contacts via thermal evaporation or screen printing, monolithic integration via mechanical scribing, and characterization of solar cell performance. Issues addressed include delamination, improving scribing precision, and damage to scribing tips. The goal is to advance the process from single solar cells to interconnected photovoltaic modules.
Device simulation of perovskite solar cells with molybdenum disulfide as acti...journalBEEI
Organo-halide Perovskite Solar Cells (PSC) have been reported to achieve remarkably high power conversion efficiency (PCE). A thorough understanding of the role of each component in solar cells and their effect as a whole is still required for further improvement in PCE. In this paper, the effect of Molybdenum Disulfide (MoS2) in PSC with mesoporous structure configuration was analyzed using Solar Cell Capacitance Simulator (SCAPS). With the MoS2 layer which having two-fold function, acting as a protective layer, by preventing the formation of shunt contacts between perovskite and Au electrode, and as a hole transport material (HTM) from the perovskite to the Spiro-OMETAD. As simulated, PSC demonstrates a PCE, ŋ of 13.1%, along with stability compared to typical structure of PSC without MoS2 (Δ ŋ/ŋ=-9% vs. Δ ŋ/ŋ=-6%). The results pave the way towards the implementation of MoS2 as a material able to boost shelf life which very useful for new material choice and optimization of HTMs
International Journal of Engineering Research and Applications (IJERA) is an open access online peer reviewed international journal that publishes research and review articles in the fields of Computer Science, Neural Networks, Electrical Engineering, Software Engineering, Information Technology, Mechanical Engineering, Chemical Engineering, Plastic Engineering, Food Technology, Textile Engineering, Nano Technology & science, Power Electronics, Electronics & Communication Engineering, Computational mathematics, Image processing, Civil Engineering, Structural Engineering, Environmental Engineering, VLSI Testing & Low Power VLSI Design etc.
Device simulation of perovskite solar cells with molybdenum disulfide as acti...journalBEEI
Organo-halide Perovskite Solar Cells (PSC) have been reported to achieve remarkably high power conversion efficiency (PCE). A thorough understanding of the role of each component in solar cells and their effect as a whole is still required for further improvement in PCE. In this paper, the effect of Molybdenum Disulfide (MoS2) in PSC with mesoporous structure configuration was analyzed using Solar Cell Capacitance Simulator (SCAPS). With the MoS2 layer which having two-fold function, acting as a protective layer, by preventing the formation of shunt contacts between perovskite and Au electrode, and as a hole transport material (HTM) from the perovskite to the Spiro-OMETAD. As simulated, PSC demonstrates a PCE, ŋ of 13.1%, along with stability compared to typical structure of PSC without MoS2 (Δ ŋ/ŋ=-9% vs. Δ ŋ/ŋ=-6%). The results pave the way towards the implementation of MoS2 as a material able to boost shelf life which very useful for new material choice and optimization of HTMs
International Journal of Engineering Research and Applications (IJERA) is an open access online peer reviewed international journal that publishes research and review articles in the fields of Computer Science, Neural Networks, Electrical Engineering, Software Engineering, Information Technology, Mechanical Engineering, Chemical Engineering, Plastic Engineering, Food Technology, Textile Engineering, Nano Technology & science, Power Electronics, Electronics & Communication Engineering, Computational mathematics, Image processing, Civil Engineering, Structural Engineering, Environmental Engineering, VLSI Testing & Low Power VLSI Design etc.
Effect of morphology on the photoelectrochemical performance of nanostructure...Pawan Kumar
Cu2O is a promising earth-abundant semiconductor photocathode for sunlight-driven water splitting. Characterization results are presented to show how the photocurrent density (Jph), onset potential (Eonset), band edges, carrier density (NA), and interfacial charge transfer resistance (Rct) are affected by the morphology and method used to deposit Cu2O on a copper foil. Mesoscopic and planar morphologies exhibit large differences in the values of NA and Rct. However, these differences are not observed to translate to other photocatalytic properties of Cu2O. Mesoscopic and planar morphologies exhibit similar bandgap (e.g.) and flat band potential (Efb) values of 1.93 ± 0.04 eV and 0.48 ± 0.06 eV respectively. Eonset of 0.48 ± 0.04 eV obtained for these systems is close to the Efb indicating negligible water reduction overpotential. Electrochemically deposited planar Cu2O provides the highest photocurrent density of 5.0 mA cm−2 at 0 V vs reversible hydrogen electrode (RHE) of all the morphologies studied. The photocurrent densities observed in this study are among the highest reported values for bare Cu2O photocathodes.
Band gap engineering of hybrid perovskites for solar cellsKiriPo
The research was conducted in summer 2014 under supervision of professor David Cahen at Optoelectronics Materials Group in Department of Materials and Interfaces at Weizmann Institute of Science (Rehovot, Israel).
Progress in all inorganic perovskite solar cellMd Ataul Mamun
Since their first introduction in the research arena, the hybrid organic-inorganic perovskite photovoltaic cells have been showing frequent record breaking power conversion efficiencies (PCEs). Despite the rapid increase in PCE by engaging new perovskite materials as active layers as well as new fabrication techniques, their stability remains too poor to go for a mass production. Mainly the organic materials in the hybrid PSCs are responsible for this instability. Consequently, very recently, different approaches are taken to replace these organic components by inorganic ones to fabricate all-inorganic PSCs. Though these first-generation all-inorganic PSCs are yet to produce competitive PCEs like their counterparts, they have already demonstrated superb stability to be a propitious bidder for solar cell energy yielding. The state-of-the-art quantum dots based cells shown efficiency as high as 10.77% and intact stability for months.
An introduction of perovskite solar cellsalfachemistry
This article introduces the development, structure and work mechanism of perovskite solar cells. Visit https://www.alfa-chemistry.com/products/perovskite-solar-cells-139.htm for more information.
this is going to show you how we convert spending into savings witch creates the shopping annuity as a customer or Unfranchies owner you can transfer your buying habits to save and create a shopping annuity shop.com is on the cutting edge of online shopping and technology and they just keep getting better. contact me for more information on becoming a business partner or customer . trish64odi@gmail.com
Effect of morphology on the photoelectrochemical performance of nanostructure...Pawan Kumar
Cu2O is a promising earth-abundant semiconductor photocathode for sunlight-driven water splitting. Characterization results are presented to show how the photocurrent density (Jph), onset potential (Eonset), band edges, carrier density (NA), and interfacial charge transfer resistance (Rct) are affected by the morphology and method used to deposit Cu2O on a copper foil. Mesoscopic and planar morphologies exhibit large differences in the values of NA and Rct. However, these differences are not observed to translate to other photocatalytic properties of Cu2O. Mesoscopic and planar morphologies exhibit similar bandgap (e.g.) and flat band potential (Efb) values of 1.93 ± 0.04 eV and 0.48 ± 0.06 eV respectively. Eonset of 0.48 ± 0.04 eV obtained for these systems is close to the Efb indicating negligible water reduction overpotential. Electrochemically deposited planar Cu2O provides the highest photocurrent density of 5.0 mA cm−2 at 0 V vs reversible hydrogen electrode (RHE) of all the morphologies studied. The photocurrent densities observed in this study are among the highest reported values for bare Cu2O photocathodes.
Band gap engineering of hybrid perovskites for solar cellsKiriPo
The research was conducted in summer 2014 under supervision of professor David Cahen at Optoelectronics Materials Group in Department of Materials and Interfaces at Weizmann Institute of Science (Rehovot, Israel).
Progress in all inorganic perovskite solar cellMd Ataul Mamun
Since their first introduction in the research arena, the hybrid organic-inorganic perovskite photovoltaic cells have been showing frequent record breaking power conversion efficiencies (PCEs). Despite the rapid increase in PCE by engaging new perovskite materials as active layers as well as new fabrication techniques, their stability remains too poor to go for a mass production. Mainly the organic materials in the hybrid PSCs are responsible for this instability. Consequently, very recently, different approaches are taken to replace these organic components by inorganic ones to fabricate all-inorganic PSCs. Though these first-generation all-inorganic PSCs are yet to produce competitive PCEs like their counterparts, they have already demonstrated superb stability to be a propitious bidder for solar cell energy yielding. The state-of-the-art quantum dots based cells shown efficiency as high as 10.77% and intact stability for months.
An introduction of perovskite solar cellsalfachemistry
This article introduces the development, structure and work mechanism of perovskite solar cells. Visit https://www.alfa-chemistry.com/products/perovskite-solar-cells-139.htm for more information.
this is going to show you how we convert spending into savings witch creates the shopping annuity as a customer or Unfranchies owner you can transfer your buying habits to save and create a shopping annuity shop.com is on the cutting edge of online shopping and technology and they just keep getting better. contact me for more information on becoming a business partner or customer . trish64odi@gmail.com
save on you energy bill save the environment build your shopping annuity we all buy light bulbs why not save money and the environment at the same time . We all need to do what we can to help the environment. for more information contact me .
www.shop.com/oddi
The best network marketing retail sales business 24 years with over 80 consecutive quarters of growths and billions in sales
not a MLM thisis an unique business plan worth looking at this could be what you have been looking for
Become a motives consultant motives is a leading brand in the cosmetics industry with over 2 million followers on Instagram
work for your self have fun doing what you love
here are the answer to the question you might have about shop energy. why not save on your energy bill while saving the environment if we all do just a little to help save the environment we will all have a better world to live in
Characterization Studies of CdS Nanocrystalline Film Deposited on Teflon Subs...IJLT EMAS
In this article, different substrates for deposition of
CdS material have been discussed. Till date glass, mica, quartz,
ceramic, etc. are commonly employed substrates in thin film
growth. In the present work, CdS is deposited on Teflon
substrate by chemical bath deposition (CBD) method. Also the
films were deposited on different substrates like glass, copper
and zinc and compared with those prepared on Teflon substrate.
The films prepared on Teflon substrate were uniform, stable and
also showed good radiating property. These films were further
characterized by UV-VIS absorption spectral studies, SEM and
EDS studies.
Morphological and Optical Study of Sol-Gel SpinCoated Nanostructured CdSThin ...iosrjce
Nanostructured CdS thin films of different thicknesses were deposited on a cleaned glass substrate
using sol-gel spin coating technique. CdS thin films were prepared using cadmium acetate as cadmium source
and thiourea as sulfur source. The Morphological, chemical composition, and optical properties of the spin- coated
CdS thin film were studied using field emission- scanning electron microscopy (FE-SEM), Energy dispersive X –ray
(EDX) spectroscopy, and a UV-Vis-NIR spectrophotometer.The morphological results revealed that the films consist
of agglomerated spherical CdS nanoparticles with diameter < 20 nm, which distributed uniformly on the substrate
surface.The films show high transmittance > 90% and very strong absorption edge at 295 nm.The absorption edge
shifts towards longer wavelength as the film thickness increased.
Nanocoating GDZ is compared with Conventional YSZ coating for Hot Corrosion Resistance in presence of V2O5 and Na2SO4 salt which are formed at high temp in gas turbines.
This paper explains the fabrication of thin film using modified Physical Vapor Deposition (PVD) Module. Physical Vapor Deposition (PVD) is a variety of vacuum deposition and is a general term used to describe any of a variety of methods to deposit thin films by the condensation of a vaporized form of the material onto various surfaces. The surface morphology of various such as Titanium Dioxide and Aluminum thin film has been studied. The Titanium Dioxide and Aluminum thin film has been fabricated on Silicon (Si) substrate using modified Physical Vapor Deposition (PVD) module system. The process started with the establishment of process flow, process modules, and process parameters. Two modules were developed. The characteristics prior to the thin film fabrication namely surface morphology, metal thickness characterization and V-I characteristic were recorded. The samples were characterized by Optical Microscope, Atomic Force Microscope (AFM),X-ray diffraction (XRD) and I - V characterization. The result and data were analyzed and applied in the fabrication of thin film using various materials. The thin film fabrication process used Titanium Dioxide (TiO2) nanopowder and Aluminum (Al2O3) nanopowder for the coating process. The result for each processes are presented in this paper.
Thin films of cadmium sulphide have been
successfully deposited by chemical bath deposition (CBD)
technique using a mixed aqueous solution of cadmium
sulphate, thiourea, and triethylamine. The films were
characterized using a variety of techniques. Powder X-ray
diffraction analysis shows that the as-deposited thin film has
the hexagonal (wurtzite) structure. Scanning electron
microscope (SEM) micrographs show the film surface consists
of clusters with a globular surface morphology. Energy
dispersive X-ray diffraction (EDAX) analysis confirmed the
film to be consistent with the formation of cadmium sulphide
on silica glass slide. The band gap, determined from optical
absorption spectroscopy, was 2.42 eV which is consistent with
other published results.
Enhancing light sources color homogeneity in high-power phosphor-based white ...TELKOMNIKA JOURNAL
Color uniformity is one of the essentials for the on-going development of WLED. To achieve a high color uniformity index, increasing the scattering events within the phosphor layers was reported to be the most efficient method and in this article, ZnO is the chosen material to apply in this method. After analyzing the scattering properties through the scattering cross-section , scattering coefficient and scattering phase function , the which outcomes comfirm that ZnO can enhance the scattered light in the phosphor layers. Moreover, the findings from the study of ZnO concentration from 2% to 26% suggest that color uniformity also depends on the fluctuation of ZnO concentration, therefore, to control color uniformity the focus should be implied on both size and concentration of ZnO. The experimental results from this research show that the luminous flux of WLED is at the peak if the concentration of ZnO is at 6%, and when the concentration of ZnO is at 18% and has 100 nm particles size, the ΔCCT reaches the lowest level. The final choice should be based on the desired characteristic of WLEDs, however, if the WLED need to excel in both luminous flux and ΔCCT then 6% ZnO concentration with particles size from 100 nm-300 nm is the optimal choice.
Impact of CuS counter electrode calcination temperature on quantum dot sensit...TELKOMNIKA JOURNAL
In place of the commercial Pt electrode used in quantum sensitized solar cells, the low-cost CuS cathode is created using electrophoresis. High resolution scanning electron microscopy and X-ray diffraction were used to analyze the structure and morphology of structural cubic samples with diameters ranging from 40 nm to 200 nm. The conversion efficiency of solar cells is significantly impacted by the calcination temperatures of cathodes at 100 °C, 120 °C, 150 °C, and 180 °C under vacuum. The fluorine doped tin oxide (FTO)/CuS cathode electrode reached a maximum efficiency of 3.89% when it was calcined at 120 °C. Compared to other temperature combinations, CuS nanoparticles crystallize at 120 °C, which lowers resistance while increasing electron lifetime.
In place of the commercial Pt electrode used in quantum sensitized solar cells, the low-cost CuS cathode is created using electrophoresis. High resolution scanning electron microscopy and X-ray diffraction were used to analyze the structure and morphology of structural cubic samples with diameters ranging from 40 nm to 200 nm. The conversion efficiency of solar cells is significantly impacted by the calcination temperatures of cathodes at 100 °C, 120 °C, 150 °C, and 180 °C under vacuum. The fluorine doped tin oxide (FTO)/CuS cathode electrode reached a maximum efficiency of 3.89% when it was calcined at 120 °C. Compared to other temperature combinations, CuS nanoparticles crystallize at 120 °C, which lowers resistance while increasing electron lifetime.
A perovskite solar cell is a type of solar cell which includes a perovskite structured compound, most commonly a hybrid organic-inorganic lead or tin halide-based material, as the light-harvesting active layer.
Ultra-optical characterization of thin film solar cells materials using core...IJECEIAES
This paper investigates on new design of heterojunction quantum dot (HJQD) photovoltaics solar cells CdS/PbS that is based on quantum dot metallics PbS core/shell absorber layer and quantum dot window layer. It has been enhanced the performance of traditional HJQD thin film solar cells model based on quantum dot absorber layer and bulk window layer. The new design has been used sub-micro absorber layer thickness to achieve high efficiency with material reduction, low cost, and time. Metallicssemiconductor core/shell absorber layer has been succeeded for improving the optical characteristics such energy band gap and the absorption of absorber layer materials, also enhancing the performance of HJQD ITO/CdS/QDPbS/Au, sub micro thin film solar cells. Finally, it has been formulating the quantum dot (QD) metallic cores concentration effect on the absorption, energy band gap and electron-hole generation rate in absorber layers, external quantum efficiency, energy conversion efficiency, fill factor of the innovative design of HJQD cells.
1. Developing Monolithically Integrated CdTe Devices
Deposited By AP-MOCVD
Eva Tejedor Alonso
Universidad de Zaragoza
Zaragoza (Spain)
Abstract: The aim of this project is to investigate the
fabrication of CdTe modules, including: (a) deposition of
CdZnS/CdTe by Atmospheric Pressure Metal-Organic
Chemical Vapour Deposition (AP-MOCVD) onto ITO
coated boro-aluminosilicate glass substrate; (b) thermal
evaporation to deposit gold back contacts or screen
printing to deposit C/Ag back contacts, including
characterisation of solar cells using IV and a solar
simulator arrangements; (c) mechanical scribing isolating
lines for achieving monolithically integrated devices and
(d) characterisation of samples/devices using
profilometry/SEM. All these with the purpose of
improving performance and efficiency of the devices and
moving towards PV modules level research.
The different steps in the manufacture of cells were
improved, making it possible to move to module level.
Keywords:
CdTe
AP-MOCVD
Monolithic Interconnection
Mechanical Scribing
TCO
1. INTRODUCTION
Due to the depletion of oil reserves and the increase in
population and consumption, the world's energy supply
and planetary health are increasingly at risk. Changes are
needed in the production of energy, to continue enjoying
it without destroying the planet, as for example, the use
of renewable energy.
Research is still needed in the renewable sector, to
improve energy storage, integrate different renewable
energies, reduce the costs and increase the efficiency.
Solar energy is the origin of all the renewables, because
the sun moves all the natural cycles, and it can be directly
converted into usable electricity with photovoltaics.
The first generation of PV (monocrystalline and
polycrystalline silicon) is well established in our lives.
However, this first generation requires high temperatures,
ultra-high vacuum and complex operations for cutting
and assembling silicon wafers, making this technology
complicated and expensive [1]. This is why a perfect
substitute would be the second generation of PV.
Second generation PV includes thin film amorphous
Silicon, CdTe/CdS and CIGS. It is a low cost technology
because it uses less raw materials, inexpensive substrates
and cheaper manufacturing processes, like MOCVD, the
method used for this study.
Thin film technology uses monolithic integration to
interconnect cells in series during the fabrication process.
This interconnection consist of 3 cuts: the first one is
realized onto the TCO (Transparent Conducting Oxide),
isolating different areas; the second one is done after the
pn junction deposition, providing an electrical path from
the TCO front contact of one cell to the back contact of
the next cell. The last scribe is done after the deposition
of the back contact, isolating the different cells [2-3].
The scribing process can be done by laser or by
mechanical scriber. One of the objectives of this research
is to improve the scribing techniques, to be able to
fabricate modules with less dependence on laser
suppliers.
Of the many thin film materials, only CIS and CdTe have
been developed enough to be able to compete with
monocrystalline silicon in terms of cost, stability and
efficiency. [4]
CdTe is used as the material of the absorber layer in low
cost solar cells. It is the cheapest thin film material
because only a few microns are needed to absorb all the
incident light. This is due to CdTe energy bandgap (~1.5
eV), which is the best for solar energy conversion.
According to the Shockley-Queisser limit for the
efficiency of different types of solar cells, shown in Fig.
1, CdTe technology has huge potential, which can be
exploited though research [5].
Figure 1. The Shockley-Queisser limit for the efficiency
of different types of solar cells.
The chronological evolution of the efficiency of solar
cells (appendix 1), made from different technologies used
2. today, is essential to evaluate the results obtained in the
investigations.
2. METHODOLOGY
For the manufacture of cells, the whole process can be
summarised by: MOCVD pn junction deposition; back
contact deposition; front contact deposition and
characterisation.
For the manufacture of micro-modules monolithic
integration process has to be added.
2.1 Atmospheric Pressure Metal Organic Chemical
Vapour Deposition (AP-MOCVD).
AP-MOCVD was the deposition method used for the pn
junction growth. This method offers high flexibility in
terms of accuracy with respect to the doping and alloying
of the layers of the devices. AP-MOCVD can be
performed to investigate various experimental parameters
for improvements in the quality of the CdTe devices.
The substrate used for the deposition was ITO (Indium
Tin Oxide) coated aluminosilicate glass substrate (1.1
mm thick glass, 4-8 Ω/sheet resistance and 5 x 7.5 cm2
size). Typical composition of the aluminosilicate glass
was 55.0% SiO2, 7.0% B2O3, 10.4% Al2O3, 21.0%
CaO, and 1.0% Na2O.
Some cleaning processes were carried out on the
substrate before the deposition to improve the layer
nucleation and the quality of the pn junction properties.
The deposition was performed in a horizontal AP-
MOCVD reactor and the carrier gas used was purified H2.
Precursors (brought with the carrier gas) flew parallel to
the static substrate. This reactor was equipped with a
triple wavelength laser reflectometer for in situ
monitoring of the layer thickness and growth rate for the
CdTe absorber and the CdS/CdZnS window layers.
Table 1 shows the order in which the different layers
were deposited along with their thickness and deposition
temperatures.
Table 1. Thicknesses and temperatures for MOCVD
deposited layers.
The process started with the CdS deposition, which is
used as nucleation (seed) layer for the following
semiconducting layers. Then a CdZnS layer was
deposited, completing the window layer growth.
CdTe:As (As doped CdTe) was grown in a bi-layer form
onto the CdZnS film, including a 2µm layer of CdTe:As
(p-type), doped with ~1018
atoms/cm3
As and a second
250 nm layer of CdTe:As+ (p+-type), using an As
concentration of ~1019
atoms /cm3
, giving the devices an
npp+ structure.
CdTe is known to be a difficult material to dope with
high carrier concentrations [6]. In these experiments up to
0.01% of the arsenic available contributed to the p-type
conductivity of the CdTe [7]. Nevertheless, these doping
levels are adequate to provide npp+ CdTe devices with
conversion efficiency in excess of 10% [8].
Towards the end, CdCl2 was grown, followed by a 10 min
anneal (420ºC) for device activation. This treatment
improves the window and absorber layers.
The excess of CdCl2 had to be washed due to its toxicity.
Finally, the device was annealed for 40 minutes at 1700
C
in a conventional oven for further activation.
2.2 Cell Structure
The cell structure most commonly used for CdTe solar
cells is the “superstrate configuration”, where the cell is
deposited on a transparent, conducting substrate from
where the light enters, as shown in Fig. 2.
The substrate, coated with ITO serves as the front contact
of the cell [2].
The next layer is CdS, which forms part of the window
layer. A very thin CdS film (typically <50 nm) raises the
current density (Jsc) in the cell [2].
Then, the growth of CdZnS (190 nm) completes the
window layer. The addition of Zn results in a wider band
gap (between 2.7-3.0 eV), so the photon transmission and
Jsc are increased and consequently, the performance [9].
CdS and CdZnS are the n-type materials of the cell [2].
Next there is the CdTe, the absorber material, formed by
2 layers (2000 nm and 250 nm) doped with different
concentration of As, forming the p and the p+ layers. The
p+
layer provides a back surface field, improving the
characteristics of the metal back contacts.
Back Contact
CdTe:As+ (p+)
CdTe:As (p)
CdZnS (n)
ITO
Glass Substrate
Figure 2. CdTe solar cell structure by MOCVD
The last layer is the metallic back contact, current-
carrying conductor, applied onto the pn junction. For this
study it was usually gold (~0.3) due to its high electrical
conductivity, its great ductility and its resistance to
corrosion during storage. Also because of the work
function of gold (~5.1 eV) which makes it a good choice
for a CdTe back contact [9-10]. Also a carbon-silver bi-
layer was used as the back contact of some devices, to
avoid vacuum based processes.
2.3 Back Contacts
Depending on the material used for the back contacts the
deposition technique was different. For gold it was
thermal evaporation.
Evaporation is a common method of thin film physical
deposition. The source material is evaporated in high
vacuum (usually better than 10-5
mbar), which allows
vapour particles to travel directly to the target object
(substrate), where they condense back to a solid state.
Layer Thickness Temperature
CdS 50 nm 315ºC
CdZnS 190 nm 360ºC
CdTe: As 2000 nm 390ºC
CdTe: As+
250 nm 390ºC
3. A thermal evaporator utilizes an electric resistance heater
to melt the material and raise its vapour pressure to a
useful range. A temperature close to melting point is
needed to make the gold evaporate.
A shadow mask was used to deposit the gold controlling
the shape and size of the contacts made (Fig. 3). The
thickness of gold had to be controlled, to keep gold
resistivity as low as possible (~300 nm).
However, evaporated gold is intended to be replaced with
low cost options with probably higher long-term stability
than gold. Thus, the deposition of a conducting bi-layer
of carbon and silver is also used, as they require non
vacuum methods, which are simple to apply.
The technique used to apply these bi-layer back contacts
to the solar devices was screen printing.
The process consisted of placing the screen pattern over
the substrate, placing then some carbon paste over the
screen. This paste was moved gently with a rubber
squeegee to cover the screen pattern, pressing down to
make the paste flow through the screen and printing this
way the pattern onto the sample below.
Then, the paste on the sample had to be oven-cured at
170 ˚C for 30 minutes. This step allowed the carbon paste
to dry by removing the solvent contained in the paste.
All the process was repeated with silver paste, depositing
it over the dried carbon paste.
Here carbon is only used because silver alone does not
perform well when deposited directly onto the pn
junction. Due to its relatively lower conductivity, carbon
film has to be as thin as possible in order to reduce its
contribution to device series resistance.
2.4 Front Contacts
Metallic front contacts to the ITO film were needed for
the CdTe devices to be characterised, in order to
complete electrical connection to the sample. For this,
part of the pn junction was removed to expose the ITO
surface, if not it results in an increase of the series
resistance. A mixture of Indium-Gallium was then
applied in that area. Other materials used to make front
contacts were silver paste (Fig. 3), a silver pen and epoxy.
Figure 3. CdTe device with 8 gold back contacts and
silver paste front contacts.
2.5 Characterisation
The characterisation of devices was done with a solar
simulator, a device that provides an artificial illumination
to approximate natural sunlight at 1000 W/m2 (i.e. AM
1.5 G), which is the STC (standard test conditions) for
testing solar cells. The test carried out was the J-V curve
(current density-voltage), where the applied voltage is
swept within a given range and the current measured.
Parameters such as Jsc (current density), Voc (open
circuit voltage), η (efficiency), FF (Fill Factor) and
Rs/Rsh (series/shunt resistances) were then calculated
from the curve to provide an overall performance of
photovoltaic devices.
Other 2 devices used for the characterisation of samples
were the profilometer, to measure the surface profile, and
the SEM (Scanning electron microscope).
2.6 Scribing Technique.
To transfer from single cells to monolithically integrated
modules, series interconnections between individual
subcells were made, using scribing techniques.
The monolithically integration scheme consists of three
scribing processes which form the interconnection of
subcells; they can be made by laser, by mechanical
scriber or just by using a mask. For this study a
mechanical scriber was used. The 3 types of scribes (P1,
P2 and P3) are shown in Fig. 4.
P1 is used to scribe within the TCO, providing isolation
and defining the individual cells [9].
P2 is the scribed line removing the full p-n junction
(CdZnS/CdTe) down to the TCO. The TCO exposed will
form the electrical contact of one cell, with the back
contact of an adjacent cell.
P3 separates different units of back contacts, isolating the
individual cells of the module.
When gold back contacts are deposited, P3 is not scribed
because a mask is used for the deposition of the back
contact (figures 8 and 9). When the bi-layer (C/Ag) back
contact is used, P3 scribing of the continuous carbon
layer is necessary to avoid shunting losses. [11]
With mechanical scribing it is more difficult to calculate
the width and depth of the cuts compared to with laser
technology. However, it provides some flexibility and
allows rapid prototyping. It was therefore necessary to
experiment, to find the appropriate scribing configuration
in order to enhance precision in the cuts. [12]
The basic mechanism of operation is the movement of the
platform on which the sample is mounted, inwards and
outwards whilst the diamond tip scribes on said sample.
The force applied by the tip over the sample is controlled
by two weights. Such a force can vary, ranging from 0 to
581cN, moving from 0 to 12 in a scale. The depth the tip
reaches in the scribing process can be also controlled by a
micrometre screw, as well as its angle of inclination (60º
towards the substrates for this research).
The first tests were made just for P1 scribes, scribing
when the platform was pulled out. After several tests, it
was seen that the P1 scribes were not homogenous and
the quality of the scribed lines was changing from one
line to another. So it was proved to shift the configuration
of the scriber so that the samples were written in address
opposite, when the platform was pushed in.
Problems were also found with P2 scribing. Sometimes
the lines were not wide and clean enough to make a good
path for the electrons.
After making all the corrections this was the
methodology employed for the different types of cuts: To
isolate the TCO with P1 it was needed to repeat the
scribes over the same line, until isolation was reached.
4. Figure 4. Scheme of the monolithic interconnection [9].
To scribe P2 only the starting height of the tip had to be
configured, touching gently the surface of the sample,
scribing a line and then moving by 0.01 mm to scribe
again and obtain this way a cleaner cut. The same
procedure was used for P3 scribes.
At the beginning the separation between these 3 cuts was
0.5 mm, later it was reduced to 0.3 mm, to maximize the
active area of the cells.
Other problem that arose in the scribing process was the
clamping of the samples on the platform to avoid
movements. This was solved by using double-sided
carbon adhesive tape on the 4 corners of the samples.
The last issue related to the mechanical scribes was the
damage done to the diamond tip caused by P1 scribing,
resulting in deep scribed lines and damage to the TCO
surface. This was investigated to find a proper solution,
as will be explained in the next chapter.
3. RESSULTS
The main objective in the last six months of study has
been to use the knowledge acquired through the research
in the manufacture of cells and apply it to the
manufacture of modules, using monolithic integration.
3.1 Delamination
Delamination was the reason why TCO was used as the
substrate for this study, because TEC glass was
impractical for the monolithic integration of cells. This
was due to delamination after the CdCl2 treatment at the
end of the MOCVD process. It mainly occurred at the
edges of the substrate and around the P1 scribes made
with mechanical scriber.
Delaminated samples were analyzed with SEM, deducing
that the affected areas have larger CdTe grains, which
come off from the glass surface easily.
Delamination did not happen over TCO glass due to its
different composition (aluminosilicate). On the other
hand, TEC glass is alkaline.
3.2 Cells Fabrication Results
Although the main purpose in these 6 months has been to
move from cell manufacturing to module manufacturing,
the investigations with cells have continued, since its
manufacturing process is simpler and faster. Hence,
possible improvements can be identified faster and then
applied in the manufacture of modules.
Tables 2 and 3 show the results of 2 devices with gold
back contacts (0.5 0.5 cm2) after the analysis with the
Solar Simulator (J-V curve), where ɳ is the efficiency
(%), Voc is the Open Circuit Voltage (V), Jsc is the
Current Density (mA/cm2), FF is the Fill Factor (%) and
and Rs/Rsh the series/shunt resistances (Ω/cm2
).The best
and the worst cell of each device are shown.
As can be seen from Tables 2 and 3, results are getting
better with time, due to the increased knowledge of the
different steps of the fabrication process.
Table 2. CSER_465 platform IV data table.
(31-07-13) Worst cell Best cell
η (%) 9.4 11.2
Jsc (mA/cm2
) 22.2 23.7
Voc (V) 0.64 0.6
FF (%) 65.4 72.9
Rs (Ω/cm2
) 1.6 1.4
Rsh (Ω/cm2
) 357.7 1702.8
Improvements have been reached by using more As
(doping more the CdTe). The result was and increment of
the efficiency. Also Zn was increased, which improved
the current density (Jsc).
Table 3. CSER_515 platform IV data table.
(27-11-13) Worst cell Best cell
η (%) 13.3 15.6
Jsc (mA/cm2
) 24.4 25.3
Voc (V) 0.7 0.7
FF (%) 73.1 78.1
Rs (Ω/cm2) 2.6 2.8
Rsh (Ω/cm2) 3323.6 2168.2
In the last results showed (Table 3) the window layer was
reduced, improving this way the current density (Jsc).
Also the post growth activation annealing was optimised
for a better open circuit voltage (Voc) and fill factor (FF).
All these experiments had also drawbacks. When a factor
is increased, another factor or factors are decreased,
because these parameters are interdependent.
3.3 Annealing Study
Not only the deposition settings affect to the final results,
there are also other parameters, like the annealing time,
that have to be taken into consideration.
An experiment was realised to study the importance of
annealing. In this experiment a device was grown on a
TCO substrate (by AP-MOCVD). Then 8 carbon back
contacts were printed (Fig. 5a). Each contact was named
with a letter and a number.
The sample was annealed during 10 min (170 ºC) for the
carbon paste to dry. Then front contacts were made with
5. silver paste and 4 of the 8 carbon back contacts were
completed with more silver, using a silver pen (Fig. 5b).
Figure 5. Sample CSER_469 with 8 carbon back contacts
(a), with 4 carbon and 4 carbon-silver back contacts (b)
and with 8 carbon-silver back contacts (c).
The sample was cured for 30 min at room temperature
and measured with the solar simulator, obtaining the
results shown in Table 4. Cells with back contacts made
just with carbon were less efficient.
Table 4. CSER_469 results sample with 4 carbon and 4
carbon-silver back contacts
1
C/Ag
2
C/Ag
3
C
4
C
A η (%) 9.3 8.8 3.1 2.4
Jsc (mA/cm2
) 21.4 20.5 15.5 12.6
Voc (V) 0.7 0.7 0.7 0.7
FF (%) 57.9 57.7 27.0 26.1
Rs (Ω/cm2
) 9.0 9.2 44.5 55.2
Rsh (Ω/cm2
) 317.1 318.6 87.8 71.8
B η (%) 10.8 10.4 3.1 3.0
Jsc (mA/cm2
) 22.5 22.2 15.8 15.3
Voc (V) 0.7 0.7 0.7 0.7
FF (%) 62.4 61.3 26.2 26.0
Rs (Ω/cm2
) 7.0 7.2 44.7 46.6
Rsh (Ω/cm2
) 451.6 378.8 78.6 75.6
More silver paste was added to the front contacts of the
same sample (Fig. 5c), curing again the silver paste in the
oven 10 min (170ºC). By using the silver pen, other 4
silver contacts were added to the 4 back contacts made
just with carbon.
The sample was tested anew with the solar simulator. The
results significantly improved, as can be seen in the
results shown in Table 5.
This test showed the importance of the annealing time.
Also the importance of the location of the sample during
the MOCVD process, because the reactor does not
deposit the compounds homogenously and the best
results are always found in the middle of the deposition
area.
Here the row B was closer to the central area of the
deposition in the reactor, having therefore better results.
Table 5. CSER_469 second solar simulator results with 8
carbon back contacts.
1 oven
C/Ag
2 oven
C/Ag
3
C/Ag
4
C/Ag
A η (%) 10.2 9.5 10.0 10.2
Jsc(mA/cm2
) 21.9 20.8 21.2 22.4
Voc (V) 0.7 0.7 0.7 0.7
FF (%) 60.8 61.1 63.0 61.1
Rs (Ω/cm2
) 6.5 7.1 6.9 5.4
Rsh (Ω/cm2
) 391.1 401.8 365.7 231.7
B η (%) 11.4 11.1 11.2 11.2
Jsc(mA/cm2
) 22.7 22.5 22.6 22.5
Voc (V) 0.7 0.7 0.7 0.7
FF (%) 65.2 64.5 64.8 65.1
Rs (Ω/cm2
) 5.9 5.7 5.9 5.9
Rsh (Ω/cm2
) 555.3 480.6 465.2 535.4
3.4 Mechanical Scriber Configuration
To completely isolate the TCO it was needed to repeat
the scribes various times over the same line. As a result
some scribes were very deep, damaging the substrate
(Fig. 6) and even producing the fracture of the sample
during subsequent processes. Profilometry tests showed
that the depths varied from ~0.09 µm to 17.2 µm (Fig. 7).
Figure 6. Mechanical scriber microscope images showing
the damage caused to the substrate by several scribes.
Figure 7. Profilometry scan result for a P1 scribe, where
the blue colour represents the deepest areas.
Numerous tests were performed by varying each of the
parameters that define the scribes. In the end, the
appropriate settings, for applied force, height of the
diamond tip and scribing direction were found.
6. After adjusting such settings outcomes were improved,
obtaining homogeneous and cleaner cuts (Fig. 8).
Figure 8. Scribed lines seen from the mechanical scriber
microscope, after the scriber diamond tip passed once (a),
twice (b), three times (c) and four times (d).
With each additional pass cuts cause more damage to the
TCO surface. By increasing the force applied with the
weights, it was found that fewer scribes were needed to
isolate the TCO and the average depth was around 3 µm.
For P2 scribes, the problem was the damage caused to the
TCO when too much force was applied. Also that the
centre of the cut was not clean, which may obstruct the
flow of electrons. This was solved by scribing P2 twice
each time, making the cut wider.
As a result of this study the best configuration for the P1
scribes was force 7, tip height calculated by lowering the
tip to touch the surface and as many number of passes as
needed to isolate the TCO. To write P2 the best option
was force 2, lowering the tip to lightly skim the surface
and repeating each scribe twice, spaced 0.1 mm.
There was also necessary to study the diamond tip, which
was damaged by P1 scribes. SEM images showed that the
face of the tip that was being used was worn. This was
solved by turning the tip to use a new face for scribing.
3.5 Monolithically integrated modules
After having upgraded all the necessary techniques, the
first mechanical scribed monolithically integrated micro-
module was made, with an average efficiency of 4.54 %.
As it was expected, the results were worse than those of
cells. This is due to factors such the increase in the
resistance when cells are connected together or the
mechanical scriber cuts which, despite the improvements,
are still not as good as the laser ones.
Further research allowed improvements in the next
manufactured modules.
4. CONCLUSIONS
Over the last 6 months various stages of the
manufacturing process of CdTe cells with MOCVD have
been investigated and improved, with the objective of
applying the acquired knowledge to the fabrication of
CdTe monolithically integrated micro-modules.
Due to delamination issues related TEC glass, TCO was
the substrate chosen for the deposition of semiconductors.
Different improvements were made in the MOCVD
settings and annealing times, obtaining better results.
Numerous tests were conducted with the mechanical
scriber to achieve a scribing quality good enough to be
applied in the manufacture of monolithically integrated
devices. These experiments have helped to know more
about the structure of CdTe devices.
All the above mentioned developments were assembled
together for the manufacture of micro-modules with
subcells interconnected in series.
Through research, efficiency has been successfully
increased, allowing jumping from cell to module level,
although research is still needed at the module level.
Monolithically integrated CdTe devices deposited by
MOCVD are promising candidates for low-cost PV.
5. ACKNOWEDGEMENTS
The author greatly thanks the host organisation (CSER)
and the academic supervisor of this study, for the
suggestions and guidance, contributing to this work.
6. REFERENCES
[1] A. Bosio et al, “Manufacturing of CdTe thin film
photovoltaic modules.,” Thin Solid Films, vol. 519,
pp. 7522-7525, 2011.
[2] B.E. McCandless; J.R. Sites, “Cadmium Telluride
Solar Cells,” in Handbook of photovoltaic science
and engineering, Wiley, 2003, pp. 61-87, 617-657.
[3] J. Perrenould et al, “Fabrication of flexible CdTe
solar modules with monolithic cell interconnection.,”
Solar Energy Materials & Solar Cells, vol. 95, pp.
S8-S12, 2011.
[4] A. B. Rujula, Sistemas fotovoltaicos (Photovoltaic
systems), Zaragoza: Prensas Universitarias de
Zaragoza, 2009.
[5] W.Shockley and J.Queisser, “Detailed Balance Limit
of Efficiency of pn Junction Solar Cells,” Journal of
Applied Physics, vol. 32, pp. 510-519, 1961.
[6] S. H. Kim et al, “The formation of ZnTe:Cu and
CuxTe double layer back contacts for CdTe solar
cells,” Current Applied Physics, vol. 10, no. 3, p.
S484–S487, 2010.
[7] A. P. Samantilleke et al, “Investigation of structural
properties of CdTe-CdS using photocurrent and
Electroreflectance spectroscopy” in PV-SAT,
Durham, 2007.
[8] R. L. Rowlands et al, “Taguchi matrix investigation
of CdCl2 activation for CdTe-CdS solar cells grown
by MOCVD,” in PV-SAT, Durham, 2007.
[9] A. Clayton et al, “MOCVD of CdZnS/CdTe PV cells
using an ultra-thin absorber layer,” S. Energy
Materials & Solar Cells, vol. 101, pp. 68-72, 2012.
[10] V. Barrioz et al, “Material utilisation when
depositing CdTe layers by inline AP-MOCVD,” J. of
Crystal Growth, vol. 354, pp. 81-85, 2012.
[11] R. W. Birkmire and B. E. McCandless, “CdTe thin
film technology: Leading thin film PV into the
future.,” Current Opinion in Solid State and
Materials Science, vol. 14, pp. 139-142, 2010.
[12] P. T. Lin, “Mechanical scriber for semiconductor
devices”. EEUU Patent US4502225 A, 1985.