A literature review on ultra thin solar cells manufactured by CdTe and CIGS. Includes manufacturing technique, their uses in practical world.

1. ULTRA THIN SOLAR CELL INTEGRATION IN BIPV:
FOCUS ON CdTe AND CIGS
Introduction:
BIPV technology revolutionizes building from energy consumer to producer of energy.
Photovoltaic modules have a large potential to serve as building exteriors such as roof, faced
and skylight. BIPV also serve as weather protection, thermal insulation and look aesthetically
good on the buildings. BIPV Thin-film are solar cells that are basically thin layers of
semiconductor materials applied to a solid backing material. Thin films greatly reduce the
quantity of semiconductor required for each cell when compared to silicon wafers. and hence
lowers the cost of production of photovoltaic cells. According to the type of photo voltaic
material used, thin film solar cells are classified as
Amorphous silicon(a-si)
Thin film silicon(TF-Si)
Cadmium telluride(CdTe)
Copper-Indium-Selenide(CIS) or Copper-Indium-Gallium-selenide(CIGS)
Dye sensitized solar cell (DSC)
And other organic solar cells.[1]
Cost of these thin film solar panel are low as compared to the older silicon wafer cells. The
functioning of these solar cells is however like that of silicon wafer cells. There are flexible
arrangements of different layer that help produce thin cells. Amorphous silicon module has a
lower efficiency of 4-10% than mono or polycrystalline but they perform better at higher
temperature. The thickness of these cells varies few nanometers to micrometers. They are
used in huge sophisticated building integrated installation. Research projects that that thin
film production would reach 22,214MW by 2020.Also, BIPV allow some light for day
lighting and thus cooling.[1]

2. Comparison of efficiency:
Out of all the thin films amorphous silicon cell produces the least efficiency and has higher
levelized cost of electricity generation. CdTe and CIGS offer the best efficiency and there are
a lot of researches on enhancing the efficiency of the CdTe and CIGS by adapting various
chemical processes. The paper will cover the manufacturing and efficiency obtained by
newest methods so far. The integration of amorphous silicon is questionable since several
arrays of amorphous silicon is necessary to produce the same power when compared to CIGS
or CdTe.[2]
Growth of thin film industry over the years
Cell material Module efficiency Surface area for 1 KW
power
Mono- crystalline silicon 15-18 7-9
Poly crystalline silicon 13-16 8-9
Micro morph tandem(aμ-si) 6-9 9-12
Thin film copper indium di-
selenide (CIGS)
10-12 9-11
Thin film cadmium
telluride(CdTe)
9-11 11-13
Thin film amorphous
silicon(α-si)
6-8 13-20
Table illustrating and comparing the efficiencies of different solar cells
Cadmium Telluride serve as the right choice for BIPV or any configuration that involve
flexible panel installation.

3. Manufacturing methods of CdTe solar cell
CdTe are made in both substrate and superstrate. Commercial CdTe modules are made in
super state configuration and has highest efficiency till date. Roll to roll process made on
metal foils is widely used for substrate configured cells. Efficiency of up to 14% and 11.5%
have been reported previously for super state and substrate configuration respectively. CdS
thin film is suitable for window layer of CdS/CdTe solar cells. They have been manufactured
using varying techniques and is classified as chalcogenide semiconductor and is used in
CdS/CdTe solar cells. It is preferred widely due to its wide and direct band gap(2.4eV). It is
therefore best suited for junction effect of CdS/CdTe solar cells because of its n-type
material. CdS is fabricated by electro deposition, RF sputtering, Chemical vapor transport
and chemical bath deposition (CBD).[3] [4]
Case study 1:
CBD is widely used for manufacture of CdS thin film for commercial CdS/CdTe solar cells.
Closed spaced sublimation (CSS) of CdCl2 heat treatment helps in grain growth, improved
electrical properties in CdTe thin film. A research carried out by Department of Materials
Science and Engineering, Korea University, studying optical properties of CdS/CdTe solar
cell used substrate glass coated with SnO2:F thin film. The film used is transparent and
possess high thermal stability. CdS thin films grown in beaker and the substrate was
deposited for 20 min, solution bath system consists of magnetic stirrer and heating system.
Beaker is then immersed into silicon oil maintaining a temperature of 75o
C. [5]
Solution consisted of CdCl2(0.002M), thiourea (SC(NH2)2 (0.003M), NH4Cl (0.015M),
NH4OH(0.640M) and deionized water. PH of ammonia bath solution is at 11. CdCl2 is used
as Cadmium precursor and Thiourea as Sulphur precursor. Ammonia is used as complexing
agent, NH3 is used as buffer. After deposition, the substrate is cleaned using dilute Hcl. CdS
thin film absorb photons and decrease the conversion efficiency of CdS/CdTe solar cells.
The schematic diagram of the working of thin film solar cell[5]

4. Case study 2:
In another research CdS and CdTe are deposited at 260o
C on a commercial SnO2 coated 3
mm thick soda-lime glass by magnetron sputtering. The thickness of CdS and CdTe layers
chosen by them was 0.13nm and 2.3nm respectively. The thickness was varied from 0.6 to
1.28nm. SnO2/CdS/CdTe cell structure is treated in vapours of CdCl2 at 390o
C in the presence
of dry air for various time durations. Cu+ Au bilayer or Au only layer is used as back contact
through a mask to produce 0.15 cm2
area dot cells followed by diffusion in room air at 1500
C.
In the study it was found out that optimum treatment time occurred at about 10 min.[6]
IV curve
Quantum efficiency versus wavelength
160
140
120
100
80
60
40
20
0
-20
6micro meter CdTe
0.87micrometerCdTe
2.3 micro meter CdTe
1.28 micro meter CdTe
-0.5 5
VoltageV
1.150.0
0.6micro meter0.87micro meter1.28micro meter
Wavelength nm
900 1000800700600500400300
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
CurrentDensity(mA/cm2)
Quantumeffeciency

5. Quantum efficiency comparison between treated and untreated CdTe
CdS(μm CdTe(μm) CdCl2(μm) Back
contact(cu)
Back contact
diffusion(min)
VOC(mV) JSC(mA/cm2
) FF% n%
0.13 2.3 30 30 45 807 23.3 72.3 13
0.13 1.28 30 30 30 714 21.3 65.9 10
0.13 1.28 10 30 30 710 22.3 61.4 9.7
0.13 1.04 10 15 18 767 22.1 66.8 11.3
0.13 1.04 10 15 18 795 21.2 70.8 11.9
0.13 1.04 30 0 No diffusion 641 20.9 60.4 8.1
0.13 0.87 10 15 18 772 22.6 69.7 11.8
0.08 0.6 30 30 30 484 20.1 44.0 4.3
0.08 0.6 10 15 18 708 20.5 64.6 9.4
0.07 0.94 10 15 10 762 22.6 67.4 11.6
0.05 0.7 10 15 10 750 21.7 69.2 11.2
Summary of the experimentation
All the above data are obtained from [4] [3]
From the above graphs thickness can be substantially reduced without losing much
efficiency. Optimization of post- deposition CdCl2 treatment and back contact diffusion
conditions lead to achievement of 11.8% efficient cells with 0.87 micro meter CdTe which is
9% relative loss from that of standard 2.3micro meter CdTe 13% cell. 11.2% efficiency using
0.05 micro meter CdS and 0.7 micro meter CdTe contributes as the thinnest CdTe of highest
efficiency.
Case study 3:
In a research conducted at National Renewable Energy Laboratory, USA a record efficiency
of 16.4% was achieved for a flexible CdTe solar cell breaking the previous record of 14.05%
previously. In their research they sputtered CdS:O window layer and evaporated and rapidly
processed ZnTe:Cu/Au as back contact. CdS:O sputter process is oxygenated process adopted
for uniformity of window layer and thereby to enhance Short Circuit current (ISC). ZnTe:Cu
Voltage
1.510.5
No treatment
Anneline treated
0.12
0.1
0.08
0.06
0.04
0.02
0
-0.02
0
-0.04
-0.5
Currentdensity(A/cm2)

6. back contact followed by rapid thermal processing leads to an increase of open circuit
voltage(VOC) and higher fill factor.[6]
In this section we look at the manufacturing of the most efficient BIPV technology involving
CdTe solar cell doped with bilayers of SnO2:F(FTO, Fluorine Tin oxide) and undoped Sn02
(TO, tin oxide). Transparent conducting oxides are deposited at a substrate temperature of
550o
C by metal organic chemical vapour deposition(MOCVD). CdS:O layer is sputtered at
room temperature using hot pressed stoichiometric CdS target. Sputtering is also done at
ambient pressure of Argon with 6% oxygen volume at a pressure of 10mTorr(1.33Pascals).
Closed spaced sublimation (CSS)is used to deposit CdTe layer at substrate temperature of
6000
C and CdTe plate is maintained at 660o
C for 2.5 minutes. Vapour CdCl2 is obtained as a
device stack and is treated by CSS (Closed spaced sublimation) at 4000
C for 10 minutes.
0.05% bromine/ethanol is used for contact pre-etching. A back layer of ZnTe:Cu is deposited
by co-evaporation from ZnTe and Cu, followed by Au on to the unheated substrate. Gold(Au)
is then evaporated through a metal mask as the contact area.[7]
From the Research the below data is obtained and is compared with efficiency of regularly
manufactured CdTe devices.
Type of manufacturing Current density
JSC (mA/cm2
)
Open circuit
voltage
VOC(mV)
Fill factor Efficiency
Sputtered CdS:O 24.3 822 70.3 14.1%
Chemical bath deposition
CdS
25.5 831 77.4 16.4%
VoltagemV
10.80.60.40.20
2
1.5
1
0.5
0
IV CURVE
CurrentmA

7. Thickness of CdS:O layer 100nm
Willow glass dimension 100µm X38.1mmx38.1mm
Device Area 0.07025cm2
Irradiance 1000W/m2
VOC (Open Circuit Current
Voltage)
0.873V
ISC (Short Circuit Current) 1.8mA
JSC (Current Density) 25.5mA/cm2
FF (Fill factor) 0.774
Efficiency 16.42%
Test module parameters
Manufacture of CIGS:
CdS chemical deposition process for Cu(In,Ga)Se2 in a research conducted by National
Energy Lab, USA varied their deposition time(varying the thickness therefore ), bath
temperature and Cd+2
partial electrolyte treatment of Chalcopyrite absorber prior to CdS
deposition. Bath solution used in the their research is 366ml of DI H2O,65.2ml of
NH4OH(Ammonium hydroxide),28-30%, 50 ml of thiourea(1.5 NH2CSNH2). Cd+2
Partial
electrolyte treatment is performed at 800
C using the same bath solution but excluding
Thiourea. ZnO bilayer (I ZnO and n-ZnO) ZnO bilayer and contacts are completed after the
CBD treatment.
ChemicalbathdepositionCdSSputtered CdS:O
300 400 500 600 700 800 900
90
80
70
60
50
40
30
20
10
0
100
90
80
70
60
50
40
30
20
10
0
IQE vsWavelength
Quantumeffeciency%

8. 0
Growth of CdS in CdTe substrate in the left and internal quantum efficiency vs wavelength in
left
It was found that thinner CdS layer allowing more light to reach the junction was not
conductive to higher short circuit current. The research concludes that device parameters was
dependent on CdS layer thickness but independent of growth temperature. Cd+2
partial
electrolyte treatment was more susceptible to CdS thickness. Thus, thinner the CdS layer,
lesser the losses of parameters like VOC and FF. In general, thin layers of CdS provide a
better device performance. At optimum thickness highest performance is achieved, increasing
the JSC due to absorption in the buffer layer. The effect of increasing CdS thickness can be
seen in the range between 350-500nm.The effect of CdS layer thickness on the spectral
response modelled using absorption co-efficient of CIGS absorber is based on:
nq=1[1=1/α x L]
Where L is diffusion length
α is absorption coefficient.
CdS layer thickness effects IV characteristics as well. It can be seen in the reverse bias.
Breakdown effect called Zener effect results in tunnelling, with thicker CdS layers a larger
breakdown voltage is seen and this efficiency limits the amount of tunnelling taking place in
reverse bias. Cd incorporation and n-type doping effects are seen on the surface of CIGS.
This is seen in the improvements of VOC and FF. A standard CdS buffer layer grown at 650
C
for 15 min and no electrolyte treatment is chosen as standard and is compared to 5-15min
deposition of thinner CdS buffer layer. Although there is decrease in the FF, it is still better
than untreated layers of CdS. Cd+2
partial electrolyte in combination with thinner buffer layer
leads to increased current density JSC. It was illustrated in the research, comparing QE f
18.8% cell with QE of Cd+2
PI treated usingequation
JT=qx∫
∞
𝑛𝑞(dT/dλ)dλ
JT is currentdensity
Q is electron charge
Nq is internal quantum efficiency

9. dT/dλ is global solar spectrum (AM 1.5)
λ is wavelength.
Cd+2
provide additional 1.2mA/cm2
JSC value for wavelength <550nm. In addition CdS layers
builds a sufficient wide depletion width that minimizes tunnelling and establishes higher
contact potential (high VOC values). It also coats absorber surface minimizing voids at
metallurgical interface and provides electronic and metallurgical Junction protection against
subsequent sputter damage from ZnO window deposition.
Conclusion:
The several state-of-the-art buildings integrated photovoltaic (BIPV) merchandise current on
the market today provide a huge vary of integration of photovoltaic (PV) systems into
buildings.
Continued research and development within both PV and BIPV materials and applied
sciences will yield higher and better BIPV solutions within the years to come back, e.g.
with reference to multiplied photovoltaic cell potency, reduced production costs and
improved building integration. New and innovative solutions might cut back prices and
increase the market share, amongst different within the retrofitting market. The chosen
solutions ought to be simply applicable, wherever one example of a future vision is paint
applications of PV cells. it's crucial that everyone new technologies and solutions area
unit completely tested and approved in accordance with existing standards,

10. and moreover, there's conjointly a desire for development of recent standards and ways,
e.g. concerning long sturdiness versus climate exposure.
References.
[1] Bjørn Petter Jelleab*, Christer Breivikb and Hilde Drolsum Røkenesb, “Building
Integrated Photovoltaic Products: A StateoftheArt Review and Future Research
Opportunities.”
[2] M. Tripathy a,n, P.K.Sadhu a, S.K.Panda b, “Acritical review on buildingintegrated
photovoltaic products and theirapplications,” Dep.
OfElectricalEngineeringIndianSchoolofMinesDhanbad826004India B Dep.
OfCivilEngineeringIndianSchoolofMinesDhanbad826004India.
[3] H.P. Mahabaduge1, W.L. Rance1, J.M. Burst1, M.O. Reese1, D.M. Meysing2, C.A.
Wolden2, and J. Li2, J.D. Beach2, T.A. Gessert1, W.K. Metzger1, S. Garner3, and T.M.
Barnes1, “High-Efficiency, Flexible CdTe Solar Cells on Ultra-Thin Glass Substrates,”
Natl. Renew. Energy Lab. Gold. CO 80401 USA.
[4] Akhlesh Gupta, Viral Parikh, Alvin D. Compaan_, “High efficiency ultra-thin sputtered
CdTe solar cells,” Dep. Phys. Astron. Univ. Toledo Toledo OH-43606 USA.
[5] Amit H. Munshia,⁎, Jason M. Kepharta, Ali Abbasb, Tushar M. Shimpia, Kurt L. Bartha,
and John M. Wallsb, Walajabad S. Sampatha, “Polycrystalline CdTe photovoltaics with
efficiency over 18% through improved absorber passivation and current collection.”
[6] Miguel A.Contr eras*, Manuel J.Romer o, Bobby To, F.Hasoon, R.Noufi, S.W ard,
K.Ramanathan, “Optimization of CBD CdS process in high-efficiency Cu(In,Ga)Se2-
based solar cells,” Natl. Renew. Energy Lab. 1617 Cole Blvd Gold. CO 80401 USA.
[7] S. Chun a, Y.Jung b, J.Kim b, D.Kim a,n, “The analysis of CdS thin film at t he processes
of manufacturing CdS/CdTe solarc ells,” Dep.
OfMaterialsScienceandEngineeringKoreaUniversity5-1Anam-DongSungbuk-
GuSeoul137-713RepublicofKorea B Dep.
OfChemicalandBiologicalEngineeringKoreaUniversity5-1Anam-DongSungbuk-
GuSeoul137-713RepublicofKorea.