Sustainable Methods for recycling of solar cells based on

compositions and manufacturability aspects

Authors: Arvind Ravi, Sathish Kasilingam, Sri Sadhan Jujjavarapu

Graduate school of Mechanical Engineering, The University at Buffalo

Abstract:

In most of the countries use of renewable

energy sources for energy production is

increasing and one such energy source is solar

energy. It is given high importance because the

latest generation solar panels can even be used

on a cloudy day to store energy and the need for

sustainability is of high importance. Sustainable

growth involves recycling, reusing and proper

design methods. There has not been much

development on recycling of solar panels and

the disposal of solar modules can lead to the

release of toxic materials into environment.

Chemicals like cadmium telluride and Indium

are the toxic substances which are harmful. For

example, the third generation solar technology

uses thin film solar cells that contain CIGS

(copper indium gallium Selenide) cells which are

highly toxic. In the US solar industry, 22.7 GW

is the installed capacity. First Solar is the only

US Company involved in solar panel collection

and recycling program. There is a need to

develop recycling methods based on economic

and ecological aspects

Index terms: solar cells, recycling, toxic

chemicals, sustainable design and life cycle

approach

Scope:

Solar cells have been in usage for more than 25

years. Much research and development has

been done to increase efficiency. Recycling and

recovery are important aspects because the

photovoltaic cells used in solar modules contain

materials which are rarely available and in order

to prevent global scarcity, recycling is very

important.

Solar panels are constructed in a manner in

which the glass panel constitutes around 60% of

the mass and the aluminum frame around 20%

of the mass, while the solar cells constitutes less

than 10% of the mass. Thus a huge portion of

the solar panel waste is only glass or aluminum

which can be reused and the rest of 10% should

be recovered in order to prevent toxic wastes

entering environment.

Motivation:

Use of solar panels for storing energy in the form

of electricity has increased in past few decades

in many of the countries. The price of solar

power, together with batteries for storage, has

continued to fall so that in many countries it is

cheaper than ordinary fossil fuel electricity from

the grid. But due to the scarcity of elements (like

indium) used in solar modules, it is important to

recover and recycle solar modules. Therefore

more importance is to be given to recycling

techniques of solar modules in order to reduce

the cost even further and improve the efficiency

of solar cells.

Research study areas:

The aim of this study is to design a methodology

for recycling of solar cells. The research area

focuses on the composition of solar cells and

detailed analysis of various chemicals in the

solar cells. Research also questions on the

manufacturability of the solar cells and the

process of recycling used till now. Analysis is

done on the life cycle approach to evaluate the

environmental impacts on the product and

materials. Extensive research on the design

method is conducted by keeping into account

various parameters which influence the property

of recycling of solar cells.

Literature Review

With the increasing awareness of the impact of energy generation from conventional sources on environment, the demand for renewable energy sources has raised in the past few decades. Solar industry plays a major role in providing an opportunity to generate clean electricity. However, the two main challenges that the solar industry is facing is firstly, the cost comparison with the conventional energy sources and secondly, to ensure sustainable manufacturing profile. The term sustainable design can be represented as a dynamic balance between economy and society which eliminates negative environmental impacts. Currently there is no federal regulation governing the recycling of solar module but, according to Silicon Valley Toxics Coalition, they fall under the regulations of hazardous wastes1. Many researches failed to consider the product’s end-of-life and coin the same as sustainable thus not making the product an impeccable design. However, in order to provide a sustainable solution, the end-of-life management of photovoltaic modules must be addressed. A PV module absorbs energy from the sun and converts it into electricity. PV module consists of myriads of solar cells which contains semiconductor materials, a protective layer, a substrate and wiring to conduct electricity2. The recent cutting edge technology deal with manufacturing of thin film PV cells which are less expensive and contains semiconductor material added to substrata like glass and metal. Second generation thin films technology also uses cadmium telluride, copper indium selenide and amorphous silicon. These materials are critical with regard to their scarcity in earth’s crust and economic impact. The other materials are polycrystalline silicon (p-si) and mono

1 Journal-Karen Ann Brouwer, Chhaya Gupta, Shelton Honda, Mahshad Zargarian, METHODS AND CONCERNS FOR DISPOSAL OF PHOTOVOLTAIC SOLAR PANELS, December 2011 2 Journal-“Golden Gate university environmental law journal, preventing waste disposal of solar photovoltaic panels- Genevieve Coyle 33pages, (7-12-2011).

crystalline silicon (c-si) which can be recycled conventionally. Researchers found many ways of recycling the solar modules from manually disassembling the module to chemical treatment and extraction of chemicals from the cells3. The life cycle approach of the product in its way to sustainability includes the following parameters: 1. Availability of materials for manufacturing 2. Manufacturing capability of the product 3. Outbound logistics 4. Consumer use and efficiency 5. Disposal.4 One such process of extensive recycling is done by the World leading solar manufacturer “First Solar”. A recycling process for CdTe thin-film modules were developed by First Solar and has been scaled to full production at all its manufacturing facilities. The same is replicated in Frankfurt an der Oder, Germany. Module collection and recycling Programme is the initiative of the First Solar and through this Programme they manage the logistics of collecting end-of-life modules and transporting them to a recycling center. In order to break lamination bonds, the modules are shredded into large pieces and then crushed by hammer mill to pieces smaller than 5 mm. slow rating leach drum is used in order to remove the semiconductor films and in the process the films are etched from the glass. To achieve an optimal solid-liquid ratio the chemicals such as weak sulphuric acid and hydrogen peroxide are added to the glass. Glass is separated from the liquids in a classifier and then it is moved to a vibrating screen where the glass is separated from Ethylene Vinyl Acetate (EVA) pieces5. The glass falls through the screen and EVA is deposited on the conveyor from where it is collected. The glass is now taken for rinsing, cleaned and sent for recycling. The rinse waters are pumped to a precipitator for the metal recovery. The

3 Journal-“ END of life management and Recycling of PV modules “- Vasilis M. Fthenakis (25 May 2000) 4 Journal-“Life cycle assessment of solar modules re-cycling process” – Muller and Wambech 5Journal- L'urederra, Fundación Para El Desarrollo Tecnológico Y Social, EP 2308919 A1, Polyvinyl butyral recycling method, April 2011

precipitation of the metal compounds are done in three stages at different pH levels using sodium hydroxide. The bottom cake comprises of rich metal which settled down during the process. The metal remains are sent to a third party where it can be processed to an extent that it can be used in new solar modules.

Discussing the success rate, about 90% of the glass and 95% of semiconductor materials are recovered to be used in new solar modules. According to the Brookhaven National Laboratory, U.S Department of Energy, the recovery of tellurium is 80% or more and can be sold as a commercial grade (99.7% Te)

In the conventional solar modules, CIGS

(Copper Indium Gallium (di) selenide) is a

critical raw material which accounts to 2% of

global solar cell market. Every recycled material

must have purity of about 99.999% for it to be

re-used for manufacturing. The CIGS material

can be recycled by using two extensive methods

which are wet chemical method and electro-

chemical method. In wet chemical method, de-

metallization of the substance is done using

HCL and HO catalyst. In electrochemical

method, the solar cell is kept in anode and the

CIGS is redeposited in cathode (substrate).6

The analysis performed after the recycling

process determine the amount of

semiconductor material recovered for 1 meter

square area of the solar module. The

polycrystalline silicon and mono-crystalline

silicon can be recycled identically. The mass of

the recovered material can be determined using

the stated equation (1):

M=A*t*ῤ*Z (1)7

Where ‘A’ is the area of the semiconductor, t is

the thickness of the semiconductor, ῤ is the

density of the semiconductor and Z is the

percentage of the semiconductor material which

can be recovered from the solar module. The

total waste from the solar module can be found

using the parameters like area (A), weight of the

solar module (w), power per unit area (e) and

nominal power (N). The equation (2) which we

derive has to be minimized in-order to get a

sustainable design product.

W=(A*e*w)/N (2)5

The product in-order to be accounted for

sustainability, researches must focus on certain

parameters like air emissions while recycling

and also the disposal of chemical wastes which

leads to pollution.

The main aspect of this research concentrates

on minimizing the environmental impact by

analyzing the design method, chemical

compositions and manufacturability of the

product and the capability of the product to be

recycled.

Fig.1. Cross-section of a solar module.

6 Journal-“Recycling of high purity selenium from CIGS solar cell waste materials”- Anna M.K Gustafsson, Mark R.Stj Foreman, Christian Ekberg 29-Dec 2013.

7 Journal-“Producer responsibility and recycling solar photovoltaic modules “- Mcdonald, J.M.Pearce (13 July 2010)

METHOD

Researches have focused on many methods in recycling the solar panels and also their economic viability. Recycling was concentrated for five of the largest volume commercialized types of the solar cells which are amorphous silicon, poly-crystalline silicon, mono crystalline silicon, cadmium telluride and copper indium gallium di-selenide8. Since thin film solar cells are making advancement in global market and they contain Cadmium Indium Gallium Selenide (CIGS), which leads to higher efficiency solar cells with lesser use of the semiconductor material which in turn reduces the cost of the module. The present methods faces many hindrances which leads to decrease in efficiency and increase in the constraints for the recycling of the CIGS photovoltaic cells. The environmental constraints consists of the gases released during the recycling process and disposal of wastes. The novel method must be designed in a sustainable way by including the environmental constraints along with the manufacturing constraints which optimizes the process of recycling.

The effects on environment due to solar panel manufacturing are addressed below:

1. The release of toxic gases during recycling of the solar cells causes inevitable environmental damage.

2. Percentage purity of materials after recovery is not 100% for all the materials, making it difficult for the rare earth minerals to be reused.

3. There is no proper method for the disposal of the wastes at the end of recovery process, causing environmental pollution.

Challenges faced during recycling: 1. In oxidation process, due to decrease in

contact time between the gas and CIGS there is less amount of selenium recovered.

8 Journal-“Recycling of high purity selenium from CIGS solar cell waste materials”- Anna M.K Gustafsson, Mark R.Stj Foreman, Christian Ekberg 29-Dec 2013 9 Gustafsson, A.M.K., Foreman, M.R.S., and Ekberg, C. Recycling of high purity selenium

2. There is a film uniformity challenge on large substrates.

3. The chemicals used to treat and the cost of processing is higher.

Recycling CIGS solar panels involves extraction of rare earth materials such as Selenium, Copper, Indium and Gallium. The extraction process is as follows:

Extraction of Selenium:

Selenium is separated as Selenium dioxide by the oxidation of CIGS material at elevated temperature and Selenium is extracted after the reduction of Selenium dioxide 9. The purity of the material achieved is 99.99 wt% with respect to the materials which decrease the efficiency of the solar cells.

a. Oxidation of Selenium: Cu(In,Ga)Se2(s)+O2(g)→SeO2(g)+MexOy(s) (3)

The Cu(In,Ga)Se2(s) (CIGS material) when oxidized results an selenide dioxide and MexOy (mixed oxide of metals)

SeO2(g)→SeO2(s) (4)

b. Reduction of Selenium: First reduction: Agent used is Deoxy benzoin

Deoxy benzoin + SeO2(aqueos) → Benzil + Se(s)+H2O(liquid) (5)

Second reduction: Agent used is Sulphur Dioxide

SeO2(aqueos)+SO2(gas)→Se(s)+H2SO4

(aqueos) (6)

Extraction of Copper, Indium and Gallium:

Pyrometallurgical treatment (such as roasting, smelting and fire refining) for sulphide ores followed by electrodeposition and electro refining can be used to extract materials, since

from CIGS solar cell waste materials. Waste Management, 2014.

PV modules contain very less percentage of metals conventional pyro- or hydrometallurgical methods are impractical. Menezes developed an electrochemical method to transfer CIS material directly from an old solar cell to a new one which can process both manufacturing and end-of-life PV module waste.

Closed-loop electrochemical recycling for PV Modules

Anode is defective glass/Mo/CIS/CdS/ZnO panel in Fig. 2. Cathode is Mo-coated glass panel and the dissolved metals can be retrieved at auxiliary electrode. The later part of the separation includes thermo-chemical removal of the ethylene vinyl chloride encapsulation. The electrodeposition at different potentials is

considered as the best method of separation of copper and indium.

Fig.2.Schematic of CIS Module Recycling System with two unit cells10 .

10 Electrochemical solutions to some thin-®lm PV manufacturing issues Shalini Menezes, 2000.

Case Study

Recycling methods of two major solar module

manufacturing companies namely ‘SunPower’

and ‘First Solar’ are mentioned:

SunPower Corporation: SunPower Corporation is an American energy company that designs and manufactures high-efficiency ‘Crystalline silicon photovoltaic cells’. Recycling of Silicon Based PV: Contains 80% of the glass and involves three main steps:

Preparation phase – removal of the frame and junction box

Shredding Processing in the flat glass recycling line

Output fractions of this flat-glass-oriented process are ferrous and non-ferrous metals, glass, silicon flakes and plastics with an average recycling quota of approximately 85% (input weight, depending on recycling technology). The glass resulting from the recycling is partly reintroduced in glass fiber or insulation products. The metals and plastics can be used for the production of new raw materials. Recycling of Non-silicon based PV:

11 Fig.3.Full Recovery End-of-Life Photovoltaic

flowchart

11 FRELP – Full Recovery End of Life Photovoltaic

These processes employ chemical baths to delaminate and separate the different PV module components:

Shredding (optional) Solubilizing in a chemical bath(like

referred to in literature review) Detaching (optional) Sorting of the materials Further processing in dedicated glass

and semiconductor recycling facilities Up to 95% of the materials used in these modules can be recovered for use in new materials.

PV Takeback, Reuse and Recycling:

Two main environmental solutions proposed are

Full Recovery End-of-Life Photovoltaic

(FRELP):

Recovery of high quality extra clear glass, to be used in the hollow and flat glass industry, thus saving energy and reducing CO2 emission in the glass melting process.

the recovery of (metallic) silicon, can be used as ferrosilicon in iron silicon alloys

or If the recovered silicon is pure enough, it can be transformed into amorphous silicon for the production of thin films.

This reduces energy consumption and CO2 emissions.

Crystalline Silicon Photovoltaic cells recovery

process

Silicon wafer recovery:

PV solar cell separation:

In thermal delamination, the ethylene

vinyl acetate(EVA) is removed and

materials such as glass, aluminum

frame, steel, copper and plastics are

separated.

Cleansing the surface of PV solar

cells:

Unwanted layers (antireflection layer,

metal coating and p–n semiconductor)

are removed from the silicon solar cells

separated from the PV modules; as a

result, the silicon substrate, suitable for

re-use, can be recovered.

The silver coating was dissolved with

40% aq. HNO3 at a temperature of 40 0C

and recovered from the waste acid by

electrolysis.

30% aqueous solution of KOH was used

to remove the Al layer from the cell’s rear

surface; the efficiency of the process was

optimal at a temperature of 80 0C.

Sun Power aim to provide a solar panel

with useful life of 40 years whereas they

are already providing a warranty for 25

years.

Fig.4. Principle of PV module recycling process

12 https://www.bnl.gov/pv/files/prs_agenda/2_krueger_ieee-presentation-final.pdf

Fig.5. Flow chart of crystalline solar cell

processing FIRST SOLAR

First Solar, Inc. is an American photovoltaic

manufacturer of rigid thin film CdTe panels.

The recycling process of First Solar is as

follows:

12

Fig.6. First Solar‘s method to collect module

from customer site, recycle it and reuse it.

Fig.7. Flowchart showing the recycling

process13.

Fig.8. Detail process of recycling of end-of-life

modules.

13 http://www.firstsolar.com/en/Technologies-and-Capabilities/Recycling-Services.aspx- First Solar recycling services.

Contains 90% of the recycled glass

(90% of the glass can be used in new panels)

Contains 95% of the recycled semiconductor

material

(90% of the semiconductor material can be

reused in new modules)

Fig.5. shows the state-of-art recycling process

of First solar

Cadmium telluride:

Cadmium telluride is manufactured from pure

Te and cd which are the byproducts of smelting

prime metals like Cu,Zn,Pb and Au.Cadmium

minerals are not found in the commercial

deposits. So it is of prime importance to recycle.

There are two methods for making the CdTe thin

films.

Electrodeposition of CdTe combined with

chemical surface deposition of CdS:

CdTe is deposited on a substrate to the cathode

of the electrolytic system. During deposition, the

concentration of Cd ions is maintained. Electro-

deposition of CdTe is usually accompanied by

chemical deposition of CdS. Precipitated Cd

solids from CBD had to recycled by converting

them to the Cd solids. The process is to stage

where the precipitated cd solids were recycled

by conversion to Cd salts . thereby 99.999% of

Cd is recycled from the CBD wastes by a

combination of leaching and electro-deposition.

This process is about 90% efficient and after

recycling of the residuals, not more than 1% of

Cd and tellurium are used in the facility would

be lost in the form of very dilute liquid and waste

streams.

Vapor Transport Deposition:

In this process the CdTe is deposited from the

compound in powder form after vaporization in

a closed-spaced reactor. The deposits are

either disposed of or recycled. Recycling is both

feasible and economic. Less than 1% of vapors

are carried in the exhaust stream. The dust

emission and the vapor are collected at 99.97%

efficiencies using different filters like HEPA.

About 10-30% is wasted in this process.

Deposition of CdTe has 83% of production

yields, 10% electrical conversion efficiency and

70% material utilization rates. But If solar cells

are thinner the production higher may be higher

than what we have assumed. PV modules are

expected to last 25-30 years. We assume that

CdTe PV modules will be either recycled or

properly disposed off at the end of their useful

life; therefore reducing the atmospheric

emissions during/after decommissioning.

Fig.9. CdTe chemical recycling process

Conclusion:

Improvement in CdTe modules:

The efficiency of the CdTe module with a

double doped PMMA (polymethyl

methacrylate) sheet with rare earth

elements (Sm3+, Eu3+, and Tb3+) in

front could be increased to 11.2%

compared to a module covered with an

undoped PMMA sheet with an efficiency

of 9.6%. These calculations make the

application of a down-shifter attractive for

photovoltaic devices.

Oxide layer formed on the surface of

CdTe modules decrease the efficiency.

Chemical etching can remove the

surface oxide layer and retrieve the

module performances.

Improvement in CIGS modules:

Alkali doping into CIGS and absorber

layers was demonstrated using alkali-

silicate glass thin layers (ASTL).

Enhanced cell efficiencies with the use of

ASTL were demonstrated regardless of

the In/Ga composition ratio in CIGS.

Alternative for CdS buffer layer:

The CdS buffer layer is replaced by other

materials like Zinc Sulphide (ZnS) and

Zinc Selenide (ZnSe). With this change

there is only a considerable change in

efficiency.

Sustainability depending upon environmental

impact:

Research is going on to extract 99.99% pure

minerals from the end-of-life solar modules and

to decrease the toxic waste generated during

the recycling process.

Green House Gas Emissions (GHG):

Comparison of the emission of Carbon:

CdTe PV

Coal-fired generation

Grid-based electricity

Oil-fired generation

X 100X 130X 1440X

Emissions from photovoltaic life cycles by

Vasilis M. Fthenakis, Hyung Chul Kim, and Erik

Alsema. Environmental science & Technology,

2008, Vol.42, No.6, pg. 2168-2174.

Fig 10: Carbon Emissions or electric

generations

Green House effect are mostly caused by

carbon-di-oxide

Durability:

The average life of solar panels is 25 – 30 years.

Research is being conducted to increase the life

of solar panel to 40 years. Deposition of

materials like Potassium or Chalcogenides or

Titanium oxides is considered in order to

increase the efficiency and to increase the

durability.

14

Energy Payback Time (EPBT):

EPBT = Einput/Esaved

Einput – Total energy required to manufacture and

install a solar panel.

Esaved – Total energy generated from the solar

module excluding the energy required to operate

the module.

Fig.10. Energy payback time for various Solar

cells.14

Other Journal Reference:

“Recycling solar panel waste glass

sintered as glass –ceramics”-kae-long lin

, tien chun chu

“Evaluation criteria to select sustainable

remediation methods”- Laura J

Gimpelson

“Recycling of thin film solar cell

modules”-Shibasaaki M, Warburg N,

Eyerer P

“Life cycle analysis of silane recycling in

amorphous silicon-based solar

photovoltaic manufacturing”- Kreiger ,

Pierce , Shonnard

“Applying analytic network process to

evaluate the optimal recycling strategy in

Upstream of solar energy industry”- Yih-

Chearng Shiue, Chun-Yueh Lin

Shogo Ishizuka, Akimasa Yamada, Koji

Matsubara, Paul Fons, Keiichiro Sakurai,

Shigeru Niki, Development of high-

efficiency flexible Cu(In,Ga)Se2 solar

cells: A study of alkali doping effects on

CIS, CIGS, and CGS using alkali-silicate

glass thin layers, Current Applied

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S156, ISSN 1567-1739

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Efficiency CIGS Solar Cells with SCAPS-

1D Software, Energy Procedia, Volume

74, August 2015, Pages 736-744, ISSN

1876-6102

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selenide (CIGS) solar cells with localized

back contacts for achieving high

performance, Solar Energy Materials and

Solar Cells, Volume 104, September

2012, Pages 152-158, ISSN 0927-0248

•Han Jun-feng, Xiao Liu, Cha Li-mei,

Jonathan Hamon, M.P. Besland,

Investigation of oxide layer on CdTe film

surface and its effect on the device

performance, Materials Science in

Semiconductor Processing, Volume 40,

December 2015, Pages 402-406, ISSN

1369-8001

•M.A. Islam, M.U. Khandaker, N. Amin,

Effect of deposition power in fabrication

of highly efficient CdS:O/CdTe thin film

solar cell by the magnetron sputtering

technique, Materials Science in

Semiconductor Processing, Volume 40,

December 2015, Pages 90-98, ISSN

1369-8001

•Franziska Steudel, Sebastian Loos,

Bernd Ahrens, Stefan Schweizer,

Luminescent borate glass for efficiency

enhancement of CdTe solar cells,

Journal of Luminescence, Volume 164,

August 2015, Pages 76-80, ISSN 0022-

2313

Website references:

https://www.bnl.gov/world/

http://www.renewableenergyfocus.com/v

iew/3005/end-of-life-pv-then-what-

recycling-solar-pv-panels/