cigs solar cell_photovoltaics research project

8/10/2019 CIGS Solar Cell_photovoltaics Research Project

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IGS Solar cellsourse Project

Photovoltaic s

Submitted by:

Basim Raza 15100065

Submitted to:

Dr. Nauman Zaffar But


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Introduction:A solar cell is a device that converts sun light into electricity through the process called thephotovoltaic effect. Solar cells are the building blocks for solar panels. Different type of solar

panels are available in the market depending upon from which solar cell are made off.

The whole working of solar cell occur in three steps:

I. Firstly, when light falls electrons-hole pairs are generated in the surface (which is n+doped)

II. The charge carriers need to be remain separated, that is recombination at the surfaceneed to be avoided.

III. Extracting those carriers to the external circuit. If we are talking in terms of excitedelectrons that have been brought from valence to conduction band by the incidentphotons, then these excited electrons need to be travel through the external connectedcircuit, due to which the current flows

Initially there was only the crystalline silicon based solar cell. But since we can optimize the

property of a material up to a certain extent and later we have to start dealing with the tradeoff so eventually people started looking into other materials. Several other materials were triedto fabricate the solar cell with wide diversity into their properties compared to the initialCrystalline Si. Based solar cell.

Our main purpose is to increase the efficiency of cell which later depends on individualefficiencies like thermodynamic and quantum and obviously the Voc and the Phil factor (FF).


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Reflection and recombination processes mainly contribute to quantum efficiencies of cell aswell as the Voc and the phil factor. Resistive losses (ESR and shunt resistance) tothermodynamic efficiency.

The materials that were used to build those solar cells classified the generations of solar cells.

For example the first generation solar cells used to be of crystalline silicon based. Secondgeneration ones were of thin film solar cells which included CIGS, amorphous Si, CdTe usedboth as stand alone photovoltaic devices as well as in integrated power systems. Thirdgeneration includes multi junction solar cells which as the terms suggest contain several of thep-n junctions. Hence if the theoretical efficiency of a single junction is 34%, that of a larger no.of series p-n junctions will be higher and reaching to 84%. Practically it has been found to bearound 43% for a Si based cell [1]. These junction can either be of same type or of differenttype of material. A junction made of two different material is called a hetero junction

While dealing with the efficiency one has be careful how the associated cost increases with theprocess. Is the material commercially available, whether its environment hazardous or not. Theprocess with which the solar cell is fabricated is expensive or are there any alternate processthat can be used for fabrication.

Since those calculations are mainly related to feasibility of the process we will not discuss it intoour further discussion and we will only be concerned with the efficiency


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What is CIGS:Out of many solar cells CIGS solar cell is one that has been commercially made around 2010.Out of the 3 fundamental thin film technologies available in the market, it is the one. Other

being the amorphous, CdTe thin film solar cells. CIGS stands for Copper (Cu), Indium (In),Gallium (Ga) and Selenide (Se) formed as thin layer the back of which is supported withelectrodes to collect current.

CIGS cell properties:So Cu is a transition metal. In,Ga are a post transition metal and Se is a nonmetal. Combinedthey formed a metal solid solution (of alloys CIS and CGS) which behaves as a semiconductorwith a direct band gap and with very high absorption coefficients of 10E5/cm^3 at 1.5evincident photons [2]. The band gap varies from 1.0 eV (CIS ) to 1.7 eV (CGS).

Hence a material with a direct dynamic band gap (thus capable of absorbing of range ofwavelengths of light), high absorption coefficient, thin film structured meaning it would beflexible, is highly favorable for us. Also, it is a hetero junction solar cell. The layer is of around1um thin, compared to the traditional 200um thick layer of the crystalline Si based solar cells.

Device structure: The picture above shows the structure of the CIGS device. It is important to understand itbefore we go into other details of improving the efficiency of solar cells. Starting from the lowermost layer, is the soda lime glass which serving as a substrate. This is coated with Mo


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Molybdenum metal which acts as a metal back contact . The hetero junction is formedbetween the CIGS and ZnO, separated by a thin layer of CdS and ZnO (intrinsic). CIGS is doped ptype by intrinsic defects and ZnO is doped n-type by Aluminium. The ZnO is doped heavilyhence the junction extends very much to depth in CIGS region.

The wide layer of CIGS serves an a absorber with a band gap ranging from 1.02 eV (CIS) to 1.65eV (CGs). In order to make upper surfaces less absorbent, their band gaps are kept relativelyhigher. For ZnO its 3.2 eV and for CdS layer its 2.4 eV.

The top most ZnO layer serves as a contact of current collection.

Modes of production:Traditionally it has been developed using co evaporation and sputtering while the more recent

techniques for fabricating these solar cells include non- vacuum solution processes. We wont discuss the modes here since that relates to synthesis side of solar cells but we will consider theeffects of various oxide layers and other semiconductor layer deposition on the solar cells.

Efficiency Comparison:Although less efficient that C-Si solar cells in terms of efficiency, CIGS are the most efficientamong the thin film solar technologies which include a-Si and CdTe cells Hitting the recordefficiency of 20.4% [3] compared to approx. 25% efficiency of mono crystalline solar cells. Agap this close makes us to investigate more towards the thin film technology to make it morecomparable to C-Si solar cells.

It is interesting to note that a German company recorded the hit 45% efficiency of a solar cellmade by C-Si solar cell with front metal contact removed to the back and the front side coatedwith thin film in order to have higher absorptions. So this kind of combinations are also underconsideration.[4]

Below be discussing the various techniques that have been employed in the past in order toenhance the efficiency of CIGS solar cells. It would include deposition of Oxide layers, quantumtunneling effects and effects of concentrated flux on the CIGS panel efficiency.


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Methods employed to improve CIGS efficiency:

1- Wide bandgap Cu(I n,Ga)Se 2 solar cell s with improved energy conversionefficiency[6]:

Attaining high efficiency (>16%) from semiconductor thin films has been difficult. Nevertheless,by using (a) the alkaline-containing high-temperature EtaMax glass substrates from Schott AG,(b) elevated substrate temperatures of 600 650 C, and (c) high vacuum evaporation fromelemental sources following a three-stage process, we have been able to improve theperformance of wider bandgap solar cells with 1.2< Eg< 1.45 eV. The current density voltage(J V) data we present includes efficiencies >18% for absorber bandgaps of ~1.30 eV andefficiencies of ~16% for bandgaps up to ~1.45 eV. The energy bandgap values for the data in

Figure 1 (and Table 1) were determined from certified quantum efficiency (QE) curves byarbitrarily assigning an effective bandgap value to the energy calculated from the wavelengthvalue where a 20% QE value is observed for the long wavelengths.

Figure 1
http://onlinelibrary.wiley.com/enhanced/doi/10.1002/pip.2244#pip2244-fig-0001http://onlinelibrary.wiley.com/enhanced/doi/10.1002/pip.2244#pip2244-fig-0001http://onlinelibrary.wiley.com/enhanced/doi/10.1002/pip.2244#pip2244-tbl-0001http://onlinelibrary.wiley.com/enhanced/doi/10.1002/pip.2244#pip2244-tbl-0001http://onlinelibrary.wiley.com/enhanced/doi/10.1002/pip.2244#pip2244-tbl-0001http://onlinelibrary.wiley.com/enhanced/doi/10.1002/pip.2244#pip2244-fig-0001


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Table 1

Here we are able to report a significant improvement to the efficiency of wide gap CIGS solarcells for 1.2 eV < E g < 1.45 e V. Nonetheless, the wide gap CIGS cells still suffer from a significantinterfacial recombination as established from our temperature-dependent J V measurementsshown in Figure below.

We have experimented with higher-than-standard substrate temperatures for the growth ofCIGS materials with 0.3 < x < 1.0. The use of higher processing temperatures (600650 C) thanthe standard 550 600 C typica lly used have led to significant improvements in the energyconversion efficiency of wide gap CIGS solar cells with bandgaps up to 1.45 eV. The main parameter that has improved is the output voltage of the cells as compared with previous CIGSmaterials grown on SLG substrates. A reduction in the reverse saturation current density value


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is responsible for the enhanced voltages attained. The high-temperature CIGS materials grownalso show improvements to their grain boundary characteristics and now display propertiessimilar to those found in high-efficiency CIGS materials

2- 19.9%-eff icient Zn O/CdS/ CuI nGaSe2 Solar Cell with 81.2% Fi ll F actor: -[4]

This technique describes a Cu(In,Ga)Se2 (CIGS) solar cell with record 19.9% total-area efficiencydemonstrated at the National Renewable Energy Laboratory (NREL) Processing of the recorddevice differed from that previously described in three respects. Most notably, The third stagewas terminated without Ga. During the last 10 s of the deposition, about 25 A of In weredelivered in the absence of Ga. Secondly, after the deposition was terminated, the sample wassubjected to a 2.5-min anneal in Se while the sample temperature was maintained at 6008C.Third, a 2-min, 2008C air anneal was performed after the CdS deposition. Similar anneals haveyielded small improvements in fill factor and voltage for devices with efficiencies greater than15%.8 We believe that these three empirical processing changes create a near-surface region inthe CIGS with reduced recombination.

Strategy:

The device exhibits significantly lower recombination and higher fill factor than earlier devices. Slight

modifications in CIGS surface are responsible for higher efficiency and greater fill factor.


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3- ZnO Doping Profi le Eff ect on CI GS Solar Cells Effi ciency and Parasiti cResistive L osses based on Cell s Equi valent Ci rcui t [6]

The window layer of the CIGS thin film solar cells plays the role of transparent front contact and

the n-side of pn-heterojunction. Thus the variation of window layers electrical and opticalproperties can affect the cell performance. Properties of Al-doped Zinc oxide (ZnO) thin film asmost common used window layer for CIGS solar cells were studied here. Structure of the cellused is below.

Table with material properties


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The quantum efficiency of simulated CIGS thin film solar cell is shown in Fig. 5. Simulation

results shows that by increasing Al content in ZnO layer, the quantum efficiency will enhance. Incell with a highly doped ZnO layer, there will be more amount of incoming light that can passthrough the TCO layer to the underlying layers and contribute to the quantum efficiency due toincrease of ZnO layers optical transmission


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The cell performance is analyzed and simulated by the function of ZnO layers doping pr ofile. Itcan be concluded that the cell with higher doped transparent conducting oxide ZnO showshigher level of efficiency and both open circuit voltage and short circuit current density

improved by increasing Al-dopant content mainly from 2% to 5%. The ZnO layer with higherdoping level will enhance the cell property in terms of series resistance but it reduces the fillfactor slightly by reducing shunt resistance. In this study the ZnO layer with doping level of upto 5% was investigated and the results show that although the shunt resistance decrease due toan increase of ZnO doping level of up to 5%, the cell efficiency has increased. Ideally the seriesresistance was to be infinity and shunt resistance was zero.

4- Cu(I n,Ga)Se2 Solar Cell s M easur ed under L ow Flu x OpticalConcentration [7]

The behavior of Cu(In,Ga)Se2 (CIGS) solar cells under low flux concentration is beinginvestigated for two complementary reasons. First, pairing photovoltaic devices withinexpensive low flux optical concentration elements can be a rational pathway towards systemsthat yield a low leveled cost of electricity (LCE). Second, characterization of photovoltaic filmsunder elevated flux levels can be an effective tool for helping to understand the nature ofrecombination and other parasitic loss mechanisms in these device structures. Here we reporton a CIGS device that achieved a record efficiency of 23.3% at 14.7 Suns optical concentration.


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The performance of these CIGS concentrator cells strongly suggest the presence of an internalseries resistance component that is limiting their ability to handle flux densities in excess oftwenty or thirty suns. Advances in the performance/cost ratio of low flux concentrating opticalsystems make it worth considering novel approaches to populating these systems with powerconverters. This work indicates that some polycrystalline thin-film devices have reached astage of development where their performance is high enough to leverage the investment inoptical enhancement while potentially taking advantage of their inherent low cost and otherqualities such as ease of fabrication of monolithic modules or deposition directly onto heatspreaders.

Conclusion:Over all, we have seen that different parameters take part to increase the efficiency of CIGssolar cells. The varying band gap, doping of ZnO layer which serves not only as a contact butalso as an absorbent layer the thickness of CIS and CdS matters when computing the powergenerated. It is suggested to model the Doping as well as the thickness of ZnO layer in CIGS to

check for optimal points for efficiency and a larger fill factor.


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References:-

[1] http://www.cnet.com/news/solar-junction-claims-cell-efficiency-record/

[2] Stanbery, B. J. (2002). "Copper Indium Selenides and Related Materials for Photovoltaic Devices". CriticalReviews in Solid State and Materials Science 27 (2): 73.

[3] http://www.empa.ch/plugin/template/empa/3/131438/---/l=2

[4] http://onlinelibrary.wiley.com/doi/10.1002/pip.822/pdf

[6] http://onlinelibrary.wiley.com/doi/10.1002/pip.2244/pdf

[7] http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=6925546&tag=1

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http://www.cnet.com/news/solar-junction-claims-cell-efficiency-record/http://www.empa.ch/plugin/template/empa/3/131438/---/l=2http://onlinelibrary.wiley.com/doi/10.1002/pip.822/pdfhttp://onlinelibrary.wiley.com/doi/10.1002/pip.2244/pdfhttp://onlinelibrary.wiley.com/doi/10.1002/pip.2244/pdfhttp://onlinelibrary.wiley.com/doi/10.1002/pip.822/pdfhttp://www.empa.ch/plugin/template/empa/3/131438/---/l=2http://www.cnet.com/news/solar-junction-claims-cell-efficiency-record/

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