solar cells and their applications (fraas/solar cells 2e) || crystalline silicon solar cells and...

PART II TERRESTRIAL SOLAR CELL ELECTRICITY

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5 CRYSTALLINE SILICON SOLAR CELLS AND MODULES

LEONID RUBIN Day4 Energy Inc.

5.1 INTRODUCTION

During the last decade, the PV industry has demonstrated an impressive 30 – 50% annual growth rate and, as a result, in 2008, the total production capacity has surpassed 4 GW with cumulative annual revenues reaching $20 billion. There are several reasons for this success.

Initially, in Japan and later in Germany, government - supported programs for the PV industry development played extremely important roles for attracting capital investments for the PV industry. Government subsidy programs helped bridge the gap in cost between PV - generated electricity and conventional electrical grid prices, thus creating a viable marketplace for PV product manufacturers. There were also a number of other factors that have contributed to the rapid growth of the PV industry. The sudden decline in the microelectronic industry due to the Internet bubble and the availability of excess silicon feedstock production capacity provided silicon feedstock for the PV industry. The growing concern about global climate change due to greenhouse gas emission combined with oil and gas price rises suggested that the development of alternative energy sources should become an important part of government policy. Substantial progress was also made in PV cell effi ciency, and PV cells were being successfully mass produced as well. One may say that it was a unique “ once - in - a - life - time ” opportunity for the PV industry and so, the PV companies took full advantage of it.

Nevertheless, the electric energy produced with solar cells continues to be noncompetitive with traditional nonrenewable sources. The PV industry is still strongly dependent on subsidies, and PV provides a negligible contribution to the

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Solar Cells and Their Applications, Second Edition, Edited by Lewis Fraas and Larry PartainCopyright © 2010 John Wiley & Sons, Inc.

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overall energy generation market. It was expected that the economy of scale would eventually result in cost reduction suffi cient to make the PV industry cost competi-tive with conventional sources of electrical power. There is no doubt that in recent years, some cost reduction associated with PV cells and module mass production has been achieved. But this progress has not been suffi cient so far to make the PV industry cost competitive.

One of the key roadblocks to further cost reduction is in the overall lack of technological innovation in PV cell and PV module manufacturing. Ironically, the very subsidy system that resulted in PV product demand explosion is also partially responsible for the lack of technological innovation during this same period. With demand exceeding supply by a wide margin, manufacturers have focused their efforts and capital on expanding production capacity, economies of scale, and profi t. Crystalline silicon technology development has been largely “ on the back burner. ” The fact is that the majority of PV cell manufacturers are utilizing identi-cal production technologies. Without manufacturing technology improvements, the difference between the effi ciencies of mass - produced and advanced PV cells has been increasing in the last decade and is now pronounced. Even less diversity is found between PV module producers. They continue to use 35 - year - old tabbing and stringing technology.

In this chapter, we will present a brief overview about the design and perfor-mance of conventional mass - produced crystalline silicon PV cells and the main factors limiting their light - to - electric energy conversion effi ciency. We will describe novel technologies that can lead to PV cell effi ciency improvement. We will then describe some advanced PV cells with outstanding effi ciency. The current status of PV module fabrication technologies and the possibilities for their improve-ment will be described. Finally, the optimization of PV cell and module designs to maximizing annual kilowatt hour generation capacity for PV power generation systems will be addressed.

5.2 INDUSTRIAL CRYSTALLINE SILICON PV CELL

Since the PV industry does not require the same silicon material purity as for microelectronics, it was initially possible to utilize the waste from microelectronic silicon production. This is referred to as solar - grade silicon for initially monocrys-tal and later for mc silicon wafer production. The most widely used technology for making monocrystal silicon is the CZ. For CZ silicon crystal growth, a silicon monocrystal seed is dipped into a crucible of molten high - purity silicon and is withdrawn slowly pulling a cylindrical monocrystal. This silicon crystal is then sliced into thin CZ wafers that are used for CZ PV cell production.

Production of mc silicon typically involves a casting process in which molten silicon of slightly lower grade, referred to as solar - grade silicon, is directly cast into a typically squared mold where it is slowly crystallized into 3D ingots that are then sliced into square mc wafers. The temperature cooling gradient profi le is one of the main factors that determine the impurity distribution divided between the

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INDUSTRIAL CRYSTALLINE SILICON PV CELL 115

silicon microcrystal and the boundaries between them. Segregation of impurities into the grain boundaries is referred to as gettering and infl uences the quality of the mc wafers.

It is known that the PV cells produced from CZ silicon typically have higher effi ciency than the PV cells from mc material. At the same time, the throughput from the CZ process is about two times lower compared with the mc process, and the mc process consumes less energy and material with a lower cost. Therefore, the price of mc silicon wafers is lower for PV cell manufacturers, but there is a compromise leading to lower PV cell conversion effi ciency.

Recently, there has been substantial progress in both single - crystal and mc process technology leading to higher cell effi ciencies for both. Currently, the market share of mc PV cells is slightly higher compared with CZ. Due to growing demand from the PV industry, a new solar - grade crystalline silicon production capacity has been established based not only on conventional solar grade but also on some new metallurgical silicon purifi cation technologies. However, while the silicon material availability has increased to meet the PV industry demand, unfor-tunately, the PV cell effi ciency produced from metallurgical silicon has been reduced. It is now evident that production of high - effi ciency and low - cost CZ or mc PV cells must be based on suffi ciently good quality initial feedstock silicon material.

Conventional crystalline silicon PV cells are generally produced from p - type monocrystal or mc semiconductor wafers of 150 - to 300 - μ m thickness. The front side of industrial PV cells is typically doped with phosphorus, thus creating an n - doped area of 0.1 - to 2 - μ m thickness and a resistivity typically of 50 – 65 Ω per square. It is also possible to produce PV cells from monocrystalline n - type crystal-line silicon with a boron - doped p - type front surface. These n - type PV cells do not suffer from light - induced effi ciency degradation as often happens with monocrys-talline p - type PV cells.

The junction between the n - doped surface area and the p - doped bulk silicon creates a charge separation region with a strong dipole electric fi eld. The surface area is referred to as the emitter and the bulk region is referred to as the base. When a PV cell is illuminated by light, photons produce electron – hole pairs and the dipole electric fi eld provides for a separation of these charges. This displacement of free charges results in a voltage difference between the p and n regions of the PV cell. The emitter technical characteristics such as thickness, doping profi le, and doped element concentration in the PV cell surface region are extremely important for PV cell effi ciency. Conventional PV cells are typically equipped with current collecting metal contacts on the front and rear sides provided for conducting elec-tric current when the p and n regions are connected through an external load. Figure 5.1 shows a drawing of a typical square mc silicon cell showing the front - side collection metallization pattern comprising screen printed fi ngers and two bus bars that collect electric current from the PV cell.

The conversion effi ciency of solar energy into electric energy is considered to be the main PV cell characteristic. Under illumination, PV cells generate a maximum voltage value referred to as the open - circuit voltage or Voc when the

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external circuit is open. If the external circuit is shorted, then the light - generated electric current reaches its maximum value referred to as the short - circuit current or Isc. The dependence of the current I versus the voltage V for different values of the external load is known as the I / V curve where the values of Isc and Voc are the intersection points with the current and voltage axis, respectively. Unfortunately, the maximum value of generated electric power is not the product of Isc × Voc. This maximum is never achieved due to inevitable power losses. The real value of generated power may be evaluated from the experimental I / V curve containing the so - called maximum power point referred to as Pmpp, at which I , V , and P reach their maximum values Pmpp = Impp × Vmpp. The ratio between Immp × Vmmp/Isc × Voc represents PV cell general power losses and is referred to as the fi ll factor (FF). The PV cell energy conversion effi ciency η is expressed as

H E= × ×Voc Isc FF ,

with FF being the fi ll factor and E being the light energy in watt per PV cell area.

5.3 EFFICIENCY LIMITATIONS

Although the theoretical effi ciency of a crystalline silicon PV cell approaches 29% and the world record for the best silicon PV cell is 24.3%, the average effi ciencies

Bus bar

Bus bar

70

Fingers

Figure 5.1. Top view drawing of a typical 156 - mm 2 MC silicon cell.

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EFFICIENCY LIMITATIONS 117

of a typical industrial monocrystalline or mc PV cells are 17% and 16%, respec-tively. There are several causes limiting PV cell effi ciency. First of all, there are limitations based on the fundamental properties of silicon semiconductors. Photons with energies less than 1.12 eV are lost due to the silicon semiconductor band gap, and photons with energies exceeding 1.12 eV loose energy via dissipation into heat. The maximum value of the PV cell open - circuit voltage Voc is substantially lower than the silicon semiconductor band gap because it is defi ned by the quasi - Fermi level separation. The typical Voc values for conventional monocrystalline and mc PV cells are about 625 and 610 mV, respectively. Even high - effi ciency PV cells demonstrate a Voc value not higher than 722 mV.

5.3.1 Optical Losses

The theoretical Isc maximum value for a crystalline silicon PV cell may reach 42 mA/cm 2 under AM 1.5 sun irradiation. In reality, it is hard to reach this value because the Isc value of a PV cell is not determined simply by the incident solar energy intensity but instead, it depends on the fraction that is absorbed by the PV cell and converted without losses into electric energy. The main problem with crystalline silicon semiconductor material is that it absorbs light very poorly due to its high refraction index of about 3.9 and corresponding high light refl ection of about 40%. The most effi cient way to solve this problem is to utilize light trap-ping. Modern wet chemical etching and laser processing technologies allow for arranging for different types of high - effi ciency textured structures like inverse pyramids or honeycomb - type structures on monocrystalline and mc PV cells. Light trapping textures can decrease light refl ection to below 10%. An additional decrease in refection is achieved after antirefl ective dielectric coating . Sample AR coatings can consist of SiN x with a refraction index of up to 2.2. They are applied on top of the initially textured surface of the PV cell. This process can decrease the light refl ection further to less then 4%. It is worth noting that most advanced technologies for achieving extremely effi cient light trapping surfaces are kept as proprietary know - how. An additional optical loss is associated with the partial transmission of light in the IR spectrum. This effect is most pronounced when the PV cell thickness is less than 250 μ m. Therefore, there is a need to utilize back - side mirror refl ective coatings, thus preventing effi ciency decline due to insuffi -cient absorption of light.

The area that is occupied by the metal current collection contacts on the front side of the PV cell is referred to as the shading area. It impacts the optical losses by preventing solar radiation from reaching the surface of the PV cell and thereby generating electric current. This shading area typically occupies up to 7 – 10% of a PV cell ’ s available front surface, thus decreasing the PV cell effi -ciency accordingly. Later in this chapter, we will describe several promising current collecting technologies that have been developed for decreasing front - side shading.

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5.3.2 Recombination Losses

The photon - generated electrical charges have to diffuse to the charge separation area and further to the current collecting contacts located on the front and rear sides of a PV cell. In general, this diffusion process encounters two main types of power losses. Charge recombination losses can occur in the semiconductor bulk and on the PV cell front and rear surfaces. Recombination in the bulk strongly depends on semiconductor impurities and crystal dislocation concentrations. These defects are responsible for energy states that function as effi cient trap and recombination centers. Free electrons captured at these centers are passing to the valence band dissipating energy as heat. This type of recombination is typical for mc and monocrystalline PV cells and strongly depends on materials purity. In the case of mc material, bulk recombination may be reduced by impurity gettering to the microcrystal boundaries during casting crystallization and/or by bulk passiv-ation, for example, with hydrogen during the SiN x AR coating application. It is obvious that PV cells produced from metallurgical silicon are particularly exposed to this type of recombination, thus demonstrating substantially lower effi ciency. The recombination on the PV cell surface depends on the density of defects on the surface due to silicon crystal edge breakdown, as well as the pres-ence of the metal current collecting contacts, and dopant concentration on the surface. Several technological methods have been developed and introduced into mass production to minimize surface recombination, which may be referred to as surface passivation. Charge recombination on the front and rear sides of a PV cell may be substantially reduced by passivation with a thin layer of a dielectric material, such as SiO 2 , SiNx, or SiC, by employing industrially available tech-nologies and equipment [1, 2] . Recombination on a p - type PV cell rear surface may also be decreased by doping, for example, with boron or aluminum, thus creating a p+ layer or BSF.

5.3.3 Resistivity Losses

Conventional high - quality silicon monocrystalline PV cells of 156 - mm 2 area with optimized optical loss minimization typically generate Isc values of up to 8.5 A. Keeping in mind that the Voc value remains practically at the relatively low level of 625 mV, it is a challenge to collect this current with minimum power losses ( I 2 × R ) because the PV cell resistivity starts to have an extremely high impact on the PV cell effi ciency. Overall, the PV cell resistivity includes the following main components: (1) the series resistance ( R ) of the current collecting pattern on the front and rear sides of the PV cell, (2) the parallel or shunt PV cell resistance ( R sh ), and (3) the contact resistance between the front and back - side metal patterns with the emitter ( R em ) and bulk silicon ( R bl ), respectively.

The typical technology for making front - and back - side metal contacts is based on a conventional screen printing process in which an electrically conductive paste, containing silver and/or aluminum powder particles, is printed through a

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EFFICIENCY LIMITATIONS 119

screen onto the front and back surfaces of a PV cell. The front - side screen is typi-cally confi gured to produce a plurality of thin parallel line contacts referred to as “ fi ngers ” connected typically to two or three wider lines referred to as “ bus bars. ” The fi ngers collect the electrical current from the front side of the PV cell and transfer it to the bus bars. Metal ribbons are typically soldered to the bus bars to conduct electric current to the neighboring PV cell and further to the electrical circuit. Typically, the width and the height of each fi nger are approximately between 110 and 120 μ m and between 10 and 20 μ m, respectively. This corre-sponds to a height - to - width aspect ratio ranging between 0.1 and 0.15. It is evident that if the ratio between fi nger width and height is improved by making them nar-rower and thicker, fi nger conductivity will be improved along with decreased shading. It has been demonstrated that the utilization of hot melt pastes with modern heated screen printing equipment allows for printing fi ngers with < 70 - μ m width and 20 - μ m heights and an aspect ratio of about 0.28. The back surface of a PV cell substrate is printed in two sequential steps. Initially, a conductive paste containing a composition of silver and aluminum powder particles is screen printed and dried by heating. This fi rst pattern consists of small areas referred to as silver pads that are to act as current collecting contacts. Afterward, a partially conductive paste containing aluminum powder particles is then spread over the entire back surface of the substrate but only partially overlapping the edges of the above - mentioned silver pads. This paste is then dried by heating. The PV cell is then “ fi red ” at high temperature in an oven. During this process, the front - side silver paste enters a silver porous metallic phase, where at least part of it diffuses through the front surface of the PV cell into the emitter area and alloys with the silicon forming a silver silicide, thus creating an ohmic contact with the emitter. The resistance of this contact depends on the emitter depth, the dopant concentration, and its profi le. The depth of the emitter must be controlled very carefully in order to prevent metallic particle penetration during the fi ring process , thus avoiding shunting. On the rear side of the PV cell, the aluminum diffuses through and alloys with the bulk silicon, thus producing a highly doped p+ layer or BSF. At the same time, the aluminum also alloys with the silver/aluminum pads in areas where it overlaps with them. These silver/aluminum pads are collecting electrical current from the rear side of the PV cell and act as back - side electrical terminals but do not provide back surface passivation.

In order to achieve low contact resistance between the metal patterns and the emitter, there is a need to have an emitter with high doping concentration in the conventional PV cell ’ s front surface. At the same time, realization of this require-ment results in higher surface recombination losses and PV cell effi ciency decline in the blue spectral region. In order to increase the conversion effi ciency of solar cells that employ a conventional screen printed metallization, emitter design parameters may be optimized such that under a screen printed fi nger, an emitter ’ s doping concentration and thickness is a suffi ciently high, whereas in the light - illuminated areas, the emitter ’ s doping concentration is lower and the thickness of the emitter is thinner. An emitter with these differing doping concentration thick-nesses is generally known as a selective emitter.

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Selective emitters may be produced, for example, in accordance with the following process. After the highly doped emitter diffusion step, the emitter area is selectively masked and exposed to wet chemical etching for removing silicon from the nonprotecting areas, thereby making the emitter thinner at the same time keeping the highly doped emitter under the mask areas protected. The screen printer metal patterns must be precisely aligned with the former mask - protected areas, thus securing low ohmic contact between metallic contacts and emitter and avoiding shunting.

Although the use of a selective emitter has proved to be effective in improv-ing PV cell effi ciency, the implementation of a selective emitter in practice is quite complicated. Another approach to reduce surface recombination and to preserve the low resistance between the screen printed metal patterns with the emitter is based on employing a deep emitter concept. After producing the p/n junction during the doping diffusion, the PV cell is exposed to high temperature, thereby driving the dopant deep into the emitter, thus lowering its surface concentration. At the same time, the increased depth of the emitter allows the metal patterns to penetrate deeper, thus increasing the contact area with the emitter, thereby lowering the contact resistance with the emitter. The negative associated with this promising approach is the necessity to use a quite expensive prolonged high - temperature process. This method also is not compatible with mc material because it does not tolerate temperatures above 900 ° C.

The shunt resistance R sh can also have a signifi cant impact on the PV cell and module performance especially under low light intensity. A low shunt resistance can lead to decreased power generation during sunrise and sunset hours. A low R sh value may also lead to PV cell irreversible damage due to a hot spot when that cell is shaded and exposed to reversed voltage. Low - quality feedstock silicon, espe-cially metallurgical - grade materials, provides substantial risk and can lead to PV cells with low R sh values.

5.4 NOVEL CURRENT COLLECTING TECHNOLOGIES

5.4.1 Electroless and Photoinduced Metal Plating

Electroless plating was discovered more than 50 years ago and allows for deposit-ing metals on substrates without the application of an electric potential but requires that the surface be autocatalytic for metal plating [3, 4] . Later, it was further dem-onstrated that it is possible to establish low ohmic contact between electroless plated nickel with n - and p - doped silicon after an initial seed layer of nickel is sintered at about 450 ° C [5] . Fortunately, the contact resistance between electroless plated nickel and the solar cell emitter is more than 10 times lower when compared with conventional screen printer fi ngers and with the same type of emitter, thus offering the potential for PV cell effi ciency improvement. The combination of electroless nickel plating with sequential electrochemical plating allows for

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NOVEL CURRENT COLLECTING TECHNOLOGIES 121

the deposition of additional layers of nickel and the creation of sandwich - type structures consisting of nickel + copper + silver, thereby increasing the metal conductivity.

Another technology for increasing the thickness of the initially deposited thin and narrow metallic patterns is known as light - induced plating. It has been dem-onstrated that when an n - type emitter of a PV cell is positioned on top of an electrolyte and the PV cell ’ s back side is kept out of contact with the electrolyte but connected to a negative potential, then under illumination, the PV cell generates an electric potential that is suffi cient to attract metal cations from the electrolyte solution and deposits them onto the n - type emitter [6, 7] . It is obvious that the metal deposition will happen exclusively on the emitter areas that are not masked by some protective insulating material. This approach allows for the creation of fi ne metallization patterns on a PV cell ’ s front - and rear - side surfaces. After the initial publications, the light - induced silver plating technology has been further improved and has allowed for higher - effi ciency PV cells [8] . It has a high potential for use in the PV industry as well.

5.4.2 Ink - Jet Printing

Ink - jet printing is a noncontact deposition technology with high line resolutions of up to 20 μ m. It has proved to be more economical and less complicated when compared with the conventional photolithography approach [9 – 11] . This technol-ogy may be extremely useful for masking and selective etching as an alternative to the quite expensive lithography technology commonly used in microelectronics. Unfortunately, this technology has not yet been able to compete with screen print-ing metallization due to strict requirements for the ink to have low viscosity in order to prevent the printing head from clogging. Unfortunately, low viscosity is achieved only when the metal particle concentration is too low to secure suffi cient conductivity in the printed metal patterns.

5.4.3 Aerosol Printing

A metal aerosol noncontact printing technology was originally developed by Optomec Inc . (Albuquerque, NM). In contrast to ink - jet printing, metal - containing ink initially is atomized pneumatically or ultrasonically. Afterward, aerosol is transported to the depositing head where continuous aerosol fl ow may be inter-rupted by a mechanical shutter. The metal aerosol printer operates via a graphical user interface that allows control of the main technological and operating param-eters. AutoCAD drawings may be translated into mechanical codes, thus allowing the printing of any metal grid design. After the metal ink is printed onto the PV cell surface, it is dried and heated at 300 – 400 ° C for 5 – 10 min. In case it is needed, the cross section of the aerosol printed fi ngers may be further increased by elec-trolytic or light - induced metal plating, thus allowing thicker current collecting metal pattern lines of less than 20 - μ m width. Extensive investigations at ISE

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Fraunhofer have demonstrated monocrystalline PV cells with 18.3% and mc PV cells with 16.1% effi ciencies [12] However, the long - term stability of PV cells with aerosol direct printed metal patterns against environmental degradation has not yet been suffi ciently investigated.

5.4.4 Laser - Fired Contact

An alternative technology for rear - side metallization is based on the earlier dis-covery that aluminum may be alloyed with the silicon semiconductor substrate after being deposited in the form of a thin fi lm and exposed to a temperature in the range between 577 and 660 ° C [13] . A thin layer of aluminum, nickel, and silver may be deposited in sequence thereafter on the aluminum surface and sintered. This multilayer coating is necessary to allow the soldering of the power collecting leads to the back side of the solar cell. This technology has been further improved at ISE Fraunhofer [14] by introducing an intermediate dielectric layer between the metal layer and the rear side of the PV cell, providing for rear - side passivation. This metallic layer is further exposed to spot radiation heating, causing a localized molten mixture of the metal layer, the dielectric layer, and the semiconductor layer. Upon solidifi cation, these regions provide an electric contact between the semi-conductor layer and the metal layer. Pulse laser sources are employed. This allows both suffi cient rear - side passivation and low - resistance current collection from the PV cell rear side. Although this promising technology has been licensed to several PV cell manufacturing companies, it has not yet been introduced into mass production.

5.5 EXAMPLES OF NOVEL PV CELLS

5.5.1 Laser Buried Grid PV Cell

We will start this section with the important results that have been achieved at The University of New South Wales, Australia, by implementing electroless nickel plating technology for developing a novel concept PV cell with “ laser buried grids ” [15] . As shown in Figure 5.2 , this technology is based on the formation of laser grooves on the front side of a PV cell with further deep emitter formation and further electroless plating of a seed nickel layer inside these grooves. In these cells, the contact resistance between the sintered nickel and the emitter is at least 10 times lower when compared with the contact resistance between the standard screen printed silver pattern and the emitter. Additional plating steps with nickel, copper, and silver layers allow the laser - formed grooves to be fi lled with a thick layer of metal, thus providing substantial reduction of the R s value. BP Solar suc-cessfully introduced this technology into mass production. Since the metal plated fi ngers are buried inside the laser - formed grooves, the metal content in the fi ngers is increased, improving the grid line height - to - width aspect ratio and simultane-

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EXAMPLES OF NOVEL PV CELLS 123

ously reducing of PV cell shading. A grid line shading as low as 4% is achieved. The buried contact technology allows substantially improving PV cell effi ciency when compared to conventional screen printed solar cells. Unfortunately, this technology failed to fi nd a widespread application in the PV industry probably due to its complexity and due to diffi culties associated with its application for thin mc silicon wafers.

5.5.2 PERC PV Cell

In 1989, the same group from University of New South Wales, Australia, devel-oped the so - called PERC with a conversion effi ciency of up to 23.2% under stan-dard terrestrial test conditions. These PERC cells were produced from high - purity silicon and have a very high Voc of 700 mV. This is achieved via very low recom-bination in the bulk and on the front and rear surfaces through effi cient passivation with silicon oxide layers. These cells also demonstrated a very high Isc value of 41 mA/cm 2 , achieved mainly due to effi cient light trapping via inverted pyramid texturing and optimized AR. The rear aluminum - silicon electrical contacts with the moderately doped silicon substrate were made through the openings in the rear - side silicon oxide passivating layer, thus preserving suffi cient passivation.

5.5.3 PERL PV Cell

On 1990, the same group further improved the PERC cell and demonstrated the novel PERL cell with an effi ciency of 24.7%. The rear - side contacts each of 10 - to 30 - micron width were aligned to the center of boron - diffused p+ areas of 30 - to 50 - micron width preserving the distance between these contacts at about 250 micron. Under standard terrestrial irradiation testing conditions, 4 - cm 2 PERL p - type monocrystalline silicon cells demonstrated Voc = 706 mV, Isc = 42.2 mA/cm 2 , FF = 82.8%, and effi ciency of 24.7% performance [16] . The main differences

Plated grid metal in laser groove

n+

p+p - silicon

Back Metal

Figure 5.2. Concept for laser grooved buried grid silicon cell.

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between the more advanced PERL cell and the PERC are (1) the introduction of novel locally highly boron - diffused areas underneath the rear - side aluminum con-tacts, thus decreasing contact resistance; (2) the introduction of a deep junction that more effectively insolates the high recombination area at the surface from the bulk region; and (3) the distance between the rear - side contacts was decreased from 2 mm in the case of the PERC cell to 0.25 mm, thus decreasing rear - side lateral resistance and improving FF value on the PERL cells. The PERL cell was dis-cussed in detail in Chapter 2 of the First Edition, and its picture was featured on the book ’ s cover fl y sheet.

5.5.4 HIT Cell

SANYO Electric Co., Ltd. started its participation in PV about 25 years ago intro-ducing fi rst commercial amorphous silicon PV cell and module production. Fifteen years later, SANYO introduced the novel HIT PV cell and arranged the mass production of high - effi ciency PV modules based on these HIT cells. In 2007, SANYO announced that it improved its own world record for industrially produced HIT cell effi ciency achieving 22%. The HIT cell of 100 - mm 2 size has a unique structure as shown in Figure 4.12 in the previous chapter. The HIT cell consists of n - type high - quality monocrystalline material surrounded by ultrathin amorphous silicon layers providing for high - effi ciency passivation of the HIT cell front and rear sides. Proprietary, well - protected know - how technologies have been intro-duced into mass production for effi cient texturing of the cell ’ s surfaces at the micron level with special surface cleaning and protection before the ultrathin amorphous silicon layer depositions, the conductive AR deposition, and the high - aspect - ratio silver pattern printing on the front and rear sides. Implementation of these technologies results in Voc values in the range from 718 to 722 mV, Isc values of 38.37 – 38.64mA/cm 2 , and FF of 78.8% and effi ciencies of η = 22%. It is expected that in 2010, SANYO will start mass production of 22% effi cient HIT cell and modules on production capacity over 600 MW. HIT cell performance provides an excellent illustration of the importance of PV cell surface passivation as the key factor for cell effi ciency improvement.

5.5.5 IBC PV Cell

On the edge of the millennium and within just 5 years, SunPower succeeded in designing and introducing into mass production both a novel substantially improved performance high - effi ciency IBC PV cell and also high - effi ciency PV modules [17] . The structure of this novel IBC cell was discussed and shown in the previous chapter (Section 4.7.1 ).

Here are the main milestones of SunPower success. On May 12, 2003, NREL verifi ed 20.4% conversion effi ciency for the 125 - mm semi - square, single - crystal A - 300 IBC PV cell that had been introduced into mass production. On June 7,

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2004, SunPower announced the debut of its fi rst mass - production line for PV modules with module conversion effi ciencies approaching about 17% utilizing back the IBC solar cells. On October 16, 2006, SunPower announced the introduc-tion into mass production of its new 22% effi cient Gen - 2 solar cells. On May 12, 2008, SunPower announced that it had produced a 5 - in. prototype Gen - 3 IBC PV cell with a world - record effi ciency of 23.4%.

By all means, this cell may be considered as a real masterpiece. It succeeded in incorporating in a practical way all of the valuable theoretical and technological approaches that had been discussed in the past. The main features of this cell include localization of current collecting contacts of both polarities exclusively on the rear side of the PV cell, thus minimizing front - side recombination due to elimination of front - side metallic current collecting contacts and optimizing light trapping by eliminating conventional PV cell shading and by introducing the most effi cient texturing. This PV cell is produced from high - purity monocrystalline n - type silicon material with a charge carrier lifetime > 1 ms that allows minority carriers to diffuse from the illuminated front surface through the entire wafer thick-ness to arrive at the junction and current collecting contacts of both polarities at the rear side. Front - side recombination has been additionally decreased by intro-ducing n+ - doped and SiO 2 passivating layers on the front side. The back side of the IBC PV cell is processed in a very special manner. Initially, “ interdigitated ” n+ and p+ parallel narrow nonoverlapping strips between the PV cell ’ s opposite edges are produced by sequential diffusion processes. An effi cient electrical insula-tion is built between these strips to guarantee high shunt resistance. The entire back - side surface of this PV cell is covered by a SiO 2 layer providing effi cient rear - side passivation. Contacting holes through this back - side SiO 2 layer are pro-duced in precision alignment with corresponding n+ and p+ strips. Metallic con-tacting narrow fi ngers are further printed in a precision - aligned manner along corresponding contacting holes, thus providing electric contact through the holes with the underlying n+ and p+ strips. Two terminal bus bars are screen printed on the opposite edges of the IBC PV cell ’ s rear side in such a way that one of them is connected to all n+ fi ngers and the other to the p+ fi ngers. These terminal bus bars are used for PV cell testing and interconnection in series by means of special tabbing during PV module production. These PV cells demonstrate high current ( > 40 mV/cm 2 ) not only due to effi cient light trapping and low shading but also due to better external quantum effi ciency. The blue response is improved due to a highly n+ - doped front diffusion and emitter localization on the back side, thus eliminating the front - side dead layer associated with a conventional PV cell struc-ture with emitters localized on the front side. A long - wavelength response in the near IR spectral region has been improved as a result of a perfect rear - side pas-sivation provided by the SiO 2 , which covers the entire rear - side area except for the contacting holes. Optimal front - and rear - side processing provides for an optical thickness for the PV cell about six times its actual thickness. Due to perfect front - and rear - side passivation, a Voc of about 700 mV has been achieved. An optimized screen printing process allows the FF to reach a value of > 80%. In contrast to conventional crystalline silicon cells that demonstrate a Voc value decline of

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2.2 mV/ ° C, the SunPower cell ’ s Voc dependence on temperature is relatively lower at about 1.87 mV/ ° C, thus decreasing its temperature dependence for power losses to 0.38%/ ° C compared to 0.5%/ ° C typical for conventional PV cells. This differ-entiation certainly will play an important role in securing annual kilowatt hour power generation.

Although SunPower asserts that the production cost of these cells is low, it is evident that the cost should be substantially higher when compared with con-ventional PV cell fabrication due to the more expensive silicon material and the technology complexity. Several PV companies have made attempts to produce PV cells employing back contact concepts, but none of them have succeeded so far in introducing their designs into mass production mainly due to production technol-ogy complexity and high cost.

The fact is that practically all crystalline silicon PV cell manufacturing com-panies are using screen printing technology for current collection, although it is evident that the limited conductivity of screen printed metallic patterns represents a real bottleneck for further PV cell effi ciency improvement and cost reduction. In reality, the PV cell is just a small electric generator. Imagine what would happen if some gas electric generating plant were renovated and then its production capac-ity is doubled, and it is then placed back in service while the corresponding trans-mission line is kept on the same level? Isn ’ t it evident that a large portion of its additional electric energy will dissipate into heat due to parasitic losses in the transmission line? The same happens when conventional screen printing technol-ogy fails to collect electric power without parasitic losses from more effi cient PV cells. Therefore, there is an urgent request from the PV industry for a more effi cient current collecting technology.

5.5.6 Day4 PV Cell

In 2003, Day4 Energy Inc. developed a novel concept for a PV cell with improved current collection hereinafter referred to as D4 technology or D4 electrode or D4 PV cell [18, 19] . This concept is based on a proprietary current collecting electrode (Fig. 5.3 ) composed of copper wires coated with low melting alloys that are spaced apart and imbedded into an optically transparent adhesive layer that in turn is fi rmly

AlloyWire

Adhesive

Film

Figure 5.3. D4 wire - tape electrode concept.

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adhered to the supporting transparent polymeric fi lm. Since the thickness of the adhesive layer is lower than the thickness of the wire it secures, then at least part of the wire protrudes above it.

One may see (Fig. 5.4 ) that the electrode wires are soldered to the copper bus bar of about 5 - to 10 - mm width and 50 - to 200 - μ m thickness. The bus bar is typi-cally positioned outside the PV cell perimeter, thus not occupying its front surface, thereby preventing shading. The D4 electrode is placed on top of the front side of the PV cell in such a way that its wires are oriented in a transverse direction in respect to the cell ’ s screen printed fi ngers, thus allowing each wire to be placed on top of each and every grid fi nger. The entire composite structure is exposed to the vacuum lamination process comprising sequential vacuuming, heating, and pressing. During this process, the air is pumped out, the alloy melts, and under pressure, the electrode wires become fi rmly soldered to the screen printed fi ngers. At the same time, the electrode ’ s adhesive layer softens and under pressure fi rmly fi xes the polymeric fi lm onto the PV cell front surface. The D4 electrode secures a very low ohmic contact between the electrode wires and the screen printed alu-minum on the rear side of a PV cell due to the alloy properties and strong mechani-cal compression during the vacuum lamination. Due to the large numbers of soldering points between the electrode wires and the screen printed fi ngers, this electrode secures a substantially lower risk of electrical layout failure when com-pared with the conventional tab spot soldering using the conventional screen printed bus bars.

It is known that resistive power losses are proportional to the square of the current fl ow distance. Figure 5.5 shows a typical screen printed cell with screen

Wires

Wires

PV cell

Fingers

Bus bar

Figure 5.4. The placement of the D4 wire - tape electrode on a PV cell.

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printed bus bars on the left in contrast to a D4 cell with a wire - tape electrode on the right.

Referring to Figure 5.5 (left), the typical length that the electric current must fl ow through the screen printed fi nger before reaching the screen printed bus bars is not less than 35 or 20 mm in the case of a 6 - in. 2 conventional PV cell that utilizes two or three screen printed bus bars . In contrast, in the case of the D4 PV cell shown in Figure 5.5 (right), with the same size PV cell with the same grid pattern, the D4 electrode allows replacing two or three screen printed bus bars with 40 D4 electrode wires, thus reducing the current fl ow distance along the screen printed grids to only 2 mm. One may see that these wires are soldered to the bus bar pro-vided for series interconnection with the following PV cell. Therefore, the resistive losses for the case of the conventional current collection through bus bar ribbons are at least 20 times higher when compared with the D4 current collecting technology.

Employment of the D4 technology provides several additional advantages. First, it allows eliminating conventional screen printed bus bars from the front side, thus increasing the PV cell ’ s R sh value and removing silver pads from the back side of PV cells, resulting in lowered recombination losses due to improved front - and rear - side passivation. Removal of silver/aluminum pads on the rear side allows eliminating one screen printing step and one drying step. It also decreases the silver paste consumption by 40% along with lowering production cost with no compro-mise in the cell effi ciency. In fact, the D4 technology improves the PV cell effi -ciency by > 0.1% absolute mainly due to higher Voc and FF values. In addition, it opens the possibility to introduce into mass production PV cells with substantially narrower screen printed fi ngers. It has been demonstrated that after industrial con-ventional multicrystalline (MC) PV cells are equipped with D4 electrodes, FF values of not less than 77% with screen printed fi ngers of 70 - μ m width and less than 6 - micron height provide an improved cell effi ciency up to 0.5% absolute due

70 4

Bus bar

Bus bar

Wires

Bus bar

Fingers

Figure 5.5. Comparison between the conventional and D4 PV cell.

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PV MODULE 129

to lower shading and better front and rear - side passivation. Recent results demon-strated that production costs of the PV cells with narrow fi ngers may be substan-tially reduced due to > 250% lower silver paste consumption relative to conventional PV cells with two bus bars.

It should be appreciated that D4 technology opens a wide range of possibili-ties to introduce into mass production novel low cost and more effi cient PV cells based on several promising technological concepts such as extremely narrow current collection metallic patterns produced either by screen printing or electroless plating, or aerosol, or ink - jet noncontacting direct printing, or laser - fi red contacts that have been already developed but failed to fi nd commercial application so far. There is realistic possibility to employ D4 technology for electric power collection and interconnection of the back contact PV cells, thus decreasing their production cost and simplifying PV module production.

5.6 PV MODULE

In contrast to the extensive progress in PV cell technology, the development of the production technology for PV modules with crystalline silicon PV cells has remained virtually unchanged for more than 30 years.

5.6.1 Conventional PV Module Production Technology

Since a PV cell is actually is a source of low - voltage ≈ 0.6 V with a DC electric current of about 34 mA/cm 2 in sunlight, there is a need to interconnect a large number of PV cells in series in order to achieve a necessary and useful value for the DC voltage that in turn may be converted into AC by means of an inverter. Neighbored PV cells typically are interconnected in series by means of tinned copper tabs that are spot soldered onto the front side of the bus bars of a fi rst PV cell and then onto the silver pads on the rear side of the next sequential PV cell. A certain number of PV cells interconnected in series are known as a PV string. Strings in their turn are interconnected in series by means of tinned copper bussing, thus producing a PV module layout.

PV modules with series - interconnected PV cells perform optimally only when all the series - interconnected PV cells are illuminated with an approximately similar light intensity. However, if even one PV cell within the PV module layout is shaded while all other cells are illuminated, the entire PV module is adversely affected, resulting in a substantial decrease in power output from the PV module. In addition to temporary power loss, the module may be permanently damaged as a result of cell shading because when a PV cell is shaded, it starts to act as a large resistor rather than as a power generator. In this situation, the other PV cells in the PV string expose the shaded cell to a reverse voltage that drives electric current through this large resistor. This process may result either in the breakdown of a shaded PV cell or in heating it to such a temperature that it can destroy even the

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entire PV module if a very high temperature persists. In order to eliminate the risk of a PV module damage in the event of shading, practically all PV modules employ BPD connected across each of the PV string and/or an entire module depending on specifi c PV module design and the quality of the input PV cells. The number of PV cells in a single PV string depends on the PV cell quality, namely, the ability to withstand back - voltage breakdown that each PV cell may be exposed to if one of them within the PV string is shaded. For example, for PV cells of good quality that can withstand back - voltage breakdown of 14 V and given that each PV cell generates a Vmax = 0.56 V , then the number of PV cells in one string should not exceed 24. Given that PV cells produced from metallurgi-cal silicon typically have lower quality and back - voltage breakdown for these cells is not higher than 7 V, using them in PV strings comprising more than 12 cells is not recommended.

Although employment of the BPD allows protecting the PV cells and PV strings against damage, it also causes substantial power losses of a PV module because the shading of just one PV cell results in an entire PV string switch - off. According to the industry estimate, almost 30% of kilowatt hour annual generation may be lost in fi eld condition due to different sources of shade. Therefore, there is a need to optimize a PV module lay out in order to secure not only suffi cient shading protection of PV cells but also to minimize annual kilowatt hour losses of PV systems as well.

Since PV modules are generally expected to operate outdoors for typically 25 years without degradation, their construction must withstand various weather and environmental conditions. The front side of a typical PV module construction involves the use of a transparent sheet of low - iron tempered glass. The PV cell strings are sandwiched between sheets of polymeric encapsulant material, such as ethylene vinyl acetate, or thermal plastic material, such as polyvinyl butyral. An array of PV cells is placed onto the polymeric encapsulant material in such a way that the front sides of the cells face the transparent glass sheet. The back side of the array is covered with an additional layer of encapsulant material and a back - sheet layer of weather protecting material, such as Tedlar ® by DuPont, or a glass sheet. The additional layer of encapsulant material and the back - sheet layer typi-cally have openings to provide for terminal electrical conductors to be passed from the PV cell strings through the back - side encapsulant layer and back sheet of weather protecting material to the outer surface for connecting with the electrical load through a junction box. For a PV module having an array of two PV strings, typically four conductors are arranged to pass through the openings so that they are all in proximity with each other so they can be terminated in a junction box mounted on the back - sheet layer. The glass, encapsulant layers, cells, and back - sheet layer are typically vacuum laminated at about 14 – 160 ° C to build a free - of - air bubbles, strong bound structure that protects the PV cells from moisture penetra-tion from the front - and back sides and also from the edges. The electrical intercon-nections of the PV strings and connections to BPDs are made in the junction box. The junction box is sealed on the back side of the PV module and is equipped with electric cables for neighbored PV module interconnection.

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PV MODULE 131

An aluminum frame extends around the perimeter of the PV module and protects it against damage, provides mechanical strength against wind and snow loads, and facilitates mounting of the module to a support. At the same time, it is possible to employ PV modules without the aluminum frame if external supporting structures are suffi cient to provide necessary mechanical strength.

The fabrication of PV modules as described above with conventional technol-ogy is quite complicated and expensive. Layout of the PV module before lamina-tion requires a separate step of “ bussing ” in which PV strings are electrically interconnected typically by means of tinned copper busses that increase the area that is occupied by the PV module, thus decreasing its conversion effi ciency.

Due to differences in thermal expansion coeffi cients between the copper and silicon and glass, there is a certain risk that the soldered spots between conventional tabs and front - side bus bars and rear - side silver pads may break causing PV module irreversible damage under inevitable changes of ambient temperature. There is an additional risk especially associated with utilization of thin PV cells. Exposing cells to spot soldering may result in PV cell breakage due to local heating and pressure.

Once PV cells are interconnected in series by means of conventional technol-ogy, the series resistance of the produced PV module eventually exceeds the sum of all PV cell resistances due to the additional impact of soldering points, tabs, and bussing resulting in PV module FF value decline and corresponding effi ciency losses when compared to the effi ciency of the PV cells utilized in the module ’ s fabrication. For example, when PV cells with an average FF value of ≈ 76% are used to produce a 48 - cell PV module, the module FF value typically will be lower than 73%. In other words, excessive series resistance associated with conventional PV module production technology is responsible for about 4% power loss when compared to the power that the input PV cells were capable of generating before being interconnected in the PV module.

These insuffi ciencies become even more pronounced when modern high - effi ciency PV cells are utilized for PV module production. This is a clear demon-stration of a growing confl ict of interest between PV cell and PV module producers. In fact, PV cell producers are motivated to build PV cells with increased power output combined with decreased production cost. The easiest way to achieve this goal is to make PV cells thinner, thus securing economy of silicon material and to increase PV cell size, thus increasing the production capacity of existing manufac-turing equipment with minimal additional investments. During the last 5 years, the size of PV cells has been increased from 100 × 100 cm 2 to 125 × 125 cm 2 and further to 156 × 156 cm 2 . Even a larger cell of 210 × 210 cm 2 area has been devel-oped and tested. PV module producers have been forced to buy these cells for an increased price based on dollar per Wp, but in return, they do not have suffi cient benefi t because thinner cells become more fragile and experience more pronounced bowing with corresponding higher - yield losses due to higher risk of breakage. To minimize this risk, there is a need for additional investment for more sophisticated production equipment for tab soldering and general handling. At the same time, conventional PV module fabrication technology does not allow increasing PV cell

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power output at PV module level if the module series resistance is not decreased accordingly. Another challenge that PV module producers have to deal with is the necessity to achieve certain voltage values that a number of interconnected PV modules must generate per occupied area in order to secure the inverter ’ s most effi cient operation. It is evident that although increased size of more effi cient PV cells results in higher power output at the PV cell level, it eventually has a negative effect on the PV module performance due to the decreased voltage value since the number of PV cells per occupied area is diminished. Yet another challenge that PV module producers have experienced is the necessity to redesign the PV module layout to allow for an increasing number of PV strings now composed of a lower number of PV cells as required for cells made with metallurgical - grade silicon. Another example of a growing confl ict of interest between PV cell and PV module producers is the employment of a PV cell with a selective emitter that demonstrates a higher effi ciency in the blue spectral region. This advantage allows PV cell manufacturers to sell their cells at a higher price. At the same time, PV module producers do not observe the same effi ciency gain on the PV module level not only because the glass and encapsulant materials substantially cut off the blue light but also due to the inevitable accumulation of dirt on the front side of PV modules. These general considerations refl ect an urgent market request for developing novel, simpler, fl exible, and cost - effi cient technologies that are capable of utilizing more effi cient PV cells in PV module production without loosing generated power.

5.6.2 Day4 ™ Technology for PV Module Production

One such promising technology has been recently developed at Day4 Energy Inc. [20, 21] . It was further demonstrated [22] that PV cells may be interconnected in series by means of electrically connecting the front - side D4 electrode of the fi rst PV cell with the back - side D4 electrode of the sequential PV cell via the tinned copper bus bar. Since the thickness of this bus bar may be ≤ 100 μ m, it securing extremely low series resistance of this interconnection. This approach allowed replacing conventional tabbing and stringing technology along with simplifying the PV module layout.

New technology provides an elegant solution to produce U - type PV strings without conventional bussing by just turning each of two sequential PV cells with front and back electrodes by 90 ° before placing it on top of the rear - side electrode of the previous PV cell. It further allows eliminating conventional string interconnection by means of bussing, thus not only simplifying and opti-mizing PV module layout but also decreasing its production cost due to material and production step reduction. At the same time, PV module effi ciency is increased due to its lower series resistance and economy of space occupied by conventional bussing. It is worth to notice that fl exibility of the Day4 technology allows designing and producing PV modules with an increased number of PV strings, thus improving protection against shading and preserving annual gener-ated kilowatt hour energy.

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CONCLUSION 133

D4 PV module technology and specialized production equipment were intro-duced into mass production in 2006 after being UL and TUV certifi ed. To date, about 40 MW of these PV modules have been installed on European, North American, and other markets and have received positive references from customers confi rming excellent and reliable performance of installation. According to neutral party test results, installations with these PV modules demonstrate higher values of annually generated kilowatt hour in comparison with neighboring installation employing PV modules from competitive producers due to suffi ciently higher shunt resistance of Day4 PV cells without bus bars, resulting in higher conversion effi -ciency at low light intensities.

Depending on the quality of the input PV cells, the average power output of a mass - produced D4 PV module containing 48 MC PV cells is not less than 175 W. The effi ciency of the best produced D4 module so far is 15%, while the average is typically > 14% relative to the < 13% average effi ciency of industrially produced 48 MC cell modules. This pronounced difference may be explained as a result of increased effi ciency of input D4 PV cells and the preservation of this effi ciency at the D4 PV module level. This conclusion is strongly supported by the experimental fact that the FF values of mass - produced D4 PV modules are very close to the FF value of the input PV cells. This is not the case with conventional PV module production technology. This observation has a clear explanation: D4 PV modules are characterized by about 30 – 40% less series resistance when compared to con-ventional PV modules.

5.7 CONCLUSION

In spite of the current fi nancial and economic crisis, the global requests for new, reliable sources of energy continue to be addressed as priority number one. Compared to other renewable sources, solar electric generation is the only one that is intrinsically universally distributed. This unique feature allows establishing PV generating capacities close to end users, thus minimizing capital investments for new electric energy transmission lines. At the same time, these advantages have zero value if not supported by lower dollar per kilowatt hour cost. During the twentieth century, there were at least two moments when interest toward PV gen-eration was extremely high and attracted substantial investments. Unfortunately, these investments ran almost dry when oil price went down. The current situation is the same. Unfortunately, the end user of electric energy does not care about the source but only about the price. The PV industry cannot continue being dependent on subsidies. The political risk associated with the government subsidies introduces a substantial amount of volatility into the PV industry demand equation. Government subsidy programs and, most importantly, political will to support them may be reduced substantially if the PV industry is unable to demonstrate its ability to reach grid - parity in a signifi cant number of applications over the course of the next few years. Therefore, the PV industry must be ready to compete with upgraded coal, nuclear, and other so - called new clean technologies where the experienced lobbyist

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has the capacity to attract practically all investment capital, thus leaving the PV industry for years ahead without suffi cient fi nancial resources.

With all respect to new types of PV cells and PV modules, we have to admit that just the crystalline silicon PV technology has suffi cient experience in > 30 years of numerous numbers of PV systems ’ reliable performance in different geographi-cal locations. That is why the 25 years warranty on the crystalline silicon PV modules is based on a real solid ground. Therefore, we believe that the crystalline silicon PV industry will continue to be attractive for the long - term investments and to be the core basis for industrial solar electric energy generation. The material presented above illustrates the high potential for cost reduction and effi ciency improvement practically at all steps of the crystalline silicon PV cell and module production that may be suffi cient to make PV electric energy cost competitive with conventional nonrenewable sources. The challenge is how to convert this potential into suffi cient cost reduction of kilowatt hour generated in - fi eld conditions.

It might sound strange but the current fi nancial crisis in fact has created a moment of truth that is forcing the PV industry either to change its business model or to die. During the last 8 months, there has been an amazing price reduction for the PV cells ( > 40%) and PV modules ( > 50%). Keeping in mind that last year the Si - based PV cell contributed about 70% of the PV module cost and now it is lowered to 55%, there is suffi cient potential for further PV module and kilowatt hour cost reduction. This trend must be combined with a modifi ed PV industry business model that must change its current focus from dollar per Wp toward dollar per kilowatt hour and ROI. The selling policy must also be changed from the current 25 - year guarantee for PV module Wp performance toward the 25 - year guarantee of a certain kilowatt hour annual generation. We believe that the success and survival of the PV industry depends on the implementation of this ambitious but feasible program.

ABBREVIATIONS

AC — alternating current AR — antirefl ective coating BPD — bypass diode BSF — back surface fi eld CAD — computer - assisted drawing CZ — Czochralski process D4 — Day4 Energy DC — direct current FF — fi ll factor Gen — generation HIT — heterojunction with intrinsic thin layer I — current IBC — interdigitated back contact Impp — current at maximum power point

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REFERENCES 135

IR — infrared Isc — short - circuit current ISE — Institute for Solar Energy max — maximum mc — multicrystalline n — negatively or donor - doped semiconductor NREL — National Renewable Energy Laboratory p — positively or acceptor - doped semiconductor PERC — passivated emitter and rear cell PERL — passivated emitter, rear locally diffused Pmmp — power at maximum power point PV — photovoltaic R — resistance R bl — bulk resistance R em — emitter resistance ROI — return on investment R s — series resistance R sh — shunt resistance SiC — silicon carbide SiN x — silicon nitride antirefl ective coating SiO 2 — silicon dioxide TUV — Technischer Ü berwachungs - Verein (Technical Inspection Association) UL — Underwriters Laboratories V — voltage Vmmp — voltage at maximum power point Voc — open - circuit voltage Wp — watt peak η — effi ciency 3D — three dimensional

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