white paper to mok- final-1

8/14/2019 White Paper to Mok- FINAL-1

1/26

The data in this report are controlled by the terms of the Non-Disclosure Agreement between MokIndustries & Spectrolab, and by the Terms and Conditions of Sale for Spectrolab Sales order # 5926.

1

Technical Report

Optimization of Multi-Junction Solar CellsFor Operation Inside

Ultra-High Concentration Photovoltaic (UHCPV) Module

October 19, 2004

Submitted to

Mok IndustriesAs the final deliverable under Spectrolab Sales Order # 5926

Prepared By

Raed A. Sherif, Ph.D.Manager, Terrestrial Photovoltaic Products

Phone (818) [email protected]

Hector L. Cotal, Ph.D.Photovoltaic Research Scientist

Richard R. King, Ph.D.Manager, Solar Cell Research & Development


2/26


2

TABLE OF CONTENTS

1.0 Background Page 3

2.0 Performance Optimization of Triple-Junction Solar cells Pages 4-14

2.1 Gridline Optimization2.2 Tunnel Junctions

2.3 Optimum Solar Cell Total Area

2.4 Thermal Effects on the Maximum Power

2.5 Wafer Power Output2.6 Heat Reduction from Attenuation of Incident Solar Spectrum

3.0 Performance Optimization of Dual-Junction Solar Cells Pages 15-173.1 Thermal Effects on the Maximum Power

3.2 Wafer Power Output

4.0 Product & Process Improvements Pages 18-23

4.1 Tunnel Junction Improvements

4.2 Pursuit of Higher Cell Efficiencies

4.2.1 Metamorphic Solar Cells4.2.2 Solar Cells with 4, 5, and 6 Junctions

4.3 Scribe & Break Process Development

4.4 Reducing the Ge Wafer Cost

5.0 Concentrator Cells Cost Projection Pages 24-25

6.0 Summary & Conclusions Page 26


3/26


3

1.0 Background

High concentration photovoltaic (HCPV) modules using high-efficiency multi- junction (MJ) cells have been sought because they have the potential ofgenerating electricity at competitive prices. The key to generating cheap

electricity from HCPV modules is to increase the module output per a givencollector area without increasing the area of the (more expensive)semiconductor. Hence, the cell efficiency is a critical parameter in reducing thecost of the overall module ($/Watt). Equally important is how much concentratedsunlight can the solar cell take without causing degradation to its performance.

As the concentration level on the cell increases, heat dissipation becomes moredemanding. The tunnel junction inside the cell will be more likely to fail as thecurrent density increases beyond a certain level. Further, series losses at thehigher current levels may cause the cell to operate below its intended peakefficiency.

Mok Industries seeks to develop Ultra-High Concentration Photovoltaic (UHCPV)modules. This study is funded by Mok Industries and is intended to addressissues associated with operation of MJ cells under concentration levels between1000 to 5000 suns. Specific issues related to this are: solar cell performanceoptimization, heat dissipation, and economics.

In this study, we consider performance optimization of triple-junction solar cells.We also consider optimization of dual-junction cells. In both cases we considerthe impact of filtering part of the infrared (IR) radiation so that less thermalenergy is absorbed in the solar cell (reducing the amount of heat that will need to

be dissipated).


4/26


4

2.0 Performance Optimization of Triple-Junction Solar Cells

Performance optimization for the solar cells under any concentration levelrequires optimization of the cell front grid lines (so that there is enough metal tocarry the current without incurring too much I2R losses, but not too much metal

so as to keep the obscuration losses to a minimum too). Under ultra-highconcentration, e.g., 1000-5000 suns, the tunnel junctions inside the solar cellsalso need to be optimized to ensure that proper tunnel junctions with high peaktunneling currents are used to support the high current densities generated withinthe electrically active junctions. Under concentration levels of 1000-5000 suns, itis expected that temperature effects will likely dictate how large the solar cell canbe, for it can be practically impossible to cool down the cells if the cell area is inthe order of 1cm x 1cm.

In this study, we will focus our attention on performance optimization of todaysGaInP/GaInAs/Ge triple-junction cells whose cross-section is shown in Fig. 1. We

will consider cell sizes that are under 0.3cm x 0.3cm. This is guided by theresults of the thermal analysis and by some power calculations that will bediscussed at later sections.

Fig. 1: Cross-Section of a triple-junction solar cell

2.1 Gridline Optimization

This entails computing a number of loss components that affect the power outputof the cell such as the amount of metal deposited on the front surface, itsobscuration (or shadowing), sheet resistance across the plane of the front face ofthe semiconductor, I2R heating through gridlines and bus bar(s), contact

Tunn

elJu

nctio

n

TopC

ell

Wide

-EgT

unnel

Middle

Cell

p-GaInP BSF

p-GaInP base

n-Ga(In)As emitter

n+-Ge emitter

p-AlGaInP BSF

n-GaInP emitter

n-AlInP window

n+-Ga(In)As

contact

AR

p-Ge baseand substrate

contact

n-Ga(In)As buffer

Bottom

Cell

p++-TJ

n++-TJ

p-Ga(In)As base

nucleation

Wide-bandgap tunnel junction

GaInP top cell

Ge bottom cell

n-GaInP window

p++-TJ

n++-TJ

Ga(In)As middle cell

Tunnel junction

Buffer region

Tunn

elJu

nctio

n

TopC

ell

Wide

-EgT

unnel

Middle

Cell

p-GaInP BSF

p-GaInP base

n-Ga(In)As emitter

n+-Ge emitter

p-AlGaInP BSF

n-GaInP emitter

n-AlInP window

n+-Ga(In)As

contact

AR

p-Ge baseand substrate

contact

n-Ga(In)As buffer

Bottom

Cell

p++-TJ

n++-TJ

p-Ga(In)As base

nucleation

Wide-bandgap tunnel junction

GaInP top cell

Ge bottom cell

n-GaInP window

p++-TJ

n++-TJ

Ga(In)As middle cell

Tunnel junction

Buffer region


5/26


5

resistance between the front metal and the semiconductor and tunnel junctionlosses. Figure 2 shows a graph of the relative power loss of triple junctionconcentrator solar cell for 1000, 3000 and 5000 suns for a 0.1cm x 0.1cm cell .

The plot for 1000 suns shows that the grid pitch (or separation) should be 157.5m apart for the solar cell to attain a combined minimum loss in power. For thiscase, the total power loss is 15.34% and can be considered as the percentageloss relative to a solar cell with ideal 0% loss. At 3000 suns, the power lossincreases to 24.69% with a 96.5 m-pitch, and at 5000 suns, the loss rises to

30.65% with a 76.2 m-pitch.

Bus BarIncluded in the calculations was a bus bar width that was selected as 60 m andremain fixed throughout the calculations. The length was allowed to vary but thewidth was selected so that it would be narrow enough to minimize obscurationand large enough to allow wire bonding. Some wire bonding characteristics willbe described below.

Gridline WidthThe modeling was performed with the narrowest gridline width possible as

allowed by Spectrolabs current processing capabilities. Note that narrow gridlinewidth reduces obscuration of which is a significant loss mechanism for the smallcells modeled in this study. To achieve further reduction in gridline width, thephotolithography process would have to be modified to account for narrowgridline width fabrication.

Fig. 2: Triple-Junction solar cell grid line optimization for a0.1cm x 0.1cm cell

0

20

40

60

80

100

0 50 100 150 200 250 300

Gridline Spacing (m)

FractionalPowerLoss

(%)

5000 Suns 3000 Suns 1000 Suns

Total Cell Area = 0.1 cm x 0.1 cm


6/26


6

Sheet ResistanceAn important parameter that contributes to power loss at high concentrationlevels is the sheet resistance. The plots in Fig. 2 are based on a sheetresistance of 500 /square. Figure 3 shows how smaller values of the sheetresistance can affect the power loss. The difference in power loss between 500

/square and 100 /square is 3.5%. For 3000 and 1000 suns the differencebecomes 2.9% and 1.8%, respectively.

Contact Resistance

Values ofc for the plots in Figs 2 and 3 were based on a value of 2x10

-4

cm

2

.Several values that ranged from 2x10-4 cm2 to 5x10-6 cm2 were included inthe models for the 1000, 3000 and 5000 suns for comparison. Significantchanges in the power loss were observed for values ofc below 2x10

-4 cm2,and were more pronounced for the modeled case of 5000 suns, which is shownin Fig. 4. Values ofc for 1000, 3000 and 5000 suns are illustrated in Table 1.The end result is that c needs be reduced by at least an order of magnitude toachieve higher solar cell performance.

Table 1: Power loss as a function of contact resistance for 1000,3000, and 5000 suns concentration

Concentration % Power LossContact Resistance Values ( cm2)

2x10-4 1x10-5 5x10-6

1000 15.34 7.90 5.503000 24.69 12.68 8.675000 30.65 15.83 10.77

Fig. 3: The effect of sheet resistance on the fractional powerloss of a triple-junction solar cell.

0

20

40

60

80

100

0 50 100 150 200 250 300


Frac

tionalPowerLoss

(%)

Sheet rho = 500 Ohms/sq




5000 SUNS MODEL


7/26


7

Wire Bond Pad ResistanceThe modeling took into account the pad resistance of the gold wire bond fortypical FP2 25 m diameter wire as described on Kulicke & Soffa (KnS)Industries web site. The wire bond pad resistance is < 1 . There are manytypes of wires of different material. Gold is selected in this study for compatibilitysince the topmost front metal layer of the solar cell bus bar is normally comprisedof gold. The wire electrical resistivity as referenced in KnS site is 3.24 -cm.Spectrolab has demonstrated that wire bonding is a viable method to use for

concentrator solar cells in the past.

2.2 Tunnel Junctions

The solar cell structure is grown in the Metal Organic Vapor Phase Epitaxy(MOVPE) reactors. The epitaxial layers grown in the MOVPE reactors mustbalance the peak current output from each electrically active junction (since eachsubcell of the triple-junction cell are in series). Each pair of junctions is bridgedby a tunnel-junction allowing current to flow with ease between the electricallyactive junctions.

The primary characteristic of a tunnel junction is its peak tunneling current. Thisis the maximum current density for which a tunnel junction operates withoutseverely hindering cell performance. Current flow between each pair ofelectrically active junctions is limited by the peak current density (Jp) of the tunnel

junction. Figure 5 illustrates a graph approximating the tunnel junction

0

10

20

30

40

50

0 50 100 150 200 250 300


FractionalPowerLo

ss

(%)

Cont Res = 0.0002 Ohms cm2




5000 SUNS MODEL

Fig. 4: Noticeable change in the total power loss withincreasing contact resistance at 5000 suns.


8/26


8

characteristics in the present tunnel junction used in the triple-junction structure.The Jp from this particular tunnel junction is approximately 75 A at 0.16 V.

For a concentrator cell operating at 5000 suns, the current density at maximumpower (Jmp) for this cell design is 69 A/cm

2. In Fig. 5, the voltage drop thatcorresponds to this Jmp is about 0.10 V, and would be manifested in the form ofpower loss in the cell performance due to this component. Although the voltagedrop is somewhat low, its Jp is not high enough as Jp should be 2- but preferably3-times higher than J

mpof 69 A/cm2. Since variations in the growth of tunnel

junctions can occur, these factors give a margin of safety to compensate for suchvariations. Since Jp is low, this particular tunnel junction would not be suitable fora concentrator cell at 5000 suns. (This was confirmed experimentally by studiesdone with concentrated light coming out of a fiber shining light on smaller areasof the cell. Occasional tunnel junction failures were reported at concentrationlevels slightly above 2200 suns.)

It is clear that we need to define a tunnel junction that has peak tunneling currentof at least 140 A/cm2 (but preferably higher than 200 A/cm2). The peak tunnelingcurrent is dependent on the band gap (Eg) of the semiconductor: the lower theEg, the higher the Jp. The choice of Eg, however, is one where Eg should be lowenough to allow for high Jp (and low voltage drops) but high enough to reducelight absorption in the tunnel junction.

The plot in Fig. 5 is representative of a tunnel junction with Eg of 1.9 to 2.0 eV.Some tunnel junctions that fall in the category of high Jp and low Eg could be theAlGaAs/InGaAlP, AlGaAs/GaInP and AlGaAs/GaAs ternary and quarternary

Fig. 5: Tunnel junction characteristics in a triple- unction cell(plot is for Eg = 1.9-2.0 eV)

0

20

40

60

80

100

0 0.5 1 1.5 2

V (V)

J(A/cm

2)

Peak Current Jp

0.10 V;

69 A/cm2


9/26


9

systems. By proper adjustment of the Aluminum, Gallium and Indiumcompositions, the desired tunnel junctions with lower Eg can be developed with Jpabove 200 A/cm2. This is clearly a development effort that must be undertakenby Spectrolab in any future work with Mok Industries to make multi-junction cellsoperate reliably under concentrations of 3000-5000 suns.

2.3 Optimum Solar Cell Total Area

It is important to keep in mind that when it comes to heat removal from a device,there are two parameters to consider: (a) the power density expressed as W/cm2,and (b) the magnitude of the power itself in Watts.

Concentration levels of 1000-5000 suns correspond to approximately 70-350Watts/cm2 (assuming 30% cell conversion efficiency). These are extremely highnumbers in terms of power density that will be very difficult to remove while

maintaining a reasonable cell operating temperature (preferably below 100 deg.C) with passive cooling. Fortunately, however, by keeping the cell size verysmall, the absolute value of power is still manageable. Figure 6 shows a plot ofthe projected cell operating temperature rise above ambient vs. levels ofconcentration for different cell sizes (assuming 1 sun = 1000 W/m2 and that thecell conversion efficiency is 35%).

Fig. 6: Finite element projections of cell temperature rise above ambient for different cellsizes under different concentration

0

20

40

60

80

100120

140

160

0 2000 4000 6000

Concentration

CellTemperatureaboveambient

(deg.

C) 1mmx1mm cell

2mmx2mm cell

3mmx3mm cell


10/26


10

The data in this curve are based on a 3-dimensional finite element model thatassumes the cell to be soldered directly to a heat spreader made of copper.Direct soldering of the cells to Cu is usually not practiced for larger cells (e.g.,1cm x 1cm) since the coefficient of thermal expansion (CTE) of Cu is muchhigher than that of the MJ cell. Under repeated temperature cycling, large

expansion mismatch can lead to breakage of the cell and/or the solder jointbetween the cell and the heat spreader (leading to loss of cooling and thermalrunaway). For smaller cell sizes, however, direct boding can take place withoutincurring too much thermal stresses.

The heat spreader has dimensions of 5cm x 5cm and its thickness is 0.2cm. Heatremoval from the back of the heat spreader occurs by natural convection (i.e.,passive cooling) with the heat transfer coefficient assumed to be 100 W/m2-degC, which is typical of natural heat convection. From the curves in Fig. 6, it is clearwhy the cell size should be limited to under 2mm x 2mm under concentrationabove 3000 suns.

Another factor to consider in the selection of cell area is the current levelproduced by the cell. Smaller cells will have smaller current generated and,hence, smaller I2R losses). Figure 7 shows the power loss vs. the total solar cellarea for 1000, 3000 and 5000 suns.

Fig. 7: Power loss vs. total cell area at differentconcentration

10

15

20

25

30

35

40

0 0.01 0.02 0.03 0.04 0.05 0.06

Total Cell Area (cm2)

%P

ower

Loss

1000 Suns 3000 Suns 5000 suns

Minimum Power Loss


11/26


11


Higher operating cell temperature reduces the cell conversion efficiency, whichoccurs mostly because of reduction in cell operating voltage. The dependence ofVmp on temperature plays a crucial role on the performance of concentrator solar

cells. If heat is not dissipated adequately, Vmp will drop and the power output ofthe cell will also drop. Table 2 shows the expected drop in cell voltage andpower as temperature goes up. The data is based on experiments conducted atSpectrolab for concentrator cells 0.55cm x 0.55cm with the maximum current(Imp) scaled to the 0.1cm x 0.1cm and 0.2cm x 0.2cm cells.

Table 2: The effect of temperature on Vmp and power at 1000 suns. Imp =0.516A for 0.2cm x 0.2cm and Imp = 0.121A for 0.1cm x 0.1cm

Parameter Operating Temperature (oC)

25 45 65 85

Vmp (V) 2.68 2.59 2.50 2.41Pmp (W)

Cell A =0.1cm x 0.1cm 0.32 0.31 0.30 0.29

Pmp (W)Cell A =0.2cm x 0.2cm

1.38 1.34 1.29 1.24

Please note that the temperature coefficient of Imp has been ignored for thecalculations in Table 2, as it is much smaller than the impact of temperature onvoltage.


Previous discussions focused on individual cells. The smaller the cells, the easierthe heat dissipation is and the smaller the I2R losses are. In smaller cells,however, the ratio of the bus bar to the active cell area is larger; hence, on awafer level, smaller cells will correspond to larger loss of wafer active area (andhence lower power). Since the ultimate objective of any concentrating PV systemis to reduce the cost of electricity generation ($/Watt), the wafer power outputmust be taken into account when deciding on the optimum cell area.

A 10cm diameter Ge wafer is used for growing epitaxial layers to form theterrestrial concentrator structure. If we eliminate the unusable wafer area (about

3mm from the outer edge of the wafer), then we can obtain:- 0.1cm x 0.1cm cells total = 6,219 cells per wafer- 0.2cm x 0.2cm cells total = 1,613 cells per wafer


12/26


12

At 5000 suns, assuming 25 C cell operation (which is impractical to achieve inactual module operation, but represents standard test conditions), the followingpower is achieved from each cell and wafer.

Standard Test Condition

Power of 0.1cm x 0.1cm cell = (Imp = 0.52A) x (Vmp = 2.68V) = 1.4 Watts/cellPower of a whole wafer = 1.4 (W/cell) x 6,219 (cells/wafer) = 8.67 kW/wafer


This clearly favors the use of the larger cell (the 0.2cm x 0.2cm) over the smallercell (the 0.1cm x 0.1cm). In actuality, however, the smaller cell will operate atlower temperature, as suggested by Fig. 6. Including the temperature effects(and assuming 25 deg. C ambient), we obtain the following power.

Real Temperature Test ConditionPower of 0.1cm x 0.1cm cell = (Imp = 0.52A) x (Vmp = 2.59V) = 1.35 Watts/cellPower of a whole wafer = 1.35 (W/cell) x 6,219 (cells/wafer) = 8.38 kW/wafer


In other words, the fact that the smaller cell will operate at lower temperaturemade the use of either cell almost equivalent in terms of total wafer power.

2.6 Heat Reduction from Attenuation of Incident Solar Spectrum

It was found from the above analysis that if the temperature were the same, a0.2cm x 0.2cm cell will be more favorable to 0.1cm x 0.1cm cells (resulting inhigher power per wafer). In this section, we investigate ways of reducing theamount of thermal radiation by cutting off the unusable (and some of the usableportion) of incident infrared (IR) in the terrestrial spectrum. Figure 8 shows theAM1.5G terrestrial spectrum along with the spectral response of each subcell inthe triple junction solar cell.

It can be seen that the GaInP/GaInAs/Ge triple-junction solar cell uses most ofthe spectrum from 350 to 1900 nm. Since there is no absorption, and thereforeno contribution to current generation above 1900 nm, this portion of the IR canbe cut off or filtered with no effect on solar cell performance. Calculation of theirradiance of this portion of the unusable spectrum at 1-sun is 0.0039 W/cm2. Inother words, by cutting off the wavelength at 1900nm, which targets only theunusable IR in the solar spectrum, we will reduce the heat generation in the solarcell by about 4%.


13/26


13

We now go back to the 3-dimensional finite element model and calculate whatthat means to the cell temperature (assuming everything else being the same).The data is shown in Table 3, which is expressed as the drop in temperature(relative to the case with no IR filtering).

Table 3: Reduction in cell operating temperature due to cuttingoff wavelength above 1900 nm

Drop in Temperature (deg. C)

Cell Size 1000 Suns 3000 Suns 5000 Suns

0.1cm x 0.1 cm 0.2 0.6 1.20.2cm x 0.2 cm 0.6 1.7 3.50.3cm x 0.3cm 1.01 2.9 5.8

Further efforts can be made to cut off some of the usable portion of the IR below

1900 nm. In fact, this can be done without degrading the performance of thetriple-junction cell because the Ge subcell in the current triple-junction cell stackproduces excess current above what the top and middle subcells are producing.Accordingly, the Ge subcell spectral response could be reduced by about 25%with minimal impact to cell performance. This corresponds to cutoff of

Fig. 8: Spectral response of top, middle and bottom subcellstogether with the standard AM1.5G terrestrial spectrum asproposed by the National Renewable Energy Laboratory (NREL.)Marker shown is at 1900 nm.

0

0.2

0.4

0.6

0.8

1

300 500 700 900 1100 1300 1500 1700 1900 2100 2300 2500

Wavelength (nm)

NormP

arameters

Top Cell

Middle Cell

B ottom Cell

Normalized Low AOD

AM1.5D Spectrum

Cut Off


14/26


14

wavelengths above 1310 nm. By doing so, we can reduce the amount of heatgeneration in the solar cell by about 12%.

We now go back to the 3-dimensional finite element model and calculate thereduction in cell operating temperature. The results are summarized in Table 4.

In this table, we assume that the cell conversion efficiency remains unchanged at35%.

Table 4: Reduction in cell operating temperature due to cuttingoff wavelength above 1310 nm

Drop in Temperature (deg. C)

Cell Size 1000 Suns 3000 Suns 5000 Suns

0.1cm x 0.1 cm 0.6 1.7 3.50.2cm x 0.2 cm 1.8 5.2 10.50.3cm x 0.3cm 3.0 8.6 17.4

Any further reduction in IR will impact the cell efficiency. The concern with this isthat it could lead to starvation of IR light in the Ge subcell, and would not producethe current needed for optimum triple-junction solar cell operation.


15/26


16/26


16

Fig. 10: Projected dual-junction cell temperature without- and with filtering of thespectrum


The thermal effects for dual-junction cells are equivalent to that of triple-junctioncells. Dual-junction cells Vmp is approximately 0.3V less than that of triple-

junction cells. Table 5 is a repeat of Table 2 with the assumption that all voltagesdecrease by 0.3 V and are independent of the concentration level.

Table 5: The effect of temperature on Vmp and power at 1000 suns. Imp =0.516A for 0.2cm x 0.2cm and Imp = 0.121A for 0.1cm x 0.1 cm

Parameter Operating Temperature (oC)

25 45 65 85Vmp (V) 2.38 2.29 2.20 2.11Pmp (W)

0.1cm x 0.1 cmDJ cell0.29 0.28 0.27 0.26

Pmp (W)0.2cm x 0.2cmDJ cell 1.23 1.18 1.14 1.09

0

20

40

60

80

100

120

140

160

180

0 2000 4000 6000Concentration

CellTemperatureabovea

mbient

(deg.

C)

0.1cmx0.1cm cell, nofiltering



0.1cmx0.1cm cell,with filtering




17/26


17


We follow a similar exercise to the one we did for the triple-junction cell at 5000suns.

Standard Test ConditionPower of 0.1cm x 0.1cm cell = (Imp = 0.52A) x (Vmp = 2.38V) = 1.24 Watts/cellPower of a whole wafer = 1.24 (W/cell) x 6,219 (cells/wafer) = 7.70 kW/wafer


This clearly favors the use of the larger cell (the 0.2cm x 0.2cm) over the smallercell (the 0.1cm x 0.1cm).

Real Temperature Test Condition



This suggests that the smaller cell is better due to the higher temperatureoperation of the larger cell.

If, however, we filter the wavelength to bring the cell temperature down, weobtain:


In other words, the larger cells become slightly more favorable or just aboutequal to the smaller cell.


18/26


18

4.0 Product & Process Improvements

The previous sections focused on performance optimization of existing celltechnologies, taking into account the impact of filtering of the unused portions ofthe solar spectrum to lower the cell operating temperature. In this section, we

discuss the development efforts that need to be undertaken in order to ensure areliable operation of MJ cells in ultra-high concentration modules and to achievelower $/Watt.

4.1 Tunnel Junction Improvements

It was already mentioned in the previous section that existing tunnel junctionswere not designed to handle the current densities associated with ultra-highconcentration regimes. Rather, they were designed to handle concentrationunder 1000 suns, although they may be able to perform at concentration levelsup to 2000 suns.

The pursuit of tunnel junctions to operate in ultra-high concentration regimes isone development effort that needs to be pursued. This will include study of wide-bandgap tunnel junctions, such as AlGa(In)As/ GaInP, AlGa(In)As/ AlGaInP,AlGa(In)As/ AlGa(In)As, AlGa(In)As/ Ga(In)As, Ga(In)As/ Ga(In)As, etc., todetermine improved doping methods in these materials, achieve the highestpractical peak tunneling currents, and the lowest voltage drop across the tunnel

junction at the incident light concentration of interest, while maximizingtransparency of the tunnel junction through the use of wide-bandgap materialsand reducing layer thickness.

4.2 Pursuit of Higher Cell Efficiencies

In the calculation of cell output power, we assumed a triple-junction cell efficiencyof 35% and a dual-junction cell efficiency of 31%. Higher cell efficiencies of 40-45% are possible and they can be pursued if sufficient funding is available.

Spectrolab proposes to accelerate its research into high-efficiency multi-junctionsolar cells for use in terrestrial concentration systems, to push the cell efficiencyup to a target efficiency of 45% under the terrestrial solar spectrum atconcentration. As high as present-day triple-junction cell efficiencies are, thetremendously leveraging effects of high efficiency to reduce the concentrating

optics and balance-of-system costs make it highly desirable to push theefficiency as close to theoretical limits as possible. There are 2-approaches thatwe plan to pursue to reach a 45% efficiency goal: (i) metamorphic solar cellstructures, and (ii) solar cells with 4, 5, or 6 junctions.


19/26


19

4.2.1 Metamorphic Solar Cells

Lattice-matched (LM) Ga0.5In0.5P/GaInAs/Ge 3-junction (3J) solar cells are thehighest-efficiency photovoltaic cells yet demonstrated for terrestrialconcentrators, as well as for space power systems. However, the GaInP/GaInAs

bandgap combination is still far from optimum for the AM1.5D terrestrialspectrum, and even higher efficiencies are possible by increasing the indiumcomposition of theGaInP and GaInAs subcells. This lowers the bandgap of eachmaterial, thereby tuning the resulting spectral responses of the GaInP andGaInAs subcells for more efficient conversion of the solar spectrum. The higherindium composition results in a larger lattice constant than the Ge substrate, sothat the GaInP and GaInAs subcells are lattice-mismatched to the Ge substrate.The lattice mismatch can produce threading dislocations in the crystal lattice, butthe dislocations can be largely accommodated in a graded buffer region, so thatthe active cell regions have relatively low dislocation density. Such devices, inwhich a new lattice constant is established at which device layers can be grown

relaxed and with a minimum of dislocations, are termed metamorphic (MM)devices.

The ideal efficiency of 3J MM solar cells is shown as a function of the middle cellbandgap in Fig. 11, for the AM1.5 Direct, low-AOD standard spectrum forterrestrial concentrator cells adopted by the National Renewable EnergyLaboratory (NREL), at 1 sun and at 500 suns. The theoretical efficiency for these3J cells limited by radiative recombination is well over 50% at 500 suns, so thateven with grid shadowing and resistive losses, efficiencies above 45% can beachieved.

Fig. 11: Efficiency of a Metamorphic triple junction cell as a function of the middle cellbandgap for space and terrestrial spectrum

0

10

20

30

40

50

60

70

80

90

100

110

0.6 0.8 1 1.2 1.4 1.6 1.8 2

Photon Energy Corresponding to Middle Cell Bandgap (eV)

IntegratedCurrentDensityinSpectrum

(mA/cm

2)

0

5

10

15

20

25

30

35

40

45

50

55

IdealEfficiency(%)and10XVoc(V)of

3JCellLimitedbyRadiativeRecomb.

AM0

AM1.5 Direct, low-AOD

Ideal Eff., AM1.5D low-AOD, 1 sun

Ideal Eff., AM1.5D low-AOD, 500 suns

Voc X 10, AM1.5D low-AOD, 1 sun

Voc X 10, AM1.5D low-AOD, 500 suns

1.305 1.414 eV


20/26


21/26


21

The bandgaps of the component subcells in a 6-junction cell along with plots ofthe available current density in the AM0 (space), AM1.5G (terrestrial one-sun),and AM1.5 Direct, low-AOD (terrestrial concentrator) solar spectra as a functionof wavelength are illustrated in Fig. 13. The area under the spectrum curve in aspecific wavelength range gives the maximum current density of a subcell for that

span of wavelengths. The areas delineated by the subcell bandgaps arecomparable in size, allowing series-interconnected subcells to be currentmatched.

Theoretical efficiencies for this type of cell at 500 suns exceed 50%, and practicalsolar cells are expected to be able to reach over 45% for terrestrialconcentrators.

Fig. 13: Division of standard solar spectra by the bandgaps of the 6 subcells in a 6-junction cell.

4.3 Scribe & Break Process Development

The ultra-high concentration modules require that the solar cell size be kept assmall as 0.2cm x 0.2cm or less, as we have seen from the temperatureprojections with- and without IR filtering. In doing so, the number of cells out of asingle wafer is in the order of thousands, with the number of cuts needed toseparate the solar cells being a major cost driver if we continue to use traditionalcutting methods; namely, saw dicing. Rather, we must pursue alternative cutting

0

10

20

30

40

50

60

70

80

90

100

350 550 750 950 1150 1350 1550 1750 1950

Wavelength (nm)

0

10

20

30

40

50

60

70

80

90

100

CurrentDensityperUnitWavelength

(mA/cm

2

m)

AM0

AM1.5G

AM1.5 Direct, low-AOD

1.41 eV 0.67 eV1.8 1.1 eV2.0 1.6


22/26


22

methods similar to those used in the microelectronics industry, i.e., scribe-and-break.

Previous preliminary studies have shown that to accomplish scribe-and-breaksuccessfully, the front and back metal thicknesses need be substantially

reduced. Typical bus bar, gridline and back metal thicknesses are severalmicrons in height. It has been recommended that the metal thickness layersshould be about several thousand Angstroms for the scribe-and-break process.This will have an impact on the gridline optimization process we discussedpreviously. Figure 14 shows an example of the relative power loss at 5000 sunswhen the metal thickness layer is reduced from 5 m to 5000 . The increasedloss from the difference in metal layer height is only 1.9%--an acceptable loss.

Once a scribe-and-break process is developed and qualified for the smallconcentrator cells, we will need to develop processes for wafer level test andpick-and-place for the cells. These activities are critical to reducing the cellfabrication cost and ensuring a competitive $/Watt.

4.4 Reducing the Ge Wafer Cost

Concentrator solar cells are grown on Ge wafers. The Ge wafers representroughly about 25-30% of the solar cell cost. So far, most of concentrator solar

Fig. 14: The effects of the front contact metal thickness on thefractional power loss of concentrator solar cells.

0

20

40

60

80

100

0 50 100 150 200 250 300


FractionalPowerLoss(%

)

Metal Thickness = 5 microns

Metal Thickness = 5000 AngstromsTotal Cell Area = 0.1 cm x 0.1 cm

5000 SUNS MODEL


23/26


23

cells are built on high-quality (i.e., low defect counts) Ge wafers. The choice oflow-cost Ge wafers may lead to lower cell fabrication costs without impacting thecell quality. This effort will entail purchase of several batches of low-cost Gewafers with different defect levels, growing epitaxial structures on these wafersand fabricating and testing them into concentrator cells. By associating the

performance and reliability of the different cells to the different wafers, we shouldbe able to set acceptable specs for terrestrial wafers.

Another radical way of reducing the cost of cell fabrication is to develop aprocess to remove the epitaxial layer grown on top of the Ge wafer, and then re-use the wafer for another growth run. This is a process that is at its infancy;hence, it will require major development to bring down to reality. However, ifsuccessful, it may have serious consequences on reducing the costs of MJ solarcells.

In this regard, scientists just developed an ultra-thin 2J GaInP/GaAs space solar

cell with 25% efficiency. The procedure involves the growth of GaInP andGaInAs epitaxial layers on a substrate, perhaps Ge. Then the epilayers are liftedoff and transferred to a metal film where the film now acts as the substrate. Thetop GaInP subcell is quite thin. The disadvantage of this process, however, isthat to make the GaAs layer as thin as possible to increase flexibility, the GaAscurrent starts to plummet due to its thin layer. Current generation is proportionalto light absorption. The thicker the layer, the more light is absorbed, and thehigher the current. Although the layers are thin, this appears to be a minorsetback as the metal substrate serves to reflect light back into the junctions of theGaInP and GaAs layers. This produces more photo-generated carriers, i.e.electrons (and holes) that contribute to the generation of current. Reports arebased on a 4cm x 7cm GaInP/GaAs dual-junction cell that corresponds to totallayer thicknesses between and 1 and 2 m (Ge wafer thickness is 0.007 inches).


24/26


24

5.0 Concentrator Cells Cost Projections

We now turn our attention to the economics equation for ultra-high concentratorcells in sizes of 0.1cm x 0.1cm and 0.2cm x 0.2cm. The cell cost is expressed interms of $/Watt. Keep in mind that in this chart, the Watts are calculated at 25

deg. C operation (standard test conditions). Figure 15 shows the projected cellcost for todays triple-junction cells, while Fig. 16 shows the projected cost for a45% cell.

Fig. 15: Projected Cell Cost (for todays triple-junction cells)

Fig. 16: Projected Cell Cost (for 45% cell)

$0.00

$0.05

$0.10

$0.15

$0.20

$0.25

$0.30

1,000 10,000 100,000 1,000,000 10,000,000

Total cell area (cm^2)

CellPrice($/Watt)

1000 suns

3000 suns

5000 suns

$0.00

$0.05

$0.10

$0.15

$0.20

$0.25

1,000 10,000 100,000 1,000,000 10,000,000

Total cell area (cm^2)

CellPrice($/Watt)

1000 suns

3000 suns

5000 suns


25/26


25

In both graphs, the cell output is calculated at 25 deg. C (standard test condition).Further, different cost reduction activities have been assumed in the generationof these charts (when the volume approaches 1,000,000 cm2), includingimplementation of scribe-and-break, automated wafer level test. Also, a gradual

reduction in the Ge wafer cost has been assumed.

From this data, it is clear that there is a real need to increase the concentrationlevel to 3000 suns (big reduction in cost between the 1000 suns case and the3000 suns case).


26/26

The data in this report are controlled by the terms of the Non-Disclosure Agreement between Mok

26

6.0 Summary & Conclusions

The operation of MJ cells in concentration regimes of 1000-5000 suns will requirethat the cells be kept smaller than 0.2cm x 0.2cm in size. This will allow directbonding of the cells to heat spreaders, which will provide for efficient heat

dissipation using passive cooling even under concentration of 1000-5000 suns.

We analyzed the impact of filtering the unused portion of the infrared radiation toreduce the cell temperature. It seems that there is benefit in cutting offwavelength above 1310 nm for 0.2cm x 0.2cm cells (10 deg C drop intemperature at 5000 suns), which will have minimal impact on cell performance.The benefit was much smaller for the 0.1cm x 0.1cm cells, with temperaturereduction under 5 deg. C. However, in dual-junction cells, where the cutoffwavelength could be as low as 900 nm, the benefit of IR filtering was much morepronounced.

There are modifications to the MJ cells that will need to take place to enable thecells to operate reliably under ultra-high concentration levels. They includemodifications to the existing tunnel junction structures since current tunnel

junction structures cannot operate reliably above 1000-2000 suns. They alsoinclude modifications to the metallization schemes to reduce series and contactresistance and make the MJ cells work efficiently and reliably in ultra-highconcentration modules.

In terms of cost reduction, the fact that the cells are going to be under 0.2cm x0.2cm in size will require changes to the existing processes from saw dicing toscribe-and-break methods. Further cost reduction activities will need to focus onthe qualification of lower cost, terrestrial-grade Ge wafers and on theimplementation of automated test methods (enabling tests of the cells on a waferlevel).

By applying all the above improvements, the solar cell cost could be as low as$0.05 per Watt or even lower at concentration levels above 3000 suns. Furthercost reduction could be driven by developing concentrator cells with 45%conversion efficiency. Although efforts to achieve 45% cell efficiency requiresubstantial funding, increasing cell efficiency is very leveraging not just inbringing down the cell cost but also in bringing down the entire system cost.

white paper to mok- final-1

Documents