Cell-to-Module Gains and Losses in Crystalline Silicon PV. Andrew Gabor Gabor Photovoltaics Consulting, LLC July 10, Intersolar NA

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1 Cell-to-Module Gains and Losses in Crystalline Silicon PV Andrew Gabor Gabor Photovoltaics Consulting, LLC July 10, Intersolar NA 1

2 Some material sourced from 2

3 Outline Background Loss/Gain Types Optical and Format Losses/Gains Electrical Losses Summary Crystalline Silicon Short/Long-term Challenges Metrology Short/Long-term Challenges 3

4 Background What really matters? Module tester results (somewhat) Energy delivery in field under real conditions Variable temperature Variable intensity (temporal, spatial) Variable angle of incidence Variable soiling Most effort goes into optimizing cell tester results Sometimes an improvement that gives a big boost at the cell level, gives a much smaller boost at the module level Sometimes an improvement that only gives a small boost at the cell level can give a larger improvement at the module level Some silicon PV road-mapping activities stop at the cell level More low hanging fruit at the module level Module efficiencies lower than cell efficiencies Cell-to-Module (CTM) Losses typically > 12% relative Higher balance of system costs 4

5 Cell-to-Module Gain and Loss Factors 14% relative CTM loss A few additional factors More factors for field performance Source: Fraunhofer ISE, Schmid

6 Module Border Area (-1.33%) Wide variation in percentage of border area between manufacturers Higher labor to hide bussing wire behind cells Different sensitivity to water ingress, dirt accumulation, and frame shading at steep angles Materials cost savings by reducing border area Larger modules Smaller percentage border area Frameless modules Eliminate frame area Large module Small border Small module Large border 6

7 Cell Spacing Area (-0.56%) Higher percentage with smaller cells Higher amount with pseudosquare vs full square cells String spacing influenced by Quality of stringing and layup automation Quality of manual adjustment of string positions Potentially closer spacing with back contacted cells (MWT and IBC) No need for stress relief bends between cells in strings pick and place with MWT eliminates tolerances related to string effective width and string placement Separate from backsheet coupling (later slide) 7

8 Glass Reflection/Absorption (-0.73%) Improve with AR coatings or glass texturing Both help energy delivery (kwh/kw p ) due to higher gains at steep angles Improve with thinner glass or polymer coversheets ~0.2% relative efficiency gain with 2mm glass 8

9 Encapsulant Absorption (-0.33%) Improve with better encapsulants Higher grade EVA Ionomer Silicone Low blue/uv absorption increasingly important Better cells have better spectral response in the blue/uv Improve with thinner encapsulant, enabled by Back contacted cells (no wires) Higher resistivity encapsulants? (reduced PID) Stronger cells (need less cushioning) 9

10 Reflection Cell Active Surface (+0.54%) High reflectivity at an abrupt cell/air interface Reduced reflectivity by encapsulating with an intermediate index of refraction material The better the texture, the smaller the gain upon encapsulation Texturing optimization needs to measure encapsulated cells or can be fooled Source ISE,

11 Reflection Fingers (+0.16%) ~50% effective width due to light trapping for typical fingers Narrow effective width for better aspect ratios Optimize based on encapsulated results Source Fraunhofer ISE,

12 Coupling Backsheet (+0.27%) Some light reflects from white backsheet, undergoes totalinternal-reflection at the glass/air interface, and then hits the cells Larger gains for larger cell spacings Typical gains over black backsheets are in the 1-3.5% relative range 12

13 Structured Backsheets Could have further gains over white backsheet both between the cells and in the border area by directing light to the cells with V-grooves or other optical approaches Source U.S.Patent #5,994,641 ASE Americas Source Exxon,

14 Resistive Loss Interconnect Wires (-0.57%) Reduce loss by Wider wires Generally no increased shading losses Possible with light trapping interconnect wires from Ulbrich or Schlenk Thicker wires Generally no increased cell cracking Possible with conductive epoxy instead of solder Conductive backsheets and backcontacted cells Wire arrays 14

15 Reduce resistive losses in wires without increasing dead area Shorter wires Lower I 2 R losses Cut rectangular bricks Even possible from mono Challenge - expensive to redesign all equipment to handle a new shape 1.6m 60-cell module 1.1m Gen5 25 square bricks Gen5 21 rectangular bricks 15

16 Reflection Interconnect Wires (+0.16%) Other measurements for coupling gain on standard wires are lower Wire/busbar shading losses in cells are ~ 2.5% relative Multiple ways to reduce shading losses Structured interconnect wires (Ulbrich, Schlenk) ~2% relative Eff improvement White paint (ipv Stuttgart) Round wire arrays (Day4/Meyer- Burger, Schmid) U. Konstanz 16

17 Cell to cell mismatch losses Calculations of losses can be in the range of 0.03% to 0.06 absolute for narrow cell sorter bins or > 0.1% for wider bins [PV Measurements 2004, Mobil Solar 1981]. Losses can be much larger at low light intensities, especially when shunt resistance is not factored into the binning algorithm [RWTH Aachen ] 17

18 Light-Induced-Degradation (-0.5%) A quick decrease of ~0.5% absolute [NREL- 2012] due to B-O defects in p-type CZ cells. 18

19 Summary ~2% absolute module efficiency gain with combined improvements 19

20 Near-term (1-3 years) metrology challenges Reflectivity measurements, Cell IV testing, and Module IV Testing should simulate in-laminate conditions and field conditions Index matching oil or encapsulated coupons for R&D Probe differently to simulate wire resistive losses Predict energy delivery for different applications at variable light intensity, temperature, and angles of incidence Separate standardized conditions and testing methods for bifacial cells/modules Cell IV testers that can contact fingers A bit silly to have busbars just for ease of cell testing Conductive adhesives or wire arrays can contact fingers directly Cell-sorter binning algorithms Take into account performance at nonstandard conditions 20

21 Long-term (4+ years) metrology challenges New performance metrics on specs sheets Predict kwh/yr for a few different standardized locations and install conditions 21

22 Near-term (1-3 years) challenges - c-si PV feedstock/crystal growth/wafering Eliminate LID for p-type mono Lower O? Ga doping? Hydrogenation solution? Inherently crack-resistant wafers/cells e.g. Solexel design Or cell designs without soldered busbars 22

23 Long-term (4+ years) challenges - c-si PV feedstock/crystal growth/wafering Rectangular wafers and updated tooling/process/handling equipment Cut rectangular wafers from CZ ingots along growth axis or from multi bricks Reduce dimension in direction of current flow Increase other dimension for larger cell size Reactive ion etching or other advanced texture Reduce resistive power loss thinner wires and reduced cracking for standard cells reduced Cu thickness for back contact cells 23