Solar Cell Chapter 4: Efficiency Limits, Losses, and Measurement

1. Solar CellChapter 4: Efficiency Limits, Losses, and Measurement Nji Raden Poespawati Department of Electrical Engineering Faculty of Engineering University of Indonesia

2. Contents 4.1. Efficiency Limits 4.2. Effect of Temperature 4.3. Efficiency Losses 4.4. Efficiency Measurement

3. Efficiency Limits There are 3 parameters could be used to characterize the performance of a p-n junction solar cell, namely • The open-circuit voltage (Voc) • The short-circuit current (Isc), and • The fill factor (FF) Short-Circuit Current The short-circuit current is the current through the solar cell when the voltage across the solar cell is zero (i.e., when the solar cell is short circuited). Usually written as ISC, the short-circuit current is shown on the IV curve in Figure 1.

4. Efficiency Limits(continued) The short-circuit current depends on a number of factors which are described below: • the area of the solar cell; • the number of photons (i.e., the power of the incident light source); • the spectrum of the incident light.For most solar cell measurement, the spectrum is standardised to the AM1.5 spectrum; • the optical properties (absorption and reflection) of the solar cell (discussed in Optical Losses); and • the collection probability of the solar cell, which depends chiefly on the surface passivation and the minority carrier lifetime in the base.

5. Efficiency Limits(continued) In a cell with perfectly passivated surface and uniform generation, the equation for the short-circuit current can be approximated as: where G is the generation rate, and Ln and Lp are the electron and hole diffusion lengths respectively. Silicon solar cells under an AM1.5 spectrum have a maximum possible current of 46 mA/cm2. Laboratory devices have measured short-circuit currents of over 42 mA/cm2, and commercial solar cell have short-circuit currents between about 28 mA/cm2 and 35 mA/cm2. …………………..(4.1)

6. Efficiency Limits(continued) Open-Circuit Voltage An ideal p-n junction cell : where IL is the light-generated current and I0 is the diode saturation current I0 is as small as possible for max. Voc For silicon : a max.Voc is about 700 mV and FF is 0.84 Eg , Voc Figure 2 shows maximum energy-conversion efficiency calculated as a function of the band gap of cell material. IV curve of a solar cell showing the open-circuit voltage is illustrated inFigure 3

7. Efficiency Limits(continued) Fill Factor The FF is defined as the ratio of the maximum power from the solar cell to the product of Voc and Isc. Graphically, the FF is a measure of the "squareness" of the solar cell and is also the area of the largest rectangle which will fit in the IV curve. The FF is illustrated in Figure 4. the FF is most commonly determined from measurement of the IV curve and is defined as the maximum power divided by the product of Isc*Voc, i.e.: ………………………..(4.4)

8. Efficiency Limits(continued) Efficiency Efficiency is defined as the ratio of energy output from the solar cell to input energy from the sun. The efficiency of a solar cell is determined as the fraction of incident power which is converted to electricity and is defined as: ……………………….(4.5) ……………………….(4.6) where Voc is the open-circuit voltage; Isc is the short-circuit current; and FF is the fill factor.

9. Effect of Temperature • The short-circuit current of solar cells is not strongly temperature-dependent • The open-circuit voltage and FF are both decrease by changing temperature Where Vg0 = Eg0/q T , Voc 

10. Effect of Temperature(continued) Example : Silicon  Vg0 1.2 V, Voc 0.6 V  3, T = 300 K, gives = - 2.3 mV/0C Hence, for silicon, Voc decreases by about 0.4 % per 0C

11. Efficiency Losses Figure 5 shows a schematic cross section of an actual p-n junction solar cell. • Short-Circuit Current Losses There are three types of losses in solar cells which could be described as being of an “optical” nature: • Reflection losses  the antireflection (AR) coating reduces such reflection losses to about 10% • The necessity of making electrical contact to both p- and n-type regions of solar cells generally results in a metal grid contact on the side of the cell exposed to sunlight. This blocks 5 to 15% of the incoming light. • Finally, if the cell is not thick enough, some of the light of appropriate energy that does get coupled into the cell will pass straight out the back.(see Figure 6) Another source of Isc loss is recombination in the bulk semiconductor and at surfaces

12. Efficiency Losses(continued) Open-Circuit Voltage Lossess • The fundamental process determining Voc is recombination in the semiconductor • The lower the recombination rate in the semiconductor, the higher is Voc

13. Efficiency Losses(continued) Fill Factor Lossess • Recombination in the depletion region can also reduce the fill factor • Solar cell generally have a parasitic series and shunt resistance associated with them, as indicated in the solar cell equivalent circuit of Figure 7 • The major contributors to the series resistance, Rs : • The bulk resistance of the semiconductor material making up the cell, • The bulk resistance of the metallic contacts and interconnections, • The contanct resistance between the metallic contacts and the semiconductor

14. Efficiency Losses(continued) • The shunt resistance, RSH, is caused by : • leakage across the p-n junction around the edge of the cell and in nonperipheral regions in the presence of crystal defects and • precipitates of foreign impurities in the junction region • Figure 8 shows effect of parasitic resistances on the output characteristics of solar cells

15. Efficiency Measurement • Figure 9(a) and (b) shows a typical experimental arrangement for measuring solar cell output characteristics and possible setup for measuring the spectral response

16. Thank You

17. Figure 1. IV curve of a solar cell showing the short-circuit current.

18. Figure 2. Solar Cell efficiency limits as a function of band gap of the cell material

19. Figure 3. IV curve of a solar cell showing the open-circuit voltage.

20. Figure 4. Graph of cell output current (red line) and power (blue line) as function of voltage. Also shown are the cell short-circuit current (Isc) and open-circuit voltage (Voc) points, as well as the maximum power point (Vmp, Imp). Click on the graph to see how the curve changes for a cell with low FF.

21. Figure 5. Major features of a solar cell

22. Figure 6.Effect of cell thickness on the percentage of full short-circuit current that may ideally be generated by a solar cell.

23. Figure 7. Equivalent circuit of a solar cell

24. Figure 8. Effect of parasitic resistances on the output characteristics of solar cells:(a) Effect of series resistance, RS(b) Effect of a shunt resistance, RSH

25. Figure 9. (a) Experimental configuration for testing solar cell and modules. (b) Possible setup for measuring the spectral response