solar cell characteristics

solar cell characteristics

1. Solar cell characteristics By 17MSE004 17MSE007 17MSE009 17MSE014 M.TECH ENERGY SYSTEMS PANDIT DEENDAYAL PETROLEUM UNIVERSITY 27 April 2018 SOLAR CELL PARAMETERS 1

2. Basic Structure of a Solar Cell

3. KEY PARAMETERS Open Circuit Voltage V oc Short Circuit Current I sc Shunt Resistance Series Resistance Internal quantum efficiency External quantum efficiency

4. SOLAR CELL – A DIODE A solar cell is a diode and hence an IV curve of a solar cell under dark conditions will look similar to that of a diode. When illuminated, the photons interact with the material to generate electron hole pairs.

5. STANDARD TEST CONDITIONS Temperature = 25 ˚C Important device characteristics can be obtained from the I-V measurements.

6. SOLAR SIMULATOR

7. OPEN CIRCUIT VOLTAGE V OC (V) In an ideal solar cell, V oc is independent of the illumination intensity. Maximum voltage generated across the terminals of a solar cell when they are kept open i.e. I=0 The open circuit voltage depends on the light generated current and the reverse saturation current Material having larger band gap have higher V oc .The band gap of silicon is 1.1 eV. Therefore maximum possible V oc is 1.1 ev .

8. SHORT CIRCUIT CURRENT I SC Short circuit current is the maximum current produced by a solar cell when its terminal are short circuited. When a photon is absorbed in a solar cells it generates an electron-hole pair which is separated by a junction and then transported to the external circuit. For maximum short circuit current, we have to assume that there is no recombination in the material . A photon is required to possess energy higher than the band gap energy of the material in order to be absorbed. A material with a large band gap will absorb less number of photons as compared to the material with low band gap. Therefore short circuit current will increase with decrease in the band gap.

9. MAXIMUM POWER Draw a rectangle with the origin, V OC and ISC as the 3 corners. The 4 th corner will give the maximum theoretical power, P T . From the origin, draw a line passing through the maximum theoretical power, P T . This is the load line The point where the load line crosses the I-V curve is the maximum power point, P MAX for the solar cell, for a given load, with maximum current and maximum voltage.

10. Calculate the efficiency &peak power of a si solar cells operating at 27˚cwith a short circuit current of 2.2 A& operating under standard illumination of 1000w/m².the area of solar cells is about 100 cm² Solution = 0.616 v

11. Consider the FF of the solar cells to be 0.75 or 75 % The efficiency is given by = 0.616*2.2*0.75/ 1000*100*10^-4 = 0.10164 = 10.16% The peak power in this case is given as P max = v oc * I sc *FF = 1.01 W

12. Parasitic Resistance

13. Characteristic Resistance The characteristic resistance of a solar cell is the output resistance of the solar cell at its maximum power point. If the resistance of the load is equal to the characteristic resistance of the solar cell, then the maximum power is transferred to the load and the solar cell operates at its maximum power point. The value of this resistance can be approximated by  

14. Parasitic Resistances Series resistance Rs of a PV module represents resistances in cell solder bonds, emitter and base regions, cell metallization, cell interconnect Bus bars and resistances in junction box terminations. The shunt resistance, Rsh , represents any parallel high-conductivity paths (shunts) across the solar cell p-n junction or on the cell edges . These are due to crystal damage and impurities in and near the junction and give rise to the shunt current, Ish . For a good Fill Factor we want Rs as low as possible and Rsh as high as possible.

15. Series Resistance

16. It is, therefore, evident that a small increase in Rs can be detrimental to the performance of PV modules due to the power loss. Dark current–voltage (I–V) measurements can be used to quantitatively evaluate increases in Rs . These measurements are also sensitive to changes in module shunt resistance and other cell parameters. Also, since the open circuit voltage and short circuit current are not affected: A straight-forward method of estimating the series resistance from a solar cell is to find the slope of the IV curve at the open-circuit voltage point   Effect of Series Resistance

17. Shunt resistance

18. Effect of Shunt Resistance Low shunt resistance causes power losses in solar cells by providing an alternate current path for the light-generated current. Such a diversion reduces the amount of current flowing through the solar cell junction and reduces the voltage from the solar cell. The effect of a shunt resistance is particularly severe at low light levels, since there will be less light-generated current.at lower voltages where the effective resistance of the solar cell is high, the impact of a resistance in parallel is large. Also, since the open circuit voltage and short circuit current are not affected:  

19. Impact of Series and Shunt Resistances In the presence of both series and shunt resistances, the I-V curve of the solar cell is given by; The overall fill factor FF is  

20. Two diode model From Fig it is clear that the shunt current, Ish , detracts from the current output and the potential drop across Rs reduces the voltage output of the solar cell. The influence of Rs and Rsh on the I–V characteristics can be determined by using Eq. In this study, all simulated I–V characteristics were generated using PV simulation software, PVSIM This software uses the two-diode model and calculates I–V characteristics based on typical solar cell parameters. The simulations were done using 1000 W/m2 irradiance and 25 C cell temperature.  

21. Measuring parasitic resistances A straight-forward method of estimating the series resistance from a solar cell is to find the slope of the IV curve at the open-circuit voltage point A straight-forward method of estimating the shunt resistance from a solar cell is to find the slope of the IV curve at the short circuit current point This, however, does not represent the true values and only gives a rough indication of respective resistance values.

22. Measuring parasitic resistances practically Rs can be determined by measuring I–V curves at different light intensities from a line drawn through points corresponding to a fixed current below the respective Isc on each curve. Rs is then obtained from the slope of this curve, with Rs = DV/DI. This result generally gives good results and is independent of Io, n and Rsh The system used is a non-intrusive technique that measures the individual cell shunt resistances of cells in encapsulated modules In this system, a variable power supply is used to bring the illuminated module to zero bias. At this condition, a small AC signal is applied to the module, the only conduction path being through the shunt resistances of the individual cells. The cells are then sequentially shaded each time ensuring the DC supply remains equal to the Voc of the module. The magnitude of the AC signal when the cells are sequentially shaded is directly proportional to the conductance of the module in the dark i.e., the shunt current.

23. QUANTUM EFFICIENCY OF SOLAR CELL

24. WHAT is QUANTUM EFFICIENCY? Function of Wavelength( λ ) or Energy(E). All photons of certain wavelength are absorbed → efficiency is unity of that wavelength. For energy below wavelength → zero.   27 April 2018 SOLAR CELL PARAMETERS 24

25. Quantum efficiency External Quantum Efficiency(EQE) EQE = = Effect of optical losses → Transmission → Reflection   Internal Quantum Efficiency(IQE) = Effect of collection probability. → charge recombination   27 April 2018 SOLAR CELL PARAMETERS 25

26. Quantum efficiency External Quantum Efficiency(EQE) By surface texturing this can be improved. For organic solar cells efficiency is high. Internal Quantum Efficiency(IQE) after optical losses, absorbed photons which can generate collectable carriers. 27 April 2018 SOLAR CELL PARAMETERS 26

27. 27 April 2018 SOLAR CELL PARAMETERS 27 Ideally square shape. Reduced due to recombination effects Collection probability

28. In blue portion → front surface recombination affects the efficiency. → due to surface passivation absorption of light is high. In green portion → absorption in bulk of solar cell. → low diffusion length will affect the collection probability Can be viewed as the collection probability due to generation profile of single wavelength, integrated over device thickness and normalized to incident number of photons. 27 April 2018 SOLAR CELL PARAMETERS 28

29. Quantum efficiency QE = First step is to measuring a laser with both photo diode and a power meter to obtain and Two methods Using Power Meter Using Radiation Pressure   27 April 2018 SOLAR CELL PARAMETERS 29

30. Using Power Meter Aligned through the Faraday isolator HWP change the amounts of light being transmitted and reflected by the PBS. Using HWP power of the reflected light coming from the PBS can be adjusted. aligned onto a photo diode or power meter to be measured. 27 April 2018 SOLAR CELL PARAMETERS 30

31. 27 April 2018 SOLAR CELL PARAMETERS 31 First step required was to measure the photo diode's output without being exposed to any laser light → V dark Then the laser light being reflected by both polarizing beam splitters is aligned onto the photo diode With the help of HWP and PBS → power is minimized. At this minimized power voltage is measured.→ V measured The power meter is inserted directly in front of the photo diode to measure the power of the laser at the same point. Then HWP and PBS adjust to increase the power and readings are taken. Then efficiency is calculated at each power level. Until the saturation voltage of photo diode is reached.

32. 27 April 2018 SOLAR CELL PARAMETERS 32 I = N e = N p = P = power v = frequency h = plank’s constant.  

33. Using Radiation Pressure 27 April 2018 SOLAR CELL PARAMETERS 33

34. 27 April 2018 SOLAR CELL PARAMETERS 34 Aligned through the Faraday isolator HWP change the amounts of light being transmitted and reflected by the PBS. Using HWP power of the reflected light coming from the PBS can be adjusted. Several focusing lenses were also placed to control the size of the laser beam. Another component of interest is the 90 % beam splitter used to direct the beam toward the interferometer. This splitter allows 10 % of incident light to be transmitted. 10 % for symmetric port 90 % towards the vacuum chamber in order to reduced the noise.

35. 27 April 2018 SOLAR CELL PARAMETERS 35 I = N e = F = F = m ∗ ω 2 ∗ X dX = * dV PD = -   P = power v = frequency h = plank’s constant Michelson interferometer

36. Spectral responsivity(r λ ) How much current comes out per incoming photon of given energy and wavelength. Unit of R λ = With the help of spectral responsivity we can find quantum efficiency of solar cell. For that, ≈ (1240 W*nm/A)   27 April 2018 SOLAR CELL PARAMETERS 36

37. PROBLEM : A photodiode is made with p-type Si doped with boron atoms/m and n-type Si phosphorus with As atoms/m . The width of the p- side is 1 m. For Si, . The intrinsic carrier concentration of Si at 300 K is given by . The index of refraction of Si is 3.5 and the absorption coefficient is m . Calculate the quantum efficiency of the photodiode at 300 K. SOLUTION: 27 April 2018 SOLAR CELL PARAMETERS 37 V B = = 0.369 v W = = 0.27 µm x p = * W = 0.03 µm x n = 0.97 µm R= So 0.31 times power is reflected a0nd 0.69 times power is transmitted. optical power which can reaches to depletion region is 0.69*P o = 0.69*P o = 0.26P o The power which is converted to electron-hole pair 0.26P o P o which implies that the efficiency of the device is less than 1%.  

38. THANK YOU 27 April 2018 SOLAR CELL PARAMETERS 38