1 © Alexis Kwasinski, 2012 PV Cells Technologies Characterization criterion: Thickness: Conventional – thick cells ( μm) Thin film (1 – 10 μm). Tend to be less costly than conventional (think) cells but they also tend to be less reliable and efficient. Crystalline configuration: Single crystal Multicrystalline: cell formed by 1mm to 10cm single crystal areas. Polycrystalline: cell formed by 1μm to 1mm single crystal areas. Microcrystalline: cell formed by areas of less than 1μm across. Amorphous: No single crystal areas. p and n region materials: Same material: homojunction (Si) Different material: heterojunction (CdS and CuInSe 2 )

2 © Alexis Kwasinski, 2012 BP SX170B PolycrystallineBP SX170B Monocrystalline Mitsubishi PV-TD 190MF5 Multicrystalline Uni-Solar Laminate PVL-136 Amorphous Uni-Solar solar shingle PV Modules at ENS PV Cells Technologies

3 © Alexis Kwasinski, 2012 PV Cells Technologies Thick film fabrication techniques: Czochraski’s (CZ): for single-crystal silicon. Costly. Float zone process (FZ): also for single-crystal silicon. Costly Ribbon silicon Cast silicon: for multicrystalline cells. Less costly. Thin film Can be used embedded in semitransparent windows. Techniques: Amorphous Silicon: can achieve higher efficiencies (in the order of 42% thanks to the multijunction (different multiple layers) in which each layer absorb photons with different energy. Gallium Arsenide (GaAs): relatively high theoretical efficiency (29 %) which is not significantly affected by temperature. Less sensitive to radiation. Gallium makes this solution relatively expensive. Gallium Indium Phosphide (GaInP): similar to GaAs. Cadmium Telluride (CdTe): Issue: Cd is a health hazard (it is very toxic). Copper Indium Diselenide (CIS or CuInSe2): relatively good efficiency) Silicon Nitrade (N 4 Si 3 )

4 © Alexis Kwasinski, 2012 The p-n junction diode p-type substrate n-type substrate Bias voltage Ideal diode Real diode IdId V d is the diode voltage I 0 is the reverse saturation current caused by thermally generated carriers At 25 C: I0I0

5 © Alexis Kwasinski, 2012 PV Cells physics I SC Reverse v-i curve for the diode I SC V OC p-n junction is equivalent to a diode Same curve The bias source (voltage source) is replaced by a current source powered by the photons The current source shifts the reversed diode curve upwards

6 © Alexis Kwasinski, 2012 PV Cell steady state characteristic From Kirchoff’s current law: The open circuit voltage is Current Power Maximum power point P max  0.7 V oc I sc

7 © Alexis Kwasinski, 2012 PV Cell steady state characteristic Dependence on temperature and insolation:

8 © Alexis Kwasinski, 2012 PV Cell steady state characteristic More on the dependence on temperature and insolation:

9 © Alexis Kwasinski, 2012 For a more realistic representation we can consider the following (equivalent to a diode’s model): 1) Effect current leakage 2) Effect of internal ohmic resistance More complex steady-state models I SC RpRp RSRS VdVd + V where V d = V+IR S This is a transcendental equation

10 © Alexis Kwasinski, 2012 Both effects can be combined to obtain the more realistic (and complex) steady state model: PV more complex steady-state model I SC RpRp RSRS VdVd V -- where V d = V+IR S This is a transcendental equation ++

11 © Alexis Kwasinski, 2012 Dynamic effects Capacitive effect As with any diode, there is an associated capacitance. However, this capacitance is relatively small, so the effects on the output can often be neglected. Therefore, PV modules can follow a rapidly changing load very well. One undesirable effect of the capacitance is that it makes PV cells more susceptible to indirect atmospheric discharges.

12 © Alexis Kwasinski, 2012 Modules combination PV cells are combined to form modules (panels). Modules may be combined to form arrays. More modules (or cells) in series More modules (or cells) in parallel When modules are connected in parallel, the array voltage is that of the module with the lowest voltage. When several modules are connected in series to achieve a higher array voltage, the array’s current equals that of the module delivering the lowest current.

13 © Alexis Kwasinski, 2012 Shading (n-1)V module (R p +R s )(n-1)I module A shadowed module degrades the performance of the entire array One module with 50% shadow One module with 100% shadow Two modules with 100% shadow

14 © Alexis Kwasinski, 2012 Bypass diode for shadowing mitigation Bypass diodes can mitigate the effects of shadows but they don’t solve the issue completely. A better solution will be presented when discussing power electronics interfaces. No shade Shaded with bypass diode Shaded without bypass diode