Behaviour of Solar Cells

Effect of light: A silicon solar cell is a diode formed by joining p-type (typically boron doped) and n-type (typically phosphorous doped) silicon. Light shining on such a cell can behave in several ways, as illustrated in Fig. 1.6. To maximise the power rating of a solar cell, it must be designed to maximise desired absorption (3) and absorption after reflection (5). In general, the closer the point of e-h (electron-hole) generation to the p-n junction, the better the chance of ‘collection’.

Fig. 1.6: Behaviour of light shining on a solar cell. (1) Reflection and absorption at top contact. (2) Reflection at cell surface. (3) Desired absorption. (4) Reflection from rear out of cell —weakly absorbed light only. (5) Absorption after reflection. (6) Absorption in rear contact.

Spectral Response: Solar cells respond to individual photons of incident light by absorbing them to produce an electron-hole pair, provided the photon energy (E) is greater than the bandgap energy (E_G). Photon energy in excess of E_G is quickly dissipated as heat. The spectral responsivity of a solar cell, given by the amperes generated per watt of incident light. Ideally, this increases with wavelength.

Effect of Temperature: The operating temperature of a solar cell is determined by the ambient air temperature, by the characteristics of the module in which it is encapsulated, by the intensity of sunlight falling on the module, and by other variables such as wind velocity. The dark saturation current I_O increases with temperature. The short circuit current (I_sc) increases with temperature, since the bandgap energy (E_G) decreases, and more photons have enough energy to create electron-hole pairs. However, this is a small effect. The main effect of increasing temperature for silicon solar cells is a reduction in V_oc, the fill factor and hence the cell output.

Effect of Parasitic Resistances: Solar cells generally have a parasitic series and shunt resistance associated with them, as shown in Fig. 1.5. Both types of parasitic resistance act to reduce the fill-factor. The major contributors to the series resistance (R_S) are the bulk resistance of the semiconductor material, the metallic contacts and interconnections, carrier transport through the top diffused layer, and contact resistance between the metallic contacts and the semiconductor. The shunt resistance (R_sh) is due to p-n junction non-idealities and impurities near the junction, which cause partial shorting of the junction, particularly near cell edges.