2.8.2 Solar Cells

Some applications can be powered by solar cells which convert the optical power of the incident light to electrical power through the photovoltaic effect [77]. A solar cell is basically an asymmetrical semiconductor diode with a pn-junction close to the surface. Incident photons with an energy $h\nu > \ensuremath{E_{\mathit{g}}}\xspace$ generate electron-hole pairs, ^2.3 which are separated by the electric field in the depletion zone. This will cause a current flow I_sc if the diode is short-circuited:

$$I_{sc} = q\eta P_{opt} / h\nu ,$$ (2.24)

where P_opt is the optical power and $\eta$ is the quantum efficiency (neglecting recombination). Typical values for I_sc / P_opt are $\rm 250mA / W$ for monocrystalline silicon cells. For a load voltage V_L the current I_L delivered to the load is

$$I_L = I_{sc} - I_s \left( e^{V_L/\ensuremath{U_{\mathit{T}}}\xspace } -1 \right) ,$$ (2.25)

where I_s is the diode saturation current. The open-circuit voltage of an illuminated solar cell is typically $V_{oc} = \rm0.55V$ and the loaded-cell voltage at maximum output power is 0.4...0.45V.

Normally, several cells must be connected in series to obtain the required supply voltage. On the other hand, Ultra-Low-Power circuits can be operated with this voltage perfectly well. This is an advantage especially in small-scale applications which need only a small solar-cell area and where series connected multiple cells would be too expensive. Furthermore, in some cases it may be possible to integrate the solar cell on the same chip.

Footnotes

... pairs,^2.3

Each electron-hole pair carries an energy of . The energy difference $\ensuremath{h}\xspace \nu - \ensuremath{E_{\mathit{g}}}\xspace$ is converted to heat.