7.4.5: PV Cells

A silicon PV cell is a thin (0.5 - 1 mm) wafer of p-type Si, on the top of which there is a thin layer of n-type Si. So, a short distance below the illuminated surface there is an np junction. The photoelectrons generated leave the cell through the surface, and return through the surface of the “dark side”. There must be an electrode on the top surface to collect the photoelectrons – but such electrode would obscure the incident sunlight. Therefore, it’s made as a grid of parallel thin wires. There is no such problem with the bottom electrode through which the photocurrent returns to the cell – it may cover the entire surface.

Figure \(\PageIndex{1}\): A scheme explaining how a photocell works. Sunlight enters through the spaces between the wires forming the top electrode (their width is much exaggerated in the plot). The n p junction is a short distance below the surface. The sunlight photons create electron-hole pairs, which are driven apart by voltage – electron towards the surface, holes towards the bottom Where does the voltage comes from? This is all the ingenuity of the design! The voltage employed for separating electrons from holes is the “built-in” voltage of the n p junction! (as shown by the graph on the left). There is no need to apply an external voltage source, like in the Fig. 7.19 – the current flows “all by itself”. So, an illuminated PV cell becomes a current source. The output voltage is close to the “built-in” voltage step, typically 0.6 Volt (source: aop).

The voltage from a single cell is far too low for applying it to some practi- cal purposes. Therefore, in PV panels several tens of single cells are connected in series to deliver a higher voltage. For instance, a typical panel of about 25 inches by 54 inches size contains 36 cells connected in series to deliver about 21.5 Volts – when no current is taken from it. As in all other types of current sources, the output voltage drops when current is taken from them. An important question is how much the voltage drops in dependence of the current (Amps) flowing through an external load. Conventionally, for pre- senting such information about a given current source one uses a plot called a with the volt scale on the abscissa (horizontal) axis, and the amp scale on the ordinate (vertical) axis. A typical shape of the I-V characteristic of a single silicon solar cell is shown in the Fig. \(\PageIndex{2}\). It’s interesting to notice that for most of the characteristic the I-V curve is almost flat (professionals say: there is a plateau region in the characteristic) – which is easy to under- stand: the output current is proportional to the number of photoelectrons leaving the cell in one second – and the number of photoelectrons generated every second is proportional to the number of photons reaching the cell every second.

Figure \(\PageIndex{2}\): A typical shape of a constant illumination I-V characteristic of a single solar cell (the blue curve). The output voltage V when the cell delivers no current (called Voc, where “oc” stands for “open circuit”) is 0.6 V. The magenta-color curve is the output power as a function of the output voltage. The power P delivered to an “external load” (e.g., such as a bulb) by electric current is simply P = V I, the voltage across the load times the current flowing through it. So, for each point of the blue curve, the corresponding power is calculated by multiplying the abscissa (volts) by the ordinate (amps). The voltage corresponding to the maximum of the power curve, V_MP is the output voltage at which a maximum power is delivered to an external load (source: aop).

The power curve at the I-V characteristic of a power cell has a distinct maximum, usually for output voltage equal to about 80% of the open circuit voltage V_oc.

In Fig. \(\PageIndex{3}\) typical I-V characteristics of PV panels for several different incident sunlight intensities are shown.

Figure \(\PageIndex{3}\): Typical IV characteristics for a PV panel for different intensities of the incident solar light (source: aop).

From the figure one can see that the output current always exhibits a plateau, with the current magnitude roughly proportional to the light intensity. The \( V_{oc} \) slightly drop with decreasing sunlight intensity as also the maximum power voltage does, but the maximum power essentially is proportional to the incident light intensity. So, a solar panel works not only on sunny days, it can also deliver power on cloudy days. Obviously, much less, in handbooks with instructions for users of PV installation they talk about 10% - 25% of their rated capacity.