cells, where the silicon is painted onto a substrate, are basically painted on as several individual cells connected in a
series, making a much more convenient solution. Such
mini-arrays are available with outputs from three to 15 volts.
Clockwise from top left are various types of solar cells:
a Sunceram five-volt cell from Solarbotics, a TX 3-25 thin
film plastic Powerfilm three-volt module from Iowa Thin
Films, a large crystalline silicon cell (0.6 volt) from a grab
bag, a smaller silicon cell (0.6 volt), a larger three-volt
Powerfilm module — this one encapsulated in an
ultraviolet-resistant Tefzel sheet, and a small
Sunceram module taken from a defunct calculator.
Powerfilm cells by Iowa Solar Films. These thin plastic cells
can be very lightweight and can be bent around various
shapes. For outdoor applications, these are available in an
ultraviolet resistant Tefzel plastic coating, though this
makes the cells heavier and more expensive.
Because a cell is just a diode, it produces a voltage that
is nearly a constant, related to the electronic bandgap of the
material. For silicon, this is about 0.6 volts. This is obviously inconveniently low, so it is usual to array cells in series (a
“string” of cells) to yield a useful voltage. One way is, of
course, to solder or clip together lots of individual cells, but
this can be tedious and delicate. Many modern amorphous
Solar cell output characteristic (I/V curve). The blue line is
the output of two Powerfilm TX 3-25 modules in series.
The short-circuit current is about 37 mA and the open
circuit voltage about 7. 5 volts. Peak power of about 0.15
watts (red line) occurs for an output of about five volts and
25 mA. Operation at three or 6. 5 volts will reduce the out-put to about 0.1 watts (green line). Peak-power conditions
correspond to a load impedance of about 200 ohms.
A solar cell’s output characteristic (the I/V curve) for
given light conditions is usually fixed by two numbers: the
open-circuit voltage and the short-circuit current. These
are what you read if you just put a high-impedance voltmeter across the cell or a low-impedance ammeter, respectively. The open-circuit voltage for individual silicon cells is
always about 0.6 volts. It depends only very slightly on
light levels, but will depend on temperature being a little
higher under cold conditions. (Solar cells on satellites also
need to worry about the characteristic being affected by
radiation in space, but, unless you are unhealthily close to
a nuclear reactor, you shouldn’t need to worry about this.)
The short-circuit current is directly proportional to the
light level and is basically set by the area of the cell. (More
area means more photons of light collected, and more
photons mean more charge carriers and more current.)
However, neither of these test conditions is useful.
The power provided by the cell is voltage x current, so the
short- and open-circuit tests give no power.
Domestic power plants and satellite power systems
use feedback control circuits to track this peak-power
point (which depends on temperature, light levels, radiation dose, and so on). However, for simple robots and
other projects, it is usually adequate to assume that peak
power will be roughly three-quarters of the open-circuit
output and design the circuit to operate fixed at that
point. Still, before finalizing a circuit, it’s a good idea to
measure the output characteristic of the cells you’re using
by putting different load resistors across it and measuring
the voltage. The output power is just the load resistance
multiplied by the square of the voltage across it.
One useful IC for small solar power supplies is the
ICL7660 voltage converter, which rapidly charges a capacitor with the supply, then switches over to charge another,
and then back to the first before it has time to substantially discharge. These two capacitors are wired so that their
voltages can be summed together to give almost double
the input. The device can handle input voltages from 1.5 to
10 volts. Maxim makes higher current versions; the MAX
860 and 660 are able to handle up to around 100 mA.
NUTS & VOLTS
Cell Output (2x
Sunlight at noon on a clear day dumps about 1,300
watts of power per square meter. A 10-cm2 cell, therefore,
intercepts about 13 watts of sunlight, but only a fraction of
this can be converted into electricity. Even the most efficient
(and expensive) multiple-junction cells on satellites convert
only about 20 percent of this into power. For the amorphous