Solar cells for amateurs

BEAM electronics, as it is known, has become quite popular. One of the central ideas is to drive small autonomous robots with solar cells, powered by ambient light.

Sunceram II

Since 1983 Panasonic has been manufacturing solar cells from the Sunceram II series. These cells were very popular for amateur electronic projects until about 2003 when the production of these cells apparently stopped.

The datasheet of the Sunceram II solar cells explicitly mentions that you should not lick these cells. This is understandable since the cells consist of a fragile glass plate covered with layers of sintered cadmium telluride and cadmium sulfide.

The Sunceram II solar cells consist of monolithic series connected cells. Panasonic chose to designate the cells by their physical dimensions in millimeters. Among the popular cells were the ones from the following table which I happen to have in my collection. The BP-242221, BP-243318 and BP-373334 were all originally designed for outdoor use under full sun illumination, which is supported by their behavior in the solar simulator.

	BP-242221	BP-243318	BP-373334
size	24×22 mm2	24×33 mm2	37×33 mm2
thickness	1.2 mm	0.8 mm	0.8 mm
no of cells	6	5	8
cell area	0.6 cm2	1.2 cm2	1.2 cm2
weight	1.7 g	1.6 g	2.4 g
Voc(1)	V	3.9 V	5.9 V
Isc(1)	mA	13.6 mA	12.9 mA
FF(1)	%	53 %	56 %

Amorton

Sanyo has been making solar cell modules of a similar size to the Sunceram series above for quite a while now. Similar to the Sunceram modules also the Amorton modules consist of a glass plate, but here the active layer is made of amorphous silicon. Amorton modules come in two different configurations: one for indoor use at low ambient light levels and one for outdoor use at full sun illumination.

The AM-1602 from my collection is an indoor module — the datasheet explicitly limits the use of these modules to illuminations of below 1000 lux. The measurement curves show the poor behavior of these modules under higher levels of illumination from 200 W/m² to 1200 W/m², caused by the high series resistance. However, at low light levels these modules work quite well.

Solarbotics

With the disappearance of the Sunceram II series of solar cells a need for an alternative source came up. The Canadian company Solarbotics created a series of small solar cell modules of similar (or better) performance to the Sunceram II series. These modules consist of small stripes of single crystalline silicon solar cells, mounted on a printed circuit board and encapsulated with clear epoxy resin.

The Solarbotics module SCC2433 for example is more or less meant to be a direct drop-in replacement for the Sunceram BP-243318 module. Compare the measurement curves and you will see that the Solarbotics modules are significantly better: a higher fill factor, higher voltage, higher current and less series resistance. However, this comes at the cost of about twice the weight as compared to the Sunceram II modules.

The Solarbotics modules are equipped with a printed circuit board on the backside, ready to build a capacitor-charging solar engine. In the inside the small solar cell stripes are series-connected at one edge and the solder joints are clearly visible. Originally the SCC3766 was planned to allow for two module configurations: two half-modules which can be connected in series for a high voltage or in parallel for a high current output by means of solder joints on the pcb. Currently this functionality is not provided by the actual modules since all individual cells are fixed in a series connection.

	SCC2422	SCC2433	SCC3733	SCC3766
size	24×22 mm2	24×33 mm2	37×33 mm2	37×66 mm2
thickness	2.6 mm	2.6 mm	2.6 mm	2.6 mm
no of cells	7	8	12	14
cell area	0.5 cm2	0.6 cm2	0.6 cm2	1.4 cm2
weight	1.6 g	2.7 g	4.3 g	8.5 g
Voc(1)	4.1 V	4.7 V	7.1 V	8.4 V
Isc(1)	16.9 mA	20.7 mA	21.9 mA	43.3 mA
FF(1)	76 %	74 %	74 %	76 %

Conrad

The German electronics distributor Conrad sells compact silicon modules of a similar type as the Solarbotics modules. The difference is the lack of the solar engine on the back of the printed circuit board. Another cosmetic difference is made by hiding the solder joints under a strip of black tape inside the epoxy package.

	YH-39×35
size	39×35 mm2
thickness	3 mm
no of cells	8
cell area	0.9 cm2
weight	4.8 g
Voc(1)	4.7 V
Isc(1)	34 mA
FF(1)	71 %

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(1) measured under standard test conditions of 1000 W/m², 25 °C and AM 1.5