The most important part of a solar charger is the solar cell itself. This overview is meant to provide the general background information on solar technology making you understand more about several types of solar cells available with all their inherent advantages and disadvantages.

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Monocrystalline solar cells (c-Si)

Silicon wafers sliced from a silicon ingot provide a high degree of energy efficiency (20%) at full sun, but little performance in overcast conditions. Monocrystalline solar cells have to face significant performance losses (- 10%) due to intense heat caused by the sun shining onto the cell. They are little (or not) tolerant to shading conditions at all. Still being expensive, they’re not bendable – thus highly fragile. UV-resistant.

Polycrystalline solar cells

Polycrystalline silicon solar cells consist of small silicon crystals being melted together. You can reconize them by a visible grain, which looks like the typical “metal flake effect”. They provide an average degree of energy efficiency (15%) at full sun and a bad performance when the sky is overcast. Great performance losses (15-20%) due to intense heat. They are little or not tolerant to shading conditions, but inexpensive. Not bendable, thus highly fragile and not UV-resistant unless placed under glass. Low-end solar cells with a PET-sealing tend to oxidize and quickly lose their performance.

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Thin film solar cells (CIS, CIGS)

Polymer films which were vapour-coated by copper-indium-gallium-di-selenide or copper-indium-di-sulfide. Average degree of energy efficiency of  12% at full sun, acceptable efficiency under a cover of clouds. Minor performance losses (- 5%) due to intense heat. They can’t be turned off. These cells are very expensive as well as UV-resistant, highly bendable and lightweight.

Dye-sensitized solar cells (Grätzel)

The dye-sensitized solar cell is a kind of thin-film cell based on the photoelectrochemical system and consists of organic dyes printed onto polymer foil. Low efficiency level (10%) at full sun, virtually no performance under a cloudy sky. Minor performance losses due to intense heat, but barely or not at all tolerant to shadings. They are cheap, bendable and lightweight. The greatest disadvantage of dye-sensitized solar cells is their limited lifespan.

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Multijunction solar cells (Ga As)

Multijunction solar cells (Triple Cell / Triple Junction) as used for aerospace. Three solar cells monolithically stacked on top of each other are sensitive to a wide range of a light spectrum (red-blue-green) thus providing the currently highest level of efficiency (30%) at full sun. The industry-leading multijunction solar cells are highly tolerant to shadowing and turn out to be the best performers under a dark sky. They are very expensive, but bendable, lightweight and resistant to UV radiation. It’s not surprising that they virtually have no performance losses (only -3%) due to intense heat.

More information on solar cells: http://en.wikipedia.org/wiki/Solar_cell

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»more information on the Triple Junction solar cell

Electrical efficiency of a solar cell: what does that mean?
The electrical efficiency rate of a solar cell and the resulting wattage or power rate are most commonly derived from standard test conditions.
The measurement is performed with a 25°C warm solar panel and a solar irradiance of 1000 watts per square meter of the panel surface, which corresponds to an efficiency of 100%.
Thus an efficiency of 15% means that the solar panel is capable of converting 15% of the current solar radiation into electric power. In accordance with that, the solar module does have a wattage rating of 150 watt per square meter of its surface.
Please note: in reality the solar panels tend to heat up to much higher temperatures then 25°C and the irradiation of 1000W/m2 is only reached in the sunniest regions of the globe near the equator. That’s why you get the best results by using a solar panel offering the highest level of efficiency AND lowest losses of power due to heating.

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The bottom line is that rigid and brittle solar cells may belong on top of the roof, but definitely not onto a backpack.

While traveling you will hardly ever find optimum conditions, so that it will certainly pay off to invest in flexible state-of-the-art solar chargers.