Practical Consequences

In Chap.2of this book, various curves of Spectral Irradiance were presented (with the units [W/m²/nm]) and for different conditions: (a) different times of the day (one noted there, that in the mornings and in the evenings, there was comparatively more red light coming in than at noon); (b) for different weather conditions (one noted there that if there was snow and sun, there was comparatively more red light coming in than if one had a cloudy day); (c) for indoor lighting with various lamps (one noted there that for most LED lamps, there was comparatively more blue light coming in than if one had direct sunlight).

Thus, if the ambient conditions to which the solar cell or module is exposed changes, not only the intensity of the light is modified—even drastically reduced in

70 A. Shah

Fig. 3.28 Typical spectral response curves for solar cells: red: amorphous silicon; green: CdTe;

blue: crystalline silicon. Courtesy of Mathieu Boccard, PV-Lab, IMT Neuchâtel, EPFL

the case of indoor lighting—this was treated in Sect.3.5.3above—but thespectrum of the incoming light is also modified: This effect will be treated here.

The correct tool to be used is the spectral response curve of the cell or module, but the EQE curve can also be used, because it contains the same information.

Figure 3.28 shows Spectral Response curves for three different types of solar cells: red: a typical amorphous silicon solar cell; green: a typical CdTe solar cell;

blue a typical crystalline silicon solar cell.

From Fig.3.28one notes that amorphous silicon solar cells are spectrally well adapted for use in Indoor Light Conditions, where one finds, especially for certain types of LEDs (see Fig.2.8in Chap. 2 and look at the curve for “cool white LED”) a preponderance of light in the range of wavelengths, between 400 and 600 nm. On the other hand, CdTe and c-Si solar cells and modules are spectrally well adapted for use in Outdoor Light Conditions.

To conclude this section, we can state that Spectral Response curves are generally used when evaluating the performance of modules in practical situations, in the

“field”, whereas EQE and IQE curves are the proper tools for analysing what is happening inside a given solar cell—they are mainly used for Research on Solar Cells.

To evaluate the efficiency of a solar cell/module, we have to consider both the spectral response curve SR (λ) of the solar cell, as well as the spectrum of the incoming light (see Chap.2).

1. In outdoor applications, we usually have, on a sunny day, an AM 1.5-like solar spectrum at noon, whereas, we have a red shift of the spectrum (towards what is called an AM 3- or AM 4-like spectrum), during morning and evening. Snow and the vicinity of large water surfaces (lakes, large rivers, the sea) shift, on the other hand, the spectrum towards the blue. The latter (blue shift of incoming light) is also true for cloudy weather. This explains why crystalline silicon solar modules

3 Solar Cells: Basics 71 have a relative advantage during mornings and evenings on sunny days, whereas CdTe modules have a relative advantage on cloudy days and in the vicinity of snow and large water surfaces.

2. In indoor applications, we often have, especially if fluorescent bulbs or light emitting diodes (LEDs) are used to illuminate the room, a spectrum with a larger blue content than the solar spectrum. This effect, together with the variation of efficiency with illumination level, explained in Sect.3.5.3, underlines the decisive advantage of amorphous silicon solar cells for indoor applications.

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opportu-nities, inProceedings of the 12th Workshop on Quantum Solar Energy Conversion (QUANTSOL 2000)

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Arvind Shahis the Founder of the Photovoltaics Research Lab-oratory (PV-Lab), at the Institute of Microtechnology (IMT), in Neuchâtel, Switzerland.

PV Lab Neuchâtel has done pioneering work in the estab-lishment of low-cost production methods for solar cells based on silicon: It introduced a novel plasma-assisted deposition method called “VHF deposition” permitting a significant increase in the deposition rate for thin-film silicon layers. It also introduced microcrystalline silicon, deposited by VHF plasma, and with very low oxygen content, as novel absorber layer, within thin-film solar cells.

From 1979 to 2005, Arvind was Professor at the University of Neuchâtel. From 1987 to 2005, he was additionally part-time professor at the EPFL Lausanne.

In 1975 he founded and co-directed the Centre for Elec-tronics Design and Technology (CEDT) at the Indian Institute of Science in Bangalore. CEDT is now one of India’s leading University Centres in the field of Electronics.

Since 2006, Arvind has been active as scientific consultant to the PV Lab and to various Industries, in Europe, India and the USA.

Arvind received the Swiss Solar Prize, together with Johannes Meier in 2005. He received the Becquerel Award in 2007.

Chapter 4

Solar Cells: Optical and Recombination