Conclusions

From the analysis of this book, it appears that there are sufficient options for improving the performance of solar photovoltaic cells beyond the single junction limits, that greatly improved performance, at some stage in the future, is very likely.

The tandem cell approach of Chap. 5 already demonstrates that such enhanced performance is feasible. Cell technologies right at the top in terms of single junction cell performance and right at the bottom have already benefited from the tandem approach. Experimental gains to date have been in the 20-25% range, relative to single junction devices, compared to theoretically achievable boosts of over 100%. For tandems involving a large number of cells, a generic approach to tandem cell design would be desirable. An example is shown in Fig. 10.1, where bandgaps within a single materials system are controlled by varying superlattice spacings is an example of such an approach (Fig. 10.1).

The hot carrier cells of Chap. 6 offer performance potential very similar to the case of an infinite number of tandem cells, but may have major operational advantages, as well as not requiring as much effort to fabricate. One major challenge is to develop suitable technology for forming energy selective contacts. Energy selective tunnelling, such as in resonant tunnelling devices, might provide a promising approach here. Narrow conductive bands within a material with a large forbidden gap, as devised for some of the other conversion options, would be another option as would conduction to valence band tunneling used in tunnel diodes (Conibeer et al 2002). The second major challenge is to find a way of maintaining hot carrier concentrators by accelerating radiative processes relative to energy relaxation.

image83

Fig. 10.1 : Generic tandem cell design based on superlattices.

Without an as yet unidentified conceptual breakthrough, performance enhancement by creating multiple electron hole pairs per photon (Chap. 7) does not seem promising. All suggested processes so far are too weak and inefficient to offer much scope. Some new concepts are required here.

The multiple energy threshold approaches of Chap. 8 would seek to offer exciting prospects due to likely improvements in the materials engineering area. Three dimensional control of material structure seems particularly relevant to concepts such as the multiband approach. Such control is likely to improve dramatically over the coming two decades. Photon up – and down-conversion without electrical contact to the elements performing this function also appears to have considerable potential.

Thermophotovoltaics (Chap. 9) is already a very active area of research. Improved energy selectivity in emission appears to be a key requirement here. The main advantage of thermophotonics appears to be larger feasible energy transfers at relative low operating temperatures. This may make the approach very well suited for the conversion of low grade heat at efficiencies close to the Carnot limit. However, considerable improvement in the radiative efficiency of both inexpensive light emitters and solar cells is required before this could become a practical option.

With the accelerating pace of materials science development, many of the ideas outlined in the preceding chapters that now appear highly speculative are likely to become implementable experimentally. By exploring desirable directions for further development, it is hoped that this treatise can provide a focus for this development and contribute to photovoltaics becoming an inexpensive, large-scale source of clean, high-grade energy worldwide.

Greek Alphabet

Letter

Lowercase

Uppercase

Letter

Lowercase

Uppercase

Alpha

a

А

Nu

V

N

Beta

в

В

Xi

z

£

Gamma

Y

Г

Omicron

0

О

Delta

8

А

Pi

П

П

Epsilon

£

Е

Rho

p

P

Zeta

%

Z

Sigma

о

X,

Eta

n

н

Tau

T

T

Theta

в

0

Upsilon

и

Y

Iota

1

I

Phi

Ф

Ф

Kappa

К

К

Chi

X

X

Lambda

я

Л

Psi

V

¥

Mu

в

м

Omega

&

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