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228 A ComPRehenSIVe GuIDe To SolAR eneRGy SySTemS

FIGURE 10.9 (A) Histogram of V OC values for more than 2400 standard polycrystalline CdTe solar cells fabricated at NREL
in the past decade. (B) Current density–voltage curves for As-doped single crystal CdTe solar cells showing higher open
circuit voltage [40].

on the production of higher quality/purity polycrystalline CdTe layers or even adopting a
different junction design to improve carrier lifetimes. It may ultimately prove though that
polycrystalline material simply cannot match the quality of the single crystal equivalent,
this has certainly proven true for silicon, but there are still a number of fundamental ques-
tions about both the material and device structure that need to be addressed. This is, at the
time of writing, one of the areas the research community is particularly focused on.

10.4 Conclusion

The recent progress of CdTe solar cells should not be taken lightly. having stagnated for
over a decade recent breakthroughs leading to improved device performances have rein-
vigorated the research field leading to a raft of new cell designs and processes. The sudden
emergence of alternative window layers such as mZo, use of absorber bandgap grading
with Se, and breaking of 1 V with single crystal As-doping have all shown that this remains
a technology in development, even at over 40 years old. Despite still being a work in prog-
ress though it has already firmly established itself as the module technology most directly
competitive with silicon. There is also tremendous scope for further module cost reduc-
tion through either performance improvements, process refinement, or simple economies
of scale. As a result CdTe seems to be a technology that will remain at the forefront of PV
research and manufacture for the foreseeable future.

References

[1] Philipps, S. Warmuth, W. photovoltaics report—Fraunhofer ISe; 2016. Available from: https://www.
ise.fraunhofer.de/en/publications/studies/photovoltaics-report.html.
[2] madelung o: Semiconductors: data handbook, Berlin, 2004, Springer.
[3] Green mA, emery K, hishikawa y, Warta W, Dunlop eD, levi Dh, et al: Solar cell efficiency tables (ver-
sion 49), Prog Photovolt Res Appl 25:3–13, 2017.