Losses due to polycrystallinity in thin-film solar cells
Section snippets
Crystalline and polycrystalline solar cells
Crystalline solar cells, particularly those made of Si and GaAs, have achieved conversion efficiencies approaching the theoretical limits for their respective bandgaps when operated at room temperature and under a standard intensity solar spectrum. Furthermore, the small remaining losses are reasonably well-understood, and additional work would appear to offer only modest additional improvement. Polycrystalline cells, in contrast, cover a broad range of physical structures, are still far from
Excess polycrystalline cell losses
The first group of individual losses to be evaluated are those due to decrease in photocurrent, or the differences in the y-axis intercepts in Fig. 2. The separation of photocurrent losses can be quite quantitative when measurements are available for grid coverage, optical reflection, absorption of window layers, and quantum efficiency of the cell [7]. Bar graphs showing the photocurrent loss differences are given in Fig. 3. These losses are:
- 1.
Grid coverage. Superstrate CdTe cells do not commonly
Discussion
Table 2 shows that there is not a clearly dominant loss for CIGS and CdTe solar cells. Fig. 6 shows a graphical comparison with the best crystalline cells, and breaks the differences down into photocurrent, open-circuit voltage, and fill-factor effects. The shaded part, about two thirds of the total difference, represents the losses directly due to polycrystallinity. Thus, there is a need for strategies to further reduce the grain impact of Fig. 1b. Such strategies might include deliberate
Acknowledgements
This work was supported by the U.S. National Renewable Energy Laboratory. The authors thank Mr. Gunther Stollwerck, Drs. Xiaoxiang Liu and Ingrid Eisgruber for the assistance with the earlier phases of this project.
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