Losses due to polycrystallinity in thin-film solar cells

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Abstract

The highest efficiency thin-film polycrystalline CuIn1−xGaxSe2 (CIGS) and CdTe solar cells are compared directly with crystalline Si and GaAs cells with similar respective bandgaps. The excess efficiency losses (6.3% for CIGS and 9.9% for CdTe) are quantitatively separated into eight categories. In each case, the impact of polycrystallinity is evaluated, and the differential losses are identified as being directly attributable to polycrystallinity or due to other causes. Approximately, two-thirds of the excess loss for polycrystalline cells is clearly due to polycrystallinity. Thus, strategies such as grain passivation to reduce the impact of polycrystallinity should be examined, but at the same time conventional approaches for incremental improvement should receive attention.

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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