Design and optimization of efficiency in quantum dot intermediate band solar cells

Intermediate band solar cells, IBSCs, with single IB provide better utilization of the solar spectrum than single-junction solar cells. In this study, the new Model-B IBSC was investigated using the Detailed Balance Model and compared with previously used two models and named here as Model-A and Model-C, respectively. For the three IBSC models under non-overlapping absorption coefficients, the maximum efficiencies were found as 63.20% for 1.95 eV and 59.90% for GaAs. With 4 eV overlap, the cell efficiencies for Model-B, Model-C, and Model-A of GaAs IBSCs were calculated as 45.86%, 41.32%, and 35.80% respectively, and Model-B was found to be more efficient than the other two models. The efficiency of IBSC increased linearly with the logarithm of the light concentration. The density of IB states should be at least 5x1017 cm-3 for maximum efficiency in GaAs IBSC. The optimum IB filling changed between 1 and 0 with 0,01 and 100 ratios of electron/holes capture cross-sections. The peak efficiency of Model-B was 35% over Model-A, at large non-equal capture cross-sections under 4 eV of overlapping. In the 1-3.5 eV range of bandgaps, Model-B reduced the negative influence of large overlapping, and the efficiency of Model-B became much higher than Model-A. Results predicted that the materials in the wide range of bandgaps could be good candidates for fabricating IBSCs using Model-B with an efficiency over 50% at 10000X under large overlapping. In this study, the efficiency of InAs/GaAs box-shaped quantum dot intermediate band solar cell (QD-IBSC) has also been investigated using the detailed balance model. The IB energy levels and sub-band absorption coefficients were determined with the single band k.p and four-band k.p methods, by changing the quantum dot size for equal effective mass and effective mass mismatch cases. The sub-bandgap absorption coefficients and the number of IB energy levels increased with the QD size for both equal effective mass and the effective mass mismatch cases. In the detailed balance model, the intermediate band photocurrent density was decreased as the number of IB energy levels increased with the QD size. With the presence of quantum dots, while there was no significant increase in short circuit current densities of QD-IBSC, the open-circuit voltage decreased. As a result, the efficiencies were found as 28.40% for InAs/GaAs QD-IBSC, 52.16% for ideal GaAs IBSC, and 34.72% for conventional GaAs solar cells.