![]() On the other hand, our QD cell with thinner SLs exhibits a steeper rise with n close to 2, as seen in Figure 3. In Figure 3, we see this n ∼ 1 situation also applied to our QD cell with thicker SLs and non-QD cell with only WLs. where V OC’s are dominated by diffusion current. Sanfacon, in Proceedings of the 22nd IEEE Photovoltaic Specialists Conference, Las Vegas, NV ( IEEE, Piscataway, NJ, 1991), pp. Typical GaAs solar cells with relatively low densities of crystalline defects have diode-ideality factors n equal to or slightly larger than unity, 11,12 11. In this present work, we used a calibrated AM1.5 G solar simulator for light I-V measurements.įigure 3 shows the dependence of V OC on the illumination intensity in suns for the high-efficiency QD cell (5-layer 1.0 μm InAs QDs, 11-nm-thick spacer layers (SLs)) described above, a QD cell with thicker SLs (5-layer 1.1 μm QDs, 40 nm SLs), and a cell with only wetting layers (WLs) (no QD, 5-layer WLs, 11 nm SLs). were slightly different from those under the standard 1-sun solar spectrum. Because of the indirect calibration, the light current-voltage ( I-V) characteristics of the QD cells measured for Ref. , we had no standard solar-simulator light source and, therefore, instead used a halogen lamp with ∼1 sun intensity calibrated indirectly with a reference GaAs cell whose photocurrent value had been known under AM1.5 G, 1 sun illumination. These thicknesses were chosen through one-dimensional electromagnetic calculations for reflectivities, shown in Figure 1, based on the Rigorous Coupled Wave Analysis. In this work, we have applied an antireflection coating with MgF 2/ZnS on the front surface of the cells with an optimized set of layer thicknesses (100-nm MgF 2/50-nm ZnS). This QD cell corresponds to QDSC-3 in Ref. The room-temperature photoluminescence spectrum from the QD cell exhibits a peak associated with the ground-state emission of the InAs QDs at 1.0 μm with a full-width at half-maximum of 65 meV. The nonmetalized part of the top p +-GaAs contact layer was removed at room temperature by selective chemical etching with 50% citric acid/H 2O 2 (4:1 vol./vol.). A part of the front and the entire back surfaces were metalized with Au/AuGeNi and Au/Cr, respectively, by electron-beam evaporation. Our solar cell devices are 4 × 4 mm 2 area cleaved pieces. The cell structure was grown on an n-GaAs (100) substrate by MOCVD. The InAs/GaAs QD solar cell we fabricated has a p-i-n GaAs structure with a 300-nm-thick i-GaAs layer embedding five layers of self-assembled InAs QDs with a density of 4 × 10 10 cm −2 per layer. For this reason, most reported efficiencies of QD solar cells have been significantly lower than those of the best single-junction cells without QD. However, so far most experimental studies for such solid-state QD cells unfortunately have resulted in lower efficiencies by incorporation of QDs presumably mainly due to QD interfacial carrier recombination, which severely reduces open-circuit voltage ( V OC). those using III-V semiconductor compound InAs/GaAs QDs (i.e., InAs QDs embedded in GaAs matrices) have exhibited the highest efficiencies and robustness. Among the wide range of semiconductor materials for QD solar cells currently under intensive study, 4–6 4. Semiconductor QD solar cells are theoretically predicted to achieve ultrahigh-efficiency solar-energy conversion in single p-n junction structures by utilizing intermediate-level energy bands.
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