Fig. 3-8 shows the illustration of the device structure for I-V measurements and the dark and light I-V curves of samples GSRO-ML and [SiO2/SRO]-ML. Both samples reveal the rectification behavior as a diode. However, the better I-V characteristics including a lower turn-on voltage and a higher forward current are obtained in sample GSRO-ML. Inset of Fig. 3-8(b) shows the corresponding light I-V curves under a halogen lamp illumination with ~1 mW/cm2 of power density. The VOC and ISC for sample GSRO-ML are 302 mV and 5.5×10-4 mA, significantly larger than 110 mV and 2.6×10-5 mA for sample [SiO2/SRO]-ML. Hence, the PV properties of the Si QD thin film are efficiently enhanced by using the GSRO-ML structure.
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Fig. 3-8: (a) Illustration of the device structure for I-V measurements and (b) the dark I-V curves of samples GSRO-ML and [SiO2/SRO]-ML. Inset shows the corresponding light I-V curves under a halogen lamp illumination.
To investigate the carrier transport mechanism, the dark forward I-V curves of samples [SiO2/SRO]-ML and GSRO-ML are plotted on a log-log and shown in Fig.
3-9. For sample [SiO2/SRO]-ML, the combination of the direct and phonon-assisted tunneling mechanisms is fitted with the experimental result, which is consistent with the conclusion from V. Osinniy et al. [55]. For sample GSRO-ML, instead of the direct and phonon-assisted tunneling mechanisms, the two-diode mode is more appropriate to describe its forward current [56]. The current increases linearly with the bias in low bias region (region I, V < 0.3 V), exponentially with the bias in the intermediate bias region (region II, V = 0.3~1.5 V), and deviates from exponential behavior in the higher bias region (region III, V > 1.5 V). The clearly distinct conductive regions indicate a corresponding change in the dominating carrier transport mechanism. The two-diode mode had also been observed by S. Park et al. from the heavily P-doped Si QD thin films integrated with B-doped Si wafer [56]. The linear relationship between log I and log V in region I indicates the presence of a parallel current path due to a shunt resistor in parallel to the junction. In region II, a feature of the current exponentially increasing with bias is dominated by the space-charge region recombination mechanism. In region III, the cause of the I-V characteristics deviated
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from the ideal behavior is still uncertain but possibly due to the trapping states distribution. From Fig. 3-8(b), the improved PV properties observed in sample GSRO-ML represent that the space-charge region recombination mechanism is a more suitable carrier transport mechanism for SC application integrating Si QDs owing to the largely enhanced tunneling probability or the mini-band formation resulting from the quite close distribution of Si QDs in sample GSRO-ML [28, 30, 52]. Besides, since the Si QD thin film utilizing the heavily-doped [SiO2/SRO]-ML structure can possess a better carrier transport property than the generally-doped one [21], we believe it is most feasible to achieve even better PV properties by using a heavily-doped GSRO-ML structure.
Fig. 3-9: Dark forward I-V curves in log-log scale for samples (a) [SiO2/SRO]-ML and (b) GSRO-ML.
The temperature-dependent dark I-V curves of samples [SiO2/SRO]-ML and GSRO-ML are also examined and shown as Fig. 3-10 for further confirmation of the carrier transport mechanism. Two kinds of transport mechanisms in sample [SiO2/SRO]-ML are gradually discriminated by increasing temperature, as shown in Fig. 3-10(a). Such temperature-dependent I-V characteristics are resulted from the direct and phonon-assisted tunneling mechanisms [55, 57]. For sample GSRO-ML, the temperature-dependent I-V curves in region II can be well-fitted in a standard diode
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equation, I(V,T)=Is(T)[exp(BV)-1], where Is is the reverse saturation current and parameter B is a coefficient dependent or independent on temperature decided by the dominant carrier transport mechanism [56]. The linearly temperature-dependent B confirms that the carrier transport mechanism is dominated by the space-charge region recombination mechanism [56]. The temperature-dependent ideality factors are larger than 2 due to the trapping defect states or the current crowding effect usually observed in the Si nano-structured thin films [58, 59]. Therefore, from the temperature-dependent I-V measurements, we confirm the different carrier transport mechanisms between samples [SiO2/SRO]-ML and GSRO-ML. The super-high density Si QD thin film achieved using the proposed GSRO-ML structure can truly improve the carrier transport efficiency through the space-charge region recombination mechanism.
Fig. 3-10: Temperature-dependent dark I-V curves of samples (a) [SiO2/SRO]-ML and (b) GSRO-ML.
Inset of (b) shows the parameter B in region II.
3-5 Summary
In chapter 3, we propose a novel deposition structure, GSRO-ML, to realize Si QD thin films with enhanced PV properties. By using a GSRO-ML structure, the super-high density Si QD thin film with good QD size control is demonstrated.
Compared with a [SiO2/SRO]-ML structure, sample GSRO-ML has improved carrier
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transport efficiency and larger optical absorption coefficient resulted from the formation of QDs with significantly higher density. The over 10 times better optical absorption means the required film thickness for SC application can be greatly reduced. As a result, considerable enhancements on electro-optical properties including the rectification, ISC, and VOC are obtained. Besides, instead of the combination of the direct and phonon-assisted tunneling mechanisms as observed in sample [SiO2/SRO]-ML, the two-diode mode is found in sample GSRO-ML even though only a generally-doped concentration is used. Therefore, the high-efficiency Si-based SCs integrating Si QDs can be most definitely expected using this GSRO-ML structure.
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