1-2.1 Energy Loss Mechanisms
So far, the performances of SCs with single-bandgap are mainly limited by five kinds of energy loss mechanisms, including the over-high photon energy loss, junction loss, recombination loss, metal-semiconductor contact loss, and over-low photon energy loss [5], as shown in Fig. 1-5. The difference of photon absorption between SCs with single- and multi-bandgap is also shown in Fig. 1-6 [5]. Better than with single-bandgap, the SCs with multi-bandgap can efficiently improve the over-high and over-low photon energy losses and significantly enhance cells’
performances. As the solar irradiation energies are widely distributed in a large wavelength range, shown as Fig. 1-7 [6], the mismatched photon energy loss will largely degrade the performances of SCs with single-bandgap. Hence, the SCs with multi-bandgap are more potential for the third-generation SCs development than that with single-bandgap.
5
Fig. 1-5: Main energy loss mechanisms in SCs with single bandgap, including the (1) over-high photon energy loss, (2) junction loss, (3) recombination loss, (4) metal-semiconductor contact loss, and (5) over-low photon energy loss.
Fig. 1-6: Difference of photon absorption between SCs with (a) single- and (b)multi-bandgap.
Fig. 1-7: Solar irradiance spectrum above atmosphere and at surface. [6]
6
1-2.2 Advantages of Using Si QD
In order to efficiently reduce the mismatched photon energy loss in the Si-based SCs, the Si QD thin films have been extensively studied owing to their many unique characteristics such as highly-tunable bandgap and better optical properties [7-9]. The Si-based dielectric materials, such as SiO2 and Si3N4, are commonly used as the matrix materials due to their simple process, high transparency, and significantly larger bandgap than c-Si material [10, 11]. Fig. 1-8 shows the band diagrams of Si QDs embedded in a wide bandgap matrix material under different QD sizes.
According to quantum confinement (QC) effect, the allowed energy levels in the valance and conduction bands will be discrete, and such results can make the effective bandgap of Si QDs (Eg,QD) being larger than that of c-Si (Eg,c-Si) material. So far, it had been demonstrated that the Eg,QD can be largely tuned from near c-Si (1.1 eV) to obviously higher than a-Si (1.7 eV) materials [12, 13]. Hence, Si QDs have great potentials on bandgap engineering for the mismatched photon energy loss improvements in the Si-based SCs.
Fig. 1-8: Illustration of band diagrams for Si QDs embedded in a wide bandgap matrix material under different QD sizes.
7
Fig. 1-9 shows an illustration of a possible design for an all Si-based tandem SC integrating Si QDs with different QD sizes. By integrating various QD sizes, the mismatched photon energy loss can be efficiently reduced, and the cell’s performance can also be significantly enhanced. Therefore, the third generation Si-based SCs with high efficiency and low cost can be highly expected by integrating Si QDs.
Fig. 1-9: Illustration of a possible design for an all Si-based tandem SC integrating Si QDs with different QD sizes.
1-2.3 Literatures Review
In 2006, M. A. Green at al. firstly proposes the idea of integrating Si QDs with Si-based SCs for the third generation SCs [10, 14]. They fabricate the Si QD thin films by radio-frequency (RF) co-sputtering and post-annealing methods, shown as Fig. 1-10. The Si QD embedded SiO2 thin films are deposited by a [silicon dioxide/silicon-rich oxide] multilayer ([SiO2/SRO]-ML) structure. During annealing, the excess Si atoms separate out and crystallize in the SRO layers, and Si QDs are formed and confined in the SRO layers. From transmission electron microscope
8
(TEM) images, shown as Fig. 1-11, the nano-crystalline Si QDs are clearly observed in the Si-rich nitride (SRN) layers [13]. Based on photoluminescence (PL) spectra analysis, the Eg,QD can be obtained. The measured PL peak energies from M. A.
Green’s and others’ results as a function of QD size are shown in Fig. 1-12 [13]. It clearly indicates the Eg,QD can be modified even larger than that of amorphous Si material (~1.7 eV) by tuning the QD size. Hence, it demonstrates that the bandgap engineering is really a feasible idea by utilizing the Si QDs.
Fig. 1-10: Illustration of the fabrication process for the Si QD thin films using a [SiO2/SRO]-ML structure. [13]
Fig. 1-11: (a) Low and (b) high magnification TEM images of the Si QD thin films using a Si3N4 matrix material. [13]
9
Fig. 1-12: Bandgap energy versus QD size using SiO2 and Si3N4 matrix materials from different groups’
works. [13]
In 2008, M. A. Green et al. also experimentally demonstrates the feasibility of SCs integrating Si QDs [15, 16]. They fabricate the n-type Si QDs embedded SiO2 thin films on p-type c-Si wafer, as shown in Fig. 1-13. The n-type Si QD thin film is consisted of 15 or 25 bilayers of SiO2 and P-doped (0.23 at. %) Si QDs with a SiO2 thickness of 1 or 2 nm. The nominal diameters of the Si QDs are 3, 4, 5 and 8 nm. The SC with 3 nm of Si QDs has the highest efficiency, with open-circuit voltage (Voc) of 556 mV, short-circuit current (Jsc) of 29.8 mA/cm2, fill factor (F. F.) of 63.8%, and conversion efficiency of 10.6%. The internal quantum efficiency (IQE) of the fabricated SCs and corresponding absorption spectra of the Si QD thin films are shown in Fig. 1-14, the well-matched relation is clearly observed. The thicker the SRO layer, the larger the QDs’ size, which results in a smaller effective bandgap.
These results indicate that the conversion efficiency of the fabricated hetero-junction SCs is indeed enhanced by integrating with Si QDs.
10
Fig. 1-13: Illustration of a n-type Si QDs/p-type c-Si SC. [16]
Fig. 1-14: Internal quantum efficiency (IQE) and corresponding absorption spectra of the fabricated n-type Si QDs/p-type c-Si SCs. [16]
Since a large difference of the conversion efficiency between the experimental result and the theoretical calculation result is observed [17], in 2009, M. A. Green et al. publish more results focusing on the Si QD uniformity, interfacial defects, built-in electrical field, etc, in order to find out the roots for this discrepancy and try to optimize this structure for higher conversion efficiency [18-20]. More results for this development are also reported by different groups after 2010. For example, S. H.
Hong et al. studied the size-dependent photovoltaic (PV) properties and developed the
11
higher efficiency SCs integrating Si QDs by optimizing the B-doped concentration [21]. However, the results reported by M. A. Green et al. for the all Si QD thin film
SCs indicate that the contributions of photo-generated carriers from QDs are apparently limited because of the naturally high-resistance property of the Si-based dielectric matrix materials [22, 23]. Therefore, more advanced and detailed studies are still necessary to solve the various issues involved using the Si-based dielectric matrices.