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Low sheet resistance (high mobility) with high transmittance in all wavelength is required for front TCO. High haze value is also required for effective light trapping. For this purpose, we have combined F-doped SnO2 (FTO) with high mobility deposited by LPCVD and reactive ion etching (RIE) processed glass substrate. However, two problems have been found. (1) The mobility of FTO on RIE substrate dropped from that on flat glass (75 to 36 cm2/Vs). To avoid this drop, thicker film is needed. (2) To keep high transmittance with thicker film, lower carrier concentration is needed. But the mobility dropped with lower carrier concentration. In order to solve these constrains, we have adopted a stacked structure using thick non-doped layer of 2700 nm and thin F-doped layer of 500 nm. With this novel approach, we have successfully achieved the high mobility (80 cm2/Vs), low carrier concentration (2.2x1019 /cm3) and high haze value (77% at wavelength of 1000 nm) at the same time. This new developed high-haze SnO2 is a new promising TCO for thin-film Si solar cells.
We report for the first time the a-Si:H/μc-Si:H/μc-Si:H triple-junction solar cells fabricated on W-textured ZnO having a very high haze value which can improve light scattering effect. For further enhancement of light confinement effects, p-type a-SiOx:H and μc-SiOx:H as wide-gap window-layers, n-type μc-SiOx:H as intermediate layers and a back reflector were employed in these solar cells too. From theoretical analysis, we have found an advantage of a-Si:H/μc-Si:H/μc-Si:H structure for an application to low-concentration photovoltaics. For the fabricated solar cells, a conversion efficiency of 8.86% at 1 sun and and 9.86% under 7.2 suns, and a total photocurrent from each subcell of 24.1 mA/cm2 were achieved although there was still a current mismatch among component subcells.
Thin-film silicon solar cells have been attracted a lot of intention as low-cost solar cells. One of the most important technologies for improving their performances is light trapping. We have demonstrated the high potential of double-textured zinc oxide (ZnO) thin films used as front transparent conductive oxide (TCO) films due to further enhancement of their light-trapping effects. Although the laser scribing method has already been well established for low-cost thin-film silicon solar cell module manufacturing, laser scribing technique on double-textured ZnO is new and still a challenging issue. In this study, we firstly demonstrated the availability of laser scribing for amorphous silicon (a-Si) solar cells fabricated on double-textured ZnO substrates. It is general to utilize lasers with wavelength of 1.06 μm and 532 nm for scribing of TCO and silicon layer, respectively. Here we attempted to scribe both of TCO and silicon layers using a 532 nm wavelength laser (green laser) for process simplifying.
The potential of chemically derived graphene as a solution-processable transparent conductive film has been explored. Synthesis of amine-functionalized graphene oxide was intended for its utilization in layer-by-layer assembly. Layer-by-layer assembly of graphene oxide was utilized to fabricate graphene based thin film in a scalable and highly reproducible way. It was found that optical transmittance and sheet resistance of the film decreases with an increase in number of LBL cycles in a reproducible way. The sheet resistance of LBL-assembled GO film improves by an order of magnitude at the same optical transparency due to more homogeneous coverage and better stacking of graphene flakes. Furthermore, we demonstrated the potential for a large-scale deposition of chemically derived graphene.
The electrical characteristics of silicon nanowire (SiNW) solar cells with p-type hydrogenated amorphous silicon oxide (Eg=1.9 eV)/n-type SiNWs embedded in SiO2/n-type hydrogenated amorphous silicon (Eg=1.7 eV) structure have been investigated using a two-dimensional device simulator with taking the quantum size effects into account. The average bandgap of a SiNW embedded in SiO2 increased from 1.15 eV to 2.71 eV with decreasing the diameter from 10 nm to 1 nm due to the quantum size effect. It should be noted that under the sunlight with AM1.5G the open-circuit voltage (Voc) of SiNW solar cells also increased to 1.54 V with decreasing the diameter of the SiNWs to 1 nm. This result suggests that it is possible to enhance the Voc by the quantum size effect and a SiNW is a promising material for the all silicon tandem solar cells.
P-type hydrogenated nanocrystalline cubic silicon carbide is a promising material for the emitter of n-type crystalline silicon heterojunction solar cell due to its lower light absorption and wider bandgap of 2.2 eV. The electrical properties of hydrogenated nanocrystalline cubic silicon carbide can be influenced by its crystallinity. In this study, we propose the use of conductive atomic force microscopy (Conductive-AFM) to evaluate the crystalline volume fraction (fc) of p-nc-3C-SiC:H thin films (20∼30 nm) as a new method instead of Raman scattering spectroscopy, X-ray diffraction, and spectroscopic ellipsometry.
Optical, electrical and structural properties of silicon films depending on hydrogen flow rate (RH), substrate temperature (TS), and deposition pressure (PD) were investigated. By decreasing RH and increasing TS and PD, the optical band gap (Eopt) of silicon thin films drastically declined from 1.8 to 1.63 eV without a big deterioration in electrical properties. We employed all the investigated Si thin films for p-i-n structured solar cells as absorbers with i-layer thickness of 300 nm. From the measurement of solar cell performances, it was clearly observed that spectral response in long wavelength was enhanced as Eopt of absorber layers decreased. Using the solar cell whose Eopt of i-layer was 1.65 eV, the highest QE at long wavelength with the short circuit current density (Jsc) of 16.34 mA/cm2 was achieved, and open circuit voltage (Voc), fill factor (FF), and conversion efficiency (η) were 0.66 V, 0.57, and 6.13%, respectively.
We prepared fine Cu(In,Ga)Se2 (CIGS) powder suitable for screen printing using a mechanochemical synthesis and wet bead milling. Particulate precursors were deposited in a layer by a screen-printing technique, and the porous precursor layer was sintered into a dense polycrystalline film by atmospheric-pressure firing in an N2 gas atmosphere. The microstructure of CIGS powder and fired CIGS film were observed in an SEM. The wet bead milling was effective for the reduction and homogenization of the average grain size of CIGS powder. The CIGS grains in the film were well sintered and the size of CIGS grains was as large as about 2 μm. The CIGS solar cell showed an efficiency of 3.1%, with Voc of 0.279 V, Jsc of 28.8 mA/cm2 and FF of 0.386.
We prepared size-controlled silicon quantum dots superlattices (Si-QDSLs) by thermal annealing of stoichiometric hydrogenated amorphous silicon carbide (a-SiC:H)/silicon rich hydrogenated amorphous silicon carbide (a-Si1+xC:H) multilayers for thin-film solar cell applications. Transmission electron microscope (TEM) observation revealed that the size of silicon quantum dots can be controlled by the thickness of the a-Si1+xC:H layers. It was found that hydrogen plasma treatment (HPT) significantly enhanced the photoluminescence (PL) of the Si-QDSLs. From the results of the PL measurement, the bandgap of the Si-QDSLs can be controlled from 1.1 eV to 1.6 eV by varying the diameter of silicon quantum dots. ESR measurement indicated that HPT reduced the defect density in a Si-QDSL from 1.83 ×1019 to 1.67 sup1018 cm-3.
The temperature dependence of Si-based thin-film single junction solar cells on the phase of the intrinsic absorber is investigated in order to find the optimal absorber at high operating temperatures. For comparison, hydrogenated amorphous, protocrystalline, and microcrystalline silicon solar cells are fabricated by plasma-enhanced chemical vapor deposition and hot-wired CVD techniques. Photo J-V characteristics are measured using a solar simulator at the ambient temperature range of 25-85°C. It is found that the cells with a higher open-circuit voltage usually show lower temperature-dependent behaviors; the protocrystalline silicon solar cells provide the lowest temperature coefficient of efficiency, while the microcrystalline silicon solar cells are highly sensitive to the temperature. Therefore, protocrystalline silicon solar cells are promising for use in high temperature regions.
Effects of boron doping on microcrystalline germanium carbon alloy (μc-Ge1-xCx:H) thin films have been investigated. We deposited boron-doped p-type μc-Ge1-xCx:H thin films by hot-wire chemical vapor deposition technique using hydrogen diluted monomethylgermane (MMG) and diborane (B2H6). A dark conductivity of 1.3 S/cm and carrier concentration of 1.7 x 1020 cm-3 were achieved with B2H6/MMG ratio of 0.1. Furthermore, the activation energy decreased from 0.37 to 0.037 eV with increasing B2H6/MMG ratio from 0 to 0.1. We also fabricated p-type μc-Ge1-xCx:H/n-type c-Si heterojunction diodes. The diodes showed rectifying characteristics. The typical ideality factor and rectifying ratio were 1.4 and 3.7 x 103 at ¡Ó 0.5 V, respectively.
We deposited a-SiCN:H films by HWCVD using a gas mixture of hexamethyldisilazane, H2 and N2, and fabricated cast polycrystalline silicon solar cells with the a-SiCN:H passivation and anti-reflection layer. N2 addition led to the reduction of the refractive index of the a-SiCN:H films due to the increase in nitrogen concentration of the films. This improved performance of the antireflection layer. The advantage of adding N2 to the process was demonstrated by the improvement in short circuit current (JSC) and efficiency of cast polycrystalline silicon solar cells. At present, the efficiency of cast polycrystalline silicon solar cell using a-SiCN:H film as a passivation layer reached 14.2%.
We have investigated properties of nanocrystalline hydrogenated cubic silicon carbide (nc-3C-SiC:H) and silicon carbide: germanium alloy (nc-SiC:Ge:H) films deposited by hot-wire chemical vapor deposition (HWCVD) at low temperatures of about 300°C. we found that the density of charged defects was strongly influenced by grain size of the films. In-situ doping into nc-3C-SiC:H films was also carried out. N-type nc-3C-SiC:H films were successfully deposited by using phosphine (PH3) and hexamethyldisilazane (HMDS) as dopants. We found that HMDS is an effective n-type dopant for low temperature deposition of nc-3C-SiC:H films by HWCVD. For the deposition of p-type nc-3C-SiC:H with trimethylaluminum (TMA), it was found that the substrate temperature of above 300°C is required to activate the acceptors. We added dimethylgermane (DMG) into mixture of MMS and H2 to prepare nc-SiC:Ge:H films. The nc-SiC:Ge:H films with Ge mole fraction of 1.9% were successfully deposited.
We proposed a new carbon source, 1,3-disilabutane (H3Si-CH2-SiH2-CH3:1,3-DSB), to grow hydrogenated amorphous silicon carbide (a-SiC:H) films by mercury-sensitized photochemical vapor deposition (photo-CVD). We described preliminary results of undoped and p-type a-SiC films deposited using 1,3-DSB. It was found that the optical energy gap of the films was changed even at very small 1,3-DSB/silane ratios of few percents. P-type doping was carried out by using diborane and we obtained the films with a darkconductivity of 1.3x10-4 S/cm at the optical bandgap of 2.1 eV. In addition, we applied this material for a p-layer of a p-i-n type a-Si based solar cell and we have achieved relatively high conversion efficiency of 9.55%.
The light-soaking effect in ZnO/ Cu(InGa)Se2 (CIGS) based solar cells has been studied. A CIGS thin film with Cu(InGa)(SeS)2 surface layer was obtained by selenization (H2Se)/sulfurization (H2S). A high resistively ZnO buffer layer deposited by the atomic layer deposition technique was used as a buffer layer. We found that the light-soaking effect mainly correlates with the properties of the CIGS surface, rather than with the properties of the ZnO buffer/window layer. This phenomenon can be eliminated by surface etching or doping CIGS surface with Zinc. Zinc diffusion using diethylzinc gas has been proposed in this work. To date, we have achieved efficiency of 13.9% (Voc: 560 mV, Jsc: 35.0 mA/cm2, FF: 0.71) without light soaking effect.
The growth mechanism of Si film at low temperature on Si(100) by photo-CVD was theoretically analyzed by using reaction models both in the gas phase and on the growing surface. We introduced three surface reactions; the growth of Si from SiH3 radicals, the dangling bond termination by atomic hydrogen and the abstraction of bonding hydrogen by SiH3 radicals. We assumed that the film structure is determined by the hydrogen surface coverage ratio “ø” and the parameters of the surface reaction model were determined from the experimental results. The theoretical analysis explained well the experimental data on the growth rate.
The Raman scattering from LO phonon–plasmon coupled (LOPC) mode in heavily carbon doped p–type InxGa1–xAs grown by metalorganic molecular beam epitaxy (MOMBE) was studied experimentally. Only one LOPC mode appears between the GaAs–like and InAs–like LO modes was observed. The peak position of the LOPC mode is near the GaAs–like TO mode frequency, and is not sensitive to the hole concentration. The intensity of the mode increases with increasing the carrier concentration while the two LO modes decrease and become unvisible under the higher doping level. The hole concentration dependence of the linewidth and intensity of the LOPC mode is very similar to that in p–type GaAs. It was shown that the plasmon damping effect plays a dominant role in the p–type doping case.
Triisobutylaluminum (TIBA) was used as an aluminum source for metalorganic molecular beam epitaxy (MOMBE). The optical absorption coefficient for TIBA was found to be larger than both tri ethyl aluminum and triethylgallium. TIBA was introduced into a laser-induced MOMBE system and selective deposition of Al and AlAs was carried out. Al metal was deposited on the area where the ArF excimer laser was irradiated and no deposition was observed without the excimer laser irradiation at a substrate temperature of 350 C. Furthermore, a laser large enhancement of the growth rate of AlAs was observed at 350 C.