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We investigated the effect of the B2H6 plasma treatment on p-type hydrogenated amorphous silicon (p-a-Si:H) surfaces for high-performance silicon heterojunction (SHJ) solar cells. Secondary ion mass spectroscopy measurements revealed that the boron concentration at the p-a-Si:H surface is increased by employing the B2H6 plasma treatment. Furthermore, specific contact resistance is decreased by about one-third after the B2H6 plasma treatment. No degradation of passivation performance is induced by the B2H6 plasma treatment. The power conversion efficiency of the SHJ solar cells with the B2H6 plasma treatment is improved by the increase in fill factor (FF) due to decreased series resistance and increased shunt resistance. From numerical simulations, the upward band bending is enhanced at the heterointerface between transparent conductive oxide (TCO) and p-a-Si:H by the B2H6 plasma treatment, which is responsible for the improved FF owing to facilitated tunneling holes from c-Si to p-a-Si:H layers and the TCO/p-a-Si:H heterointerface.

The development of photovoltaics (PV)-powered vehicles are expected to contribute to reduce CO2 emission of vehicles and create a clean energy society. This paper presents the impact of high-efficiency solar cell modules on reduction in CO2 emission, charging cost reduction for electric vehicles, and reducing storage capacity of PV-powered electric vehicles. In this paper, the effects of solar cell module efficiency upon driving distance of PV-powered vehicles are also shown. Especially, the potential of Si tandem solar cells for PV-powered vehicle applications is discussed. This paper presents that the III-V/Si 3-junction solar cell modules with an efficiency of more than 37% have the potential of longer driving distance of 30 km/day average and more than 50 km/day on a clear day compared to an average 16 km/day driving by vehicles powered by 20% efficiency Si solar cell modules.

A mechanism of potential induced degradation (PID) delay effect by ultraviolet (UV) light irradiation during PID test for p-type crystalline silicon (c-Si) solar cells was proposed in this work. The degradation rate of the solar cell performances is slowed down by the UV light irradiation in the 300–390 nm wavelength range during PID test. The conductivity increase of the silicon nitride (SiNx) anti-reflection coating (ARC) layer on the solar cell surface under UV light irradiation during PID tests, which relates to the mechanism preventing the penetration of sodium ions into the active cell layer, induces the PID delay effect for the p-type c-Si solar cells. The PID delay effect was also analyzed by a microwave photo-conductance decay (μ-PCD) technique in this work. The reduction behavior of the components in the μ-PCD signal curves including rapid (τ1) and slow (τ2) decay time constants and the effective lifetime (τeff) presents a good correlation with the performance degradation behavior of the solar cells over PID test duration. Moreover, the reduction rate of these components is also slowed down under the UV light irradiation in the 300–390 nm wavelength range during PID tests. Notably, their reduction behavior was compatible with the mechanism of the conductivity increase of the SiNx ARC layer under UV light irradiation.

The cost of solar cell production can be reduced by wafer thinning. A thinner wafer provides flexibility, and crystalline silicon solar cells are promising as flexible solar cells due to their flexibility. However, as wafers become thinner, production yield decreases due to wafer breakage caused by sawing damage; thus, to further reduce wafer thickness, it is necessary to suppress sawing damage. We investigated the flexibility of wafers under various slice conditions by conducting biaxial bending tests and clarified the dominant factor causing sawing damage to further reduce the wafer thickness for crystalline silicon solar cells. The results of damage observation by scanning electron microscopy and evaluation of the crystal structure by Raman spectroscopy confirm that the damage structure changes significantly depending on wire specifications. The results from the biaxial bending tests indicate that the three-dimensional flexibility of a wafer is determined by wire specifications.

Sodium (Na) and lithium (Li) in the silver (Ag) paste cause the potential-induced degradation (PID), while the PID of p-type crystalline silicon (Si) photovoltaic modules is caused by Na in the front cover glass. Some Ag pastes contain these elements to control the firing properties in solar cells fabrication. In order to eliminate the effect of Na and other elements in the front cover glass on PID, PID tests for crystalline Si photovoltaic modules without front cover glass and using cells with various electrode pastes of controlled additive contents were performed. When the Ag paste with Na is used, the shunt resistance decreases and the PID occurs. This phenomenon is similar to that induced by Na in the front cover glass, but, it requires the shorter time duration as compared with the case of Na in the front cover glass. In addition, it is found that Li in the paste also causes the PID equivalent to that by Na in the paste.

Development of high-efficiency solar cell modules and new application fields such as PV (Photovoltaics)-powered vehicles are significant for the further development of PV and the creation of new clean energy infrastructure based on PV. In this paper, analytical results for impact of high-efficiency solar cell modules on increasing driving distance, reducing CO 2 emission and saving charging cost of electric vehicles by PV-powered vehicles. Because the Si tandem solar cells are expected to have significant potential for PV-powered vehicle applications, potentials of high-efficiency and driving distance are also analyzed. The III-V/Si 3-junction solar cell modules have potential of driving distance of 30 km/day average and more than 50 km/day on a clear day.

In this study, we evaluated sawing damage of thin silicon wafers. Ultra-thin silicon wafers with great advantages of bendability and lightweight are suitable for needs such as to realize PV-powered vehicles. For thin wafers, it is important to realize high precision slicing to prevent breakage. We evaluated the distribution of sawing damage and crystallinity in order to clarify the cause of wafer breakage and establish low damage slice conditions. We confirmed phase transformation of silicon and crystalline defects on wire exit side on the conventional slicing surface, while the advanced slicing surface shows almost only single-crystalline Si phase.

We investigated the degradation factor of the conversion efficiency of the silicon heterojunction solar cells. In particular, we clarified the effect of the transparent conductive oxide film deposition conditions on the film quality and interface states, and the relationship between these factors and the conversion efficiency. Heat process causes a decrease in carrier mobility due to a decrease in the grain size of the TCO film, but has an effect of improving the TCO/a-Si:H(p) interface states. In particular, the silicon oxide layer between the TCO film and a-Si:H(p) increases the series resistance and moderates the band bending at the TCO/a-Si:H(p) interface, leading to FF degradation. Since In2O3:H film easily forms a silicon oxide layer at the TCO/a-Si:H(p) interface, the interface treatment is important factor in the cell manufacturing process.

Komatsu NTC developed ultra-thin wafer slicing with low kerf-loss by multi diamond-wire saw. We modified slicing conditions and diamond-wire specifications to keep the straightness of wire for the fine pitch slicing and established 150 μm pitch slicing technology. The as-cut wafer thickness is 90 μm, and the kerf-loss is 60 μm. As a result, the highly flexible ultrathin wafer can be obtained with well-controlled surface crystallinity. The PERT cell fabrication trial using ultra-thin wafers showed that the relative deference of cell efficiency between 166 μm thick and 84 μm thick solar cells was just only 1.2%.

In order to investigate the role of SiNx anti-reflection coating in potential-induced degradation (PID) for p-type crystalline Si cells, PID tests were carried out using the cell without SiNx. PID did not occur with the module using the cell without SiNx even if the voltage of -2000 V was applied for 1 week. The simulation results of electric field distribution for the module without SiNx under applied voltage suggest that electric field is not be applied to cell-surface. Therefore, it is concluded that no electric field applied to SiNx is the most important to prevent PID.

The theoretical lateral current of the surface inversion layer in a crystalline silicon (cSi) surface for a p-aSi:H/i-aSi:H/cSi heterojunction (SHJ) solar cell was calculated using computer simulation and was compared with the experimental one to study defects/traps at the aSi:H/cSi interface and/or in the cSi surface and to detect the acceptor concentration (N a) in p-aSi:H. To experimentally extract the lateral surface inversion layer current, a field-effect transistor type test element group device was co-integrated with SHJ cells on the same wafer. From the correlation between the experimental and calculated lateral surface inversion layer current, the density of defects/traps (D it) at the aSi:H/cSi interface and/or in the cSi surface and the value of N a were extracted. The calculated lateral surface inversion layer current stayed unchanged for various minority carrier lifetimes in the substrate, suggesting that this method is not suffered from the variation in the material parameters in the substrate.

Mimicking the biological function of the extracellular matrix is an approach to developing cell adhesive biomaterials. The RGD peptide, derived from fibronectin (Fn), mainly binds to integrin αvβ3 and has been widely used as a cell adhesive peptide on various biomaterials. However, cell adhesion to Fn is thought to be mediated by several integrin subtypes and syndecans. In this study, we synthesized an RGD-containing peptide (FIB1) and four integrin α4β1-binding-related motif-containing peptides (LDV, IDAPS, KLDAPT, and PRARI) and constructed peptide-chitosan matrices. The FIB1-chitosan matrix promoted human dermal fibroblast (HDF) attachment, and the C-terminal elongated PRARI (ePRARI-C)-conjugated chitosan matrix significantly promoted HDF attachment through integrin α4β1 and syndecan binding. Next, we constructed a mixed ePRARI-C- and FIB1-chitosan matrix to develop a Fn mimetic biomaterial. The mixed ePRARI-C/FIB1-chitosan matrix promoted significantly better cell attachment and neurite outgrowth compared to those of either ePRARI-C- or FIB1-chitosan matrices. HDF adhesion to the ePRARI-C/FIB1-chitosan matrix was mediated by integrin, α4β1, α5β1, and αvβ3, similar to HDF adhesion to Fn. These data suggest that an ePRARI-C/FIB1-chitosan matrix can be used as a tool to analyze the multiple functions of Fn and can serve as a Fn-mimetic biomaterial.

Effects of an ITO capping layer on the interface workfuncion of MoO x was studied for an ITO/MoO x /SiO2/Si structure device from the viewpoint of the behavior of In atoms. The workfunction of MoO x at the MoO x /SiO 2 interface, which were extracted by capacitance-voltage (C-V) analysis, was largely different from the bulk MoO x film in its value and post-deposition forming gas annealing (PDFGA) temperature dependency. The interface workfunctoin decreased by 0.13 and 0.35 eV by 200 and 400 °C PDFGA, respectively, while bulk workfunction decreased by only 0.1 eV by 400 °C PDFGA. A secondary ion mass spectroscopy (SIMS) analysis showed that In atoms were penetrated from the ITO layer into the MoO x layer by PDFGA. The concentration of In near the MoO x /SiO 2 interface increased from ∼1019 cm−3 for the as-grown ITO sample to ∼1020 cm−3 for the sample with 400 °C PDFGA. The increase in the In concentration near the MoO x /Japan>SiO 2 interface is a possible cause to decrease the interface workfunction.

The behavior of oxygen precipitates under solar cell fabrication processes and the effect on device performance were investigated using TEM observation. Samples were prepared with different carbon concentration and with two sets of growth conditions. The number of precipitates correlates monotonically with the carbon concentration. When the initial carbon concentration is high, the cell efficiency is improved by decreasing the carbon concentration. When the initial carbon concentration is reduced to smaller than 1016 cm−3, the oxygen precipitate grows largely and introduces dislocation. Precipitates grown in a plate form introduce dislocations to the surroundings at a high density, while dislocation density is relatively small around the precipitates polyhedral-grown on the basis of an octahedron. Under the growth conditions for introducing a plate-like precipitate, the cell efficiency was not improved even if the carbon concentration was reduced.

When our novel Ag paste is used, the high solar cell performance is obtained. A thick SiON layer is formed between Ag line and Si substrate after the firing process. To understand the effect of this layer on the electrical properties, the current-voltage analysis of the contact is carried out. The obtained results indicate that there might be locally contacted areas between Ag and Si, and these regions are distributed in laterally, which determined the carrier transportation at the contact. This SiON layer with local contacts prevents the Ag diffusion into Si crystal during the firing process, and decreases the minority carrier recombination, resulting in the relative high performances.

The high quality and thin Si wafer technology for the future higher conversion efficiency and lower cost crystalline silicon solar cells are realized. The high minority carrier lifetimes even after the processes are obtained by controlling the Czochralski growth condition, which prevents the interstitial oxygen segregation enhanced by the substitutional carbon. The Cz ingot is sliced by the advanced diamond multi-wire saw technology and the thin wafers with relatively thin damaged layer and 100μm kerf-loss are realized. The thin wafer with low kerf-loss decreases the wafer cost and improve the cell performance. The thin wafer solar cells are fabricated using the PERC processes, and these technologies are evaluated from the view point of photovoltaic.

We applied aluminum-induced crystallization (AIC) as a technique to realize heavily doped p-type polycrystalline Si layer at low temperature for the rear side of tunnel oxide passivated contact concept solar cell. Systematic temperature variation of AIC revealed that annealing at a temperature slightly lower than the eutectic temperature (577°C) is effective to improve solar cell performance. This is explained by reduced resistivity for low contact resistance while avoiding meltinz of AI-Si.

By stacking a III - V cell to the Si cell, it is expected to reach to more than 30 % of efficiency. However, the contribution of the Si bottom cell may be reduced to 5 to 6 % in maximum. Another problem is that the III-V cell cost is still expensive and high concentration without relying on tracking is expected. For solving these two problems, a new configuration that places small III-V multi-junction solar cells at four corners of a single-crystalline Si solar cell was invented.

An energy conversion efficiency of 25.1% was achieved in heterojunction back contact (HBC) structure Si solar cell utilizing back contact technology and an amorphous silicon thinfilm technology. A new patterning process was established, and it was applied to the fabrication process of HBC cells. In addition, the unique technology of the surface mount technology concept contributed to the superior performance of HBC cell. A short circuit current density (J sc ) and an open-circuit voltage (V oc ) were 41.7 mA/cm2 and 736 mV, respectively. The high J sc as well as the high Voc indicates the strength of HBC structure cell. Besides, a high fill factor of 0.82 was obtained, which shows that HBC structure cell does not have any fundamental critical losses caused from series resistance or shunt resistance. Such high values of I-V parameter means that the patterning process was properly performed.

We investigated the effect of silicon dioxide (SiO_2) film in n-type front-emitter (n-FE) crystalline Si (c-Si) solar cells on the potential-induced degradation (PID) of n-FE photovoltaic (PV) modules. After PID tests by applying a bias of −1000 V at 85 °C for a few min, the modules with the cells without SiO_2 did not degrade in the short-circuit current density (J_) and the open-circuit voltage (V_). Since the degradation is known to be due to positive charge accumulation in SiN_x films, the result suggests that such SiO_2 acts as barriers to retain accumulated positive charges. After further PID tests, modules without SiO_2 show faster and more significant degradation by a decreases in the fill factor (FF) and the V_. It has been proposed that the degradation in the FF and V_ is caused by sodium (Na) introduction into cells. The results therefore suggest that SiO_2 delays Na migration.

We present the measurement results and analyses of a InGaP/GaAs//Si mechanical stack four-terminal solar cell. From these results, we performed detailed discussions of the loss mechanisms and efficiency limits of InGaP/GaAs//Si mechanical stack solar cells.

Peptide-conjugated polysaccharide matrices using bioactive laminin-derived peptides are useful biomaterials for tissue and cell engineering. Here, we demonstrate an easy handling preparation method for peptide-polysaccharide matrices using polyion complex with both alginate and chitosan. First, aldehyde-alginate was synthesized by oxidization of alginate using NaIO

In this study, we investigated progression of potential-induced degradation (PID) in photovoltaic modules fabricated from n-type-based crystalline-silicon cells with front p^+ emitters. In PID tests in which a bias of −1000 V was applied to the modules, they started to degrade within 5 s and their degradation saturated within 60 s. This behavior suggested that the PID was caused by positive charge accumulation in the front passivation films. Performing PID tests with a bias of −1500 V revealed that the degradation rate strongly depended on the applied bias whereas the saturation value was independent of the applied bias. Regeneration tests on degraded modules previously subjected to PID tests for durations of 5 and 10 min were performed by applying a positive bias of +1000 V. All the degraded modules completely recovered their performance losses within 60 s regardless of the degradation test duration. On the basis of these results, we proposed that these positive charges originate from positively charged K centers formed by extracting electrons from neutral and negatively charged K centers. This model readily explains the observed degradation and regeneration behavior. To test our model, we determined the fixed positive charge densities(Q_f ) of a silicon nitride passivation film before and after PID, for which it was found that Q_f showed similar saturation behavior. Additionally, the saturated Q_f value was of the same order as K center density. These results support our model involving a charging process of K centers.

We investigated to apply thermal SiO2/APCVD SiO2/PECVD SiNx stacking layer to the rear surface passivation of PERC. After rough optimization of process conditions of LCO and firing steps, the cell efficiency reached 19.3% and it was 1.1% absolute higher than that of the conventional cell. The results of CV measurements suggest that the hydrogen released from the PECVD SiNx during the firing reduces Dit and the deposition of SiO2 by APCVD is effective to reduce Qf. By combining these two effects, both Dit and Qf of the thermal SiO2/APCVD SiO2/PECVD SiNx stacking layer are largely reduced. Therefore, the thermal SiO2/APCVD SiO2/PECVD SiNx stacking layer is suitable for rear surface passivation of PERC.

The structural and electrical properties of the CdS/CdTe(S) interface region of 2-µm-thick CdS/CdTe solar cells have been studied in conjunction with photovoltaic (PV) performances with a focus on the influence of the annealing temperature (TCdCl2) and oxygen concentration of the CdCl2 treatment. When the CdCl2 treatment with the same conditions for a 5-µm-thick solar cell is applied to the 2-µm-thick solar cells, the PV performances largely decrease due to the structural deterioration at the CdS/CdTe(S) interface; however, they are improved by the rapid treatment with high TCdCl2 under a higher oxygen concentration. Similar behaviors are observed between the open-circuit voltage (VOC) and the signal intensity of the electromodulated photoluminescence against TCdCl2 or oxygen concentrations. These results suggest that the n+-CdTe1-xSx layer adjacent to the CdS/CdTe(S) interface plays a crucial role in the increase of VOC because of a decrease in the nonradiative recombination rate at and near the CdS/CdTe(S) interface and/or an increase in the built-in electric field.

Workfunction of reactive-plasma deposited indium-tin-oxide (RPD-ITO) at the ITO/SiO2 interface, which is referred as interface workfunction, and the process-induced damage are experimentally extracted for the first time based on capacitance-voltage (C-V) analysis. The estimated interface workfunction value of 4.74 eV for as-deposition condition is about 0.4 eV higher than that in the bulk determined by ultraviolet photoelectron spectroscopy (UPS). The RPD process induces the damage at the Si/SiO2 interface, and the degree of the damage is evaluated as the interface defect density (Dit) to be around 1012 cm-2eV-1. The effects of forming-gas annealing on the interface workfunction and recovery of the damage are also studied. The interface workfunction value once decreases to 4.53 eV by the annealing up to 250 oC and then turns to increase to 4.77 eV after 400 oC annealing. The damage is annihilated by the low-temperature forming-gas annealing at 200 oC.

The effects of residual C on cell properties are investigated from the view point of thermal budget in the n-type bifacial cell processes. Implied Voc obtained from wafers with same Oi concentration depend on the thermal budgets decreases as the Cs concentration increases. The Voc values vary depending on the wafer with different growth cooling rate. To analyze the effect of thermal budget correspond to solar cell fabrication process, CZ wafers with almost the same Oi concentrations are prepared. One of the wafers with relatively high residual Cs concentration shows the longer lifetime than the initial value after the 950 oC annealing step. On the other hand, the lifetime of a wafer with relatively low Cs concentration dramatically decreased by the same process due to the O segregation. These results suggest that it is important to choose appropriate wafer specification, starting with feedstock material, for increasing the solar cell efficiency.

Peptide-conjugated polysaccharide matrices using bioactive laminin-derived peptides are useful biomaterials for tissue and cell engineering. Here, we demonstrate an easy handling preparation method for peptide-polysaccharide matrices using polyion complex with both alginate and chitosan. First, aldehyde-alginate was synthesized by oxidization of alginate using NaIO4 , and then, reacted with Cys-peptides. Next, the peptide-alginate solution was added to a chitosan-coated plate, and the peptide-polyion complex matrices (peptide-PCMs) were prepared. The peptide-PCMs using an integrin αvβ3-binding peptide (A99a: ALRGDN, mouse laminin α1 chain 1145-1150) and an integrin α2β1-binding peptide (EF1XmR: RLQLQEGRLHFXFD, X = Nle, mouse laminin α1 chain 2751-2763) showed strong cell attachment activity in a dose-dependent manner. When we examined the effect of various spacers on the biological activity of A99a-PCM, hydrophobic and long spacers enhanced the cell attachment activity. Further, the A99a-PCM with the spacers strongly promoted neurite outgrowth. The polyion complex method is an easy way to obtain insolubilized matrix and is widely applicable for various polysaccharides. The peptide-PCM is useful as a biomaterial for cell and tissue engineering.

We performed workfunction mapping of the cleaved interface of silicon heterojunction (SHJ) solar cell. While it has been widely accepted that the efficiency of solar cells depends on their electric contacts at various interfaces, no direct information on the electronic properties had been obtained on nm-scale. In this work, we employed Kelvin probe force microscopy (KFM) and simultaneously measured the workfunction and morphology of the cleaved interfaces on nm-scale for the first time. We show the measured workfunction differences between the surface layers on the cleaved SHJ solar cell, and discuss how such information can be used to improve the quality of the devices.

We investigated a new high efficiency low cost bifacial cell based on n-type substrate using a new simplified inline ion doping system, the so called ion shower. We were able to successfully form the boron doping layer and control the sheet resistance of the doped layer by controlling the doping and annealing parameters. Furthermore, we fabricated a bifacial Passivated Emitter and Rear Totally diffused (PERT) cell using the ion shower doping system to form a boron emitter. At first, PV performances were relatively low and the bifaciality was higher than 1. This result suggested that the recombination on the P+ side was greater than on the N+ side. Therefore, we applied the Alignment Free-Selective Emitter (AF-SE) technique to our cell to reduce the recombination loss of the emitter. As a result, all parameters were improved, and especially the increase in Jsc on the P+ side was remarkable, with the bifaciality decreased to 0.987. This clearly shows the effectiveness of AF-SE. In conclusion, we confirmed the feasibility of newly proposed high efficiency low cost n-type bifacial cell fabrication process using an ion shower doping system.

The width of screen printed lines can be decided by the emulsion opening of a screen mask of screen printing, but actually the printed width is very much affected by the roughness of a substrate and the rheology of the paste used. Finger lines of solar cells should be as narrow as possible to increase sunlight usage efficiency. Hence the printed condition to realize fine lines should be optimized by adjusting the mesh counts of the screen mask used and the rheology of the paste depending on the texture size of wafers. Screen print has to print on a flat surface of a substrate, but the screen print in solar cells is required to print on a rough surface like a textured surface, which causes widening printed lines due to the bleeding of paste leaking out through the gap between the flat emulsion surface of the screen and the textured surface of a silicon wafer. The effect of texture size on printed line width is studied by changing etching condition for texturing to make a large texture (roughly 8μm) and a small texture (roughly 3μm) on mono-crystalline wafers. And, the proper adjustment on screen printing conditions are accomplished to realize 30μm lines on the small textured wafer with normal silver paste by using screen mask of ultra-fine mesh.

We studied the texturization mechanism of mono crystalline silicon wafer using some alternative additives. We successfully controlled the texture size with change of concentration of KOH in the etchant. We also proposed one appropriate model of role of additives in texturization process. In our proposed mechanism, the additives should play a role as micro-mask when the micro pyramid is formed and the texturization process should be dominated by the correlation of etching rate and adhesion and desorption rate of additives. We also fabricated solar cells with several texture sizes and evaluated them. The decrease of F.F. with the decrease of texture size was mainly caused by the increase of Rs and this result suggested that we had to optimize the screen printing and firing conditions for the small texture. In addition, there are thick damaged layer on the top and bottom of texture, and these damaged layer cause the deterioration of cell efficiency with texture miniaturization. Reduction of the damage is important to realize high efficiency solar cell using miniaturized texture.

Various solar cells and materials have great potential as future generation solar cells and materials including crystalline Si. Si tandem solar cells such as III-V/Si tandem, II-VI/Si tandem, Chalcopyrite/Si tandem and Perovskite/Si tandem solar cells are expected to have great potential of space and terrestrial application because of high efficiency, light weight and low cost potential. Quantum and Nano structures such as MQWs ‘(Multi Quantum Wells) and QDs (Quantum Dots) are also expected as high-efficiency solar cells. However, there are some problems to be solved. Because point defects in crystalline Si, high density of dislocations and interface recombination in Si tandem solar cell materials and interface recombination in MQWs and QDs affect on solar cell properties, understanding and controlling those defects are very important for realizing higher efficiency. This paper presents analytical results for high efficiency potential of single crystalline Si solar cells, Si tandem solar cells, and MQW and QD solar cells. Although 25.6% efficiency has been attained with hetero-junction and back contact structure and by using bulk minority-carrier lifetime of around 10 msec, high efficiency of more than 27.5% is expected to be realized by increasing bulk lifetime of more than 100 msec Si. Really, high bulk lifetime of around 100 msec has been obtained by the MCZ (Magnetic-field-applied Czochralski) method and decreasing carbon concentration to less than 1014 cm-3 in Si. Understanding behavior of point defects such as Cs-Oi and so forth in Si and decreasing those defects density are suggested for realizing higher bulk lifetime and higher efficiency Si solar cells of more than 27.5%. High efficiency potential of Si tandem solar cells such as III-V/Si tandem, II-VI/Si tandem, Chalcopyrite/Si tandem and Perovskite/Si tandem solar cells is also analyzed by using measure of external radiative efficiency: 43% for III-V/Si 3-J(Junction) tandem, 38% for III-V/Si 2-J tandem, 33% for II-V/Si 2-J tandem, 32% for Chalcopyrite/Si tandem, and 32% for Perovskite/Si tandem solar cells. Effects of dislocations upon minority-carrier lifetime in III-V/Si tandem solar cells are also presented. Our recent result for mechanically stacked InGaP/GaAs//Si 3-junction tandem solar cell with efficiency of 27.4% is shown. Although MQW and QD solar cells are expected as high efficiency solar cells, one of big problems is higher interface recombination near MQWs and QDs because of their lattice mismatching. Lattice mismatch (Δa/a0) dependence of interface recombination velocity (SI) in MQW and QD solar cells is analyzed. Open-circuit voltage drop by interface recombination in MQWs and QDs and their efficiency potential is discussed.

A simulation of interdigitated back-contact silicon heterojunction (IBC-SHJ) solar cells was performed using a quantum transport model to consider the quantum effect at the crystalline/amorphous (c/a) heterojunction interface. It was found that the impact of the quantum effect on the open-circuit voltage is comparable to that of the interface defect density at the c/a interface, indicating the importance of implementation of the quantum model. The optimal back-contact design was also discussed from the viewpoint of mass production, in which the design rule is relaxed. The degradation of the conversion efficiency by widening the gap between the p- and n-aSi:H layers can be compensated by improving passivation quality at the c/a interface.

We have grown 50 cm2 mono Si ingots by the seed cast technique. The carbon and oxygen concentrations of the ingots have been significantly reduced by improving the gas flow condition and coating. The dislocation density was also reduced by eliminating the extra dislocation generation sources. Owing to these developments, the lifetime of wafers has reached 465 µs. Finally, the efficiency of 18.7% has been achieved, which is comparable to 18.9% of the reference Czochralski (CZ) Si wafer.