Ryuji Oshima

To evaluate the compatibility of 1.5 eV Ga0.9In0.1As0.85P0.15 solar cells with the space environment, we investigated their radiation degradation characteristics and examined the behavior of the resulting defects under heat light soaking (HLS). The open-circuit voltage (Voc) of Ga0.9In0.1As0.85P0.15 solar cells was initially higher than that of GaAs solar cells; however, this relationship reversed following irradiation with 1 MeV electrons at a fluence of 3 × 1014 cm−2. On the other hand, only the Voc of Ga0.9In0.1As0.85P0.15 solar cells recovered after HLS and eventually surpassed that of GaAs solar cells. Light soaking improved the defect recovery rate and reduced the activation energy of defect recovery in Ga0.9In0.1As0.85P0.15 solar cells, suggesting that light soaking accelerates the defect recovery phenomenon that occurs under dark conditions. Furthermore, by performing HLS while varying the operating point of the solar cell, we clarified that the extent of recovery is determined by the density of recombination carriers in the solar cell. We discuss the overall radiation resistance of Ga0.9In0.1As0.85P0.15 solar cells, including their recovery phenomenon in the orbital environment.

Commercially available formulations of the popular conductive polymer, poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) are aqueous dispersions that require the addition of secondary dopants such as dimethyl sulphoxide (DMSO) or ethylene glycol (EG) for fabricated films to have the desired levels of conductivity. Clevios^TM F HC Solar, a formulation of PEDOT:PSS produced by Heraeus, GmbH, achieves over 500 S/cm without these secondary dopants. This work studies whether secondary dopants such as DMSO have any additional effect on this type of PEDOT:PSS. The temperature dependencies of the conductivity of F HC Solar spin-coated thin films measured using a four-probe method seem to exhibit different charge transport properties compared with secondary doped PH1000. Observations made using atomic force microscopy (AFM) show that different concentrations of DMSO affect the orientation of the PEDOT domains in the thin film. These morphological changes cause room temperature conductivity to reduce from 640 S/cm in pristine films to as low as 555 S/cm after adding 7 wt% of DMSO along the film. Such tuning may prove useful in future applications of PEDOT:PSS, such as nanoprobes, transistors and hybrid solar cells.

PEDOT:PSS/Si hybrid photovoltaic cells have been attracting attention as a potential way to simplify the manufacturing process and democratize solar energy production. Control of the PEDOT/Si interface is also one of the primary ways to ensure the improved performance and lifetimes of multijunction devices, such as perovskite/Si tandem solar cells. In this work, the effects of the interfacial silicon oxide layer were investigated by creating a novel and controllable neutral beam oxide interlayer with different thicknesses. A novel self-doped PEDOT (S-PEDOT) was used to improve interfacial contact and avoid the secondary doping of PEDOT:PSS. X-ray photoelectron spectroscopy (XPS) showed that the saturation of interfacial silicon atoms in SiO_x-Si bonds as well as a very thin, (~1 nm) damage-free oxide interlayer were the keys to maintaining good passivation with a high tunneling current. Lifetime measurements also showed that the interlayers with the most SiO₂ content degraded the least. The degradation of the devices was due to the continued growth of the oxide layer through reactions with silicon sub-oxides and the degradation of S-PEDOT.

High production costs are a significant obstacle to expanding applications for III-V solar cells. Therefore, we studied to combine the substrate reuse technique and hydride vapor phase epitaxy (HVPE) toward realizing low-cost III-V solar cells. For HVPE, a low epitaxial growth cost can be realized because of the use of group-III precursors through in situ high-efficiency reactions involving less expensive pure metals and HCl gas. The substrate reuse was performed through the epitaxial lift-off (ELO). All samples were grown via HVPE. For the ELO process, a 100 nm-thick AlAs release layer was grown on GaAs substrate. The AlAs was grown using AlCl3 precursor. The GaAs epitaxial layer grown on reused substrate showed good surface flatness comparable to that grown on new epi-ready substrates. The arithmetic mean roughness values were approximately 0.5 nm for GaAs layers grown on new epi-ready and reused substrates. Further, the GaAs single-junction solar cells grown on reused substrate exhibited similar performances with samples grown on new substrates. For the GaAs cell grown on the new substrate, the short-circuit current density (JSC), open-circuit voltage (VOC), fill factor (FF), and conversion efficiency (η) were 19.6 mA cm-2, 0.991 V, 0.817, and 15.8%, respectively. Whereas, for the GaAs cells grown on the reused substrate, JSC, VOC, FF, η were 19.4 mA cm-2, 0.984 V, 0.817, and 15.6%, respectively. The combination of substrate reuse and HVPE can reduce solar cell fabrication costs. This study is a major achievement in reducing the production cost of III-V solar cells.

In photovoltaic (PV) power generation, highly efficient III-V solar cells are promising for emerging mobile applications, such as vehicle-integrated PVs. Although hydride vapor phase epitaxy (HVPE) has received attention due to its lower fabrication costs, realization of high throughput performance while maintaining solar-cell characteristics using this growth method is essential. In this study, the effect of atmospheric-pressure triple-chamber HVPE growth conditions on GaAs solar-cell properties were carefully investigated in conjunction with defect analysis using deep-level transient spectroscopy (DLTS). Based on the analysis on GaAs reaction processes, the suppression of arsine thermal cracking in the HVPE hot-wall reactor was important to achieve fast GaAs growth using a low input V/III ratio. Moreover, the DLTS results revealed that the reduced input V/III ratio was effective in suppressing the generation of EL2 traps, which is a common GaAs midgap complex defect involving arsenic antisites. Although the EL2 trap density increased with the growth rate, the performance of GaAs solar cells that were grown under reduced arsine thermal cracking exhibited almost no considerable cell parameter deterioration at a growth rate of up to 297 μm/h. Consequently, a conversion efficiency of 24% with a high open-circuit voltage of 1.04 V was achieved for the cells that were grown at 200 μm/h.

Multijunction solar cells (MJSCs) have attracted attention as next-generation solar cells. In particular, GaAs//Si-based MJSCs are highly efficient with low cost and are expected to gain new applications, such as on-vehicle integrations. In this article, we examined a highly efficient In _0.49 Ga _0.51 P/Al _0.06 Ga _0.94 As//Si three-junction solar cell. The bottom Si cell has a tunnel oxide passivated contact structure. The key technology used to fabricate this solar cell is a stacking method that uses Pd nanoparticles (Pd-NPs) and metal-assisted chemical etching (MacEtch) for the bonding interface, which is improved from our previous “smart stack” technology. The MacEtch method has a feature of selective etching for Si around a metal body. Pd-NPs selectively invade the Si cell through the surface of the Si oxide layer, thereby improving the bonding resistivity between the GaAs-based cell and Si cell. Further, this technology aids the management of the bonding gap width by controlling the Pd-NP invasion depth. As a result, an efficiency of 27.6% for the aperture area was attained. The proposed technology is useful for the connection of Si-based cells, enhancing the development of GaAs//Si-based tandem solar cells.

Multijunction (MJ) solar cells have attracted considerable attention as next‐generation solar cells. III–V‐based MJ solar cells connected to heterogeneous cells, such as GaAs//Si and GaAs//CIGSe (CuIn_1−yGa_ySe₂), are expected to be highly efficient and inexpensive. Consequently, we have developed a mechanical stacking method with Pd nanoparticles and silicone adhesives for the bonding interface, which was modified for our previous “smart stack” technology. Using this “modified smart stack,” we previously demonstrated an InGaP/Al_0.06Ga_0.94As//CIGSe three‐junction solar cell with an efficiency of 28.06%. In this study, we fabricated an InGaP/GaAs//CIGSe three‐junction solar cell. The total efficiency is 29.3% for the aperture area (31.0% for the active area) under AM1.5G solar spectrum illumination, which is the optimal value reported for two‐terminal GaAs//CIGSe‐based tandem solar cells thus far. This superior performance was realized by using a specialized CIGSe cell with a flattened surface via a wet etching process and a thin In₂O₃;Ce,H transparent conducting oxide layer. Using an area current‐matching technique, the efficiency increased to 30%. In addition, the bonding resistivity of the fabricated solar cell was estimated to be less than 1 Ωcm²from solar‐concentrated measurements. These results suggest the potential of III–V//CIGSe‐based tandem solar cells with modified smart stack technology as next‐generation photovoltaic cells for applications such as vehicle‐integrated photovoltaics.

Multijunction (MJ) solar cells haveattracted attention as next-generation solar cells. In particular, III–V-based MJ solar cells connected with heterogeneous cells, such as GaAs//Si and GaAs//CIGSe (CuIn _1−y Ga _y Se ₂ ), are expected to be highly efficient and low cost. In this article, we demonstrate a highly efficient InGaP/Al _0.06 Ga _0.94 As//CIGSe three-junction solar cell. The key technology used to fabricate this solar cell is a bonding method that uses a Pd nanoparticle array and an adhesive material (silicone adhesive) for the bonding interface, which is modified for our previous “smart stack” technology. This modified smart stack technology contributes to enhancing the bonding strength between the GaAs-based cell and the CIGSe cell. Using this technology, total efficiency of 28.06% was attained, as certified by the National Institute of Advanced Industrial Science and Technology. This value is superior to that obtained in our previous result (27.2%) and represents the highest efficiency ever reported for two-terminal GaAs//CIGSe-based tandem solar cells.

Hydride vapor phase epitaxy (HVPE) is attracting attention as a technology for reducing the epitaxial cost of III–V solar cells. To further reduce manufacturing costs, it is effective to combine HVPE with substrate-reuse technology using epitaxial lift-off (ELO). However, ELO of solar cells grown via HVPE has not yet been demonstrated. Although Al(Ga)As is typically used as the release layer for the ELO process, there is a difficulty in growing Al-containing materials on HVPE because aluminum monochloride reacts with the reactor. Here, we present the growth of AlAs via HVPE using aluminum trichloride generated by the reaction between Al metal and HCl gas at a temperature of 500 °C. The AlAs layer grown via HVPE exhibits sufficient crystal quality to achieve the ELO process. We obtained a conversion efficiency of 21.63% for a HVPE-grown GaAs single-junction solar cell peeled off from a GaAs substrate. This performance is almost similar to that of HVPE-grown solar cells on GaAs substrates without ELO.