Brief introduction to the silicon nanowire fabrication technologies which have been being developed in Future PV Innovation project is given in this article. The final target of this project is to develop high efficiency silicon-based tandem solar cells. To realize the siliconbased tandem solar cells, a silicon-based material with a bandgap of about 1.7 eV is required for the top cell. The potential of silicon nanowire for the top cell material is discussed. In addition, efforts to realize bandgap controlled silicon-based nanowire is also reviewed.
Photonic nanostructures coupled with Ge quantum dots (QDs) are integrated with crystalline Si solar cells for improvement of light trapping, photon absorption in nearinfrared wavelength, and carrier recombination suppression due to spatial separation of excited carriers. We demonstrate that the photonic nanostructures coupled with Ge QD multilayers can be fabricated by combination of a crystal growth technique and a maskless wet etching process. The geometry in the photonic nanostructures can be tuned by changing structural parameters in Ge QD multilayers and a kind of etchant. We found that external quantum efficiency in the entire wavelength is enhanced for a solar cell with a photonic nanostructure coupled with a Ge QD multilayer compared with a crystalline Si solar cell. It is demonstrated that a solar cell with photonic nanostructures formed by maskless wet etching process has a high potential for enhancement of solar cell properties.
We have examined the crystal growth of pseudomorphic epitaxial layers of Ge_<1-x-y>Si_xSn_y ternary alloy on Ge(001) substrates. We have investigated the electronic and optoelectronic properties of Ge_<1-x-y>Si_xSn_y/n-Ge heterostructures for photovoltaic applications. A Ge_<1-x-y>Si_xSn_y layer with a high Sn content of 12% can be prepared with lattice matching epitaxy on Ge at a low temperature below 250 ℃. The small misfit between Ge_<1-x-y>Si_xSn_y and Ge effectively impacts on the high crystalline quality and high thermal stability of substitutional Sn atoms in Ge matrix. Engineering of the absorption energy in solar cell of Ge_<1-x-y>Si_xSn_y/n-Ge hetero-structure is also demonstrated.
Semiconductor nanowires (NWs) are promising candidate for light-absorbing material in next generation photovoltaic which can reduce cost and materials consumption compared to planar devices. Furthermore, III-V NW-based multi-heterojunction solar cells using lattice-mismatched material system are expected as high energy-conversion efficiencies under concentrated light. Here, we report on III-V compound semiconductor nanowire solar cells. After brief introduction of crystal growth of semiconductor nanowires, we discuss on optical properties and device performance of nanowire solar cells compared to planer ones. Next we review on current research on nanowire solar cells. Finally we discuss future prospects, especially how to reduce cost and materials consumption.
This paper will summarize our approach toward efficiency enhancement of multijunction cells using InGaAs/GaAsP quantum wells with both the quasi-lattice-match relationship with GaAs host and the extended absorption edge to a longer wavelength than the value of GaAs. The optimized structure for efficient collection of photo-generated carriers in the InGaAs wells employed both thin (≤3 nm) barriers for tunneling-assisted carrier transport and a stepwise potential to assist thermionic carrier escape. In spite of its small thickness, 100 stacks of InGaAs wells in GaAs pin junction evidenced up to 80% internal quantum efficiency in the sub-bandgap region of GaAs. The growth of such a structure necessitates elaborate control of strain accumulation, for which in situ monitoring of wafer curvature proved to be quite effective. The heterointerfaces between compressive InGaAs and tensile GaAsP imposes difficulty in keeping surface morphology up to >100 stacks. The interlayer should be inserted at such interfaces to mitigate straininduced crystal imperfection, as evidenced by in situ surface reflectance monitoring.
A method to crystallize organic semiconductor blends in organic photovoltaic cells is reviewed. This method utilizes a liquid as a non-sticking co-evaporant of small molecular organic semiconductors and crystallizes their single and blend films. Blend films based on H_2Pc and C_<60> with much improved crystallinity have been produced by this method and confirmed by analysis using UV-Vis, XRD and FESEM. Used in organic photovoltaic cells, a variety of blend films, such as blends of fullerene (acceptor) and H_2Pc, PbPc, AlPcCl, and rubrene (donors), have been produced by this method and have achieved striking enhancement of short-circuit current density. In this review, I also present the principle of this new method, co-evaporant induced crystallization. Since it drastically improve the principle of vacuum deposition of organic semiconductors, this method would not be limited for organic photovoltaic cells but also useful for other organic devices.
Inserting quantum wells (QWs) into GaAs p-i-n solar cells could be a potential solution for the beyond the Shockley-Queisser limit. This is because the QWs can extend the absorption region and enhance the short-circuit current. However, they function as the recombination center leading to degradation in open-circuit voltage. We then evaluate the carrier behaviors by detecting the non-radiative recombination utilizing the piezoelectric photothermal (PPT) technique. The PPT spectra were clearly decomposed into excitonic and inter-subband transitions. From the temperature dependencies of radiative and nonradiative recombination and carrier escape from the well, we estimated the activation energies for non-radiative recombination and thermally carrier escape. The usefulness of the PPT methodology for investigating the QWs-embedded solar cells was clearly demonstrated.