AVS2012 Session EN+NS-MoM: Nanostructured Solar Cells

Si quantum dots (QDs) imbedded in a SiO₂ matrix is a promising material for the next generation optoelectronic devices, such as solar cells and light emission diodes (LEDs). However, low conductivity of the Si quantum dot layer is a great hindrance for the performance of the Si QD-based optoelectronic devices. The effective doping of the Si QDs by semiconducting elements is one of the most important factors for the improvement of conductivity. High dielectric constant of the matrix material SiO₂ is an additional source of the low conductivity.

Active doping of B in Si nano structures and the effect of internal polycrystalline bridge layer were investigated by secondary ion mass spectroscopy (SIMS) depth profiling analyses. Phosphorous and boron doped-Si / SiO₂ multilayers on Si wafers were fabricated by ion beam sputtering deposition as a model structure for the study of the diffusion behavior of the dopants. The distributions of the dopants after annealing at high temperatures were analyzed by SIMS depth profiling analyses.

The pressing need for massively scalable carbon-free energy sources has focused attention on both increasing the efficiency and decreasing the cost of solar cells. Quantum-dot (QD) solar cells employing multiple exciton generation (MEG) have attracted much attention as a candidate for the third generation solar cells, because MEG represents a promising route to increased solar conversion efficiencies up to about 44 % in single junction. Our interest has been concerned with QD sensitized solar cells using Si nanoparticles [1]. The main purpose of this study is to discuss the characteristic of the quantum yield in view of the MEG effect.

QD thin films composed of size-controlled Si nanoparticles were deposited using double multi-hollow discharge plasma chemical vapour deposition (CVD) of a SiH₄/H₂ and CH₄ or N₂ gas mixture [2]. Short-circuit current density of Si QD sensitized solar cells increases by a factor of 2.5 by irradiation of CH₄ or N₂ plasma to Si nanoparticle surface. We also have measured incident photon-to-current conversion efficiency (IPCE) in the near-ultraviolet range using quartz-glass plates as front panels of QD sensitized solar cells. IPCE gradually increases by light irradiation in a wavelength range less than 600 nm around optical band-gap (E_g) of Si nanoparticle films, and then steeply increases below 280 nm around 2E_g. This rapid increase of IPCE under the ultraviolet light incidence may be explained by the theoretically predicted MEG, the creation of two electron-hole pairs from one high-energy photon incidence, in Si nanoparticle QDs.

[1] G. Uchida, et al., Phys. Status Solidi C 8 (2011) 3021.

[2] G. Uchida, et al., Jpn. J. Appl. Phys. 51 (2011) 01AD01-1.

In light of recent advances in synthesis, characterization, and the emerging understanding of their size-dependent properties, there are many exciting opportunities for semiconductor nanomaterials to contribute to the development of next-generation energy conversion technologies. Semiconductor nanocrystal quantum dots are particularly attractive material candidates for the efficient capture of solar emission in inexpensive, thin film photovoltaic devices due to their large absorption cross sections, low-cost solution-phase processing and size-tunable energy gaps. The prospect of exploiting colloidal nanostructures for the creation of low-cost multi-junction solar cells has garnered immense scientific and technological interest. We recently demonstrated demonstrate solution-processed tandem solar cells created from nanocrystal quantum dots with size-tuned energy levels. Bringing this prospect to fruition requires the connection of absorber layers with cascaded energy gaps subject to stringent electrical and optical constraints. We show that interlayers composed of ZnO/Au/PEDOT provide appropriate carrier density and energy-level alignment to resolve this challenge. With such interlayers we have been able to create nanocrystal quantum dot tandem cells that exhibit IR sensitivity and a open circuit voltage approaching 1V. These advances provide guidelines for the design of an effective interlayer in tandem cell devices and suggest a promising future for solution-processed nanocrystal quantum dot solar cells.

PbSe quantum dots (QD) in the size range of 4-8 nm are promising candidates for solar energy harvesting as they exhibit multi-exciton generation with ultraviolet (UV) photon absorption. While generation of multi-excitons has been demonstrated, dissociation of excitons to enhance current densities has not been realized. One of the main bottlenecks has been the difficulty in removing the surfactants on QDs to form a clear interface between the QD and the polymer matrix. We have developed a Laser Assisted Spray (LAS) deposition technique to deposit uniform coatings of surfactant-free QDs on substrates. This technique involves the transient heating of aerosols containing PbSe QDs by a CO₂ laser-gas interaction to burn the organic surfactants. Transmission electron micrographs and absorption spectroscopy show, under optimum conditions, the particles remain as single crystals and maintain quantum confinement. Growth parameters are optimized by monitoring the degree of surfactant removal by studying the Fourier Transform Infrared (FTIR) spectra of coatings grown by LAS technique. Two-layer solar cell structures of PbSe/polymer that is sandwiched between ITO and Al electrodes have been fabricated. Comparison of the IV characteristics of these cells and cells fabricated by PbSe QDs with ligand-exchange will be presented.