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We discuss results of surface photovoltage (U) measurements for d = 10 µm thick layers of undoped hydrogenated microcrystalline silicon (µc-Si:H). By applying excitation with low energetic photons (down to 1.1 eV), i.e., with small absorption coefficient a and a large penetration depth α[sup -1], a photovoltage peak appears on a curve U= U(α). This peak is located at α...d[sup -1] and its occurrence depends critically on the substrate material. The peak is present in a µc-Si:H film grown on p + crystalline silicon (c-Si), on the other hand it is missing in the µc-Si:H samples grown on n[sup +] c-Si or ZnO film. We present a mathematical model that enables us to link this peak to photocharge separation in the bottom space charge region at the interface µc-Si:H/substrate. Besides the magnitude of the ambipolar carrier diffusion length L, a parameter particularly critical for the occurrence of the peak turns out to be the ratio n of reverse saturation current densities of the two diodes representing surface and bottom space charge regions. The peak can be observed only when n is below a certain threshold value n[sub th]. In the simplified case when d/L ... 1 and when the thicknesses of the upper and lower depletion layers can be neglected, we have found n[sub th] ≈ 0.135 or 1, depending on the orientation of the top and bottom barriers to each other. However, the magnitude of the peak increases exponentially with further lowering of n. Therefore, the surface photovoltage method is suitable not only for evaluating the minority carrier diffusion length L, but also for detecting the occurrence and properties of the bottom space charge region in thin film solar cells. [ABSTRACT FROM AUTHOR]

Abstract: We report on the role of selected silicon nanocrystal (Si-nc) synthesis techniques on the photoelectric properties of Si-ncs and conjugated poly(hexylthiophene) (P3HT)-based blends. Nanosecond laser processing – both ablation and fragmentation – in liquids and electrochemical etching are employed to fabricate surfactant-free Si-ncs. We demonstrate that the transport properties of the blend can be tuned by the processing conditions. The blends containing Si-ncs produced by laser ablation clearly show superior photovoltaic properties due to the enhanced bulk heterojunction surface area and improved charge transport. Finally, we argue that the nanosecond laser processing in liquid medium could be a flexible and efficient tool to tailor the photoelectric properties of the Si-ncs/P3HT nanocomposites. [ABSTRACT FROM AUTHOR]

Abstract: We have investigated the formation of silicon nanocrystals (Si-nc) ex situ of the silicon dioxide host matrix as well as their structural and optical properties. Two different methods were used: Si-nc prepared by electrochemical etching as reference and Si-nc fabricated by ball milling. We show that both methods are able to prepare Si-nc with quantum confinement size (<10nm). The Si-nc fabrication ex situ of host matrix allows investigating additional properties in colloidal ethanol dispersion by light scattering. The identification of Si-nc in the ethanol-based colloids has been performed by Malvern autosizer experiments. We show the clustering effect of Si-nc and as well as declustering step by adding ammonia to ethanol-based colloids. Reached Si-nc sizes from light scattering experiment corresponds to that evaluated by other independent techniques. Furthermore, we have prepared spin on glass-based self-supporting samples with high Si-nc concentration layers which allowed PL analysis. In all cases visible photoluminescence at room temperature was detected. However, for Si-nc prepared by ball milling a weak PL was detected, which likely originates rather from surface states then quantum confinement size effect. [Copyright &y& Elsevier]

In this work, we investigate the potential use of silicon nanocrystals (Si-nc) into photovoltaics technology as one possible way to increase the silicon solar efficiency at low cost. The Si-nc were prepared ex situ (pulverizing of electrochemical etched porous silicon), embedded into spin-on-glass antireflecting SiO2 based solution and then spun onto standard silicon solar cells. The Si-nc/SiO2 layer serves as a luminescence down-converter. Indeed, the high energetic photons are absorbed within the converter ‘Si-nc’ and transformed via its photoluminescence (PL) to red ones (∼700 nm) which are then converted much more efficiently in silicon solar cell. We first quantify the size of the Si-nc. Then we present investigations of Si-nc based converter with different PL intensities and its influence on solar cell performances (internal quantum efficiency (IQE), current–voltage characteristic). We observe increase in IQE in the region where the PL of Si-nc appears. We also report the correlation between the converter PL intensity and IQE. To get insight into the potential of such converter, we introduced a simplified one-dimensional model. The results of the modelling are shown and compared with experimental data. [Copyright &y& Elsevier]

Photoluminescence (PL) from silicon nanocrystals (Si-nc) prepared from pulverised porous silicon and embedded in undoped (SOG) or phosphorus-doped spin-on-glass (SOD) solutions was studied. Effects of rapid thermal annealing on the PL was also investigated. A strong room temperature PL signal was observed at 710 nm due to the recombination of electron–hole pairs in Si-nc and the PL maximum shifts to the blue region as the phosphorus concentration in the spin on glass increases. However, the rapid thermal annealing process (30 s, 900°C) quenches the PL response. These results suggest that for Si-nc/SOG (SOD) the surface termination is efficient but high phosphorus doping of Si-nc is detrimental to the PL. [Copyright &y& Elsevier]

Porous silicon grains embedded in the phosphorus doped SiO2 matrix exhibit improved photoluminesce properties and better stability in comparison with native porous silicon samples. We have tested this material for the presence of room temperature optical amplification under femtosecond (100 fs, 395 nm) excitation. Combined variable stripe length and shifted excitation spot experiments reveal positive optical gain, the net modal gain coefficient reaching 25 cm-1 at a pump intensity of 1.1 W/cm2 (mean power). The gain spectrum is broad (full width at half maximum ∼130 nm), peaked at ∼650 nm, and is slightly blueshifted with regard to the standard photoluminescence emission. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Charge transport in microcrystalline silicon is strongly influenced by its heterogeneous microstructure composed of crystalline grains and amorphous tissue. An even bigger effect on transport is their arrangement in grain aggregates or possibly columns, separated by grain boundaries, causing transport anisotropy and/or depth profile of transport properties. We review special experimental methods developed to study the resulting transport features: local electronic studies by combined atomic force microscopy, anisotropy of conductivity and diffusion length and also their thickness dependence. A simple model based on the concept of changes of transport path for description of the observed phenomena is reviewed and its consequences for charge collection in microcrystalline based solar cells are discussed. [Copyright &y& Elsevier]