Effective passivation of crystalline silicon surfaces by ultrathin atomic-layer-deposited TiOx layers

We characterize the surface passivation properties of ultrathin titanium oxide (TiOx) films deposited by atomic layer deposition (ALD) on crystalline silicon by means of carrier lifetime measurements. We compare different silicon surface treatments prior to TiOx deposition, such as native silicon oxide (SiOy), chemically grown SiOy and thermally grown SiOy. The best passivation quality is achieved with a native SiOy grown over 4 months and a TiOx layer thickness of 5 nm, resulting in an effective lifetime of 1.2 ms on 1.3 Ωcm p-type float-zone silicon. The measured maximum lifetime corresponds to an implied open-circuit voltage (iVoc) of 710 mV. For thinner TiOx layers the passivation quality is reduced, however, samples passivated with only 2 nm of TiOx still show a lifetime of 612 μs and an iVoc of 694 mV. The contact resistivity of the TiOx including the SiOy interlayer between the silicon wafer and the TiOx is below 0.8 Ωcm2. The combination of excellent surface passivation and low contact resistivity has the potential for silicon solar cells with efficiencies exceeding 26%

Oxidation as Key Mechanism for Efficient Interface Passivation in Cu (In,Ga)Se2 Thin-Film Solar Cells

Copper-indium-gallium-diselenide (CIGS) thin-film solar cells suffer from high recombination losses at the back contact and parasitic absorption in the front-contact layers. Dielectric passivation layers overcome these limitations and enable an efficient control over interface recombination, which becomes increasingly relevant as thin-film solar cells increase in efficiency and become thinner to reduce the consumption of precious resources. We present the optoelectronic and chemical interface properties of oxide-based passivation layers deposited by atomic layer deposition on CIGS. A suitable postdeposition annealing removes detrimental interface defects and leads to restructuring and oxidation of the CIGS surface. The optoelectronic interface properties are very similar for different passivation approaches, demonstrating that an efficient suppression of interface states is possible independent of the metal used in the passivating oxide. If aluminum oxide (Al2O3) is used as the passivation layer we confirm an additional field-effect passivation due to interface charges, resulting in an efficient interface passivation superior to that of a state-of-the-art cadmium-sulfide (CdS) buffer layer. Based on this chemical interface model we present a full-area rear-interface passivation layer without any contact patterning, resulting in a 1% absolute efficiency gain compared to a standard molybdenum back contact

Reassessment of the intrinsic bulk recombination in crystalline silicon

Characterisation and optimization of next-generation silicon solar cell concepts rely on an accurate knowledge of intrinsic charge carrier recombination in crystalline silicon. Reports of measured lifetimes exceeding the previous accepted parameterisation of intrinsic recombination indicate an overestimation of this recombination in certain injection regimes and hence the need for revision. In this work, twelve high-quality silicon sample sets covering a wide doping range are fabricated using state-of-the-art processing routes in order to permit an accurate assessment of intrinsic recombination based on wafer thickness variation. Special care is taken to mitigate extrinsic recombination due to bulk contamination or at the wafer surfaces. The combination of the high-quality samples with refined sample characterisation and lifetime measurements enables a much higher level of accuracy to be achieved compared to previous studies. We observe that reabsorption of luminescence photons inside the sample must be accounted for to achieve a precise description of radiative recombination. With this effect taken into account, we extract the lifetime limitation due to Auger recombination. We find that the extracted Auger recombination rate can accurately be parameterized using a physically motivated equation based on Coulomb-enhanced Auger recombination for all doping and injection conditions relevant for silicon-based photovoltaics. The improved accuracy of data description obtained with the model suggests that our new parameterisation is more consistent with the actual recombination process than previous models. Due to notable changes in Auger recombination predicted for moderate injection, we further revise the fundamental limiting power conversion efficiency for a single-junction crystalline silicon solar cell to 29.4%, which is within 0.1%abs compared to other recent assessments

The pH dependence of phosphate sorption and desorption in Swedish agricultural soils

A number of previous studies have reported the existence of a minimum in phosphate solubility between pH 5.5 and 7 in non-calcareous soils. Different hypotheses have been forwarded to explain this phenomenon. In this study, ten soil samples with varying textures and phosphorus status were subjected to batch experiments in which dissolved phosphate was measured as a function of pH and phosphate load. Soil samples with more than 20% clay all had a minimum phosphate solubility between pH 6 and 7, whereas for samples with &lt;10% clay, no such minimum was observed. Further experiments involving additions of phosphate and arsenate showed an increasing adsorption of these anions with decreasing pH also below pH 6 in clay soils, suggesting that the pH dependence on adsorption and desorption in short-term experiments was not the same. Kinetic experiments showed that the increased phosphate desorption at lower pH values in non-calcareous clay soils was a quick process, which is consistent with adsorption/desorption being the most important mechanism governing the retention and release of inorganic P. Moreover, by comparing extraction results with batch experiment results for samples from a long-term fertility experiment, it was concluded that more than 60% of the accumulated phosphate was occluded, i.e. not reactive within 6 days. Additional evidence for an important role of occluded phosphate comes from an analysis of the Freundlich sorption isotherms for the studied soils. It is hypothesized that interlayered hydroxy-Al and hydroxy-Fe polymers in clay minerals may be important for P dynamics in clay soils by trapping some of the P in an occluded form. The results also suggest that improved knowledge on the speciation and dynamics of phosphorus in soils is required for consistent mechanistically based modeling of phosphate sorption/desorption reactions.QC 20130125</p