Low Surface Recombination Velocity using amorphous Silicon on industrial-type cleaned Surfaces

AbstractThe surface passivation ability of a hydrogenated amorphous silicon (a-Si:H) layer is investigated on industrial-type cleaned p-type (2Ωcm) FZ and Cz silicon wafers. The cleaning sequence consists of laser/saw damage removal and immersions in aqueous HCl and HF solutions. After this cleaning an effective surface recombination velocity Seff of 1.0cm/s is achieved with a deposited and annealed a-Si:H-layer. No RCA or similar elaborate cleaning steps are needed to achieve this low surface recombination velocity. After a firing step in a belt furnace at a wafer temperature of up to 670°C the passivation ability of the a-Si:H layer is fully restored during hydrogen annealing. It is shown that the amount of silicon hydrogen bonds in the a-Si:H layer is correlated to the degradation and recovery of the passivation quality of the a-Si:H layer

Light scattering and diffuse light propagation in sintered porous silicon

The scattering coefficient and the refractive index of sintered porous silicon are deduced from measurements on 1-4 mu m thick freestanding films. Mie's theory is applied to describe the light scattering by the spherical pores. Using a reduced effective refractive index for the host medium in Mie's theory accounts for the close spacing of the pores and results in an agreement between the measured and calculated scattering coefficients. A coherent calculation for the specular nonscattered radiation is combined with a model that describes the propagation of the scattered diffuse light flux. For this diffuse model two approaches, the Kubelka Munk theory and a Lambertian model developed in this work, are compared. The combined model reproduces both, the specular as well as the diffuse component of the measured reflection and transmission

Influence of the dopant on the contact formation to p+-type silicon

AbstractIn this contribution we investigate the influence of the doping element on the contact formation to p+-type Si. Contacting B doped layers with Ag thick film paste leads to very few Ag crystallites at the contact interface and results in poor contact resistances [1–3]. Using Ag/Al thick film paste for contact formation, the contact is not only formed by Ag crystallites, but by diversely shaped Ag/Al contact spots and the contact resistance is reduced by more than one order of magnitude [2]. Al melting at the Si wafer surface forms Al doped rectangles on the Si wafer where the growth of Ag/Al/Pb spikes is enhanced.When contacting Al doped layers with Ag thick film paste a larger number of Ag crystallites is observed than for B doped layers. If the contact is formed with Ag/Al paste the number of Al-rich rectangles is enhanced and we detect higher doped areas under the contact spots. The contacts detected have an ellipsoidal, pyramidal or “L-formed” shape. We conclude that not only the acceptor impurity concentration under the contact area is crucial for the contact formation, but also the properties of the specific acceptor present

Review on screen printed metallization on p-type silicon

AbstractAdvanced solar cell contacts feature local contacts to p- and p+- Si. In this review existing models for contact formation to p and p+-Si are presented.The formation of the local Al BSF as applied in PERC like solar cell structures is inhibited by the formation of voids. It is shown that the formation of voids depends on process parameters and their influence on the contact formation is reviewed. Using an analytical model it is possible to predict the depth of the resulting Al BSF in dependence of the contact geometry and the peak firing temperature.Contact formation to B doped Si with pure Ag paste results in high contact resistances whilst contact formation to Al doped Si with Ag paste is less difficult. The contact formation with Ag paste to B doped Si is enhanced by the addition of Al to the Ag paste

Screen-Printed Al-Alloyed Rear Junction Solar Cell Concept Applied to Very Thin (100μm) Large-Area n-Type Si Wafers

AbstractReducing the thickness of crystalline Si wafers processed to solar cells returns two significant benefits. Firstly, processing cost is reduced by saving cost- and energy-intensive Si material. Secondly, the required diffusion length of minority carriers is smaller, thus, wafers with a smaller carrier lifetime (e.g. due to higher base doping) can be utilized. In this work, the industrially feasible “PhosTop” cell concept is employed by manufacturing large-area n-type rear junction solar cells with a screen-printed Al-alloyed emitter featuring a selective phosphorous front surface field and a SiO2/SiNx passivation on the front.PC1D simulations for substrates with different base doping concentrations show that the range of base resistivities utilizable for those PhosTop solar cells is extended towards higher doping concentrations with decreasing wafer thickness. PC1D forecasts a conversion efficiency of the chosen 2.8Ωcm n-type Czochralski-Si wafers of 19.2% for 100μm thickness, merely 0.1% less than for standard thickness but saving ∼25% of the Si material. The manufactured thin large-area solar cells achieve a maximum efficiency of 19.0%

Al2O3/SiNx-Stacks at Increased Temperatures: Avoiding Blistering During Contact Firing

AbstractWe investigate the passivation stability and blister formation during firing at 800°C of an Al2O3/SiNx stack deposited on p-type float zone silicon at different Al2O3 deposition set temperatures ranging from 170°C to 400°C. The actual wafer temperatures during Al2O3 deposition in the FlexAL reactor are determined using spectroscopic ellipsometry. After the firing step blistering can be observed for stacks featuring 15nm thick Al2O3 layers grown at 170°C set temperature. We show that the deposition of the layer at higher set temperatures of 250°C, 300°C and 400°C reduces blister formation significantly. After firing, stacks with 15nm thick Al2O3 layers deposited at set temperatures of 250°C and 300°C show the best passivation resulting in effective surface recombination velocities below 5cm/s without significant blister formation

Annealing and oxygen role in the structural, optical and electrical properties of nc-SiOxNy

Research in the renewable energy field is mandatory to provide alternatives to fossil fuels in energy production and photovoltaics is very promising as solar energy is widely distributed. Moreover, the related technology is nowadays easily available, it can be integrated with buildings and presents limited environmental issues. Silicon Oxi-Nitride (SiOxNy) thin films have been characterized in view of the application in thin-film solar cells as well as in wafer based silicon solar cells, like Silicon HeteroJunction (SHJ) solar cells. Material properties like optical and electronic properties of the film, composition and structure of the nanocrystals have not been studied yet. In addition, the role of deposition conditions and precursor gas concentrations on these properties is also not clear. The results presented in the contribution aim to clarify several aspects concerning nc-SiOxNy film properties