Ernst Bucher

A simple and effective light trapping technique for polycrystalline silicon solar cells

Abstract Mechanical grooving using a standard dicing saw in combination with bevelled blades is shown to be a promising texturing technique for polycrystalline silicon surfaces. A minimum total reflectance of R = 5.6% at 950 nm and an average R = 6.6% between 500 and 1000 nm have been obtained on SILSO silicon. The reflection coefficient obtainable is limited by blade tip curvature and a non-perfect surface after damage etching.

Origin of the deep center photoluminescence in CuGaSe2 and CuInS2 crystals

Photoluminescence (PL) of CuGaSe2 and CuInS2 single crystals, either as grown or Cu annealed, reveals a broad and clear deep emission band at hν≈Eg−0.6 eV. In both of these as-grown materials this band has a similar doublet structure with the two D1,D2 subbands separated by about 100 meV. After the Cu annealing all samples became highly compensated and an additional deep PL band (W band) appeared on the high energy side of these D bands. This suggests a closely similar origin of the emission for the both materials. By a straightforward model calculation we show that the changes in the shape and intensity of these emission bands—due to variation of temperature, excitation intensity or due to the Cu annealing—are well explained if we assume that the D1 and D2 PL subbands originate in the recombination between the closest and the second closest donor–acceptor pairs, with the essential ingredient of the emission center being an interstitial donor defect, i.e., either Cui or Gai in CuGaSe2 and Cui or Ini in Cu...

Mechanical wafer engineering for high efficiency solar cells: an investigation of the induced surface damage

During mechanical structuring of crystalline silicon an electronically active surface damage layer is induced whose complete removal is a prerequisite for the preparation of highly efficient mechanically textured multicrystalline silicon solar cells. In order to evaluate the presently unknown damage layer thickness of mechanically textured silicon, electron microscopy studies and microwave reflection lifetime measurements in combination with a step etching procedure were performed using mono and multicrystalline silicon as base material. The influence of the diamond grain size and the lateral cutting speed of the beveled sawing blades on the surface damage was studied to obtain a better understanding of the mechanical structuring of silicon. In order to confirm the results obtained from lifetime measurements, screenprinted mechanically V-grooved solar cells were processed with different etching times during the sawing damage removal process step. It could be shown that the electronically active surface damage layer has a thickness of about 3 /spl mu/m when applying standard grooving parameters and a diamond grain size of 4-6 /spl mu/m within the abrasive.

Low-cost back contact silicon solar cells

Back-contacted solar cells offer multiple advantages in regard of reducing module assembling costs and avoiding grid shadowing losses. The investigated emitter-wrap-through (EWT) device design has an electrical connection of the front emitter and the rear emitter grid in form of small holes drilled into the crystalline silicon wafer. The obtained cell structure is especially suitable for low-cost base material with small minority carrier diffusion lengths. Different industrially applicable solar cell manufacturing processes for EWT devices are described and compared. The latest experimental results are presented and interpreted; the photocurrent is found to be distinctly increased. The relation between open circuit voltage and rear side passivation is discussed based on two-dimensional (2-D) computer simulations.

Characterization of novel mono- and bifacially active semi-transparent crystalline silicon solar cells

This paper presents the latest cell results for semi-transparent mono- as well as bifacially active POWER (Polycrystalline Wafer Engineering Result) solar cells of different cell sizes on Cz and multicrystalline silicon substrates. Top efficiencies of 10.4% for monofacial and 12.9% for bifacial cells are reported. Attention has been paid to apply a fully industrially compatible production process. It uses dicing saw based mechanical texturization of the front and rear side of the silicon wafer and screen printing metallization. In the POWER solar cell concept, perpendicular grooves on the front and rear side create holes with a variable diameter at their crossing points. This results in a partial optical transparency of the solar cell. In this study, holes of 200 /spl mu/m diameter lead to a transparency of 16-18% on average for the total cell area. The cell characteristics for the different cell types are compared by means of illuminated and dark current-voltage (I-V), spectral response, and Laser Beam Induced Current (LBIC) measurements. While bifacial POWER cells need a more elaborate production process, they reveal better I-V characteristics and a higher efficiency as compared to monofacial cells. This is mainly explained by a better surface passivation due to an active emitter and a passivating silicon nitride ARC both on the front and rear surface.

CVT-growth of AgGaSe2 single crystals: Electrical and photoluminescence properties

Abstract Single crystals of AgGaSe 2 were grown by the Chemical Vapor Transport (CVT) -method using iodine as transport agent. Growth temperatures of 770°C and concentrations of the transport agent of 1.6–1.7 mg I 2 /cm 3 yielded compact single crystals with a size of up to 8 X 5 X 5 mm 3 and a habitus dominated by the {112}-faces. The as-grown crystals were highly insulating (σ −8 (Ω cm) −1 ). Annealing of the crystals in vacuum at 700°C resulted in n-type conductivity of 2 · 10 −1 (Ω cm) −1 with a dominant peak in the photoluminescence spectra at 4 K associated with a donor level of 164 meV. Annealing in Se-atmosphere at 600°C lead to p-type conductivity of 6 · 10 −6 (Ωcm) −1 within a surface layer of the AgGaSe 2 single crystals. The corresponding photoluminescence spectra and the activation energy of the electrical conductivity (between 100 K and 300 K) suggest the presence of an acceptor (V cation ) with an activation energy of 60 meV and a donor (V Se ) with an activation energy of 100 meV.

Optical behavior of textured silicon

The optical behavior of polycrystalline SILSO silicon wafers with a mechanically V‐grooved surface has been studied between 300 and 1500 nm. The texturization was carried out by a conventional dicing saw using beveled blades. For a 35° V‐grooved surface and a nonmetallized backside the optical path length in the weakly absorbing part of the spectrum (1100–1200 nm) was found to be enhanced by a factor of 33 as compared to a nongrooved wafer. The enhanced reflectance in the nonabsorbing spectral region for the former is analyzed and explained. The different loss contributions due to a nonideal grooved structure are discussed.

Effects of pn-junctions bordering on surfaces investigated by means of 2D-modeling

Several new solar cell designs, among them the emitter wrap through (EWT) and the POWER solar cell, suffer from reduced fill factors. These cells have interdigitated p- and n-type regions. At the margin of these regions, the p-n junction borders on the surface causing additional recombination. We investigate by means of two-dimensional modeling the recombination mechanisms occurring in such device regions, and we give an experimental example. It is shown that a poor quality of the surface passivation near to where the pn-junction borders, is mainly responsible for the observed losses in fill factor and open-circuit voltages.

Spin dynamics and magnetic ordering in mixed valence systems

Neutron scattering measurements are reported on the mixed valence compounds Ce1−xThx and TmSe. The χ″ (Q,ω) as derived from the inelastic spectra of Ce0.74Th0.26 shows a peak in the γ phase near 20.0 meV and shifts abruptly to greater than 70.0 meV at the transition to the α phase. The temperature independence of the susceptibility within the γ phase cannot be simply reconciled with the temperature dependence of the valence within the γ phase. TmSe is shown to order in a type I antiferromagnetic structure below TN∼3.2 K. The magnetic phase diagram is understood as a successive domain reorientation and a metamagnetic phase transition for T<3 K with increasing field. The mixed valence nature manifests itself in a reduced moment and a markedly altered crystal field. Another sample of TmSe with a lattice parameter implying 100% Tm3+ exhibits spin correlations characteristic of a type II structure but never achieves long range order.

Enhanced Carrier Collection observed in Mechanically Structured Silicon with Small Diffusion Length

The diffusion length of minority charge carriers in the silicon bulk Ldiff is an important characteristic of optoelectronic devices fabricated from low cost silicon wafers. In this study computer simulations have been carried out to calculate the beneficial effects of a macroscopic surface texturization on the charge carrier generation and the collection probability. Textured solar cells should be able to collect charge carriers more effectively resulting in an increased current due to the special emitter geometry resulting from the texture, decreased reflection losses, and the inclined penetration of the light. In order to prove this expected behavior, deeply V-textured solar cells have been processed and characterized on low cost silicon reaching an Ldiff of about 25 μm. Spatially resolved high resolution measurements of the internal quantum efficiency exhibit a strongly increased signal in the texture tips which is the first experimental proof of the increased charge carrier collection probability of d...