G. Willeke

Band‐band impact ionization and solar cell efficiency

The effect of impact ionization has been taken into account in the calculation of the maximum solar cell efficiency in the thermodynamic limit. A red shift of the optimum band gap is observed with respect to the Shockley–Queisser result. A maximum solar cell efficiency of 60.3% at Eg=0.8 eV is predicted, which compares with 43.9% at Eg=1.1 eV for the usual case. In order to obtain a significant increase in η, an impact ionization probability of at least a few percent is required. A most pronounced effect is observed for bandgap energies below about 1 eV.

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.

Polycrystalline silicon solar cells with a mechanically formed texturization

Polycrystalline (Wacker SILSO) silicon has been mechanically textured using a conventional dicing saw and beveled blades for V‐groove formation. The minimum optical reflectivity achievable is limited by the blade tip radius and surface roughness after damage etching. Solar cells were prepared using a conventional diffusion and screen printed metallization. Grooved cells without an additional antireflection coating (jsc=31.8 mA/cm2, Voc=536 mV, FF=69%, η=11.8%) showed a 20% increase in jsc and a 1.1% absolute efficiency improvement as compared to a nongrooved reference cell with an antireflexion coating (jsc=26.4 mA/cm2, Voc=547 mV, FF=74.1%, η=10.7%). In grooved cells the efficiency is found to be limited mainly by the fill factor due to a nonoptimized front grid design.

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.

Theoretical and experimental behavior of monolithically integrated crystalline silicon solar cells

A concept for the fabrication of monolithically integrated silicon solar cells is presented. The concept is based on standard Si wafer technology and does not use thin-film approaches. A key feature is isolation trenches dividing the wafer into several unit solar cells. Due to the imperfect isolation between unit cells defined on the same conductive wafer, some device aspects deviating from an ordinary series connection of solar cells arise. For the theoretical description, a model proposed by Valco et al. [G. J. Valco, V. J. Kapoor, J. C. Evans, Jr., and A. T. Chin, in Proceedings of the 15th IEEE Photovoltaic Specialists Conference, Orlando, FL (1981), p. 187] has been generalized by using a two-diode concept for the unit cells and by weakening the assumption of identical unit cells. The model was used to simulate the cell performance in dependence on light intensity, isolation resistance, cell area, and number of unit cells. As a result, general design rules for these truly monolithically integrated so...

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.

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.

Two- and three-dimensional optical carrier generation determination in crystalline silicon solar cells

Two- and three-dimensional analyses of the distribution of optically generated charge carriers in textured crystalline silicon solar cells of arbitrary geometry have been performed. The simulation algorithm, developed for that purpose, is based on geometrical optics and ray tracing. It determines the dominant contributions to the optical generation within textured silicon exactly. The contribution of weakly absorbed long-wavelength photons is calculated using a Monte-Carlo simulation. The presented algorithm is fast and accurate and can also be used to calculate reflectance and transmittance spectra in excellent agreement with measurements. Two- and three-dimensional generation profiles in single- and double-sided textured solar cells are presented and discussed in detail. Examples for applications are given. Finally, the presented algorithm is compared with a pure Monte-Carlo algorithm.

Passivation properties of amorphous and microcrystalline silicon layers deposited by VHF-GD for crystalline silicon solar cells

Abstract A comparative study of crystalline silicon (c-Si) surface passivation techniques is presented. The passivation is essential in increasing the total solar cell efficiency by enhancing the open-circuit voltage ( V oc ). Apart from the thermally grown oxides or nitrides widely used sofar, recent publications report on excellent passivating properties of amorphous silicon (a-Si:H)- and microcrystalline silicon (μc-Si:H) layers, deposited on n-type crystalline silicon [1–3], forming thereby heterojunctions. We present first results of a new structure called ⪡BAP (Both Sides Amorphous Passivated) cell⪢, where the wafer was sandwiched between two intrinsic amorphous silicon layers. These two layers, as well as the μc-Si:H emitter, and the layer forming the BSF (Back Surface Field) were all deposited by the VHF-GD (Very High Frequency Glow-Discharge) process which was developed in our laboratory [4]. BAP cells showed V oc -values as high as 635 mV, proving thus the excellent passivating properties of a-Si:H and the validity of this cell concept which makes a fully in-line processing possible (wafer in-cell out).