晶硅制绒光学设计

Presented at the 26th European PV Solar Energy Conference and Exhibition, 5-9 September 2011, Hamburg, Germany OPTICAL CHARACTERIZATION OF RANDOM PYRAMID TEXTURIZATION Katrin Birmann1, Matthias Demant1,2, S. Rein1 1Fraunhofer Institute for Solar Energy Systems (ISE)

2Freiburg Material Research Center (FMF)

Heidenhofstrasse 2, D-79110 Freiburg, Germany

ABSTRACT: The reflection property of alkaline textured wafers is an important parameter for texture evaluation. Nevertheless, the geometric properties of the surface structure influence cell performance and cannot be distinguished by reflection measurements only. The pyramid size distribution can affect, e.g. contact formation or emitter passivation. Confocal images allow the recognition of shiny areas, the pyramid number and pyramid size distribution. The laser intensity and height information images were evaluated by image processing techniques. The pyramid segments of the pyramids were found to be a good indicator for peak size and recognition of long pyramid edges. The derived pyramid height distribution can be used to distinguish different pyramid homogeneity. Keywords: alkaline texturization, image processing, confocal microscopy

1 INTRODUCTION Alkaline etching with potassium, or sodium hydroxide and an additive like a surfactant or an alcohol is the usual process to texture monocrystalline wafers for silicon solar cells. This alkaline texturing results in random pyramids on the wafer surface which reduces its reflection. These pyramids, in contrast to inverted pyramids, have the characteristic of size inhomogeneity due to the process since pyramids are never initiated at the same time. In order to control the quality of the texture and the wafer surface, a suitable characterization method is needed. The reflection of the wafer surface is a commonly used method. Flat parts that are not covered with pyramids can be detected by higher reflection. Pyramid sizes in the micrometer range cannot be distinguished, as the reflection value is still low and the wafers appear macroscopically homogeneous. The influence of the pyramid size on the silicon solar cell performance has been investigated by Ximello [11]. Inhomogeneous pyramidal textures with a wider pyramid size distribution, e.g., more large pyramids (5 to 6 µm) and small pyramids (0.2 to 1 µm), showed a lower short circuit current, open circuit voltage, fill factor and cell efficiency than homogeneous textured silicon solar cells. Mäckel [4] studied textures and solar cells with different reflection and different angular resolved reflection recorded as a texture quality factor. Even for low reflections between 10 to 12% (at 625 nm) the factor varied from 10 to 70 relating to different texture qualities. Mäckel showed a trend for increasing short circuit current with increasing texture quality factor. Scanning electron microscope (SEM) pictures are frequently used for pyramidal texture characterization due to the strong signal contrast, the lateral resolution and the depth of sharpness [1, 4, 9, 11]. Next to SEM, further microscopic optical methods such as confocal imaging have been improved recently to allow a fast, contactless and non-destructive characterization of the pyramid structure. The spatially resolved height information of the confocal images is used for evaluation [10]. The parameter arithmetic quadratic deviation of the mean (Sq) or Root Mean Square (RMS) gives an impression of pyramid size, but not of the pyramid size distribution [8]. Whereby, the inhomogeneous size distribution has an influence on solar cells [11]. 2 EXPERIMENTAL 2.1 Alkaline texturing and characterization Monocrystalline Czochralski silicon (Cz-Si) wafers (125x125 mm², 1-3 Ohm cm) were textured at a laboratory wet chemical bench in KOH and additive (XV090075 from Dow) using a bath volume of 15 liters. The different pyramid sizes were accomplished by changing the chemical concentration and the process time resulting in a surface with small, big and mixed pyramids on a completely and a not completely covered wafer surface. These textured surfaces have been studied by SEM with 1 k magnification (Fig. 1). The SEM pictures have been divided into six classes: 80% and 96% coverage, and a complete coverage with small, medium, big and mixed pyramids. The reflection was measured at 600 nm after texturing by using a Varian spectral photometer (Table I). For the completely covered surface the reflection is lower than 12%. The little higher reflection for big pyramids results from a lack of coverage of 0.2%.