In advanced solar cell concepts, where the reduction of the silicon material thickness and the increase of cell efficiency necessitates the improvement of the rear surface design, the rear side incorporates a passivation layer that is interrupted by small-area local aluminum contacts. The potential of this cell structure is discussed in several publications [11–16].
The industry shows strong interest in these solar cell concepts with a passi-vated, locally contacted rear side, due to their higher efficiency on thinner wafers as a key requirement for the next generation of solar cells, in order to decrease the costs per power output. The use of thinner wafers means an important saving potential for silicon in the industry, since more wafers per ingot can be produced [17].
The main results presented in this thesis are based on the rear passivated solar cell concept. New experimental observations on the contact formation at the rear side of this device will be presented.
For this work, screen-printed aluminum pastes andp-type crystalline silicon surfaces were tested. The contact between aluminum and silicon is formed locally through the openings of a rear passivation layer (dielectric). The application of these physical observations gave rise to high efficiency rear passivated solar cells in laboratory. Since conventional processes can be used for fabrication, this solar
1.2. Motivation: Contact Formation for Rear Passivated Solar Cells 3
cell device presents a high potential for industrial application in the nearby future.
Important contributions to rear surface passivation techniques have come up recently. They have been motivated by the improvement of solar cell efficiency in industrial production [18–20]. The dielectrically coated rear surface enables an improvement of the internal light reflection compared to a fully covered aluminum area. Another advantage is the minimization of the rear surface recombination velocity by a reduced Al metallization. As a consequence, the open circuit voltage increases by reducing the recombination at the rear side. Another aspect is that a decrease of metallized area reduces the mechanical bow of the wafer due to the different thermal expansion coefficients of Si and Al-Si alloys [21]. The spectral response in the long wavelength is strongly enhanced for this type of solar cell device, resulting in a gain in short circuit current and therefore, in an increase of solar cell efficiency.
There are still open questions, however, regarding the understanding of the local Al-Si contact formation and its influence on the local back surface field formation for industrial screen printed aluminum pastes. The main experimental work presented in this thesis is based on the local contact formation between aluminum and silicon (see Chapter 6).
Blakerset al.[12] were the first to present a high efficiency rear side passivated solar cell. This device, called the passivated emitter and rear cell (PERC) showed an increase of the rear internal reflection up to 97 %. A SiO2barrier together with a locally alloyed Al/Si interface formed the rear structure. Although the result was not obtained on industrial large area silicon material, thep-type float zone wafer of 4 cm2size showed an impressive energy conversion efficiency of 22.8 %. With high quality bulk material an open circuit voltage of 696 mV was achieved. Due to the high rear internal reflectivity, the short circuit current was increased up to 40.3 mA/cm2, with a fill factor of 81.4 %.
Figure 1.1 shows a schematic model of the industrial PERC structure (after reference [14]). This solar cell has a passivatedn+-layer (P-diffused emitter) at the front side and a dielectrically passivatedp-type silicon layer with local contact openings (LCOs) at the rear. The interaction between Al and Si occurs locally, and, as a result, sharply delimited dark lines in the thick aluminum layer appear. Thus, the Al-Si alloy and thep+-doped Al layer (also known as back surface field, BSF) are formed in the LCOs at the back contact area. Contrary to that, state-of-the-art solar cells present a fully covered Al-BSF layer at the back, which gives the name of Al-BSF solar cells.
The analysis of several solar cell structures with rear passivation (for industrial application) is presented in the last chapter of this thesis (Chapter 7). During the
4 Chapter 1: Introduction
Figure 1.1: Solar cell structure of an industrial rear passivated solar cell (PERC structure), showing the textured and passivated front side emitter, with LCOs at the rear and local back surface field. The spread of Si inside Al is shown in an optical microscope picture, which will be discussed in Chapter 6.
processing of the PERC device, crucial questions appeared in the field of local contact formation between aluminum pastes and silicon surfaces. This thesis was motivated by the following questions (scientific analysis is presented in Chapter 6):
• What is the influence of the contact geometry of the rear side pattern (contact spacing, contact size, and contact opening) on the contact formation and mass transport, and what is the impact on solar cell level?
• What is the explanation for the formation of the observed sharply limited dark lines within the thick aluminum layer, and what is their relevance concerning the Al-Si interaction? How is the distribution of Si in the screen-printed Al?
• Is there an influence of the firing orientation of the cells during firing on the local Al-Si eutectic morphology?
• Why are voids formed after sintering of the contacts instead of Al-Si eutectic layers? Which effect influences the appearance of voids and how can they be minimized?