THE ALU + CONCEPT: N-TYPE SILICON SOLAR CELLS WITH SURFACE- PASSIVATED SCREEN-PRINTED ALUMINUM-ALLOYED REAR EMITTER Robert Bock 1 , Jan Schmidt 1 , Susanne Mau 1 , Bram Hoex 2 , Erwin Kessels 2 , and Rolf Brendel 1 1 Institut für Solarenergieforschung Hameln (ISFH), Am Ohrberg 1, D-31860 Emmerthal, Germany 2 Dep. of Applied Physics, Eindhoven University of Technology, 5600 MB Eindhoven, The Netherlands ABSTRACT Aluminum-doped p-type (Al-p + ) silicon emitters fabricated by means of screen-printing and firing are effectively passivated by plasma-enhanced chemical- vapor deposited (PECVD) amorphous silicon (a-Si) and atomic-layer-deposited (ALD) aluminum oxide (Al 2 O 3 ) as well as Al 2 O 3 /SiN x stacks, where the silicon nitride (SiN x ) layer is deposited by PECVD. While the a-Si passivation of the Al-p + emitter results in an emitter saturation current density J 0e of 246 fA/cm 2 , the Al 2 O 3 /SiN x double layers result in emitter saturation current densities as low as 160 fA/cm 2 , which is the lowest J 0e reported so far for screen-printed Al-doped p + emitters. Moreover, the Al 2 O 3 as well as the Al 2 O 3 /SiN x stacks show an excellent stability during firing in a conveyor belt furnace at 900°C. We implement our newly developed passivated Al-p + emitter into an n + np + solar cell structure, the so-called ALU + cell. An independently confirmed conversion efficiency of 20% is achieved on an aperture cell area of 4 cm 2 , clearly demonstrating the high-efficiency potential of our ALU + cell concept. INTRODUCTION For solar cells where n-type Czochralski-grown (CZ) silicon bulk material is used, a p + -emitter is needed for the formation of the pn-junction. This p + -region can e.g. be formed by a high-temperature boron-diffusion or simple a screen-printing process, where Al-paste is screen-printed on the Si wafer and subsequently fired in a conveyor belt furnace, resulting in an Al-p + emitter. The high-temperature p + boron-diffusion has in the past mainly been used for the fabrication of high-efficiency laboratory solar cells, because it is technologically more demanding and tends to induce crystallographic defects in the bulk material degrading its recombination lifetime [1,2]. On the other hand, there is a simple screen- printing-based process for the formation of the p + -region which is mainly used for the formation of back surface fields (BSFs) on industrial p-type crystalline silicon solar cells [3,4] and as Al-p + rear emitter on n-type crystalline silicon solar cells [5]. However, the full-area metallization of the screen-printed Al-p + emitter causes a relatively high emitter surface recombination velocity which limits the solar cell conversion efficiency. To overcome this limiting factor our ALU + n-type silicon solar cell concept is based on a surface-passivated screen-printed Al-p + emitter [6]. PASSIVATED Al-p + EMITTERS In this contribution, we determine the emitter saturation current density J 0e of passivated aluminum- doped p + emitters. Fig. 1. (a) SEM micrograph of a cross section of a screen-printed Al-p + emitter. The Al-p + region is clearly visible as brighter contrast beneath the residual Al paste [7]. (b) SEM tilted plan-view image of an Al-p + Si surface after the residual Al-paste and the Al-Si eutectic have been removed. AI-rich surface structures appear as bright contrast. (c) Cross-sectional TEM bright-field image of one individual surface structure. Positions of Al-rich material covered by a thin Si layer are marked with arrows [8]. For this purpose we fabricate asymmetric test structures using single-crystalline shiny-etched (100)- oriented 300 µm thick p-type float-zone (FZ) silicon wafers of 200 Ωcm resistivity, where first one side of the wafer is passivated with PECVD-SiN x and second, on the other wafer surface, the p + -emitter is prepared by applying a conventional screen-printing process. Subsequently, the samples undergo a firing process in an infrared conveyor belt furnace at 900°C for 13 s. In