IMPROVING PERFORMANCE OF SUPERSTRATE p-i-n a-Si SOLAR CELLS BY OPTIMIZATION OF n/TCO/METAL BACK CONTACTS Steven S. Hegedus, Wayne A. Buchanan, Erten Eser Institute of Energy Conversion University of Delaware Newark DE 19716 ABSTRACT A comprehensive study of the n-layer and back contact for superstrate (glass/textured SnO 2 /p-i- n/TCO/metal) a-Si solar cells is presented. In particular, the difference between a-Si and μc-Si n-layers are compared. These results show that the efficiency can be improved from 7% to 10% (absolute) by optimizing the back contact layers to incorporate a good optical back reflector. A rectifying contact is formed between the TCO and a-Si n-layer which reduces FF. A μc-Si n-layer eliminates the blocking n/TCO contact. Results suggest that the n/TCO interface has a controlling influence. ZnO gives ~1 mA/cm 2 higher J sc compared to ITO. The best contacts are μc-Si/ZnO/metal. INTRODUCTION Most a-Si solar cell research has focused on the i-layer, p-layer, and p/i and TCO/p interfaces. The n-layer and its contact has received relatively little attention. However, the n-layer and its contact can have a significant influence on performance [1,2]. We have performed a comprehensive study of the n-layer and back contact for superstrate a-Si solar cells. The goal is to determine the optimum contact having high back reflection and low absorption which is needed for high J sc , along with low contact resistance which is needed for high FF. This work identifies and separates the critical roles of the n- layer conductivity, the TCO and the metal layer. As a result of this effort, we succeeded in fabricating a device having 10.4% efficiency, verified at the National Renewable Energy Laboratory (NREL), having parameters: V oc =0.880 V, J sc =16.2 mA/cm 2 , and FF=72.7%. DEVICE FABRICATION Single junction a-Si solar cells were deposited in a single chamber plasma CVD system with the configuration of glass/textured SnO 2 /p-b-i-n/TCO/metal. The textured SnO 2 was Asahi type U. The p and b (buffer) layers were a-SiC. There was no H 2 dilution of the p, b, or i- layers. The i-layers were 0.5 μm thick. All devices analyzed in this paper had identical p-b-i layers. The only differences were the types of n-layer, a-Si or μc-Si, and the back contacts. Film properties of the two types of n- layers are shown in Table 1. The μc-Si n-layer has a significantly lower activation energy and lower resistivity compared to the a-Si. Deposition conditions and properties of the ITO and ZnO films used as back TCO contacts are shown in Table 2. Metal contacts typically 0.5 μm thick of Ag, Ti(25Å)/Ag and Al were evaporated either directly on n-layers or on ZnO. The electrical and optical behavior of the back contacts have been studied using temperature dependent current voltage (J-V) measurements in the light and dark, and quantum efficiency (QE) and reflection measurements. Table 1 . Gas flow rates (sccm) and properties of the a-Si and μc-Si n-layers J-V RESULTS AND ANALYSIS Table 3 shows the results from a series of devices having either a-Si or μc-Si n-layers with either Ti/Ag, ITO/Ag, or ZnO/Ag back contacts. The ITO and ZnO layers were ~70 nm. The a-Si n-layers make very poor contacts with either ITO or ZnO as indicated by the low FF and high R oc (dV/dJ at open circuit). Figure 1 shows the J- V curves for the four devices in Table 1 with a-Si n-layers. The device with the Ti/Ag metal contact is the only one with a “normal” J-V curve, having high FF and low R oc . The non-ideal curvature around V oc , characterized by large R oc in Table 3, occurs for all three a-Si n-layer devices with a TCO/Ag contact. The curvature is greater for ZnO (Ar/O 2 ) than for ITO (Ar/O 2 ), and it is greater for ZnO (Ar/O 2 ) compared to ZnO (Ar). Table 2 indicates the ITO and ZnO resistivities differ by only a factor of 2. This suggests that a critical role of the sputtering atmosphere may be to n-layer H 2 /SiH 4 ρ(Ω -cm) E A (eV) E 04 (eV) a-Si 0/30 1000 0.30 1.90 μc-Si 200/2 1 0.05 2.05 Table 2 . Sputtering conditions and properties of ITO and ZnO TCO layers. TCO layer % O 2 in Ar resistivity ( Ω-cm) avg. absorption λ =l500-900 nm ZnO:Al 0 6 E-4 0.011 ZnO:Al 0.2% 10 E-4 0.006 ITO 0.9% 4 E-4 0.012