A New Method to Characterize TCO/P Contact Resistance in a-SI Solar Cells
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interface, forming a non-ohmic contact. Various schemes to improve the ZnO/p electrical contact have been discussed (7). However, characterization of the TCO/p interface is difficult since it is in series with the dominant p-i-n junction. We have developed a method to characterize the TCO/p contact based on the work of Shafarman and Phillips (8). This method requires having a two adjacent TCO regions. A device is biased to have standard current flow through its TCO/p contact and TCO region while the voltage is measured on the adjacent TCO pad which is electrically "floating." The second pad is thus a voltage sensing contact giving internal access to the potential in the biased device. An additional benefit to this technique is that the SnO sheet resistance in a completed device structure is also obtained. SAMPLE FABRICATION The substrates studied here were textured Sn0 2-coated glass made by Asahi Glass (Type U), AFG, or Solarex. The devices were fabricated at Solarex. The Sn0 2 on the 3x3 inch square pieces was laser scribed to create long parallel strips approximately 8-10 mm wide and 76 mm (3") long. Then, single junction a-Si p-i-n layers were deposited by PECVD. The p-layers were a-SiC:H. A row of six individual devices were fabricated on each Sn0 2 strip by depositing a ZnO/AI back contact (5x5 mm 2) mm through a mask on the n-layer. A low resistance contact to the SnO2 strip was made with Ag paste which was baked at 200'C.
737 Mat. Res. Soc. Symp. Proc. Vol. 557 ©1999 Materials Research Society
ANALYSIS A top view and side view of the resulting structure is shown in Figure 1. The two SnO 2 strips of width W, with Ag-paste contacts labeled A and B, each have a row of square devices. The current in cells 1, 2, 3, etc. travels in the SnO 2 a distance L from each device to the SnO 2 contact A. The series resistance of the SnO2 between the device and the contact is
(1)
RTCo = RSH x (L/W)
where RSH is the sheet resistance of the SnO 2. As L increases, the series resistance of the TCO between the device and its SnO 2 contact increases. Assuming each device is identical, and its TCO/p contact is identical, this increase in RTCO will be the only difference between devices in the same row.
ton view S S n0 n22
scribe lines
to(defines p cell contacts area)
I
SnO 2 contact (Ag paste)
side view TCOIp
interface &-ý
a-Si
- ---- -
p-i-n
SnO, glass
Figure 1. Top view and side view of row of a-Si devices (1-2-3) on scribed SnO 2 strips with contacts labeled A and B.
B
Figure 2. Cross-section view looking down SnO 2 strips A and B. Current flow is established in pi-n cell 1 between 1 and A while voltage is measured between I and A, 1 and B, or A and B. SnO 2 strip B is floating at same potential as the p-layer over strip A.
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The three different measurement configurations will be discussed in reference to Figure 2 which shows a cross-sectional view looking down the two Sn02 strips having contacts A and B. For example, assume that cell I in the row of devices labeled 1-2-3 in Figure 1 is to be characteri
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