ANALYSIS
structures that form the series connections. Electrical anode and cathode connections are then formed at each end of the series of cells.
The PN junction apparently forms near the top surface of the CIGS layer as a result of surface
segregation of indium rich materials, such as CuIn3
Se5
, causing the conductivity type to invert
from P-type to N-type, and thus forming a PN junction.
The molybdenum film provides the interconnection and the anode back contact for each cell while the N-type, ZnO:Al TCO is the cathode contact. A layer of CdS and an undoped ZnO buffer layer are often used to reduce lattice mismatch between the CIGS layer and the TCO.
The cleaved edge of one of the CIGS solar panels recently analyzed by Chipworks is shown in Figure 2. The bottom glass is usually (cheap, easily available) soda-lime window glass and appears green in color, due to the presence of sodium. Diffusion of sodium into the stack from the glass has been shown to improve the performance of the cell.
26
Figure 3 P1 Scribe Lane
Figure 3 shows a cross-sectional view of a P1 scribe line, which provides the electrical isolation between anodes of adjacent cells, as shown schematically in Figure 1. The TCO, CIGS and Mo layers are easily distinguished in the cross-section. The Mo layer was likely laser scribed, which results in a thickening for the layer near the edges. A detailed SEM cross-section of the CIGS stack is shown in Figure 4. The top surface of the CIGS layers has a very rough morphology, which is partly re-planarized by the TCO. The CdS and ZnO buffer layer were also resolved in the analysis. The compositions were determined by SEM and TEM- based energy dispersive X-ray analysis.
While the essential nature of the CIGS cells was the same, Chipworks’ analyses of the two competing panels inevitably found differences in the details of their structure, including variation in the layers used for the optically active CIGS stack, the materials used for the transparent polymer layer, and the methods of contacting the active layers.
Figure 4 CIGS Stack
Such is the stuff of competition; as with the broader photovoltaic market, cost per installed watt will be the final arbiter.
While the essential nature of the CIGS cells was the same, Chipworks’ analyses of the two competing panels inevitably found differences in the details of their structure, including variation in the layers used for the optically active CIGS stack, the materials used for the transparent polymer layer, and the methods of contacting the active layers
www.solar-pv-management.com Issue III 2010
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56