Taking the results shown in the previous section, it is clear that solar cell performance is improved with the addition of one bus more to its front contact grid, but what happens when a bigger number of buses are added to the front grid design? Can we expect further improvement?. To answer these questions we need to modify the mathematical expressions of the analytical model to evaluate the new situation that is shown in Fig. 11.
|
Fig. 11. Basic calculus cell for a variable number of bus-bar, it is shown the basic unitary cell size for the case of having N collecting bus-bars with n bonding points per bus-bar in the solar cell |
With this configuration, expressions of the different components of the series resistance will change as it is shown in Table 6, where N is the number of buses and the rest of symbols represent the same parameter as in Table 3.
|
Component |
Expression |
|
Emitter |
R n’ s Re ЕшШег 3L / L 4 [ N Wbu J |
|
Base |
R _p 4 N ■ n ■ wbase base base L |
|
Metallic finger |
R _ П’ S p metal ( L w ^ llngei 3L wfhf [ N busJ |
|
Bus bar |
mi_ Pmetal L bus 3П Wbushbus |
|
Rear contact resistance |
R _ 4 N’ П ‘ s ‘ RFrontPaste f LwrL + N ■ wbus -(s – wf )) |
|
Front contact resistance |
R _ 4 N’ n’ RBackPaste bc L |
|
Table 6. Analytical expressions of the series resistance components for a multi bus-bar cell (with a number of bus-bars equal to N) |
Total solar cell series resistance corresponds in this case to the expression:
IR
4n • N
 |
With these new analytical modelling and the set of values for each technological parameter presented in the previous section, it is possible to extend the study of the performance of cells with two and three buses to a bigger number of buses, as it is plotted in Fig. 12, where the maximum possible efficiencies (reached with the optimal design of grid) are shown as a function of the covering factor for a grid with 100 microns finger width.
When the number of bus-bars increases an improvement in efficiency is reached, but this improvement, due to the addition of one new bus-bar, makes lower when the total number of buses increase as can be seen in Fig. 12.
As in the case of using three bus-bars, when the number of bus-bars is increased a further reduction of the interconnection module power losses is obtained. It can be calculated from the example referred in Fig. 12 (for a shadowing factor of 7.5%) that a relative power loss reduction of 11.7% (with respect to the two bus-bars configuration) would be reach with three bus – bars, while it would be 15.5%, 18.3%, and 18.3% with four, six and eight bus-bars respectively.
Nowadays technological parameters make possible the improvement of solar cells with the addition of more buses mainly thanks to the reduction of the finger resistance. The improvement is not so high for more than three or four buses, and it must be taken into account that a future improvement in conductivity of metal pastes or its resulting finger cross section’s aspect ratio would produce a lower improvement, due to the addition of more buses.
