Solar integration takes a page from the semi wafer CSP playbook
maximum theoretical output of a silicon offer a low resistance, reliable system for at energies less than the bandgap
based cell is about 33%. The highest interconnect. energy. The silicon is unable to absorb
reported commercial cell is 23.4%
4
. In terms of equipment, techniques such the radiation and it passes through the
In terms of improving cell perfor- as sputtering or evaporation generate too silicon.
mance, many of the solutions are related much wasted material to be viable for solar. 2. Short wavelengths correlate to photons
to interconnect, including series resistance Electroplating or print deposition technolo- at energy higher than the bandgap.
and recombination at the contacts (intrinsic gies are the preferred approaches to deposit- This radiation is only partially con-
recombination is a fundamental property of ing metal. There is also an environmental verted to electricity and most of the
the crystalline material). Efforts to improve consideration for the current packaging energy results in heating the cell.
cell performance have centered on improv- systems: traditional solder technologies such
ing recombination lifetime. Changing bulk as PbSn are implemented in cells, whereas There are many organizations pursuing ways
material type (from p to n), thinning wafers the semi industry has already implemented to expand the spectrum of useable energy
7
.
and optimizing the doping levels are ap- lead free technologies. One of the drawbacks of the current ap-
proaches being pursued to raise cell output. proach of using EVA materials for encapsu-
However, there are certain trade-offs in- Heat sinking lation is that it raises the thermal resistance
volved, including yield loss (thinned wafers) High temperature degrades the performance around the cells. Thus, as the cells increase
and increased resistance from reducing dop- of silicon material. A rise in temperature in temperature, it becomes more difficult to
ing levels (reducing doping levels decreases decreases the bandgap of the silicon. This dissipate the heat. Energy losses can be as
the number of defects in the crystal which gives rise to effects such as increased leakage high as 0.5%/˚C.
can form recombination sites). Although currents, memory bit and logic errors and Many of the heat dissipation technolo-
solar cells can be viewed as commoditized increased noise. For solar cells, increased gies are applicable to semi can be imple-
goods, the advanced solutions developed by temperature results in increased recombina- mented for solar. BGA heat sink arrays,
the semi industry are very much applicable. tion. thin films or solder attach assemblies can
These solutions include low cost materials, In the case of integrated circuits, the be implemented directly on cell. However,
thermal dissipation, high performance inter- rise in temperature comes from the power this approach would require a different
connect, system integration and standards. dissipation of a large number of transistors kind of sealing system than EVA. Thermally
or high current devices to drive large loads. conductive PCB assemblies also offer a low
Materials cost For solar cells, heat is generated through ab- cost method for reducing temperature.
After the silicon, the most expensive mate- sorbed radiation from the sun. The amount
rial on the solar cell is the screen printed of photon energy that is converted into Interconnect
silver paste. Although silver is the most elec- electricity is dependent on the wavelength Although silver paste is unproblematic to
trically conductive metal, it is highly reactive of the light. The optimal wavelength for a implement and has low start-up cost, it
with moisture and is only slightly stronger solar cell is dependent on the bandgap and presents three problems:
than aluminum as a mechanical material. is given by λ = h /E where λ is the optimal
c g
The semi industry has developed its wavelength, h is Planck’s constant, and Eg 1. High series resistance: The junction
c
own low cost solution: copper. Copper is the bandgap (eV). formed by the paste and silicon has
has sixty percent more strength than silver For wavelengths outside the optimal significant resistance. Traditional
and has only 1-2% of the cost ($0.22/oz region, there are two effects: silicide or aluminum type contacts
compared to $13.66/oz for silver)
5
. Further- have resistances on the order of 1E-6
more, multi-metal stacks such as Ni-Cu-Sn 1. Long wavelengths correlate to photons Ωcm
2
while the paste contacts can have
resistances as high as 1E-
3
Ωcm
2 7-8
.
2. Recombination at the contact: Incom-
plete metallurgical bonding or surface
defects can act as recombination sites
for the carriers.
3. Large metal line width: One solution
to the resistance problems is to make
the silver paste thicker. However, the
!

Figure 8. Challenges in meeting IC performance
2
. Figure 9. Efficiency factors in solar cells
3
.
8 – Global Solar Technology – March/April 2009 www.globalsolartechnology.com
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