Amorphous Diamond Solar Cell

The merit of amorphous diamond to convert either light or heat to electricity can be applied to solar cell panels or thermal electrical generators. For example, amorphous diamond was over covered coated on indium tin oxide (ITO), the transparent electrical conductor that was coated on a glass substrate. This panel was separated from another ITO coated glass by glass bead spacer. The gap was sealed around and the space was pumped down to high vacuum (10-6 torr). This panel was exposed to a xenon light that irradiated

Figure 9.10. The schematic of DEL design and the brightness of DEL panel (A4 size).

a spectrum with an energy output similar to solar constant (AM1.0 or 0.1 W/cm2). An external bias was applied and the electric current was monitored. It was demonstrated that the current increased substantially when light shone through or when amorphous diamond was heated up (Fig. 9.11).

When the applied bias was gradually reduced to zero, the current enhanced by xenon lamp was not dependent on the bias. Hence, the field emission could be triggered by sunshine directly without adding an external bias. However, the current density was too low to be useful as a solar panel unless the vacuum gap could be reduced further from 7 ¡x.

When the vacuum was back filled with iodine, the current could be generated noticeably without applying the external field. This current was further increased by sensitizing amorphous diamond with a light absorbent dye. But even so, it was still low with the conversion efficiency (Fig. 9.12).

Figure 9.11. Amorphous diamond can enhance electron emission in vacuum by light absorption and by thermal agitation.
Figure 9.12. The field emission became spontaneous when the external bias was reduced to zero.
Figure 9.13. The photo-electric effect of amorphous diamond when exposed to a xenon lamp (AM1.0) of about 0.1 W/cm2. In the experiment, the vacuum gap was back filled with liquid electrolyte of iodine.

The amorphous diamond solar cell was also constructed with a silicon layer in a hybrid design (Fig. 9.13). In this case, no vacuum was needed. In one example, the nitrogen doped amorphous diamond was coated on boron doped silicon substrate. This hybrid design showed a dramatic increase in photo electricity, much higher than using a vacuum gap or back filed with a liquid electrolyte (Fig. 9.14).

When a monochrometer was used to filter the broad spectrum of the xenon lamp and the photocurrents of amorphous diamond coated silicon and silicon solar cell were measured and compared, the former exhibited a much higher value. Upon cooling the semiconductors to a cryogenic temperature of liquid nitrogen (70 K), the electrical current generated by light increased. Moreover, the increase was higher with shorter wavelengths (i.e. with higher energy). However, amorphous diamond coated silicon showed much higher cooling enhancement and also the blue shift of the peak wavelength.

The above observation demonstrated that amorphous diamond could absorb light and generate electricity more effectively than silicon. This is particularly attractive as amorphous diamond

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Figure 9.14. The photo electric effect of nitrogen doped amorphous diamond coated on boron doped silicon layer.

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Figure 9.14. The photo electric effect of nitrogen doped amorphous diamond coated on boron doped silicon layer.

is radiation superhard and it would not be susceptible to UV damage. Amorphous silicon solar cells have the advantages of thin film and low cost, but the aging problem of UV damage makes it less useful so more costly crystalline silicon plates are used as solar panels. It would appear that amorphous diamond coating of amorphous silicon solar cells could boost both the energy conversion efficiency and the longevity of the service.

Due to the high electric resistance of amorphous diamond, the electric current it generated was dissipated as heat so the final output of electricity was significantly reduced. The dampening effect was greatly reduced by cooling the device at liquid nitrogen temperature. However, alternative methods by channeling out electricity rapidly once it is formed may also be effective to preserve the electricity generated. One example is to coat amorphous diamond on amorphous silicon layers and stack them together. Due to the thinness (e.g. 100 nm) of the amorphous diamond and amorphous silicon, the light absorbed by each layer can generate electricity independently. The electricity can then be channeled out readily due to the short distance of travel to reach the electrode. The combined electricity would retain most of the energy derived from sunlight (Fig. 9.15).

Figure 9.15. The photo electrical current as a function of optical wavelength. Note that amorphous diamond could convert more current with IR irradiation than pure silicon. Moreover, the cooling enhancement of both energy and intensity was more obvious.

In summary, amorphous diamond has the highest density of discrete electronic states. This unique feature makes amorphous diamond particularly useful as energy converters, such as field emitters, solar cells, thermal generators, radiation coolers, and heat absorbers.

Amorphous diamond is highly complimentary to thin film solar cells (Fig. 9.16). There are two major contenders for solar cells without using expensive crystalline silicon. One of them is the coating of amorphous silicon. This solar cell typically incorporate boron-doped silicon as the P-type. However, boron atoms are much smaller than silicon, so the substitutional boron atoms in silicon pseudo lattice is limited. As a result, the whole concentration may not be sufficient. In contrast, boron atoms are only slightly larger carbon atoms, so the solid solution can be much more extensive than boron-doped silicon. The p-n junction made of phosphorus doped silicon and boron doped carbon can be more comparable for a higher efficiency output as solar cells.

Another complimentary design is to couple nitrogen-doped amorphous diamond as N-type with CIGS as P-type. CIGS is rapidly rising as the leading solar cell without using silicon. CIGS stands for copper-indium-gallium-selenide (sulfide), it has the highest absorption energy of all thin films, except amorphous diamond. CIGS may also be coated on flexible substrate (e.g. aluminum foil) so it can be mounted on cars. The other flexible

Figure 9.16. The solar absorbed energy of CIGS is the highest among various thin films (shown in figure) with a theoretical conversion efficiency of 25% (top diagram). One example of CIGS solar panel (bottom diagram) is depicted with CdS replaceable by amorphous diamond to boost the overall performance including reliability.

Figure 9.16. The solar absorbed energy of CIGS is the highest among various thin films (shown in figure) with a theoretical conversion efficiency of 25% (top diagram). One example of CIGS solar panel (bottom diagram) is depicted with CdS replaceable by amorphous diamond to boost the overall performance including reliability.

panel made of dye sensitized titanium dioxide has a reliability issue as it contains unstable organic materials.

Although CIGS is rising rapidly as commercial solar panels (e.g. NanoSolar venture, IBM/TOK joint development), it has an Achilles heel. This is because CIGS uses CdS as the N-type coating to furnish electrons. CdS is not only poisonous but emits damaging as well. If nitrogen-doped amorphous diamond replaces CdS, then the above mentioned weakness of CIGS is reinforced. In addition, amorphous diamond can also serve as the antireflaction layer. Furthermore, amorphous diamond is electrically-conductive so it could avoid using transparent cathode (e.g. indium tin oxide). Alternatively, amorphous diamond may also support thinner coating of aluminum/silver as electrode.

Getting Started With Solar

Getting Started With Solar

Do we really want the one thing that gives us its resources unconditionally to suffer even more than it is suffering now? Nature, is a part of our being from the earliest human days. We respect Nature and it gives us its bounty, but in the recent past greedy money hungry corporations have made us all so destructive, so wasteful.

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