Cadmium Sulfide/Copper Sulfide

Solar cells made from these two П-VI semiconductors (CdS and Cu2S) are clearly heterojunction diodes with the ре-CdS having an energy gap of 2.3 eV and the pc-Cu2S having an energy gap of 1.2 eV. Considering the nature of sunlight, it is clear that the Cu2S layer is
responsible for the bulk of photocurrent generation. As with heterojunctions in general, these solar cells have a tendency towards multiple recombination centers located in the vicinity of the pn-junction. This leads to a situation wherein the photocurrent increases in a non-linear manner as the light intensity is increased. This is a result of the fact that with higher light intensities there is an increased concentration of photo­generated charge carriers and they succeed in saturating the recombination centers.

These thin film polycrystalline solar cells were first fabricated in 1954 [9]. Then and now CdS/Cu2S solar cells are most often constructed by the Clevite process [10]. Here, approximately 20 pm of pc-CdS is vacuum deposited (evaporated) onto a metal, a metal coated plastic or a metal covered glass sheet. This layer, with its ~5 pm sized crystallites, is then dipped into a cuprous chloride solution ("20 seconds at 90° C). A chemical process exchanges copper for cadmium in a thin surface region some 2000 A thick forming the pc-Cu2S layer and a heterojunction. Efficiencies in excess of 9% have been reported for these solar cells [11].

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The current voltage characteristics of these solar cells differ from single crystal devices in a number of ways. For example, as shown in Figure IX.2, the non-illuminated (dark) and illuminated (lighted) forward biased current-voltage characteristics of pc-CdS/Cu2S cross each other

Figure IX.2. The current-voltage characteristic for illuminated and non – illuminated pc-CdS/Cu2S solar cells and illuminated and non-illuminated c-Si solar cells.

while those of most solar cells (for example, c-Si) do not. In addition, the photovoltage depends on the spectral characteristics of the incident
light and the solar cell capacitance depends, in a major way, upon the illumination level[60]. All of these variations from single crystal material device behavior are dependent on the fabrication processes employed.

The principal advantage of these polycrystalline solar cells is one of cost. Being polycrystalline and thin, they can be fabricated on a wide variety of substrates by mass production techniques. Note that the effective optical absorptivity of most polycrystalline materials is considerably higher than that for the single crystal form of the same material. This enables us to construct much thinner solar cells and still capture the incoming photons.

The major disadvantages of these solar cells are their relatively low efficiency [12], coupled with an inherent instability [13]. A number of degradation or destabilization modes have been observed in cadmium sulfide/copper sulfide solar cells: (1) air, particularly moist air, can oxidize the pc-Cu2S; (2) high temperatures (which may be as low as 60°C) can alter the stoichiometry of the pc-Cu2S to pc-Cu^S where x is less than two; and (3) the performance decreases whenever the load voltage exceeds 0.33 volts [14].

In summary, cadmium sulfide/copper sulfide thin film

polycrystalline solar cells suffer from poor efficiency and instability. However, if conversion efficiencies in excess of 10% can be reliably sustained, then such cells, owing to their low fabrication cost, offer a strong possibility of being economically feasible. For solar cells of efficiencies less than 10% the costs of the supporting structure, the energy conditioning equipment and other sub-systems are sufficient to make single crystal semiconductor based systems preferable.

Updated: August 20, 2015 — 7:41 pm