Abstract
As an example of the thin heterojunction polycrystalline solar cell, the CdS/Cu2S cell is chosen. The current voltage characteristic is computed. Space charge effects in the heterojunction are discussed. The influence of electron traps in CdS is shown. The influence of a field enhanced depletion of hole traps is experimentally demonstrated. The influence of field quenching is computed. The Frenkel-Poole effect is identified. Experimental results of kinetic effects of the solar cell characteristics are shown. A Voltage drop kinetic method is introduced. The influence of interface recombination is calculated. Boundary conditions at the interface are introduced. The quality factors A and B are introduced and measured. Lessons learned from the CdS/Cu2S solar cell.
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Notes
- 1.
We have consistently referred to this cell as a Cu2S solar cell even though this would indicate that the copper sulfide is a chalcocite, while in actuality it is Djurleite with a stoichiometry closer to 1.98 rather than 2. This also is done in order to indicate that we do not want to use the examples discussed here for more than as possible phenomena rather than staying too close to an actual cell in all the detail discussed here.
- 2.
Even though as a heterojunction cell this type seems to miss the obvious advantage of optical absorption close to the heterojunction, it may still be of technical interest because of the ease of fabrication resulting in relatively inexpensive devices that may still show acceptable conversion efficiencies.
- 3.
This is a typical characteristic of copper doped CdS by creating a high-field domain.
- 4.
In actuality, there may be some discontinuities, as discussed in several previous sections, which can be easily introduced but are omitted here to avoid confusion with other effects that are emphasized in this chapter.
- 5.
Here we assume that the conductivity in the Cu2S is high enough that any voltage drop here can be neglected.
- 6.
The Schottky barrier approximation is well suited for the CdS part of the heterojunction since in the entire CdS one has p≪n, and the Cu2S is nearly degenerate, thereby acting in some respects as pseudo-electrode.
- 7.
When using the relation
$$ \tilde{V}_{Dn}=\frac{kT}{e}\ln\biggl(\frac{N_{c}}{n_{j_0}}\biggr) $$(33.3)and, inserting for zero current and steady state \(n _{j_{0}} = g_{o} \tau _{n}\).
- 8.
It should be emphasized that a more comprehensive discussion of the characteristic must relate to the distribution of both quasi-Fermi levels that describe the net current. However a realistic computation is exceedingly difficult in typical heterojunctions since the trap distribution and related transition coefficients are insufficiently known.
- 9.
Even considering frozen-in steady state for the minority carriers in the dark and reasonable generation rates and lifetimes under sunlight, the minority carrier density within the CdS will remain well below the electron density within the entire barrier region.
- 10.
For a more precise evaluation of the sequential trap depletion see the corresponding Sect. 27.2.2 that deals with the dark-diode.
- 11.
Such field quenching is of interest for technical applications, since it permits working with semiconductors of lower purity, allowing less expensive fabrication methods. Without field quenching, such semiconductors would easily be driven into a range of excessive barrier fields, with detrimental influence on performance due to tunneling through the barrier, thereby creating leakage currents.
- 12.
Only in the bias range between the Boltzmann and the saturation branch can such kinetics be observed. Otherwise the structure of interest becomes hidden in the horizontal current saturation branch.
- 13.
Observe that we split off an exponential with the diffusion voltage that does not contain A.
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Böer, K.W. (2013). The Heterojunction with Light. In: Handbook of the Physics of Thin-Film Solar Cells. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-642-36748-9_33
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