The distinct crossover anomalies are observed in my recent samples. It's explained in the book that there might be a barrier at the interface or in the absorber's near interface region.The barrier is sensitive to illumination.
My questions are:
(1) If the barrier is induced by the conduction band offset, is the band offset caused by the large difference between CIGS and CdS? For instance, too low GGI for CIGS.
(2) If the barrier is induced by a high density of acceptors in the absorber, does it mean that the Cu is too insufficient at the surface?
(3) Except for two reasons above, will too high Ga content in the surface of CIGS also act as a barrier and lead to such a crossover anomaly?
[1] Roland Scheer & Hans‐Werner Schock. Chalcogenide Photovoltaics. (John Wiley & Sons, Ltd, 2011). doi:10.1002/9783527633708.
Dear Yifan Kong ,
Hope you are well and the colleagues who answered the questions.
I observed such behavior while measuring silicon solar cells with aluminum metallization in the front layer. It was so that the top n-layer which must be heavily doped found to be having relatively low doping such that the aluminum built a rectifying nonlinear Schottky contact instead of an ohmic contact.
I investigated this crossover and modeled it and found it is due to the nonohmic behavior of the metal contact to the semicondcutor.
I would like that you refer to the paper in the link:Article A. Zekry and G. El-Dllal, “ effect of MS contact on the elec...
Newly I studied the possible formation to the this cross over in heterojunction solar cells because the effect of positive forward solar cell voltage on the built in electric field where it reduces it such that the electric field can get weak to dispatch all photogenerated carriers before they recombine in this space charge region.
It follows that the collected photo current will decrease by forward biasing.In this case the illuminated current will cross the dark current.
The difference between the two effect that the first the dark current has too large current resulting from the main junction.
For the second effect please follow the paper in the link:Conference Paper Generic Analytical Models for Organic and Perovskite Solar Cells
Best wishes
Dear Yifan,
In my opinion, the temperature-dependent J-V crossover can be explained by the temperature- dependent impact of the secondary barrier between CdS and CIGS at lower temperature in the dark, the maximum distortion-free barrier is reduced and results in a more serious current limitation, indicating a greater J-V crossover.
It is also found that, under low-intensity illumination, there is a lower doping density of the window layer due to a smaller amount of photons with which can excite the buffer layer to release the trapped electrons from the deep-level defect state. The result is a greater secondary barrier to limit the electron current through the interface and shift the light J-V curve right towards the dark J-V curve at high bias which reduces the J-V crossover.
in attached I am sending you the titles of some scientific contributions that may be useful to you :
https://mountainscholar.org/bitstream/handle/10217/81083/Song_colostate_0053N_12036.pdf?sequence=1
Good luck.
Dear Yifan.
Yes, I think the book you are referring to is right. This anomalous effect is likely to be due to an illumination dependent potential barrier related to defect states at semiconductor-semiconductor junction interface as well as at metal-semiconductor contacts. Indeed, deep defect states are quite often separated with respect to allowed band states by a potential barrier. It ends up as if you are dealing with simultaneous effects of illmination dependent shunt and series resistance both due to defects.
Why does the potential barrier depend on the illumination intensity? This is so because of illumination induced quasi-Fermi level increase in semiconductor conduction and valence bands as a result of increasing free electron-hole pairs photo-generation.
This quasi-Fermi levels increase means a decrease of the effective potential barrier to be overcome by free carriers under bias effect.
To make the point clear, it may be quite interesting to perform I-V measurements under different illumination intensities.
I hope this could help you,
Best regards,
Professor A. Kadri
I join my answer to Prof. Kadri and Prof. Bouchama answers .
You should go with a systematic study (try simulation), however it's a bit difficult, it's all related to defects and auto-doping properties of CIGS during their synthesis. These defects, as mentioned above, are mainly responsible for light dependent potential barriers.
Good luck.
Nassim
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