On the beneficial effect of Al2O3 front contact passivation in Cu(In,Ga)Se2 solar cells

- An Al2O3 front contact passivation layer is introduced to Cu(In, Ga)Se2 solar cells.
- The ALD Al2O3 is thin enough to allow for tunneling.
- Detrimental effect of openings in buffer layer is mitigated by passivation.
- CIGSe/Al2O3 interface exhibits lower defect density compared to CIGSe/ZnO interface.
- Heat treatment during Al2O3 deposition induces a larger space charge region width.

This study reports on the beneficial effect of an absorber surface passivation by Al2O3 on the performance of Cu(In, Ga)Se2 (CIGSe) solar cells. Here the Al2O3 layer is deposited by atomic layer deposition (ALD) subsequently to a CdS buffer layer. It is shown that a very thin film of about 1 nm efficiently reduces the interface recombination rate if the buffer layer is too thin to not fully cover the CIGSe absorber. An Al2O3 thickness of 1 nm is sufficiently low to allow current transport via tunneling. Increasing the thickness to >1 nm leads to a detrimental blocking behavior due to an exponentially decreasing tunnel current. Losses in open circuit voltage (Voc) and fill factor (FF) when reducing the buffer layer thickness are significantly mitigated by implementing an optimized Al2O3 layer. It is further shown, that the heat treatment during the ALD step results in an increase in short circuit current density (Jsc) of about 2 mA/cm2. This observation is attributed to a widening of the space charge region in the CIGSe layer that in turn improves the collection probability of electrons. For not fully covering CdS layers the decrease in interface defect density by the passivation contributes as well, leading to a gain of about 5 mA/cm2 for cells without a buffer. Finally, the leakage current of the solar cell devices could be reduced when applying the Al2O3 layer on insufficiently covering CdS films, which proves the capability of mitigating the effect of shunts or bad diodes.

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