Preserving the performance of photovoltaic devices exposed to atmospheric factors such as humidity, oxygen, high temperature or UV irradiation is key for practical application. For PbS quantum dot (QD) solar cells, the presence of oxygen promotes the formation of PbO or PbSO_x, often resulting in device failure.^[1,2] To address this issue, we developed a coating strategy consisting of spin-coating a PbS-silane functionalized reduced graphene oxide (PbS-rGO) hybrid material^[3] as protective layer. Measurements of the power conversion efficiency (PCE) over time demonstrate that solar cells with a PbS-rGO film integrated into the absorber layer show an enhanced stability, especially under humid conditions.

The solar cells absorber layer consists of PbS:EMII (200nm)^[4] / PbS:EDT (10 nm) / PbS-rGO:EDT (44 nm) films. An average (best) PCE of 5.7% (7.6%) was achieved, comparable to corresponding PbS reference cells where we obtained 8.2% (9.0%). Continuous illumination of the devices and exposure to a saturated water vapor environment demonstrated the unique advantage of the PbS-rGO layer. Device stability in the first case was slightly improved, with a relative decrease of PCE of 37% after 168h of continuous illumination, compared to 46% for the PbS reference cells. More importantly, when exposing the PbS-rGO solar cells to a saturated water vapor atmosphere for 5 days, they retained 96% of the initial PCE, while the PCE of the PbS reference cells was reduced to about 50% of its initial value. As result, even the absolute PCE of the PbS-rGO solar cells surpassed the reference cells after 24h. Scanning electron microscopy and energy dispersive X-ray spectroscopy revealed that device failure in the PbS reference cells under humid conditions is due to formation of PbSO_x crystals and cracks in the PbS film. Such degradation was not observed in the PbS-rGO devices.

Our results contribute toward the practical implementation of QD solar cells and, given the versatility of the silane-functionalized rGO, the approach can also be extended to other solution-processed devices which suffer from atmospheric stability, as organic or perovskite solar cells.

Acknowledgements

This project has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No-696656 (GrapheneCore1).

References

[1]       J. Tang et al., ACS Nano 2010, 4, 869.

[2]       G. Zhai et al., Appl. Phys. Lett. 2011, 99, 63512.

[3]       B. Martin-Garcia et al., J. Mater. Chem. C 2015, 3, 7088.

[4]       Y. Cao, et al., Nat. Energy 2016, 1, 16035.