**4. Conclusions**

In conclusion, we developed an easy-to-handle, low-cost, and solution-processable method to fabricate photoactive functional coating for sandwich-structured PSCs to enhance their light harvesting ability and improve device performance. The photoactive functional coating consists of a PMMA matrix embedded with CQDs that possess down-conversion fluorescent properties. The application of this coating on PSC devices showed an improved photocurrent and fill factor, resulting in an overall performance increase. The method is simple and straightforward to apply and can afford further fine-tuning and/or modification to fabricate functional thin films with the desired properties and morphology. In addition, this low-cost and solution-based method offers ease of scale-up of fabrication, which makes it very attractive for future industrial manufacturing.

**Author Contributions:** Conceptualization, A.N.J. and T.S.A.; methodology, A.N.J., T.S.A. and A.N.; investigation, A.A.M., S.N.S., D.A. and A.N.J.; resources, C.S., A.N., Y.-H.H., T.S.A. and A.N.J.; writing—original draft preparation, A.A.M., S.N.S, and A.N.J.; writing—review and editing, C.S., A.N., Y.-H.H., T.S.A. and A.N.J.; visualization, A.N.J.; supervision, A.N.J.; All authors have read and agreed to the final version of the manuscript.

**Funding:** This research was funded by Nazarbayev University, gran<sup>t</sup> number 110119FD4512, 090118FD5326, 110119FD4506 and Ministry of Education and Science, Kazakhstan, gran<sup>t</sup> number BR05236524.

**Acknowledgments:** A.N.J. acknowledges the financial support from the FDCRG (Grant Number: 110119FD4512) and SPG fund (Nazarbayev University). C.S. thanks NU (Grant Number: 090118FD5326) for the support. A.N. acknowledges the gran<sup>t</sup> 110119FD4506 (Nazarbayev University) and the targeted program BR05236524 funded by Ministry of Education and Science, Kazakhstan. The authors thank Rakhima Shamenova from Nazarbayev University for her help with SEM imaging.

**Conflicts of Interest:** The authors declare no conflict of interest.
