**1. Introduction**

Hybrid organic–inorganic lead halide perovskites are considered to be one of the most promising materials for photovoltaic (PV) applications. In just a decade, the power conversion efficiency (PCE) of perovskite solar cells (PSCs) has improved 6-fold, reaching remarkable values of 25.2% in 2019 [1]. This is the result of tremendous efforts spent on optimization of the device fabrication process, careful tailoring of material properties used for device functional layers (perovskite photo-absorber layer, electron- and hole-transporting layers (ETL and HTL, respectively), and device structure (mesoporous, planar *p*-*i*-*n*, and planar *<sup>n</sup>*-*i*-*p*)) [2,3]. Theoretical calculations sugges<sup>t</sup> that PSCs have the potential to reach efficiencies as high as ~31% [3]. Approaching the theoretical performance limit would require even further development and optimization in all aspects of device fabrication. Light managemen<sup>t</sup> for effective utilization of incident light would be one of them.

One of the drawbacks of PSCs in terms of their light harvesting ability is the sandwich structure of the devices, which induces parasitic light absorption by the substrate (fluorine- or indium-doped tin-oxide-coated glass (glass/FTO or glass/ITO, respectively)) and other functional layers, such as ETL and HTL [4]. For instance, most of the UV light is absorbed by the ETL made of wide band gap

materials, such as TiO2 and SnO2, or by the HTL (e.g., NiOx in devices with an inverted structure). This, on the one hand, is beneficial for the perovskite photo-absorber layer, since it prevents degradation of the perovskite material under UV illumination. However, on the other hand, absorption of UV light by the substrate and charge-transporting layers does not allow for an e ffective utilization of the incident light. One way to overcome this issue without changing the structural composition of PSCs is to employ fluorescent materials with down-converting properties. These are materials that can absorb photons with higher energy (e.g., in the near-UV range) and re-emit them with lower energy (e.g., in the visible light range). Application of such functional materials to improve the light harvesting ability of the photo-absorber layer in PSCs can a fford enhanced charge carrier generation and improved device performance.

Carbon quantum dots (CQDs) with excellent fluorescent properties absorb UV light and re-emit in the visible light range. Such a unique optical property of CQDs combined with their low-cost synthesis, non-toxicity, and chemical inertness make them very attractive for electronics, sensing, and bioimaging applications [5–7]. For example, carbon and graphene quantum dots were utilized recently for light managemen<sup>t</sup> in silicon-based solar cells [8,9]. In both cases, the authors reported enhancement in the performance of silicon-based solar cells, owing to the carbon/graphene quantum dots coatings that emit additional down-converted photons in the visible spectrum. A recent study reported that CQDs with a down-conversion property introduced into a mesoporous TiO2 film in PSCs can improve the stability (convert UV radiation to visible light) as well as the PCE of the devices [10]. Therefore, by considering the favorable down-conversion optical e ffects, we embedded the CQDs in a thin poly(methyl methacrylate) (PMMA) film (CQD@PMMA) and used it as an additional coating layer for PSC devices. This coating will be able to convert the UV part of the incident light to visible light, thus allowing for more light to be transmitted to the perovskite photo-absorber layer (see Figure 1).

**Figure 1.** Schematic image of perovskite solar cells (PSCs) (**a**) without and (**b**) with the poly(methyl methacrylate) with embedded carbon quantum dots (CQD@PMMA) coating used for improving light harvesting in a PSC due to the down-conversion e ffect.

#### **2. Materials and Methods**
