*2.1. Materials*

High-purity materials were purchased from Alfa Aesar or Sigma-Aldrich and used as received. The FTO-coated glass substrates (~7 ohm/sq) were obtained from OPV-tech (Yingkou City, China). The organic precursors for synthesis of perovskite thin films were obtained from the GreatCell Solar.

#### *2.2. Synthesis of CQDs*

Phosphorus-doped CQDs were prepared according to a previously reported protocol [7]. Briefly, 0.1 g of Na2HPO4 and 1 g of dextrose were dissolved in 25 mL of deionized water. Next, the solution was placed into an Erlenmeyer flask (50 mL capacity) with a screw cap, closed, and kept at 200 ◦C under vigorous stirring for 1 h. The prepared black solution was passed through a 0.1 μm filter, and samples were purified and collected using a lyophilization process.

#### *2.3. Preparation of CQD@PMMA Solution*

As-prepared CQDs with sizes around 3–7 nm were mixed with 5 wt% PMMA in chlorobenzene (10 mg of CQDs in 10 mL of 5 wt% PMMA/chlorobenzene). The resulting mixture was sonicated for 30 min in an ultrasonic bath, and then stirred vigorously for 48 hrs at 40 ◦C to disperse CQDs and obtain the final brownish-colored CQD@PMMA solution.

#### *2.4. Fabrication of PSCs*

A glass/FTO/ETL/perovskite/HTL/gold architecture was used to fabricate PSCs. The FTO-coated glass substrates were cleaned by consecutive sonication in the detergent, deionized water (DI), and organic solvents. The substrates were then dried under a nitrogen flow and treated by UV-ozone for 30 min. The ETL was composed of a bilayer structure formed by consecutive depositions of SnO2 quantum dot (QD-SnO2) and SnO2 nanoparticles (NP-SnO2) as described below. The 0.15 M colloidal SnO2 quantum dot (QD-SnO2) solution was prepared by using SnCl2·2H2O as the precursor dissolved in 30 mL of DI water, and left overnight under vigorous stirring in an open beaker. The prepared solution was spin coated at 3000 rpm for 30 s and thermally annealed at 150 ◦C for 10 min and 200 ◦C for 60 min. SnO2 nanoparticles (NP-SnO2) were prepared by using SnO2 colloidal dispersion solution. The prepared solution was spin coated at 3000 rpm for 30 s, and the samples were annealed at 150 ◦C for 30 min. The perovskite precursor solution was prepared by forming 1.1 M PbI2, 1 M formamidinium iodide (CH5IN2), 0.2 M PbBr, and 0.2 M methylammonium bromide (CH3NH3Br) in the mixture of dimethyl formamide (DMF) and dimethyl sulfoxide (DMSO) (4:1 volume ratio). Then, CsI (1.5 M stock solution) was added into the precursor solution in a volume ratio of 8:92 to form 1 mL of final perovskite precursor solution. The perovskite thin film was prepared in a nitrogen-filled glove box by a four-step process: (i) spin coating of the precursor solution at 2000 rpm for 30 s; (ii) cryogenic treatment; (iii) blow-drying under a nitrogen gas flow for 60 s; and (iv) thermal annealing at 105 ◦C for 30 min on a hotplate. The precursor solution for the hole transport layer (HTL) was prepared by dissolving 80 mg of 2,2',7,7'-Tetrakis [N,N-di(4-methoxyphenyl)amino]-9,9'-spirobifluorene (Spiro-OMeTAD), 954 μL of chlorobenzene, 29 μL of 4-tert-Butylpyridine, and 17.5 μL of Bis(trifluoromethane)sulfonimide lithium salt in acetonitrile with a concentration of 520 mg/mL. The prepared solution was spin coated at 3500 rpm for 30 s. Gold electrodes (70 nm) were thermally evaporated through a shadow mask at a vacuum pressure of 10−<sup>7</sup> Torr to complete fabrication of PSCs (Glass/FTO/SnO2/Cs0.05(MA0.17FA0.83)0.95Pb(I0.84Br0.16)3/spiroMeOTAD/Au) [11].

#### *2.5. Deposition of CQD@PMMA Coating on PSCs*

In order to deposit CQD@PMMA coating on PSCs, freshly prepared CQD@PMMA solution was spin coated (30 s under various spinning speed conditions) on the glass slide of already fabricated PSC devices at room temperature. For each deposition speed, at least 15 devices were tested. The estimated number of CQD particles per 1 × 1 cm<sup>2</sup> geometric area is ~1.48 × 1012.
