Molecularly Engineered Multifunctional Bridging Layer Derived from Dithiafulavene Capped Spiroxanthene for Stable and Efficient Perovskite Solar Cells

This study introduces a novel approach centered around the design and synthesis of an interfacial passivating layer in perovskite solar cells (PSCs). This architectural innovation is realized through the development of a specialized material, termed dithiafulvene end-capped Spiro[fluorene-9,9′-xanthene], denoted by the acronym AF32. In this design architecture, dithiafulvene is thoughtfully attached to the spiroxanthene fluorene core with phenothiazine as the spacer unit, possessing multiple alkyl chains. AF32 passivates interfacial defects by coordinating the sulfur constituents of the phenothiazine and dithiafulvene frameworks to the uncoordinated Pb2+ cations on the surface of the perovskite film, and the alkyl chains construct a hydrophobic environment, preventing moisture from entering the hydrophilic perovskite layer and improving the long-term stability of PSCs. Furthermore, this conductive interlayer facilitates hole transport in PSCs due to its well-aligned molecular orbital levels. Such improvements translated into an enhanced power conversion efficiency (PCE) of 22.6% for the device employing 1.5 mg/mL AF32, and it maintained 85% of its initial PCE after more than 1800 h under ambient conditions [illumination and 45 ± 5% relative humidity (RH)]. This study not only marks progress in photovoltaic technology but also expands our understanding of manipulating interfacial properties for optimized device performance and stability.


Synthesis of SFX:
A 100 mL round bottom flask was charged with 2,7 dibrom-9-fluorenone (1.750 g, 5.177 mmol), resorcinol (2.280 g, 20.710 mmol), p-TsOH (0.100 g,0.051 mmol) and toluene (40 mL).The mixture was refluxed for 12h and then cooled to room temperature.After completion of the reaction, water (100 mL) was added and the mixture was stirred at room temperature for 0.5h.The yellow colour precipitated from the reaction mixture was isolated by filtration and washed with water several times.The crude was dissolved in ethyl alcohol (100 mL) and the organic solvent was concentrated by vacuum evaporation and purified by column chromatography to give the white solid (1.90g, 67% yield).The characterization is well-matched with the reported one 1 .

Synthesis of SFX-EH:
Compound 2 was prepared as per the literature. 2 SFX (1.8g, 3.45 mmol, 1equiv), 2-ethylhexyl bromide (2 mL, 10.35 mmol, 3 equiv), K2CO3 (0.773g, 13.79 mmol, 4equiv) and DMSO (10 mL) were added to a single-necked round bottom flask.The mixture was refluxed for 12h and then cooled to room temperature.The mixture was extracted with ethyl acetate.The organic layer was dried with sodium sulphate, and evaporated under vacuum and the residue was further purified with column chromatography to give a colourless solid.

Synthesis of SFX-Bpin:
To a dry round bottom flask (100 mL) the compound 2 (0.912g, 1.22 mmol, 1equiv), Bis(pinacolato)diboron (0.932g, 3.672 mmol, 3equiv), potassium acetate (0.480g, 4.896 mmol, 4equiv) were added and dissolved in 1,4-dioxane (40 mL) under the nitrogen atmosphere.The mixture was degassed for 30 min.After that, we added Pd(dppf)Cl2 catalyst (10 mol%) and then the mixture was refluxed for 24h at 110°C.The reaction mixture was cooled to room temperature, filtered and washed with DCM several times.The reaction mixture was extracted with dichloromethane.The organic layer was dried over sodium sulphate and the solvent was evaporated under vacuum.The residue was further purified by column chromatography to give the white solid (1.15g, 92%).

Synthesis of PTZ-CHO:
PTZ-CHO was synthesized in two steps 3 Step-1: Dry phosphorus oxychloride (11.9 mL, 128.4 mmol) was added dropwise very slowly to the dry N, N-dimethylformamide (DMF) (8.0 mL, 102.7 mmol) at 0 0 C in a 100 ml two neck round bottom flask under nitrogen atmosphere with stirring.After the formation of a white solid, 10-(2ethylhexyl)-10H-phenothiazine (8.0 g, 25.6 mmol) was dissolved in 50 mL of dry 1,2dichloroethane (DCE) was added dropwise for 0.5 h to the reaction mixture.The reaction was heated to reflux for 24 h.After cooling to room temperature diluted sodium hydroxide solution was added and extracted with ethyl acetate (200 ml × 3).The organic layer was dried with anhydrous Na2SO4 and then the solvent was removed using a rotatory evaporator.The residue was purified by silica gel column chromatography using ethyl acetate and hexane as the eluent to yield phenothiazine aldehyde as a yellow liquid.Yield (7.5g, 86.3%).
Step-2: In a round bottom dry flask (100 mL) phenothiazine aldehyde (0.930 g, 2.74 mmol) was dissolved in acetone (20 mL) and NBS (0.488 g, 2.74 mmol) was added portionwise up to 2h.The reaction was stirred for 6h at room temperature under dark conditions.After completion of the reaction, the solvent was evaporated under vacuum.The residue was further purified by column chromatography to give the yellow solid compound 4 (0.850 g, 75% yield).The characterization is matched with the reported literature.

S20 11 .
Fig. S10: Water contact angle of perovskite films without and with different concentrations of AF32.
Optical and electrochemical properties of Spiro-xanthene-based AF32.

Table S2 :
Summary of the carrier lifetime estimated from fitting TRPL spectra.