Mechanistic Insights from Functional Group Exchange Surface Passivation: A Combined Theoretical and Experimental Study

Wei Zhang,Lars Kloo,Azar Sadollahkhani,Valentina Leandri,James M Gardner,Yuanyuan Li

doi:10.1021/acsaem.9b00050

Abstract

Four different functional groups including amino (−NH2), phosphine (−PH2), hydroxyl (−OH), and thiol (−SH) were combined with POSS (polyhedral oligomeric silsesquioxane) molecules to investigate how functional groups affect the surface passivation of POSS systems. Results from density-functional theory (DFT) calculations indicate that functional group amino (−NH2) with adsorption energy 86 (56) kJ mol–1 is consistently better than that of thiol (−SH) with adsorption energy 68 (43) kJ mol–1 for different passivation mechanisms. Theoretical studies on the analogous POSS–OH and POSS–PH2 systems show similar adsorption energies. Two of the systems were also investigated experimentally; aminopropyl isobutyl POSS (POSS–NH2) and mercaptopropyl isobutyl POSS (POSS–SH) were applied as passivation materials for MAPbI3 (MA = methylammonium) perovskite and (FA)0.85(MA)0.15Pb(I3)0.85(Br3)0.15 (FA = formamidinium) perovskite films. The same conclusion was drawn based on the results from contact angle studies, X-ray dif...

Highlights

Because of simple fabrication methods and rapidly increasing power conversion efficiencies (PCEs), perovskite solar cells have attracted much attention in recent years.[1−4] Methylammonium lead(II) iodide (MAPbI3), one type of commonly used perovskite materials, was first structurally characterized in 1978.5 it was not applied into solar cells as a light absorber until 2009 by Miyasaka et al.[6] showing an efficiency of around 3.8%
To understand the mechanisms behind surface passivation, we considered three different hypotheses illustrated in Figure 2, and density-functional theory (DFT) calculations were used for modeling
These results indicate that polyhedral oligomeric silsesquioxane (POSS)−NH2 adsorbs more strongly to the surface of MAPbI3, alternatively covers the surface more effectively, than POSS−SH

Summary

■ INTRODUCTION

Because of simple fabrication methods and rapidly increasing power conversion efficiencies (PCEs), perovskite solar cells have attracted much attention in recent years.[1−4] Methylammonium lead(II) iodide (MAPbI3), one type of commonly used perovskite materials, was first structurally characterized in 1978.5 it was not applied into solar cells as a light absorber until 2009 by Miyasaka et al.[6] showing an efficiency of around 3.8%. The effects of adding a polymer directly into the MAPbI3 perovskite precursor solution were studied by Liu et al.[13] A polymer denoted J71 was used as additive in the precursor solution and resulted in perovskite films with less pin holes and large grain size They found that using this method, they could obtain devices showing efficiencies over 19% and with improved stability versus moisture. Apart from the above-described methods, surface passivation represents another effective way to increase the device stability.[16] Yang et al.[17] demonstrated that hydrophobic tertiary and quaternary alkyl ammonium cations can successfully be assembled on top of the MAPbI3 perovskite surface as water-resisting layers via a facile surface functionalization technique Such layers can protect the perovskite films under high relative humidity (90 ± 5)% over 30 days.

■ RESULTS AND DISCUSSION

■ CONCLUSIONS

■ REFERENCES