1. Introduction

Despite the advantageous optical bandgap and encouraging development in hybrid organic-inorganic PSC, the instability from moisture and heat arising from the organic constituents seriously impedes its commercialization[1,2]. As one of the prospective alternatives based on inorganic cation, CsPbIBr2 has been investigated in recent years with suitable bandgap (~ 2.08 eV) and reinforced thermal stability (> 460℃), becoming a preferable candidate in all-inorganic PSC community[3].

High-performance CsPbIBr2 PSCs based on n-i-p structure is in demand of organic materials such as Spiro-OMeTAD HTL. However, its high cost and air instability derived from the additives hinder the commercialization. Therefore, a solution-processed NiOx was firstly employed in inverted CsPbIBr2 PSCs by Liu with PCE of 5.5%[4]. Interestingly, an emerging low-cost HTL material, self-assembled monolayer (SAM) has been gradually employed in a series of efficient inverted PSCs. In this work, a bilateral strategy is proposed in achieving high-performance all-inorganic CsPbIBr2 PSCs.

2. Result and Discussion

Figure 1a illustrates the effect between ITO substrates and SAMs in inverted CsPbIBr2 PSCs. The phosphonic anchoring groups of HSMs strongly bind with the hydroxyl from the surface of metal oxide substrates. Meanwhile, compact and smooth surface of HSMs contribute to full-coverage and uniform perovskite film. The modified HSM exhibits equally good transmittance with ITO substrate, as shown in Figure 1b.  In addition, Figure 1c and d show the enhanced optical absorption and PL quenching.  In conjunction with the gradient energy level alignment by introducing HSM, bilayer eliminates the energy barrier between HTL/perovskite and reinforce the hole injection. Therefore, SAMs-based device obtains improved Voc because of the suppressed recombination. Optimal energy level alignment also leads to higher Jsc for bilayer device. Simultaneously, we successfully employ an interlayer on perovskite film to suppress the interface recombination, which shows highly increased FF. As a result, the bilateral modification towards CsPbIBr2 perovskite layer achieve the champion PCE over 10% with highly improved Jsc of 10.14 mA/cm2, Voc of 1.22 V and extra high FF of 82.26%.

3. Conclusion

In summary, we have employed a bilayer HTL with gradient energy level alignment and preferable surface, which is beneficial to carrier transfer and inorganic perovskite growth. Thus, the bilayer-device obtains highly increased Jsc. Importantly, an interlayer is deposited on perovskite surface to accelerate electron extraction and suppress interface recombination, resulting in remarkable FF. This work proposes an effective bilateral modification method in achieving high-performance inverted CsPbIBr2 PSCs.