LAUSR

DOI: 10.1002/admi.201901372 challenges for commercializing the perovskite solar cells is relatively poor longterm stability.[7–10] Perovskite films tend to degrade into a hydrate form in humid atmosphere, and decompose into PbI2 in the presence of oxygen under illumination via the reaction with superoxide.[11–13] Many strategies have been proposed to mitigate the intrinsic degradation of perovskite films, and they include augmentation of grain size and alloying of cations with Cs or Rb and anions with Br.[14–17] Another approach involves encapsulation of the devices that prevents O2 and H2O in the atmosphere from interacting with the perovskite films. However, thermal stress, which is one of the main factors affecting stability, is still an issue even for well-encapsulated devices. Not only does thermal stress degrade perovskite films, but also it can damage 2,2′,7,7′-tetrakis-(N,N-di-4-methoxyphenylamino)9,9′-spirobifluorene (spiro-OMeTAD), one of the most common hole-transport layers (HTLs) used in high-performing perovskite solar cells.[18–21] To improve the electrical conductivity of spiro-OMeTAD, it is common to incorporate dopant additives such as lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI) and 4-tert-butylpyridine (TBP) to spiro-OMeTAD. When a doped spiro-OMeTAD layer is subjected to a temperature above 85 °C, pinholes begin to form in the film, leading to the deterioration of device performance.[19–21] As a remedy, polymer-modified spiro-OMeTAD has recently been introduced and demonstrated some success in improving the long-term thermal stability.[22] Still, the use of costly organic hole-transport materials is ultimately not desirable for the commercialization.[23–25] Intrinsic thermal instability of organic charge-transport layers leads to serious research efforts on the inorganic HTLs.[26–34] Among the potential candidates as an inorganic HTL, CuSCN is an attractive choice because it is cheap and solution-processible with solvents such as diethyl sulfide and dipropyl sulfide.[24,25,30,34] Although the thermal stability of a CuSCN layer itself is excellent, it is known to react with the underlying perovskite to form PbI2 and CuI impurities when the layers are subjected to a temperature above 85 °C.[34] In order to reduce the interfacial reaction between CuSCN HTL and a perovskite layer, Snaith and co-workers have inserted a mesoporous layer of Al2O3 nanoparticles before the CuSCN Herein, solution-processible inorganic hole-transport layer (HTL) of a perovskite solar cell that consists of CuGaO2 nanoparticles and CuSCN, which leads to an improved device performance as well as long-term stability, is reported. Uniform films of CuGaO2 are prepared by first treating CuGaO2 nanoparticles with aminosilane that leads to well-dispersed CuGaO2 solution, followed by spin-coating of the suspension. Subsequent spin-coating of CuSCN solution onto the CuGaO2 forms a smooth HTL with excellent coverage and electrical conductivity. Comparing to the reference device with CuSCN HTL, the CuGaO2/CuSCN device improves carrier extraction and reduces trap density by ≈40%, as measured by photoluminescence and capacitance analysis. Excellent thermal stability is also demonstrated: ≈80% of the initial efficiency of the perovskite solar cells with the CuGaO2/CuSCN HTL is retained after 400 h under 85 °C/85% relative humidity environment.