and electron transport materials (ETMs) are considered to be fundamentally crucial in improving PSCs performance, because HTMs and ETMs can selectively extract and transport charge from perovskite layers. Most PSCs have a sandwich structure that a perovskite absorber is located between ETM and HTM, which can be divided into n-i-p structures and p-i-n structures according to the sequence of the stacked layer. [16,17] Compared with conventional n-i-p structured devices, inverted p-i-n planar structured devices have advantages of the lower processing temperatures and the negligible hysteresis effect. [18,19] Despite this, the reported champion PCEs of the PSCs have been achieved using the n-i-p structure, and the PCEs of the state-of-the-art inverted p-i-n structure still lag behind the PCEs of n-i-p structure due to the interfacial nonradiative recombination loss. [20] It has been well recognized that charge transfer layer is fundamentally important in further controlling carrier transport and improving performance. For the inverted planar PSCs, the HTM not only plays a significant role in the crystallinity and crystal orientation of perovskite layers, but also can effectively extract/transport holes and block electrons, which has significant impacts on the performance of PSCs with inverted structure. [21] Organic HTMs, such as PEDOT: PSS, PTAA, etc., have been widely used in inverted planar PSCs and achieved excellent performances, [22][23][24] but the temperatureinduced inactivation and hygroscopic feature still hampers the stability of the device. [25,26] Compared with organic HTMs, inorganic p-type materials, such as copper compounds (Cu 2 O, CuSCN, CuGaO 2 ), [27][28][29] nickel oxide (NiO) and some other inorganic HTMs like MoO x and V 2 O 5 , [30][31][32] have earned a lot of attention due to their excellent carrier mobility, low cost, outstanding stability, and simple processing. [33][34][35] Among them, NiO has been extensively investigated because its better band alignment with most perovskite absorbers. [36] The strategies enhancing NiO properties have been proposed, including new fabrication methods, [37,38] surficial modification/passivations, [39][40][41] and new deposition routes. [41,43] For example, the PSCs with a KCl-treated NiO hole transport layer (HTL) show Perovskite solar cells (PSCs) have been recognized as fascinating optoelectronic devices with a rapid progress of the power conversion efficiency (PCE). However, serious carrier recombination at charge transport layer (CTL)/ perovskite interface limits further development of PSCs. Therefore, carrier dynamics at interface should be finely regulated to achieve a satisfied performance. Herein, a hole transport layer (HTL) is developed with a bilayer film composed of Li:NiO/NiO, in which NiO directly contacts with perovskite film. The prepared HTL is a p-p + homojunction as Li:NiO film has higher concentration of carrier than NiO. Energy level alignment in Li:NiO/NiO HTL reflects a hole transport improvement by both built-in electric field and ...

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Perovskite solar cells (PSCs) are the most promising commercial photoelectric conversion technology in the future. The planar p–i–n structure cells have advantages in negligible hysteresis, low temperature preparation and excellent stability. However, for inverted planar PSCs, the non-radiative recombination at the interface is an important reason that impedes the charge transfer and improvement of power conversion efficiency. Having a homogeneous, compact, and energy-level-matched charge transport layer is the key to reducing non-radiative recombination. In our study, NiO x /Sr:NiO x bilayer hole transport layer (HTL) improves the holes transmission of NiO x based HTL, reduces the recombination in the interface between perovskite and HTL layer and improves the device performance. The bilayer HTL enhances the hole transfer by forming a driving force of an electric field and further improves J sc. As a result, the device has a power conversion efficiency of 18.44%, a short circuit current density of 22.81 mA⋅cm−2 and a fill factor of 0.80. Compared to the pristine PSCs, there are certain improvements of optical parameters. This method provides a new idea for the future design of novel hole transport layers and the development of high-performance solar cells.

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