Interface engineering of nickel oxide charge transporting layer for solution-processed perovskite solar cells

The metalorganic halide perovskite solar cells (PSCs) have attracted a great deal of attention of solar cell research community due to an incredible device efficiency improvement from 3.8% to 25% since 2009 [1][2] , exceeding the record efficiency of Cu(In,Ga)Se2 solar cells [3] and approaching the efficiency of solar cells based on crystalline silicon [4]. The high potential of perovskite solar cells is based on the semiconductor material properties and device technology: tunable bandgap value (1.24 - 3.53 eV) [5], [6], high absorption coefficient (α = 5 × 103 cm-1 at 700 nm) [7], micron-range carrier diffusion length [8], ambipolar charge carrier transport [9], [10], low trap density (8.6 × 1016 cm-3 ) [11],[12] and weak exciton binding energy (≃10-15 meV for MAPbI3 and MAPbBr3) [13],[14]. Moreover, functional layers (except metal electrode) can be deposited by low-temperature and low-cost solution-processed deposition techniques [15], [16]. However, crystallization of the halide perovskite films tends to the formation of various structural defects interstitials, dislocations, grain boundaries, which could act as a trigger for the degradation of functional films[17]. Further efficiency gains of PSС requires as influence on the internal material's processes (changing the chemical composition, application method), as impact on interfaces between perovskite and transporting layers because due to different lattice constants at the perovskite/transport layer interface forms defects and creates energy barriers, which leads to charge accumulation and low stability and recombination in PSCs. Thus, it is vital to understand and control charge extraction across the interfaces in order to minimize energy loss and improve device performance. The perovskite solar cell architecture has two variations: n-i-p (direct) and p-i-n (inverted), where the perovskite photoactive layer (i) is located between the n-type transport layer (ETL) and the p-type transport layer (HTL). In addition, the transporting layers divided on planar and mesoscopic structures. In fact that the highest efficiency of solar light conversion has been recorded on devices with mesoscopic n-i-p configuration [12]. However, this proposed architecture has disadvantages that limit industrial production capabilities, such as (a) the presence of hysteresis effect on the current-voltage plot, arising from ion migration and recombination effects at grain [19], [20]; (b) the high temperature annealing process (480 °C) of mesoscopic TiO2; and (c) the photo-aging effect in titanium dioxide under ultraviolet (UV) light activates the degradation processes in perovskite [21], [22]. Thus, the performance control of the device remained incomplete. Most of the issues raised can be solved by change to planar p-i-n configuration [23]. Nickel oxide was considered as one of the most promising hole transporting materials for PSCs due to perfect semiconductor properties: high mobilities of the holes (up to 101 cm2V -1 s -1 )[37], optimal valence band position (in range of - 5.4...5.2 eV) and large band gap (>3.4 eV)[38]. Moreover, NiOX can be fabricated vial solution processing [39,40] from cheap precursors [35,41] in opposite to the high performing organic small molecules and polymers, that requires excessive costed and complex synthesis. However, the intrinsic and unpassivated NiOX film does not provide a stable bond to the perovskite, which requires appropriate interface modification. Therefore, passivation with thin polymer films or self-organising layers of the NiOx/perovskite interface provides a reduction of structural defects on the surface by adapting the surface morphology (cross-linking of uncoordinated ions at grain boundaries in perovskite) and preventing the diffusion of ionic defects along the device [30], [31]. Also, the crystallinity of the photoactive layer, composition and deposition method have an important feature. The two-step perovskite deposition method is characterized by deficiency of defect states, the large grain perovskite size, the large diffusion length of the carriers, the scalability and the simplicity of the deposition process. The sequential deposition method of perovskite crystallization can be performed in an inert atmosphere without reducing productivity under high humidity and oxidation processes, and it is also suitable for the production of modules with larger area [39]. Therefore, it is essential to design and fabricate low-defect structure in order to obtain highefficiency and stable PSCs. Herein, we highlight the roles of interface engineering in devices' performance and crystal engineering approach for perovskite harvesting layer.