Indoor photovoltaics (PV) is a perspective application for solar cells in domotic applications and  low-power devices for the internet of things [1], [2]. The emerging and low-cost solution-processed halide perovskite solar cells (PSC)[3]–[5] is currently  demonstrating unprecedented progress for power conversation efficiency (PCE) at 1 sun conditions exceeding 23% [6]. At the same time, PSC are showing competitive output characteristics for indoor (>40 uW/cm²) at 400 lux [7] in comparison to commercially available Si, GaAs and DSSCs devices[8]–[10]. However, the typical architecture for high performing indoor PSCs, that is the n-i-p structure, has inherent hysteresis of the JV characteristics[11] due to parasitic accumulations effects[12], that can negatively contribute to maximum power performances. Thus, optimization of PSC structure for indoor use is still a challenging task. As reported for operation of PSC with inverted planar configuration at standard conditions (1.5 AM G, 100 mW/cm²), the use of p-i-n structure significantly reduces hysteresis effect[13]. However,  there are only few reports presented in the literature for p-i-n indoor PSC[14] and no any published results for the device structure with stable inorganic hole transport layer (HTL).

In this work we developed p-i-n indoor PSC with nickel oxide (NiO) HTL deposited from the tris (ethylene diamine) nickel acetate precursor (T_process=300 ̊C, 10 nm compact film) and nanoparticles (NP) dispersion (T_process=100 ̊, 60 nm nanoparticles film). We provided comparison of device performance fabricated on different transparent conductive electrodes (TCOs - planar FTO and ITO) with the two types of NiO HTL  to define the impact of cell architecture on output characteristics. For all types of NiO HTL we observed the absence of hysteresis during JV sweeps, but a strong correlation to J_sc and V_oc. In general, at 200 and 400 lux of illumination (white LED), devices with compact NiO HTL showed 20-26 % larger J_sc and, at the same time, 4-10 % lower V_oc with respect to NiO NP HTL. Maximum power densities (P_max) extracted from JV plots at 200 lux achieved at level of 15.9 and 16.8 µW/cm² for compact NiO HTL devices on FTO and ITO substrates, respectively. On the other hand, cells with NiO NP HTL provided 13.9 µW/cm² on ITO and 8.8 µW/cm² on FTO. At 400 lux, compact NiO HTL devices reached a P_max value of 43.7  µW/cm² on ITO and 39.6 µW/cm² on FTO, while NP HTL cells demonstrated 39.2 µW/cm² on ITO and 27.3 µW/cm² on FTO (see Fig. 1).

Such difference in J_sc values was investigated with measurement of dark-current characteristics, where devices with NiO NP HTL demonstrated larger shunting leakages on both types of used TCOs. Transient photovoltage study showed that heterojunction with NP has faster dynamics of relaxation due to lower concentration of defects on the interface, that corresponds to higher values of Voc for the devices with NiO NP HTL. We found that achieved values for generated power densities under dim-light illumination are currently the best for inverted planar structures presented in the literature. The stability of indoor solar cells structure was proved during maximum power point tracking under 200-400 lux conditions several hundreds of hours.