We present new architectures in CH 3 NH 3 PbI 3 based planar perovskite solar cells incorporating solution processed SnO 2 /MgO composite electron transport layers that show the highest power outputs ever reported under typical 200-400 lx indoor illumination conditions. When measured under white OSRAM LED lamp (200, 400 lx), the maximum power density values were 20.2 µW/cm 2 (estimated PCE = 25.0% ) at 200 lx and 41.6 µW/cm 2 (PCE = 26.9%) at 400 lx which correspond to a  20% increment compared to solar cells with a SnO 2 layer only. The thin MgO overlayer leads to more uniform films, reduces interfacial carrier recombination, and leads to better stability. All layers of the cells, except for the two electrodes, are solution processed at low temperatures, thus low cost processing. Furthermore, ambient indoor conditions represent a milder environment compared to stringent outdoor conditions for a technology that is still looking for a commercial outlet also due to stability concerns. The unparalleled performance here demonstrated, paves the way for perovskite solar cells to contribute strongly to the powering of the indoor electronics of the future (e.g. smart autonomous indoor wireless sensor networks, internet of things etc). KEYWORDSelectron transport layer, SnO 2 layer, SnO 2 /MgO composite layer, planar perovskite solar cell, maximum power density, indoor light illumination.

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Paper is a flexible material, commonly used for information storage, writing, packaging or specialized purposes. It also has strong appeal as a substrate in the field of flexible printed electronics. Many applications, including safety, merchandising, smart labels/packing, chemical/biomedical sensors, require an energy source to power operation. Here we review progress regarding development of photovoltaic and energy storage devices on cellulosic substrates where one or more of the main material layers are deposited via solution processing or printing. Paper can be used simply as the flexible substrate or, exploiting its porous fibrelike nature, as an active film by infiltration or co-preparation with electronic materials. Solar cells with efficiencies of up to  4% on opaque and 9% on transparent substrates have been demonstrated. Recent developments in paper-based supercapacitors and batteries are also reviewed with maximum achieved capacity of 1350 mF cm -2 and 2000 mAh g -1 respectively.Analysing the literature, it becomes apparent that more work needs to be carried out in continuing to improve peak performance, but especially stability and the application of printing techniques, even roll-to-roll, over large areas. Paper is not only environmentally friendly and recyclable, it is thin, flexible, low-weight, biocompatible, and low-cost.

Figure S1 Photovoltaic parameters, namely power conversion efficiency (a), open circuit voltage (b), short circuit current density (c) and fill factor (d), of planar and mesoscopic flexible solar cells measured at STC (25 °C , AM 1.5G spectrum, 1,000 W·m -2 ).

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Flexible perovskite solar cells (FPSCs) are prime candidates for applications requiring a highly efficient, low-cost, lightweight, thin, and even foldable power source. Despite record efficiencies of lab-scale flexible devices (19.5% on a 0.1 cm 2 area), scalability represents a critical factor toward commercialization of FPSCs. Large-area automized deposition techniques and efficient laser scribing procedures are required to enable a high-throughput production of flexible perovskite modules (FPSMs), with the latter being much more challenging compared to glass substrates. In this work, we introduce the combined concept of laser scribing optimization and automatized spray-coating of SnO 2 layers. Based on a systematic variation of the incident laser power and a comprehensive morphological and electrical analysis of laser-based cell interconnections, optimal scribing parameters are identified. Furthermore, spray-coating is used to deposit uniform compact SnO 2 films on large-area (>120 cm 2 ) plastic substrates. FPSCs with spray-coated SnO 2 show comparable performance as spin-coated cells, delivering up to 15.3% efficiency on small areas under 1 sun illumination. When upscaling to large areas, FPSMs deliver 12% power conversion efficiency (PCE) and negligible hysteresis on 16.8 cm 2 and 11.7% PCE on a 21.8 cm 2 active area. Our perovskite devices preserved 78% efficiency when the active area increased from 0.1 to 16.8 cm 2 , demonstrating that our combined approach is an effective strategy for large-area manufacturing of perovskite devices on flexible substrates.

We present planar perovskite solar cells (PSCs) incorporating thin SnO2/Al2O3 double electron transport layers between the perovskite and an indium tin oxide (ITO) bottom electrode. When measured under 1 sun illumination, we obtained a maximum power conversion efficiency (PCE) of 20.1% and a steady state efficiency of 17.8% for the best cell. These values were ~ 20-30% higher in relative terms than that of cells with SnO2 only (i.e. a maximum PCE of 15.3% and a steady state PCE of 14.9%). Insertion of the thin UV-irradiated solution-processed nanoparticle Al2O3 interlayer effectively enhanced the wettability of the electron transport layer, provided enhanced interface area, as well as a lower work function, leading to improved charge extraction. Incorporation of an Al2O3 layer between the perovskite and SnO2 layers also improved the rectification ratios of the diodes as well as both series and shunt resistances. Our devices are fabricated using fully solution-processed transport and active semiconducting layers processed at low temperatures (≤ 150 °C).

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