Near-Ultraviolet Indoor Black Light-Harvesting Perovskite Solar Cells

Indoor light-energy-harvesting solar cells have long-standing history with perovskite solar cells (PSCs) recently emerging as potential candidates with high power conversion efficiencies (PCEs). However, almost all of the reported studies on indoor light-harvesting solar cells utilize white light in the visible wavelength. Low wavelength near-ultraviolet (UV) lights used under indoor environments are not given attention despite their high photon energy. In this study, perovskite solar cells have been investigated for the first time for harvesting energy from a commercially available near-UV (UV-A) indoor LED light (395–400 nm). Also called black lights, these near-UV lights are commonly used for decoration (e.g., in bars, pubs, aquariums, parties, clubs, body art studios, neon lights, and Christmas and Halloween decorations). The optimized perovskite solar cells with the n–i–p architecture using the CH3NH3PbI3 absorber were fabricated and characterized under different illumination intensities of near-UV indoor LEDs. The champion devices delivered a PCE and power output of 20.63% and 775.86 μW/cm2, respectively, when measured under UV illumination of 3.76 mW/cm2. The devices retained 84.10% of their initial PCE when aged under near-UV light for 24 h. The effects of UV exposure on the device performance have been comprehensively characterized. Furthermore, UV-stable solar cells fabricated with a modified electron transport layer retained 95.53% of its initial PCE after 24 h UV exposure. The champion devices delivered enhanced PCE and power output of 26.19% and 991.21 μW/cm2, respectively, when measured under UV illumination of 3.76 mW/cm2. This work opens up a novel direction for energy harvesting from near-UV indoor light sources for applications in microwatt-powered electronics such as internet of things sensors.

S-3 was dropped 15 seconds before the end of the spin cycle. The substrates were then annealed at 100 ℃ for 10 minutes to obtain the MAPbI 3 films. For PCBM:BPhen interlayer between SnO 2 and MAPI, 12.5mg PCBM was dissolved in 1 ml chlorobenzene and 200 µl Bphen (0.5 mg in 1 ml chlorobenzene) was added into it. The precursor was stirred overnight and spin coated on SnO 2 coated substrate at 3000 RPM for 30 seconds followed by annealing at 100 ℃ for 10 minutes. To deposit hole transport layer (HTL), Spiro-OMeATD precursor consisting of 72.5 mg/ml chlorobenzene with 17.5 µl of LiTFSI (520 mg/ml of 1-butanol) and 27.5 µl 4-tert butylpyridine was spin coated on MAPbI 3 layer at 3000 rpm for 30 seconds. Finally, to complete the device, silver electrodes (100 nm) were thermally evaporated (in 5 × 10 -7 mbar vacuum) through a shadow mask.

UV LED source and intensity measurements
A commercially available near-UV LED light [TBE Lighting, A60 UV LED, 395-400 nm, 9W], generally used as an indoor decorating lamp, was used as the indoor light source. As the near-UV LED used in the present study emits within the visible region (380-700 nm), it was possible to measure the intensity in terms of lux. A dark box fitted with the near-UV LED lamp was used for the intensity and solar cell characterizations. A photospectrometer (model ILT 350, with measurement range of 380-780 nm) was used to measure the light intensity. The intensity and the irradiance of the near-UV LED could be directly read from the spectrophotometer. The spectrophotometer recorded the near-UV LED source spectrum with the peak at ~399 nm which is in consistent with the UV-LED specification from the manufacturer. The term UV used throughout manuscript and supporting information represent near-UV region (395-400 nm) with the peak position at ~399 nm.

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For the UV degradation studies, MAPbI 3 films and solar cells were exposed (referred to as near-UV pre-treatment) to near-UV light [TBE Lighting, A60 UV LED, 395-400 nm, 9W] at 100 lux (2.78 mWcm -2 ) for different time durations. All the samples were kept under ambient condition (RH:40-45 % and temperature of 20-24 ℃) for 24 hours irrespective of the near-UV pre-treatment duration. This is to guarantee same environmental impact on all the samples/devices.

Characterization
The surface morphology of MAPbI 3 films were imaged using the field-emission scanning electron microscope (FE-SEM, FEI Quanta 250 FEG). The optical absorption of the films was measured using UV-Vis spectroscopy (UNICAM UV-300). Fourier transform infrared (FTIR) spectroscopic measurements were carried out in attenuated total reflection mode (Agilent 0147A). The XRD measurements were performed at room temperature on a Bruker D2 Phaser system using monochromatic CuK α radiation with a wavelength of 1.5406 Å. The samples were scanned in the range 5-80 with an increment of 0.04 on the 2θ scale. The substrates were set to a rotation speed of 8 rpm throughout the measurement. Photoluminescence (PL) measurements were done using Horiba Fluorolog 3-22 with xenon lamp at an excitation wavelength of 503 nm. TRPL measurements were done using Horiba Deltaflex TCSPC lifetime fluorometer with 503 nm laser source. The PL decay time was determined by fitting the curve using Origin pro software. The Electrochemical impedance spectroscopic measurements were performed using AUTOLAB PGSTAT 302N and FRA32M. External quantum efficiency (EQE) spectrum was taken using Bentham (PVE300) in transformer mode.
Silicon reference diode was used for calibration.

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The current-voltage measurements were carried out using Ossila Solar Cells I-V test system.
For 1 sun measurement, the solar spectrum at AM 1.5 was simulated with a Xenon lamp and filters (Oriel Sol 1A, class ABB, 94021A) with the measured intensity at 100 mW cm −2 . The illumination intensity was calibrated using reference Si solar cell (Newport, 91150V). For indoor near-UV LED, a low power (9W) A60 UV LED lamp (TBE lighting) with the wavelength of 395-400 nm was used as a source. ILT350 spectrophotometer was used to measure the light intensity, incident power, and related spectral distribution of the LED light.
The current density versus voltage (J-V) curves of the resulting devices were acquired in both forward-scan (FS; -0.2 to1.2 V) and reverse scan (RS: 1.2 to -0.2) with the scan speed of ~80 mV/s. The active area of the devices is 0.026 cm 2 determined by the standard aperture mask.  The hysteresis index (HI) of the perovskite solar cells is calculated using the relation     Figure S5. Optical absorption spectra of CH 3 NH 3 PbI 3 films on glass substrate measured after near-UV (100 lux) pre-treatment for different time durations.