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The personal ultraviolet (UV) dosimeter is a useful measurement tool to prevent UV induced dermal damages; however, conventional digital dosimeters are either bulky or require external power sources. Here, a wearable, colorimetric UV film dosimeter that provides color transition, from purple to transparent, is reported to indicate the UV dose. The film dosimeter is made of a purple photodegradable dye ((2Z,6Z)-2,6-bis(2-(2,6-diphenyl-4H-thiopyran-4-ylidene)ethylidene)cyclohexanone or DTEC) blended in low density polyethylene film. The DTEC film discolored 3.3 times more under the exposure of UV light (302 nm) than visible light (543 nm), and a UV bandpass filter is developed to increase this selectivity to UV light. The DTEC film completely discolors to transparency in 2 h under an AM 1.5 solar simulator, suggesting the potential as an indicator for individuals with types I-VI skin to predict interventions to avoid sunburn. Finally, the DTEC film is integrated with the UV bandpass filter on a wristband to function as a wearable dosimeter for low cost and convenient monitoring of sunlight exposure.

Author(s): Luo, Yanqi; Aharon, Sigalit; Stuckelberger, Michael; Magana, Ernesto; Lai, Barry; Bertoni, Mariana I; Etgar, Lioz; Fenning, David P

Optoelectronic devices based on hybrid perovskites have demonstrated outstanding performance within a few years of intense study. However, commercialization of these devices requires barriers to their development to be overcome, such as their chemical instability under operating conditions. To investigate this instability and its consequences, the electric field applied to single crystals of methylammonium lead bromide (CH3 NH3 PbBr3 ) is varied, and changes are mapped in both their elemental composition and photoluminescence. Synchrotron-based nanoprobe X-ray fluorescence (nano-XRF) with 250 nm resolution reveals quasi-reversible field-assisted halide migration, with corresponding changes in photoluminescence. It is observed that higher local bromide concentration is correlated to superior optoelectronic performance in CH3 NH3 PbBr3 . A lower limit on the electromigration rate is calculated from these experiments and the motion is interpreted as vacancy-mediated migration based on nudged elastic band density functional theory (DFT) simulations. The XRF mapping data provide direct evidence of field-assisted ionic migration in a model hybrid-perovskite thin single crystal, while the link with photoluminescence proves that the halide stoichiometry plays a key role in the optoelectronic properties of the perovskite.