RESUMO
An efficient synthetic protocol to functionalize the cyanoacrylic acid anchoring group of commercially available MK-2 dye with a highly water-stable hydroxamate anchoring group is described. Extensive characterization of this hydroxamate-modified dye (MK-2HA) reveals that the modification does not affect its favorable optoelectronic properties. Dye-sensitized solar cells (DSSCs) prepared with the MK-2HA dye attain improved efficiency (6.9%), relative to analogously prepared devices with commercial MK-2 and N719 dyes. The hydroxamate anchoring group also contributes to significantly increased water stability, with a decrease in the rate constant for dye desorption of MK-2HA relative to MK-2 in the presence of water by as much as 37.5%. In addition, the hydroxamate-anchored dye undergoes essentially no loss in DSSC efficiency and the external quantum efficiency improves when up to 20% water is purposefully added to the electrolyte. In contrast, devices prepared with the commercial dye suffer a 50% decline in efficiency under identical conditions, with a concomitant decrease in external quantum efficiency. Collectively, our results indicate that covalent functionalization of organic dyes with hydroxamate anchoring groups is a simple and efficient approach to improving the water stability of the dye-semiconductor interface and overall device durability.
RESUMO
The relatively low stability of solar cells based on hybrid halide perovskites is the main issue to be solved for the implementation in real life of these extraordinary materials. Degradation is accelerated by temperature, moisture, oxygen, and light and mediated by halide easy hopping. The approach here is to incorporate pristine graphene, which is hydrophobic and impermeable to gases and likely limits ionic diffusion while maintaining adequate electronic conductivity. Low concentrations of few-layer graphene platelets (up to 24 × 10-3 wt %) were incorporated to MAPbI3 films for a detailed structural, optical, and transport study whose results are then used to fabricate solar cells with graphene-doped active layers. The lowest graphene content delays the degradation of films with time and light irradiation and leads to enhanced photovoltaic performance and stability of the solar cells, with relative improvement over devices without graphene of 15% in the power conversion efficiency, PCE. A higher graphene content further stabilizes the perovskite films but is detrimental for in-operation devices. A trade-off between the possible sealing effect of the perovskite grains by graphene, that limits ionic diffusion, and the reduction of the crystalline domain size that reduces electronic transport, and, especially, the detected increase of film porosity, that facilitates the access to atmospheric gases, is proposed to be at the origin of the observed trends. This work demonstrated how the synergy between these materials can help to develop cost-effective routes to overcome the stability barrier of metal halide perovskites, introducing active layer design strategies that allow commercialization to take off.
RESUMO
The poor photostability under ambient conditions of hybrid halide perovskites has hindered their recently explored promising nonlinear optical properties. Here, we show how Bi3+ can partially substitute Pb2+ homogeneously in the commonly studied MAPbI3, improving both environmental stability and photostability under high laser irradiation. Bi content around 2 atom % produces thin films where the nonlinear refractive (n2) and absorptive coefficients (ß), which modify the refractive index (Δn) of the material with light fluence (I), increase up to factors of 4 and 3.5, respectively, compared to undoped MAPbI3. Higher doping inhibits the nonlinear parameters; however, the samples show higher fluence damage thresholds. Thus, these results provide a road map on how MAPbI3 can be engineered for practical cost-effective nonlinear applications by means of Bi doping, including optical limiting devices and multiple-harmonic generation into optoelectronics devices.