Solar cells on paper have the potential to be inexpensive and portable due to several unique features of the substrate: paper is cheap, flexible, lightweight, biodegradable, and manufactured by roll-to-roll processing. Here, we report the first nanocrystal photovoltaic devices (PVs) made on paper. Using spray-deposited CuInSe2 nanocrystals as the absorber material on substrates composed of bacterial cellulose nanofibers synthesized by the microorganism Gluconacetobacter hansenii, these devices demonstrate exceptional electrical and mechanical integrity. There is no significant loss in PV device performance after more than 100 flexes to 5 mm radius, and the devices continue to perform when folded into a crease. The practical use of these paper PVs is demonstrated with a prototype device powering liquid crystal displays (LCDs) mounted to various kinds of surfaces.
Grazing incidence small angle X-ray scattering (GISAXS) measurements reveal that superlattices of 1.7 nm diameter, gold (Au) nanocrystals capped with octadecanethiol become significantly more ordered when heated to moderate temperatures (50-60 °C). This enhancement in order is reversible and the superlattice returns to its initially disordered structure when cooled back to room temperature. Disorder-order transition temperatures were estimated from the GISAXS data using the Hansen-Verlet criterion. Differential scanning calorimetry (DSC) measurements of the superlattices exhibited exotherms (associated with disordering during cooling) and endotherms (associated with ordering during heating) near the transition temperatures. The superlattice transition temperatures also correspond approximately to the melting and solidification points of octadecanethiol. Therefore, it appears that a change in capping ligand packing that occurs upon ligand melting underlies the structural transition of the superlattices. We liken the heat-induced ordering of the superlattices to an inverse melting transition.
Uniform silicon nanocrystals were synthesized with cuboctahedral shape and passivated with 1-dodecene capping ligands. Transmission electron microscopy, electron diffraction, and grazing incidence wide-angle and small-angle X-ray scattering show that these soft cuboctahedra assemble into face-centered cubic superlattices with orientational order. The preferred nanocrystal orientation was found to depend on the orientation of the superlattices on the substrate, indicating that the interactions with the substrate and assembly kinetics can influence the orientation of faceted nanocrystals in superlattices.
We recently observed that a disordered assembly of octadecanethiol-capped gold (Au) nanocrystals can order when heated from room temperature to 60 °C [Yu, Y.; Jain, A.; Guillaussier, A.; Voggu, V. R.; Truskett, T. M.; Smilgies, D.-M.; Korgel, B. A. Faraday Discuss. 2015, 181, 181–192]. This “inverse melting” structural transition was reversible and occurred near the melting-solidification temperature of the capping ligands. To determine the generality of this phenomenon, we studied by in situ grazing incidence small-angle X-ray scattering (GISAXS) the structure of assemblies of Au nanocrystals with shorter C12 and C5 alkanethiol capping ligands that form ordered superlattices at room temperature and have a ligand melting-solidification temperature below room temperature. Superlattices of dodecanethiol-capped Au nanocrystals disorder when cooled below 260 K, which is the melting-solidification temperature for dodecanethiol. Au nanocrystals capped with even shorter pentanethiol ligands that have a melting transition below 100 K (the lowest experimentally accessible temperature) do not undergo the disorder transition.
Plastic photovoltaic devices (PVs) were fabricated by spray-depositing copper indium diselenide (CuInSe2) nanocrystals into micrometer-scale groove features patterned into polyethylene terephthalate (PET) substrates. Each groove has sidewall coatings of Al/CdS and Au and performs as an individual solar cell. These PV groove features can be linked electrically in series to achieve high voltages. For example, cascades of up to 15 grooves have been made with open-circuit voltages of up to 5.8 V. On the basis of the groove geometry, the power conversion efficiencies (PCEs) of the devices reached as high as 2.2%. Using the active area and photovoltaic response of devices determined from light-beam-induced current (LBIC) and photoreflectivity measurements gave PCE values as high as 4.4%.
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