A reduction of material consumption in thin-film photovoltaic devices can make solar energy economically more viable.
However, since thin films essentially absorb less light, there is an imminent need for existing technology to improve light harvesting. We present an effective approach of better light absorption, enhanced photocurrent generation and therefore higher quantum efficiency of poly (3-hexylthiophene): 1-(3-methoxycarbonyl) propyl-1-phenyl-[6, 6]-methanofullerene (P3HT:PCBM) bulk heterojunction photovoltaic/photodetector devices. We have integrated a thin semi-continuous gold film (SCGF) (~20nm) sputtered at percolation threshold between the active P3HT:PCBM layer and the indium-tin-oxide (ITO) electrode. At critical metal concentrations, i.e. percolation threshold, the light reaching the SCGF undergoes a broadband trapping with characteristic time of 200 fs, through complex interactions with fractal gold clusters. This thin SCGF together with the ITO serves as an anode. The interface between SCGF and the polymer represents the metaldielectric composite (MDC) that supports broad-band surface plasmon resonances that store electromagnetic radiation at the nanoscale and acts as an effective bulk type of concentrator without the need of increasing the photovoltaic device physical collection area. Here we report a six-fold enhancement in the integral quantum efficiency over the solar spectrum for device employing plasmon-active gold layer. Such enhancement is an important contribution for the future design of more efficient photodetecting/photovoltaic devices. The experimental results are supported by the theoretical modeling of metal-dielectric composites by block elimination method in 3D. The AC and DC responses of MDC, local field distribution, broad optical response and photon trapping in the percolating MDC were numerically calculated.
A polymer-based dynamic microlens system that can provide variable focal length and field-of-view (FOV) is fabricated and tested for its optical imaging characteristics. A flexible polydimethylsiloxane (PDMS) polymer membrane is used to form the lens surface. Two such membranes are combined with a spacer in between to form the fluidic lens chamber. The entire assembly is actuated by fluidic pressure using an external syringe pump to form either a double convex (DCX) or double concave (DCV) lens. The relationship between the focal length (f) and FOV of this dynamic lens as a function of the volume of the fluid pumped into or out of the lens chamber is investigated. The focal length of the single dynamic lens system can be tuned over the range of 75.9 to 3.1 mm and -75.9 to -3.3 mm, respectively, for the DCX and DCV lens configurations. The FOV that could be achieved using this dynamic lens system as DCX and DCV lenses is in the range of 0.12 to 61 degrees and 7 to 69 degrees, respectively. The smallest f-number (f/#) of 0.61, which corresponds to a numerical aperture of 0.64, could be achieved for a single dynamic lens system. An integrated two or three variable focal length DCV microlens system to provide wide FOV has also been fabricated and tested. The effective focal length of the integrated dynamic microlens system with two and three DCV lenses can be tuned in the range of -37.9 to -2.1 mm and -25.3 to -1.8 mm, respectively. The FOV achieved using the integrated two and three variable focal length DCV microlens systems were in the range of 8 to 76.7 degrees and 11.5 to 90.4 degrees, respectively.