Monochromatic Conversion Efficiency Research Articles

Light trapping for photovoltaic cells used for optical power transmission

Photovoltaic (PV) cells used for optical power transmission convert laser light incident from limited directions to electricity. This illumination condition is in contrast to that for solar cells. A combination of an angular selective filter consisting of a dielectric multilayer on the front surface and a diffuse reflector on the rear surface significantly traps both the incident laser light into and radiation from the PV cell. This light trapping effect can improve the monochromatic conversion efficiency of a GaAs PV cell by 9% (absolute) compared with that for the ×4n2 trapping, under 872 nm and 1 W cm−2 laser illumination.

Application of sandwich-type polyoxometalates modified TiO<sub>2</sub> in dye-sensitized solar cells

<p indent=0mm>Nowadays, the rapid consumption of fossil energy has led to global energy shortage. At present, more than 80% of the energy consumed globally is derived from non-renewable fossil fuels such as coal, oil and natural gas. The burning of fossil fuels will also bring about a series of problems such as the greenhouse effect, the falling quality of global air, the formation of acid rain and the pollution of water bodies. Therefore, whether new energy sources that can replace fossil energy can be found has become an increasingly popular concern problem. Dye-sensitized solar cells (DSSCs) as a new type of renewable energy source have attracted wide attention in recent years, because of their simple fabrication process, low production costs, relatively high photoelectric conversion efficiency (PCE), and being environmentally safe. But the recombinations of photogenerated electrons holes are seriously affected the photoelectric conversion efficiency of DSSCs. Polyoxometalates (POMs) are a class of metal oxide clusters with the highest oxidation state of tungsten, molybdenum or vanadium. Due to their structural diversity and unique physicochemical properties, they have been applied in catalysis, photochemistry, biology and medicine. However, due to POMs have excellent photoelectrochemical properties, which can be used as an excellent electronic extractant in the photoanode of DSSCs. Here, the sandwich-type polyoxometalates compound K₁₅{K₃[(A-α-PW₉O₃₄)₂Fe₂(C₂O₄)₂]}·29H₂O with the intramolecular electron transfer properties is composited with TiO₂ by sol-gel method to prepare a POM@TiO₂ composite photoanode. The energy band structure of the polyoxometalate is studied by optical and electrochemical measurements. The result shows that the LUMO energy level <sc>(-0.06 V)</sc> of the sandwich-type polyoxometalates is lower than that of TiO₂ and the optical band gap is <sc>2.28 eV.</sc> The performance test shows that the DSSC efficiency of POM@TiO₂ composite photoanode production reached 6.33%, which is 15% higher than that of battery whose anode is pure TiO₂ (5.52%). Electrochemical impedance spectroscopy (EIS), dark current tests and voltage decay curve tests demonstrate that sandwich-type POMs effectively suppress electron-hole recombinations and increase electron lifetime. The incident monochromatic photon-electron conversion efficiency (IPCE) test further demonstrates that the introduction of the POMs increases the monochromatic conversion efficiency (from 35% to 53%).

Challenges and strategies for implementing the vertical epitaxial heterostructure architechture (VEHSA) design for concentrated photovoltaic applications

Monochromatic conversion efficiencies in excess of 60% have been achieved with Vertical Epitaxial HeteroStructure Architechture laser power converters (with anywhere from 5 to 20+ n/p junctions stacked vertically). We are presently investigating the applicability of this design to solar cells, whereby the individual junctions of a multi-junction cell are replaced with a current matched stack of subcells. If viable, such a design offers the potential for efficiency gains via reduced I2R losses and elevated Voc. Moreover, splitting the short-circuit current over additional junctions opens up the possibility of operation under concentration ratios otherwise considered impractical for conventional cells.

Laser (λ = 809 nm) power converter based on GaAs

Laser-power converters with a wavelength of λ = 809 nm are fabricated on the basis of single-junction AlGaAs/GaAs heterostructures grown by the method of liquid-phase epitaxy (LPE). Photovoltaic modules with an operating voltage of 4 V for converting radiation of various densities are developed and tested. Two approaches—without the use of optical concentrating systems and with the use of Fresnel lenses are investigated. A monochromatic conversion efficiency exceeding 44% is attained with a photovoltaic module based on 64 laser-radiation converters 0.04 cm2 in area and a concentrating system made of a quartz-lens matrix.

Light trapping for emission from a photovoltaic cell under normally incident monochromatic illumination

We have theoretically demonstrated a new light-trapping mechanism to reduce emission from a photovoltaic (PV) cell used for a monochromatic light source, which improves limiting conversion efficiency determined by the detailed balance. A multilayered bandpass filter formed on the surface of a PV cell has been found to prevent the light generated inside by radiative recombination from escaping the cell, resulting in a remarkable decrease of the effective solid angle for the emission. We have clarified a guide to design a suitable configuration of the bandpass filter and achieved significant reduction of the emission. The resultant gain in monochromatic conversion efficiency in the radiative limit due to the optimally designed 18-layerd bandpass filters is as high as 6% under normally incident 1064 nm illumination of 10 mW/cm2 ∼ 1 kW/cm2, compared with the efficiency for the perfect anti-reflection treatment to the surface of a conventional solar cell.