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The possibility to tune chemical and physical properties in nanosized materials has a strong impact on a variety of technologies, including photovoltaics. One of the prominent research areas of nanomaterials for photovoltaics involves spectral conversion. Modification of the spectrum requires down- and/or upconversion or downshifting of the spectrum, meaning that the energy of photons is modified to either lower (down) or higher (up) energy. Nanostructures such as quantum dots, luminescent dye molecules, and lanthanide-doped glasses are capable of absorbing photons at a certain wavelength and emitting photons at a different (shorter or longer) wavelength. We will discuss upconversion by lanthanide compounds in various host materials and will further demonstrate upconversion to work for thin-film silicon solar cells.

A near-infrared to visible upconversion phosphor (b-NaYF 4 :Yb 3 + (18%), Er 3 + (2%)) has been applied at the back of a thin film hydrogenated amorphous silicon (a-Si:H) solar cell in combination with a white back reflector to investigate its response to sub-bandgap infrared irradiation. Current-voltage measurements were performed on the solar cells. A maximum current enhancement of 6.2 mA was measured on illumination with a 980 nm diode laser at 28 mW. This corresponds to an external quantum efficiency (EQE) of 0.03% of the solar cell. A small part, 0.01%, was due to the direct absorption of sub-bandgap radiation but the larger part originates from upconversion. These experiments constitute a proof-of-principle for the utilization of photon upconversion in thin film solar cells. A close match between the non-linear behavior of the upconversion material and the EQE was found by varying the intensity of the laser light.

For substrate n-i-p-type cells rough reflecting back contacts are used in order to enhance the short-circuit currents. The roughness at the electrode∕silicon interfaces is considered to be the key to efficient light trapping. Root-mean-square (rms) roughness, angular resolved scattering intensity, and haze are normally used to indicate the amount of scattering, but they do not quantitatively correlate with the current enhancement. It is proposed that the lateral dimensions should also be taken into account. Based on fundamental considerations, we have analyzed by atomic force microscopy specific lateral dimensions that are considered to have a high scattering efficiency. Textured back reflectors with widely varying morphologies have been developed by the use of sputtered Ag and Ag:AlOx layers. For these layers we have weighted the rms roughness of the surface with the lateral dimensions of the effective scattering features. A clear correlation is found between the current generation under (infra)red light in microcrystalline (μc-Si:H) n-i-p solar cells and the weighted rms value of the Ag back contacts. Furthermore, the surface plasmon absorption of the rough Ag back contact has been found to be a significant limiting factor for the current enhancement. Using Ag:AlOx layers on glass, deposited at substrate temperatures below 300°C, a μc-Si:H n-i-p solar cell is obtained with an efficiency of 8.1%. Using textured Ag layers made at a higher substrate temperature on a stainless steel substrate we have developed a hot-wire chemical vapor deposited μc-Si:H n-i-p-type solar cell with 8.5% efficiency.

Microcrystalline silicon‐based single‐junction p–i–n solar cells have been fabricated by very high‐frequency plasma enhanced chemical vapor deposition using a showerhead cathode at high pressures and under silane depletion conditions. The i‐layers are made near the transition from amorphous to crystalline. It was found that, especially at high crystalline fractions, the open‐circuit voltage and fill factor are very sensitive to the morphology of the substrate. At an i‐layer deposition rate 0·45 nm/s, we have measured a stabilised efficiency of 10% (Voc = 0·52 V, FF = 0·74) for a cell made on texture‐etched ZnO:Al. The performance is stable under light soaking. The defect density of the absorber layer is in the 1015 cm−3 range. In spite of the presence of oxygen contamination, good electrical properties and good infrared cell response are obtained. Copyright © 2006 John Wiley & Sons, Ltd.

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We present a silicon nitride/polymer hybrid multilayer moisture barrier for flexible electronics made entirely by hot wire chemical vapor deposition (HWCVD) at substrate temperatures below 100 °C. Using the initiated CVD (iCVD) variant of HWCVD for the polymer layers, these can be extremely thin, while efficiently decoupling the defects in consecutive inorganic layers. Although a single layer of low temperature SiNx is more prone to have pinholes than its state‐of‐the‐art high temperature equivalent, we have achieved a simple three‐layer structure consisting of two low‐temperature SiNx layers with a polymer layer in between, which is pinhole free and shows a water vapor transmission rate (WVTR) as low as 5 × 10–6 g/m2/day at a temperature of 60 °C and a relative humidity of 90%. This WVTR is low enough for organic devices. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

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