Search

The drift mobility of electrons and holes was studied in a standard time-of-flight (TOF) experiment, as a function of temperature and applied electric field, for several series of a-Si:H samples that were deposited at high growth rate in an expanding thermal plasma (ETP) at substrate temperatures near 400°C. Room-temperature electron mobilities are somewhat lower than for standard plasma-enhanced chemical vapor deposited (PECVD) material, but the hole mobility is more than one decade higher. The dispersion of the photocurrent transients is comparable to the one in PECVD samples, but both electron and hole mobilities of the ETP material are remarkably insensitive to the magnitude of applied electric fields in the measured range of 0.1 to 5.0 × 104 V/cm. The temperature dependence of the mobilities, on the contrary, is high, with resolved activation energies of ∼0.24 eV for electrons and ∼0.40 eV for holes. Standard multiple-trapping analysis of the above results in terms of the underlying density of localized states suggests rather wide distributions of tail states on both sides of the gap. To account simultaneously for the weak field dependencies it is necessary to consider non-negligible recombination or deep trapping of the carriers. Measurements and analysis of the post-transit TOF signals do support the presence of a strong deep-trapping component in the transient signal.

To further increase the efficiency of multijunction thin-film silicon (TF-Si) solar cells, it is crucial for the front electrode to have a good transparency and conduction, to provide efficient light trapping for each subcell, and to ensure a suitable morphology for the growth of high-quality silicon layers. Here, we present the implementation of highly transparent modulated surface textured (MST) front electrodes as light-trapping structures in multijunction TF-Si solar cells. The MST substrates comprise a micro-textured glass, a thin layer of hydrogenated indium oxide (IOH), and a sub-micron nano-textured ZnO layer grown by low-pressure chemical vapor deposition (LPCVD ZnO). The bilayer IOH/LPCVD ZnO stack guarantees efficient light in-coupling and light trapping for the top amorphous silicon (a-Si:H) solar cell while minimizing the parasitic absorption losses. The crater-shaped micro-textured glass provides both efficient light trapping in the red and infrared wavelength range and a suitable morphology for the growth of high-quality nanocrystalline silicon (nc-Si:H) layers. Thanks to the efficient light trapping for the individual subcells and suitable morphology for the growth of high-quality silicon layers, multijunction solar cells deposited on MST substrates have a higher efficiency than those on single-textured state-of-the-art LPCVD ZnO substrates. Efficiencies of 14.8% (initial) and 12.5% (stable) have been achieved for a-Si:H/nc-Si:H tandem solar cells with the MST front electrode, surpassing efficiencies obtained on state-of-the-art LPCVD ZnO, thereby highlighting the high potential of MST front electrodes for high-efficiency multijunction solar cells. Copyright (c) 2015 John Wiley & Sons, Ltd.

A profound comprehension of the nanostructure of hydrogenated amorphous silicon (a-Si:H) and the defect states in this material is currently still lacking despite several decades of research on this topic. Investigating the nature of defects in a-Si:H devices is especially important in view of the poorly understood Staebler–Wronski effect (SWE). Therefore we present the latest insights into the gap states in the a-Si:H bandgap by manipulating these sub gap states in three different ways: via the application of a voltage bias, via in situ light soaking, and via in situ annealing. To monitor the changes in the sub gap absorption we employ an advanced application of Fourier Transform Photocurrent Spectroscopy (FTPS). A recently published optical model that removes interference fringes from the photocurrent spectrum is presented to obtain the absorption coefficient. For FTPS measurements on solar cells the photocurrent spectrum is scaled to the external quantum efficiency (EQE) of the solar cell. Further, a mathematical data fitting routine is employed to accurately quantify the sub gap absorption and the changes therein during light soaking and annealing. The high sensitivity of FTPS is particularly helpful in identifying the sub gap states that play a role in the SWE in a-Si:H devices.

Thin oxide interlayers are commonly added to the back reflector of thin-film silicon solar cells to increase their current. To gain more insight in the enhancement mechanism, we tested different back reflector designs consisting of aluminium-doped zinc oxide (ZnO:Al) and/or hydrogenated silicon oxide (SiOx:H) interlayers with different metals (silver, aluminium, and chromium) in standard p-i-n a-Si:H solar cells. We use a unique inverse modeling approach to show that in most back reflectors the internal metal reflectance is lower than expected theoretically. However, the metal reflectance is increased by the addition of an oxide interlayer. Our experiments demonstrate that SiOx:H forms an interesting alternative interlayer because unlike the more commonly used ZnO:Al it can be deposited by plasma-enhanced chemical vapour deposition and it does not reduce the fill factor. The largest efficiency enhancement is obtained with a double interlayer of SiOx:H and ZnO:Al.