Abstract
In solar cell technology, the current trend is to thin down the active absorber layer. The main advantage of a thinner absorber layer primarily is the reduced consumption of material and energy during production, but also the increased production rates and lower cost. While this is of interest to all photovoltaic technologies, for thin-film silicon technology thinning down the absorber layer is of crucial importance since both the device throughput of vacuum deposition systems and the stability of the devices need to be significantly enhanced. These features lead to lower cost per installed watt peak for solar cells, provided that the (stabilized) efficiency is the same as for thicker devices. However, merely thinning down inevitably leads to reduced light absorption. Therefore advanced light-trapping schemes are crucial to increase light absorption in the ultrathin devices. The use of nanorods/nanowires is an innovative method for advanced light trapping. The enhanced light absorption performance originates from the multiple scattering between individual nanostructures and an improved anti-reflection effect thanks to the three-dimensional geometric configuration. These advantages potentially allow for high efficiency at significantly reduced material quantity, and even at reduced material quality of the semiconductor.
In this work we propose a simple, low-cost, and scalable approach for ultrathin nanostructured three-dimensional (nano-3D) solar cells. Self-assembled ZnO nanorods are synthesized by chemical bath deposition at 80°C and are used as the scaffolds for the nano-3D cells. With an only 100 nm thick a‑Si:H light absorbing layer, an efficiency of 7.1% has been achieved for the nano-3D cells. Increasing the absorber layer thickness to 200 nm the efficiency goes up to 8.4%, significantly higher than that of 6.4% for the flat counterparts. The light-trapping mechanisms in the nano-3D cells are systematically investigated by both experiments and three-dimensional finite-difference time-domain simulations (FDTD). The nanorod cells show a substantially enhanced optical performance with respect to conventional thin film solar cells with flat or textured structures. Corrugation at the top of the nanorod-based devices leads to an enhanced response in the blue part of the spectrum and light trapping is observed in the red part. The nano-3D cells have very little total reflection. Although parasitic absorption is enhanced in the nanostructured Ag layer at the back, the nano-3D concept provides net gain over cell concepts that utilize conventional light-scattering textured interfaces. Our simulations provide approaches to reduce absorption in the Ag while maintaining the nano-3D concept, thus further enhancing the utilization of incident light. The inexpensive design proposed in this work opens up a new platform for novel efficient cell design that can be made at low cost. With proper back contact material the nanorod substrate is applicable for a variety of thin-film PV technologies based on e.g. multi-junction thin-film Si and CuInxGa1−xSe2 (CIGS).
Original language | English |
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Qualification | Doctor of Philosophy |
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Award date | 4 Jul 2014 |
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Print ISBNs | 978-94-6259-203-2 |
Publication status | Published - 4 Jul 2014 |