Radial-junction silicon nanowire solar cells in a low-cost thin film technology – SOLARIUM

First generation of solar cells, based on high quality c-Si wafers, has dominated the PV market over half-century. Indirect crystalline silicon band gap (around 1.12 eV) causes the reduced light absorption in near- infrared part of spectrum. For the sufficient absorption of all the photons above bandgap energy, 180 300 µm thick and expensive high quality c-Si absorbers are necessary. This has a strong impact on the costs of all first generation solar cells. Second generation solar cells adopt low-cost thin film materials to reduce cost-per-watt, although they sacrifice some of the performance and stability. The new generation of solar cells aims towards the nanostructuring of the silicon material in order to boost performance of solar cells with thin absorber layers. In this context, silicon nanowire (SiNW) radial junction architecture represents an innovative approach, where the light absorption and carrier extraction can be decoupled.

The Solarium project aims at fabricating of tandem radial junction (TRJ) SiNW based solar cells combining hydrogenated amorphous silicon (a-Si:H) and microcrystalline silicon (µc-Si:H) materials. Such solar cells represent a promising path for photovoltaic technology, especially because of the low cost and the industrial compatibility of proposed low temperature fabrication processes. Optical and electrical numerical modeling will be used to identify parameters which may limit the device performance cells. Besides developing and optimizing thin film TRJ SiNW solar cells, the Solarium project will also cover their integration into PV modules : cell interconnection, encapsulation and testing.

In the Solarium project, the first device architecture will be focused on n-i-p radial junction with a Si:H as an active material. Indeed numerical modeling results have shown that the n-i-p structure (n-core NW) gives better performances (as high as 12 %) [Dio12] than the p-i-n (p-core NW). In addition the n-i-p structure also appears to be more robust against light-soaking [Dio12]. Another approach to reduce the light-soaking degradation is to use the µc-Si:H intrinsic layer instead of a-Si:H. The realization of this n-i-p structure will rely on the strong expertise of LPICM group who has recently achieved 8 % efficiency on p-i-n SiNW-based a-Si:H layers. In order to improve solar cell stability and to use photons from the near-infrared part of the spectrum, radial junctions solar cells based on the µc-Si:H absorber layer are proposed in a second step. The development will be driven by coupling optical and electrical modeling which will define the optimized structure for µc-Si:H radial junction solar cells. From the point of view of the fabrication, there have been important advancements in controlling the conformal deposition of µc Si:H that will benefit the realization of these last structures. In the third stage of the project, the proof-of-concept tandem device, combining a Si:H and µc-Si:H radial junctions will be realized. According to the photo-current generation prediction of ~30 mA/cm2 for c-Si NWs [Fol11], and the typical characteristics of Si thin film solar cell, the goal of this stage is set to achieve a conversion efficiency of >10% for a single junction and to >15% in a tandem radial junction cell.

Part of the Solarium project will be oriented to the demonstration of pre-industrial prototypes of SiNW based solar cell modules. In particular, it will focus on the development of robust processes for structuring the module into a number of cells that will be interconnected in series. A viable encapsulation will be also developed in order to produce 10 x 10 cm² modules.

The established research consortium for Solarium project consists of four laboratories (LPICM, LGEP, PMC and GPM) and one industrial PV partner (SOLEMS) with relevant experience and appropriate expertise in several areas of research, notably in photovoltaics and nanotechnology.