Electron microscopy studies of Silicon Radial junction for stable and highly efficient thin film solar cells

Abstract

Owing to their enhanced light trapping and anti‐reflection effect, silicon nanowires (SiNWs) provide an effective platform for developing a new generation of low‐cost and efficient solar cells. By decoupling the light absorption and carrier collection directions, SiNWs enable the use of ultra‐thin intrinsic layers for high efficiency PIN radial junction solar cells fabricated on the NW matrix. Apart from reducing the material consumption, using thinner absorber layers provides higher built‐in field and hence a better separation of carriers. Consequently, the cells operate as random arrays of microscopic PIN diodes connected in parallel with optical and electrical properties of microstructural elements strongly depending on their dimensions, occurrence of defects or structural imperfections. [1,2] From a fundamental point of view, it is expected that overall performances of the cells could be limited by weak diode elements related to local variations of microstructure. Along this line, the goal of this work is to perform a comprehensive analysis using both TEM and the STEM‐HAADF imaging modes, together with the STEM‐HAADF electron tomography. This type of analysis provides reliable information regarding the morphology and the crystallographic structure of the radial junction (RJ) which allows us to propose hypotheses, firstly on the nucleation and the growth of SiNWs processes and second how conformaly the hydrogenated amorphous (a‐Si:H) layer and ITO(Indium Tin Oxide) layers cover the SiNWs. In order to access the characteristics of each component prior to the TEM observations, an FIB (Focused Ion Beam) preparation technique was used to obtain thin lamellas on each studied radial junction sample. Figure 1(a) shows an SEM image of the complete PIN radial junction solar cell after the deposition of intrinsic and n‐type a‐Si:H and Fig. 1(c) after the ITO sputtering. Both images were taken at positions marked on the sample photo in Fig. 1(b). The TEM observations performed in bright field mode on transversal FIB cross‐sections (see Fig. 2) for both samples allowed the identification of the SiNW‐ core, the a‐Si:H layers (intrinsic and n‐type) as well as the ITO. The typical total thickness of the the a‐Si:H was estimated to be around 120 nm. Concerning the sputtered ITO layer, the observation allowed seeing that the deposition is done in an inhomogeneous manner with thickness varying from 50 to 90 nm around the RJ. Regarding the morphology of the radial junction, several tomographic studies were performed in STEM‐HAADF imaging mode on a radial junction solar cell without an ITO layer. A detailed analysis of the reconstructions suggests that the diameter of the Si core NWs is varying from 22 nm at the bottom to 10‐12 nm at the top. Concerning the a‐Si:H layers, we have observed that they are not perfectly homogeneous, but have their thickness varying from 140 nm at the top of the radial junction to 65 nm at its bottom.