| AVS 58th Annual International Symposium and Exhibition | |
| Energy Frontiers Focus Topic | Tuesday Sessions |
| Session EN+TF-TuA |
| Session: | Thin Films for Solar Cells |
| Presenter: | Mariadriana Creatore, Eindhoven University of Technology, the Netherlands |
| Authors: | M. Creatore, Eindhoven University of Technology, the Netherlands K. Sharma, Eindhoven University of Technology, the Netherlands M.C.M. van de Sanden, Eindhoven University of Technology, the Netherlands |
| Correspondent: | Click to Email |
Polycrystalline silicon (poly-Si) is considered to be a promising candidate for thin film PV, coupling the high quality crystalline Si technology with large area and low- cost manufacturing. Our initial studies [1] on poly-Si layers have shown grains extending through the whole thickness (1 μm) upon solid phase crystallization (SPC) of high growth rate plasma deposited amorphous silicon (a-Si:H) films. Furthermore, larger grains are promoted by an increase in the a-Si:H microstructure parameter R* [2], which represents the order (low R*)/disorder (high R*) in the matrix according to the Si-H bond distribution in mono-/di-vacancies (–low stretching mode-LSM) and nano-sized voids (–high stretching mode-HSM), and it is quantified by the integrated IR absorption band ratio IHSM/ (ILSM+IHSM).
The SPC of a-Si:H follows the steps of incubation, nucleation and grain growth. With the purpose of providing insight on the crystallization process, this contribution addresses a detailed crystallization kinetic study of plasma deposited a-Si:H films by means of in-situ X-ray diffraction (XRD). a-Si:H films having R* in the range of 0.05-0.6, with an hydrogen content of 5-14 at. %, were annealed at 600 ℃.
The medium range order (MRO) of the a-Si:H layers, quantified by the XRD line-width, and representing the most ordered regions in the matrix (up to 15-25 Å from the mono-vacancies), is found to affect the incubation time (to), in agreement with [3]: low R* and high MRO promote a faster nucleation (to in the range of 50-100 min), since the most ordered regions act as nucleation centers; as the structural disorder increases, the MRO decreases and the incubation step is delayed up to 450 min. However, for R*> 0.3 and an hydrogen content above 9%, the incubation time unexpectedly decreases. Therefore, the R* and the MRO evolutions during the annealing step are studied. High R* layers, characterized by hydrogen mainly bonded to nano-sized voids, are more prone to hydrogen out- diffusion upon annealing, as inferred by the quantitative decrease of the HSM mode with respect to the LSM mode. The hydrogen evolution is then followed by the rearrangement of the a-Si:H into more ordered regions, as witnessed by the increase of the MRO upon annealing, promoting a decrease in incubation time. In conclusion, next to the established role of the MRO , the nano-sized voids play also a role in the crystallization kinetics, as they affect the overall microstructure and medium range order upon annealing.
[1] Illiberi et al., Material Letters 2009, 63, 1817.
[2] Sharma et al., Advanced Energy Materials 2011, DOI: 10.1002/aenm.201000074
[3] Mahan et al., Adv. Funct. Mater. 2009, 19, 2338.