Impact of a-Si:H structural properties on the annealing behavior of a-Si:H/c-Si heterostructures used as precursors for
- PDF / 271,088 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 1 Downloads / 160 Views
1268-EE01-07
Impact of a-Si:H structural properties on the annealing behavior of a-Si:H/c-Si heterostructures used as precursors for high-efficiency solar cells T. F. Schulze1, H. N. Beushausen1, C. Leendertz1, A. Dobrich2, T. Hannappel2, L. Korte1, B. Rech1 1 Institute for Silicon Photovoltaics, Helmholtz-Zentrum Berlin, Kekuléstr. 5, D-12489 Berlin, Germany 2 Materials for Photovoltaics, Helmholtz-Zentrum Berlin, Hahn-Meitner-Platz 1, D-14109 Berlin, Germany ABSTRACT We analyze the dependence of the interface defect density Dit in amorphous/crystalline (a-Si:H/c-Si) heterostructures on the microscopic properties of the ultrathin (10nm) undoped aSi:H films. It is shown that the hydrogen bonding configuration, probed by infrared spectroscopy, determines the a-Si:H network disorder, which in turn governs the annealing behavior of these structures upon a short thermal treatment at moderate temperatures (T ≤ 200°C). While the as-deposited Dit seems to be determined by the local structure at the interface, the final state of the annealed samples is determined by the bulk a-Si:H network strain as reflected in the valence band tail slope. Thus it appears valid to treat the equilibrated a-Si:H/c-Si interface as a 2D-layer that has the projected defect properties of the 3D a-Si:H bulk. INTRODUCTION Solar cells based on amorphous/crystalline silicon (a-Si:H/c-Si) heterojunctions have gained much attention in recent times due to their high conversion efficiency, lately reaching 23.0% [1]. In order to increase the open circuit voltage Voc of these solar cells, the prime objective is to ‘passivate’ the a-Si:H/c-Si interface, i.e. to suppress interface recombination of photogenerated charge carriers by saturating recombination-active dangling bonds at the heterointerface. Thin (~3-10nm) undoped amorphous ((i)a-Si:H) interlayers are commonly used to reduce the density of interface defects Dit. Although the potential for improving the c-Si surface passivation by low-temperature post-deposition annealing has been widely explored, the details of the annealing mechanism and its dynamics remain under dispute [2, 3]. In the present study we explore the microscopic preconditions for a low Dit in the asdeposited and the annealed state of a-Si:H/c-Si structures with 10nm (i)a-Si:H thickness using Fourier transform infrared spectroscopy (FTIRS) and near-UV photoelectron spectroscopy (NUVPES). It is shown that the potential for a reduction of Dit upon annealing is crucially determined by the microscopic configuration of hydrogen (H) in the thin amorphous layers. Depending on the a-Si:H deposition parameters (substrate temperature, chamber pressure, H dilution), distinct H bonding configurations are observed, resulting in different silicon-network microstructures which in turn govern the as-deposited properties and annealing behavior of the heterointerface. By comparison with pertinent data on thick a-Si:H films we surmise that the asdeposited state is dominated by the local network structure at the interface (possibly inhomogeneous and dep
Data Loading...
