Dependence of a-Si:H Degradation on i-Layer Thickness and Photon Energy
- PDF / 251,457 Bytes
- 5 Pages / 414.72 x 648 pts Page_size
- 88 Downloads / 142 Views
DEPENDENCE OF a-Si:H DEGRADATION ON i-LAYER THICKNESS AND PHOTON ENERGY H. PAES, C. ACHETE and W. LOSCH LEMI/COPPE/Universidade Federal do Rio de Janeiro, P. 0. Box: 68.505 Rio de Janeiro -RJ - Brazil - 21.945-970 - fax: (005521)290.6626
ABSTRACT Metastable light-induced changes in hydrogenated amorphous silicon (a-Si:H) have been monitored by stationary single photocarrier charge collection in p-i-n diodes under reverse bias. The influence of the following parameters has been systematically investigated: i-layer thickness, applied voltage and illumination -time and energy. A strong dependence of cell degradation on i-layer thickness is observed. In the case of degraded diodes superlinear current vs. voltage (Iph a Vm) in the low voltage regime were observed. These results can be related to space-charge-limited photocurrents. The results from variation of photon energy and single and/or double carrier injection indicate that recombination rather than single-carrier trapping plays a key role in metastable defect creation. INTRODUCTION Despite numerous studies, light-induced defect formation in a-Si:H, the StaeblerWronski (SW) effect [1], is still one of the most serious problems for device application of this material. Charge collection experiments penformed on solar cell structures are a powerful tool in this field of research since they permit a direct correlation between light-induced defects and changes in electron and/or hole transport [2]. Among several
models proposed to explain the SW effect two are of major importance: the weak-bond to dangling-bond conversion model [3] and Adler's model [4], which is based on the coexistence of charged defects (D- and D+) with neutral (DO) states. D- and D+ states are predicted even for undoped a-Si:H in some models for gap-state distribution [5]. It has been also suggested that the properties of the i-layer in p-i-n solar cells are spatiallyvariant [6]. D- states are the dominant species of i-layer space charge near the n-i interface. This space charge increases the electric field near the interface, improving cell performance [7]. It should be mentioned that after degradation the DO band is larger than the charged bands, contrary to the annealed state [8]. In this paper, we report changes in primary electron and hole photocurrents due to light-induced defects and demonstrate that recombination plays a key role in metastable defect creation. SAMPLE PREPARATION AND EXPERIMENTAL PROCEDURE A-Si:H solar cells were deposited at 250 oC on glass/specular SnO2 substrates in a conventional RF glow-discharge system. The p and n doped layers were 100 and 400A thick, respectively. The i-layer was deposited using a flow-rate of 25 sccm silane, and its thickness ranges from 0.4 to 3.0 jim. A NiCr semitransparent film was used as back contact in order to permit n-side illumination. The p-i-n cells were characterized by primary electron (Jeo) and hole (Jho) photocurrent densities versus applied voltage curves Mat. Res. Soc. Symp. Proc. Vol. 297. '1993 Materials Research Society
596
Data Loading...
