Recombination and Resistive Losses in Amorphous Silicon / Crystalline Silicon Heterojunction Solar Cells
- PDF / 225,950 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 39 Downloads / 201 Views
Recombination and Resistive Losses in Amorphous Silicon / Crystalline Silicon Heterojunction Solar Cells NILS JENSEN, UWE RAU, AND JÜRGEN H. WERNER Institut für Physikalische Elektronik, Universität Stuttgart, Pfaffenwaldring 47, D-70569 Stuttgart, Germany, [email protected] ABSTRACT This contribution investigates the electronic properties of a-Si:H/c-Si solar cells and explains their electrical output parameters open circuit voltage, short circuit current, and fill factor. Our device analysis is based on measurements of the internal quantum efficiency, of current/voltage and capacitance/voltage curves. We find carrier recombination within the crystalline silicon absorber material to be responsible for the limitation of the open circuit voltage. The short circuit current is restricted by collection losses in the absorber material and by absorption in the electrically inactive a-Si:H emitter. Resistive losses affecting the fill factor originate from the transport of minority carriers across the interface. The I/V curves measured at low temperatures reveal a characteristic S-shaped behavior. This effect increases with decrasing temperature and stems from the minority carrier transport, which is hindered by the band offset between a-Si:H and c-Si. We propose a new analytical model to describe this anomalous behavior. INTRODUCTION Production of crystalline silicon solar cells usually requires several high temperature processing steps. The low temperature formation of heterojunctions between a-Si:H and c-Si promises a cost-saving alternative. In our group cell efficiencies of 15% are achieved using ntype a-Si:H and p-type c-Si. These results stem from cells that have no high efficiency features such as back surface fields or surface texturing. This contribution presents investigations of the electronic loss mechanisms in a-Si:H/c-Si heterojunction solar cells. Detailed analysis of current/voltage (I/V) curves, quantum efficiency spectra, and capacitance/voltage (C/V) curves demonstrates that diffusion and subsequent recombination of minority carriers in the c-Si bulk is the dominant recombination mechanism, limiting the open circuit voltage of the devices. The second part introduces a new closed-form model for the anomalous S-shape behavior of low temperature I/V curves. SAMPLE PREPARATION We prepare a-Si:H/c-Si heterojunction solar cells in a plasma enhanced chemical vapor deposition chamber by deposition of a thin layer of doped a-Si:H with or without an instrinsic aSi:H layer on a crystalline silicon absorber. Ultrasonic treatment in acetone and isopropanole as well as removal of the native oxide by a HF-dip serves as a cleaning procedure of the wafers prior to emitter formation. On top of the a-Si:H emitter we deposit an indium tin oxide (ITO) layer with a thickness of 100 nm by RF magnetron sputtering. Both antireflection coating and good lateral conductivity are thus achieved. Contact to the ITO is provided by a silver front grid.
A13.1.1
40
20 10 0 0
n =1.05 2 Rs=0.48Ω cm
(b)
-1
10 Current I
Data Loading...
