Rahul Dewan - Jacobs University

2.2. Thin-film silicon solar cells
hν
TCO p i n
E C
Metal
Back Contact
Electrons
Energy
E F
E V
Holes
Distance
Figure 2.7: Schematic sketch of a p-i-n solar cell. The charge separation of the
electron-hole pairs are shown in the band diagram of the semiconductor device.
tended by inserting an intrinsic i-layer of silicon. Inside the i-layer, for each absorbed
photon with energy higher than the silicon bandgap (1.1 eV for µc-Si and 1.7 eV for
a-Si) an electron-hole pair is generated. By means of the electric field, these electrons
and holes are then driven to the n- and p- layers, respectively. The diffusion lengths
of the charge carriers in the intrinsic a-Si:H and µc-Si:H are typically 100 nm [33]
and 200 nm [34] only. Since their diffusion length of the charge carriers are much
smaller than the i-layer thickness, the extraction from the p-i-n diode results mainly
from the drift of the electric field. Contrary to diffusion based crystalline silicon solar
cells, the charge extraction in thin-film silicon solar cells is dictated by the electric field.
Therefore thin-film silicon solar cells are also called drift cells.
The transport of the photogenerated carriers in a drift based p-i-n solar cell can be
understood from the electron and hole continuity equations. There have been several
publications in the early 1980s where the authors have derived and modeled the transport
mechanism in a-Si based p-i-n solar cells [35–38]. Several authors have proposed
the role of mobility×lifetime product, µτ,in order to evaluate the material property
and performance of p-i-n solar cells [39, 40]. But as Shah et al. have noted in their
work [41]: if in an actual p-i-n solar cell, the effective mobility×lifetime product were
as high as those reported in various measurents [40, 42], it would have been feasible
to fabricate a-Si:H solar cells with thickness of the i-layer larger than 1 µm even for
solar cells in the degraded state. In practice, however, the i-layer thickness of solar
cells based on a-Si:H are kept lower than 400 nm. Because of the non-uniform electric
field in the i-layer of the p-i-n solar cell, the collection of charges in actual a-Si:H solar
cells is very poor [41]. This detrimental effect imposes an electrical limitation on the
performance of p-i-n solar cells which limits the thickness of the i-layer.
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