Abstract
Ph.D.
A one-dimensional numerical simulation of photovoltaic (PV) cells has been written,
and has been designated RAUPV2. An algorithm for determining the optical
generation rate profile, taking into account multiple internal reflections in a multilayer
cell has been developed. A method which enables realistic boundary values to
be calculated, using RAUPV2 itself, has been developed. This method allows all three
boundary values (', Fn and Fp) at each surface, to be determined, without the need
to specify any additional input parameters.
A comprehensive set of input parameters for aSi:H PV cells has been established,
in consultation with the literature. Dangling-bond theory has been described and
input parameters for dangling-bond defects have been presented.
The effect of surface states in the p-layer on the contact potential at the TCO/p
interface has been investigated. It was found that there is an intimate relationship
between the contact potential and the parameters pertaining to the surface states.
A simple method has been demonstrated, which has allowed RAUPV2 to reproduce
the J-V curve of an existing aSi:H PV cell. The method requires that only the
dangling-bond concentration in the i-layer and the contact potential at the Sn02/P
interface needs to be adjusted.
Once the J- V curve had been generated, the simulation results were used to characterise
the empirical cell, in both thermodynamic- and steady-state equilibrium. This
simulated cell was designated the realistic cell.
The effect of asymmetries in the input parameters, under carrier band mobility interchange,
on the performance of p-i-n cells has been investigated. The results indicate
that, while asymmetries in the gap state distributions do give rise to asymmetrical
behaviour in the J- V curve, the effect is slight, and it is the positional asymmetry of
the optical generation profile that is mostly responsible for the observed asymmetry
in the J- V curve under mobility interchange.
An investigation of the limiting carrier effect has led to the conclusion that, in
a p-i-n aSi:H cell under forward bias, the electron is the limiting carrier. This has
been explained by appealing to the form of the optical generation profile, since most
electron-hole pairs (EHPs) are generated near the front of the cell, and it is electrons
that must be collected at the back contact. Investigations of the n-i-p aSi:H cell,
under forward bias, have shown the hole to be the limiting carrier. It was found that
the introduction of positional symmetry into the optical generation rate profile greatly
reduced the limiting carrier effect, and it was concluded that the limiting carrier effect
arises due to the asymmetries in the material parameters of the cell, particularly the
_ positional asymmetry of the optical generation profile.
It was observed that the nature of the optical generation profile actually plays an
important role in determining the identity of the limiting carrier, in a p-i-n cell. The
same effect was not observed in the n-i-p cell. The effective carrier collection length has been defined, and it was seen that the
limiting carrier possesses the larger effective collection length.
The effect of boron and phosphorous profiling of the i-layer was studied. It was
found that boron profiling led to a decrease in cell performance, while phosphorous
profiling improved cell performance. It was found that there was a P concentration at
which cell performance peaked.
The dependence of the spectral response of the realistic cell on device length L,
was investigated, showing a general improvement in the spectral response as L was
decreased.
The spectral response has been interpreted in a novel way. It was assumed that
the form of the monochromatic optical generation profiles in the vicinity of the peak
in the spectral response represented optimal generation profiles. These profiles were
subjected to a linear transformation, such that their form was preserved but that their
integrated value was the same as that of the realistic optical generation profile, under
global AM1.5 illumination. Using these transformed optical generation profiles, J- V
curves were obtained. The maximum power output PM of these J- V curves was seen
to exhibit a maximum some 17% greater than that of the realistic cell with a realistic
optical generation profile.
The spectral response of the phosphorous profiled cell was obtained. In a manner
similar to that for the non-P profiled cell, the optimal generation profile was found.
The PM for this profile was found to be 7.86mWcm -2 , considerably larger than the
5.60mWcm-2 for the phosphorous profiled cell with a realistic optical generation profile.
The effect on the simulation output of variations in numerous dangling-bond defect
input parameters has been investigated. It was found that the energy position
and concentration of the doped layer defects need not be known to a high degree of
precision. On the other hand, it was found that the energy position of the i-layer defects,
the standard deviation of the defect distributions, and the defect carrier capture
cross-sections, do need to be known with certainty.