Abstract:
Recently, solar cells have gained significant interest due to renewed attention to sustainability. The
energy harvesting effectiveness of a solar cell centres are critically influenced by surface and interface
properties inherent in the manufacturing of these devices. Conversion efficiencies over 40% have been
achieved using conventional III–V semiconductor compounds as photovoltaic materials. The fundamental
bandgap of the group III-nitride alloy system covers over a wider spectral region (from 0.64 to 6.2 eV) with
tunable bandgap and strong absorption coefficient. However, bottleneck limiting the performance of such
device arises from the potential barrier at the hetero interface due to the electron affinity difference.
Another important factor is the existence of significant interface charges induced by spontaneous and
piezoelectric polarizations due to non-centrosymmetric crystal structure. In turn, results in surface band
bending depending on the interaction with surrounding atmosphere. Heterojunction (HJ) between n-ZnO
and p-Si has a potential to perform as efficient and inexpensive solar cell. However, the relation between
the polarization bound charges and the electronic properties of the HJ interfaces is not yet well understood.
Calculated work function, ϕZnO (or barrier height) for ZnO varied from 5.02 to 0.33 eV as a function of
Zn/O molar ratios. Surprisingly, a non-centrosymmetric crystal structure can develop a giant photovoltage.
Specifically, the electron processes: photo-excitation, scattering, and relaxation occur with different
probabilities. Considering a hetero-structure with cubic and non-centrosymmetric material, each noncentrosymmetric
layer can act as a photovoltaic, so that the overall open-circuit voltage across the multijunction
is large, potentially much larger than the bandgap. Fundamental issues considering wurtzite-
GaN/cubic GaN and wurtzite-ZnO/cubic-CdO hetero-structures will be discussed towards next-generation
solar cells.
