For a long time attention has been paid to the bottom cell of a micromorph cell, i. e., the nc-Si i-layer, because of its large thickness and the dependence of the cell efficiency on deposition rate. However, more attention should be paid to the top cell, considering that the top cell generates 2/3 of the total power of a micromorph cell. The thickness of the top cell is actually an issue that was previously glossed over. The standard top cell (a-Si) i-layer with a typical thickness of ~300 nm not only consumes considerable time in its deposition, due to the fact that the fabrication of this layer is carried out in the protocrystalline condition, but also suffers from degradation. A thinner top cell will increase the throughput of solar-cell production and improve the stabilized efficiency. The third beneficial effect of a small thickness of the top cell is that the Voc is increased.
Hence, the recent trend is towards also thinning down this top cell i-layer. This is achieved by using an intermediate reflector at the tunnel recombination junction (TRJ), a concept that was proposed already in 1996 by the EPFL group [107] by the use of ZnO as IR. The success of such a design to a large extent depends on the quality of the IR, which has the role to confine short-wavelength light within the top cell. The refractive index (n) and the thickness (d) of this high-bandgap conducting layer (the n x d product) are so chosen as to provide the maximum reflection of the small wavelengths light suitable for the top cell and to allow the long-wavelength light to completely pass to the bottom cell. Hence, a wavelength-selective reflector is being developed for the IR. The distribution of light is finely tuned at the TRJ. Pioneering works carried out on this topic by many groups, notably by Kaneka [108, 109], in addition to EPFL [110] and FZ Juelich [111] have shown that the thickness of the top cell can be reduced by almost 40%. All these reports on solar cells used n-type nc-SiOx IRs, however, one can also use p-type SiOx that may help to reduce defect formation in the bottom cell, as mentioned earlier (Section 9.4.4.1). Another type of material, n-type SiNx also serves as a good IR [112]. The highest recorded thin-fllm silicon solar-cell efficiency of 16.3% has been, in fact, achieved by using nc-SiOx as a dual function layer (as n-layer and IR) in the middle of a triple junction (a-Si/a-SiGe/nc-Si) cell. The advantage of such an IR layer is to give the possibility of reducing the thickness of the a-SiGe layer or increasing its bandgap; both leads to better FF and Voc. The total current generated by the three cells is 28.6mA/cm2, which is only slightly lower than the highest current generated in a single junction nc-Si cell, i. e., 29.1mA/cm2 [113].
The IR has to satisfy the following criteria:
1. The reflection of the light from the IR should be spectrally selective. The light in the spectral region for which the absorption of the top cell is most efficient must be reflected, whereas the transmittance of the long-wavelength light that reaches the backreflector should be near 100%. An ideal solution could be a 1D photonic layer, (distributed Bragg reflector). In fact, with the use of such an IR a reduction of the top layer thickness by 50% has been predicted by simulations [114]. There are suggestions to use 2D and 3D photonic layers as IR [115].
2. The mobility/conductivity characteristics of the IR should be anisotropic, i. e., a high conductivity (low ohmicity) in the direction of the current flow and a lower conductivity in the lateral direction. An ideal solution is the use of quantum dots embedded in a matrix.
3. An optimum combination of the front TCO texture and the IR should be obtained, such that the reflected light to the top cell does not suffer from optical loss due to escape at the front surface.
4. The resistance increase due to the IR has to be below 2^ cm2 to avoid voltage loss at the tunnel recombination junction.
