CHAPTER 4 RESULTS AND DISCUSSIONS
4.3 T ANDEM S OLAR C ELL WITH AND WITHOUT X L AYER
The Tandem solar cell is constituted by the optical and electrical series connection of an amorphous silicon (a-Si:H) top cell and a microcrystalline silicon (µc-Si:H) bottom cell. As a consequence of the electrical series connection, the short-circuit current density Jsc of the whole tandem is limited by the absorber (top or bottom cell) with the lower current generation capabilities. The thickness of the a-Si:H intrinsic layer must be made as thin as possible to minimize the Staebler-Wronski effect. It is generally thin (about 250 nm) in order to collect a maximum of the photo-induced electrons. The µc-Si:H intrinsic layer has to be thicker (about 2 µm) because of its indirect band-gap and the necessity to match the photo-current generated by the two stacked cells.
Fig. 4-9 Tandem solar cell without x layer.
Tandem solar cell with the following structure as shown in Fig. 4-9 was used in the simulations: flat glass / ITO / p-a-SiC:H / i-a-Si:H(di,top) / n-a-Si:H / p-µc-Si:H /
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i-µc-Si:H(di,bot) / n-a-Si:H / Ag. The Ag forms the BR. The absorber layers were relatively thin (di,top 200 nm and di,bot 2.2µm), and no interlayer was applied. The calibrated optical and electrical parameters of undoped, doped a-Si:H and µc-Si:H layers were used in the simulations.
4.3.1 Simulation Model
Since the tandem cell was fabricated under the same conditions as the single-junction cells we used this set of parameters for modeling of the tandem cells. Also, The physical models we used here are identical with a-Si:H and µc-Si:H solar cell as mentioned before except thickness.
Fig. 4-10 Tandem solar cell with x layer.
The physics controlling the electric transport in n-a-Si:H and p-µc-Si:H interface is generally called tunneling recombination junction (TRJ) as shown in Fig. 4-9 which is
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explored with atlas. When modeling the tandem cell as a complete stacked structure (pinpin device), and not using a x layer for the TRJ between the two-component cells, we could not obtain an realistic J-V curve for illuminated tandem cell unless adding a strong-recombination layer (which we call an x-layer) sandwiched between the n- and p-layers of the two inter-cell contact regions as shown in Fig. 4-10. We also have found that the parameters of the TRJ, such as doping concentration and the defect density in the doped layers of the TRJ, must be optimized in order to get the realistic illuminated characteristics of the tandem cell. The parameters for models, such as the mobility gap and the defect density of the x-layer, were also sensitive for obtaining the realistic tandem cell characteristics. After determining the parameters of the TRJ models, we obtained an excellent J-V curve of tandem cell with added-x-layer models. A typical J-V curves for both cases (with and without x layer) are shown in Fig. 4-11.
Fig. 4-11 The J-V curves of tandem solar cells with and without x layer.
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The parameters which were used to describe TRJ are reported in Table 4-4 and Fig. 4-12 shows the energy band diagram of the tandem cell in thermal equilibrium for insertion of special layers (x-layer) in the tunnel/recombination junction on tandem cells. Fig. 4-13 shows (a) the distribution of hole current density, (b) electron current density and (c) the recombination rate in the cell. In the cell carrier's current density increases as the carriers move toward the "tunnel junctions". But at "tunnel junction" the hole current is seen to drop very low in the n region, while correspondingly the electron current is seen to drop very low in the p region. This indicates a very strong recombination process is happening in the "tunnel junction" which manifests itself in the Fig. 3c, the plot of recombination rate in the contacts.
This good recombination in contacts is needed for continuity of currents. Careful examination of Fig. 4-13(b) shows that there actually is a small component of electron current in the second sub-cell moving toward the first "tunnel junction", which is due to the unbalance of the net photo-carrier generation. Hence, the modeling shows the delicate balancing going on at these contacts [28]. The crucial role of recombination in the "tunnel junction" contacts of multi-junction solar cells is very different than the physics in the true tunnel junctions of tunneling diodes, in which electrons tunnel through the band gap from valence band to conduction band. Here electrons must fall from the conduction band to the valence band through a recombination process and fill in holes. Because the electrical field in the
"tunneling junction" is so strong due to our wanting to dope heavily to increase the field across the absorbers and because of this field's orientation (see Fig. 4-12), it acts against the holes and the electrons moving into the x-layers in contact region. Hence, our modeling shows that supplying carriers to this recombination can be a problem. It is in this supply role that tunneling is needed through the n-layer for electrons to supply the x-layer and through p-layer for holes to supply the x- layer. The key process in the functioning of the contact region is recombination. Any material layer that enhances this recombination will reduce the dipole modification and will enhance cell performance if it does not strongly absorb light.
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Hence, the TRJ region cannot be represented with a resistor or diode model, and such a layer could have the state distribution properties of a metal, a narrow gap semiconductor, or a very heavily defective semiconductor [28].
Table 4-4 TRJ (a-Si:H n-layer, x layer, µc-Si:H p-layer) parameters.
Parameter n-a-Si:H x p-µc-Si:H
Layer thickness (nm) 10~30 1~5 10~30
Mobility gap (eV) 1.8 1.1 1.3
Donor doping density (cm-3) 9x1018
Acceptor doping density (cm-3) 1x1019
Electron mobility(cm2/Vs) 20 0.1 100
Hole mobility(cm2/Vs) 4 0.1 25
Electron life time (μs) 0.01 0.01 0.01
Hole life time (μs) 0.1 0.1 0.1
Effective DOS in the valance and
conduction bands (cm-3) 2x1020 2x1020 2x1020
Exponential tail Prefactors NTD, NTA
(cm-3eV-1) 4x1021 4x1021 4x1021
Characteristic energy WTD (VB tail) (eV) 0.05 0.05 0.02
Characteristic energy WTA (CB tail) (eV) 0.03 0.03 0.01
Gaussian distribution density NGD, NGA
(cm-3eV-1) 9x1018 1x1019 5x1018
Characteristic energy for Gaussian
distribution WTD (donor like state) (eV) 0.2 0.2 0.2
Characteristic energy for Gaussian
distribution WTA (acceptor like state) (eV) 0.2 0.2 0.2
Peak of donor like Gaussian distribution
EGD (meas. From Valance edge) (eV) 0.78 0.59 0.4
Peak of acceptor like Gaussian distribution
EGA(meas. From Conduc. edge) (eV) 0.52 0.16 0.6
Correlation energy U (eV) 0.5 0.35 0.3
a measured from the conduction band edge ; b measured from the valance band edge, x is tunnel recombination layer.
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Fig. 4-12 The added-x layer TRJ (tunneling recombination junction) band diagram.
The strategy is to simulate the tunneling effect around x region in band gap.
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Fig. 4-13 Charge transport in tandem cells under AM1.5G illumination conditions.
(a) Hole current density distribution;
(b) Electron current density distribution;
(c) Recombination rate distribution.
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