Optimization of Recombination Layer in the Tunnel Junction of Amorphous Silicon Thin-Film Tandem Solar Cells

1 Department of Materials Science and Engineering, National Chung Hsing University, Taichung 402, Taiwan 2 Department of Materials Science and Engineering, MingDao University, ChungHua 52345, Taiwan 3 Graduate Institute of Precision Engineering, National Chung Hsing University, Taichung 402, Taiwan 4 Department of Mechanical Engineering, Yuan Ze University, 135 Yuan-Tung Road, Chungli, 320 Taoyuan, Taiwan 5 Graduate Institute of Materials Science and Green Energy Engineering, National Formosa University, Huwei, Yunlin 632, Taiwan


Introduction
Amorphous silicon (a-Si)/a-Si tandem solar cells have attracted extensive interest among solar cell because of the less light-induced degradation [1,2] (Stabler-Wronski effect) compared to their single-junction solar cell counterparts.The n-p junction between the two subcells is often referred to as a tunnel junction but actually functions as a recombination junction in electrically connecting the two p-i-n junctions of the tandem structure.For high stabilized efficiency tandem cell applications, a good n/p junction must have very high recombination rates, negligible optical absorption, and an ohmic characteristic with a low series resistance in order to improve the carrier transport [3][4][5][6].Various recombination layers, such as a-SiC:H [7], metal oxides [8], microcrystalline n + layer [9], and n + /p + recombination layer [10] have been introduced between the n and p layers to promote carrier recombination.
The thickness of intrinsic-(i-) layer of individual subcell is another key parameter because of the current matching limitation imposed by series connection.In addition, reducing the i-layer thickness of top cell as possible as it is important to stabilize against light degradation [11,12].
In this paper, we use a p + recombination layer as the recombination layer inserted in a tandem solar cell to investigate the effect on the cell performance.Furthermore, the tandem cells with different i-layer thickness matching ratio are also fabricated and their photovoltaic characteristics are also discussed.

Experimental
In this study, we prepared double-junction (a-Si/a-Si) solar cells by high-frequency (27.1 MHz) plasma-enhanced chemical vapor deposition (HF-PECVD).The HF-PECVD reac tion chamber is equipped with a load-lock for the transport and placement of the substrate into the chamber including a substrate holder, and a system with temperature control.The size of chamber was 20 × 20 cm 2 , and the electrode distance ranged from 7 mm to 40 mm.We fabricated tandem cells without recombination layer (p  with p + recombination layer and with n + /p + double recombination layers inserted between two subcells, which were designated as sample A, sample B, and sample C, respectively.The schematic structure is shown in Figure 1.The substrate used in this paper was SnO 2 -coated glass (Asahi U-type).The substrate temperature was fixed at 200    measured for a wavelength range of 300 to 900 nm.Crosssection micrographs of the solar cell were obtained by transmission electron microscopy (TEM).The light soaking test was performed at the open circuit condition under one-sun light intensity using a metal halide lamp at 50 • C for 500 h.

Results and Discussion
Figure 2 shows the I-V characteristics of sample A, sample B, and sample C. It can be seen that the open-circuit voltage (V oc ) increases in the case of adding either p + recombination layer or n + /p + double recombination layers.For the shortcircuit current density (J sc ), the sample A, which is without recombination layer, does have a rectifying property, resulting in a reduction in J sc [10].The sample B shows the highest J sc due to it not only has the smallest series resistance (R s ) offering a good ohmicity of the tunneling junction, but also the largest shunt resistance (R sh ).For the sample C, the n + /p + double layer causes a buildup of trapped holes on the p + side and electrons on the n + side, and hence yields a lower J sc compared to the sample B. From our result, the best cell conversion efficiency of 8.52% occurs when the p + recombination layer is used.The variation in the previous J sc result is also reflected in the QE measurements, shown in Figure 3.The QE curves of these samples remain the same in the short-wavelength region but evidently vary in the long wavelength region, implying that the adding recombination layer could mainly improve the absorption at long wavelength.
Figure 4 shows the experimental I-V characteristics of the double-junction solar cells subject to different i 2 /i 1 thickness ratio.It can be seen that the V oc almost maintains a stable value of 1.59 V, approximately equaling to the sum of the V oc of the two subcells, showing a good series connection.The J sc increases from 6.06 to 7.96 mA/cm 2 with increasing the i 2 /i 1 thickness ratio from 2 to 6, and then it drops to a lower value with further increasing the ratio.This result can be explained by the QE result, as shown in Figure 5.The tandem cell with an i 2 /i 1 thickness ratio of 6 presents the highest QE response, showing the optimum current matching.By using the optimum i 2 /i 1 thickness ratio, a conversion efficiency of 9% is obtained.
Figure 6(a) shows TEM cross-sectional images of the tandem cell with the p + recombination layer.The AZO back reflectors layer of 80 nm and Ag layer of 300 nm on the a-Si: H solar cell can be observed demonstrates the whole crosssection morphology.The photograph depicting the interface between p-layers and SnO 2 (TCO) of Asahi (U-type) substrate deposited the high-resolution TEM (HR-TEM) is shown in Figure 6(b), and Figure 6(c) demonstrates the TRJ (n/p + /p) and recombination layer (p + -layer) between top and bottom cells.In general, a better interface treatment and uniform films are important factors for double-junction solar cells.It seems that each layer and interface of cells as shown Figure 6 is deposited densely and uniformly.Therefore carriers are able to travel longer path thus good for the conversion efficiency.
Figure 7 demonstrates the maximum output power of the a-Si/a-Si tandem solar cells with the p + recombination layer and the thickness ratio of 6 as a function of the exposure time.The relative power is also shown for the convenience of observing the degradation.Only very little decrease in power (less than 5% degradation) is observed, showing that the a-Si/a-Si tandem solar cell is more stable than a traditional single junction a-Si cell.This result can be attributed to a thinner i-layer (50 nm) of the top sub-cell compared to that 300 nm thickness i-layer of the single-junction solar cell, might leading to a reduction in escape of hydrogen atoms from Si matrix.

Conclusions
In this paper, the effects of the recombination layer and the i-layer thickness matching on the cell performance have been investigated.The results show that inserting the recombination layer can increase the V oc and J sc .Furthermore, the ilayer thickness ratio strongly affects the J sc , and the i-layer of bottom sub-cell thicker than that of top sub-cell by a factor

Figure 2 :
Figure 2: I-V characteristics of sample A, B, and C. Sample A is with the standard n/p junction; sample B has the p + recombination layer; sample C has the n + /p + double recombination layer.

Figure 4 :
Figure 4: I-V characteristics of a-Si/a-Si tandem solar cells with different i 2 /i 1 ratio.

Figure 5 :
Figure 5: Measured QE of solar cells with different i 2 /i 1 ratio.

of 6
is found to be the optimum thickness matching.Finally, an initial efficiency of 9.0% and the stabilized efficiency of 8.7% are obtained, showing the good stability compared to a typical single-junction a-Si:H solar cell.

Table 1 :
The deposition conditions of intrinsic layer of the tandem solar cell.