Optical Design of Multi-Stacked Intermediate Layers with Wavelength-Selective Reflectance for a-Si:H/µc-Si:H Tandem Solar Cells

Bancha Janthong1*, Ihsanul Afdi Yunaz1, Hidetada Tokioka2, Shinsuke Miyajima1, and Makoto Konagai1,3

1Department of Physical Electronics, Tokyo Institute of Technology

2Advanced Technology R&D Center, Mitsubishi Electric Corporation 3Photovoltaics Research Center (PVREC), Tokyo Institute of Technology

*Corresponding E-mail: janthong.b.aa@m.titech.ac.jp

ABSTRACT The optical study of multi-stacked intermediate layers in an a-Si:H/µc-Si:H tandem solar cell using simulator called Optical was performed. We have theoretically investigated the effect of refractive index and thickness on light reflectance spectrum at the n-top layer/intermediate layer interface. It was found that the triple or four intermediate layers whose high reflectance at a short wavelength and low reflectance at a long wavelength are applicable to a-Si:H/µc-Si:H tandem solar cells. To verify the numerical results, single and triple layers were fabricated, and their transmittance spectra were measured. Also, the experimental results show that a triple intermediate layer is beneficial as a reflector layer for tandem solar cells. However, optimization of a thickness combination is urgently required especially in multi-stacked intermediate layers.

INTRODUCTION

A multi-junction thin film solar cell, e.g. hydrogenated amorphous silicon (a-Si:H) /hydrogenated microcrystalline silicon (µc-Si:H) tandem cell, is a promising candidate to obtain higher efficiency than a traditional single junction solar cell. One of main requirements for multi-junction solar cells is a proper current matching of the series connected cells, since the component cell with the lowest current limits the current of complete device. This problem can be overcome by employing intermediate layer(s) between the component cells. The intermediate layer (IL) is needed to improve light-trapping and to increase the current of the top cell with the thinner top cell. However, introducing an intermediate layer generally causes a drop in current of bottom cells due to high reflection at a long wavelength region (LWR). Therefore, careful design of the IL with high reflection at the short wavelength region (SWR) and low reflection at the LWR as shown in Fig. 1 is required to avoid this problem. In this study, an optical analysis of intermediate layers by device simulator was performed. We have systematically investigated the effects of refractive index and thickness of multi-stacked intermediated layers on the light-reflectance in the tandem solar cells. The reflectance spectrum representing the light-trapping at interface of the top cell and the IL was used to evaluate the performance of the ILs to find the optimum intermediate layer with reflectance spectrum close to the ideal one given in Fig. 1.

Experimental study was also conducted to confirm the simulation results.

Fig. 1 Desired reflectance spectrum of intermediate layer for use in a-Si:H/µc-Si:H tandem cell

SIMULATION MODEL

We have calculated the reflectance of intermediate layers by using the simulation program called Optical, a computer program that can treat multilayer with any number of coherent and incoherent layers in any position and for any incidence angle using a generalized scattering matrix method [1]. The schematic diagram of the calculated tandem cells is shown in Fig. 2. The optical properties of all layers were described by wavelength-dependent complex refractive indices (N(λ) = n(λ) – ik(λ), where n(λ) is wavelength-dependent real refractive index and k(λ) is wavelength-dependent extinction coefficient). The media of incident light and transmitted light were a-Si:H and µc-Si:H, respectively. The reflective indices of a-Si:H and µc-Si:H at 600 nm were both assumed to be 3.9. The single intermediate layer as well as the multi-stacked intermediate layers which include double, triple and more intermediate layers was investigated. Effects of the wavelength-dependent refractive index and the thickness of each layer on the reflectance spectrum at a-Si:H/IL interface were analyzed. The refractive indices of all intermediate layers used in this numerical analysis were obtained by measuring the wavelength-dependent refractive index of the fabricated silicon oxide (SiOx) intermediate layers. It should be noted that the refractive

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indices mentioned in this paper, e.g. table and figures, were those measured at the wavelength of 600 nm.

Fig.2 Schematic diagram of calculated a-Si:H/µc-Si:H tandem cell with intermediate layer

EXPERIMENTAL DETAILS To confirm the numerical results, samples with single IL and triple ILs were fabricated on Corning 7059 glass substrates. The structure of fabricated samples was glass substrate/n-top layer/IL (single or triple ILs)/p-bottom layer. All layers were prepared by using a multi-chamber conventional radio frequency plasma enhanced chemical vapor deposition (RF-PECVD) technique. Here, SiOx was employed as single IL, which thickness and refractive index were assumed to be ~60 nm and 1.9, respectively. The triple IL, whereas, was grown under the same SiOx deposition condition by using n-SiOx/n-µc-Si/n-SiOx structure with refractive indices assumed as 1.9/3.9/1.9 respectively and estimated total thickness of 160 nm. The thickness of individual layer of fabricated samples was deduced from fitting of variable-angle spectroscopic ellipsometry (SE) measurement results. The transmittance spectra measurement was also performed using UV-VIS-NIR Spectrophotometer.

RESULTS AND DISCUSSION

Simulation of Single Intermediate Layer Firstly, the reflectance spectra of single intermediate layer with variation of refractive indices and thicknesses were calculated. Fig. 3 shows the reflectance spectra of single IL with refractive index varied from 1.9 to 2.6. Here, the thickness of IL was assumed to be 50 nm. As can be clearly seen, the reflectance spectra at the whole wavelength region increased as the refractive index of intermediate layer decreased. Fig. 4 gives the reflectance spectra of single IL as a function of IL thickness. Here, the refractive index was assumed to be 1.9 nm. We can observe that the value of maximum reflectance for each IL thickness firstly increased with increasing the IL thickness to 40 nm. Then, in case of IL thickness of more than 50 nm, the value of maximum reflectance decreased. However, the reflectance spectra at the long wavelength region could not be suppressed. Such reflectance spectra

are not desired in a multi-junction solar cell due to a drop of the current in the bottom cell. Therefore, a further optimum design has to be performed.

Fig. 3 Reflectance spectra of single IL as a function of IL refractive index

Fig. 4 Reflectance spectra of single IL as a function of IL thickness Simulation of Triple Intermediate Layer To further improve single SiOx intermediate layer, the reflectance spectra of triple intermediate layer were analyzed by varying the refractive index and thickness combinations of first, second, and third layers. Fig. 5 illustrates effects of thickness combinations on reflectance of triple ILs. Here, the thicknesses of the first and third layers were both varied from 40 to 100 nm with the refractive index combination of the first/second/third layers kept constant as 1.9/3.9/1.9, respectively. It can be seen that the maximum value of reflectance slightly increased up to 70 nm, then, decreased. It also can be seen that individual reflectance peak moved to the long wavelength region as the thickness of first and third layer increased.

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Then, the dependence of refractive index combinations on reflectance of triple intermediate layer was observed. Here, the thickness combination of first/second/third layers was kept constant as 70/17/70 nm, respectively. As can be seen from Fig. 6, among these refractive index combinations, the combination of 1.9/3.9/1.9 (low/high/low) exhibited the best reflectance spectrum. By using this refractive index combination, the reflectance at the long wavelength region also could be suppressed since the refractive index at the interface of a-Si:H top cell/intermediate layer/µc-Si:H bottom cell became high/low/high/low/high (3.9/1.9/3.9/1.9/3.9), which was close to a photonic crystal (PC) structure [2]. Moreover, the reflectance at short wavelength region could be higher than of previous both of ILs.

Fig. 5 Reflectance spectra of triple IL as a function of IL thickness

Fig. 6 Reflectance spectra of triple IL as a function of IL refractive index combination Simulation of Two, Four and More Intermediate Layers For detailed optical study of multi-stacked intermediate layer, we have also investigated effects of index and

thickness combinations on reflectance spectra of different layers i.e. two, four, five, six and seven intermediate layers. The optimum refractive index and thickness combination in individual intermediate layers was summarized as shown in the table 1. Fig. 7 shows the best reflectance spectra for each intermediate layer. From Fig. 7, we can see that the value of maximum reflectance was about 85% obtained from five or six intermediate layer. Unfortunately, in either intermediate layer, the high reflectance at long wavelength region could still not be suppressed. The high reflectance at long wavelength region decreased with triple or four intermediate layers even if the value of maximum reflectance insignificantly decreased. Table 1 Summary of the optimum refractive index and thickness combination in individual intermediate layer

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Four Layers 1.9/3.9/1.9/3.9 60/20/60/20

Five Layers 1.9/3.9/1.9/3.9/1.9 60/30/60/30/60

Six Layers 1.9/3.9/1.9/3.9/1.9/3.9 60/30/60/30/60/30

Seven Layers 1.9/3.9/1.9/3.9/1.9/3.9/1.9 40/20/40/20/40/20/40

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Fig. 7 Reflectance spectra of various multi-stacked ILs Experimental Results Since it is difficult to measure the reflectance spectra at the interface of n-top layer and ILs, instead of reflectance spectra, the transmittance spectra of the fabricated samples (glass substrate/n-top layer/IL/p-bottom layer) were used to compare the optical property of fabricated single and triple intermediate layers. As previously

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mentioned, intermediate layer with high reflection at the SWR and low reflection at the LWR is needed. In other words, we need to obtain intermediate layer with low transmittance at the SWR and high transmittance at the LWR. Fig. 8 shows the transmittance spectra of the single intermediate layer with the structure of glass/n-top layer/ n-SiOx/p-bottom layer. For comparison, the estimated transmittance spectrum, which was calculated using Optical simulator based on the estimated thickness of the samples, is also shown. For simulation, here, the media of incident light and transmitted light were glass and air respectively. As we can see from the experimental result, the higher transmittance at the short wavelength of 500~700 nm and the lower transmittance at the long wavelength of more than 700 nm were observed. On the other hand, the transmittance of lower than 20% at the short wavelength region of 450~700 nm and the high transmittance increasing up to ~90% at the long wavelength region were shown from the estimated result.

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Fig. 8 Estimated and experimental transmittance spectra of the single IL The estimated and experimental transmittance spectra of the triple layer whose structure is glass/n-top layer/n-SiOx/n-µc-Si/n-SiOx/p-bottom layer/air configuration were also investigated as shown in the Fig. 9. As we can see from the result, although the transmittance in the wavelength region of over 850 nm decreased, the transmittance at the short wavelength was quite low and the maximum transmittance at a long wavelength increased up to 90%. It can also be seen that the maximum peak of transmittance obtained from experimental result shifted to the long wavelength region compared to calculated one. Moreover, from the Fig. 9, it was found that the transmittance of fabricated triple intermediate layer in wavelength region of 600~900 nm decreased. According to both figures shown above, the experimental results were partly different from the estimated result. One considerable reason was that the optimum thickness

combination could not be gained from experimental study. From SE measurement, it was found that the thickness of the fabricated sample with n-top layer/n-SiOx single IL/p-bottom layer was 68/71/33 nm, which is slightly close to the estimated value; 35/69/39 nm. However, for the samples with triple IL, the thicknesses of n-top layer/n-SiOx/n-µc-Si/n-SiOx/p-bottom layers was 80/77/36/92/40 nm, respectively, which is largely different from the estimated thickness (35/69/28/69/39).

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Fig. 9 Estimated and experimental transmittance spectra of the Triple IL For further study of thickness of samples, the comparison of transmittance spectra of single and triple intermediate layers with experimental result as well as calculation result is presented in Fig. 10. As might be known, it is difficult to determine an accurate thickness of a multi-stacked intermediate layer by SE. To confirm that the thicknesses of the single and triple layers obtained from SE were accurate, here, calculation of their transmittance spectra by Optical simulator based on the experimental thickness obtained from SE was also performed. The result shows that the thicknesses of the single and triple layers obtained from SE were reliable due to the same calculation and experimental result. It can certify that our assumption, i.e. experimental thickness combinations was not optimized, is truth. Thus, optimization of experiment condition to realize better thickness combination as estimated from simulation results is urgently needed. However, from the estimated result, it is also observed that the triple layer allowed the lower transmittance at a short wavelength and higher transmittance at a long wavelength in totally compared to the single layer in spite of decreasing transmittance at some part of a long wavelength.

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Fig. 10 Calculation and experimental transmittance spectra of fabricated samples with single and triple IL. Both calculation results are based on experimental thickness.

CONCLUSIONS We have studied the effect of refractive index and thickness combinations of multi-stacked intermediate layers on the light-reflectance in the tandem solar cells and the further experimental investigation was also performed. From the simulation results, it is clear that the wavelength region of reflection could be freely modified by adjusting the thickness and or the refractive index of multi-stacked intermediate layers. Compared to the single, the reflectance of SiOx multi-stacked intermediate layers at short wavelength region could be increased, whereas at the long wavelength region could be suppressed. Thus, a multi-stack intermediate layer is beneficial for use in a-Si:H /µc-Si:H tandem cell.

REFERENCES [1] E. Centurioni,”Generalized Matrix Method for Calculation of Internal light Energy Flux in Mixed Coherent and Incoherent Multilayers” , Appl. Optics 44 (2005) pp.7532-7539. [2] H.-Y. Lee and T. Yao, “Design and Evaluation of Omnidirectional One-Dimensional Photonic crystals”,J. Appl. Phys. 93 (2003) pp.819-830 [3] Katsuya Tabuchi, Wilson W.Wenas, Akira Yamada, Makoto Kongai, and Kiyoshi Takahashi “Optimization of ZnO Films for Amorphous Silicon Solar Cells”, Jpn. J. Appl. Phys. 32 (1993) pp.3764-3769

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