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Use of microcrystallinity depth profiling in an actual tandem silicon solar cell by polishing to achieve high conversion efficiency
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2015 Jpn. J. Appl. Phys. 54 052302 (http://iopscience.iop.org/1347-4065/54/5/052302) View the table of contents for this issue, or go to the journal homepage for more
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REGULAR PAPER
Japanese Journal of Applied Physics 54, 052302 (2015) http://dx.doi.org/10.7567/JJAP.54.052302
Use of microcrystallinity depth profiling in an actual tandem silicon solar cell by polishing to achieve high conversion efficiency Mitsuoki Hishida1†, Hiroyuki Ueno2, Takeyuki Sekimoto1, and Akira Terakawa3 1
R&D Division, Panasonic Corporation, Seika, Kyoto 619-0237, Japan Eco Solutions Division, SANYO Electric Co., Ltd., Eco Solutions Company of Panasonic Group, Kaizuka, Osaka 597-0094, Japan 3 Eco Solution Company, Panasonic Corporation, Kadoma, Osaka 571-8686, Japan E-mail:
[email protected] 2
Received November 18, 2014; revised January 7, 2015; accepted January 20, 2015; published online April 1, 2015 In order to perform an accurate evaluation of the crystallinity (Xc) of hydrogenated microcrystalline silicon (µc-Si:H), polishing was performed from the electrode layer side after measuring the current–voltage characteristics of thin-film silicon tandem solar cells. The angle of polishing was about 1.4°, and the µc-Si:H was exposed in the horizontal direction. The polishing could facilitate the measurement of Xc in the vertical direction of µcSi:H by Raman analysis, and it succeeded in revealing a variation of Xc in the depth direction of the solar cell, which even further clarified its characteristics of the solar cells. Additionally, an Xc-adjustment film was used to adjust the Xc profile of i-µc-Si:H. It could control Xc in the depth direction of µc-Si:H, and a high Xc was obtained, which did not decrease during the process of forming a thick film. As a result, the conversion efficiency was improved by 1.8% (0.21 points) compared with that under normal conditions. In this paper, we propose an index of the Xc profile of µc-Si:H for obtaining high conversion efficiency. © 2015 The Japan Society of Applied Physics
1.
Introduction
We achieved the world’s highest-level conversion efficiency of hydrogenated amorphous silicon (a-Si:H)=hydrogenated microcrystalline silicon (µc-Si:H) tandem solar cells:1) 10.7% for fifth-generation substrate modules (Gen-5) and 12.2% for small cells.2–4) This was achieved by improving the µc-Si:H quality and device structure, including optical confinement.5,6) Recently, École Polytechnique Fédérale de Lausanne has developed thin-film silicon solar cells with 12.63% efficiency.7) In addition, Kaneka also developed highefficiency triple-junction solar cells with 13.4% efficiency with a p–i–n configuration.8) In a multi junction solar cell, µc-Si:H was employed for the long-wavelength region near infrared.9) Toward our achievements, electrical conductivity analysis, atomic force microscopy, Raman analysis of silicon crystallinity, and transmittance=reflectance analyses have been widely used to evaluate of µc-Si:H.10–15) However, it was difficult to use crystallinity (Xc) in Raman spectrometry as an index to confirm the high conversion efficiency, since the Xc of µc-Si:H varies depending on its underlying shape or film thickness.16,17) This variation was attributed to the shadowing effect as well as the changing orientation of the grain.16–18) As the bottom cell of thin-film silicon tandem solar cells, µc-Si:H is deposited on transparent conductive oxide (TCO)= top layer (a-Si:H). On the other hand, to enhance the optical confinement effect, the shape of TCO tends to be more complex. Conversely, the more complex the shape of TCO, the more difficult it is to attain a stable quality of µc-Si:H.5) Regarding the tin oxide (SnO2) as the TCO, it is pyramidal in shape. Its angles in the region of the valley are sharp. These features with a complex shape affect the characteristics of µc-Si:H.19–21) To avoid such a complex structure, some research institutes use a technique with which the TCO surface is changed to have a blunt angle or a sweep shape using zinc oxide.22–24) Furthermore, the condition of the top †
Present address: Automotive & Industrial Systems Company, Panasonic Corporation, Seika, Kyoto 619-0237, Japan.
layer changes at times. Consequently, it can be very difficult to evaluate µc-Si:H by analyzing only one condition of a µcSi:H film. Under various conditions, such as a complex shape or a high-haze substrate,25) there is a strong demand for an index of Xc that can be used to evaluate µc-Si:H accurately. To evaluate µc-Si:H stably and easily, a stable substrate such as bare glass or a commercial TCO substrate has often been employed. However, such a substrate is only relevant to the analysis of the average crystallinity of films. In this study, actual thin-film silicon solar cells were used for the evaluation of the Xc of µc-Si:H without using stable substrates. After evaluating their current–voltage (I–V ) characteristics, the thin-film silicon tandem solar cells were smoothly polished to have a gentle angle of about 1.4° on the electrode side. The cells of the µc-Si:H layer were exposed in a horizontal direction, which facilitated the evaluation of the crystallinity of µc-Si:H along with thickness.26) Raman spectrometry analysis after cells were obliquely polished revealed the profile of Xc in the thickness direction. Furthermore, an Xc-adjustment film is proposed when using this structure. This new Xc-adjustment layer was inserted before the i-µc-Si:H layer.27) The aim is to control the Xc of the µc-Si:H film and absorb the complicated structure from TCO to obtain a high conversion efficiency. In fact, when the Xc-adjustment layer was used in the thin-film silicon tandem solar cells, the conversion efficiency of the cells improved by 1.8% (0.21 points) compared with that of conventional cells. In the results of Xc profiling of the test cells with the Xc-adjustment film, there was no reduction in crystallinity in the thickness direction. In addition, crystallinity increased with increasing film thickness, and a maximum crystallinity was confirmed. Finally, we propose an optimum Xc profiling indicator of µc-Si:H for obtaining a high conversion efficiency. 2.
Experimental methods
In this study, actual thin-film silicon tandem solar cells were used for the Xc evaluation of µc-Si:H, without using stable substrates. The structure of these cells was glass=tin oxide (SnO2)=a-Si:H (p–i–n)=µc-Si:H (p–i–n)=back electrode. For TCO, a commercial SnO2-based TCO substrate (Asahi VU)
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Table I. Cell and film thicknesses of Xc-adjustment film and i-µc-Si:H; all other conditions of the compared cells were the same (unit: nm). Cell Normal Xc-adjustment filma)
—
Raman spectroscopy / Step gauge laser / probe
Film
A
B
Normal
A
A
B
7
70
—
7
70
Normal i-µc-Si:H
1500
1493
1430
500
493
430
Total i-layer
1500
1500
1500
500
500
500
Scaned direction
Back electrode (250 nm)
B
µc-Si:H (1500 nm)
Shaved area TCO
a-Si:H (240 nm)
Glass
a) Lower Xc than i-µc-Si:H.
Scanned direction
A
B
TCO a-Si:H µc-Si:H SnO2 50µm
Fig. 2. (Color online) Image of actual polished cell as viewed from electrode side.
Shaving depth (µm)
was adopted. µc-Si:H and a-Si:H were deposited by plasmaenhanced chemical vapor deposition (PECVD).28) In each cell, an Xc-adjustment film is inserted to control the Xc of the i-µc-Si:H film. The Xc of µc-Si:H was lower than that of i-µc-Si:H. The low Xc of this film is due to the presence of a large component of the amorphous phase, and it was expected to restructure the defective region in µcSi:H.29,30) Two Xc-adjustment film thicknesses were adopted (7 and 70 nm) in addition to the normal condition of i-µcSi:H. These thicknesses were very small compared with that of i-µc-Si:H (1500 nm). Test cells with Xc-adjustment films of 7 and 70 nm thicknesses were labeled cells A and B, respectively. Then, the normal-condition i-µc-Si:H was deposited after depositing each Xc-adjustment film until a thickness of 1500 nm was reached, including the Xc-adjustment film. In addition, three types of cell were compared: the normal-condition i-µc-Si:H and the two types of Xc-adjustment films used with the normal-condition i-µc-Si:H. Aside for the i-layer and Xc-adjustment film, all other conditions were the same in all the tests. For each condition, five cells were fabricated. The active area of each cell was 1 cm2. The I–V characteristics of open circuit voltage (Voc), short circuit current (Isc), fill factor (FF ), and conversion efficiency (Eff ) were measured using a solar simulator under an air mass of 1.5 (AM 1.5) at 25 °C. A summary of the conditions is shown in Table I. The thicknesses of the entire cell structure and i-µc-Si:H layer were about 2.2 and 1.5 µm, respectively. After measuring the characteristics of the solar cells, the cells were polished with a circular drum from the back electrode side. A circular drum with a diameter of 20 mm and an abrasive material (BAIKALOX CR MC0.1cr) were used at the same time. The mount where the cells were set also rotated simultaneously. The cells after polishing showed a depression with a globular shape. The polishing depth was about 2.2 µm in order to expose the entire µc-Si:H. The diameter of the polished sphere was about 170 µm, and a gentle slope of about 1.4° was formed. The cell structure formed with this polishing process is shown in Fig. 1, and an image of an actual polished cell, as viewed from the electrode side, is shown in Fig. 2. Three measurement results obtained using a step gauge are shown in Fig. 3: normal-condition, cell A and cell B. The center of the image in Fig. 2 is the TCO area. Furthermore, we confirmed that the top layer of a-Si:H, the bottom layer of µc-Si:H, and the back electrode layer were donut-shaped. Xc was measured by Raman spectrometry through the center of the polished region at the point trajectory from A to B, as shown in Figs. 1 and 2. It was automatically measured at intervals of 1.0 µm. The positions were analyzed by matching the step gauge data with the
Fig. 1. Structure of cell polished with circular drum. Measurement direction of A to B in measurements using Raman spectrometry and step gauge.
0.0 -0.5 -1.0 -1.5
Normalcondition A
-2.0
B
-2.5 0
50
100
150
200
Shaving width (µm) Fig. 3. (Color online) Shaving depth and width in the case of using the step gauge: normal-condition cell, cell A, and cell B.
Raman measurement data because the shape of the polished cells was convex in an upward direction and not in a straight line. The diameter of the laser spot in the Raman measurement was about 1.0 µm, and the wavelength of the laser was 514 nm. The depths of the laser for a-Si:H and µc-Si:H reached about 100 and 300 nm, respectively.10) There are some places that are thin and polished diagonally, and a portion of the laser sometimes irradiated other layers at the edge of µc-Si:H. However, the effects were not addressed in this study. Also, a comparison of Xc among these samples showed that the errors in Raman measurements due to polishing were negligible, because the angles of each polished sample, which varied within 0.02°, as shown in Fig. 3, were almost the same. The roughness of the polished surface is also very small compared with the diameter of the laser spot (1.0 µm) since a 0.1-µm-diameter abrasive was used for polishing.
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M. Hishida et al. Table II. Results for each test cell. Each of the values of Eff, Voc , Isc , and FF was averaged for each condition. Cell Normal
A
B
Voc (V)
1.393
1.397
1.410
Isc (mA)
10.80
10.75
10.75
FF
0.777
0.781
0.784
Eff (%)
11.68
11.72
11.89
12
Eff (%)
11.9 11.8 11.7 11.6 11.5 1.43
Voc (V)
1.42 1.41 1.4 1.39 1.38
Isc (mA)
11.1 10.9 10.7 10.5 10.3 0.790 0.785
FF
The solar cell and film qualities were evaluated simultaneously. In these evaluations, Corning 7059 was used as the substrate for stabilization rather than the solar cell. A 20 nm p-layer under the same conditions as the cell was first deposited on a bare substrate. Then, two types of Xc-adjustment film were deposited at thicknesses of 7 and 70 nm, and these were labeled as test films A and B. Next, films with the normal-condition i-µc-Si:H were deposited at their respective thicknesses. The total i-layer thickness was adjusted to 500 nm, including the thickness of the Xc-adjustment film; this thickness has been used because the film quality changes with an increasing film thickness.16–18) These three types of film (film A, film B, and normal-condition film) were measured by Raman spectrometry and Fourier transform infrared spectroscopy. To distinguish the crystallinity of the cell expressed by “Xc”, the crystallinity of the film is referred to as “X0c ”. A summary of the conditions of each film is shown in Table I. The crystallinity of µc-Si:H was evaluated using a 514 nm wavelength and a Via Reflex Raman microscope (Renishaw). The crystallinity evaluated by Raman spectroscopy was defined as Xc = I520=I480, where the peak heights of I480 and I520 are the wavenumbers of 480 and approximately 520 cm−1, respectively.10,30) FTIR measurements by the attenuated total reflection (ATR) method were carried out using a Spectrum 100 (PerkinElmer).31) The following stretching modes (SM) of Si–H bonds were used to fit the IR spectra: oxide silicone (OySiHx) at about 2250 cm−1, high SM (HSM) at 2100 cm−1, low SM (LSM) at 2040 and 2000 cm−1, and extremely low SM (ELSM) at 1925 and 1895 cm−1.11–13) Each SM was analyzed using the Gaussian mode,32) and each SM fraction was defined as the ratio of the total area of all SMs divided to the area of an individual SM. The LSM peaks were classified into two levels at 2000 and 2040 cm−1. The sum of two SMs was expressed as the LSM value, while the value of two ELSMs was also expressed as ELSM.
0.780 0.775 0.770
Results and discussion
A
B
Fig. 4. Results of each test cell: normal-condition cell, cell A, and cell B. For each cell type, five cells were fabricated.
LSM ELSM X c'
1
0.8
0.8
0.6
0.6
0.4
0.4
0.2
0.2
Nomalized Xc'
1
HSM OySiH x
0
film B
0
film A
Test results for the cells with the Xc-adjustment film inserted are shown in Table II and Fig. 4. Each of the values of Eff, Voc , Isc , and FF was the average for each condition in Table II. All results for the cells are plotted in Fig. 4. Here, the Eff, Voc , and FF of cell A slightly improved compared with those of the normal-condition cell. For cell B, Eff improved by 1.8% (0.21 points), Voc by 0.017 V, and FF by 0.21 compared with those for the normal-condition cell. From these two sets of results, the Xc-adjustment film led to the formation of a high-quality the µc-Si:H film. In addition, the thicker the Xc-adjustment film became, the more the quality of the µc-Si:H film improved. On the other hand, increasing the thickness of the Xc-adjustment film slightly reduced Isc . The results of analysis by Raman spectrometry and FTIR are shown in Fig. 5. The value of X0c was normalized by that of the normal-condition film. Despite inserting the Xcadjustment film with a very small thickness, X0c was greatly reduced. The reduction ratios of X0c were proportional to the thickness of the Xc-adjustment film. The X0c of film B with the
Normalcondition
3.1 Conversion efficiency and film quality in the case of using Xc-adjustment film
Normalcondition
SM fraction
3.
Fig. 5. (Color online) Analyzed results of film tests by Raman spectrometry and FTIR. The values of X0c were normalized by the value of the normal-condition film.
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M. Hishida et al.
1
Normalized Xc
0.8 0.6 0.4
B A Normal-condition
0.2 0 0
250 500 750 1000 1250 1500
µc-Si:H thickness (nm) Fig. 6. (Color online) Results of Raman analysis of polished test cells. Values of Xc were normalized by the maximum value of Xc in this test. Xc was expressed from the TCO side.
Xc-adjustment film of 70 nm thickness dropped by 35% that of the normal-condition film. On the other hand, the LSM value increased with decreasing X0c . Moreover, the LSM value was proportional to Eff. Much of the LSM film contains a large amount of monohydride silicone (Si–H), so this component may compensate for the grain boundary between the crystalline phase and the amorphous phase,29,33) and it may terminate the dangling bonds in µc-Si:H.34,35) This may be the reason why the condition with a large amount of Xc-adjustment film resulted in good Eff. 3.2 Actual crystallinity profile of µc-Si:H along with depth in the case of using Xc-adjustment film
The results of the Raman analysis of the polished cells (normal-condition cell, cell A, and cell B) are shown in Fig. 6. The relationship between film thickness and crystallinity is shown from the µc-Si:H thickness of zero. The obtained figure was expressed from the TCO side, and it also includes the thickness of the p-layer. This is because the player of the bottom cell was µc-Si:H, but its thickness was very small (20 nm). The values of Xc were normalized by the maximum value of Xc in this test. Xc increased with film thickness until 500 nm under normal conditions. However, it decreased at an approximately 1000 nm film thickness. It has been proposed that the decrease in Xc is due to the formation of a defective region due to the shadowing effect of incident radicals.18,20) In cell A, the Xc of µc-Si:H was affected by the Xcadjustment film thickness being only 7 nm. The Xc of cell A was slightly lower than that of the normal-condition cell up to 250 nm thickness. Furthermore, the Xc of cell A was higher than that of the normal-condition cell when the film thickness ranged from 500 to 1250 nm. Furthermore, in cell B, Xc was even lower than those in the other cells up to 250 nm thickness. The Xc-adjustment film was more influenced by the thicker Xc-adjustment film than by the other cells. Xc increased, reaching the maximum at the film thickness of 1500 nm without any decrease. This result also led to obtaining good values of Eff. 3.3
Discussion The Eff of cell A was slightly higher than that of the normalcondition cell owing to the slightly higher Xc of µc-Si:H, although Xc decreased during the formation of a thick film.
Furthermore, the Eff of cell B was significantly higher than that of the normal-condition cell also owing to the significantly higher Xc of µc-Si:H. The high Xc of i-µc-Si:H led to achieving a high Eff of the solar cell. The decreasing Xc during the formation of a thick film was considered an obstructive factor for a high Xc of µc-Si:H. The Xc-adjustment film prevented the decrease in Xc. The Xc-adjustment film eventually led to a µc-Si:H film of higher Xc. The Xc-adjustment film was used to adjust the Xc of i-µcSi:H. The increase in Xc up to 250 nm thickness slowed down in the presence of the Xc-adjustment film. The reduction in Xc up to 250 nm thickness was proportional to the thickness of the Xc-adjustment film. However, it was possible to fabricate high-quality µc-Si:H for high-conversion-efficiency solar cells as a result. The film with a low Xc contains a large component of the amorphous phase, which is rich in monohidride. The monohidride was expected to terminate the dangling bonds in µc-Si:H.34,35) This might be an effect of the defective region of µc-Si:H, which was affected by the complex texture of the TCO, and this film smoothed the complex structure of TCO. These changes in the quality of µc-Si:H increased Voc and FF, but slightly reduced Isc .6) The thickness of the i-layer (a-Si:H) of the top layer is sufficient at 240 nm; as a result, the top layer current has no effect on the bottom layer current. The bottom layer current has a rate-controlling effect on Isc. The reduction in the Xc of µc-Si:H reduced the external quantum efficiency in the high-wavelength region.36) Consequently, Isc decreased. From this comparison of Xc, the profiling of Xc to achieve high-conversion-efficiency solar cells has proven to be superior to the conventional method. Using a low Xc, pretreatments such as the insertion of an Xc-adjustment film led to a high conversion efficiency. However, even without using an Xc-adjustment film, it may be possible to obtain a highconversion-efficiency cell using the Xc conditions for µcSi:H, which were the same as those for cell B with a very low Xc in the early stages that eventually became very high. 4.
Conclusions
Evaluating the quality of µc-Si:H by a conventional measurement method, namely, Raman analysis, was not sufficient for obtaining high-conversion-efficiency solar cells. This is because the analysis of µc-Si:H varied depending on its underlying shape, and it is only relevant to the analysis of the average crystallinity of films. In this study, the Xc of the cell was directly evaluated by Raman analysis after polishing the cells rather than using a stable substrate. Xc can be accurately confirmed in the profile of µc-Si:H along with the depth. The use of an Xc-adjustment film prior to i-µc-Si:H is suggested as a method of fabricating a high-quality µc-Si:H film. It achieves a high Xc of µc-Si:H that does not decrease the value of Xc during the process of forming a thick film. As a result, the cell achieved high conversion efficiency. Finally, an index of the Xc profile of µc-Si:H for confirming a high conversion efficiency was proposed. In order to enhance the optical confinement effect, the shape of TCO will need to be more complex in the future. Even under such difficult conditions, this new evaluation method and index of the Xc profile will facilitate evaluations for confirming a high conversion efficiency.
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Acknowledgments
This work was in part supported by Professor Yukiharu Uraoka and Associate Professor Yasuaki Ishikawa of Nara Institute of Science and Technology. The authors thank them for constructive discussions.
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