Solar Energy Materials & Solar Cells

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1 Solar Energy Materials & Solar Cells 113 (2013) Contents lists available at SciVerse ScienceDirect Solar Energy Materials & Solar Cells journal homepage: Large area Si thin film solar module applying n-mc-sio x :H intermediate layer with low refractive index Sun-Tae Hwang, Dong Joo You n, Sun Ho Kim, Sungeun Lee, Heon-Min Lee Solar Energy Team, Materials & Components R&D Lab. LG Electronics Advanced Research Institute, Seoul , South Korea article info Article history: Received 29 October 2012 Received in revised form 25 January 2013 Accepted 25 January 2013 Available online 28 February 2013 Keywords: Si thin film solar cell n-mc-sio x :H intermediate layer Large area solar module abstract We present our development of an n-mc-sio x :H intermediate layer with low refractive index for a highly efficient Si thin film solar module in a large area ( m 2 ). We try to increase the electron temperature (T e ) in the plasma to activate the oxygen reaction during the deposition of the n-mc-sio x :H intermediate layer. Thus, the T e -related process conditions are controlled, such as the gap between the electrodes, the working pressure and the H 2 flow rate in the plasma-enhanced chemical vapor deposition (PECVD) system. An intermediate layer with a lower refractive index is fabricated in narrower gap between the electrodes, lower working pressure and lower H 2 flow rate. As a result, we obtain a superior intermediate layer with a refractive index of 1.73 on a large area substrate, vertically conductive enough for a favorable junction in the cell, as confirmed by conductive atomic force microscopy (C-AFM) analysis. This intermediate layer is applied between the middle and the bottom layers in the triple junction (a-si:h/a-sige:h/mc-si:h) cell. A low refractive index leads to a higher current of the middle cell by the superior reflection. Our large area module gains 0.3% conversion efficiency, with an improvement in the open circuit voltage (V oc ) and the fill factor (FF) by the thinner middle absorbing layer owing to the intermediate layer with a lower refractive index. & 2013 Elsevier B.V. All rights reserved. 1. Introduction Si thin film solar cells have the advantages of low cost, large area and an aesthetic design for building integrated photovoltaic (BIPV) applications [1,2]. Nevertheless, the market of Si thin film solar cells is limited nowadays due to low conversion efficiency [3]. To improve the conversion efficiency, many groups have adopted multi-absorption layers with various band-gaps to efficiently absorb a wide range of wavelengths of light [4 10]. Recently, Yan et al. reported a record of 16.3% cell efficiency for a triple-junction cell with a-si:h/a-sige:h/mc-si:h absorbing layers [7]. In multi-junction Si thin film solar cells, the thickness of the amorphous absorbing layers, such as a-si:h or a-sige:h, has to be thin to minimize the impact of light-induced degradation, and its current can limit the current of the device. To overcome this issue, an intermediate layer is introduced between the cells to increase the current of the amorphous cells [11]. For the triple junction (a-si:h/a-sige:h/mc-si:h) cells, the intermediate layer is introduced between the middle (a-sige:h) and bottom (mc-si:h) cells to compensate for the middle cell current [7]. Several groups have used n-mc-sio x :H as the material of the intermediate layer [5,12 14] because of its superior reflection due n Corresponding author. Tel.: þ addresses: djryu93@hanmail.net, dj.you@lge.com (D.J. You). to its low refractive index and lower absorption of light in the long wavelength region compared with other materials, such as ZnO:Al and ZnO:B [15,16]. n-mc-sio x :H does not need any additional ex situ deposition step and an additional laser scribe for a monolithic series interconnection when fabricating modules, which is different from ZnO-based intermediate layers. [17]. There are two main properties of the n-mc-sio x :H intermediate layer, which are its refractive index and conductivity. An intermediate layer with a low refractive index can enhance the reflection of light, which can reduce the thickness of the middle (a-sige:h) absorption layer. An intermediate layer below a certain level of conducvitity can deteriorate the contact performance, resulting in a higher series resistance (R s ) and lower fill factor (FF) of the solar cell. Generally, it is known that the intermediate layer should have the vertical conductivity greater than 1E 5 S=cm [13]. However, due to the difficulty in measuring the vertical conductivity, many groups consider the vertical conductivity to be strongly related to the lateral conductivity with co-planar electrodes on the thin film [7,11,14]. Some researchers have tried Raman spectroscopy [5] or conductive atomic force microscopy (C-AFM) analysis [2] to compensate for the lateral conductivity. These analyses are based on the multi-phase microstructure of the n-mc-sio x :H, which refers to the mc-si:h grains embedded in the a-sio:h matrix [18,19]. The refractive index and the conductivity of the n-mc-sio x :H intermediate layer can be determined by the oxygen content in /$ - see front matter & 2013 Elsevier B.V. All rights reserved.

2 80 S.-T. Hwang et al. / Solar Energy Materials & Solar Cells 113 (2013) the films. When the oxygen content increases in the film, in general, both the refractive index and the conductivity decrease [20]. Many groups have tried to control the oxygen content by adjusting the CO 2 /SiH 4 gas ratio [13,14,20] and the H 2 /SiH 4 gas ratio [12] in the plasma. The reactivity of CO 2 is lower than that of SiH 4 in the plasma because the dissociation energy of CO 2 (532 kj/mol for CO O and 1076 kj/mol for C O) is higher compared with SiH 4 (298 kj/mol) [21,22]. How well the oxygen source gas (CO 2 ) can be dissociated to the radicals in the plasma is very important. For an intermediate layer with a low refractive index, there should be a sufficient oxygen reaction, which means that the electron temperature above a certain level is necessary in the plasma. However, an excessive oxygen reaction can result in low conductivity, which can lead to contact problems in the device. Thus, it is important to control the electron temperature in the plasma. In this study, we controlled the properties of the n-mc-sio x :H intermediate layer by adjusting some conditions of the plasmaenhanced chemical vapor deposition (PECVD) system related to the elecron temperature in the plasma. The conditions include the gap between the electrodes, the working pressure and the H 2 flow rate. The intermediate layer with low refractive index and sufficient conductivity for favorable contact in the device is applied in the triple-junction (a-si:h/a-sige:h/mc-si:h) Si thin film solar module in a large area. 2. Experimental n-mc-sio x :H intermediate layers are deposited by mixing H 2, SiH 4, PH 3 (1 at% diluted by H 2 gas), and CO 2 gases in the capacitively coupled radio frequency PECVD system on large area ( m 2 ) substrate. The PECVD chamber has a multi-hollow cathode and a single radio frequency feedthrough. The deposition conditions of the PECVD system are as follows: working pressure of 3 4 Torr, gap between the electrodes of mm, H 2 flow rate of slm, the H 2 /SiH 4 gas ratio of 650 and power density of 0.14 W/cm 2 at MHz radio frequency power. The n-mc- SiO x :H films are 100 nm thick. The refractive index and thickness of the film are obtained by using a spectroscopic ellipsometer. The conductivity is measured by two parallel Ag electrodes on the film on glass. The measured properties of an n-mc-sio x :H thin film can differ depending on the measurement position in the large area substrate. Therefore, three positions in the substrate are sampled ( Center for the center of the substrate, Edge for the edge, and Middle for the position between the Center and the Edge ). Conductive atomic force microscopy (C-AFM) is used to investigate if the film is vertically conductive, and auger electron spectroscopy (AES) is used to analyze the oxygen content in the film. For C-AFM and AES, the n-mc-sio x :H thin film is on the ZnO:Al (conductive) layer on glass. Raman spectroscopy is used to analyze the crystalline (mc-si:h) fraction in the n-mc-sio x :H film [12]. Largearea( m 2 ) Si thin film solar modules with three absorbing layers (a-si:h/a-sige:h/mc-si:h) were fabricated. The modules have p i n structure on the TCO (SnO 2 :F) glasses. The thicknesses of the absorbing layers are 90 nm for the top cell (a-si:h), nm for the middle cell (a-sige:h) and 3.5 mm for the bottom cell (mc-si:h). An n-mc-sio x :H intermediate layer was applied between the middle (a-sige:h) and bottom (mc-si:h) absorbing layers. Module efficiency was measured by a PVS1222i- L solar simulator by Nissinbo. The external quantum efficiency (EQE) of each absorbing layers was measured from a 1 cm 2 electrode sampled from the large area cell. 3. Results and discussion 3.1. Properties of the n-mc-sio x :H intermediate layers Properties of the n-mc-sio x :H intermediate layers as a function of the gap between the electrodes Fig. 1 shows (a) the refractive index and (b) the conductivity as a function of the gap between the electrodes in the PECVD system. As shown in Fig. 1(a), the refractive index increases as the gap between the electrodes gets wider. In Fig. 1(b), the conductivity increases as the gap gets wider from 24 mm to 28 mm, but it then slightly decreases when the gap gets wider than 28 mm. As the gap increases, the electric field between the electrodes decreases, leading to a lower electron temperature in the plasma [23]. The oxygen reaction is then expected to decrease, which can result in a lower oxygen content and higher refractive index in the film. In general, the conductivity is strongly related to the crystalline fraction of Si in the film. As the crystalline fraction gets higher, the conductivity also gets higher [24]. It is reported that oxygen hinders the crystallization of Si in n-mc-sio x :H films [20]. As the gap gets wider from 24 mm to 28 mm, the increase of the conductivity is considered due to the easier crystallization of Si by the low oxygen content. There can be other reasons for the decreasing conductivity as the gap gets wider than 28 mm. Auger electron spectroscopy (AES) and Raman spectroscopy were performed to investigate the reason mentioned above. Fig. 2 shows the oxygen content in the film measured by AES and the crystalline fraction measured by Raman spectroscopy as a function of the gap between the electrodes in the PECVD system. The oxygen content in the film (black squares) decreases as the gap gets wider, which means that the increase in the refractive index is related to the oxygen content in the film. The crystalline Fig. 1. (a) The refractive index and (b) the conductivity as a function of the gap between the electrodes in the PECVD system.

3 S.-T. Hwang et al. / Solar Energy Materials & Solar Cells 113 (2013) Fig. 2. The oxygen content measured by auger electron spectroscopy (AES) analysis (black square) and the crystalline fraction (Xc) measured by Raman spectroscopy (red circle) as a function of the gap between the electrodes in the PECVD system. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) fraction (red circles) increases as the gap gets wider from 24 mm to 28 mm, and it slightly decreases when the gap gets wider than 28 mm. At the gap wider than 28 mm, the crystalline fraction slightly decreases as the gap gets wider, in spite of the lower oxygen content in the film. In this region, it is considered that the decrease of the electron temperature has more effect on the change in the crystalline fraction than the slight decrease of the oxygen content in the film, as shown in Fig. 2. Therefore, it is understood that the crystallization gets easier due to lower oxygen content in the film for the gap range from 24 mm to 28 mm, and it gets more difficult due to a lower electron temperature when the gap gets wider than 28 mm. The crystalline fraction in Fig. 2 shows a similar tendency to the conductivity gap shown in Fig. 1(b). It shows that the conductivity is strongly related to the crystalline fraction in the film. As a result, an adequate gap is mm because it has a low refractive index and sufficient conductivity for a favorable junction. In addition, Fig. 1 also shows the difference in the refractive index and the conductivity depending on the position in the large area substrate. The refractive index and the conductivity at the center of the substrate are lower than at the edge regardless of the gap. At the center of the substrate in the large area PECVD, the residual time of the supplied gases is relatively long, which means there is more reaction. On the other side, the short residual time of the supplied gases causes relatively less reaction at the edge. From this difference, the relatively high oxygen content is expected at the center due to the increased oxygen reaction, which leads to a lower refractive index and lower conductivity. The AES result also shows accordance with the expectation above. As an example with a 28 mm gap, the oxygen content is 52% at the center and 48% at the edge Properties of the n-mc-sio x :H intermediate layers as a function of the working pressure Fig. 3 shows (a) the refractive index and (b) the conductivity as a function of the working pressure in the PECVD system. As shown in Fig. 3(a), the refractive index increases as the working pressure increases. In Fig. 3(b), the conductivity increases as the working pressure increases from 3 Torr to 3.5 Torr, but it slightly decreases above 3.5 Torr. It is reported that as the working pressure gets higher, the electron temperature generally gets lower in the plasma [25]. This means that the oxygen reaction decreases at higher working pressures, which results in lower oxygen content and a higher Fig. 3. (a) The refractive index and (b) the conductivity as a function of the working pressure in the PECVD system. refractive index of the film. In Section 3.1.1, we already showed the strong connection between the conductivity and the crystalline fraction in the film. It is understood that crystallization gets easier as the oxygen content in the film gets lower for working pressures from 3 Torr to 3.5 Torr, while crystallization gets more difficult due to the lower electron temperature for pressures higher than 3.5 Torr. In this experiment, a low refractive index and sufficient conductivity for a junction can be achieved at around Torr Properties of the n-mc-sio x :H intermediate layers as a function of the H 2 flow rate To verify the effect on the electron temperature, we investigated the properties of the intermediate layer as a function of the H 2 flow rate in the PECVD system. The gas ratios, such as H 2 /SiH 4,PH 3 /SiH 4, and CO 2 /SiH 4, are unchanged for all conditions of the H 2 flow rates. Fig. 4 shows (a) the refractive index, (b) the conductivity, and (c) the crystalline fraction measured by Raman spectroscopy as a function of the H 2 flow rate. As the H 2 flow rate gets higher, the refractive index and the conductivity also gets higher, and the crystalline fraction slightly increases. It is reported that an increase in the H 2 flow rate leads to a shorter residual time of the process gases in the plasma, which causes a lower electron temperature due to a decreased reaction [26]. In this experiment, as the H 2 flow rate increases, the electron temperature is considered to get lower, and a decreased oxygen reaction causes lower oxygen content in the film, and also leads to easier crystallization, as shown in Fig. 4(c). Therefore, it is understood, as discussed in Section 3.1.1, that when the H 2 flow rate becomes higher, the refractive index becomes higher due to lower oxygen content, and the conductivity also becomes higher due to easier crystallization of Si. In this experiment, the condition of

4 82 S.-T. Hwang et al. / Solar Energy Materials & Solar Cells 113 (2013) bias voltage ( 1.5 V) is applied; darker points indicate more current flowing at those spots. Dark areas are rare in Fig. 5(a), while light grey areas are distributed in (b). Dark grey spots are distributed in the overall areas of (c) and (d). As shown from the average current in Fig. 5, the vertical conductivity increases in the order of (a), (b), (c) and (d). Sample (a), with a refractive index of 1.67 and conductivity of 3E 9 S/cm, has few areas vertically conductive, and its average current is similar to the noise level (0.003 pa). It is considered that it would be difficult to apply sample (a) in the cell. The light grey area of sample (b), with a refractive index of 1.73 and conductivity of 3E 7 S/cm, means that it is somewhat vertically conductive in that area. The average current of sample (b) also increases by 50 times (0.14 pa) compared with sample (a). This light grey area is considered to be mc-si:h, based on the multi-phase structure of n-mc- SiO x :H [18,19]. The vertical conductivity is improved by this mc-si:h area, which can make the junction stable in the cell. For sample (c), with a refractive index of 1.84 and conductivity of 1E 4S/cm,the grey areas are larger and darker than in sample (b). This shows that sample (c) is more vertically conductive than sample (b), which means there are more and larger mc-si:h grains distributed in the film. Sample (d) can be explained in a similar way as sample (c): it is more vertically conductive than sample (c). As a result, three intermediate layers with refracive indices of 1.73, 1.84, 1.96 are sufficiently vertically conductive to apply in cells. Among them, the refracive index of 1.73 is expected to have the best reflection property. This refractive index (1.73) is similar to the lowest refractive index (1.7) reported by other groups [27]. In particular, our development has significant meaning because the low refractive index is achieved in a large area ( m 2 ) substrate Triple junction cells and large area modules Fig. 4. (a) The refractive index, (b) the conductivity, and (c) the crystalline fraction (Xc) measured by Raman spectroscopy, as a function of the H 2 flow rate in the PECVD system. the H 2 flow rate with a low refractive index and sufficient conductivity for favorable junctions is slm. We explained the change in the properties of the n-mc-sio x :H intermediate layer by controlling some main process conditions of the PECVD system from Sections As a result, we obtained a condition that has a low refractive index and sufficient conductivity for favorable junctions. Four intermediate layers are then selected by changing the CO 2 /SiH 4 gas ratio in the PECVD system. The four intermediate layers have refractive indices of 1.67, 1.73, 1.84, 1.96 and conductivities of 3E 9, 3E 7, 1E 4, 4E 3 S/cm, respectively Evaluation of the vertical conductivity of the n-mc-sio x :H intermediate layers In order to evaluate if the intermediate layers in Section are sufficiently vertically conductive for favorable junctions, we performed conductive atomic force microscopy (C-AFM) analysis. Fig. 5 shows the current images of n-mc-sio x :H thin films measured by C-AFM in a 5 5 mm 2 area. In the images, the contrast of each pixel shows the current of the spot when the Middle (a-sige:h) cell current of the triple junction cells as a function of the refractive index of the n-mc-sio x :H intermediate layer n-mc-sio x :H intermediate layers with refractive index of 1.73, 1.84, 1.96, which were mentioned in Section 3.1.4, are applied in the triple junction (a-si:h/a-sige:h/mc-si:h) cells with a thickness of 75 nm. We have shown they are sufficiently vertically conductive in the device. Fig. 6 shows the external quantum efficiency (EQE) of each absorbing layer of the solar cell as a function of the refractive index of the intermediate layer. Table 1 indicates the current density of each absorbing layer corresponding to the cells shown in Fig. 6 and the total current density of each cell. The EQE of the middle cell (a-sige:h) increases at the nm wavelength range when the refractive index decreases, as expected in Section The current density of the middle cell also increases by ma/cm 2 for the cell with the lower refractive index of intermediate layer. This means that the intermediate layer with low refractive index effectively reflects light in the nm wavelength range to the a-sige:h intrinsic layer. It also means that the current density of the middle cell can be maintained even if a thinner a-sige:h absorbing layer is applied when the refractive index of the intermediate layer gets lower. The open circuit voltage (V oc ) and fill factor (FF) at the triple junction cell is also expected to be improved due to a thinner a-sige:h absorbing layer Large area ( m 2 ) modules Table 2 shows some conditions and properties of the large area ( m 2 ) solar modules. The intermediate layers in this experiment are the same as those shown in Section The thickness of the a-sige:h absorbing layer gets thinner and the

5 S.-T. Hwang et al. / Solar Energy Materials & Solar Cells 113 (2013) Fig. 5. Current images of the n-mc-sio x :H thin films analyzed by conductive atomic force microscopy (C-AFM). (a) refractive index [n]¼1.67, conductivity [s]¼5e 9 S/cm, (b) n¼1.73, s¼3e 7 S/cm, (c) n¼1.84, s¼1e 5 S/cm, and (d) n¼1.96, s¼4e 4 S/cm. Table 2 The initial conversion efficiencies and some other properties of the large area ( m 2 ) modules as a function of the refractive index of the intermediate layer (IL) and the thickness of the a-sige:h layer. Sample name Refractive index of IL Thickness of a-sige:h layer (nm) V oc (V) I sc (A) Fill factor Efficiency (%) A B C Fig. 6. External quantum efficiency (EQE) of 1 cm 2 size triple-junction cells prepared with different refractive indices (n) of the n-mc-sio x :H intermediate layer. Table 1 The cell current densities of each component cell and total current densities measured by external quantum efficiency (EQE) of 1 cm 2 size triple-junction cells prepared with different refractive indices (n) of the n-mc-sio x :H intermediate layer (IL). Refractive index of IL Current density measured by EQE (ma/cm 2 ) Top Middle Bottom Total refractive index of the intermediate layer gets lower in the order of sample A, B and C. Sample C has similar short circuit current (I sc ) as A and B, even though it has a thinner a-sige:h layer by 20 nm compared with that of A and by 10 nm compared with that of B. Sample C also has a higher open circuit voltage (V oc ) and fill factor (FF) compared with the other samples. The conversion efficiency of C is 12.02%, which is higher than A by 0.3%, and higher than B by 0.1%. This result is due to the low refractive index (1.73) of the intermediate layer, which has the superior reflection of light to the a-sige:h absorbing layer, resulting in the current increase in the a-sige:h layer. Therefore, the I sc value can be maintained even if a thinner a-sige:h absorbing layer is applied when the refractive index of the intermediate layer gets lower. In addition, a thinner middle layer increased the V oc and FF values, resulting in a higher efficiency for sample C, as expected in Section Conclusions An n-mc-sio x :H intermediate layer is applied to the triple junction Si thin film solar cell with a large area ( m 2 ). We controlled the T e related process conditions such as the gap between the electrodes, the working pressure and the H 2 flow

6 84 S.-T. Hwang et al. / Solar Energy Materials & Solar Cells 113 (2013) rate in the plasma enhanced chemical vapor deposition (PECVD) system. The intermediate layer with a lower refractive index is fabricated in conditions of a narrower gap between the electrodes, a lower working pressure and a lower H 2 flow rate. It was difficult to apply in the device due to low conductivity despite a low refractive index in the conditions of an excessively narrow gap, a low working pressure and a low H 2 flow rate. The CO 2 /SiH 4 ratio is optimized to acquire some conditions with different refractive indices and conductivities. As a result, we obtained the superior intermediate layer with a refractive index of 1.73 in a large area substrate, and which was sufficiently vertically conductive for a favorable junction in the cell, which was confirmed by C-AFM analysis. This intermediate layer is adopted in the triple junction (a-si:h/a-sige:h/mc-si:h) cell between the middle and the bottom layers, confirming the increase in the middle current at the lower refractive index of the intermediate layer. This intermediate layer is applied in a large area module. The short circuit current is maintained even if a thinner a-sige:h absorbing layer is applied when the refractive index of the intermediate layer is reduced. The open circuit voltage and fill factor increase when the middle absorbing layer gets thinner, resulting in an increase in the conversion efficiency by 0.3% in the large area module by applying an intermediate layer with a lower refractive index. References [1] A.V. Shah, H. Schade, M. Vanecek, J. Meier, E. Vallat-Sauvain, N. Wyrsch, U. Kroll, C. Droz, J. Bailat, Thin-film silicon solar cell technology, Progress in Photovoltaics: Research and Applications 12 (2004) [2] C.R. Wronski, B. Von Roedern, A. 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