Scaling PECVD for Large-Area Solar Cells
Thin-film solar modules based on amorphous (a-Si:H) and microcrystalline (µc-Si:H) silicon are emerging as leading solutions for low-cost, large-scale photovoltaic (PV) applications. Gigawatt-scale production of these modules requires large-area uniformity and high deposition rates of the absorber layers.
Shuran Sheng, Yong Kee Chae, Liwei Li, Jeff Su and Xiesen Yang, Applied Materials, Santa Clara, Calif. -- Semiconductor International, 11/1/2008
Thin-film solar modules based on amorphous (a-Si:H) and microcrystalline (µc-Si:H) silicon are emerging as leading solutions for low-cost, large-scale photovoltaic (PV) applications. Gigawatt-scale production of these modules requires large-area uniformity and high deposition rates of the absorber layers. Applied’s SunFab thin-film line manufactures 5.7 m2 solar modules on glass — 4× larger than conventional thin-film modules. These lines can manufacture both single-junction (SJ) and tandem-junction (TJ) solar modules.
This article reports on recent developments of a-Si:H SJ and a-Si:H/µc-Si:H TJ solar cells and modules fabricated using proprietary, field-proven, large-scale RF-PECVD cluster tools. High quality a-Si:H and µc-Si:H absorber films were deposited at high deposition rates of ~200-400 Å/min and ~300-600 Å/min, respectively. Excellent uniformity of film thickness and properties over large area substrates was achieved, with better than ±10% for all of the substrate sizes (0.43-5.7 m2). Better than 5% uniformity of device parameters for both SJ and TJ cells were also achieved. a-Si:H SJ p-i-n laboratory cells with initial and stabilized efficiencies of 9.2 ±0.4% and 7.3 ±0.4%, respectively, were obtained on low-cost, commercially mass-fabricated floatline transparent conducting oxide (TCO) glass. a-Si:H 30 × 30 cm SJ modules were demonstrated with initial and stabilized efficiencies of >8% and ~6.6%, respectively. Incorporating high-quality and stable a-Si:H on the top cells in a-Si:H/µc-Si:H tandem structure, initial and stabilized TJ cell efficiencies of 11.2 ±0.4% and 9.7 ±0.3%, respectively, were obtained on floatline TCO. Moreover, 11.6% initial tandem cell efficiency and 9.3% stabilized module (28 × 30 cm) efficiency were demonstrated on high-quality TCO.
Our earlier work reported on a-Si:H SJ cells fabricated on high-quality offline transparent conducting oxide (TCO) with absorber-layer deposition rates of 120-300 Å/min. These cells showed high initial fill factor (FF=70-75%) and high open circuit voltage (Voc=0.90-0.95 V), with <25% light-induced degradation in cell efficiency. This indicated excellent film properties and interface quality, as well as good light stability.
In this paper, our process development is focused on low-cost, commercially mass-fabricated floatline TCO glass. By carefully controlling the quality of cell interfaces (TCO/p and p/i interfaces, etc.) and optimizing the doping profile and thickness of the window p-layer, we fabricated high-efficiency SJ cells. The FF and Voc, as well as light stability were comparable to that obtained with the high-quality offline TCO.
Device optimization for TJ cells was performed by extensive investigation of the nucleation and crystallinity evolution of µc-Si:H film deposition to achieve homogeneous and device-quality absorber layers. Furthermore, interconnecting n-p tunnel junction, current matching between top and bottom cells, and light-induced degradation effects were studied to gain in-depth understanding of optimum TJ device design. Here we present high-efficiency a-Si:H SJ and a-Si:H/µc-Si:H TJ solar cells and modules fabricated on large-area 5.7 m2 substrates.
Experimental
The proprietary AKT plasma-enhanced chemical vapor deposition (PECVD) tool enables film deposition at high rates and with excellent uniformity, as well as high throughput and process flexibility. This has been demonstrated with layer uniformity control of better than ±10% (20 mm edge exclusion) across substrates ranging in size from 0.43 to 5.7 m2 (Fig. 1). The PECVD process was initially developed on the smaller Gen3.5 (0.43 m2) system and then transferred to the larger Gen7.5 (4.4 m2) and Gen8.5 (5.7 m2) systems for the development of mass production processes.
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| 1. The AKT PECVD tool has demonstrated layer uniformity control of better than ±10% (20 mm edge exclusion) across substrates ranging in size from 0.43 to 5.7 m2. |
A wide range of deposition parameters such as pressure, hydrogen dilution, RF power, gas flow and electrode spacing were optimized to deposit stable a-Si:H and homogeneous µc-Si:H films at high deposition rates. The a-Si:H and µc-Si:H absorber layers were deposited at ~200-400 and ~300-600 Å/min, respectively, at temperatures of 200ºC or lower. Because of the significant variations in the properties of commercial TCO glass, appropriate window p-layers were developed. Sputtered TCO/metal stacks of ZnO/Al and ZnO/Ag were used as back reflectors for SJ and TJ cells, respectively, to enhance the light trapping from the back side of device.
The cell areas were defined by using a shadow mask (0.545 cm2) or by laser scribing (0.49 cm2). SJ and TJ modules of dimension 30 × 30 cm were fabricated using laser scribing to effect monolithic series interconnection. The cells and modules were characterized using current-voltage (I-V) testing under simulated sunlight (AM1.5G, 100 mW/cm2, 25ºC) and quantum efficiency (QE) measurements. Light-soaking was performed using metal-halide lamps under 1-sun or 2-sun simulated illumination close to AM1.5G at 50ºC and at open circuit condition.
Results: a-Si:H single junction solar cells
Because the properties of commercial TCO may vary significantly, we developed an ultrathin heavily doped p- type interface layer to improve the TCO/p-layer interface quality. By optimizing the doping profile and thickness of the window p-layer, the cell current was improved while maintaining high FF and Voc. The p++/p+ layer structure provides a wider process window, enabling use of much thinner window p-layer. Consequently, short-circuit current (Jsc) and FF were considerably enhanced with only a small drop in Voc. Nominal SJ cell initial efficiency of >9% was obtained on production floatline TCO.
Figure 2 shows typical initial and stabilized I-V characteristics for an a-Si:H SJ cell deposited at ~220 Å/min. The initial and stabilized efficiencies measured for more than 200 laboratory cells demonstrated that the SJ cell process was reliable and robust, producing initial and stabilized SJ cell efficiencies of 9.2 ±0.4% and 7.3 ±0.4%, respectively. Cells fabricated in a manufacturing line are expected to have a tighter efficiency distribution. The light-induced degradation was ~20%, indicating very good light stability.
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| 2. Light I-V characteristics of high-quality a-Si:H SJ cells deposited on production floatline TCO demonstrated that the SJ cell process was reliable and robust. |
Results: a-Si:H/µc-Si:H tandem-junction solar cells
Current matching between the series-connected top and bottom cells is a critical requirement for two-terminal tandem solar cells to maximize stable efficiency. Optimal matching is realized when the current density of each component cell is equal at the maximum power point for each cell in the light stabilized state. High-quality and stable a-Si:H film was optimized for the top cell of an a-Si:H/µc-Si:H TJ structure with different degrees of current matching by varying the top and bottom cell i-layer thicknesses.
Figure 3 shows the impact of bottom µc-Si:H i-layer (deposited at 410 Å/min) and top a-Si:H i-layer (deposited at 300 Å/min) thickness on tandem cell initial and stabilized efficiencies. The thickness values have been normalized to the top a-Si:H i-layer thickness and bottom µc-Si:H i-layer thickness, respectively. QE measurements indicated that for thick bottom cells (µc-Si:H to a-Si:H thickness ratio >~5.7), the top cells with the i-layer thickness optimized for SJ cells could not deliver enough current to obtain the bottom-limited condition desired for maximum stable tandem cell output power. As shown in Figure 3, tandem cells with thin bottom cells combined with relatively thick top cells (a-Si:H to µc-Si:H thickness ratio >~0.18) are most stable with the highest stabilized efficiency. Such a bottom-limited tandem cell structure produced the highest initial/stabilized efficiencies of 11.5%/9.8% on low-cost floatline TCO glass, and 11.6%/10.1% on high-quality offline TCO (Fig. 4).
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| 3. Tandem cells with thin bottom cells combined with relatively thick top cells (a-Si:H to µc-Si:H thickness ratio >~0.18) are most stable with the highest stabilized efficiency. Thickness numbers have been normalized. |
tandem cells than for SJ cells because the slightly reduced or unaltered tandem cell current overcompensates a drop in FF (Fig. 4 ). Figure 5 shows SJ cell performance for more than 190 laboratory cells on production floatline TCO, demonstrating initial and stabilized efficiencies of 11.2 ±0.4% and 9.7 ±0.3%, respectively. The efficiency distribution in a manufacturing line is expected to be tighter.
Large-area uniformity of solar cells
In addition to the prerequisite layer uniformity over large-area substrates (Fig. 1), the uniformity of solar cell performance parameters is critical for high-efficiency cells. We have successfully developed solar PECVD processes on the AKT PECVD platform for Gen 8.5 (2.2 × 2.6 m) substrate size. Excellent device performance and uniformity have been achieved for both SJ and TJ solar cells over a 5.7 m2 substrate, as demonstrated in Figures 5 and 6, respectively. Better than 3% uniformity was obtained for the SJ cell I-V parameters, and better than 5% uniformity for the tandem cell I-V parameters.
SJ and TJ solar modules
The state-of-the-art quality of a-Si:H and µc-Si:H materials and the excellent uniformity of layer thickness and properties over large-area substrates were further manifested by the fabrication of highly efficient a-Si:H SJ modules and a-Si:H/µc-Si:H TJ modules. After prolonged light-soaking, only ~19% and ~14% light-induced degradation were observed for high-rate deposited SJ and TJ modules, respectively, indicating good light stability (Fig. 7). The SJ and TJ modules were demonstrated with initial/stabilized efficiencies of 8.18%/6.6% and 10.85%/9.3%, respectively.
Summary and conclusions
Applied Materials has successfully developed high-deposition-rate processes for the fabrication of high-efficiency thin-film silicon solar modules over large-area substrates. a-Si:H single junction cells with 9.2 ±0.4% (initial) and 7.3 ±0.4% (stabilized) efficiencies and a-Si:H/µc-Si:H tandem junction cells with 11.2 ±0.4% (initial) and 9.7 ±0.3% (stabilized) efficiencies have been achieved on low-cost, commercially mass-fabricated floatline TCO glass.
The PECVD processes for the absorber layer deposition for both SJ and TJ cells have demonstrated excellent uniformity (<5% variations for cell I-V characteristics) on a substrate size of 2.2 × 2.6 m. a-Si:H SJ modules and a-Si:H/µc-Si:H TJ modules with 6.6% and 9.3% stabilized efficiencies, respectively, have been produced.
Acknowledgements
The authors thank Ankur Kadam, David Tanner, Chris Eberspacher, Tae Kyung Won, Soo Young Choi and John White of Applied Materials’ Solar Business Group for their contributions.
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| 7. After prolonged light-soaking, only ~19% and ~14% light-induced degradation were observed for high-rate deposited SJ and TJ modules, respectively, indicating good light stability. |
| Author Information |
| Shuran Sheng is a senior process engineer with Applied Materials’ Solar Business Group and leads efforts in the development of advanced and industrial-worthy processes for high-efficiency and high-deposition-rate thin-film silicon-based single junction and tandem junction solar cells and large-area PV modules. He has a B.Sc. in physics and M.Sc. in magnetism and magnetic materials and devices, both from Lanzhou University, China, and a Ph.D. in semiconductor physics and devices from the Chinese Academy of Sciences. |
| Yong Kee Chae is a process development manager in Applied Materials’ Solar Business Group, responsible for process optimization and solar cell characterization. He has a Ph.D. in chemical engineering from the University of Tokyo. |
| Liwei Li is a senior process engineer in Applied Materials’ Solar Business Group, where he is engaged in process and hardware development for PV modules. He has a B.S. and M.S. in materials science and engineering from Xi’an Jiaotong University, China, and a Ph.D. in materials science and engineering from the New Jersey Institute of Technology. |
| Jeff Su is a member of technical staff in Applied Materials’ Solar Business Group, working on thin-film solar module processes for the front end, including washer/cleaning, laser scribe, shunt bust and solar simulator. He has a Ph.D. from the University of Wisconsin-Madison. |
| Xiesen Yang is a process engineer with Applied Materials’ Solar Business Group, focused on process and hardware development for the thin-film PV modules. He has a Ph.D. in physics from the University of Toledo. |
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