Design and fabrication of a SiOx/ITO double-layer anti-reflective coating for heterojunction silicon solar cells

doi:10.1016/j.solmat.2013.05.044

Solar Energy Materials and Solar Cells

Volume 117, October 2013, Pages 132-138

https://doi.org/10.1016/j.solmat.2013.05.044 Get rights and content

Highlights

•
Use ASA software to precisely simulate, analyze and design optics of HIT cells.
•
J_sc is mainly dominated by the c-Si wafer absorbance.
•
Maximizing c-Si absorbance more relevant than minimizing reflectance for AR design.
•
Fabricated designed HIT cell with J_sc of 40.5 mA/cm² and an efficiency of 19.0%.

Abstract

In this contribution optical simulations of both flat and textured heterojunction silicon solar cells are presented and verified experimentally. Using Advanced Semiconductor Analysis (ASA) software, we optimize a double-layer anti-reflective (AR) coating, which has an additional SiO_x film on the top of the existing indium tin oxide (ITO) coating. Our approach is based on maximizing the absorbance of the crystalline silicon (c-Si) wafer, which is strongly correlated with the solar cell′s short circuit current (J_sc). Our simulations show that for a flat heterojunction silicon solar cell c-Si absorbance can increase by using a double-layer AR coating instead of a single-layer AR coating. As predicted by the simulations, experimental devices show corresponding J_sc increase, leading to the increase of the solar cell efficiency. On a textured heterojunction silicon solar cell the incident light travels an oblique path through the AR coating and we use an advanced ray-tracing model to optimize the single and double-layer AR coating for this case. Our simulations show that for the textured heterojunction silicon solar cell, reflection losses are lower but parasitic absorption losses in the ITO and amorphous silicon layers play a more important role. Using a double-layer AR coating not only reduces reflection losses further, but because a thinner ITO layer can be used it also reduces parasitic absorption losses. Experimentally, our textured heterojunction silicon solar cell with a double-layer AR coating shows that the J_sc (active area) of 40.5 mA/cm² and an efficiency of 19.0%.

Introduction

The heterojunction silicon solar cell is attracting more and more attention in both photovoltaic research and industry. The reason is that the heterojunction silicon solar cell has high efficiency [1], [2], lower thermal budget, shorter processing time, and potentially lower processing cost compared to the conventional diffused homojunction crystalline silicon (c-Si) solar cell [3]. As is the case for homojunction silicon solar cells, there are also optical losses such as reflection losses and metallic shading losses in heterojunction cells. The high refractive index of the hydrogenated amorphous silicon (a-Si:H) emitter in the heterojunction cell and the c-Si emitter in the homojunction cell results in high reflection losses (>40% in the wavelength range from 300 to 1200 nm). Therefore, anti-reflection measures are very important for reducing reflection losses and increasing the light absorption in c-Si. In the case of homojunction silicon solar cells, silicon nitride is normally used as anti-reflective (AR) coating. For heterojunction silicon solar cells, the transparent indium tin oxide (ITO) front contact layer is also designed for this AR purpose.

For the heterojunction silicon solar cell, ITO can absorb light in the ultraviolet regime due to its large band gap as well as in the infrared regime due to its free carrier absorption. In addition the high absorption coefficient of a-Si:H leads to high absorption in these thin layers at wavelengths shorter than 700 nm, further contributing to the parasitic absorption losses [4]. These absorption losses can reduce the benefits gained from the AR coating, making the design of an AR coating more complicated for a c-Si homojunction cell. In 2012, Holman et al. showed that how ITO, and p-type and intrinsic a-Si:H layers in the front side of the heterojunction silicon solar cell contribute to the current losses by comparing EQE and simulated absorption in c-Si of different solar-cell structures at wavelengths from 300 to 600 nm [5]. They concluded that no light was absorbed in ITO and p-type a-Si:H contributes to the photocurrent, whereas 20–50% of the light absorbed in intrinsic a-Si:H can contribute to this current. In this contribution, we present an accurate optical analysis of heterojunction silicon solar cells in the wavelength range from 300 nm to 1200 nm. Based on our optical analysis, we show that for AR coating design the short-circuit current density (J_sc) can be maximized by optical simulations alone, provided that the c-Si wafer absorbance is maximized since this absorbance dominates the J_sc. We show that this procedure is more accurate than simply minimizing the reflectance due to the influence of parasitic absorption of front layers. In this contribution, we illustrate this approach for designing a double-layer AR coating of both a flat and textured heterojunction silicon solar cell. In order to validate our model, solar cells with the optimized AR coating are fabricated.

Section snippets

Experimental

The structure and layer thicknesses of the heterojunction silicon solar cells with single and double layer AR coating that we process in our facility are shown in Fig. 1. The same structure is also used for simulations using ASA computer software [6], [7]. The area of the solar cells is 0.16 cm². For the solar cells, n-type c-Si (111) FZ wafers are used with a thickness of about 280 µm and selected from a batch with resistivity between 2 and 5 Ω cm. On the back side of the c-Si wafer, a 9 nm thick

Results and discussion

In order to compare the effectiveness of an AR coating on a solar cell, it is important to take the AM 1.5 solar spectrum into account. For this investigation we have chosen the wavelength range from 300 nm to 1200 nm of the solar spectrum. The reason is that for wavelengths shorter than 300 nm, the spectral power density in the AM 1.5 spectrum is almost zero, while photons with wavelengths longer than 1200 nm are hardly absorbed by the c-Si. In this work, we define the weighted average reflectance

Conclusion

In this contribution, we present optical simulations for flat and textured heterojunction silicon solar cells with our ASA software. Our simulations show excellent agreement with reflectance measurements on real devices, indicating the accuracy of the model used. We have shown that optical optimization of textured heterojunction solar cells needs to be carried out especially in relation to the ${\bar{A}}_{c - Si}$ and not in relation to the $\bar{R}$ since parasitic absorption plays an important role. Based on this

Acknowledgments

The work is funded by Energy Research Centre of The Netherlands. We would like to thank M. Tijssen and S. Heirman for their technical support. We also would like to thank H. Tan, J. Meerwijk, D. Deligiannis and other colleagues for their helpful discussion.

References (15)

T. Yagi et al.
Ray-trace simulation of light trapping in silicon solar cell with texture structures
Solar Energy Materials and Solar Cells
(2006)
R. Santbergen et al.
The absorption factor of crystalline silicon PV cells: a numerical and experimental study
Solar Energy Materials and Solar Cells
(2008)
T. Kinoshita, D. Fujishima, A. Yano, A. Ogane, S. Tohoda, K. Matsuyama, Y. Nakamura, N. Tokuoka, H. Kanno, H. Sakata,...
A. Descoeudres et al.
>21% efficient silicon heterojunction solar cells on n- and p-type wafers compared
IEEE Journal of Photovoltaics
(2013)
S. De Wolf et al.
High-efficiency silicon heterojunction solar cells: a review
Green
(2012)
E. Maruyama, A. Terakawa, M. Taguchi, Y. Yoshimine, D. Ide, T. Baba, M. Shima, H. Sakata, M. Tanaka, Sanyo′s challenges...
Z.C. Holman et al.
Current losses at the front of silicon heterojunction solar cells
IEEE Journal of Photovoltaics
(2012)

There are more references available in the full text version of this article.

Cited by (80)

Feasibility test of drastic indium cut down in SHJ solar cells and modules using ultra-thin ITO layers
2023, Solar Energy Materials and Solar Cells
In this work, we investigate the possibility to drastically decrease the indium (In) consumption in silicon heterojunction (SHJ) solar cells, using ultrathin (<15 nm) ITO layers on both cell sides, in combination with SiN:H capping dielectric layers. The best ITO/dielectric combinations were assessed using optical simulations. The optimal stacks were integrated on front, rear, and both sides of complete SHJ cells. Two types of nanocrystalline layers (nc-Si:H and nc-SiO_x:H) were implemented as selective layers on the front side, and proved to relax the constraints on the front ITO conductivity. Among all conditions including 100 nm references, the highest module efficiencies were obtained with 15 nm front ITO (22.4%), while modules with ITO layers below 10 nm on nc-Si:H showed very low efficiency losses compared to the reference. Integration of such ultrathin ITO layers on the rear side proved to be non-optimal since the Fill Factor (FF) losses are not counterbalanced with current gains (contrary to what is observed on the front side with ultra-thin ITO). Finally, UV reliability tests were performed and showed an enhanced reliability for modules with thinner ITO layers. After 60 kW h.m⁻² of UV exposure, modules with 5 nm ITO on nc-Si:H showed the best efficiencies among all tested conditions. Subsequent damp heat tests were also performed and did not show a clear warning against using ultrathin ITOs on the front side, and in the end, modules with nc-Si:H layers feature the highest efficiencies at the end of the reliability sequence.
Strategies for realizing high-efficiency silicon heterojunction solar cells
2023, Solar Energy Materials and Solar Cells
Silicon heterojunction (SHJ) solar cells have achieved a record efficiency of 26.81% in a front/back-contacted (FBC) configuration. Moreover, thanks to their advantageous high V_OC and good infrared response, SHJ solar cells can be further combined with wide bandgap perovskite cells forming tandem devices to enable efficiencies well above 33%. In this study, we present strategies to realize high-efficiency SHJ solar cells through combined theoretical and experimental studies, starting from the optimization of Si-based thin-film layers to the implementation of electrodes with reduced indium and silver usage. Advanced opto-electrical simulations, which enable comprehensive theoretical understandings of the main physical mechanisms governing carriers’ collection and light management, provide clear pathways for device designs and experimental optimizations. We present the fabricated FBC-SHJ solar cells in both monofacial and bifacial configurations with the best efficiencies of 24.18% and 23.25%, respectively. We point out that to achieve optimum device performance, the compositional materials should be holistically optimized and evaluated as part of the contact stacks with adjacent layers. As an outlook beyond the classical FBC-SHJ solar cell architecture, we propose various novel SHJ-based solar cell architectures. Their potential performance was assessed and compared via rigorous opto-electrical simulations and a maximal efficiency of 27.60% was simulated for FBC-SHJ solar cells featuring localized contacts.
Influence of Al<inf>2</inf>O<inf>3</inf>/IZO double-layer antireflective coating on the front side of rear emitter silicon heterojunction solar cell
2022, Vacuum
Transparent conductive oxide (TCO) thin films play a significant role in silicon heterojunction (SHJ) solar cell and perform both as antireflection coating (ARC) and carrier transport layer. Indium zinc oxide (IZO) films have attracted much attention in opto-electronic industry as an alternative TCO material owing to their superior optical and electrical characteristics as compared to conventionally used indium tin oxide (ITO) films. The optical and electrical properties of IZO films were studied and compared with ITO films. Moreover, the dielectric layer of Al₂O₃ was deposited on the IZO films using an atomic layer deposition (ALD) system to prepare a double layer antireflection coating (DLARC) structure. The wafer ray tracer simulation tool was used to optimize the thickness of Al₂O₃/IZO DLARC for SHJ solar cells and experimental results showed its validation. The performance of Al₂O₃/IZO DLARC on the front surface of rear emitter SHJ solar cell recorded to increase in the current density of 1.37% with enhancement in the efficiency of 0.8%, showing noticeable improvement as compared to cell fabricated with single-layer IZO film.
Optoelectrical analysis of TCO+Silicon oxide double layers at the front and rear side of silicon heterojunction solar cells
2022, Solar Energy Materials and Solar Cells
Silicon Heterojunction has become a promising technology to substitute passivated emitter and rear contact (PERC) solar cells in pursuance of lower levelized cost of electricity through high efficiency devices. While high open circuit voltages and fill factors are reached, current loss related to the front and rear contacts, such as the transparent conductive oxide (TCO) layers is still a limiting factor to come closer to the efficiency limit of silicon based solar cells. Furthermore, reducing indium consumption for the TCO has become mandatory to push silicon heterojunction technology towards a terawatt scale production due to material scarcity and costs. To address these issues dielectric layers, such as silicon dioxide or nitride cappings are implemented to reduce TCO thicknesses both diminishing parasitic absorption and material consumption. However, reducing the TCO thickness comes in cost of resistive losses. Furthermore, the TCO properties do vary with thickness and neighboring layer configuration altering the optimization frame of the device. In this paper we present a detailed analysis to quantify the optoelectrical losses trade-off associated to the TCO thickness reduction in such layer stacks. Through the analysis we show and explain why experimental bifacial cells with 20 nm front and rear TCO perform at a similar level to reference cells with 75 nm under front and rear illumination reaching efficiency close to 24% at 92% bifaciality. We present as well a simple interconnection method via screen printing metallization to implement a thin TCO/silicon dioxide/silver reflector enhancing current density from 39.6 to 40.4 mA/cm² without compromising resistive losses resulting in a 0.2% absolute solar cell efficiency increase from a bifacial design (23.5–23.7%). Finally, following this approach we present a certified champion cell with an efficiency of 24.6%.
Improved optical and electrical properties for heterojunction solar cell using Al<inf>2</inf>O<inf>3</inf>/ITO double-layer anti-reflective coating
2021, Results in Physics
Citation Excerpt :
However, cells made with a thin wafer has low absorption efficiency in the red and near-infrared portions of the solar spectrum resulting in a lower Jsc. The hydrogenated a-Si (a-Si:H) emitter layer causes excessive reflection losses in SHJ solar cells owing to their high refractive index [11]. Therefore, lowering the optical losses would improve the absorption properties of a solar cell, which is the crucial aspect for attaining high efficiency.
Silicon heterojunction solar cells have been gaining remarkable attention in the photovoltaic industry in recent years owing to their low temperature coefficient and high efficiency. This study aimed to maximize the short circuit current density (J_sc), which is directly correlated with the absorbance of the solar cells. An advanced ray tracking model and hall effect measurement was used to improve the optical properties of Al₂O₃/ITO as a double-layered anti-reflection coating (DLARC) on the solar cell. RF/DC power sputtering system was used to deposit ITO layer, while atomic layer deposition was used to deposit Al₂O₃ on ITO to create a DLARC. An average decrease in reflection from 9.33% to 4.74% and enhancement in EQE from 76.89% to 84.34% were observed for the DLARC in the wavelength spectrum at 300–1100 nm. It also exhibited a higher J_sc value of 41.13 mA/cm² and maximum conversion efficiency of 21.6%. The findings of both simulation and experiments showed that the Al₂O₃/ITO DLARC has better anti-reflection properties than a single-layer ITO coating.
Room-temperature sputtered tungsten-doped indium oxide for improved current in silicon heterojunction solar cells
2021, Solar Energy Materials and Solar Cells
The window layers limit the performance of silicon heterojunction (SHJ) solar cells with front and back contacts. Here, we optimized tungsten-doped indium oxide (IWO) film deposited by radio frequency magnetron sputtering at room temperature. The opto-electrical properties of the IWO were manipulated when deposited on top of thin-film silicon layers. The optimal IWO on glass shows carrier density and mobility of 2.1 × 10²⁰ cm⁻³ and 34 cm² V⁻¹s⁻¹, respectively, which were tuned to 2.0 × 10²⁰ cm⁻³ and 47 cm² V⁻¹s⁻¹, as well as 1.9 × 10²⁰ cm⁻³ and 42 cm² V⁻¹s⁻¹, after treated on i/n/glass and i/p/glass substrates, respectively. Using the more realistic TCO data that were obtained on thin-film silicon stacks, optical simulation indicates a promising visible-to-near-infrared optical response in IWO-based SHJ device structure, which was demonstrated in fabricated devices. Additionally, by adding an additional magnesium fluoride layer on device, the champion IWO-based SHJ device showed an active area cell efficiency of 22.92%, which is an absolute 0.98% efficiency gain compared to the ITO counterpart, mainly due to its current gain of 1.48 mA/cm².

View all citing articles on Scopus

View full text

Design and fabrication of a SiOx/ITO double-layer anti-reflective coating for heterojunction silicon solar cells

Highlights

Abstract

Introduction

Section snippets

Experimental

Results and discussion

Conclusion

Acknowledgments

Solar Energy Materials and Solar Cells

Solar Energy Materials and Solar Cells

>21% efficient silicon heterojunction solar cells on n- and p-type wafers compared

IEEE Journal of Photovoltaics

High-efficiency silicon heterojunction solar cells: a review

Green

Current losses at the front of silicon heterojunction solar cells

IEEE Journal of Photovoltaics

Design and fabrication of a SiO_x/ITO double-layer anti-reflective coating for heterojunction silicon solar cells