Elsevier

Solar Energy

Volume 214, 15 January 2021, Pages 319-325
Solar Energy

High-efficiency CdTe-based thin-film solar cells with unltrathin CdS:O window layer and processes with post annealing

https://doi.org/10.1016/j.solener.2020.12.008Get rights and content

Highlights

  • ā€¢

    A post annealing process is developed and applied to CdTe solar cells with thinner CdS:O.

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    Carrier concentration (NCV), built-in potential (Vbi) are increased, and back contact barrier (Ī¦b) is decreased.

  • ā€¢

    Open circuit voltage (Voc) and fill factor (FF) of cells are improved.

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    The Cu diffusion and redistribution are studied and partially responsible for the improvement.

Abstract

The thickness of the CdS:O window layer plays an important role in CdTe-based thin-film solar cell performance. Normally with thinner CdS:O layer, the cellsā€™ short wavelength response can be enhanced to achieve higher short-circuit current density (Jsc), while the open-circuit voltage (Voc) and fill factor (FF) would degrade. Previous research finds that reducing the CdS:O layer thickness to less than 40Ā nm would decrease the cellsā€™ efficiency (Eff.). In this work, 18% efficiency CdTe thin-film solar cell with only 30Ā nm CdS:O window layer is demonstrated. A post annealing process is developed to fabricate high efficiency cells with ultrathin CdS:O layers. Our results indicate that the high defect density in CdTe absorber, low built-in potential (Vbi) and high back contact barrier (Ī¦b) are partially responsible for the reduced Voc and FF of cells with ultrathin CdS:O layers. The post annealing process can increase the carrier concentration (NCV) and Vbi, and decrease the Ī¦b, in which way improves Voc and FF for higher cell efficiency.

Introduction

One of the challenges in CdTe-based thin-film solar cell development is minimizing the absorption loss from the CdS:O window layer [Wu, 2004]. Researchers reported that adopting MgZnO or CdSe window layers can improve the short wavelength response of the solar cells [Kephart et al., 2016, Ablekim et al., 2020]. The problem is that, the evaporation process of CdSe is not suitable for production, while the MgZnO window layer is highly sensitive to ambient moisture and oxygen influence [Bittau et al., 2019, Li et al., 2019], which would make the high efficiency process more challenging and complicated, and also brings potential stability issue. Indeed, low efficiency MgZnO/CdTe cells are reported [Delahoy et al., 2017, Bittau et al., 2018, Artegiani et al., 2019], the FF remains low even though the oxygen is eliminated from the cell process [Ablekim et al., 2019]. It is also arguable that eliminating oxygen deteriorates the CdTe absorber quality and thus the cell performance is low. The CdS:O layer can prevent the MgZnO from the influence of H2O and O2, and allow for the oxygen introducing into cell process, this promotes the CdTe deposition and cell preparation. Therefore, the process of MgZnO/CdS:O/CdTe solar cell is more suitable for the large scale production. High Jsc of 25.8Ā mA/cm2 and Eff. of 16.2% have been achieved by the MgZnO/CdS:O/CdTe solar cell [Menossi et al., 2017]. However, the short wavelength response is very low (less than30% at 500Ā nm). In mass production, the most straightforward solution to improve the short wavelength response is reducing the CdS:O window layer thickness. However, reducing the CdS:O thickness to lower than 40Ā nm will cause significant cell efficiency loss [Wang et al., 2018].

In this study, we developed one post annealing process to further reduce the CdS:O thickness to 30Ā nm and still achieved 18% efficiency cell performance. The underlying mechanism of the post annealing process on cells with ultrathin CdS:O window layers was discussed.

Section snippets

Experimental

SnO2:F/glass (FTO) was used as substrate for device fabrication. A 20Ā ~Ā 40Ā nm MgZnO buffer layer and a 0Ā ~Ā 60Ā nm CdS:O window layer were sputtered sequentially on the FTO glass at room temperature, from the Mg0.23Zn0.77O and CdS targets, respectively. For MgZnO sputtering, the RF power was 300Ā W, the pressure was 10 mTorr, and the ambient was O2/Ar mixtures in a O2/(O2Ā +Ā Ar) ratio of 24%. For CdS:O sputtering, the RF power was 100Ā W, the pressure was 15 mTorr, and the ambient was O2/Ar mixtures

Results and discussion

The J-V characteristics of CdTe solar cells versus CdS:O thickness are presented in Fig. 1. Each box represents 4 cells and these 4 cells are from two substrates. The device fabrication process is optimized respectively for each CdS:O thickness. The Jsc increases with the thickness of CdS:O window decreased from 50Ā nm to 30Ā nm, as shown in Fig. 1(a), which can be attributed to improved short wavelength response. Further reducing the CdS:O thickness will drop the Jsc. When the CdS:O thickness

Conclusion

The performance of CdTe based thin-film solar cells adopting the MgZnO/CdS:O is always degraded when the CdS:O thickness reduced to below 40Ā nm. The performance loss with thinner CdS:O partially comes from the higher defects in the CdTe absorber, lower Vbi and higher back contact barrier. A post annealing process is developed in this work which improves the NCV and Vbi, and reduces the Ī¦b of cells with thinner CdS:O. As a result, the Voc and FF of the cells are improved. By optimizing the cell

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

We would like to thank Manyu Zhang, Haibing Ying and Xiaowei Hou for the help of cell preparation and I-V and EQE measurements, thank Yue Wu, Weiwei Wang, Dan Liu and the R&D and production groups for the technical support at this work, thank Mr. Ben Wu and Advanced Solar Power for financial support, thank Dr. Chaowei Xue for the text calibration. Some of the characterization work was also supported by the National Nature Science Foundation of China (Grant no. 61721005).

Xuanzhi Wu, the chairman of Advanced Solar Power (Hangzhou) Inc. He has been engaged in the research and development on solar cells for more than 40 years.

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    Xuanzhi Wu, the chairman of Advanced Solar Power (Hangzhou) Inc. He has been engaged in the research and development on solar cells for more than 40 years.

    Deren Yang is dean of the Faculty of Engineering, and director of the Institute for Semiconductor Materials at Zhejiang University. He has been engaged in the research on silicon materials for microelectronic devices, solar cells and nano-devices.

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