Research

Fundamental and Practical Limits of Heterojunction Solar Cells

PhD Committee: Prof. M A. Alam, Prof. J. L. Gray, Prof. M. S. Lundstrom, Prof. D. Janes

A brief description of my thesis is presented here.

Thesis Title: Process to Panel Modeling and Multi-probe Characterization of Silicon Heterojunction Solar Cells

The large-scale deployment of PV technology is very sensitive to the material and process costs. There are several potential candidates of p-n heterojunction (HJ) solar cells competing for higher efficiencies with lower material and process costs. However, these systems are generally more complex, involve diverse material systems and are not as well understood. Often, the direct translation of classical p-n homojunction theory to understand a p-n HJ system, causes misinterpretation, suffers from lack of self-consistency and ultimately renders it useless for modeling and characterization of these solar cells. Hence, there is a strong need to redefine/reinterpret the modeling and characterization of HJ solar cells, which would lead to a material system independent and self-consistent framework. Towards this goal, we explore the physics, interpret characterization experiments of p-n HJs, using Silicon HJ (HIT) cell as an example. The list of accomplished tasks is as follows.

1. Multi-probe characterization of Silicon HJ solar cells

Here identify the device bottlenecks for these cells and explain their role on the dark and light I-V characteristics in a correlated manner. In this work, we provide a self consistent characterization approach for identification of key device parameters using capacitance studies using frequency, temperature and applied bias as the probes.

[J1] R. V. K. Chavali, J. R. Wilcox, B. Ray, J. L. Gray, and M. A. Alam, IEEE J. Photovoltaics, vol. 4, no. 3, pp. 763 – 771, 2014.

[J2] R. V. K. Chavali, S. Khatavkar, C. V Kannan, V. Kumar, J. L. Gray, and M. A. Alam, IEEE J. Photovoltaics, 2015.

[C1] R. V. K. Chavali, J. R. Wilcox, B. Ray, J. L. Gray, and M. A. Alam,in 2013 IEEE 39th Photovoltaic Specialists Conference (PVSC), 2013, vol. 1, pp. 0652–0657.

[C2] R. V. K. Chavali, S. Khatavkar, B. M. Arora, P. R. Nair, J. L. Gray, and M. A. Alam, in 2014 40th IEEE Photovoltaic Specialists Conference, 2014. (Best Student Paper Finalist)

2. Frozen potential approach to separate diode and photo currents

In this work, we develop a novel numerical method to calculate the exact photo current and diode injection current in any classical solar cell. This method is useful for all solar cells when the principle of superposition fails. (click on figure to enlarge)

[J1] R. V. K. Chavali, J. Moore, X. Wang, M. Alam, M. Lundstrom, and J. Gray,” IEEE J. Photovoltaics, 2015.

[J2] J. E. Moore, S. Dongaonkar, R. V. K. Chavali, M. A. Alam, and M. S. Lundstrom, IEEE J. Photovoltaics, vol. 4, no. 4, pp. 1138–1148, Jul. 2014.

[W3] J. L. Gray, X. Wang, R. V. K. Chavali, X. Sun, A. Kanti, and J. R. Wilcox, “ADEPT 2.1,” Mar-2014. [Online].

[C1] R. V. K. Chavali and J. L. Gray, in 2014 IEEE 40th Photovoltaic Specialist Conference (PVSC), 2014, pp. 0238–0242. (Best Poster Award)

3. Process-to-Panel Modeling of Silicon HJ solar cells to Investigate the Cell-to-Panel Efficiency Gap

The cell-to-panel efficiency gap observed in a-Si/c-Si heterojunction solar cells is one of the key challenges of this technology. To systematically address this issue, we describe an end-to-end modeling framework to explore the implications of process and device variations at the module level. First, a process model is developed to connect the a-Si deposition parameters to the material properties. Next, a physics based device model is presented which captures the essential features of photo-current and diode injection current using the thermionic-diffusion theory. Using the process and device models, the effects of process conditions on cell performance are explored. Finally, the performance of the panel as a function of device and process parameters is explored to establish panel limits. The insights developed through this process-to-panel modeling framework will improve the understanding of the cell-to-panel efficiency gap of this commercially promising cell technology.

[J1] R. V. K. Chavali, E. C. Johlin, J. L. Gray, T. Buonassisi and M. A. Alam, IEEE J. Photovoltaics, 2016

[C1] R. V. K. Chavali, E. C. Johlin, J. L. Gray, T. Buonassisi and M. A. Alam, IEEE 42nd Photovoltaic Specialist Conference (PVSC), 2015, (Accepted). (Best Student Paper Finalist)

4. Bifacial Si Heterojunction-Perovskite Organic-Inorganic Tandem to Produce Highly Efficient Solar Cell

In this paper, we have explored the performance potential of Perovskite-HIT tandem cell based on state-of-the-art sub-cells. We find that a traditional tandem design requires an optimized Perovskite thickness of to provide a modest 25% efficient cell. Unfortunately, the efficiency gain is compromised due to sensitivity to and both the cells under-performs compared to their individual efficiencies. Through a novel bifacial tandem design one can resolve the current matching problem and improve the performance to 33%. As an added advantage, we observe that this efficiency gain is insensitive to thicknesses of Perovskite and c-Si layers. Further, it out-performs the bifacial HIT cells over a practical range of albedo reflection. Therefore, it offers a viable, robust HIT-Perovskite tandem for low-cost, highly-efficient PV technology.

[J1] R. Asadpour*, R. V. K. Chavali*, M. R. Khan*, M. A. Alam, Appl. Phys. Lett. (2014), vol 106, 24.

(* equal contribution)

5. Generalized Theory to Analyze Suns-Voc Technique in HJ solar cells

This study suggests that the Suns-Voc technique, when appropriately interpreted, can be used to obtain a broad range of information about the solar cells. For example,

1. The HJ theory presented here is consistent with our earlier work on dark and light J-V, impedance spectroscopy and C-V analysis of SHJ solar cells. Therefore, the Suns-Voc technique can be used, in combination with other techniques, to extract device parameters, such as the HJ band offset (Δ𝐸_𝑉), surface potential (𝜙_𝑁) etc., parameters that are not yet routinely accessed in a device-fabrication laboratory environment.

2. We study the Suns-Photovoltage curves as a generalization of Suns-Voc measurement at various bias conditions to help analyze the above features in a systematic and consistent manner. Finally, we validate our HJ hypothesis using experiments on two HJ device types, namely, a ‘standard’ high-efficiency SHJ and a substoichiometric molybdenum oxide (MoOx) based hole collector HJ device.

[J1] R. V. K. Chavali, J. V. Li, C. Battaglia, S. De Wolf, J. L. Gray, M. A. Alam, IEEE J. Photovoltaics, 2016

[C1] R. V. K. Chavali, J. V. Li, C. Battaglia, S. De Wolf, J. L. Gray, M. A. Alam, PVSC, 2016

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