Heterojunction and Passivating Contacts on Solar Cells

Introduction

To further improve the efficiency, it is necessary to reduce the electrical losses at the surfaces and specifically the contact between the semiconductor and the metallisation. Current technology has pushed this to their limit and new disruptive solutions are now required. As a result, carrier selective or passivating contacts have recently attracted significant interest in the photovoltaic scientific community [1]. These passivating or carrier-selective contacts by design exhibit extremely low recombination currents for minority carriers (i.e. passivation) while having a low resistivity for majority carriers (i.e., carrier selectivity). Silicon heterojunction (HJ) solar cells are one such passivated contact cell.

HJ cells are typically formed with an n-type bulk between intrinsic amorphous silicon (a-Si) layers. The passivating contacts are then completed by a p-type doped a-Si layer for the hole contact and an n-type doped a-Si layer for the electron contact. As the lateral conductivity of the doped a-Si layers is relatively poor, a transparent conductive oxide (TCO) layer is grown on top of both doped a-Si films. This TCO layer has a similar refractive index and thickness as a silicon nitride layer so it can simultaneously serve as antireflection coating.  All these layers are less than 100 nm thick and affect the optical properties of the solar cell.

Learning Objectives

• Understand the differences between heterojunction and standard Al-BSF cells
• Be able to perform experiments to observe differences between heterojunction and standard Al-BSF cells
• Be able to perform a main factor response to identify the most important thin film layer to optimise
• Be able to perform a single factor response to optimise the most important thin film layer
• Understand the function of the thin film layers in HJT silicon solar cells

Tutorial Exercise

The focus of this tutorial is to observe the optical losses in the thin film layers of the heterojunction cell provided in the default SunSolve template. Please review the relevant PVmanufacturing.org pages prior to attempting this tutorial to have a better understanding of the context of the exercises. You will be using the c-Si SSP Cell and c-Si HJ Cell SunSolve default templates, describing and relating their differences in performance to their differences in structure. You will then investigate the bifaciality of the HJ cell and then attempt to optimise the optical losses in the HJ cell, prioritising the thin film layers.

REMINDER

Make sure to save and organise any templates/simulations as you proceed throughout this tutorial, any unsaved progress will be lost if the SunSolve page is closed/changed/refreshed.

Part One – Comparing Default Cell Performance

You will be conducting a simple experiment to observe key differences in cell performance between a HJ cell and an Al-BSF cell under standard testing conditions. The aim is to identify the reasons behind these differences.

The responses you will be observing include:

Table 1 - List of responses to be observed in the comparison experiment

Response	Unit
Front reflection photon current density (JR,Front)	mA/cm2
Front escape photon current density (JE,Front)	mA/cm2
Rear escape photon current density (JE,Rear)	mA/cm2
Total rear metal series resistance (RS,Grid)	Ωcm2
Short circuit current density (Jsc)	mA/cm2

Conducting the Experiment

1. Open a new simulation using the c-Si SSP Cell template and run without changing any settings, record the responses outlined in Table 1 above
2. Open a second simulation using the c-Si HJ Cell template, take note of the layer differences and run without changing any settings, record the responses outlined in Table 1
3. Tabulate the results from Steps 1 and 2, highlighting the differences observed
4. Make sure to save your simulations, any unsaved data will be lost once SunSolve is closed

Questions

1. Why is there no ‘rear escape’ current observed for the Mono-Facial cell?
2. Which cell shows a lower series resistance of rear metals? What material is used for this to occur?
3. Investigate other sources of photon current losses in the different layers of the HJ cell. Is there an indication of which layers could be optimised to increase the performance of the Cell? (Hint: ‘Fraction of J_inc’ is a good indicator)

Part Two – Rear Performance

The HJ cell is bifacial in nature, therefore, you will investigate the rear performance of the cell. You will also compare the rear performance to the front. To save on rays, you can run a single simulation with rear illumination and compare the results to your HJ cell simulation in Part One. For bifacial PV modules, a bifacial gains plot can be used to estimate its performance in the field, therefore, measuring the bifaciality of the module is an important aspect of bifacial PV characterisation.

The responses you will be observing include:

Table 2 - List of responses to be observed in the bifacial experiment

Response	Unit
Rear reflection photon current density (JR,Rear)	mA/cm2
Front escape photon current density (JE,Front)	mA/cm2
Rear escape photon current density (JE,Rear)	mA/cm2
Short circuit current density (Jsc)	mA/cm2
Bifaciality coefficient	(calculate)

Conducting the Experiment

1. Open a new simulation using the c-Si HJ Cell template and under the illumination tab, apply a zenith angle of 180º. Run without changing any other settings
2. Record the responses outlined in Table 3 above
3. Using the J_sc results of this simulation and the front illumination simulation of the HJ cell in Part One, calculate the short circuit current bifaciality coefficient:

φ_Isc = I_sc,Rear / I_sc,Front

Questions

1. Compare the reflectance and escape photon current densities of the illuminated surfaces, which simuation had better results?
2. How might bifaciality be used to predict the performance of bifacial PV in the field?

Part Three – Main Factor Response

In Part One, you should have observed considerable parasitic absorption in the films surrounding the silicon substrate. In this section, you will be conducting a main factor response experiment to determine the most important film to optimise on the front of the HJ cell. This experiment still utilises front illumination only, therefore, the factors in question are the thicknesses of the front amorphous-Si layers and TCO layer. The default HJ solar cell uses an indium tin oxide (ITO) layer as its TCO. Furthermore, under Inputs -> Options, you can tick the box for “Absorption in each film” to observe the different amounts of parasitic absorption occurring in each film layer. However, the main factor response experiment will only require recording the total parasitic absorption.

The factor settings are listed in Table 3 below.

As described in previous tutorials, “-“, “0” and “+” are used to indicate “a lower setting”, “the baseline setting” (or “default”) and “a higher setting” for the factors, respectively. The actual values for these simulation settings are provided in Table 3 above.

The responses that you will be observing for the main factor response experiment:

Table 4 - List of responses to be observed in the main factor response experiment

Response	Unit
Front reflection photon current density (JR,Front)	mA/cm2
Parasitic absorption at the ‘Solar Cell front (non-contact Interface)’ photon current density	mA/cm2
Short circuit current density (Jsc)	mA/cm2

Conducting the Experiment

1. Open a new c-Si HJ Cell Template and using the sweep function, create a single simulation that will run the main factor response experiment. Use the run summary to accurately set up the experiment
2. Produce main factor response curves for all 3 responses
3. Identify the factor that is most important to optimise depending on its effect on J_sc
4. Make sure to save your simulation, any unsaved data will be lost once SunSolve is closed

Questions

1. Which factor is the most important to optimise?
2. Why is short circuit current density (J_sc) the easiest response to compare when identifying the most important factor?
3. Does this main factor response experiment also identify which layer is most responsible for the parasitic absorption observed in the ‘front (non-contact Interface)’ layer?
4. Does the main factor response experiment indicate any optimum values for the thin film layers?
5. SunSolve allows the thin film layers to be extremely thin (<2 nm), what issue may occur if such a thin film is used?

Part Four – Single Factor Response

After identifying the most important factor to optimise in part two, a single factor response experiment is used to find the optimum value for that factor. You will be conducting a single factor response experiment, optimising your factor identified in the previous part. Make sure to record the same responses as in Part two.

Conducting the Experiment

1. Make sure to use the default settings (“0” setting) for other factor(s)
2. Using the sweep function again, run the simulation with at least 8 steps starting from the “-“ and ending at the “+” setting of your factor. SunSolve can create equal intervals automatically
3. Record the 3 responses for each run of the simulation
4. Sketch an X-Y scatter plot for each response (y-axis) versus your factor of interest (x-axis).
5. Describe the relationships between your factor and each of the responses
6. Identify and record the optimum value for your factor. Create a new template with this optimised value
7. Save your simulations as any unsaved data will be lost once SunSolve is closed

Part Five – Understanding HJ and Passivated Contact Cells

The tutors will ask you the following questions on the tutorial. Make sure that you understand the processes and prepare your answers while completing the above tasks

1. The a-Si layers provide surface passivation for the HJ solar cell, what aspect of the thin layers allows this to occur?
2. The default HJ cell template in SunSolve uses a thicker ITO thin film layer on the rear. Why might this be the case? (Hint: the silicon substrate is thinner than in other silicon solar cell design)
3. Screen-printing and cofiring of solar cells require temperatures >800ºC, why might this not be the case for HJ silicon solar cells. How is this issue overcome?
4. The HJ cell used in this tutorial is bifacial in nature, describe any improved PV manufacturing methodologies that can utilise these cells.
5. How do carrier selective (or passivating) contacts differ from diffused contacts. Please explain with a band diagram.

References

[1] – Cuevas, A., T. Allen, J. Bullock, Y. Wan, D. Yan, and X. Zhang, Skin Care for Healthy Silicon Solar Cells, in 42nd IEEE PVSC. 2015. p. 10.1109/PVSC.2015.7356379.