IBC solar cells have the potential to achieve high efficiency by eliminating front-side metallization, reducing shadow loss, and increasing light absorption area, but are complex and expensive to manufacture. In this paper, IBC solar cells are investigated using Quokka3 simulations, focusing on the effects of front and back surface passivation and thin layer resistance on cell performance.

Passivation is critical for optimizing IBC solar cell performance, with front surface passivation reducing surface complexation losses and back surface passivation reducing complexation in the metal contact and diffusion regions; doping concentration affects charge transport and complexation, and optimizing doping improves cell efficiency; and gate design reduces resistive losses and improves current harvesting efficiency. 

Simulation Methods

Schematic diagram of IBC solar cell structure

Using Quokka3 simulation software, an IBC solar cell model was constructed with n-type silicon wafers as the substrate to study the effects of c-Si body lifetime, wafer resistivity, post-boron thin layer resistance, front and back surface passivation and reflection-reducing layer on the performance of the cell, and to simulate the parameters of the output open-circuit voltage (Voc), short-circuit current density (Jsc ), fill factor (FF), and efficiency (η ). 

Effect of carrier lifetime on electrical parameters

Simulation results of n-type c-Si wafer body life on electrical parameter performance of IBC solar cells

As the n-type c-Si body lifetime increases, the lower the compound loss, the open circuit voltage (Voc), short circuit current density (Jsc ), fill factor (FF), and cell efficiency (Eff) all show an increasing trend.

The fill factor (FF) improves due to lower series resistance and the efficiency (η) peaks at 24.64% at 5 ms. 

Effect of wafer resistivity on electrical parameters

Simulation results of electrical parameter performance of IBC solar cell with resistivity of n-type c-Si wafer

The highest efficiency (24.65%) is achieved at resistivity 1.2 Ω-cm due to the balance of carrier separation (high resistivity) and conductivity (low resistivity).Voc rises with increasing resistivity (widening of depletion region and reduction of compounding); Jsc increases due to enhanced light absorption. 

Effect of square resistance of backside boron doped layer

Results of post-boron thin layer resistance with FF and η for IBC solar cell simulation

The increase in Rsh decreases FF from 83.89% to 83.77% and efficiency from 24.66% to 24.61%, while Voc (707.8 mV) and Jsc (41.52 mA/cm2 ) remain unchanged.

Because the increase of Rsh increases the series resistance, which hinders the charge collection and transport and reduces the FF; Voc depends on the material bandgap and junction built-in potential, and Jsc depends on the light absorption and quantum efficiency, which are not directly affected by Rsh, and the doping concentration needs to be optimized in order to balance the conductivity with the composite loss. 

The effect of front surface J0

Composite current density (J0) results simulated using Quokka3 for IBC solar cell front passivation layer

As J0 increases, Voc, Jsc, FF, and Eff decrease; Voc decreases from about 710 mV to about 685 mV, Jsc decreases from about 41.7 mA/cm2 to about 40.2 mA/cm2, FF decreases from about 84.0% to about 83.4%, and Eff decreases from about 24.80% to about 22.94%. The high J0 indicates that the front surface passivation quality is poor and the compounding is intensified, leading to overall degradation of performance.

The effect of rear surface J0

Effect of post-surface passivation complex current density (J0) on the performance of IBC solar cells

After J0 increases, the parameters decrease. High J0 intensifies the rear surface composite, quasi-Fermi energy level separation decreases, Voc decreases; carrier collection decreases, Jsc decreases; composite deteriorates the IV curve, FF decreases; and finally Eff decreases due to deterioration of parameters.

 In IBC solar cells, the quality of back-passivation has a significant impact on the cell performance, and a low J0 value is essential to maintain efficient cell operation. 

Optimization of emitter coverage and backside boron doping

Effect of post-boron thin-layer resistance and post-boron area fraction on the performance of IBC solar cells

The cell performance is optimized when the post-boron thin layer resistance is 100 Ω/Sq and the emitter fraction is about 40%. At this point, Voc is 719.2 mV, Jsc is 41.66 mA/cm2 , FF is 84.71%, and efficiency is 25.2%. At this point, the cell strikes a balance between reducing compounding and maintaining low contact resistance. 

In terms of BC cell structure and process, the charge transfer and collection efficiency inside the cell can be optimized to reduce energy loss by precisely regulating the resistance of the thin layer of rear boron. Meanwhile, the quality of front and rear surface passivation is closely related to the compound current density J0. A high-quality passivation layer can effectively reduce J0, reduce surface compounding, and play a key role in improving the battery's open-circuit voltage, short-circuit current and fill factor. 

Millennial TLM Contact Resistance Tester

Millennial TLM contact resistance tester is equipped with contact resistivity test function, which can realize fast, flexible and accurate detection.

✔  Static test repeatability ≤1%, dynamic test repeatability ≤3%

✔  Line resistance measurement accuracy up to 5% or 0.1Ω/cm

✔  Contact resistivity test and line resistance test switching

✔  Customize a variety of probes for measurement and analysis

The Millennial TLM contact resistance tester is capable of accurately measuring the contact resistance of the cell, which is crucial for an in-depth understanding of the charge transport mechanism in IBC solar cells. As found in this study, the resistance of the post-boron thin layer has a significant impact on the cell performance, and the contact resistance is one of the key aspects. With the help of this tester, the change of contact resistance under different process conditions can be accurately determined, which can provide direct data to support the optimization of the electrode design and preparation process of the cell.

Original source: Improved passivation and antirefection techniques for higherefciency interdigitated back contact (IBC) solar cells