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Comprehending and Mitigating Backside Recombination in Cu(In,Ga)Se2 Solar Cells

Author: WANG Taowen
Publication venue: Unilu -Universität Luxemburg
Publication date: 13/07/2023
Field of study

The aim of this thesis is to comprehend and mitigate backside recombination in Cu(In,Ga)Se2 solar cells. The record Cu(In,Ga)Se2 solar cell has the highest efficiency reaching up to 23.35%[1] at the time of writing this thesis. To achieve such an efficiency level, CsF post-deposition treatments are used to improve the absorber quality in the bulk and on the front surfaces. A good quality absorber and front interface is necessary to make a high efficiency solar cell. Meanwhile, the interface between the metal back contact (molybdenum) and the absorber must be passivated to avoid non-radiative recombination losses due to high backside recombination. Traditionally, this is accomplished by constructing a Ga gradient that creates a higher conduction band gradient towards the backside. Unfortunately, the Ga gradient also introduces an inhomogeneous absorber, leading to higher non-radiative and radiative losses in the solar cells, limiting further improvement in power conversion efficiency. To solve this issue, a functional hole selective transport structure is proposed in this work. One of the critical issues of using hole selective transport layers for Cu(In,Ga)Se2 is that they must withstand the harsh growth condition of Cu(In,Ga)Se2, e.g. high temperature and Se pressure. It means that the introduced hole selective layers should have a good thermal stability to avoid massive diffusion. In this work, we developed a thermally stable hole transport layer which shows comparable passivation effects and transport of holes to the Ga gradient, but with a homogeneous absorber. Since the Ga gradient is not required with the hole selective transport structure, the absorber thickness can be reduced to less than 1.0 μm, thereby lowering manufacturing costs and making Cu(In,Ga)Se2 more cost-competitive with other solar cell technologies. Firstly, to gain a better understanding of how backside recombination affects the quasi-Fermi level splitting or open circuit voltage of solar cells, this study investigates the traditional backside passivation strategies of Ga gradient and metal oxide dielectric layers. The results confirm that reducing backside recombination can improve quasi-Fermi level splitting by at least 40 meV, even with a short minority carrier lifetime of only dozens of nanoseconds. These findings are supported by SCAPS simulations, which also demonstrate similar results. Secondly, after gaining an understanding of the quasi-Fermi level splitting losses caused by backside recombination, this study investigates several candidate hole selective transport layers, which are supposed to mitigate the backside recombination and transport holes simultaneously. Some single layers prove to be thermally unstable due to the harsh growth conditions of Cu(In,Ga)Se2, which causes a negative impact on quasi Fermi-level splitting or open-circuit voltage on solar cells. Others show good thermal stability but provide negligible passivation. To address this issue, the study proposes a combination of a hole transport layer with a metal oxide stabilizer, CuGaSe2/In2O3, which significantly improves thermal stability and provides a good passivation effect that enhances quasi-Fermi level splitting by approximately 80 meV. Beside the good passivation effect, we found that the hole transport properties depend on the excess Cu of the hole selective transport layer. The Cu annealing of CuGaSe2/In2O3 can remove the current blocking effects, which improves the FF from ~40% to ~77%. Thirdly, backside recombination can also impact the optical diode factor, and thus fill factor of solar cells. The optical diode factor (ODF) discussed in this thesis is based on injection level dependent metastable defects that transition from donors to acceptor, which additionally shifts down the Fermi level of the holes, thus leading to a higher ODF. Both experiments and simulation found that the higher backside recombination and doping density can lower the optical diode factor. In general, a lower optical diode factor is desired to achieve a higher fill factor. However, it has been found that a lower optical diode factor resulting from higher backside recombination is unfavorable due to significant losses in quasi-Fermi level splitting. Conversely, improving the doping density has been found to be preferable, as it enhances quasi-Fermi level splitting while simultaneously lowering the optical diode factor. This thesis presents a thorough investigation of the impact of backside recombination on the quasi-Fermi level splitting of Cu(In,Ga)Se2 solar cells. Using this understanding, a novel hole selective transport structure is proposed that enables the construction of a high-efficiency solar cell with a homogeneous absorber, making a significant shift in the paradigm of Cu(In,Ga)Se2 solar cells. With this shift, the non-radiative and radiative loss of solar cells due to inhomogeneity, e.g. Ga profile, can be removed. Additionally, the results presented in this thesis shed light on the relationship between the optical diode factor, backside recombination, and doping level, providing a direction for further optimization of Cu(In,Ga)Se2 solar cells to achieve even higher power conversion efficiency.7. Affordable and clean energ

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