Atomic layer deposited (ALD) Al2O3 films on Cu(In,Ga)Se2 (CIGS) surfaces have been demonstrated to exhibit excellent surface passivation properties, which is advantageous in reducing recombination losses at the rear metal contact of CIGS thin-film solar cells. Here, we report, for the first time, experimentally extracted electronic parameters, i.e. fixed charge density (Qf) and interface-trap charge density (Dit), for as-deposited (AD) and post-deposition annealed (PDA) ALD Al2O3 films on CIGS surfaces using capacitance–voltage (C-V) and conductance-frequency (G-f) measurements. These results indicate that the AD films exhibit positive fixed charges Qf (approximately 1012 cm−2), whereas the PDA films exhibit a very high density of negative fixed charges Qf (approximately 1013 cm−2). The extracted Dit values, which reflect the extent of chemical passivation, were found to be in a similar range of order (approximately 1012 cm−2 eV−1) for both AD and PDA samples. The high density of negative Qf in the bulk of the PDA Al2O3 film exerts a strong Coulomb repulsive force on the underlying CIGS minority carriers (ns), preventing them to recombine at the CIGS/Al2O3 interface. Using experimentally extracted Qf and Dit values, SCAPS simulation results showed that the surface concentration of minority carriers (ns) in the PDA films was approximately eight-orders of magnitude lower than in the AD films. The electrical characterization and estimations presented in this letter construct a comprehensive picture of the interfacial physics involved at the Al2O3/CIGS interface.
This work proposes gallium oxide grown by plasma‐enhanced atomic layer deposition, as a surface passivation material at the CdS buffer interface of Cu(In,Ga)Se2 (CIGS) solar cells. In preliminary experiments, a metal‐insulator‐semiconductor (MIS) structure is used to compare aluminium oxide, gallium oxide, and hafnium oxide as passivation layers at the CIGS‐CdS interface. The findings suggest that gallium oxide on CIGS may show a density of positive charges and qualitatively, the least interface trap density. Subsequent solar cell results with an estimated 0.5 nm passivation layer show an substantial absolute improvement of 56 mV in open‐circuit voltage (VOC), 1 mA cm−2 in short‐circuit current density (JSC), and 2.6% in overall efficiency as compared to a reference (with the reference showing 8.5% under AM 1.5G).
Atomic-layer-deposited (ALD) aluminum oxide (Al2O3) has recently demonstrated an excellent surface passivation for both n-and p-type c-Si solar cells thanks to the presence of high negative fixed charges (Q f ∼ 10 12 −10 13 cm −2 ) in combination with a low density of interface states (Dit). This paper investigates the passivation quality of thin (15 nm) Al2O3 films deposited by two different techniques: plasmaenhanced atomic layer deposition (PE-ALD) and Thermal atomic layer deposition (T -ALD). Other dielectric materials taken into account for comparison include: thermally-grown silicon dioxide (SiO2) (20 nm), SiO2 (20 nm) deposited by plasma-enhanced chemical vapour deposition (PECVD) and hydrogenated amorphous silicon nitride (a-SiNx:H) (20 nm) also deposited by PECVD. With the above-mentioned dielectric layers, Metal Insulator Semiconductor (MIS) capacitors were fabricated for Q f and Dit extraction through Capacitance-Voltage-Conductance (C-V -G) measurements. In addition, lifetime measurements were carried out to evaluate the effective surface recombination velocity (SRV). The influence of extracted C-V -G parameters (Q f ,Dit) on the injection dependent lifetime measurements τ (Δn), and the dominant passivation mechanism involved have been discussed. Furthermore we have also studied the influence of the SiO2 interfacial layer thickness between the Al2O3 and silicon surface on the field-effect passivation mechanism. It is shown that the field effect passivation in accumulation mode is more predominant when compared to surface defect passivation.
This work provides a rapid overview for the current state of surface passivation layer schemes for thin film solar cells: From its fundamentals to solar cell applications, and their perspective. It provides an overview of important literature and prospect considerations based on simulations.
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