Surface passivation effects of aluminium oxide on ultra-thin CIGS solar cells

Among all thin-film (TF) technologies, photovoltaic (PV) cells based on copper indium gallium diselenide (CIGS) absorbers yield the highest efficiency (>22%). Current approaches and future priorities within the CIGS TF PV community are focused on CIGS thickness reduction to further lower material costs and surface passivation concepts to reduce the electronic recombination at interfaces and further enhance the solar cell performance. These approaches involve novel methods to passivate the front and rear surfaces of the CIGS absorber by implementing (i) alkali post-deposition treatments at the front surface and (ii) rear surface field-effect passivation using gallium grading schemes within the CIGS absorber layer. However, above-mentioned surface passivation approaches have been shown less effective when considering ultra-thin ( 2µm). Additionally, in-depth analyses on the materials and devices were carried out using advanced opto-electrical and material characterization techniques to understand, correlate and optimize these properties towards stable, efficient solar cells. Lastly, to generalize these electronic and interface passivation effects on the CIGS solar cell performance, for the first time, a simulation model has been developed using SCAPS TF PV software.(FSA - Sciences de l'ingénieur) -- UCL, 201

Surface passivation effects of aluminium oxide on ultra-thin CIGS solar cells

Among all thin-film (TF) technologies, photovoltaic (PV) cells based on copper indium gallium diselenide (CIGS) absorbers yield the highest efficiency (>22%). Current approaches and future priorities within the CIGS TF PV community are focused on CIGS thickness reduction to further lower material costs and surface passivation concepts to reduce the electronic recombination at interfaces and further enhance the solar cell performance. These approaches involve novel methods to passivate the front and rear surfaces of the CIGS absorber by implementing (i) alkali post-deposition treatments at the front surface and (ii) rear surface field-effect passivation using gallium grading schemes within the CIGS absorber layer. However, above-mentioned surface passivation approaches have been shown less effective when considering ultra-thin ( 2µm). Additionally, in-depth analyses on the materials and devices were carried out using advanced opto-electrical and material characterization techniques to understand, correlate and optimize these properties towards stable, efficient solar cells. Lastly, to generalize these electronic and interface passivation effects on the CIGS solar cell performance, for the first time, a simulation model has been developed using SCAPS TF PV software.(FSA - Sciences de l'ingénieur) -- UCL, 201

METHOD FOR PRODUCING IMPROVED BLACK SILICON ON A SILICON SUBSTRATE

Brevet concernant la fabrication et la passivation d'une microstructure antireflet, appelée "black silicon"

Electronic properties of negatively charged SiOx films deposited by Atmospheric Pressure Plasma Liquid Deposition for surface passivation of p-type c-Si solar cells

Here we demonstrate the influence of firing temperatures on the electronic properties of Atmospheric Pressure Plasma Liquid Deposition (APPLD) silicon dioxide films due to reformed material composition and its overall impact on surface passivation quality. Experimentally extracted electronic parameters using electrical capacitance voltage-conductance (C-V-G) measurements on aMetal-Oxide-Semiconductor (MOS) structure reveal that films fired at 810 °C show a slightly higher concentration of negative fixed charges (−Qf) and interface trap charges (Dit) compared to films fired at 940 °C. Such a dependency on the firing temperature can be attributed to variation in the net concentrations of silanol and carbon groupswithin the films, subsequently influencing the type of passivation mechanism involved. We show that for films fired at 810 and 940 °C, the predominant passivation mechanisms are related to field effect induced by excess silanol groups and chemical passivation due to the absence of electrically active carbon related defects, respectively. Additionally, dielectric constant (K) extraction from C-V measurements at 1 kHz returns an almost 2-fold higher K-value for films fired at 810 °C (K ~ 21) compared to films fired at 940 °C (K ~ 12), due to excess silanol concentration in the former films

Influence of Ga/(Ga + In) grading on deep-defect states of Cu(In,Ga)Se2 solar cells

The benefits of gallium (Ga) grading on Cu(In,Ga)Se2 (CIGS) solar cell performance are demonstrated by comparing with ungraded CIGS cells. Using drive-level capacitance profiling (DLCP) and admittance spectroscopy (AS) analyses, we show the influence of Ga grading on the spatial variation of deep defects, free-carrier densities in the CIGS absorber, and their impact on the cell’s open-circuit voltage Voc. The parameter most constraining the cell’s Voc is found to be the deep-defect density close to the space charge region (SCR). In ungraded devices, high deep-defect concentrations (4.2 × 1016 cm–3) were observed near the SCR, offering a source for Shockley–Read–Hall recombination, reducing the cell’s Voc. In graded devices, the deep-defect densities near the SCR decreased by one order of magnitude (2.5 × 1015 cm–3) for back surface graded devices, and almost two orders of magnitude (8.6 × 1014 cm–3) for double surface graded devices, enhancing the cell’s Voc. In compositionally graded devices, the free-carrier density in the absorber’s bulk decreased in tandem with the ratio of gallium to gallium plus indium ratio GGI = Ga/(Ga + In), increasing the activation energy, hindering the ionization of the defect states at room temperature and enhancing their role as recombination centers within the energy band

Passivation effects of atomic-layer-deposited aluminium oxide

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 (Qf ∼ 1012−1013 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: plasma enhanced 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 Qf 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 (Qf ,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

Investigating the electronic properties of Al2O3/Cu(In, Ga)Se2 interface

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

Adressing the impact of rear surface passivation mechanisms on ultra-thin Cu(In,Ga)Se2 solar cells performances using SCAPS 1-D model

We present a (1-D) SCAPS device model to address the following: (i) the surface passivation mechanisms (i.e. field-effect and chemical), (ii) their impact on the CIGS solar cell performance for varying CIGS absorber thickness, (iii) the importance of fixed charge type (+/−) and densities of fixed and interface trap charges, and (iv) the reasons for discrete gains in the experimental cell efficiencies (previously reported) for varying CIGS absorber thickness. First, to obtain a reliable device model, the proposed set of parameters is validated for both field-effect (due to fixed charges) and chemical passivation (due to interface traps) using a simple M-I-S test structure and experimentally extracted values (previously reported) into the SCAPS simulator. Next, we provide figures of merits without any significant loss in the solar cell performances for minimum net −Qf and maximum acceptable limit for Dit, found to be ∼5 × 1012 cm−2 and ∼1 × 1013 cm−2 eV−1 respectively. We next show that the influence of negative fixed charges in the rear passivation layer (i.e. field-effect passivation) is more predominant than that of the positive fixed charges (i.e. counter-field effect) especially while considering ultrathin (< 0.5 μm) absorber layers. Furthermore, we show the importance of rear reflectance on the short-circuit photocurrent densities while scaling down the CIGS absorber layers below 0.5 μm under interface chemical and field-effect passivation mechanisms. Finally, we provide the optimal rear passivation layer parameters for efficiencies greater than 20% with ultra-thin CIGS absorber thickness (< 0.5 μm). Based on these simulation results, we confirm that a negatively charged rear surface passivation with nano-point contact approach is efficient for the enhancement of cell performances, especially while scaling down the absorber thickness below 0.5 μm

Hemispherical cavities on silicon substrates: an overview of micro fabrication techniques

Abstract Hemispherical photonic crystals found in species like Papilio blumei and Cicendella chinensis have inspired new applications like anti-counterfeiting devices and gas sensors. In this work, we investigate and compare four different ways to micro fabricate such hemispherical cavities: using colloids as template, by wet (HNA) or dry (XeF2) isotropic etching of silicon, and by electrochemical etching of silicon. The shape and the roughness of the obtained cavities have been discussed and the pros/cons for each method are highlighted

Passivation study of aluminium oxide deposited by atomic layer deposition

In the recent years, it was reported that surface passivation of crystalline solar cell with aluminum oxide (Al2O3) deposited by atomic layer deposition (ALD) seems to be a good candidate for c-Si Solar cells for both n-type and p-type Si due to its very high negative fixed charge density (Qf ~ 1012-1013 cm-2) . This paper assesses the study of passivation quality of different dielectrics. We have considered three different dielectric materials for this work namely: thermal SiO2(20nm), SiO2 deposited by plasma enhanced chemical vapor deposition (PECVD)(20nm), Al2O3 deposited by ALD with two different techniques (thermal(20nm) and plasma(15nm)). MOS-capacitors has been fabricated and characterized to extract the interface trap charge densities (Dit) at the silicon/dielectric interface, fixed charge (Qf) in the dielectric and life time measurements were also performed with quasi-steady-state photo-conductance decay , transient techniques