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

Abstract

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-IS 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 ~5x10 12 cm-2 and ~1x10 13 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 ultra-thin (< 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.

Rear Optical Reflection and Passivation Using a Nanopatterned Metal/Dielectric Structure in Thin-Film Solar Cells

“…The extracted J-V figures of merit, with average values, standard deviation values and the values of the cell with the highest efficiency cells, are shown in Table II. The reference device has an average efficiency value of 6.2 %, which is expectedly low due to rear interface recombination and incomplete light absorption, characteristic of ultrathin CIGS solar cells and comparable with other works [5], [8], [18], [24], [32], [35], [36]. With the Al2O3 passivation layer, an increase in Voc from 535 mV (reference) to 558 mV is observed, which is a good indication of the passivation effect created by the Al2O3 layer [4], with mitigation of rear interface recombination.…”

Passivation of Interfaces in Thin Film Solar Cells: Understanding the Effects of a Nanostructured Rear Point Contact Layer

“…Semiconductor surfaces, especially semiconductor-metal interfaces, can host several interface defects, leading to high surface recombination velocities, which in turn lower the electrical performance of solar cells. [36,37] These findings have motivated device simulations that have predicted gains up to 3% (in absolute power conversion efficiency) in fully passivated solar cells. The field effect passivation occurs by the presence of a built-in electric field that arises from the high density of fixed charge from the commonly used dielectric materials of the passivation layer.…”

High‐Performance and Industrially Viable Nanostructured SiOx Layers for Interface Passivation in Thin Film Solar Cells

“…[ 30,31 ] Such values were chosen to understand if the SiO x layer has the ideal Q f values, which should be around 10 12 –10 13 cm −2 or if the material has a Q f value below the ideal one ≈10 11 cm −2 , as reported in the literature. [ 31,70,71 ] The contact width values were varied, and for a better understanding, the contact width values were converted into a percentage of passivation area using the following equations [ 72 ] where d is the contact width value and P is the pitch value. Equation () is for point contact architectures and Equation () is for line contact architectures.…”