Reducing absorber layer thickness below 500 nm in regular Cu(In,Ga)Se2 (CIGS) solar cells decreases cell efficiency considerably, as both short-circuit current and open-circuit voltage are reduced because of incomplete absorption and high Mo/CIGS rear interface recombination. In this work, an innovative rear cell design is developed to avoid both effects: a highly reflective rear surface passivation layer with nano-sized local point contact openings is employed to enhance rear internal reflection and decrease the rear surface recombination velocity significantly, as compared with a standard Mo/CIGS rear interface. The formation of nano-sphere shaped precipitates in chemical bath deposition of CdS is used to generate nano-sized point contact openings. Evaporation of MgF2 coated with a thin atomic layer deposited Al2O3 layer, or direct current magnetron sputtering of Al2O3 are used as rear surface passivation layers. Rear internal reflection is enhanced substantially by the increased thickness of the passivation layer, and also the rear surface recombination velocity is reduced at the Al2O3/CIGS rear interface. (MgF2/)Al2O3 rear surface passivated ultra-thin CIGS solar cells are fabricated, showing an increase in short circuit current and open circuit voltage compared to unpassivated reference cells with equivalent CIGS thickness. Accordingly, average solar cell efficiencies of 13.5% are realized for 385 nm thick CIGS absorber layers, compared with 9.1% efficiency for the corresponding unpassivated reference cells.

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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.

This is the accepted version of a paper published in Solar Energy. This paper has been peerreviewed but does not include the final publisher proof-corrections or journal pagination.

The unique architecture of iridescent Morpho butterfly scales is known to exhibit different optical responses to various vapours. However, the mechanism behind this phenomenon is not fully quantitatively understood. This work reports on process developments in the micro-fabrication of a Morpho-inspired photonic structure in atomic layer deposited (ALD) materials in order to investigate the vapour optical sensitivity of such artificial nanostructures. By developing recipes for dry and wet etching of ALD oxides, we micro-fabricated two structures: one combining Al2O3 and TiO2, and the other combining Al2O3 and HfO2. For the first time, we report the optical response of such ALD Morpho-like structures measured under a controlled flow of either ethanol or isopropyl alcohol (IPA) vapour. In spite of the small magnitude of the effect, the results show a selective vapour response (depending on the materials used).

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 × 1016cm–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. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)

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