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Observation of robust hot carrier effects in quantum-well structures has prompted hopes to increase the efficiency of solar cells beyond the Shockley–Queisser limit (33% for single junction solar cells at AM1.5G). One of the main studies in hot carrier effects is the determination of carrier temperature, which provides information on the thermalization mechanisms of hot carriers in semiconductor materials. Here, we investigate the spatial distribution of photo-generated hot carriers in a InGaAs multi-quantum-well structure via hyperspectral luminescence imaging. We discuss proper methods of extracting the temperature of carriers from a photoluminescence spectrum. Robust hot carrier effects are observed at the center of the laser spot at various lattice temperatures. In addition, it is seen that the local carrier temperature scales linearly with the local laser intensity as long as the illumination exceeds a threshold power; the carrier temperature at regions with local intensities below the threshold drops to the lattice temperature, i.e., experiences no hot carrier effects. Moreover, at large distances from the concentrated light, where the level of illumination is negligible, evidence of carrier radiative recombination is observed, which is attributed to carrier diffusion in the planar structure. The results of this study can be applied to investigate the influence of carrier diffusion and thermoelectric effects on the thermalization of hot carriers. [ABSTRACT FROM AUTHOR]

Cu(In,Ga)Se2‐based (CIGS) solar cells with ultrathin (≤500 nm) absorber layers suffer from the low reflectivity of conventional Mo back contacts. Here, we design and investigate ohmic and reflective back contacts (RBC) made of multilayer stacks that are compatible with the direct deposition of CIGS at 500°C and above. Diffusion mechanisms and reactions at each interface and in the CIGS layer are carefully analyzed by energy dispersive X‐ray (EDX)/scanning transmission electron microscopy (STEM). It shows that the highly reflective silver mirror is efficiently encapsulated in ZnO:Al layers. The detrimental reaction between CIGS and the top In2O3:Sn (ITO) layer used for ohmic contact can be mitigated by adding a 3 nm thick Al2O3 layer and by decreasing the CIGS coevaporation temperature from 550°C to 500°C. It also improves the compositional grading of Ga toward the CIGS back interface, leading to increased open‐ circuit voltage and fill factor. The best ultrathin CIGS solar cell on RBC exhibits an efficiency of 13.5% (+1.0% as compared to our Mo reference) with a short‐circuit current density of 28.9 mA/cm2 (+2.6 mA/cm2) enabled by double‐pass absorption in the 510 nm thick CIGS absorber. RBC are easy to fabricate and could benefit other photovoltaic devices that require highly reflective and conductive contacts subject to high temperature processes. [ABSTRACT FROM AUTHOR]

Abstract Because of poor light absorption, Cu(In,Ga)Se 2 -based (CIGS) solar cells with an ultrathin absorber layer (<500 nm) require the development of reflective back contacts. To enhance rear reflectance in CIGS ultrathin devices, we investigate novel back contact architectures based on a silver metallic mirror covered with a thin layer of In 2 O 3 :Sn (ITO), which is fully compatible with nanopatterning for further light trapping improvements. First, numerical electromagnetic simulations of complete solar cells have been performed for a 490 nm thick CIGS absorber with various back contacts. We predict a short-circuit current density of J SC = 34.0 mA/cm2 for a 490 nm thick CIGS absorber with a silver nanostructured mirror. Second, we have fabricated and characterized 490 nm thick CIGS solar cells with transparent back contacts made of ITO, and reflective back contacts made of silver covered with ITO. Solar cells with a transparent ITO back contact exhibit an average efficiency of 10.0%, compared to 9.3% for standard molybdenum back contacts. A 5 nm thick Ga 2 O 3 layer is revealed at the ITO/CIGS interface by transmission electron microscopy and energy dispersive X-ray spectroscopy. When silver is added, the reflective back mirror leads to a J SC improvement of 4.6 mA/cm2 (from 22.4 to 27.0 mA/cm2). These results pave the way for efficient ultrathin CIGS solar cells on reflective back contacts. Highlights • Simulated absorption of ultrathin CIGS enhanced by a nanostructured back mirror. • ITO is a suitable back contact for ultrathin CIGS solar cells. • Experimental EQE of ultrathin CIGS devices improved with a reflective back contact. [ABSTRACT FROM AUTHOR]

Ultrathin c-Si solar cells have the potential to drastically reduce costs by saving raw material while maintaining good efficiencies thanks to the excellent quality of monocrystalline silicon. However, efficient light trapping strategies must be implemented to achieve high short-circuit currents. We report on the fabrication of both planar and patterned ultrathin c-Si solar cells on glass using low temperature (T < 275 °C), low-cost, and scalable techniques. Epitaxial c-Si layers are grown by PECVD at 160 °C and transferred on a glass substrate by anodic bonding and mechanical cleavage. A silver back mirror is combined with a front texturation based on an inverted nanopyramid array fabricated by nanoimprint lithography and wet etching. We demonstrate a short-circuit current density of 25.3 mA/cm² for an equivalent thickness of only 2.75 μm. External quantum efficiency (EQE) measurements are in very good agreement with FDTD simulations. We infer an optical path enhancement of 10 in the long wavelength range. A simple propagation model reveals that the low photon escape probability of 25% is the key factor in the light trapping mechanism. The main limitations of our current technology and the potential efficiencies achievable with contact optimization are discussed. [ABSTRACT FROM AUTHOR]

Fabrication of high-quality ultrathin monocrystalline silicon layers and their transfer to low-cost substrates are key steps for flexible electronics and photovoltaics. In this work, we demonstrate a low-temperature and low-cost process for ultrathin silicon solar cells. By using standard plasma-enhanced chemical vapor deposition (PECVD), we grow high-quality epitaxial silicon layers (epi-PECVD) from SiH4/H2 gas mixtures at 175 °C. Using secondary ion mass spectrometry and transmission electron microscopy, we show that the porosity of the epi-PECVD/crystalline silicon interface can be tuned by controlling the hydrogen accumulation there. Moreover, we demonstrate that 13-14% porosity is a threshold above which the interface becomes fragile and can easily be cleaved. Taking advantage of the H-rich interface fragility, we demonstrate the transfer of large areas (∽10 cm2) ultrathin epi-PECVD layers (0.5-5.5 µm) onto glass substrates by anodic bonding and moderate annealing (275-350 °C). The structural properties of transferred layers are assessed, and the first PECVD epitaxial silicon solar cells transferred on glass are characterized. Copyright © 2016 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]