Broadband light trapping patterns for ultrathin CIGSe and CZTS solar cells

Thin film solar cells offer a potential reduction in fabrication and material costs compared to conventional solar cell geometries, together with the possibility of flexible devices. Nevertheless, the reduced absorber thickness, usually below 1 μm, implies a decrease in the absorption of light and, therefore, a reduction of the efficiency. We address this absorption loss by studying the coupling of light incident on the solar cells into the optical waveguide modes supported by the absorber layer. In previous works this coupling was only shown either at specific wavelengths or in a very broad spectral range, with periodic and random arrays of nanoparticles, respectively. In contrast, in this thesis we focus on optimized arrays of nanoparticles and textures, zebra patterns, that allow for the excitation of waveguide modes in the desired range of wavelengths. We fabricate periodic, random and optimized arrays of cylindrical nanoparticles with substrate conformal imprint lithography, while the zebra patterns are fabricated by means of two different techniques: focus ion beam (FIB) and electron-beam lithography. All structures are constituted of Si patterns on top of a Si substrate. We measure the power spectral density (PSD) of all patterns with a home-built Fourier microscope, which allows us to recover the angular distribution of the light scattered by the structures. The results show that the optimized array of nanoparticles and zebra pattern exhibit a distribution of the intensity of the diffracted light in the desired range of momenta, k | |, which can be tailored to match with the wavevectors of the waveguide modes. In particular, the zebra pattern shows a high concentration of diffracted light within the selected range of momenta, thus being more desirable for photovoltaic applications. As a next step, we demonstrate the coupling of light into waveguide modes in thin film solar cells by periodic and optimized structures. First, we consider a CuInxGa1−xSe2 (CIGSe) solar cell with a periodic pattern of SiO2 nanoparticles on an indium tin oxide (ITO) substrate. We find that the measured external quantum efficiency (EQE) exhibits an overall improvement and, in particular, we can distinguish peaks at the wavelengths in which the momentum provided by the grating matches with the wavevectors of the modes. Secondly, we evaluate absorption enhancement of ultrathin Cu2ZnSnS4 (CZTS) solar cells containing the optimized zebra patterns, fabricated of SiO2 on top of a molybdenum substrate, by performing finite-difference time-domain (FDTD) simulations. The results show an increase in absorption over broad ranges of wavelengths, leading to a predicted 6 % rise of the short current density with respect to the unpatterned cell. Finally, we investigate the optical properties of CZTS solar cells through cathodoluminescence (CL) spectroscopy. We observe that our sample, which consists of a CZTS layer with a thin film of CdS on top, does not exhibit significant variations in the luminescence spectra over different regions, with separations up to a few millimetres. Main intensity peaks are observed at 520 and 920 nm, corresponding to emission from CdS and CZTS, respectively. An additional peak at 760 nm is observed, which is attributed to CdS or the CdS/CZTS interface. Overall, this thesis provides insight into the scattering of light from nanostructured interfaces, which can lead to further light trapping enhancement in thin film solar cells.