Carmen M. Ruiz

Impact of electronic defects on the Raman spectra from electrodeposited Cu(In,Ga)Se2 solar cells: Application for non-destructive defect assessment

This work reports on the electrical and Raman scattering analysis of Cu(In,Ga)Se2 cells synthesised with different densities of Se and Cu related point defects. The analysis of the Raman spectra from the surface region of the absorbers shows a direct correlation between the spectral features of the main Raman peak and the density of Se vacancies detected by admittance spectroscopy, being sensitive to the presence of vacancy densities higher than 1015 cm−3. These results corroborate the potential of Raman scattering for the non-destructive detection of electronic defects with potential impact on the characteristics of the solar cells.

Modeling the Back Contact of Cu 2 ZnSnSe 4 Solar Cells

In this paper, we analyze the theoretical impact of the Molybdenum (Mo) back contact and the MoSe2 interfacial layer on the performances of a Cu2 ZnSnSe4 (CZTSe)-based solar cell. MoSe2 layers are formed spontaneously in the Mo/CZTSe interface during the annealing of the absorber, but disagreeing interpretations about their actual role in affecting the device figures of merit (VOC , JSC , FF, and η) have been proposed in the literature. In our approach, we have simulated three structures presenting different conditions at the back contact: ideal-contact/CZTSe (flat-band), Mo/CZTSe, and Mo/MoSe2/CZTSe. For these three layers, an accurate explanation of the selection of critical material parameters is given. The numerical simulations, performed with SCAPS 3.2.01, show that the low values of Mo work function (≤4.95 eV) would have a strong detrimental effect on the VOC and FF of the cell if no interfacial layers were present at the Mo/CZTSe interface. On the other hand, a beneficial effect of the MoSe2 layer on the VOC of the device is demonstrated when this layer is included in the structure. This trend is confirmed by experimental measurements. The expected band diagram of the full ZnO/CdS/CZTSe/MoSe2/Mo structure is provided.

Overcoming the V oc limitation of CZTSe solar cells

The Voc deficit issue is considered to be the most significant limitation of high-performance kesterite devices [1]. Secondary phases, defects, band tailing and interface recombination at the kesterite/CdS heterojunction are possible reasons for the low Voc. Raman spectroscopy and PL imaging are applied here for the detection of these phenomena. Their impact on Voc and the efficiency of CZTSe devices is investigated. Process steps are optimized so as to limit their negative influence. Control of secondary phase removal and optimization of the CZTSe/CdS interface lead to significant Voc enhancement. CZTSe devices with 466 mV open circuit voltage and 8.2% power conversion efficiency are achieved. An even higher Voc of 490 mV is obtained for a 4.5% CZTSe/In2S3 device.

Using combined photoreflectance and photoluminescence for understanding optical transitions in perovskites

In this work, the optical properties of organometallic perovskites with different band gaps have been analyzed with photoluminescence and photoreflectance techniques. Band gap of the perovskites has been modified by bromine substitution of iodine in the structure. With these characterizations, the main optical transitions, both inside the band gap and higher in energies, and luminescence properties of these materials have been studied in order to further comprehend their photovoltaic properties.

High efficiency Cu2ZnSnSe4:In doped based solar cells

In this work we investigate the indium doping of CZTSe thin films. For this purpose, CZTSe was synthesized by a sequential process with different nominal In concentrations ranging from 0 to 2.6×1020 at/cm3. Absorbers and devices were characterized using XRF, PL, TOF-SIMS, SEM, J-V AM1.5 illuminated curves, EQE and CV. Results suggest the formation of InSn defects, which have a negligible impact on the carrier concentration of the absorber due to the deep character of the level introduced by this defect. This leads also to the presence of a new PL band. The main effect of the doping is reflected in changes on the morphology, where the increasing indium concentration leads to a deterioration of the absorber quality. Efficiencies in the range of 7-7.5% were obtained for In concentrations below 2.6×1019 at/cm3. This suggests that CZTSe is very tolerant to In doping, and high efficiency devices are obtained even with high In concentrations. A defect model based on the experimental results will be presented, explaining the apparently innocuous effect of In doping on the CZTSe electro-optical properties to a certain concentration.

1D and 2D numerical simulations of Cu2ZnSnSe4 solar cells

1D and 2D numerical simulations can be employed to perform optimizations of thin film solar cells and analysis of the physical mechanisms affecting the performances. In this work we discuss optical optimizations of a CZTSe solar cell predicted by a 1D model and a 2D model implementing grain boundaries as p-type phases with low band gap. The results of the simulations point out the significant impact that these GBs can have on the Voc loss and possible correlations with other experimental results obtained by K-AFM and C-AFM.

Understanding CIGS device performances through photoreflectance spectroscopy

Cu(In1-x,Gax)S2 was studied using photoreflectance spectroscopy. In this study, efforts are devoted to optimizing PR set-up for measuring CIGS grown by electrodeposition: issues such as photoluminescence perturbation, high roughness and scattering are addressed. Dual frequency photoreflectance, where both probe and pump beams are modulated, is proposed here to over come the poor signal to noise ratio. Considering the low electric field regime, material parameters are extracted by employing the third derivative functional form of dielectric functions to fit data. The reliability of the technique is finally tested by measuring PR spectra on a specific 15 x 15 cm2 wafer and explanations of PR line-shape evolution on this wafer are discussed.

Effect of shell thickness of gold-silica core-shell nanospheres embedded in an organic buffer matrix for plasmonic solar cells

The integration of metal nanoparticles in an organic buffer matrix for plasmonic organic solar cells (OSCs) has been explored as a route for improving the photovoltaic performance, with localized electromagnetic field enhancement around nanoparticles. We investigate the optical behavior of gold-silica core-shell nanospheres (Au@SiO2 NSs) with different shell thicknesses integrated into a 30 nm-thick poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) layer which is traditionally used as a buffer layer in OSCs. The morphology and size of the chemically synthesized Au@SiO2 NSs are determined by TEM, indicating that the average diameter of the Au core is about 50 nm, while the thickness of the dielectric shell can be adjusted to around 5 or 10 nm. The effect of Au@SiO2 NSs on the surrounding electromagnetic field in such a heterogeneous matrix and subsequent multilayers is examined using a numerical simulation based on a 3D-FDTD method. Furthermore, a broadband absorption enhancement in the films, which ...

Specific tools for studying the optical response of heterogeneous thin film layers

Abstract. The extraordinary progresses in the design and realization of structures in inorganic or organic thin films, whether or not including nanoparticles, make it possible to develop devices with very specific properties. Mastering the links between the macroscopic optical properties and the optogeometrical parameters of these heterogeneous layers is thus a crucial issue. We propose to present the tools used to characterize and to model thin film layers, from an optical point of view, highlighting the interest of coupling both experimental and simulation studies for improving our knowledge on the optical response of the structure. Different examples of studies are presented on copper indium gallium selenide, perovskite, P3HT:ZnO, PC70BM, organic layer containing metallic nanoparticles, and colored solar cells.

Toward a nanoimprinted nanoantenna to perform optical rectification through molecular diodes

This work presents investigations about the realization and modelization of rectenna solar cells. Rectennas are antennas coupled with a rectifier to convert the alternative current originating from the antenna into direct current that can be harvested and stored. By reducing the size of the antennas to the nanoscale, interactions with visible and near-infrared light become possible. If techniques such as nanoimprint lithography make possible the fabrication of sufficiently small plasmonic structures to act as optical antennas, the concept of rectenna still faces several challenges. One of the most critical point is to achieve rectification at optical frequencies. To address this matter, we propose to use molecular diodes (ferrocenyl-alkanethiol) that can be self-assembled on metallic surfaces such as gold or silver. In this paper, we present a basic rectenna theory as well as finite-difference time-domain (FDTD) optical simulations of plasmonic structures and experimental results of both nanoimprint fabrication of samples and characterizations by electron microscopy, Raman spectroscopy, and cyclic voltammetry techniques.