Damien Barakel

Charge trapping dynamics in PbS colloidal quantum dot photovoltaic devices.

The efficiency of solution-processed colloidal quantum dot (QD) based solar cells is limited by poor charge transport in the active layer of the device, which originates from multiple trapping sites provided by QD surface defects. We apply a recently developed ultrafast electro-optical technique, pump-push photocurrent spectroscopy, to elucidate the charge trapping dynamics in PbS colloidal-QD photovoltaic devices at working conditions. We show that IR photoinduced absorption of QD in the 0.2-0.5 eV region is partly associated with immobile charges, which can be optically detrapped in our experiment. Using this absorption as a probe, we observe that the early trapping dynamics strongly depend on the nature of the ligands used for QD passivation, while it depends only slightly on the nature of the electron-accepting layer. We find that weakly bound states, with a photon-activation energy of 0.2 eV, are populated instantaneously upon photoexcitation. This indicates that the photogenerated states show an intrinsically bound-state character, arguably similar to charge-transfer states formation in organic photovoltaic materials. Sequential population of deeper traps (activation energy 0.3-0.5 eV) is observed on the ~0.1-10 ns time scales, indicating that most of carrier trapping occurs only after substantial charge relaxation/transport. The reported study disentangles fundamentally different contributions to charge trapping dynamics in the nanocrystal-based optoelectronic devices and can serve as a useful tool for QD solar cell development.

n-p Junction formation in p-type silicon by hydrogen ion implantation

Hydrogen ion implantations at an energy of 250 keV and a dose of 3 × 10 16 cm -2 were applied to float zone, Czochralski grown silicon wafers and to multicrystalline samples. It was found that after annealing at 350°C <T<550°C for 1 h a n-p junction is formed and a photovoltaic behaviour is observed. Spectral responses show that the photocurrent in the near infrared part of the spectrum is comparable to that given by a standard silicon solar cell. The depth of the junction is about 2μm and C-V measurements show that the junction is graduated. Hydrogen plasma immersion leads to similar results. The conversion of p- to n-type silicon is explained by the formation of shallow donor levels associated to a high concentration of hydrogen.

N-type multicrystalline silicon wafers for solar cells

In n-type silicon the capture cross sections of metallic impurities, are neatly smaller than in p-type. So, lifetime and also diffusion length of minority carriers should be neatly higher. This is of a paramount interest for multicrystalline silicon wafers, in which the impurity-defects interaction governs the recombination strength of minority carriers. In 1.2 /spl Omega/cm wafer, lifetime is found around 200 /spl mu/s and diffusion lengths around 220 /spl mu/m, These values increase strongly after gettering treatments like phosphorus diffusion or AI-Si alloying. Scan maps reveal that extended defects are poorly active, even when the density of dislocation is higher than 10/sup 5/ cm/sup -2/. High quality abrupt p/sup +/n junctions are obtained by Al-Si alloying and annealing at 850 or 900 /spl deg/C, which could be used for rear junction cells.

ZnS|Ag|TiO2 multilayer electrodes with broadband transparency for thin film solar cells

A ZnS|Ag|TiO2 (ZAT) multilayer architecture is proposed as an alternative to symmetric TiO2|Ag|TiO2 (TAT) electrodes. The TAT electrodes were proved to be among the best ITO-free transparent conductive electrodes. It is demonstrated that choosing ZnS instead of TiO2 as a substrate for Ag allows better optical and electrical performances. The complex refractive indices of both dielectric materials are determined by spectroscopic ellipsometry and implemented into a transfer matrix algorithm to optimize the optical transmittance of ZAT and TAT multilayers in the visible part of the spectrum. It is shown that both types of electrodes are equivalent in terms of optical behavior. Manufactured electrodes with symmetric 40 nm dielectric thicknesses are then fabricated on glass substrates by e-beam evaporation and the effect of the silver layer thickness on performances is studied. It is found that ZAT multilayer performances are systematically better, and on a broader transmittance range, when the Ag layer is very thin than TAT stacks. A state of the art 90.23% maximum transmittance is reached at λ = 460 nm for a ZnS(36 nm)|Ag(12.7 nm)|TiO2(37 nm) multilayer, with a sheet resistance RS of 5 Ω Sq−1. Over 80% transmittance is achieved in the [380–855] nm wavelength range for a ZnS(36 nm)|Ag(7 nm)|TiO2(40 nm) multilayer, with a RS of 11.3 Ω Sq−1. Scanning electron microscopy (SEM) reveals continuous silver films grown on ZnS as opposed to those grown on TiO2, thus justifying the better performances of the ZAT.

Tuning of Light Trapping and Surface Plasmon Resonance in Silver Nanoparticles/c-Si Structures for Solar Cells

In this work, we investigate silver (Ag) nanoparticle-related plasmonic effect on light absorption in Si substrate. Ag nanoparticles (Ag-NPs) deposited on top of Si were used to capture and couple incident light into these structures by forward scattering. We demonstrate that we can control nanoparticle size and shape while varying deposition time and annealing parameters. By the increase of the total time of the reaction process, morphology of Ag-NPs evolutes affecting the number and the width of surface plasmon resonance peaks, whereas for changed annealing parameters (temperature and time), the effect is more pronounced on the broadening and the position of peaks. Specific morphology of Ag-NPs can exhibit an interesting enhancement of optical properties which enables plasmon-related application in photovoltaic solar cells.

Optical and electrical properties of structured multilayer with tunable transparency rate

An experimental study has been carried out on structured multilayer with tunable transparency rate. In this paper, we present the optical and electrical characterization of an oxide | metal | oxide structured electrode manufactured by e-beam deposition and patterned by a lift-off process. The obtained samples are made of grids with different geometrical parameters that lead to varying surface coverage rate on glass. The electrical and optical parameters of SnOx|Ag|SnOx grids are investigated to determine the efficiency, sustainability and limitations of such structures. A linear relationship between the transmittance of the electrodes and the increase of the surface coverage rate is obtained. Coupled to an optimization process, we are able to define a high transparency in a chosen spectral range. Electrical results show a relative stability of the resistivity from 2.9 × 10 − 4 Ω.cm for an as-grown electrode to 5.6 × 10 − 4 Ω.cm for a structured electrode with a surface coverage rate of 59%.

Optical role of the thin metal layer in a TiOx/Ag/TiOx transparent and conductive electrode for organic solar cells

One possible alternative to ITO, the most commonly used transparent and conductive electrode (TCE) for Organic Solar Cells (OSCs) and other optoelectronic components, is to use an oxide|metal|oxide multilayer. Glass|cathode|ZnO (20 nm)|P3HT:PCBM (250 nm)|PEDOT:PSS (50 nm)|Ag (150 nm) inverted OSC structures are realized, where the cathode can be a TiOx|Ag|TiOx or ITO reference TCE. The sizing of the TiOx|Ag|TiOx (TAT) TCE structure is numerically realized by optimization of the normalized squared electric field inside the active P3HT:PCBM layer. The optimized TAT design in the whole design is different from the one involving optimization of transparency at the output of the trilayer structure in air. A photovoltaic efficiency of 2.7% is obtained for OSC with the TiOx (22 nm)|Ag (15 nm)|TiOx (19 nm) structure and can be compared to the 3.14% of efficiency obtained with the ITO reference. The short-circuit current density is identified as the crucial photoelectrical parameter. The morphology of the silver layer in TAT can give rise to an exaltation of the electromagnetic field, leading to an enhanced and undesirable absorption inside the metal layer. This exaltation is dependent of the thickness of the metal layer and induces changes in current density proportional to the normalized squared electric field inside this layer. The lost in short-circuit current density is estimated between 0.3 and 0.6 mA cm−2, and is comparable to a thickness variation of 20 nm for both TiOx layers or 2.5 nm of the silver layer. We define an exaltation coefficient of the bare electrode, which can be considered as a factor of merit to qualify the quality of the optical role of the silver layer and thereby of the trilayer electrode.

Minority Carrier Lifetime Measurements in Specific Epitaxial 4H-SiC Layers by the Microwave Photoconductivity Decay

We report on measurements of the minority carrier lifetime for different epitaxial 4H-SiC layers by using the microwave photoconductivity decay (µ-PCD) method. This is a non-contacting, non-destructive method very useful for the monitoring of recombination processes in semiconductor material. Distinct samples have been analyzed, giving different lifetime values. Transmittance and absorption spectra have also been carried out. The n-type layers, giving rise to a specific absorption peak near 470 nm, are not sensitive to optical excitation for the used wavelengths, as opposite to p-type layers whose lifetime values depend on thickness and doping.

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.

Numerical and experimental study of SnOx | Ag | SnOx multilayer as indium-free transparent electrode for organic solar cells

We propose a SnOx | Ag | SnOx multilayer, deposited in a continuous vacuum atmosphere by E-beam evaporation, as transparent anode for a (poly-3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) bulk heterojunction based Organic Solar Cell (OSC). Optical characterization of the deposited SnOx is performed to determine the dispersion of the complex refractive index. A Transfer Matrix Method (TMM) numerical optimization of the thicknesses of each layer of the electrode is realized to limit the number of manufactured samples. A numerical study using the morphology of the silver inserted between the oxide layers as input data is performed with a Finite Difference Time Domain (FDTD) method to improve the accordance between measurement and optical model. Multilayers are manufactured with the objective to give to the electrode its best conductivity and transparency in the visible spectral range by using the results of the optical optimization. These bare tri-layer electrodes show low sheet resistance (<10 Ω/□) and mean transparency on [400-700] nm spectral band as high as 67 % for the whole Glass | SnOx | Ag | SnOx structure. The trilayer is then numerically studied inside a P3HT:PCBM bulk heterojunction based OSC structure. Intrinsic absorption inside the sole active layer is calculated giving the possibility to perform optical optimization on the intrinsic absorption efficiency inside the active area by considering the media embedding the electrodes.