Thin-film solar cells based on liquid phase crystallized silicon (LPC Si) with 8-20 mu m thick absorber layers demand for advanced light management to achieve high photocurrent densities. Open-circuit voltages (V-oc) > 600 mV underline the high silicon material quality of LPC silicon thin-films on nano-textured glass superstrates. We present a 500 nm-pitched sinusoidal nano-texture which outperforms larger pitched gratings with respect to light incoupling at the buried glass-silicon interface. In the wavelength range of interest reflection of incident light is minimized to values close to 4%, which is the reflection at the sun-facing air-glass interface. Further, the electronic material quality of sinusoidally textured devices is analyzed on basis of a comparison of maximum achieved open-circuit voltages on different texture types. The Voc on sinusoidally textured glass superstrates could be raised to 630 mV by changing the interlayer deposition method from a PVD to a PECVD process. Thus, we are able to unify high optical and electronic properties of silicon absorber layers on sinusoidaly textured glass substrates. These results constitute a crucial step toward fully exploiting the optical potential of LPC silicon thin-film solar cells.
Pure selenide Cu2ZnSnSe4 solar cells were fabricated with a maximum achieved open-circuit voltage (V-OC) of 454 mV, which is on par with the highest known V-OC for this technology (Bourdais et al., Adv. Energ. Mater. 2016, 6). A simple, non-vacuum, spin coating technique was developed for the introduction of alkali ions (Li+, K+, Rb+) before two annealing steps. The presence of Rb and Li ions in particular, was observed to reduce recombination in the bulk and increase minority charge carrier decay time. This resulted in the reduction of the redshift in the photoluminescence peak from the bandgap and the V-OC deficit. Bandgap variations and electrostatic potential fluctuations in conduction and valence bands were investigated as aspects likely affected by alkali ions.
Carrier multiplication is a non-radiative recombination mechanism that leads to the generation of two or more electron-hole pairs after absorption of a single photon. By reducing the occurrence of dissipative effects, this process can be exploited to increase solar cell performance. In this work we introduce two different theoretical fully ab-initio tools that can be adopted to study carrier multiplication in nanocrystals. The tools are described in detail and compared. Subsequently we calculate carrier multiplication lifetimes in H- and OH- terminated silicon nanocrystals, pointed out the role played by the passivation on the carrier multiplication processes.
The use of an analytical model on luminescent downshifting layers (LDSLs) has been used to estimate the short-circuit current. Various LDSL have been tested with two different polymers and luminescent materials. These devices have been coupled with a CIGS solar cell. The CdSe/ZnS quantum dots (QD) and Lumogen organic dyes are added in a polymeric scaffold to do a luminescent waveguide increasing the absorption in the UV range. The experimental short-circuit current are compared to the calculated short-circuit current. In this model, only the effects of the dyes were taken into account. It has been demonstrated a good correlation between the experimental and the theoretical values. Moreover, different coupling architectures have been experimented. The impact of the interface between LDSL and the solar device is not negligible. It has been experimentally proved the interface change can improve about 40-80% with an ethanol interface. The increase can reach 60-90% with a glycerine interface. The change of this interface impacts directly on the scattering effects and decrease the losses due to the surface defects.
Constructing tandem junction devices consisting of a bottom cell based on prevailing solar cell techniques such as silicon (Si) or Cu(In,Ga)(Se,S)(2) (CIGS) combined with a perovskite top cell is one effective approach to improve photovoltaic device performance. An intermediate transparent conducting layer as one component of the tunnel junction connecting the two single devices, at the same time serving as inorganic hole transport material for the perovskite solar cell is one of the key components to realize a perovskite-CIGSe tandem structure. In this contribution, we describe the deposition of p-type Cu2O films at room temperature by magnetron sputtering using nitrogen doping to form a homogenous layer with high transparency and good conductivity, and the modeling of a monolithic perovskite-CIGSe tandem solar cell utilizing this layer. We applied the in-house software RefDex to illustrate the impact of Cu2O on the performance of a tandem device, and calculated the reflection, transmission, and absorption.
We have theoretically investigated the 1.55 mu m p-type doped GaNAsBi-based double quantum wells (DQWs) using the (16 x 16) BAC model combined with a self-consistent calculation. We have found that the coupling effect becomes more pronounced by reducing barriers width. The optical performance of the structure is enhanced when the DQWs are coupled and doped. Moreover, a p-i-n heterojunction based on GaAs/GaNAsBi/GaAs DQWs designed for infrared photodetection was developed. The optimization of well parameters such as the bismuth composition, the well width, and the doping densities give rise to p-i-n heterojunction emitting at the wavelength 1.55 mu m. The computed gain can reach the value 3 10(4) cm(-1). The quantum confined stark effect on the optical properties of studied structures is also discussed.
In this work, undoped and 1–5 at.% In-doped SnO2-δ films are deposited onto glass substrates at 350 °C by spray pyrolysis technique. The influence of dopant concentration is investigated using X-ray diffraction (XRD), UV-Visible spectroscopy, and Hall Effect measurements using van der Pauw method. X-ray diffraction studies indicate that all films had preferred orientation along (200) plane and are polycrystalline with tetragonal rutile structure. The calculated average crystallite sizes increased after doping. Substitution of In into SnO2-δ thin films can be confirmed by the shifting of the peaks in the XRD patterns. Optical transmittance of the films show high average transparency ∼80–90% in the visible region. Hall measurements show that the conduction type is dependent on In content. For low-doped films (In ≤3 at.%), the films are n-type, while at higher doping concentration the films are p-type. The calculated values of the mean free path are very small compared to the average crystallite sizes calculated using XRD measurements. Therefore, we suggest that ionized and/or neutral impurity scattering are the main scattering mechanisms in these films. The above-mentioned characteristics render these In-doped SnO2 films potential candidates for their use in light-emitting diode and in optoelectronic devices, with the advantage that they are prepared by a simple and economical technique.
High sensitivity photodetectors based on PbS colloidal quantum dots have been demonstrated by several research groups in the last years with performance comparable to commercial III-V semiconductor devices. Nevertheless, investigation of the noise performance of such new photodetectors is still lacking. Here we report on the characterization of PbS colloidal quantum dot near infrared photoconductors including a preliminary analysis of noise power spectra. Devices have been characterized focusing on the low frequency regime (up to 10 kHz) investigating the noise dependence on the voltage bias.
First-principles calculations of work function tuning induced by different chemical terminations on Si(100) surface are presented and discussed. We find that the presence of halogen atoms (I, Br, Cl, and F) leads to an increase of the work function if compared to the fully hydrogenated surface. This is a quite general effect and is directly linked to the chemisorbed atoms electronegativity as well as to the charge redistribution at the interface. All these results are examined with respect to previous theoretical works and experimental data obtained for the (100) as well as other Si surface orientations. Based on this analysis, we argue that the changes in the electronic properties caused by variations of the interfacial chemistry strongly depend on the chemisorbed species and much less on the surface crystal orientation.
Zinc oxide thin films co-doped with rare earth (Nd and Tb) have been grown on heated glass substrates (350 degrees C) by chemical spray pyrolysis method. The effect of doping rate on structural, optical, and electrical properties is studied. X-ray diffraction confirmed that all the prepared films have the hexagonal wurtzite structure with a preferred orientation toward the c-axis. No peaks belonging to rare earth or their oxides are observed in the limit of XRD technique detection. UV-visible-NIR spectra show a significant optical transmission above 75%, which decrease with the increase of Tb concentration and an increase of the optical band gap with the insertion of the rare earth dopants. Photoluminescence spectra are dominated by an emission band attributed to the radiative recombination of excitons and a large band attributed to the various defects in ZnO matrix. In addition band emissions from Nd3+ ions are observed in the (Nd, Tb) co-doped films. Electrical resistivity of about 3.7 10-2 Omega . cm was achieved.
