C1N1 Thin Films from Guanine Decomposition Fragments

Polymeric semiconductors are finding a wide range of applications. In particular, graphitic carbon nitride g C3N4 has been investigated extensively in the past decade. However, the family of carbon nitrides is not limited to C3N4 and new CXNY are now being explored due to their different bandgap energy, morphology, and overall physicochemical properties. Here, homogenous and semi transparent C1N1 thin films are fabricated using guanine as a nontoxic molecular precursor. They are synthesized in a simplified chemical vapor deposition process on top of fused silica and fluorine doped tin oxide coated glass substrates. The chemical and structural studies reveal that C N ratio is close to target 1, triazine vibrations are visible in vibrational spectra and stacking of the film is observed from glancing incidence X ray diffraction data. The photo electrochemical properties are studied, the film is a p type semiconductor with a good photoresponse to visible light and a suitable catalyst for hydrogen evolution reaction. A simple and safe way of synthesizing C1N1 films on a range of substrates is presented her

Correlating facet orientation, defect level density and dipole layer formation at the surface of polycrystalline CuInSe2 thin films

Individual grains of chalcopyrite solar cell absorbers can facet in different crystallographic directions at their surfaces. To gain a deeper understanding of the junction formation in these devices, we correlate variations in the surface facet orientation with the defect electronic properties. We use a combined ana lytical approach based on scanning tunneling spectroscopy STS , scanning electron microscopy, and elec tron back scatter diffraction EBSD , where we perform these experiments on identical surface areas as small as 2 2 amp; 956;m 2 with a lateral resolution well below 50 nm. The topography of the absorber sur faces indicates two main morphological features micro faceted, long basalt like columns and their short nano faceted terminations. Our STS results reveal that the long columns exhibit spectral signatures typ ical for the presence of pronounced oxidation induced surface dipoles in conjunction with an increased density of electronic defect levels. In contrast, the nano faceted terminations of the basalt like columns are largely passivated in terms of electronic defect levels within the band gap region. Corresponding crystallographic data based on EBSD experiments show that the surface of the basalt like columns can be assigned to intrinsically polar facet orientations, while the passivated terminations are assigned to non polar planes. Ab initio calculations suggest that the polar surfaces are more prone to oxidation and resulting O induced defects, in comparison to non polar planes. Our results emphasize the correlation between morphology, surface facet orientations and surface electronic properties. Furthermore, this work aids in gaining a fundamental understanding of oxidation induced lateral inhomogeneities in view of the p n junction formation in chalcopyrite thin film solar cell

Effects of KF and RbF treatments on Cu In,Ga Se2 based solar cells A combined photoelectron spectroscopy and DFT study

In this work, the alkali induced chemical and electronic modifications observed at the KF and RbF treated Cu In,Ga Se2 CIGSe CdS interfaces are correlated to a Density Functional Theoretical DFT model of the alkali metal induced point defects at a CuInSe2 CdS interface. Analysed with hard X ray photoelectron spectroscopy HAXPES , the near interface regions showed a Cu poor, In rich and stoichiometric CdS composition for the KFCIGSe CdS interface and a Cu poor, In, S rich composition for the RbF CIGSe CdS interface. The DFT calculated defect formation energies and valence band offsets VBO at the defect induced interfaces indicate towards possible formation of specific defects at the KF and RbF treated CIGSe CdS interfaces. Cu vacancies indicated by the Cu poor stoichiometry of the alkali treated interfaces contribute to an increase in the acceptor densities NA . Possible formation of KCu and RbCu defects could result in lower NA at the interfaces because of the Cu vacancies being filled up by K and Rb atoms. NaCd and excess CdCu defects at the KF CIGSe CdS interface and only CdCu defects at the RbF CIGSe CdS interface might have formed that would result in higher donor densities ND at the interfaces. These factors, which showed enhanced type inversion when applied in device simulations, resulted in fill factor FF and open circuit voltage Voc gains in device

Properties of Co Evaporated RbInSe2 Thin Films

The formation of an Rb containing In Se compound at the surface of Cu In, Ga Se2 CIGS thin films is assumed to be part of the mechanism of RbF post deposition treatments PDTs performed on these absorber layers. Alkali PDTs have acquired attention lately as they significantly enhance the efficiency of CIGS solar cells. In this contribution the formation of various phases during the RbF PDT has been investigated. The results indicate that RbInSe2 is the most probable phase to form. Combining theoretical and experimental investigations, fundamental properties of a thermally co evaporated RbInSe2 thin film are reported in order to serve as reference values in further studie

Surface Passivation and Detrimental Heat Induced Diffusion Effects in RbF Treated Cu In,Ga Se2 Solar Cell Absorbers

Alkali postdeposition treatments of Cu In,Ga Se2 absorbers with KF, RbF, and CsF have led to remarkable efficiency improvements for chalcopyrite thin film solar cells. However, the effect of such treatments on the electronic properties and defect physics of the chalcopyrite absorber surfaces are not yet fully understood. In this work, we use scanning tunneling spectroscopy and X ray photoelectron spectroscopy to compare the surface defect electronic properties and chemical composition of RbF treated and nontreated absorbers. We find that the RbF treatment is effective in passivating electronic defect levels at the surface by preventing surface oxidation. Our X ray photoelectron spectroscopy XPS data points to the presence of chemisorbed Rb on the surface with a bonding configuration similar to that of a RbInSe2 bulk compound. Yet, a quantitative analysis indicates Rb coverage in the submonolayer regime, which is likely causing the surface passivation. Furthermore, ab initio calculations confirm that RbF treated surfaces are less prone to oxidation in the form of Ga, In, and Se oxides than bare chalcopyrite surfaces. In addition, elemental diffusion of Rb along with Na, Cu, and Ga is found to occur when the samples are annealed under ultrahigh vacuum conditions. Magnetic sector secondary ion mass spectrometry measurements indicate that there is a homogeneous spatial distribution of Rb on the surface both before and after annealing, albeit with an increased concentration at the surface after heat treatment. Depth resolved magnetic sector secondary ion mass spectrometry measurements show that Rb diffusion within the bulk occurs predominantly along grain boundaries. Scanning tunneling and XPS measurements after subsequent annealing steps demonstrate that the Rb accumulation at the surface leads to the formation of metallic Rb phases, involving a significant increase of electronic defect levels and or surface dipole formation. These results strongly suggest a deterioration of the absorber window interface because of increased recombination losses after the heat induced diffusion of Rb toward the interfac