Roberto Félix

Na incorporation into Cu(In,Ga)Se2 thin-film solar cell absorbers deposited on polyimide: Impact on the chemical and electronic surface structure

Na has deliberately been incorporated into Cu(In,Ga)Se2 (“CIGSe”) chalcopyrite thin-film solar cell absorbers deposited on Mo-coated polyimide flexible substrates by adding differently thick layers of NaF in-between CIGSe absorber and Mo back contact. The impact of Na on the chemical and electronic surface structure of CIGSe absorbers with various Cu-contents deposited at comparatively low temperature (420 °C) has been studied using x-ray photoelectron and x-ray excited Auger electron spectroscopy. We observe a higher Na surface content for the Cu-richer CIGSe samples and can distinguish between two different chemical Na environments, best described as selenide-like and oxidized Na species, respectively. Furthermore, we find a Cu-poor surface composition of the CIGSe samples independent of Na content and — for very high Na contents — indications for the formation of a (Cu,Na)–(In,Ga)–Se like compound. With increasing Na surface content, also a shift of the photoemission lines to lower binding energies cou...

Unveiling the Hybrid n-Si/PEDOT:PSS Interface

UNLABELLED We investigated the buried interface between monocrystalline n-type silicon (n-Si) and the highly conductive polymer poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) ( PEDOT PSS), which is successfully applied as a hole selective contact in hybrid solar cells. We show that a post-treatment of the polymer films by immersion in a suitable solvent reduces the layer thickness by removal of excess material. We prove that this post-treatment does not affect the functionality of the hybrid solar cells. Through the thin layer we are probing the chemical structure at the n-Si/ PEDOT PSS interface with synchrotron-based hard X-ray photoelectron spectroscopy (HAXPES). From the HAXPES data we conclude that the Si substrate of a freshly prepared hybrid solar cell is already oxidized immediately after preparation. Moreover, we show that even when storing the sample in inert gas such as, e.g., nitrogen the n-Si/SiOx/ PEDOT PSS interface continues to further oxidize. Thus, without further surface treatment, an unstable Si suboxide will always be present at the hybrid interface.

Hard x-ray photoelectron spectroscopy study of the buried Si/ZnO thin-film solar cell interface: Direct evidence for the formation of Si–O at the expense of Zn-O bonds

The chemical structure of the interface between silicon thin films and the transparent conductive oxide ZnO:Al has been investigated by hard x-ray photoelectron spectroscopy. By varying the excitation energy between 2010 and 8040 eV, we were able to probe the Si/ZnO interface buried below 12 nm Si. This allowed for the identification of changes induced by solid phase crystallization (SPC). Based on in-situ SPC annealing experiments, we find clear indications that the formation of Si–O bonds takes place at the expense of Zn–O bonds. Hence, the ZnO:Al acts as the oxygen source for the interfacial Si oxidation.

CdS/Cu(In,Ga)Se2 Interface Formation in High-Efficiency Thin Film Solar Cells

The evolution of the CdS/Cu(In,Ga)Se2 interface in high-efficiency thin film solar cells was monitored by chemically sensitive x-ray emission spectroscopy as a function of CdS chemical bath deposition time. We find direct experimental evidence that, in the initial deposition steps, the sulfur atoms on the Cu(In,Ga)Se2 surface exist in at least two distinct chemical environments, namely CdS and a compound involving Ga and In. The findings indicate the complexity of the CdS/Cu(In,Ga)Se2 interface structure at the atomic scale.

The silicon/zinc oxide interface in amorphous silicon-based thin-film solar cells: Understanding an empirically optimized contact

The electronic structure of the interface between the boron-doped oxygenated amorphous silicon “window layer” (a-SiOx:H(B)) and aluminum-doped zinc oxide (ZnO:Al) was investigated using hard x-ray photoelectron spectroscopy and compared to that of the boron-doped microcrystalline silicon (μc-Si:H(B))/ZnO:Al interface. The corresponding valence band offsets have been determined to be (−2.87 ± 0.27) eV and (−3.37 ± 0.27) eV, respectively. A lower tunnel junction barrier height at the μc-Si:H(B)/ZnO:Al interface compared to that at the a-SiOx:H(B)/ZnO:Al interface is found and linked to the higher device performances in cells where a μc-Si:H(B) buffer between the a-Si:H p-i-n absorber stack and the ZnO:Al contact is employed.

Microstructure of vanadium-based contacts on n-type GaN

Atomic force microscopy, wavelength-dispersive x-ray spectroscopy and photoemission electron microscopy were used to study the contact formation of Au/V/Al/V-based contacts on n-type GaN. After a rapid thermal annealing contact formation step, we find that the surface is composed of dendritic structures. The dendrites are Au-rich, while the voids between the branches of the dendrites are V-rich, and cracks in the voids are Ga-rich. A detailed model of the chemical structure and morphology of V-based contacts on n-GaN is given and discussed in view of the ohmic-like behaviour of such contacts.

p-Type a-Si:H/ZnO:Al and µc-Si:H/ZnO:Al thin-film solar cell structures—A comparative hard X-ray photoelectron spectroscopy study

The chemical and electronic properties of a-Si:H(B)/ZnO:Al and µc-Si:H(B)/ZnO:Al thin-film solar cell structures are studied by hard X-ray photoelectron spectroscopy (HAXPES). Using a combination of different X-ray excitation energies and deliberate sample design, we were able to select the probed volume, i.e., the silicon capping layer only or the silicon and zinc oxide layer (including the buried interface). For the a-Si:H(B) material, we find a higher deposition rate and a smaller value for the modified Auger parameter than for µc-Si:H(B). In addition, we find indications of a pronounced band bending limited to the very surface of the a-Si:H(B) and the µc-Si:H(B) layers, which is more distinct in the latter case.

NaF/KF post-deposition treatments and their influence on the structure of Cu(In, Ga)Se 2 absorber surfaces

To determine the influence of NaF/KF-post-deposition treatments (PDT) on the chemical and topographical surface structure of Cu(In,Ga)Se2 (CIGSe) solar cell absorbers, we have used synchrotron-based hard x-ray photoelectron spectroscopy (HAXPES) and scanning electron microscopy (SEM). Variations of the PDT parameters can be used to tune thickness and degree of surface nanopatterning; here we find that the nanopatterning is more pronounced on CIGSe surfaces having more potassium and less copper and gallium. Detailed analysis of Se 3d and In 4d photoemission spectra reveals the presence of (at least) two different species, which indicate the formation of a (nanopatterned) K-In-Se-type surface layer.

Exciton-Dominated Core-Level Absorption Spectra of Hybrid Organic–Inorganic Lead Halide Perovskites

In a combined theoretical and experimental work, we investigate X-ray absorption near-edge structure spectroscopy of the I L3 and the Pb M5 edges of the methylammonium lead iodide (MAPbI3) hybrid inorganic-organic perovskite and its binary phase PbI2. The absorption onsets are dominated by bound excitons with sizable binding energies of a few hundred millielectronvolts and pronounced anisotropy. The spectra of both materials exhibit remarkable similarities, suggesting that the fingerprints of core excitations in MAPbI3 are essentially given by its inorganic component, with negligible influence from the organic groups. The theoretical analysis complementing experimental observations provides the conceptual insights required for a full characterization of this complex material.

The heavily intermixed In 2 S 3 /Cu(In, Ga)Se 2 interface as Revealed by photoelectron and soft x-ray emission spectroscopy

We report on the characterization of the chemical and electronic interface structure of the heavily intermixed In2S3/Cu(In, Ga)Se2 (CIGSe) interface. Discussing our findings inferred from direct and inverse photoemission as well as soft x-ray emission spectroscopy, we particularly focus on the impact of the interfacial intermixing/interdiffusion processes on the electronic band alignment at the interface. Furthermore, we present deposition-temperature-dependent data on the (In, Al)2S3/CIGSe interface. We find that the chemical processes at the buffer/absorber interface are thermally activated with a threshold temperature of approximately 200°C.