Th. Schedel-Niedrig

CuGaSe2 solar cell cross section studied by Kelvin probe force microscopy in ultrahigh vacuum

Kelvin probe force microscopy under ultrahigh vacuum conditions has been used to image the electronic structure of a Mo/CuGaSe2/CdS/ZnO thin film solar cell. Due to the high energy sensitivity together with a lateral resolution in the nanometer range we obtained detailed information about the various interfaces within the heterostructure. The absolute work function of the different materials was measured on a polished cross section. To obtain a flat and clean surface we optimized the sputtering process with Ar ions. The presence of an additional MoSe2 layer between the Mo backcontact and the CuGaSe2 absorber layer was observed.

Formation of an interfacial MoSe2 layer in CVD grown CuGaSe2 based thin film solar cells

Thin polycrystalline films of CuGaSe 2 (CGSe) have been grown on Mo coated glass substrates by halogen supported chemical vapor deposition (CVD) with two different binary source materials, Cu 2 Se and Ga 2 Se 3 . Solar cells based on these absorber films prepared in a sequential two-stage process show efficiencies exceeding 6%. High resolution transmission electron microscopy investigation of the complete solar cell structure reveals a 170-nm thick MoSe 2 interfacial layer at the CGSe/Mo back contact. The crystallites of the MoSe 2 layered structure are found to be mainly oriented perpendicular to the Mo surface. The main focus of this investigation was to study the influence of the CVD process on the growth of MoSe 2 and the role the interfacial layer may have in the performance of the solar cell. For a detailed analysis we studied the growth and morphology of the interfacial layer dependent on the [Cu]/[Ga]-ratio in the gas phase during the CGSe deposition process and the Na content of the glass substrate. It was found that Na influences the growth of the MoSe 2 layer. By means of temperature dependent IV (IVT)-measurements the electrical properties of the CGSe/MoSe 2 /Mo heterostructure were investigated. In the heterostructure under investigation the MoSe 2 interfacial layer mediates an ohmic contact to the CGSe film.

Tunable optical transition in polymeric carbon nitrides synthesized via bulk thermal condensation

Polymeric derivatives of dicyandiamide were synthesized via a bulk thermal condensation method, using a range of process temperatures between 400 and 610 °C. The obtained carbon nitride powders exhibit an optical transition in the UV-green range that has been assigned to the direct bandgap of a semiconductor-like material. Within this context, the apparent bandgap is linearly tunable with increasing process temperatures, showing a temperature coefficient of - 1.7(1) meV K(-1) between 2.5 and 3.0 eV. The obtained results show a predominant optical transition within the tri-s-triazine unit of the polymer, with a bathochromic shift originating from a gradually increasing degree of polymerization.

Lift-off process and rear-side characterization of CuGaSe2 chalcopyrite thin films and solar cells

An alternative approach to the so-called “lift-off” technology is presented, in which a CuGaSe2 solar cell absorber film is detached from a Mo-coated glass substrate. The proposed lift-off takes advantage of an interfacial MoSe2 layer, acting as a sacrificial layer, which forms at the rear contact during the growth of the CuGaSe2 film. No additional processing step is thus required to proceed with the lift-off. The lift-off was carried out in ultrahigh vacuum for quality assessment, and the rear CuGaSe2 and top MoSe2 surfaces were characterized by means of surface-sensitive techniques, namely, Kelvin probe force microscopy and photoelectron spectroscopy. The cleanness of the CuGaSe2 rear surface was confirmed by the absence of Mo remnants, thus demonstrating the suitability of the proposed method for further processing of the absorber film onto alternative substrates. In addition, a quantitative analysis of surface photovoltage, doping concentration, and interface charge at grain boundaries on the absorbe...

Monitoring chemical reactions at a liquid–solid interface: Water on CuIn(S,Se)2 thin film solar cell absorbers

The chemical and electronic structure of the interface between liquid water and a CuIn(S,Se)2 thin film surface was studied with synchrotron-based, high energy-resolution soft x-ray emission spectroscopy (XES). By probing the local environment around the sulfur atoms, an x-ray-induced sulfate formation at the CuIn(S,Se)2 surface can be monitored, correlated with a substantial enhancement of sodium impurity atoms from the CuIn(S,Se)2 film and its glass substrate. The results demonstrate that, with XES, an experimental probe is available to in situ study chemical reactions at liquid–solid interfaces or at surfaces in a high-pressure gas environment in a chemically sensitive and atom-specific way.

Optical spectra and energy band structure of single crystalline CuGaS2 and CuInS2

The reflection spectroscopy of chalcopyrite CuGaS2 and CuInS2 single crystals has been applied for light polarized perpendicular () and parallel () to the optical axis in the photon energy range between 1.5 and 6 eV at 77 K. By using the Kramers–Kronig relations, the spectral dependences of the real e1 and imaginary e2 components of the complex dielectric function e(E) = e1(E)+ie2(E) have been calculated for the investigated materials. As a result, the energy band structure of CuGaS2 and CuInS2 at photon energies higher than the fundamental band gap is derived from the analysis of the structures observed in e(ω) spectra. Additionally, the spectral dependences of the complex refractive index, extinction coefficient and absorption coefficient s of CuGaS2 and CuInS2 single crystals are determined in the 1.5–6 eV photon energy range.

Electronic structure of secondary phases in Cu-rich CuGaSe2 solar cell devices

Kelvin probe force microscopy in ultrahigh vacuum was used to image the electronic structure of thin-film solar cell cross sections based on as-grown Cu-rich CuGaSe2 absorbers. We observe different secondary phases in the absorber film. A p-type degenerate Cu2−xSe phase is identified by a higher work function (Φ∼5.35eV) than CuGaSe2 (Φ∼5.1eV), allowing good contrast mappings of both phases within the absorber film. Besides entire Cu2−xSe crystallites we also observed this secondary phase segregated as an interfacial layer along CuGaSe2 grain boundaries. An additional high-work function phase at the CuGaSe2∕window junction is attributed to the formation of an improper CuS buffer layer during chemical bath processing. The detrimental effect of these secondary phases on the solar cell performance is discussed.

Cd2+∕NH3 treatment-induced formation of a CdSe surface layer on CuGaSe2 thin-film solar cell absorbers

CuGaSe2 (CGSe)-based high-gap thin-film solar cells have to date not reached their potential level of electrical performance. In order to elucidate possible shortcomings of the electronic interface structure, we have studied the initial stage of the CdS∕CGSe interface formation by use of a simple Cd2+∕NH3 treatment. As in the case of low-gap chalcopyrites, we find a Cd-containing surface layer, in the present case comprised of approximately one monolayer of CdSe. The results indicate that the CdS∕CGSe interface is not abrupt, but contains intermediate layers. Furthermore, they shed light on possible surface modification schemes to enhance the overall performance of high-gap CGSe chalcopyrite solar cells.

Deposition and characterization of Ga2Se3 thin films prepared by a novel chemical close-spaced vapour transport technique

Single-phase Ga2Se3 films have been deposited with a growth rate of about 30?nm?min?1 on clean and Mo-coated soda-lime glass substrates by chemical close-spaced vapour transport. The use of HCl/H2 as a transport agent results in a stoichiometric volatilization of the binary Ga2Se3 powder source material and the growth of Ga2Se3 films with reproducible composition. The films have been characterized using x-ray diffraction measurements, scanning electron microscopy observations, energy dispersive x-ray analysis, x-ray fluorescence spectrometry and elastic recoil detection analysis. A p-type conductivity was determined by means of the thermoelectric probe method. A Ga2Se3 band gap energy Eg = 2.56?eV has been found by optical measurements.

Three-dimensional simulations of a thin film heterojunction solar cell with a point contact/defect passivation structure at the heterointerface

Thin film heterojunction solar cells such as those based on the chalcopyrites or amorphous silicon are often limited by interface recombination at the active heterointerface. A new strategy to overcome this limitation is described, replacing the conventional wider band gap contact material with a combination of a passivation layer plus the conventional contact in a point contact type structure. This is similar to the established method to minimize rear contact recombination in crystalline silicon solar cells. Here point contacts at the heterointerface of a CuInS2 based solar cell are modeled using the WIAS-TeSCA code. The importance of the donor defect energy level at the absorber/passivation interface is shown, and a way to improve the cell efficiency by >25% (relative) is outlined.