D. Fuertes Marrón

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.

Direct measurement of Urbach tail and gap state absorption in CuGaSe2 thin films by photothermal deflection spectroscopy and the constant photocurrent method

The applicability of photothermal deflection spectroscopy (PDS) and the constant photocurrent method (CPM) to chemical vapor deposited and physical vapor deposited CuGaSe2 (CGSe) thin films is investigated. Process dependent variations in direct band-gap energies, Urbach tail widths, bulk and surface defect densities of the films are given as a function of composition. Differences in the PDS and CPM absorption spectra of more than one order of magnitude are found. Using the standard a-Si theory of PDS and CPM analysis, a two-layer system consisting of a defect-rich near-surface layer and a less defective bulk layer is found in device-quality films. Optical improvements due to the Ga+Se annealing step as a second stage of the growth process result in a reduced bulk defect density while the near-surface defect density remains unchanged.

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...

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.

Photoluminescence and sub band gap absorption of CuGaSe2 thin films

Abstract A detailed study on the optical properties of polycrystalline thin film CuGaSe 2 as a function of chemical composition has been carried out using photoluminescence and photothermal deflection spectroscopy. The CuGaSe 2 thin films were grown by a novel open-tube chemical vapor deposition process. This method is suitable for the preparation of films within a wide range of composition. Luminescence due to shallow defect levels can be directly compared with the photoluminescence spectra of MOCVD-grown epitaxial CuGaSe 2 thin films. In addition, we have observed a broad excitonic luminescence peak 31±2 meV below the band gap correlated with a donor–acceptor pair-like luminescence transition at approximately 1:29±0:01 eV. By a detailed analysis of the binding nature of the exciton, we were able to identify an acceptor-like defect band at 243±20 meV, presumably caused by Ga-vacancies in slightly Cu-rich films. The composition dependence of the luminescence intensities is correlated with composition-dependent sub band gap absorption measurements performed by photothermal deflection spectroscopy. These data show shallow defect structure-dependent Urbach-tail energies (varying from 25±2 to 35±2 meV) as well as variations of defect absorption far below the band edge. By comparing the photoluminescence data with the absorption data, we could attribute this defect absorption to the same acceptor-like defect band.

Photoreflectance analysis of a GaInP/GaInAs/Ge multijunction solar cell

We have analyzed the photoreflectance spectra of a GaInP/GaInAs/Ge triple junction solar cell. The spectra reveal signatures from the window layer and middle and top subcells included in the stack. Additional contributions from the multilayer buffer introduced between the mismatched bottom and middle cells have been detected. Franz–Keldysh oscillations (FKOs) dominate the spectra above the fundamental bandgaps of the GaInP and GaInAs absorbers. From the FKO analysis, we have estimated the dominant electric fields within each subcell. In light of these results, photoreflectance is proposed as a useful diagnostic tool for quality assessment of multijunction structures prior to completion of the device or at earlier stages during its processing.

Hot carrier solar cells: Challenges and recent progress

The limiting efficiency on the conversion efficiency of terrestrial global sunlight is not circa 31%, as commonly assumed, but 74%. To reach the lowest possible costs and hence to attain its intrinsic potential as a major source of future sustainable energy supplies, it would appear photovoltaics has to evolve to devices targeting the latter efficiency rather than the former. The hot carrier solar cell, although presenting substantial device challenges, is arguably the highest efficiency photovoltaic device concept yet suggested and hence worthy of efforts to investigate its practicality. Challenges in the implementation of hot carrier cells are identified and progress in overcoming these are discussed.

Long-range structure of Cu(InxGa1−x)3Se5: A complementary neutron and anomalous x-ray diffraction study

Distinguishing the scattering contributions of isoelectronic atomic species by means of conventional x-ray- and/or electron diffraction techniques is a difficult task. Such a problem occurs when determining the crystal structure of compounds containing different types of atoms with equal number of electrons. We propose a new structural model of Cu(InxGa1−x)3Se5 which is valid for the entire compositional range of the CuIn3Se5–CuGa3Se5 solid solution. Our model is based on neutron and anomalous x-ray diffraction experiments. These complementary techniques allow the separation of scattering contributions of the isoelectronic species Cu+ and Ga3+, contributing nearly identically in monoenergetic x-ray diffraction experiments. We have found that CuIII3Se5 (III=In,Ga) in its room temperature near-equilibrium modification exhibits a modified stannite structure (space group I4¯2m). Different occupation factors of the species involved, Cu+, In3+, Ga3+, and vacancies have been found at three different cationic pos...

Radiative recombination in CVT-grown CuGaSe2 single crystals and thin films

Abstract The theoretically expected high open circuit voltage of CuGaSe 2 (CGSe) based thin film solar cells is partly limited by their high deep defect density. To gain more insight on this aspect, we have analyzed the radiative recombination mechanisms of intrinsic CGSe single crystals and polycrystalline films with various compositions using the steady state photoluminescence spectroscopy technique. The single crystals were grown by chemical vapor transport (CVT) in a closed system using iodine as transport agent and polycrystalline CGSe as the raw charge. The CGSe thin films were grown by an open-tube chemical vapor deposition (CVD) technique using Cu 2 Se and Ga 2 Se 3 as source materials. Iodine and chlorine were used as transport agents for source materials in the growth process. The observed low temperature photoluminescence is described by a well-known defect model based on two shallow acceptors and one shallow donor. However, the model is widened in this study to include additional deep luminescence transitions, observed approximately at h ν=1.3 eV in nearly stoichiometric CGSe grown with halogen support. The characteristic chemical reactions of the CVD/CVT growth process, analyzed with thermodynamic equilibrium calculations, are shown to be responsible for the presence of these deep defects.