Combined photo- and electroreflectance of multijunction solar cells enabled by subcell electric coupling

Electric coupling between subcells of a monolithically grown multijunction solar cell in short circuit allows their simultaneous and independent characterization by means of photo- and electroreflectance. The photovoltage generated by selective absorption of the pump beam in a given subcell during photoreflectance measurements results in reverse biasing the complementary subunits at the modulation frequency set on the pump illumination. Such voltage bias modulation acts then as external perturbation on the complementary subcells. The spectral separation of the different subcell absorption ranges permits the probe beam to record in a single spectrum the response of the complete device as a combination of photo- and electroreflectance, thereby providing access for diagnosis of subcells on an individual basis. This form of modulation spectroscopy is demonstrated on a GaInP/GaAs tandem solar cell.Comment: 5 pages, 4 figures. This article has been accepted by Appl. Phys. Lett. After it is published, it will be found at https://doi.org/10.1063/1.506260

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

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 positions of the structure (Wyckoff sites 2a, 2b, and 4d) depending on the composition of the compound. Significantly, Cu+ does not occupy the 2b site for the In-free compound, but does for the In-containing case. Structural parameters, including lattice constants, tetragonal distortions, and occupation factors are given for samples covering the entire range of the CuIn3Se5–CuGa3Se5 solid solution. At the light of the result, the denotation of Cu-poor 1:3:5 compounds as chalcopyrite-related materials is only valid in reference to their composition

Transport properties of CuGaSe(2)-based thin-film solar cells as a function of absorber composition

The transport properties of thin-film solar cells based on wide-gap CuGaSe(2) absorbers have been investigated as a function of the bulk [Ga]/[Cu] ratio ranging from 1.01 to 1.33. We find that (i) the recombination processes in devices prepared from absorbers with a composition close to stoichiometry ([Ga]/[Cu] = 1.01) are strongly tunnelling assisted resulting in low recombination activation energies (E(a)) of approx. 0.95 eV in the dark and 1.36 eV under illumination. (ii) With an increasing [Ga]/[Cu] ratio, the transport mechanism changes to be dominated by thermally activated Shockley-Read-Hall recombination with similar E(a) values of approx. 1.52-1.57 eV for bulk [Ga]/[Cu] ratios of 1.12-1.33. The dominant recombination processes take place at the interface between CdS buffer and CuGaSe(2) absorber independently from the absorber composition. The increase of E(a) with the [Ga]/[Cu] ratio correlates with the open circuit voltage and explains the better performance of corresponding solar cells

Recent progress with hot carrier solar cells

Hot carrier solar cells offer one of the most promising options for high performance “third generation” photovoltaic devices. For successful operation, these need to be thin, strongly absorbing, radioactively efficient devices in a simple 2-terminal configuration. Nonetheless, they offer potential performance close to the maximum possible for solar conversion, equivalent to a multi-cell stack of six or more tandem cells possibly without some of the limitations, such as spectral sensitivity. However, hot carrier cells offer some quite fundamental challenges in implementation that our team is addressing in an internationally collaborative effort

Application of photoreflectance to advanced multilayer structures for photovoltaics

Photoreflectance (PR) is a convenient characterization tool able to reveal optoelectronic properties of semiconductor materials and structures. It is a simple non-destructive and contactless technique which can be used in air at room temperature. We will present experimental results of the characterization carried out by means of PR on different types of advanced photovoltaic (PV) structures, including quantum-dot-based prototypes of intermediate band solar cells, quantum-well structures, highly mismatched alloys, and III?V-based multi-junction devices, thereby demonstrating the suitability of PR as a powerful diagnostic tool. Examples will be given to illustrate the value of this spectroscopic technique for PV including (i) the analysis of the PR spectra in search of critical points associated to absorption onsets; (ii) distinguishing signatures related to quantum confinement from those originating from delocalized band states; (iii) determining the intensity of the electric field related to built-in potentials at interfaces according to the Franz?Keldysh (FK) theory; and (v) determining the nature of different oscillatory PR signals among those ascribed to FK-oscillations, interferometric and photorefractive effects. The aim is to attract the interest of researchers in the field of PV to modulation spectroscopies, as they can be helpful in the analysis of their devices

Pricing American stock options by linear programming

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