Jan Schöne

Current-matched triple-junction solar cell reaching 41.1% conversion efficiency under concentrated sunlight

A metamorphic Ga0.35In0.65P/Ga0.83In0.17As/Ge triple-junction solar cell is shown to provide current-matching of all three subcells and thus composes a device structure with virtually ideal band gap combination. We demonstrate that the key for the realization of this device is the improvement of material quality of the lattice-mismatched layers as well as the development of a highly relaxed Ga1−yInyAs buffer structure between the Ge substrate and the middle cell. This allows the metamorphic growth with low dislocation densities below 106 cm−2. The performance of the approach has been demonstrated by a conversion efficiency of 41.1% at 454 suns (454 kW/m2, AM1.5d ASTM G173–03).

Metamorphic GaInP/GaInAs/Ge triple-junction solar cells with ≫ 41 % efficiency

The use of lattice-mismatched materials offers an additional degree of freedom to the design of an optimum bandgap combination in a multi-junction solar cell. This allows for achieving higher conversion efficiencies compared to state-of-the-art lattice-matched systems if the material quality of the pn-junctions can be kept sufficiently high. Work performed at the Fraunhofer ISE on metamorphic GaInP/GaInAs/Ge triple-junction solar cells with 1.2% lattice-mismatch between the middle and bottom cell recently yielded a new record efficiency of 41.1% under 454-fold concentration of the AM1.5d spectrum. This paper summarizes the main benefits of the metamorphic solar cell approach and discusses some of the challenges associated with the development of this device structure.

Present Status in the Development of III–V Multi-Junction Solar Cells

During the last yearshigh-concentration photovoltaics (HCPV) technology has gained growing attention. Excellent operatingAC-system efficiencies of up to 25% have been reported. One of the driving forces for this high system efficiency has been the continuous improvement of III–V multi-junction solar cell efficiencies. In consequence, the demand for these solar cells has risen, and strong efforts are undertaken to further increase the solar cell efficiency as well as the volume of cell output. The production capacity for multi-junction solar cells does not constitute a limitation. Already now several tens of MWp per year can be produced and the capacities can easily be increased. The state-of-the art approach for highly efficient photovoltaic energy conversion is marked by the Ga0.50In0.50P/Ga0.99In0.01As/Ge structure. This photovoltaic device is today well established in space applications and recently has entered the terrestrial market. The following chapter presents an overview about the present research status in III–V multi-junction solar cells at Fraunhofer ISE regarding cell design, expected performance, numerical simulation tools, adaptation of devices to different incident spectra and the fabrication of these devices. Finally, an outlook on future developments of III–V multi-junction solar cells is given.

Development of metamorphic triple-junction solar cells for low temperature, low intensity operation in space

Space missions to outer planets such as Mars or Jupiter place special requirements on the solar generator as intensities and temperatures can be much lower compared to the earth orbits. Additionally, the spectral conditions on a planet like Mars are depending on the specific landing point and are changing significantly over time. This leads to the question about the best solar cell technology for such low intensity, low temperature (LILT) missions. In this paper the performance of triple-junction solar cells was investigated for five typical Mars scenarios. A state-of-the-art lattice-matched triple-junction solar cell is compared to a metamorphic cell consisting of Ga0.35In0.65P, Ga0.83In0.17As and Ge. Theoretical calculations suggest that the efficiency of the metamorphic devices can be up to 21 % higher under extreme Mars operating conditions with temperatures down to −120 °C and intensities of only 22 W/m2. Experimentally this was confirmed with even 25 % higher efficiencies measured for the metamorphic 3-junction solar cell under these conditions. The IV-characteristics of the metamorphic devices were found to be well behaved even at the lowest intensities, suggesting that dislocations due to the lattice-mismatched buffer structure are not leading to low shunt resistances. The metamorphic triple-junction solar cell turns out to be an extremely interesting alternative to the lattice-matched structure for LILT conditions.

Lattice‐Matched GaInAsSb on GaSb for TPV Cells

GaInAsSb layers and TPV cells were grown by metalorganic vapor phase epitaxy (MOVPE) on GaSb substrates. GaInAsSb layers show two forms of phase separation, one inclined and the other one perpendicular to the surface. The latter was found to be sensitive to the reactor pressure and correlated to the rotation frequency of the wafers in the AIXTRON‐2600G MOVPE reactor.TPV cell structures grown at different growth temperatures, V/III ratios and susceptor rotation frequencies have been investigated. The cells show efficiencies between 0.5 and 1.7 % under AM1.5g one‐sun illumination conditions. Unfortunately, all cells have a low parallel resistance. The relative External Quantum Efficiency shows no significant difference between cells with different parallel resistance and short‐circuit current density. This can be explained either through the inclined phase separation or by crystal defects resulting in a low shunt resistance at the pn‐junction.

Defect Formation and Strain Relaxation in graded GaPAs/GaAs, GaNAs/GaAs and GaInNAs/Ge Buffer Systems for high-efficiency Solar Cells

Transmission electron microscopy of cross-section specimens and high-resolution X-ray diffraction analyses have been applied to investigate the formation of defects and the relaxation of layer strain in step-graded GaPxAs1−x and GaNyAs1−y buffer layer systems grown by metal-organic vapour phase epitaxy on GaAs (001) substrates with 6° miscut towards (111)A. The investigations have been complemented by characterization of the layer surfaces employing optical microscopy. The comparison of the different buffer concepts reveals characteristic differences in the formation of defects and in the relaxation of tensile layer strain. For GaPAs layers dislocations and microtwins form, releasing the major part of the tensile misfit strain. In contrast, for GaNAs dislocations and microtwins are largely absent, at least in the upper part of the buffer structure, and microcracks are generated. Consequently, during subsequent growth of layers with tensile strain, strain relaxation and defect formation can be effectively hindered by introducing intermediate GaNyAs1−y layers with concentrations y > 2 % into a GaAs1−xPx buffer structure [1]. A similar concept can be used for layer systems with compressive strain, however, modified by using layers of differing alloy composition. The use of dilute nitride layers appears to offer a new concept for engineering defect distributions and layer strain in lattice-mismatched compound semiconductor layer structures. Such concepts are of particular interest not only but especially also for applications in high-efficiency III-V solar cells.

TEM study of strain and defect engineering with diluted nitride semiconductors

Optoelectronic semiconductor devices fabricated by hetero-epitaxy, such as heterostructure lasers and III-V multi-junction solar cells, need high material qualities and low dislocation densities. We have developed a technique controlling strain relaxation and dislocation population in metamorphic semiconductor heterostructures using diluted nitride intermediate layers [1].