Development of metamorphic buffer structures for inverted metamorphic solar cells

Lattice-mismatched materials can help to improve the theoretical efficiency potential of today's III-V multi-junction solar cells. Carefully designed buffer structures are incorporated in these solar cells to grade the lattice constant and relax the crystal. These buffer layers need to result in low surface roughness, low threading dislocation density and high relaxation. Ga 1-xIn xP buffer structures grown on GaAs substrates with different orientation were investigated regarding surface roughness, lattice tilt and relaxation. The buffer structures were varied in stress and strain by a tuning layer with altering composition. Close to 100 % relaxation of the crystal lattice could be reached for tuning layers with a 0.3 % larger lattice-constant compared to the target value. From the AFM data, a preference for (111)B misoriented substrates was found, since these buffer structures show the smallest RMS roughness

Improved grating monochromator set-up for EQE measurements of multi-junction solar cells

Grating monochromator set-ups are widely in use for the measurement of the external quantum efficiency (EQE) of solar cells. This paper describes the grating monochromator setup for EQE measurements of multi-junction and concentrator cells at the calibration laboratory (ISE CalLab) at Fraunhofer ISE. The set-up has been updated with innovative features. This includes an optional LED based bias illumination system which is of particular interest for the measurement of multi-junction cells whose subcells show narrow or overlapping absorption bands. Additionally the set-up has been improved in order to allow for the reliable EQE measurement of concentrator cells with areas below 1 mm2. Moreover, the set-up has been modified to measure the I-V-curve of the solar cell under bias light condition. In the case of multi-junction cells this allows to retrieve information about the shape of the I-V-curves of the individual subcells. Low temperature EQE measurements can be performed by replacing the measurement chuck through a cryostat

CPVIndia - Energy Yield Forecasting with PVsyst

In this work first results of the CPVIndia project are presented. In this project a 53 kW CPV power plant has been partly installed in Greater Noida, India together with a test facility for single (C)PV modules. The test facility enables a long term ambient conditions data logging which allows for testing of energy yield forecasting models in India. The CPV module technologies investigated in this project are AZUR C3PV and BSQ D280. For this reason the power plant consists of each two CPV systems equipped with AZUR SPACE Solar Power and with BSQ SOLAR CPV modules. First results in this project are the creation and testing of PVsyst input parameter files (PAN-files). These PAN-files have been derived from one year of I-V and ambient conditions recording performed in Freiburg, Germany using one BSQ and one AZUR CPV module. The main parameters used for the PAN-files have been derived from applying the IEC 62670-3 power rating procedure and by fitting a given equation for the so called ‘CPV Utilization Factors’ (UF). The UFs implement the dependency of the CPV module power output on spectral irradiance and lens temperature into the energy yield forecasting software PVsyst. The PAN-files created in this work have been tested using the outdoor I-V and ambient conditions data. Agreements of -2.3 % for the AZUR module and of +0.5 % for the BSQ module have been found in this manner

Wafer bonded four junction GaInP GaAs GaInAsP GaInAs concentrator solar cells with 44.7 efficiency

Triple-junction solar cells from III–V compound semiconductors have thus far delivered the highest solar-electric conversion efficiencies. Increasing the number of junctions generally offers the potential to reach even higher efficiencies, but material quality and the choice of bandgap energies turn out to be even more importance than the number of junctions. Several four-junction solar cell architectures with optimum bandgap combination are found for lattice-mismatched III–V semiconductors as high bandgap materials predominantly possess smaller lattice constant than low bandgap materials. Direct wafer bonding offers a new opportunity to combine such mismatched materials through a permanent, electrically conductive and optically transparent interface. In this work, a GaAs-based top tandem solar cell structure was bonded to an InP-based bottom tandem cell with a difference in lattice constant of 3.7%. The result is a GaInP/GaAs//GaInAsP/GaInAs four-junction solar cell with a new record efficiency of 44.7% at 297-times concentration of the AM1.5d (ASTM G173-03) spectrum. This work demonstrates a successful pathway for reaching highest conversion efficiencies with III–V multi-junction solar cells having four and in the future even more junctions