C. Baur

Effects of optical coupling in III-V multilayer systems

A method to visualize and investigate radiative recombination processes in compound semiconductor materials by utilizing the effect of optical coupling in III-V multilayer systems is presented. For this purpose, a semiconductor material of interest is grown on an activated germanium (Ge) substrate which then serves as a photodiode. By means of spectral response measurements of the Ge photodiode, a response signal from the upper layers can be detected. It is proven both by experiment and by modeling that the signals from these layers can only be explained by optical transport mechanisms, i.e., radiative recombination.

3-6 junction photovoltaic cells for space and terrestrial concentrator applications

Multi-junction solar cells made from III-V compound semiconductors are the highest efficient photovoltaic devices today. Different solar cell structures have been developed for space as well as terrestrial concentrator applications in a close collaboration between the Fraunhofer ISE and RWE-SSP in Germany. Efficiencies up to 29.1% (AM0) have been recently achieved for a 30.2 cm/sup 2/ GaInP/GaInAs/Ge device at RWE-SSP. Triple-junction solar cells with remaining factors up to 88 % after irradiation with 1 MeV electrons at a fluence of 10/sup 15/ cm/sup -2/ have been demonstrated at Fraunhofer ISE. Multi-junction solar cells with 5 and even 6 junctions for even higher radiation hardness are under development. On the other hand lattice mismatched triple-junction solar cells are an excellent solution for terrestrial concentrator applications. Efficiencies up to 35.2 % at a concentration ratio of 443-637 (AM1.5d, low AOD) have been obtained.

Doping, Electrical Properties and Solar Cell Application of GaInNAs

The present chapter addresses several important topics in the framework of carrier transport in dilute nitride films, having a potential application in multijunction solar cells. Minority carrier devices are sensitive to carrier localization and trapping. Hence, in a complex material system like GaInNAs results of transport, optical and structural properties should be interpreted together to obtain a picture of the intrinsic and the growth-related properties of the material system. The metastable GaInNAs was grown as bulk layers on GaAs under nonequilibrium conditions at low temperatures by metal organic vapour phase epitaxy with high structural quality using all-liquid MO precursors. Results on doping with various elements and their transport properties, as determined by Hall-measurements in van der Pauw geometry as well as by magnetoresistance measurements are discussed. Because of the low growth temperature, the post-growth annealing procedure is of key importance to establish optoelectronic properties in these metastable dilute N-containing materials with improved device performance via removing of defects. These defects are – if not removed – shown to have a significant influence on minority carrier devices like solar cells. The annealing conditions were varied systematically and the related changes in the microstructural N-environment were quantified. In addition, the correlation of these characteristics with improvements in 1 eV solar cell performance is established.

Results from an International Measurement round Robin of III-V Triple-Junction Solar Cells Under Air Mass Zero

This paper reports the results of an international measurement round robin of monolithic, triple-junction, GaInP/GaAs/Ge space solar cells. Eight laboratories representing national labs, solar cell vendors and space solar cell consumers, measured cells using in-house reference cells and compared those results to measurements made where each lab used the same set of reference cells. The results show that most of the discrepancy between laboratories is likely due to the quality of the standard cells rather than the measurement system or solar simulator used

Thin 5-Junction Solar Cells with Improved Radiation Hardness

Besides the efficiency, the radiation hardness of a solar cell is one of the key parameters for space applications. Todays GaInP/GaInAs/Ge triple-junction solar cells achieve remaining factors for pmpp of 88 % after 1 MeV electron irradiation at a fluence of 1015 cm-2. The degradation is dominated by the GaInAs middle cell. New solar cell structures with 5 pn-junctions have been developed to further improve the radiation resistance and excellent remaining factors of 95 % for Voc and 93 % for pmpp are reported in this paper. The structure consists of AlGaInP, GaInP, AlGaInAs, GaInAs and Ge active pn-junctions. A 1.1 mum thin Ga0.99In0.01As 4th subcell with a radiation hard layer structure was developed. This subcell has now a remaining factor for Jsc of 95 %. This proves the high radiation hardness of the 5-junction space solar cell concept

Analysis of the radiation hardness of triple- and quintuple- junction space solar cells

The Fraunhofer ISE has developed a characterization tool called spectrometric characterization. In this paper we discuss that this tool is extremely powerful to characterize triple-junction (3J) EOL cells. The current-mismatch of the subcells can be determined and the performance for current-matched subcells can be predicted. Additionally, lattice-matched AlGaInP/GaInP/AlGaInAs/GaInAs/Ge quintuple-junction (5J) cells are being developed at Fraunhofer ISE as a possible next-generation of space solar cells. This material combination aims at the same BOL efficiency as state-of-the-art 3J cells but higher EOL efficiency. Based on spectral response measurements it will be demonstrated that the 5J cells do in fact show a higher radiation hardness compared to the 3J cell.

Investigation of Ge component cells

In this paper we investigate germanium (Ge) component cells with respect to their suitability as test structures for irradiation experiments and as reference cells. It will be demonstrated by means of spectral response measurements that Ge component cells do not exhibit the same behavior as a Ge subcell in the complete triple-junction (3J) device. Photon recycling from upper layers (GaInP, GaInAs) results in large signals in shorter wavelength regions. This leads to a higher photocurrent in the Ge component cell compared to the Ge subcell. In this paper it will be shown that this effect places restrictions on the suitability of Ge component cells for the above mentioned purposes. Possible solutions for each of the mentioned applications of Ge component cells are presented.