Procedures for measuring the spectral response of multi-junction cells in general require variation of the bias spectrum and voltage biasing. It is shown that a refined procedure including optimization of bias spectrum and voltage is necessary to minimize a measurement artifact, which appears if the subcell under test has non-ideal properties, such as a low shunt resistance or a low reverse breakdown voltage. This measurement artifact is often observed on measuring the spectral response of the Ge bottom cell of GaInP/Ga(In)As/Ge triple-junction cells. The main aspect of the measurement artifact is that the response of another subcell is simultaneously measured, while at the same time the signal of the Ge subcell is too low. Additionally, the shape of the spectral response curve is influenced under certain measurement conditions. In this paper the measurement artifact is thoroughly discussed by measurement results and simulation. Based on this analysis, a detailed procedure for the spectral response measurement of multi-junction cells is developed, specially designed to minimize such measurement artifacts

III-V monolithic multi-junction (MJ) solar cells reach efficiencies exceeding 30% (AM1.5 global) and have applications in space and in terrestrial concentrator systems. The subcells of monolithic MJ cells are not accessible separately, which presents a challenge to measurement systems and procedures. A mathematical approach is presented which enables a fast way of spectral mismatch correction for MJ cells, thereby significantly reducing the time required for calibration. Moreover, a systematic investigation of the I-V parameters of a MJ solar cell with variation of the incident spectrum is possible, herein called 'spectrometric characterization'. This analysis method visualizes the effects of current limitation and shifting of the operating voltage, and yields precise information about the current-matching of the subcells. MJ cells can hereby be compared without the need to match the current of the structures to a reference spectrum in advance. Further applications of the spectrometric characterization are suggested, such as for the determination of the radiation response of the subcells of MJ space solar cells or for the prediction of the annual power output of terrestrial MJ concentrator cell

The use of Ga 1Àx In x As instead of GaAs as a bottom solar cell in a Ga y In 1Ày P/ Ga 1Àx In x As tandem structure increases the¯exibility of choosing the optimum bandgap combination of materials for a multijunction solar cell. Higher theoretical ef®ciencies are calculated and different cell concepts are suggested for space and terrestrial concentrator applications. Various Ga y In 1Ày P/Ga 1Àx In x As material combinations have been investigated for the ®rst time and ef®ciencies up to 24Á1% (AM0) and 27Á0% (AM1Á5 direct) have been reached under one-sun conditions. An ef®ciency of 30Á0±31Á3% was measured for a Ga 0Á35 In 0Á65 P/Ga 0Á83 In 0Á17 As tandem concentrator cell with prismatic cover at 300 suns. The top and bottom cell layers of this structure are grown lattice-matched to each other, but a large mismatch is introduced at the interface to the GaAs substrate. This cell structure is well suited for the use in next-generation terrestrial concentrators working at high concentration ratios. For the ®rst time a cell ef®ciency up to 29±30% has been measured at concentration levels up to 1300 suns. A small prototype concentrator with Fresnel lenses and four tandem solar cells working at C 120 has been constructed, with an outdoor ef®ciency of 23%.

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