Current matching

Another important issue in multijunction cells is current matching. The individual currents must exactly balance otherwise a loss in efficiency occurs. A current mismatch can be easily revealed by measuring the spectral response [595]. If the currents are matched, then the quantum efficiency is flat over a wide range of wavelengths. If one of the cells is limiting the current, then the observed quantum efficiency is not flat, and in fact is the quantum efficiency of the current-limiting cell. 

Olson JM, Kurtz SR (1993) Current-matched high-efficiency, multijunction monolithic solar cell. US patent US... 

Schueppel R, Timmreck R, Allinger N, Mueller T, Fumo M, Uhrich C, Leo K, Riede M (2010) Controlled current matching in small molecule organic tandem solar cells using doped spacer layers. J Appl Phys 107 6... 

To illustrate the technique, let s consider the case in which the system is stimulated by an applied voltage at a discrete frequency and the response current is measured at the same frequency. At zero frequency, or d.c., impedance is equivalent to resistance as defined by Ohm s law R = V/I. When the impressed voltage is oscillated at a particular frequency, the system responds by passing an oscillating current. If the amplitude of the input voltage is sufficiently small (typically < 10 mV), the system is linear, and the frequency of the response wave (current) matches the frequency of the perturbation (voltage). However, the response current wave may differ from the perturbation in amplitude and phase (Fig. 1). The ratio of the amplitudes of the perturbation to the response waveforms and the phase shift between the signals define the impedance function. 

Table I shows dut measurable gains in efficiency are possible using a high performance spectrum shifting dye (high quantum yield and low emission loss) for a dual-junction GaAs based crystalline solar cell. Since die two junctions were likely well current-matched, die additional photons were assumed to be emitted at two different wavelengths, with each wavelengdi being absorbed by a different junction. This was necessary because the two junctions are connected in series and therefore an increase in current production by one junction will be limited by the current production of the other junction. Therefore, the photons where divided such that the addition current added to each junction was equivalent. Quantum efficiency measurements showed that the top junction was active with minimum bottom junction absorption at 490 nm and that the bottom junction was active at 800 run widi minimal top junction absorptioiL The shifted photons were therefore split between 490 nm and 800 nia The external quantum efficiency of die device was 0.8 and 0.88 at 490 nm and 800 nm, respectively.

There needs to be enough flexibihty in selection of semiconductor bandgap, absorption properties and thickness to allow for current-matching in the stack design. 

Outer, W., et al. Current-matched triple-junction solar cell reaching 41.1% conversion efficiency under concentrated sunlight. Appl. Phys. Lett. 94, 223504 (2009)... 

From these basic experiments, it was found that silica aerogels had very low acoustic impedances compared to those of the current matching layers [7-9], and are therefore very promising as materials for the acoustic matching layers of high-sensitivity airborne ultrasonic transducers. 

The material with the smaller energy gap defines the current matching condition. Therefore, in Fig. 18, the current at the maximum power point of the photoanode is matched with the current of opposite sign from the photocathode. Since the... 

Hence, the current matching occurs at a potential where this cell component does not have its optimum performance, e.g., at potentials larger than that at the maximum power point. 

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