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Summary form only given. Optical studies of semiconductor quantum dots (QDs) have been a subject of very intensive recent investigations. It has been experimentally and theoretically established that the number of carriers which occupy a photoexcited QD greatly affect its photoluminescence (PL) spectrum. In spite of its neutral nature, optical spectroscopy has very recently proved to be a useful means for investigating and preparing charged QD systems. We report here on continuous wave (cw) and pulsed optical PL spectroscopy of single self-assembled QDs (SAQDs) embedded within a mixed type quantum well (QW) structure. This specific design, which facilitates charge separation by optical means, is used here to tune the charge state of the QD under study. We compare the PL emission spectra for a single In(Ga)As/GaAs SAQD occupied with an increasing number of neutral multiexcitons, and the emission of the same type of dots embedded in a GaAs/AlAs mixed type structure. Using time-resolved spectroscopy and a comparison between cw and pulse-excited PL spectra we are able to accurately determine the charge state of the single QDs at a given excitation intensity level. We determine the collective carrier state from which each spectral line originates.