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Modified method of surface photovoltage (SPV), spectral response measurement, and constant photocurrent method (CPM) were applied to thin film CdS/CdTe solar cells with the aim of finding diffusion length of minority carriers (L) in the CdTe material. The SPV signal was theoretically calculated without constraints of absorption coefficients for the incident radiation and thickness of the sample assuming one space charge region (SCR) located on the CdS/CdTe interface. In addition to the diffusion length, the SPV is a function of the surface recombination velocity and the parameters of the SCR, which complicates the evaluation. Illuminating the back side of the solar cell (without ohmic contact) we obtain a photovoltage spectrum predominantly influenced by the diffusion length. On the other hand, the standard measurement using light penetrating from the CdS side strongly depends on the thickness of the SCR. The small signal approximation model presented here successfully explains both measured spectra and permits extraction of the diffusion length of minority carriers and thickness of the SCR in CdTe absorber. The CPM is used for determination of absorption coefficients in the CdTe layer. The absorption of this material depends on its preparation and must be known for correct evaluation of experimental data.

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Cadmium sulphide and cadmium telluride thin layers are sprayed and electrodeposited, respectively, for n-CdS/p-CdTe solar cells. Approximately equal concentrations of ionized donors and acceptors (4 x l O I 5 cm-3) in n-and p-parts, respectively, and the thickness of space charge region in CdS which extends over the full width of the CdS film, are derived from the 1/C2-U, specially shaped, dependence. The dark current transport mechanism of the cells is consistent with the tunnelling/intcrface recombination model. The best photovoltaic parameters deduced from I -U characteristics under illumination equivalent to AM 1.5 are: J,, = 22 mA/cmZ, U,, = 675 mV, FF = 0.54, q = 7.4%. The spectral response of the short-circuit current agrees well with the generalized theory of the photovoltaic effect published previously. The main contribution to the photocurrent originates from the space charge region in CdTe.Couches minces de CdS et CdTe sont preparees par la mkthode de pulverisation chimique reactive et electrodeposition, respectivement. Presque les memes concentrations en donneurs et accepteurs (4 x 10" cm-3) dans n-et p-parties, respectivement, et la largeur de zone de charge d'espace dans lc CdS expandue par son volume entier sont determinees en evaluant la dependence de l/Cz-U qui sc deroule en forme spkciale. O n explique le mecanisme de transport d u courant dans l'obscurite par le model tunnel/interface recombinaison. Les parametres photovoltai'ques meilleurs obtenus par le mesure de caracteristiques courant-tension sous eclairement equivalent au AM 1,5 sont: J,, = 22 mA/cm2, U,, = 675 mV, FF = 0,54, q = 7,4%. La reponse specrale du courant de court-circuit suit bien la theorie gtnt.ralisee d'effet photovoltaique qui a ete publiee auparavant. C'est de la zone desert& du CdTe d'od le contribution au photocourant est plus grand.') Ke Karlovu 5, 121 16 Prague 2, Czech Republic.