Electrical Properties of CdS Films

CdS films for solar cell applications should have low resistivity in order to reduce the series resistance of the cell. Films which have been evaporated at high rates and at low substrate temperatures are usually n-type and exhibit a resistivity of 1 - loo Qcm /28/. Typical values are lO f2cm. The residual gases and the purity of the starting material also show an important effect on the resistivity. Low temperature resistivity measurements indicated an activation energy of 25 meV for shallow donor levelds. The carrier concentration is 10 - lO /cm, the mobility is about 10 cm /Vs. 

Films with low resistivity can be obtained by In doping /29/. Highly photoconductive films with high dark resistance can be made by Cu or Ag doping. These elements act as deep acceptors at an energy level between 0.8 and 1.2 eV above the valence band and hence cause a fractional compensation of the n-type conductivity /30/. 

---

Doping can be divided into two parts native doping (e.g., S vacancies) and extrinsic doping by foreign elements. This section deals with the latter, not because it is more important but because there is httle in the literature to link native doping with the electrical properties of CD films. It will be enough to note that the few measurements of No (donor density) carried out tend to give values typically... 

---