  The doping of wide gap semiconductors is an interesting problem both from the
scientific and technological point of view. A well known example of this
problem is the doping of Cu$_2$O. The only element which has produced an order
of magnitude increase in the conductivity of Cu$_2$O bulk samples is chlorine,
as previously reported by us and others. However the solar cells produced with
this material do not show any improvement in performances because of the
reduction in the minority carrier diffusion length. In this paper we
investigate the effect of other impurities in order to check their possible use
as dopants and to assess their effects on the minority carrier diffusion
length. Seven impurities have been introduced by evaporation on the starting
copper sheet before the oxidation used to produce Cu$_2$O: chromium (Cr), iron
(Fe), silver (Ag), silicon (Si), sodium (Na), sulfur (S) and phosphorus (P).
The experiments show that a 20 ppm of concentration of these dopants does not
give any relevant effect neither on the resistivity, nor on the mobility. The
effect on minority carrier diffusion length is also negligible except for
sodium which produces a slight degradation of the samples.
