TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Device: The hole transport material is of formula (I), (II) or (III);R = hexyl or ethylhexyl, with the wt.% ratio of hexyl:ethylhexyl being 40:60.The oxidized hole transport material is present in the mixture with the hole transport material at 0.01-10 (preferably 0.08-1.2) wt%.The device further comprises a semiconductor oxide layer sensitized with a dye (preferably a ruthenium complex). The semiconductor oxide layer is preferably porous and comprises nanoparticles, preferably nanoparticles of TiO2.Preferred Method: The hole transport material in step (i) is chemically oxidized. Unreacted reactants and reduced oxidants are removed prior mixing step (ii). The oxidation is carried out using silver hexafluoroantimonate (AgSbF6) or nitrosonium tetrafluoroborate (NOBF4). By oxidation, radical cations of the hole transport material are obtained. The method further comprises connecting electrodes to the semiconductor oxide layer and to the mixture. EXAMPLE - To compare the stability of a doped hole transport system according to the invention with a system known in the prior art, UV spectra of hole transport material doped with various dopants were taken on a spectrophotometer.    In the UV spectra of pristine 2,2',7,7'-tetrakis(diphenylamino)-9,9'-spirofluorene (spiro-MeO-TAD) and dopant N(PhBr)3SbCl6 (in chloroform at a concentration of 10-5M), the absorption value of dopant was found to be at 309 nm and that of spiro-MeO-TAD at 297 and 357 nm.    The UV-Vis spectra of the spiro-MeO-TAD doped with 10 wt.% of the dopant in chloroform showed that there was not much shift in these absorption when the spiro compound was mixed with the dopant, but in addition to the above two strong absorption values at 297 and 357 nm, there was a new absorption at 518 nm. This may have been due to the formation of a charge-transfer complex between the hole transport material and the dopant resulting in higher conductivity of the hole transport material due to the creation of positive centers which is responsible for the increase in the efficiency of the solar cell with the addition of the dopant. The new species may have been a radical cation. It was observed that the color of the solution was diminishing with time and became colorless within hours. This indicated that the new species obtained by the addition of the dopant to the hole transport material was unstable.    The UV-Vis spectra of the spiro compound doped with the dopant after four hours showed that the absorption at 518 nm had vanished after four hours and that the intensity of the absorption at 377 nm had also lowered. This showed that the newly formed species was very unstable and the life time of this species was less than four hours.    To make the system stable, it was necessary to have a stable material which has a similar structure and physical properties of the hole transport material. To achieve this, the hole transport material was chemically oxidized into a radical cation salt and then mixed with the unmodified hole transport material.    The UV-Vis spectra of pristine spiro compound, spiro compound doped with dopant N(PhBr)3SbCl6 (10% by weight) and oxidized spiro compound (oxidized with AgSbF6). The similarity in absorption at 519 nm of the radical cation salt spiro-SbF6 with that of the prior art doped spiro compound indicated that the species formed by the chemical oxidation is similar in nature. In addition, there were two more new absorption at 677 and 887 nm found for the oxidized spiro compound.    To show the stability of the oxidized spiro compound, UV-Vis spectra were measured at different intervals (zero hours, after one day and after one week). Unlike the spiro compound doped with the dopant, only slight changes in intensity of absorption could be found after one day. The oxidized spiro compound was still stable in solution after one week. This clearly indicated that the radical cation of the spiro compound obtained by chemical oxidation led to a doped conjugated semiconductor, which showed improved characteristics compared to the known doped conjugated semiconductors, and therefore to an improved device having improved stability and no phase separation and degradation.