As attractive energy converters, solar cells are considered to play a vital role in covering the constantly increasing energy demands in the future. Despite the strong success in the field, a variety of solar cells including the commercially available ones are utilizing mainly the visible region of the solar spectrum, and thus are lacking the capacity to exploit effectively the infrared region. This issue gives a possibility to improve the solar cell efficiencies by using the unutilized energy of the infrared radiation. One solution to overcome this issue is to use upconverting luminescent materials which are capable of converting infrared radiation to visible by stacking photons. However, despite the potential, the rather weak conversion efficiency still limits the use of these materials. On the other hand, even small portions of this converted radiation would have significant effect on the solar cell efficiency which is why the development of these materials is desirable.

The main aims of this thesis work were to investigate and develop different fabrication methods and properties of the upconverting materials that could be used in solar cells. These methods included the co-precipitation method to prepare crystalline inorganic upconverting fluoride materials, the atomic and the molecular layer deposition techniques to fabricate upconverting oxide and hybrid thin films, and the direct particles doping method for preparation of upconverting luminescent glasses. In addition, another goal was to study the possibilities to enhance the upconversion luminescence which was done by adding transition metal ions (Cr3+ or Mnn+) into the Yb3+ and Er3+ doped fluoride material. Improvement in the upconversion luminescence intensity was obtained by using Cr3+ ions.

Another enhancement possibility is to use efficient NIR absorbers together with a strong upconverting lanthanide ion. The combined ALD/MLD technique was shown to enable the combination of NIR harvesting organic moiety and upconverting lanthanide ions to form a hybrid thin film. The ALD and the combined ALD/MLD techniques were demonstrated to be well suitable for upconverting thin film fabrication. Moreover, the direct particles doping method was shown to offer a promising way to introduce a variety of crystalline luminescent materials into different glass matrices.