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Thin-film solar cells have the potential to require only a fraction of the material, and energy in comparison to the widely used silicon cells, deserving attention of the scientific community. Indeed, thin-film solar cells of Cu(In,Ga)Se2 (CIGS) and CZTS offer the highest Schokley-Queisser limit, above to the level achieved with c-Si cells. Besides being essential competitors, CIGS and CZTS solar cells have additional advantages compared to Si cells, such as the possibility of fabricating flexible modules, having a coefficient of temperature lower than the one of Si, a higher response under low irradiance conditions and lower production costs even with low CAPEX investments. Also, all these advantages make CIGS and CZTS technology a compelling candidate for several applications other than flat modules like, for instance, building-integrated PV. The current efficiency for CIGS solar cells is 23.35% and CZTS is 10.0%, and Shockley–Quessey limit is 33% and 32.4%, respectively, so there is still a high potential for the development of the thin-films solar cell architecture.

The atomic layer deposition (ALD) is not only an ultrathin film technology used in semiconductor industries. Lately, it has been found in many applications in the renewable energy field due to its precise control of thickness of up to a few angstroms and its unique characteristic of grown conformal and uniform films in any 3D structure of random geometry. ALD has long-range applications in this field, including electrochemical storage, fuel cells, photovoltaic solar energy, and photoelectrochemical (PEC) water splitting to produce hydrogen as a green fuel. In PEC water splitting, ALD is now being extensively used as an efficient tool to deposit surface passivation layers, absorber, and barrier and buffer layers in several kinds of PEC cells. This chapter aims to briefly review PEC technology, as well as to introduce the main ALD-based materials used to improve the solar-to-hydrogen efficiency that help to make this technology commercially viable.

This work proposes bifacial tandem solar panels with MOS cells on the backside aiming at applications in deserts. MOS solar cells were fabricated using Al(200nm)/ Mg(30nm)/SiO 2 (1.73nm)/Si-p structures. The gate oxide was grown by rapid thermal processing (RTP) and the main parameters studied were extracted by means of electric characterization through IxV curves of the MOS solar cells. For the operation temperature of the MOS cell varying from 25°C to 70°C, it was shown that the loss of the conversion efficiency ($\eta$) was at least 25% lower compared to conventional solar modules based on PN junctions and multi-crystalline-Si [9, 12]. As a result, the use of MOS solar cell on the backside of two different generations of CdS_CdTe cells with different conversion efficiencies at 25° C (15.8% and 21.0%), operating at the typical temperature of 70°C in deserts, promotes the increase of the conversion efficiency of 10.0% for CdS_CdTe1 (15.8%) and 6.0% for CdS_CdTe2 (21.0%).

Titanium dioxide (TiO( 2 ) thin films were deposited on conductive (titanium and fluorine tin oxide glass), insulant (mica, cover glass and thermal SiO( 2 thin film on silicon) and semiconductive (silicon (100) and 4H-SiC) substrates by atomic layer deposition (ALD) technique. The metal and ligand precursors used were titanium tetrachloride and water, respectively. Grazing incidence x-ray diffraction (GIXRD) analysis was performed to investigate the dependence of crystalline phase of the as-deposited thin films on different substrates for process temperatures ranging from $150-450 ^{\circ}\mathrm{C}$. Results indicate that the ALD TiO( 2 crystalline phase is dependent on the substrate nature which modifies the required temperature for phase change, i.e. from amorphous to anatase to rutile. For example, for conductive substrates the temperature for formation of rutile phase is around $350 ^{\circ}\mathrm{C}$ while for semiconductor substrates it was observed only from $400 ^{\circ}\mathrm{C}$. By other hand, when the substrate has an amorphous structure there is not a common rule, i.e. for mica and thermal SiO( 2 thin film on silicon only anatase phase was formed in all temperature range investigated while, for cover glass, it was possible to observe all stages of TiO( 2 phase change, highlighting the formation of brookite phase for temperatures between 300 and $350 ^{\circ}\mathrm{C}$. Moreover, it is shown that rutile phase can be obtained, in pure phase, at temperatures higher than $400 ^{\circ}\mathrm{C}$, however only for glass and titanium substrates. These results allow us to infer that less expensive Ti thin film could act as a good seed layer for growth of good quality rutile TiO( 2 phase, by using ALD process on Si substrate and using precursors such as TiCl 4 and H 2 O.

This paper discusses the metal-oxidesemiconductor (MOS) solar cells for energy harvesting applications using Al(200nm)/SiO 2 (1.73nm)/Si-p and Al(200nm)/Mg(30nm)/SiO 2 (1.73nm)/Si-p structures. P-type (100) silicon wafers with resistivity of 10 $\Omega$.cm were used. The gate oxide was obtained by rapid thermal processing (RTP) and the main parameters studied were extracted by means of electric characterization through IxV curves of the MOS solar cells with areas of 3.24 cm2. A different behavior compared to the cells in literature was observed with lower fill factor (~30)% against (~85)%) attributed to the MOS solar cells under the effect of series resistance (~ 12 $\Omega$) and low dark current density (~0.1 $\mu$ A/cm2), which determined reproducible electrical characteristics for energy harvesting in indoor environments with conversion efficiencies up to 12.8% and generated power up to 1mW/cm2.