A strategy that facilitates a substantial increase of carbon ionization in magnetron sputtering discharges is presented in this work. The strategy is based on increasing the electron temperature in a high power impulse magnetron sputtering discharge by using Ne as the sputtering gas. This allows for the generation of an energetic C + ion population and a substantial increase in the C + ion flux as compared to a conventional Ar-HiPIMS process. A direct consequence of the ionization enhancement is demonstrated by an increase in the mass density of the grown films up to 2.8 g/cm 3 ; the density values achieved are substantially higher than those obtained from conventional magnetron sputtering methods.

The discharge current behavior in reactive high power impulse magnetron sputtering (HiPIMS) of Ti-O and Al-O is investigated. It is found that for both metals, the discharge peak current significantly increases in the oxide mode in contrast to the behavior in reactive direct current magnetron sputtering where the discharge current increases for Al but decreases for Ti when oxygen is introduced. In order to investigate the increase in the discharge current in HiPIMS-mode, the ionic contribution of the discharge in the oxide and metal mode is measured using time-resolved mass spectrometry. The energy distributions and time evolution are investigated during the pulse-on time as well as in the post-discharge. In the oxide mode, the discharge is dominated by ionized oxygen, which has been preferentially sputtered from the target surface. The ionized oxygen determines the discharge behavior in reactive HiPIMS.

The effect of peak power in a high power impulse magnetron sputtering (HiPIMS) reactive deposition of TiO 2 films has been studied with respect to the deposition rate and coating properties. With increasing peak power not only the ionization of the sputtered material increases but also their energy. In order to correlate the variation in the ion energy distributions with the film properties, the phase composition, density and optical properties of the films grown with different HiPIMS-parmeters have been investigated and compared to a film grown using direct current magnetron sputtering (DCMS). All experiments were performed for constant average power and pulse on time (100W and 35 μs, respectively), different peak powers were achieved by varying the frequency of pulsing. Ion energy distributions for Ti and O and its dependence on the process conditions have been studied. It was found that films with the highest density and highest refractive index were grown under moderate HiPIMS conditions (moderate peak powers) resulting in only a small loss in massdeposition rate compared to DCMS. It was further found that TiO 2 films with anatase and rutile phases can be grown at room temperature without substrate heating and without postdeposition annealing.

This study evaluates the potential of hydrogen-doped In 2 O 3 (IOH) as a transparent back contact material in (Ag y ,Cu 1-y )(In 1-x ,Ga x )Se 2 solar cells. It is found that the presence of Na promotes the creation of Ga 2 O 3 at the back contact during (Ag y ,Cu 1-y )(In 1-x ,Ga x )Se 2 growth. An excessive Ga 2 O 3 formation results in a Ga depletion, which extends deep into the absorber layer.Consequently, the beneficial back surface field is removed and a detrimental reversed electrical field establishes. However, for more moderate Ga 2 O 3 amounts (obtained with reduced Na supply), the back surface field can be preserved. Characterization of corresponding solar cells suggests the presence of an ohmic back contact, even at absorber deposition temperatures of 550°C. The best solar cell with an IOH back contact shows a fill factor of 74% and an efficiency contacts are further needed in bifacial devices, which can be semitransparent if desired (eg, for "solar windows"). The upper cells in a multijunction tandem structure need to be grown on TBCs as well to allow for light propagation to the bottom cell. An advantage of these approaches over a superstrate configuration is that the required buffer layer does not have to undergo the thermal stress during absorber formation. Up to now, there are no buffer materials found, which are chemically stable at high temperatures, which results in a significant efficiency drop compared to substrate-configured solar cells. 4,5 Nevertheless, there are some requirements to a TBC as well. After the thermal stress during absorber formation, it still needs to (1) create an ohmic contact to the absorber, (2) be highly transparent, and (3) exhibit a low sheet resistance (R SH ). In the case of CIGS-based solar cells, it also needs to exhibit a certain permeability for alkaline ions, if they are supposed to be supplied from the underlying glass substrate.In the past, different transparent conductive oxide (TCO) layers have been investigated as potential TBC materials for CIGS solar cells. In this study, we use (Ag,Cu)(In,Ga)Se 2 (ACIGS) as an absorber material, which is proven to result in large grain sizes 19 and highefficiency solar cells. 20,21 The silver was added intentionally, since V OC losses for higher absorber band gap energies are less pronounced in ACIGS compared to CIGS. 21 This is in particular interesting for top cells in a tandem configuration where the optimum band gap energy E G is about 1.6 eV (two junctions).It should be emphasized that this work is supposed to act as a first potential evaluation for IOH as a TBC. Hence, the cell structure was designed for maximized efficiencies at normal front side illumination, which means that the absorber layer is about 2 μm thick and has a band gap energy of about 1.2 eV. | MATERIALS AND METHODS | Sample fabrication and configurationsThe ACIGS solar cells investigated in this study were manufactured as a stack of soda lime glass (SLG)/back contact/ACIGS/CdS/i-ZnO/AZO.As a back contact either IOH or standard Mo (DC-sp...

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