Study of the surface properties of ZnO nanocolumns used for thin-film solar cells

Densely packed ZnO nanocolumns (NCs), perpendicularly oriented to the fused-silica substrates were directly grown under hydrothermal conditions at 90 °C, with a growth rate of around 0.2 μm/h. The morphology of the nanostructures was visualized and analyzed by scanning electron microscopy (SEM). The surface properties of ZnO NCs and the binding state of present elements were investigated before and after different plasma treatments, typically used in plasma-enhanced CVD solar cell deposition processes, by X-ray photoelectron spectroscopy (XPS). Photothermal deflection spectroscopy (PDS) was used to investigate the optical and photoelectrical characteristics of the ZnO NCs, and the changes induced to the absorptance by the plasma treatments. A strong impact of hydrogen plasma treatment on the free-carrier and defect absorption of ZnO NCs has been directly detected in the PDS spectra. Although oxygen plasma treatment was proven to be more efficient in the surface activation of the ZnO NC, the PDS analysis showed that the plasma treatment left the optical and photoelectrical features of the ZnO NCs intact. Thus, it was proven that the selected oxygen plasma treatment can be of great benefit for the development of thin film solar cells based on ZnO NCs.


Introduction
The widely accepted design of thin-film silicon (TF-Si) solar cells, used for mass production, is composed of a transparent conductive oxide with roughness at the nanoscale on the front (TCO), e.g., tin oxide (SnO 2 ) or zinc oxide (ZnO), followed by p-i-n Si layers (amorphous and/or nanocrystalline) in the cell and a back reflector [1,2]. In such a layer arrangement, the light scattering and the consequent light trapping, caused by the interfaces with nano-scale roughness (front TCO-active layer and active layer-back reflector), increase the optical path inside in the thin silicon layer. These effects are observed in the weakly absorbing spectral region of silicon above approximately 650 nm, leading to efficiencies well above 13% at the cell level and above 12% at the module level [3,4]. However, the photo-generated current, determined by light absorption, is limited by the drift of generated electrons and holes across the absorber layer. Thus, the highest performances are expected for solar cells having a sufficiently large "optical thickness" and a sufficiently short distance between the electrodes, the "electrical thickness". In common planar TF-Si solar cells, it is impossible to simultaneously fulfil these two conditions. Recently developed solar cells based on a three dimensional (3-D) design, in which periodically ordered zinc oxide nanocolumns (ZnO NCs) are used as a front electrode, have been of great interest, because they would exceed in the ultimate light trapping and provide excellent charge separation [5][6][7]. Due to the vertical geometry of these textures, the optical thickness is dictated by the height of the NCs, such that most of the light traversing the cell sees an absorber-layer thickness approximately equal to the NC height. In contrast, as the front and back TCO contacts are interpenetrating, the inter-electrode distance, given by the thickness of the Si layers on the walls of the NCs, is generally substantially thinner than that applied for state-ofthe-art a-Si:H solar cells; the lateral carrier transport provided by this type of texture should thus ensure an optimal current collection. Therefore, it is envisaged that in comparison to thinfilm planar cells with nano-scale roughness, the 3-D solar cells might lead to higher efficiency providing important assets such as minimal material consumption [8][9][10]. The proposed 3-D concept is not limited to thin-film silicon solar cells, but could be advantageously used for all other thin-film solar cells.
So far, a wide diversity of methods have been used for the preparation of ZnO nanocolumns such as metal organic chemical vapor deposition (MOCVD) [11], electrochemical deposition [12], sputtering [13], reactive ion etching [5] and the hydrothermal method [6,14,15]. The last mentioned is an attractive and preferable method for growing one-dimensional structures of ZnO, as it is simple, does not require expensive equipment, is safe and environmentally friendly since water is used as a solvent, and it is easy to scale-up for further mass production. Solar cell deposition is a multistep process during which different plasma processes are being used. Oxygen plasma is applied for both, activation of the surface and stripping of the polymer mask used for fabrication of periodically ordered ZnO NCs, while hydrogen plasma is usually used immediately before the deposition of the active solar-cell layer for directly increasing the electrical conductivity. The employed plasma treatments could significantly influence the concentration of defects and free carriers, reflected in the defect and free-carrier adsorption, and consequently impact the efficiency of the solar cell. Therefore, the investigation of the effects of the different plasma treatments on the ZnO nanocolumns is of crucial importance.
Herein, a low-temperature hydrothermal method is used to synthesize densely packed NCs on fused silica substrates covered with a ZnO seed layer, which were prepared before by DC reactive magnetron sputtering. The optical absorption of the pristine ZnO layers as well as that of the substrates bearing the dense ZnO NCs was investigated by photothermal deflection spectroscopy (PDS) [16,17]. Furthermore, we investigated the changes in the PDS spectrum of the dense NCs induced by hydrogen and oxygen plasma treatment under conditions typical for plasma processing of thin-film silicon solar cells. XPS was used to determine the changes in surface composition as a result of the different plasma treatments. Figure 1 reports SEM images of densely packed ZnO nanocolumns grown at 90 °C for 180 min. As it can be seen from the SEM cross-sectional view, the ZnO nanocolumns are not interconnected and are well spaced with gaps of several nanometers. Notably, it is evident that each column has well defined boundaries ( Figure 1a). The position of the individual nanocolumns is random with diameters varying from around 30 to 180 nm, as revealed by the top-view SEM image (Figure 1b).

Results and Discussion
An average nanocolumn length of 650 nm was measured from the cross section, as shown in the SEM image of Figure 1a. The thickness of the seed layer is about 150 nm. It should be noted, that after the hydrogen or oxygen surface plasma treatments the morphology of the nanocolumns does not change (see Figure S1 and Figure S2, Supporting Information File 1 for details).
The chemical bonding structure of the ZnO films prepared by hydrothermal growth from a seed layer on fused silica carriers was examined by XPS. Figure 2    The changes induced to the synthesized ZnO NCs by the exposure to different plasma treatments was further probed by PDS. Figure 3 shows the optical absorptance spectra of as-grown ZnO nanocolumns and NCs treated in H-plasma for 1, 5, 10 and 25 min. The measured PDS absorptance spectra reflect the absorption edge, Urbach tail, absorption on defects and freecarrier absorption (proportional to the concentration of free carriers).
All optical absorptance spectra show the optical absorption edge at a photon energy of 3.3 eV and the free-carrier absorption in the red and the infrared part of the spectrum below photon ener- gies of 2 eV. The infrared optical absorption increases with hydrogen plasma treatment indicating the increase of the freecarrier concentration as described in the Drude model. The increase of the free-carrier concentration is reasonably expected to increase the electrical conductivity of the ZnO NCs. Nevertheless, precise measurement of electrical conductivity is a difficult task and we plan to approach it by direct measurement on individual ZnO nanocolumns. The major changes appear within several minutes of exposure to hydrogen plasma and the effect saturates after about 10 min. We suppose that hydrogen diffuses into ZnO creating shallow donors [19,20]. We note that the hydrogen doping does not shift the optical absorption edge [21], which means that the lattice as well as the occupancy of valence and conductive states does not change significantly (up to the degenerate conduction band) [22].
While the hydrogen plasma treatment induced significant changes in the optical absorptance spectra, notably, the PDS spectra show that there is no detectable change of the infrared optical absorptance (Figure 4). This observation strongly suggests that the O-plasma treatment does not have any detrimental effects on the free-carrier concentration in the ZnO NCs.

Conclusion
In this work, randomly arranged densely packed and preferentially perpendicularly oriented ZnO nanocolumn arrays were grown from seed layers on fused silica substrates.  free-carrier adsorption of up to two orders of magnitude, thus seriously affecting the optical and photoelectrical characteristics of the ZnO NCs. The oxygen-plasma treatment led to negligible changes in the PDS absorptance spectra. The concomitant increase in the presence of active surface species and only minor influence on the optical and photoelectrical features of the ZnO NCs absorptance spectra, make the O-plasma a preferred treatment for the preparation of thin-film solar cells based on ZnO NCs. The reported ZnO nanocolumns layer with a proper spacing could be used as a 3-D scaffold not only for amorphous silicon solar cells, but also for other absorbers with a short lifetime such as CuO, CuO 2 , FeS 2 , quantum dots or nanocrystalline Si.

Experimental
The growth of densely packed ZnO nanocolumns was performed on fused silica (Suprasil ® ) substrates that were covered with an undoped thin seed layer of ZnO by DC reactive magnetron sputtering. The parameters of magnetron sputtering were as follows: processing temperature of 400 °C at a target voltage of 400 V, ratio between gas species of Ar/O = 2/0.5 for 10 min.
The dimensions of all substrates were 10 × 10 mm 2 . Before seed layer deposition, the substrates were cleaned in an ultrasonic bath with acetone for 10 min, then rinsed with deionized water and dried under nitrogen flow.
The hydrothermal growth of ZnO nanocolumns was performed from an equimolar aqueous solution of 25 mmol zinc nitrate hexahydrate (Zn(NO 3 ) 2 ·6H 2 O) and hexamethylenetetramine ((CH 2 ) 6 N 4 ) in an aqueous bath at 90 °C for 3 h [15,23]. During the nanocolumns growth, the substrate was mounted upsidedown on a sample holder. After termination the sample was thoroughly washed in deionized water and dried in nitrogen flow. The surface morphology of the samples was characterized by SEM (MAIA3, TESCAN). The electron energies were