Impact of the TCO Microstructure on the Electronic Properties of Carbazole-Based Self-Assembled Monolayers

Carbazole-based self-assembled monolayers (PACz-SAMs), anchored via their phosphonic acid group on a transparent conductive oxide (TCO), have demonstrated excellent performance as hole-selective layers in perovskite/silicon tandem solar cells. Yet, whereas different PACz-SAMs have been explored, the role of the TCO, and specifically its microstructure, on the hole transport properties of the TCO/PACz-SAMs stack has been largely overlooked. Here, we demonstrate that the TCO microstructure directly impacts the work function (WF) shift after SAM anchoring and is responsible for WF variations at the micro/nanoscale. Specifically, we studied Sn-doped In2O3 (ITO) substrates with amorphous and polycrystalline (featuring either nanoscale- or microscale-sized grains) microstructures before and after 2PACz-SAMs and NiOx/2PACz-SAMs anchoring. With this, we established a direct correlation between the ITO crystal grain orientation and 2PACz-SAMs local potential distribution, i.e., the WF. Importantly, these variations vanish for amorphous oxides (either in the form of amorphous ITO or when adding an amorphous NiOx buffer layer), where a homogeneous surface potential distribution is found. These findings highlight the importance of TCO microstructure tuning, to enable both high mobility and broadband transparent electrodes while ensuring uniform WF distribution upon application of hole transport SAMs, both critical for enhanced device performance.


MAIN TEXT
Transparent conductive oxides (TCOs) are omnipresent in a range of high-efficiency optoelectronic devices, including perovskite solar cells (PSCs), both in their single junction and tandem implementations 1,2 .Among the various available TCOs, indium tin oxide (ITO) remains a common choice as a transparent electrode for optoelectronic applications as it is well established, scalable, and commercial glass/ITO substrates are readily available.However, further progress in PSC performance could benefit from additional TCO optimization.Moreover, with the rise of monolithic perovskite/perovskite and perovskite/silicon tandem solar cells, TCOs are also often used as an interband recombination junction 3 deposited onto the bottom cell (i.e., a perovskite or silicon cell), connecting the subcells in series.Both in the single-junction and in the tandem case for inverted, p-i-n devices, the hole transport layer (HTL) is deposited onto the TCO, followed by the perovskite absorber and the electron selective contact stack.Depending on the TCO deposition method and process conditions, different microstructures can be achieved which may also influence the optoelectronic properties.For instance, amorphous TCOs generally feature a narrower band gap as compared to polycrystalline TCOs due to their distorted absorption edge 4,5 .
Furthermore, we investigate the potential distribution and its link to the WF along the surface of various types of ITO substrates with different microstructures.We focus in this work on ITO as a model system, but the drawn conclusions are valid for other TCOs.Specifically, we studied ITO substrates with comparable sheet resistance but distinct microstructures, namely: commercial ITO, featuring a polycrystalline microstructure with small (nm-scale) grains, and pulsed laser deposited (PLD) ITO, either amorphous or polycrystalline with large (μm-scale) grains.Moreover, the effect of introducing a sputtered NiOx layer between the different ITO electrodes and 2PACz-SAMs was analysed.The potential distribution was mapped using Kelvin probe force microscopy (KPFM), while the grain orientation of the ITO for the same area was measured by electron backscatter diffraction (EBSD) analysis.Ultraviolet photoelectron spectroscopy (UPS) was used to determine absolute WF values and verify the values determined by KPFM.Based on these experiments, we demonstrate how the ITO crystalline grain orientation and grain size influence the potential distribution in the ITO/2PACz-SAMs electrodes, compare the WF values achieved for the distinct ITOs and discuss the role of a NiOx buffer layer on achieving a uniform potential distribution for hole extraction.By optimizing the TCO/PACz-SAMs interface, valuable insights for enhancing solar cell designs can be gained, thus further improving their efficiency and reliability.
EXPERIMENTAL SECTION 100 nm Sn-doped In2O3 (10/90 wt% SnO2/In2O3) thin-films with sheet resistance (Rsh) below 50 Ω/sq, representing typical TCO device requirements, with three different microstructures were selected for this study.The ITO films were deposited on glass substrates by PLD at room temperature.As-deposited ITO films were found to be amorphous, subsequent annealing for 20 min at 450˚C (Figure S2), resulted in a polycrystalline structure, as confirmed by XRD (Figure 1a, Table S1).Top-view SEM and AFM scans (Figure 1c-d) show a flat, featureless surface for the amorphous ITO (RMS of 0.29 nm), and large micron-sized grains for the annealed ITO films (RMS of 0.30 nm).This observed change in crystallinity and microstructure is due to a solid phase crystallization, as previously reported for sputtered In-based TCOs 7,8 .In the process of physical vapor deposition of In-based TCOs at room temperature (either by PLD or sputtering), nanocrystals are generated within an amorphous matrix.These nanocrystals act as nucleation sites, facilitating the growth of grains during a subsequent annealing step 7 .Commercially available ITO substrates (Ossila Ltd.), featuring a polycrystalline structure and nano-scale grains microstructure (RMS of 3.20 nm), as depicted in Figure 1a and e, respectively, were used to compare the influence of the grain size in the WF distribution.Furthermore, all studied ITO films demonstrate >80% transmittance in the wavelength range of 350-750 nm (Figure 1b).For wavelengths above 750 nm, the commercial ITO samples exhibited a larger absorption (>15%) as compared to PLD ITO.This can be explained by free carrier absorption as the concentration of free carriers, Ne, is significantly higher (10x10 20 cm -3 ) for commercial ITO films as compared to PLD films (up to 4.5x10 20 cm -3 ).The electrical properties of the ITO films are summarized in the inset of Figure  For conciseness, as-deposited ITO films will further be referred as 'a-ITO', annealed ITO polycrystalline films with large (μm-sized) grains as 'poly-ITO-μm-grain' and commercial ITO polycrystalline films with small (nm-sized) grains as 'poly-ITO-nm-grain'.
To gain insight into the surface potential distribution of the ITO films with distinct microstructural properties, KPFM was performed, which directly maps the contact potential difference (CPD) between a conducting AFM tip and the sample (Figure S4).Here, a relatively large area of 10x10 μm 2 was scanned.The resulting CPD maps and the corresponding topography image (inset) are shown in the left column in Figure 2a.In the case of a-ITO and poly-ITO-nm-grains, a relatively consistent surface potential across the scanned film area is measured, indicating an overall homogeneous WF distribution.However, in the case of poly-ITO-μm-grain films, the CPD is not uniform across the scanned area, as observed by the lighter and darker domains in Figure 2a, representing areas of higher and lower WF values, respectively.The domains match with the corresponding large grains in the topography inset as marked with white arrows.Subsequently, 2PACz-SAMs were deposited onto the three ITO films described above (details in SI).The right column in Figure 2a displays the CPD maps, accompanied by insets of topography AFM images of the ITO/2PACz-SAMs substrates.Generally, the topography remained virtually unaltered with the introduction of 2PACz-SAMs, but a systematic overall reduction in CPD confirms the presence of 2PACz-SAMs on the surface and implies an increase in the WF.Notably, in the case of the poly-ITO-μm-grain films, the presence of domains with distinct CPD values remain even after the 2PACz-SAMs application.NiOx is a high work function metal oxide and has been demonstrated as an effective hole transport layer in inverted PSCs 41,42 .However, it is widely reported that its direct contact with the perovskite absorber, leads to a defective interface, leading to the development of several NiOx surface passivation approaches, including the use of PACz-SAMs 37,43,44 .While similar performance have been reported for PSCs with ITO/2PACz-SAMs and ITO/NiOx/2PACz-SAMs, it has been proposed 40,44 that the use of a thin NiOx buffer layer deposited on top of ITO helps to homogenize morphological and energetical differences on the ITO substrates, enabling higher reproducibility in devices for the ITO/NiOx/2PACz-SAMs stack as compared to the ITO/2PACz-SAMs counterpart 37,39 .It is hypothesized that the presence of a NiOx buffer, could minimize the impact of the presence of pin holes in the 2PACz-SAMs layer 38,40 .Moreover, most of the reported NiOx hole transport layers, are either amorphous or nanocrystalline, with randomly oriented nanometre-sized grains.We suggest that this amorphous or nanocrystalline microstructure is also beneficial for homogenizing the surface roughness and the surface potential as will be elaborated below.
To confirm this, here we sputtered an amorphous NiOx layer (14 nm) onto the studied ITOs.
Figure S5 displays a featureless X-ray diffraction pattern, confirming the formation of an amorphous NiOx film.For elaborate characterizations and properties of the NiOx layer, we refer the reader to reference 41 .We note that NiOx subsequently underwent a treatment with a potassium chloride (KCl) solution to passivate its surface defects 45  The WF values were later confirmed by UPS (Figure 4).

Co-localization KPFM and EBSD mapping: correlating ITO grain orientation with work function
The KPFM findings reveal a notable correspondence between the CPD domains and the respective ITO grains in the poly-ITO-μm-grain films.To ascertain whether the identified domains originate from distinct crystalline orientations, a combined approach employing EBSD mapping and KPFM measurements is adopted for simultaneous topographical, electronic and microstructural imaging of the ITO surface on the same point of interest (POI).This is schematically represented in Figure 3a (details on protocol procedure 46

in SI).
It is well-established that crystal orientations can lead to diverse atomic arrangements on a material's surface, influencing the electronic structure and impacting the WF.It has been proposed that generally, closely packed planes (high atomic density) display higher WF compared to loosely packed planes (low atomic density) 47 .From reported surface densities based on density functional theory calculations for In2O3 (100) > (110) > (111), it is speculated that ITO (100) planes possess higher WF 48 .Another theory that may elucidate the phenomenon of crystalline orientation's influence on local WF variation is the surface polarity concept, introduced by Tasker 49 .While the initial observations were for In2O3 48,50,51 this concept can be extended to ITO.In detail, (111) plane is a Tasker II type of surface without surface dipole perpendicular to the surface normal.On the other hand, (100) plane corresponds to a Tasker III type of surface, characterized by alternating charged planes that lead to a dipolar moment on the uppermost surface layer pointing away from the surface in the normal direction.Consequently, this induced surface dipole makes the removal of an electron more challenging, resulting in an increased WF 14 (Figure 3f).
Topography and CPD maps obtained from KPFM for poly-ITO-μm-grain films are presented in Figure 3b-c.Using the dash-framed grains and arrows as guiding marks, we note that the indicated grains exhibit a lighter coloration, implying a higher applied CPD and thus a lower WF.The respective grains in the EBSD map (out-of-plane, z-direction) are coloured green, indicating (111) orientation (Figure 3d-e, S6).Conversely, the darker-coloured grains identified on the CPD map, reflecting a low applied CPD, thus a high WF, align with the red/blue coloured grains on the EBSD map, which correspond to the (001) family of planes.The preferred surface termination of a single grain along 4×(001) was also confirmed with high-resolution transmission electron microscopy (HR-TEM) and selected area diffraction pattern (SADP) as presented in Figure 3g-h.
Furthermore, we conducted EBSD on poly-ITO-nm-grain films, as shown in Figure 3i-j and   Figure S7.The distinct crystal orientations of the nm-scale grains are visible, suggesting that local WF variations induced by the grain's orientation could also be expected.However, due to the nmscale of the grains, the spatial resolution of the KPFM tip was not sufficient to detect such nanoscale variation.Despite this, it is suggested that local WF variations, even at the nanoscale, are expected on any polycrystalline sample 47,51 .The presence of significant energetic variations across a material can lead to unwanted effects on a device level, such as an uneven charge distribution, altered electronic transport properties, and even limitations in device efficiency and performance as previously reported 37,38,52 .To verify the WF values determined by KPFM and to extract the valence band maximum (VBM) and the highest occupied molecular orbital (HOMO) levels, UPS measurements were conducted.
Figure 4a showcases the WF values determined by KPFM for the studied ITOs, in their initial state (right upon solution cleaning procedure), after UV-O3 treatment, and subsequent 2PACz-SAMs deposition.The large error bar for the poly-ITO-μm-grain film, could be ascribed to the presence of the distinct crystalline grain orientations.However, the overall WF of all bare ITOs are found within the same range, and a systematic increase in the WF after UV-O3 treatment and upon 2PACz-SAMs anchoring is evident across all studied ITO films.This trend is expected, as any form of surface treatment inherently alters the surface potential and thereby directly impacts the WF 47 .
UPS results complement these findings, indicating a pronounced shift of the secondary electron cut-off (SECO) towards lower binding energies.This shift strongly implies a substantial WF increase upon the anchoring of 2PACz-SAMs, a phenomenon consistent across all the examined cases.In addition, the VBM region spectra show a significant modification after 2PACz-SAMs deposition.Specifically, the characteristic sharp, linear-like form typical of TCOsattributed to localized electronic states changes to a hump-like HOMO edge, characteristic of organic molecules with delocalized π-electrons 53 (Figure 4b).The presence of 2PACz-SAMs on the surface was further confirmed via X-ray photoelectron spectroscopy (XPS) analysis (Figure S8-9).S2.An overall good correlation between two independent techniques (each operating at different conditions, i.e., UPS at UHV and KPFM at ambient air) and a consistent trend in the WF increase upon ITOs surface modification can be observed (for a detailed explanation, check Figure S10).
As expected, all three ITO films have pronounced n-type character as their Fermi level is far from VBM. Modified ITOs, either with 2PACz-SAMs or NiOx only, or a combination NiOx/2PACz-SAMs shifted the Fermi level closer to the ionization potential level (HOMO or VBM, respectively), indicating the p-type characteristics and enhanced hole selectivity.It is worth highlighting that the values of the WF and VBM for ITO/NiOx agree with previously reported values for ALD deposited NiOx (4.6 -4.7 eV and 5.3 eV, respectively) 37 .Upon depositing 2PACz-SAMs, the WF is further increased, due to the molecular dipole moment that 2PACz-SAMs introduce 44 , contributing to the surface energy term 47 .Notably, the WF values for the ITO/2PACz-SAMs case and the ITO/NiOx/2PACz-SAMs case are quite similar.Going back to the KPFM mapping, we suggest that the main advantage of NiOx in addition to its hole transport properties is its amorphous nature, minimizing pin hole formation and ensuring a uniform WF distribution due to the lack of preferential grain orientation.We therefore argue that the use of an amorphous metal oxide with adequate WF for hole (or electron) extraction, or an amorphous TCO buffer layer ensures an enhanced coverage of the 2PACz-SAMs and with it, a uniform WF distribution.This combination holds the potential to improve energy level alignment and charge extraction on a device scale, thereby enhancing overall device performance, reproducibility, and stability.

Conclusion
In summary, we studied the impact of the ITO microstructure on the electronic properties of hole-selective transport layers, NiOx and 2PACz-SAMs.Three different types of ITO thin-films morphology and microstructure were characterized.Correlated KPFM and EBSD mapping revealed that polycrystalline ITO films present a non-uniform distribution of surface potential, subsequently impacting WF uniformity.The application of 2PACz-SAMs was not sufficient to overcome the lateral inhomogeneity in WF inherent to the polycrystalline ITO films.However, this challenge can be successfully addressed by employing either an amorphous TCO or an amorphous NiOx buffer layer.While polycrystalline TCOs present the potential for high mobility, P. De Wolf for discussions on AFM and KPFM measurements, A. A. Said for guidance on NiOx sputtering and E. Aydin on fruitful discussions on ITO/2PACz properties.

1b.
Additionally, an increase in the optical band gap for PLD ITO annealed films from ~3.4 eV for as deposited ITO to ~3.7 eV for annealed ITO was estimated from the Tauc plot in FigureS3.).

Figure 2 .
Figure 2. KPFM mapping for the stacks: (a) ITO with and without 2PACz-SAMs; (b) ITO/NiOx with and without 2PACz-SAMs (scanning area 10x10 μm 2 , larger image: CPD mapping, inset: . Topography and surface potential images for the resulting ITO/NiOx stack with and without 2PACz-SAMs are shown in Figure 2b.The left-hand column illustrates CPD mappings and topography insets for the ITO/NiOx configuration.AFM images reveal that the NiOx layer follows the topological features of the distinct ITO substrates.However, the presence of NiOx on the surface of the ITO films clearly reduces the variations in CPD values along the surface of all ITOs.Consequently, a uniform surface potential distribution emerges, regardless of the underlying microstructure of the ITO.This underscores the efficacy of utilizing an amorphous NiOx buffer layer as a surface modifier, effectively countering potential non-uniformities and promoting uniform electrical response in the films, particularly in the case of poly-ITO-μm grain films.Following the KCl-treated NiOx, 2PACz-SAMs were deposited on ITO/NiOx substrates (Figure2b, right column).With the incorporation of 2PACz-SAMs, the uniform CPD was preserved for all the studied ITOs/NiOx, but a reduction in CPD values serves as evidence of the presence of 2PACz on the surface, also indicating a WF increase.

Figure 3 .
Figure 3. Grain orientation influence on local WF.(a) Schematic representation of the point of

Figure S2 .
Figure S2.Post-annealing treatment of PLD ITO films at different temperatures and under

Figure S3 .
Figure S3.Tauc plot for studied ITO films with extracted band gap value

Figure 5 .
Figure 5. XRD of NiOx (grey) and KCl treated NiOx film (blue) indicating the presence of KCl

Table S2 .
Summary of average work function values determined by UPS and KPFM