Functionalized Nickel Oxide Hole Contact Layers: Work Function

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Functionalized Nickel Oxide Hole Contact Layers: Work Function versus Conductivity Sebastian Hietzschold, Sabina Hillebrandt, Florian Ullrich, Jakob Bombsch, Valentina Rohnacher, Shuangying Ma, Wenlan Liu, Andreas Köhn, Wolfram Jaegermann, Annemarie Pucci, Wolfgang Kowalsky, Eric Mankel, Sebastian Beck, and Robert Lovrincic ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b12784 • Publication Date (Web): 20 Oct 2017 Downloaded from http://pubs.acs.org on October 21, 2017

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ACS Applied Materials & Interfaces

Functionalized Nickel Oxide Hole Contact Layers: Work Function versus Conductivity Sebastian Hietzschold1,2,3,*, Sabina Hillebrandt1,3, Florian Ullrich1,4, Jakob Bombsch1,3, Valentina Rohnacher1,3, Shuangying Ma1,6, Wenlan Liu1,6, Andreas Köhn1,6, Wolfram Jaegermann1,4, Annemarie Pucci1,3,5, Wolfgang Kowalsky1,2, Eric Mankel1,4, Sebastian Beck1,3, and Robert Lovrincic1,2,* 1

InnovationLab, Speyerer Str. 4, 69115 Heidelberg, Germany

2

Institute for High-Frequency Technology, TU Braunschweig, Schleinitzstr. 22, 38106 Braunschweig, Germany 3

Kirchhoff-Institute for Physics, Heidelberg University, Im Neuenheimer Feld 227, 69120 Heidelberg, Germany 4

Surface Science Division, TU Darmstadt, Jovanka-Bontschits-Str. 2, 64287 Darmstadt, Germany 5

Centre for Advanced Materials, Heidelberg University, Im Neuenheimer Feld, 69120 Heidelberg, Germany 6

Institute for Theoretical Chemistry, University of Stuttgart, Pfaffenwaldring 55, 70569 Stuttgart, Germany *Corresponding authors, E-mail: [email protected], [email protected]

Keywords: metal oxides, self-assembled monolayers, hybrid interfaces, solar cells, density functional theory

ACS Paragon Plus Environment

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Abstract Nickel oxide is a widely-used material for efficient hole extraction in opto-electronic devices. However, its surface characteristics strongly depend on the processing history and exposure to adsorbates. To achieve controllability of the electronic and chemical properties of solutionprocessed nickel oxide (sNiO), we functionalize its surface with a self-assembled monolayer (SAM) of 4-cyanophenylphosphonic acid. A detailed analysis of infrared and photoelectron spectroscopy shows the chemisorption of the molecules with a nominal layer thickness of around one monolayer and gives insight into the chemical composition of the SAM. DFT calculations reveal possible binding configurations. By the application of the SAM, we increase the sNiO work function by up to 0.8 eV. When incorporated in organic solar cells, the increase in work function and improved energy level alignment to the donor does not lead to a higher fill factor of these cells. Instead, we observe the formation of a transport barrier, which can be reduced by increasing the conductivity of the sNiO through doping with copper oxide. We conclude that the widespread assumption of maximizing the fill factor by only matching the work function of the oxide charge extraction layer with the energy levels in the active material is a too narrow approach. Successful implementation of interface modifiers is only possible with a sufficiently high charge carrier concentration in the oxide interlayer to support efficient charge transfer across the interface.


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Introduction Transition metal oxides (TMOs) are commonly used in their non-stoichiometric phase as efficient charge selective transport materials in opto-electronic devices.1,2 Nickel oxide (NiO) is a particularly promising candidate for hole injection and extraction,3–8 as it combines p-type conductivity, high transparency, and a low electron affinity (EA).9–21 However, the work function of NiO films depends on their crystalline orientation, surface composition, processing history, and is not always sufficiently high to align well with the frontier orbitals of organic donor materials with high ionization potential (IP) for ohmic contact. It has previously been shown that a mixed NiO/MoO3 layer can overcome this problem and increase the built-in voltage and thereby the fill factor (FF) in organic solar cells.22 Another convenient way to increase the work function of NiO is an oxygen plasma treatment,23 but its adjustment range is limited. A more variable approach is the use of dipolar self-assembled monolayers (SAMs),24–29 with which the work function30 and also the surface energy (wettability) can be regulated by tailoring the backbone and functional group of the interface modifier.31–33 Phosphonic acids (PA) are known to chemisorb and form highly dense monolayers on hydroxylated oxide surfaces via a mono-, bior tridentate bonding mode.26,34,35 While there are reports on molecular modification with phosphonic acids on nickel foil36, e-beam evaporated NiO37 and pulsed laser deposited Ni:CoO38, we are aware of only one short report of SAM deposition on solution-processed NiO (sNiO).39 For these reasons it is of high interest to investigate the formation of SAMs on sNiO surfaces and its impact on device performance in more detail. Here, we show that phosphonic acids indeed chemisorb on sNiO surfaces and form a SAM. We use the commercially available 4-cyanophenylphosphonic acid (CYNOPPA) as SAM precursor, as it possesses a strong permanent dipole moment to increase the work function.31 Furthermore,


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its well conjugated backbone is thought to enable efficient charge transport40 and the cyano group allows to improve the wettability for printing applications (see Figure S8). Through the growth of a CYNOPPA monolayer on the sNiO, we successfully increase the work function by up to 0.8 eV. However, despite the proper energy level alignment to the donor, the low intrinsic charge carrier density in the sNiO in combination with the SAM leads to a transport barrier in organic solar cells. This barrier can be reduced by increasing the conductivity of the sNiO HTL with copper oxide (CuO). While oxide contact layers are often treated like metals in terms of energy level alignment,13,41 our work highlights that the semiconducting properties of transition metal oxides, e.g. doping level and band bending, are still important for proper interface engineering. This differentiated consideration is crucial for the enhancement of charge transport across hybrid interfaces in general.

Results and discussion sNiO films of ~25 nm thickness were fabricated according to Manders et al.42 by spin-coating nickel acetate tetrahydrate from an ethanol solution and subsequent annealing in air at 325 °C and 400 °C, respectively (see methods for details). At these temperatures the precursor was shown to be decomposed to a high degree.43 The resulting films show excellent substrate coverage and consist of small grains with a typical size of 10 nm. High resolution AFM and SEM images revealing this grainy surface structure are attached in the supporting information (see Figure S14). To gain insight into the growth mechanism of CYNOPPA on the sNiO surface a complementary set of infrared and photoelectron spectroscopy measurements were performed. CYNOPPA was


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applied to the surface as suggested elsewhere.31 Figure 1 shows infrared transmission spectra of CYNOPPA on sNiO films for sNiO annealed at two different temperatures. As reference, the exact same substrates were measured before and after the deposition of the SAM to get as detailed information as possible. The angle of incidence (AOI) was set to 10°, which we correspond to as normal incidence. Various absorption bands of CYNOPPA arise in the spectra on both sNiO layers. Modes marked with black dashed lines are assigned to vibrational modes of the molecule with the help of DFT calculations (see Figure S1 and Table S1).

α-Ni(OH)2

C N

rel. transmission

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


β-Ni(OH)2

325°C-sNiO/CYN 400°C-sNiO/CYN 1000

1250

1500

1750

2000

0.2% 2250

wave number [cm-1]

3500

4000

Figure 1. Mid-infrared measurements of CYNOPPA on 325 °C (blue) and 400 °C (red) annealed sNiO. The sNiO substrate prior treatment was used as a reference. The characteristic absorption bands of CYNOPPA are marked with black dashed lines. At 833 cm-1, 966 cm-1, and 1250 cm-1 absorption bands of the phosphonate group, at 1130 cm-1 the stretching vibration of the phenyl ring and at 2231 cm-1 the absorption of the (C≡N) cyano group stretching vibration are observed. The reduction of β-Ni(OH)2 through the SAM treatment is marked with a light grey dashed line. Characteristic modes of α-Ni(OH) have been highlighted with grey diagonally striped regions. At 1130 cm-1 the stretching vibration of the phenyl ring is present. Another characteristic mode can be seen at 2231 cm-1, which is assigned to the stretching vibration of the cyano group (C≡N). The weak vibrational modes at 833 cm-1, 966 cm-1, and 1250 cm-1 are attributed to absorption bands of the phosphonate group. The existence of all these characteristic peaks is a clear evidence that CYNOPPA is present at the surface. Additional positive absorption bands are observed in the spectral range between 1280 and 1700 cm-1 and between 3500 and 3700 cm-1.


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These features are assigned to changes in the sNiO layer, as discussed in the following. Incomplete conversion of the precursor leads to residual nickel hydroxide (Ni(OH)2), which exhibits absorption bands in this range.44,45 The very pronounced upwards peak at 3670 cm-1 can be attributed to a reduction of β-Ni(OH)2 and the broad mode around 3500 cm-1 implies a change in α-Ni(OH)2. The weak signals of the phosphonate group, especially the broadened P=O mode, and the reduction of Ni(OH)2 indicate a chemical interaction of the molecules with the sNiO. Furthermore, the overall intensity of the vibrational modes of the molecule, which lies below 0.4%, is in good accordance with a monolayer formation. The C≡N stretching vibration can directly be related to the amount of molecules if there is no orientation change in the two compared samples. As the peak height does not vary under different angle of incidence (see Figure S2), a smaller amount of CYNOPPA is present on sNiO films annealed at higher temperatures. This goes along with a decreasing amount of Ni(OH)2 in the films with increasing annealing temperature of the NiO precursor.43,46 Both experimental observations allow us to conclude that, here, the phosphonic acid binds mostly to the nickel hydroxide at the sNiO surface. To evaluate possible binding configurations, we employed density functional theory (DFT) calculations on a fully hydroxylated ideal NiO(111) surface considering a series of adsorption modes. A comparison of the adsorption energies (see Figure S15) shows that the reactions leading to adsorption modes with more than one water molecule involved in the dehydration process are energetically unfavorable. This is supported by ab initio molecular dynamics simulations which demonstrate that only mono- and bidentate binding modes are stable under the conditions of the surface treatment (ethanol solution). The purely hydrogen bonded mode will not be stable under the conditions of the CYNOPPA annealing step, which is introduced to


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activate the phosphonate bonding. In the absence of protic solvent molecules to reverse the bonding process the other binding modes become increasingly stable. Hence, the most probable adsorption mode is a monodentate binding between the dehydrated CYNOPPA and the NiO surface. The formation of a bidentate binding mode might be possible, too, but the tridentate bonding, which would lead to a strong net orientation towards the surface normal, is not accessible.

Figure 2. Background corrected and normalized XP detail spectra a) Ni 2p3/2 and b) O 1s core level spectra for 325 °C (blue) and 400 °C (red) annealed sNiO films with and without CYNOPPA treatment. Spectra were shifted to the maximum of the 325 °C sample. In both spectra, the intensity of the peak shoulder is altered after PA adsorption, highlighted in the zoomed inset of a). c) The spectral components P 2s, C 1s and N 1s are fitted with Voigt functions, exemplary for a substrate annealed at 325 °C. The C 1s signal exhibits two different species, and N 1s and P 2s only one.


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We used X-ray photoelectron spectroscopy to determine the stoichiometry of the molecules, layer thickness and electrostatic properties of CYNOPPA on sNiO. In Figure 2a the Ni 2p3/2 core level spectra for both annealing temperatures are given with and without CYNOPPA modification, normalized to the intensity of the main peak to enable a clear comparison of the spectral shape. The overall spectral features of sNiO are in good agreement with previous results.42,20,47,48 Unanimously, the maximum at 854.1 eV is addressed to the Ni2+, while the shoulder around 856 eV is commonly assigned to a superposition of Ni bulk hydroxide (Ni(OH)2), surface hydroxide (-OH) and oxy-hydroxide (NiOOH).42,47–51 Several shake-up satellites appear at higher binding energies. The spectra for both temperatures show small but significant changes of the peak shapes after CYNOPPA adsorption, which indicates a chemical reaction of sNiO in the surface region. To prove that the peak changes are not only caused by the exposure to ethanol, the sNiO films were immersed into pure anhydrous ethanol without PA for the same process parameters and investigated afterwards. No variation in the peak shapes could be observed, which clearly evidences that the peak changes are due to a chemisorption of CYNOPPA on sNiO. The alteration is weaker for sNiO annealed at 400 °C. This finding is in good agreement with the IR measurements revealing a smaller amount of CYNOPPA for higher annealing temperatures. A similar trend can be observed in the according O 1s core levels (see Figure 2b). It must be emphasized that unambiguous conclusions about substrate changes cannot be drawn from the O 1s spectra, since the signals of the different PA oxygen species overlap with the side-peak of the sNiO spectra. We take a closer look at the C 1s, N 1s and P 2s core level spectra depicted exemplarily for a 325 °C sample in Figure 2c. The P 2s and N 1s signals exhibit single distinct features at binding


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energies of 191 eV and 400 eV, indicating that none of both species underwent any chemical reaction. The carbon spectral response is split into two different species: a major component at 285 eV belonging to the aromatic carbon with hydrogen termination27 and a minor response from contributions of the C≡N group52 as well as from the carbon connected to the phosphonic anchor.53 By integrating the spectral areas of Voigt fits and dividing by the respective atomic sensitivity factors, a carbon ratio of 1:2.6 is determined, which well aligns with the carbon ratio in the molecule of 1:2.5. An overall molecular stoichiometry of 8.4 carbon to 1 nitrogen to 1.2 phosphor is found (reference: N 1s). This adopts the molecular structure of CYNOPPA with a ratio of 7:1:1 also quite well. The nominal layer thickness = − ln can be determined by

measuring the intensity of the background-corrected Ni 2p spectrum before (I0) and after (Id) PAtreatment. The inelastic mean free path (1.84 nm) of electrons contributing to the Ni 2p signal ( ≈ 617 eV) was calculated according to the equation of Tanuma, Powell and Penn.54 This yields nominal layer thicknesses for CYNOPPA of around 0.75 nm (325 °C) and 0.50 nm (400 °C), respectively. These qualitative values are close to the molecular length of a single CYNOPPA molecule (0.81 nm). Their variation presumably results from the lower coverage of molecules at 400 °C. At this point the chemical analysis of IR, DFT and XPS results allows us to assume that the chemisorbed CYNOPPA layer is indeed a SAM, with a chemical bonding of the phosphonic acid anchor, predominantly to the Ni(OH)2. Furthermore, we can deduce that a layer of superposed mono- and bidentate binding modes is formed and that the net orientation of the SAM molecules to the surface normal is dominated by the accessible binding configurations as well as the grainy microstructure of the sNiO surface.


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Table 1. Work functions and valence band maxima of differently annealed and treated sNiO films. WF values were measured with XPS using the secondary electron cut-off edge and the VBM was determined via the valence band onset relative to the Fermi level. Complementary Kelvin probe data can be found in Figure S7.

T

WF

VBM

[°C]

[eV]

[eV]

325

4.81 ± 0.19

0.86

400

4.40 ± 0.05

0.97

325

5.21 ± 0.12

0.94

400

5.09 ± 0.18

1.00

325

5.55 ± 0.08

0.68

400

5.63 ± 0.13

0.75

325

5.20 ± 0.11

0.92

400

5.18 ± 0.11

0.88

sNiO:CuO

325

4.53 ± 0.12

0.34

sNiO:CuO/CYN

325

5.05 ± 0.11

0.44

sNiO

sNiO/CYN

sNiO/OP

sNiO/OP/CYN

In a next step, the electronic properties, especially work function (WF) and valence band maximum (VBM), of the sNiO surface before and after the SAM growth are investigated with XPS and compared to the treatment with an oxygen plasma (OP). All values are summarized in Table 1. For higher sNiO annealing temperatures the WF of untreated films decreases from (4.81 ± 0.19) eV down to (4.40 ± 0.05) eV. This is probably the result of a higher degree of precursor conversion and thus, NiO stoichiometry.55 As expected, the OP treatment leads to a strong increase in the work function and to a decrease in the valence band maximum of around 0.2 eV for both annealing temperatures. Such an onset decrease can be ascribed to a higher hole carrier concentration due to the oxidation of the surface.


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More importantly, the CYNOPPA SAM successfully increases the work function to 5.1-5.2 eV on the as-deposited sNiO. We note that, as a further result from our DFT calculations according to the method shown in previous reports,56,57 the main change in the work function upon CYNOPPA chemisorption is probably not due to the net dipole moment of CYNOPPA, but due to a charge transfer between the hydroxylated NiO surface and the phosphonate group (see Figure S15). Interestingly, this final value of around 5.2 eV is almost independent of the pretreatment (i.e. oxygen plasma) and initial WF. A possible explanation could be that EF of the sNiO becomes pinned to surface states that are not present until the chemical bonding of the molecules takes place. The WF change of 0.8 eV seen on sNiO annealed at 400 °C fits well to results reported for CYNOPPA on ITO by Koh and coworkers.31 To study the influence of work function change on device performance, F4ZnPc:C60 flatheterojunction (FHJ) solar cells with sNiO annealed at 325 °C on pre-structured ITO substrates were built and tested under AM 1.5G illumination. Obviously, this is a rather low efficiency photo-active system, but it is very reproducible and well-defined, which is crucial for our investigated interface phenomenon. Devices with 400 °C sNiO could not be fabricated, as the high temperature would reduce the ITO conductivity and thus, the overall device performance.


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Figure 3. a) Linear J-V characteristics of F4ZnPc:C60 FHJ devices with untreated sNiO (square), oxygen plasma treated (dot), CYNOPPA modified (triangle), and CYNOPPA modified oxygen plasma treated (diamond) sNiO HTL. The inset depicts the used solar cell stack. Dark currents are displayed in Figure S9. b) Schematic energy band situation before contact of the different materials utilized in the solar cell device stack. The Fermi level of the functionalized sNiO hole contact layer (green) should perfectly align with the HOMO of F4ZnPc through the dipole of the CYNOPPA SAM. Figure 3a and Table 2 sum up the representative J-V characteristics and corresponding device performance data (average values can be found in Table S2). The plasma treated sNiO with a strongly increased work function shows a significantly improved FF compared to the untreated sNiO HTL.26,61 The lower series resistance (Rs) might be a consequence of the oxidative growth of

a

thin

NiOOH

surface

layer

with

increased

hole

carrier

concentration

and

conductivity.13,15,46,58 This would agree with the decrease of the VBM value mentioned before. The Voc around 640 mV stays unchanged within statistical errors for different treatments, presumably because it is already dominated by the effective bandgap (∆EDA) of the donoracceptor system.59,60 All devices with CYNOPPA modified sNiO show a second photodiode behavior (s-kink) in the operating regime. This also applies to co-evaporated bulk-heterojunction (BHJ) cells built in comparison to the FHJ devices (see Figure S10). An oxygen plasma treatment prior to the SAM deposition cannot hinder the development of the s-kink.


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Often the occurrence of an s-kink is ascribed to hole extraction barriers caused by the energetic misalignment of the HTL’s IP or Fermi level to the donor’s HOMO. Figure 3b depicts the energy band situation before the sNiO and the F4ZnPc (HOMO = 5.3 eV) are brought into contact. Fermi levels and the IPs are adapted from the values in Table 1. The electron affinities were estimated by the according bandgaps given in literature.47 If the sNiO surface is not further treated, the Fermi level of the as-deposited HTL exhibits an offset of around 0.5 eV to the HOMO of the donor. When tuned with CYNOPPA, we succeed to minimize the offset to only 0.1 eV. The increased sNiO work function and decrease in energy offset does not lead to an increase in fill factor compared to the OP treated sNiO. Theoretically, the oxygen plasma treatment shifts the work function so far that an energetic hole extraction barrier is formed. However, this is not reflected in the J-V characteristics. If the s-kink here was solely a consequence of energetic mismatch of the modified HTL’s Fermi level with the HOMO of F4ZnPc, the oxygen plasma treated sNiO devices should exhibit an s-kink as well. This is clearly not the case. To understand this apparent contradiction, we performed a set of different experimental approaches. Table 2. Device performance parameters for FHJ cells including differently post-treated sNiO hole transport layers annealed at 325 °C. HTL

FF

Voc

Jsc

Rs

(325 °C)

[%]

[V]

[mA cm-2]

[Ω cm2]

sNiO

52

0.634

3.93

39

sNiO/OP

61

0.640

4.01

15

sNiO/CYN

34

0.646

3.60

166

sNiO/OP/CYN

36

0.645

3.85

140

sNiO:CuO

56

0.633

3.87

25


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sNiO:CuO/CYN

49

0.641

4.06

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The first approach refers to a simple test introduced by Tress et al. to distinguish between extraction and injection barriers at the electrodes of hybrid solar cells with s-shaped J-V characteristics by varying the illumination intensity during the measurement.61 Following this procedure, we analyzed the s-kink under different light intensities (see Figure S11). At first glance, the characteristics are very similar to the authors model of an energetic extraction barrier. The s-kink strongly depends on the illumination intensity, because the more photo-carriers are generated, the more charges pile up at the HTL/donor interface barrier. The Voc nearly stays the same for high enough intensities. When the intensity is drastically reduced, the Voc cannot be maintained and the open-circuit voltage gradually decreases over the light intensity with a slope of around 0.07 V per decade (see Figure S13). This is a typical indicator that the device is mainly limited by non-geminate recombination in the active layer. The J-V data show that recombination at the sNiO/CYN interface is not limiting the Voc here. We were also able to reproduce the s-kink by using 1H,1H,2H,2H-perfluorooctanephosphonic acid (FHOPA), which shifts the work function of the sNiO surface to 5.1 eV (see Figure S11). The hydrophobic character of its backbone and thus, change in surface energy, has no significant influence on the


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s-shape. Hence, the s-shaped J-V is not correlated to a possible change of the molecular orientation of the donor.62 A further method to probe the nature of the barrier is to vary the thickness of the underlying HTL. Reducing the layer thickness behind the barrier impacts the electric field and leads to a higher probability for charges to be extracted. This explains why the s-kink is reduced for thinner sNiO layers, depicted in Figure S12, again confirming the existence of an extraction barrier. As the energy levels seem to be perfectly aligned after the functionalization, we propose that the combination of the SAM with the low intrinsic carrier density in the as-deposited sNiO turns into a bottleneck. The considerable increase of Rs for all sNiO/CYN devices (see Table 2) indicates the creation of a poorly conductive interface, which we see as the reason for the appearance of the s-kink. To support this hypothesis, we intentionally increased the charge carrier density and the conductivity of the sNiO by blending the sNiO with copper acetate to form copper oxide (CuO, see experimental details) doped NiO. Adding CuO shifts the VBM of the sNiO 0.5 eV closer to EF (see Table 1 and Figure S4). This trend reminds of the typical EF shift induced by pdoping. However, from the location of Cu relative to Ni in the periodic table, doping a NiO crystal with Cu should lead to an n-doping. Therefore, we do not understand the doping here as a replacement of Ni vacancies by Cu,63,64 but rather as a charge transfer due to the higher work function of CuO (WF = 5.4 eV).65 Nevertheless, we stick to the terminology of doping in this context. The doping is accompanied by a decreasing specific resistance from >30 Ωm (sNiO) down to 6.6 Ωm (sNiO:CuO) measured by the four-point probe method on planar films on glass.


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Figure 4. a) Linear J-V characteristics of F4ZnPc:C60 FHJ devices with CuO doped sNiO (square), CYNOPPA treated CuO doped sNiO (dot), and CYNOPPA modified sNiO (triangle). The CYNOPPA reference device is always built in the same batch of investigation. The inset depicts the solar cell stack with sNiO:CuO interlayer. b) Linear I-V characteristics of F4ZnPc hole-only devices incorporating the different sNiO hole contact layers. The active area was the same as in the solar cells (4 mm²) and the layer thicknesses were 50 nm for F4ZnPc and 10 nm for the MoO3, respectively. As a consequence of the doping and decreasing specific resistance of the HTL films, the series resistance of the sNiO:CuO device decreases slightly and the FF increases from 52% to 56% compared to the as-deposited sNiO device (see Figure 4 a)), which is consistent with earlier observations by Kim and coworkers.64,66,67 More importantly, the s-kink gets repressed such that the FF is almost fully recovered. We can rule out that this is due to a lower CYNOPPA coverage on the sNiO:CuO as the characteristic IR absorption modes show similar intensities and the thickness estimated from the damping of the Ni 2p3/2 and Cu 2p3/2 core levels are almost identical to the results on undoped sNiO (see Figure S3 and S5). To further prove the transport barrier, F4ZnPc hole-only devices incorporating the functionalized undoped and doped sNiO hole contact layers were fabricated and evaluated. The corresponding I-V characteristics are shown in Figure 4 b). It can clearly be seen that the CYNOPPA SAM hinders hole extraction at negative bias, but also impedes charge injection to a certain extent at


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positive bias. However, the doping with CuO facilitates charge transport over the barrier, which is in very good agreement with the previous observations in the solar cells. These results lead us to the conclusion that the combination of low doped sNiO together with the SAM causes a kinetic transport barrier, similar to the observations by Cowan et al. for phosphonic acid SAMs on ZnO.68 Such a barrier can lead to a reduced field-dependent surface recombination velocity of photo-generated majority carriers at the anode.69 In other words, photo-generated holes are not able to pass the sNiO/CYNOPPA at sufficiently high rates. This also explains why the s-kink in the BHJ architecture (see Figure S10) with a higher interfacial area to the acceptor is more pronounced. We reveal that this barrier can be overcome by doping the oxide HTL and increasing its conductivity.66 This might also reduce the width of a possible space charge region, which rises due to the pile up of carriers in front of the transport barrier.70 Conclusion In summary, we demonstrate the phosphonate based SAM formation on sNiO and successfully tune the work function of sNiO. CYNOPPA increases the sNiO work function by up to 800 meV to 5.2 eV, which is among the highest reported values for SAM induced WF changes on this material. While IR and XP-spectroscopy clearly reveal the SAM formation, the IR results give further evidence that the molecules preferably attach to the Ni(OH)2. DFT calculations show that the monodentate binding is energetically more favorable than a bidentate configuration and that the tridentate binding mode is not accessible. We apply these monolayers in devices and find that the poorly conductive sNiO/SAM interface induces a kinetic transport barrier, which is not a result of energy level mismatch. This barrier can be overcome by increasing the charge carrier density in the sNiO through doping. From a more general perspective, our results emphasize that for device optimization with interface modifiers it is not sufficient to concentrate only on the


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energy levels of charge extracting layers. Especially in the case of low conductivity oxides great care must be taken to avoid the creation of transport barriers. With our work, we open a pathway for future surface functionalization of hydroxylated oxides, especially sNiO, and their application in hybrid opto-electronic devices.


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Supporting Information. The following files are available free of charge. Experimental methods, DFT calculations, device structures, IR, XPS, AFM and device data. (PDF) AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ORCID, Sebastian Hietzschold, 0000-0003-4781-9251 *E-mail: [email protected] ORCID, Robert Lovrincic, 0000-0001-5429-5586 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT We acknowledge the German Federal Ministry of Education and Research (BMBF) for financial support within the INTERPHASE project (N. 13N13656, 13N13657, 13N13658, 13N13663). We also acknowledge the support by the state of Baden-Württemberg through bwHPC and the German Research Foundation (DFG) through grant no. INST 40/467-1 FUGG. The authors also acknowledge Paul Heimel for the support during fabrication of the hole-only devices.


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TOC


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Functionalized Nickel Oxide Hole Contact Layers: Work Function

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