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X-ray absorption
X-ray absorption spectroscopy (XAS) is a well-established analytical technique used extensively for the characterization of semiconductors in solid or liquid, crystalline or amorphous, bulk or nanoscale form. If X-rays of intensity I0 are incident on a sample, the extent of absorption depends on the photon energy h and the sample thickness l (Figure I.17). According to Beer’s Law, the transmitted intensity It is: It = I0 exp−μ(h)l
where μ (h) is the energy-dependent X-ray absorption coefficient, and the material density. Over large energy regions, the absorption coefficient (μ (h)) is a smooth function of the photon energy, varying approximately as Z4 m(h)3 . [77] Here Z and m are the atomic number and mass, respectively. Thus, μ (h) decreases with increasing photon energy.
DIPO-Ph4 growth mode on indium tin oxide
As it was previously described, the interactions between the substrate and the organic layer, and inside the layer itself, lead to three different growth modes.
To understand the ITO/DIPO-Ph4 interface, it is important to characterize the electrode surface: roughness, contaminations, defaults. Before all further analyses, atomic force microscopy (AFM) was used to characterize both the ITO substrates and the organic layer morphology on ITO (Appendix B.3.1). In this first part, we will present the morphology of the ITO surface and the efficiency of the used cleaning process (Appendix A.1). AFM images of the bare ITO show many light dots, which are attributed to contamination clusters. After the cleaning process, no cluster appears on the ITO surface. The chemical cleaning is efficient as the ITO surface appears without particle contamination. The ITO roughness is 1nm and the surface shows a granular morphology. This was already observed in the literature for sputtered ITO. [78, 79] Besides, we performed some
current-sensing AFM (CS-AFM). The sample is biased and the AFM tip is grounded. This means that for a negative bias, electrons are travelling from the substrate to the tip, and in the opposite direction for a positive bias. Concerning the current image color appearance, the color scale for the negatively biased image has been reversed in comparison to the positively biased image so that the current minimum is blue in each case and the absolute current maximum is red. The current images are represented between 0 and (−)1 nA to correlate the observation with the ITO granular morphology, but the observed maximum current is +25 nA and −25 nA for the positively and the negatively biased image, respectively. CS-AFM images show that the conduction is clearly happening through the ITO grains.
We are thus able to get a cleaned ITO surface with a small roughness despite a granular surface. The CS-AFM experiments confirm the metallic behaviour of the ITO although it is a doped n-type SC. The Sn doping of the commercial ITO is efficient to make ITO conductive. We will now focus the analysis of DIPO-Ph4 on ITO samples in order to investigate the organic layer morphology. Several samples were prepared with a change in the deposited organic material DIP amount. The weight, followed by a quartz balance (QB), increases per surface unit during the time evaporation (Appendix B.1). It is then converted into a molecular surface density using the density given by XRD experiment. [58] The different samples are named by their QB-coverage We studied three different samples: a “thin” layer of 0.5 × 1015 molecule · cm−2, an intermediate layer of 2 × 1015 molecule · cm−2, and a “thick” layer 10 × 1015 molecule · cm−2. AFM images were used to determine the real value of the coverage and make correlations with the QB-coverage.
Access to the first deposited layer and to the crystallized material
Two kinds of treatment were performed:
• One after the evaporation, in ambient pressure condition at a temperature near the evaporation temperature of the DIPO-Ph4 (T 170 C) to perform some molecular desorption and access to the first deposited layers (ITO/DIPO-Ph4 interface).
• One during the evaporation, i.e. evaporation on hot substrate (T = 100 C), to bring some energy to the system and increase the growth speed (thermodynamic equilibrium).
This study was performed while replacing the ITO substrate by a very thin (100nm) nitride silicon (Si3N4) substrate. No cleaning treatment has been performed on these substrates before the molecular evaporation. The interface was then studied via XAS absorption measurements to determine the molecular orientation. AFM results are presented in Figure II.3.
First, it is important to notice that the layer morphology on a Si3N4 is similar to the one on ITO substrate (no thermal annealing, presented in Figure II.3). The un-annealed sample morphologies are the same as the ones presented in the previous section: slightly elongated clusters with an average height of 30nm for the 1 × 1015 molecule · cm−2 sample. AFM-calculated coverages are 1.1 × 1015 molecule · cm−2 (25% of the Si3N4 covered) and 11 × 1015 molecule · cm−2 for the 1 × 1015 and the 10 × 1015 molecule · cm−2 samples, respectively. A similar morphology between the two substrates may result from a similar surface energy.
After a post-annealing treatment at T 170 C on the 1 × 1015 molecule · cm−2 sample (post thermal annealing, presented in Figure II.3a)), the average height of the clusters decreases to 13nm. The mounds still cover 20% of the Si3N4 surface, with an average diameter of 230nm. The molecular surface density calculated, from the AFM image, is divided by 3 (0.3 × 1015 molecule · cm−2). The crystallization on a hot substrate (under thermal annealing, presented in Figure II.3a)) leads to the formation of twinned DIPO-Ph4 structures that are more elongated than in the previous cases.
Carrier concentration
The In 3d peak highlights that there would be a tin segregation at ITO surface. As the carrier concentration depends on the Sn doping, it is crucial to probe a possible in-depth variation of the Sn concentration (see Ref. 46). We present Sn 4d and In 4d spectra of the chemically cleaned samples recorded at different photon energies, 60, 825, and 1486.6 eV (Figure III.3), corresponding to calculated values of 0.5nm, 1.5nm, and 2.5nm respectively, see Ref. 75.
The In 4d and Sn 4d peaks, found at 18.5 eV and 26.5 eV respectively, flank a smaller peak at 22.5 eV (hardly visible at h = 60 eV) attributed to O 2s, considering the local density of state (LDOS) given in Ref. 91. The O 2s contribution cannot affect the measurement of the Sn/In atomic ratio rSn/In) for two reasons: (i) the calculated photoemission cross-section [92] ratio In 4d/O 2p is 34 at h = 60 eV and 16 for photon energies of 825 eV and 1486.6 eV; (ii) the admixture of O 2s states with the In 4d level is almost negligible judging from the LDOS given in Ref. 91. Taking into account the photon energy variation of the cross-sections and asymmetry parameters of In 4d and Sn 4d, [92] we have calculated the rSn/In values reported in Table III 2.
For the chemically cleaned ITO, we find a value of 0.100 – 0.108 in surface sensitive conditions (h 825 eV), in excellent agreement with the rSn/In value of 0.102 calculated for tin oxide weight of 10% in indium oxide. The higher rSn/In value of 0.128 obtained in surface sensitive conditions h = 60 eV), indicates Sn segregation at the substrate surface. This segregation was already reported by Gassenbauer. [46]
Table of contents :
Acknowledgement
Abbreviations
Introduction
I Metal/organic interfaces
I.1 Semiconductor description
I.1.1 Inorganic semiconductor
a) Theoretical description
b) Electrode for organic electronics
I.1.2 Organic semiconductor
I.2 Metal/organic interface
I.2.1 Layer interactions
I.2.2 Energetic level alignment
I.3 X-ray characterizations
I.3.1 Physical principles
I.3.2 Sampling depth
I.3.3 X-ray absorption
II DIPO-Ph4 layer on ITO substrate
II.1 ITO/DIPO-Ph4 layer morphology
II.1.1 DIPO-Ph4 growth mode on indium tin oxide
II.1.2 Access to the first deposited layer and to the crystallized material
II.2 Molecular orientation
II.2.1 Molecular description
II.2.2 Absorption spectroscopy
IIIITO/DIPO-Ph4 interface
III.1 ITO characterization
III.1.1 Electronic properties
III.1.2 Carrier concentration
III.1.3 Electron energy level scheme
III.2 ITO/DIPO-Ph4 interface
III.2.1 Core levels XPS spectroscopy
a) Indium and tin core levels
b) Carbon and oxygen levels
c) DFT calculation correlation
III.2.2 Valence band energy level
III.2.3 Electron energy level scheme
III.3 Charge transfer from DIPO-Ph4 to ITO
III.3.1 Resonant photoemission spectroscopy
III.3.2 ITO/DIPO-Ph4 interface
a) C K-edge
b) O K-edge
III.3.3 Pump-probe experiments
IV DIPO-Ph4 as interfacial layer
IV.1 Solar devices
IV.1.1 Photovoltaic mechanism
IV.1.2 Photovoltaic devices
a) Photovoltaic generations
b) Organic solar cells
c) Interfacial layer
IV.2 Organic electronics application
IV.2.1 Energetic alignment
IV.2.2 Photovoltaic response
V DIP heteroatom effect
V.1 ITO/DIP layer morphology
V.1.1 Molecular description
V.1.2 DIPS-Ph4 and DIPSe-Ph4 growth mode
V.2 Electronic properties
V.2.1 Core levels XPS spectroscopy
V.2.2 Valence band energy level
V.2.3 Electron energy level scheme
Conclusion
A Materials A-1
A.1 Indium tin oxide
A.2 Dipyranylidenes
B Experimental setups A-3
B.1 Deposition
B.2 Fabrication of photovoltaic devices
B.3 Spectroscopy and microscopy analysis
B.3.1 AFM
B.3.2 STXM
B.3.3 XPS
B.4 Theoretical calculation
B.5 Data analysis
B.5.1 STXM stack analysis
B.5.2 Core levels analysis
C Complementary results A-9
C.1 Morphology at the TEMPO beamline
C.2 Electronic properties after air exposure
C.3 Solar cell characterization
French summary S-1
Introduction
S.1 Interfaces métal/organique
S.2 DIPO-Ph4 sur ITO
S.3 Interface ITO/DIPO-Ph4
S.4 DIPO-Ph4 comme couche interfaciale
S.5 Effet de l’hétéroatome du DIP
Conclusion
Bibliography R-1
