IOSC with 80 °C TiO x

In Fig. 15.2 the TiO_x/C₆₀energy line-ups, before UV illumination derived from experiments at Bessy and at Daisy-Fun are depicted (left and middle). Included is the E_{V B} and E_{C B}of TiO_x and the HOMO/LUMO position of C₆₀. Before UV illumination the IOSC with TiO_x as ETL showed S-shaped I-V characteristic with a reduced fill factor F F (see Fig. 7.2). The energy band line-up derived at Bessy displays that the E_{C B} of TiO_x is 0.3 eV above E_F and 0.2 eV above the LUMO_onset of C₆₀. This offset displays a barrier for electrons and is an electron extraction barrier. The energy line-up derived from the interface experiment at the Daisy-Fun in Darmstadt shows a different situation. While the E_{C B}-LUMO_onset distance is 0.2 eV as well, the relative position of E_{C B} of TiO_xand the LUMO_onset of C₆₀ are lower compared to E_F. E_{C B} of TiO_x is right at E_F and the C₆₀ LUMO_onset at the interface is 0.2 eV below E_F. In contrast to the Bessy derived line-up, the energetic line-up concluded from Daisy-Fun experiments before UV, does not show an electron extraction barrier for electrons. Furthermore E_F of TiO_x is on the same level as E_{C B}, which

160 15. Comparing energy band diagrams and I-V characteristics

is evidencing a good conductivity of TiO_x.

The differences in both line-ups (Bessy and Daisy-Fun before UV) origin probably mostly from the different method of VBM determination. While the VBM of TiO_x at the Daisy-Fun experiments were determined after the method of Kraut by using a linear extrapolation of the leading edges to the baselines of the XPS spectra,^[150] the VBM of TiO_x measured with 90 eV photon energy at Bessy, were derived as proposed by Pekkola and Kumarasinghe for small photon energies using the emission onset.^[25,218] The differences in the VBM determination methods result in differences of 0.4-0.5 eV in the position of the VBM (see Fig. 14.1). After UV illumination an energy level line-up was only derived at the Daisy-Fun (see Fig. 15.2 right). The Ti 2p_3/2-C 1s distance increases from 173.4 eV to 173.8 eV. As a result also the E_{C B}-LUMO_onset changes and the LUMO_onset of C₆₀rises above (0.1 eV) E_{C B} of TiO_x. This line-up indicates good electron extraction behavior at the interface as well.

The comparison of the experiments performed at Bessy and the Daisy-Fun make it evident how crucial the VBM position determination is, to derive a proper energy level line-up. As a result two similar experiments derived with different photon energies lead to two different energetic line-ups of the TiO_x/C₆₀ interface. While the line-up derived at Bessy gives rise to an electron extraction barrier at the interface and low conductivity of TiO_x, the line-up obtained at the Daisy-Fun does not explain S-shaped I-V characteristics before UV illumination as neither an extraction barrier for electrons is observed nor is a low conductivity of the TiO_x(see Section 2.5).^[54]In the following paragraphs influences of adsorbates on the energy level line-up on the basis of the Daisy-Fun experiments are discussed.

Figure 15.2. – Energy band diagrams of 80 °C TiO_x/C₆₀interface derived at Bessy before UV (left), derived at Daisy-Fun before UV (middle) and derived at the Daisy-Fun after UV (right).

Air influence: The results in Section 11.1 indicate that air exposure causes adsorbates at the surface inducing a downward band bending at sc-(101) anatase surface of 100-200 meV. Therefore the energy levels in the bulk of the TiO_x may are at different positions, than determined at the surface (as XPS is surface sensitive). As the bulk properties of the ALD prepared sample are not accessible by XPS, it is assumed that air induces a similar surface band bending on TiO_xas on the sc-(101) anatase surface (see Section 11.1). Such a situation is depicted in Fig. 15.3. In the energy diagram it is assumed, that air induces a downward band bending of 200 meV from the bulk of TiO_x towards the surface. The line-up at the interface does not change compared to Fig. 15.2. In the bulk of TiO_x the CBM is now above E_F indicating a reduced conductivity of the TiO_x. After UV illumination the band bending towards the interface increases to 400 meV and the TiO_xCBM at the surface is below E_F.

As many others Ecker et al. observed S-shaped I-V characteristics of IOSC with TiO_x ETL’s before UV illumination and diode like I-V characteristics after UV illumination. Ecker et al. performed impedance measurements and attributed the S-shape to a reduced conductivity of TiO_x, which improves after illumination with UV light.^[54] The derived energy diagram in Fig. 15.3 with the air induced downward band bending towards the surface supports the model, that a reduced conductivity of the TiO_x before UV illumination is existent. However XPS only allows a statement about the CBM at the surface and no statement about a possible change of CBM position in the bulk after UV illumination can be made.

In this context Pomoni et al. reported an increased conductivity of nc-TiO₂ after UV irradiation.^[73]

Concluding, the line up shown in Fig. 15.3 could support the assumption that an increased conductivity of TiO_xafter UV is responsible for the improved I-V characteristics of the IOSC with TiO_xas ETL.

Figure 15.3– Energy diagram with an assumed accumulation layer at the TiO_xsurface due to the adsorption of hydroxyls and oxygen at the surface.

It is assumed that oxygen causes an accumulation layer at the surface.

In Fig. 15.4 dark current voltage curves measured on IOSC with TiO_x as ETL are displayed on a logarithmic scale. The red and blue curve represent dark I-V characteristics before and after UV

162 15. Comparing energy band diagrams and I-V characteristics

10^-5 10^-4 10^-3 10^-2 10^-1 10⁰ 10¹

ln |j|

-0.8 -0.4 0.0 0.4

U [V]

before UV after UV

I * U for ohmic behavior

Figure 15.4– Logarithmic plot of the dark I-V curves of IOSC with TiO_x as ETL. They correspond to the I-V curves shown in Fig. 7.2. The red curve is before UV and the blue curve after UV illumination. The black curve follows an ohmic behavior.

illumination. The black curve represents the ohmic behavior of a resistor. The blue and black curve are in good accordance in negative bias direction, while the red curve is different to the black, ohmic, curve. In an ideal solar cell all contacts/interfaces, besides the absorbing p-n contact, should be ohmic contacts. At negative voltages the diode is in reverse direction and only the dark current flows, while the I-V curve is mainly determined by the contacts. The good accordance of the blue and the black curve shows, that at negative voltages (in reverse direction) the I-V characteristics are ohmic, indicating a good blocking behavior of the p-n diode and ohmic contacts. The difference of the red I-V curve before UV and the black I-V curve indicates an additional non ohmic contact within the solar cell. This could be an evidence for the existence of a diode in reverse direction to the photo diode (counter diode) as it is described in Section 2.4. The origin of such a counter diode could be the TiO_x/PC₆₁BM interface, if the HOMO_onset of the organic material is above E_{C B} of the ETL. As the S-shape disappears with AZO as ETL it is unlikely that any other interface, than the ETL/organic interface, occurring in the IOSC device is responsible for the S-shape.

O₂influence: A third option is that the energy diagram observed under UHV condition does not rep-resent the actual line-up, which occurs under ambient pressure conditions. Porsgaard et al. reported, that molecular oxygen with a partial pressure of 1.3 mbar induces an upward band bending of 0.4 eV on a rutile (110) surface, compared to UHV conditions.^[171] Hence there might be a significant upward band bending at the surface of TiO_x at atmosphere, which is not observed in XPS due to desorption of O₂ under UHV conditions. Therefore an upward band bending of 0.4 eV towards the interface is added in the line-up depicted in Fig. 15.5. This is reflected by an upward shift of all TiO_x energy levels at the surface by 0.4 eV. Now E_{C B} is 0.4 eV above E_F. The TiO_x energy level are drawn short, as the band positions within the bulk are not discussed. Compared to line-up in Fig. 15.2 this changes the line-up at the interface significantly. Before UV illumination, an extractions barrier occurs, as now E_{C B} of TiO_x at the surface is 0.4 eV above E_F. An offset of the TiO_x E_{C B} and the C₆₀ LUMO of 0.6 eV is apparent at the surface. This leads to an electron extraction barrier of 0.4 eV (as the C₆₀ LUMO at the interface is 0.2 eV below E_F). Assuming oxygen species adsorption at the surface, the E_{C B} position suits well to the observed low conductivities of amorphous TiO_x.^[223] After UV illumination the line-up is the same as in Fig. 15.2 right and no electron extraction barrier is observed.

Figure 15.5– TiO_x/C₆₀energy diagram with an assumed upward shift of the energy levels by 0.4 eV due to adsorbed oxygen species. The oxygen species desorbs under UHV conditions.