Role of the Metal-Oxide Work Function on Photocurrent Generation in Hybrid Solar Cells

ZnO is a widely used metal-oxide semiconductor for photovoltaic application. In solar cell heterostructures they not only serve as a charge selective contact, but also act as electron acceptor. Although ZnO offers a suitable interface for exciton dissociation, charge separation efficiencies have stayed rather poor and conceptual differences to organic acceptors are rarely investigated. In this work, we employ Sn doping to ZnO nanowires in order to understand the role of defect and surface states in the charge separation process. Upon doping we are able to modify the metal-oxide work function and we show its direct correlation with the charge separation efficiency. For this purpose, we use the polymer poly(3-hexylthiophene) as donor and the squaraine dye SQ2 as interlayer. Interestingly, neither mobilities nor defects are prime performance limiting factor, but rather the density of available states around the conduction band is of crucial importance for hybrid interfaces. This work highlights crucial aspects to improve the charge generation process of metal-oxide based solar cells and reveals new strategies to improve the power conversion efficiency of hybrid solar cells.


Electron Microscopy & XRD
By incorporating Sn we do not find any apparent macroscopic structural changes in nanowires as seen in SEM images. Furthermore, high-resolution transmission electron microscopy (HRTEM) images on ZnO NWs show the interplanar spacing of 0.26 nm, which corresponds to the (002) plane of the wurtzite structure of ZnO. Also XRD data reveals that nanowires were preferentially oriented in (002) direction with its c-axis perpendicular to the substrate and indexed to the hexagonal Wurtzite structured of ZnO according to the JCPDS card No. 36-1451. Moreover, there is no peak shift observable and SnO or SnO2 peaks are absent. It is evident that Sn 4+ substituted successfully in native Zn 2+ site without any aggregate formation.
In contrast, we see that the intensity of the (002) plane peak is reduced which we attributed to a lower crystallinity. This can be done in this comparison, since geometry and morphology differences are minimal.

XPS:
In Figure S2, XPS Wide Scans are shown. For all samples, we observe signals originating from C, O, and Zn. The presence of C we attribute to precursor residuals and atmospheric contamination, due to the exposure of the samples to the atmosphere before their introduction to the UHV chamber for the characterization. Indium from the ITO substrate, is not present, which implies that the nanowires are closely packed, as it is confirmed by electron microscopy observations. The Zn 2p peak appears as a doublet due spin orbit splitting. The Zn 2p3/2 peak is at a binding energy of 1021.7±0.1 eV for all the samples, which is attributed In Figure S3, the Sn 3d peak is presented. At the pristine sample a wide peak can be observed, which is attributed to the presence of Auger peaks of Zn. With increasing dopant concentration, only a small broadening of that peak is observable with a slight shift towards higher binding energies that could possibly be attributed to SnOx though the low intensity indicates that only traces are present. Figure S4 presents the O 1s peak. The peak is asymmetric and large which we attribute to a superposition of two peaks, the first one with a binding energy of 530.4±0.1 eV, which is attributed to the Zn-O bond and the second one at about 532.0±0.1 eV, which is due to the OH (hydroxyls) from the atmosphere. No significant change in the overall shape and the two-peak component ratio is observed at the O1 s peak with Sn doping.

Space-charge analysis
Light intensity dependent J-V measurements were carried out to investigate the device behaviour on varying charge carrier densities. This is particularly important for investigations on space charge effects. If the photocurrent density is linearly dependent on light intensity I, (and, thus, on the charge generation rate), there is no space charge region present in a working device and the photocurrent density can be expressed by: where Jph is the photogenerated current density, q is the electric charge, G is the generation rate of electron-hole pairs, L is the specimen thickness, µ h is the charge carrier mobility of holes, τ h is the charge carrier lifetime, and V is the voltage across the entire photoactive layer. Note, G is direct proportional to I. The current density in the space charge limited conduction regime (Jscl) can be expressed by Mott-Gurney's law: (2)

Figure S7| Charge collection probability calculated for J-V curves by J(V)/J(V=-0.2V). Measurements have been performed under different light intensities between 1-100 mW/cm² (from red to black color) for all devices under investigation. No matter how strong ZnO is doped, at JSC charges can reach the electrodes efficiently and a voltage dependent photocurrent is basically absent.
where εr and ε0 represent the relative dielectric constant and the vacuum permittivity, respectively. Note that almost the entire voltage V drops on a region L1 of hole accumulation therefore V =V1 and the generated photocurrent in this region is essentially the total current. In the case of a space charge limited photocurrent, the width of L1 can be determined from As can be seen in Figure S8, doping does not result in a narrowing of the space charge region and also the transition to the saturation regime is identical between a non-doped and a strongly doped device. More importantly, however, this observation does not change if the incident light intensity is decreased by a factor of 4.
Although we could successfully decrease the electron mobility by Sn 4+ doping and the NW conductivity, a reduction of space charge effects is virtually absent.