Removal of Surface Oxygen Vacancies Increases Conductance Through TiO2 Thin Films for Perovskite Solar Cells.

We report that UV-ozone treatment of TiO2 anatase thin films is an efficient method to increase the conductance through the film by more than 2 orders of magnitude. The increase in conductance is quantified via conductive scanning force microscopy on freshly annealed and UV-ozone-treated TiO2 anatase thin films on fluorine-doped tin oxide substrates. The increased conductance of TiO2 anatase thin films results in a 2% increase of the average power conversion efficiency (PCE) of methylammonium lead iodide-based perovskite solar cells. PCE values up to 19.5% for mesoporous solar cells are realized. The additional UV-ozone treatment results in a reduced number of oxygen vacancies at the surface, inferred from X-ray photoelectron spectroscopy. These oxygen vacancies at the surface act as charge carrier traps and hinder charge extraction from the adjacent material. Terahertz measurements indicate only minor changes of the bulk conductance, which underlines the importance of UV-ozone treatment to control surface-based defects.

The Interplay of Contact Layers: How the Electron Transport Layer Influences Interfacial Recombination and Hole Extraction in Perovskite Solar Cells.

Charge-selective contact layers in perovskite solar cells influence the current density-voltage hysteresis, an effect related to ion migration in the perovskite. As such, fullerene-based electron transport layers (ETLs) suppress hysteresis by reducing the mobile ion concentration. However, the impact of different ETLs on the electronic properties of other constituent device layers remains unclear. In this Kelvin probe force microscopy study, we compared potential distributions of methylammonium lead iodide-based solar cells with two ETLs (planar TiO2 and C60-functionalized self-assembled monolayer) with different hysteretic behavior. We found significant changes in the potential distribution of the organic hole transport layer spiroMeOTAD, suggesting the formation of a neutral spiroMeOTAD/iodide interface due to a reaction between iodide with p-doped spiroMeOTAD in the TiO2 cell. Our results show that the ETL affects not only the mobile ion concentration and the recombination at the perovskite/ETL interface but also the resistance and capacitance of the spiroMeOTAD.

Know your full potential: Quantitative Kelvin probe force microscopy on nanoscale electrical devices.

In this study we investigate the influence of the operation method in Kelvin probe force microscopy (KPFM) on the measured potential distribution. KPFM is widely used to map the nanoscale potential distribution in operating devices, e.g., in thin film transistors or on cross sections of functional solar cells. Quantitative surface potential measurements are crucial for understanding the operation principles of functional nanostructures in these electronic devices. Nevertheless, KPFM is prone to certain imaging artifacts, such as crosstalk from topography or stray electric fields. Here, we compare different amplitude modulation (AM) and frequency modulation (FM) KPFM methods on a reference structure consisting of an interdigitated electrode array. This structure mimics the sample geometry in device measurements, e.g., on thin film transistors or on solar cell cross sections. In particular, we investigate how quantitative different KPFM methods can measure a predefined externally applied voltage difference between the electrodes. We found that generally, FM-KPFM methods provide more quantitative results that are less affected by the presence of stray electric fields compared to AM-KPFM methods.

Local Time-Dependent Charging in a Perovskite Solar Cell.

Efficient charge extraction within solar cells explicitly depends on the optimization of the internal interfaces. Potential barriers, unbalanced charge extraction, and interfacial trap states can prevent cells from reaching high power conversion efficiencies. In the case of perovskite solar cells, slow processes happening on time scales of seconds cause hysteresis in the current-voltage characteristics. In this work, we localized and investigated these slow processes using frequency-modulation Kelvin probe force microscopy (FM-KPFM) on cross sections of planar methylammonium lead iodide (MAPI) perovskite solar cells. FM-KPFM can map the charge density distribution and its dynamics at internal interfaces. Upon illumination, space charge layers formed at the interfaces of the selective contacts with the MAPI layer within several seconds. We observed distinct differences in the charging dynamics at the interfaces of MAPI with adjacent layers. Our results indicate that more than one process is involved in hysteresis. This finding is in agreement with recent simulation studies claiming that a combination of ion migration and interfacial trap states causes the hysteresis in perovskite solar cells. Such differences in the charging rates at different interfaces cannot be separated by conventional device measurements.

Surface Modification of TiO2 Photoanodes with Fluorinated Self-Assembled Monolayers for Highly Efficient Dye-Sensitized Solar Cells.

Dye aggregation and electron recombination in TiO2 photoanodes are the two major phenomena lowering the energy conversion efficiency of dye-sensitized solar cells (DSCs). Herein, we introduce a novel surface modification strategy of TiO2 photoanodes by the fluorinated self-assembled monolayer (F-SAM) formation with 1H,1H,2H,2H-perfluorooctyltriethoxysilane (PFTS), blocking the vacant sites of the TiO2 surface after dye adsorption. The F-SAM helps to efficiently lower the surface tension, resulting in efficient repelling ions, e.g., I3(-), in the electrolyte to decrease the electron recombination rate, and the role of F-SAM is characterized in detail by impedance spectroscopy using a diffusion-recombination model. In addition, the dye aggregates on the TiO2 surface are relaxed by the F-SAM with large conformational perturbation (i.e., helix structure) seemingly because of steric hindrance developed during the SAM formation. Such multifunctional effects suppress the electron recombination as well as the intermolecular interactions of dye aggregates without the loss of adsorbed dyes, enhancing both the photocurrent density (11.9 → 13.5 mA cm(-2)) and open-circuit voltage (0.67 → 0.72 V). Moreover, the combined surface modification with the F-SAM and the classical coadsorbent further improves the photovoltaic performance in DSCs.

Real-space observation of unbalanced charge distribution inside a perovskite-sensitized solar cell.

Perovskite-sensitized solar cells have reached power conversion efficiencies comparable to commercially available solar cells used for example in solar farms. In contrast to silicon solar cells, perovskite-sensitized solar cells can be made by solution processes from inexpensive materials. The power conversion efficiency of these cells depends substantially on the charge transfer at interfaces. Here we use Kelvin probe force microscopy to study the real-space cross-sectional distribution of the internal potential within high efficiency mesoscopic methylammonium lead tri-iodide solar cells. We show that the electric field is homogeneous through these devices, similar to that of a p-i-n type junction. On illumination under short-circuit conditions, holes accumulate in front of the hole-transport layer as a consequence of unbalanced charge transport in the device. After light illumination, we find that trapped charges remain inside the active device layers. Removing these traps and the unbalanced charge injection could enable further improvements in performance of perovskite-sensitized solar cells.