Solid oxide cells are promising devices for decarbonizing the energy economy, as they offer efficient energy storage and generate green alternatives to fossil fuels. In such electrochemical cells, the interfaces between various components are vital for the charge transfer kinetics and therefore for the overall performance of the cell. Although there are well-established models describing interfacial space charge regions for either electronic or ionic species, models describing space charge regions for both ionic and electronic charge carriers are far less developed. The latter type of space charge is, for example, present in high temperature solid oxide solar cells based on the interfaces between SrTiO₃ (STO) and other mixed ionic and electronic conductors (MIECs) [1]. Experiments yield photovoltages up to > 1 V, but the interplay of ionic and electronic defects in determining the corresponding space charge potential is not truly understood.

This work focuses on the investigation of space charge regions between highly electronic conducting MIECs and STO. YBa₂Cu₃O_7-δ (YBCO), La_0.6Sr_0.4FeO_3-δ (LSF), La_0.6Sr_0.4CoO_3-δ (LSC), La_0.65Sr_0.35MnO_3-δ (LSM) and La_0.9Sr_0.1CrO_3-δ (LSCr) thin films were grown by pulsed laser deposition on (nominally) undoped STO single crystals. The resulting interfacial space charge zones are investigated by several approaches: First and most prominently, the resistances of space charge regions at the MIEC│MIEC interfaces were determined in a broad range of oxygen partial pressures at 500°C by means of electrochemical impedance spectroscopy. According to these measurements, investigated materials are distinguishable into MIECs leading to very pronounced space charges (LSM, LSCr), MIECs with moderate space charges (LSF, LSC), and oxides that do not lead to any measurable space charge resistance (YBCO). Interestingly, experiments of nanometer thin interlayers of LSM revealed that already a layer thickness of a single unit cell leads to the formation of a space charge with properties very similar to much thicker films. In a second approach, ¹⁸O tracer diffusion across the corresponding interfaces revealed information on the depletion of ionic defects (oxygen vacancies). Third, work function measurements by means of X-ray photoelectron spectroscopy were performed for LSCr, LSM and LSF layers on STO, which strongly support the space charge data obtained from impedance measurements. The same is true for the fourth approach, the analysis of space charges by means of photovoltages measured upon UV illumination of these MIEC│STO hetero-interfaces [1]. Finally, a model is developed, which correlates the ionic and electronic driving forces determining the space charge potentials. This model predicts a relation between space charge potentials and reducibilities of the MIECs, i.e. the transition points in Brouwer diagrams. Comparison with literature data on defect energies indeed confirms the predicted trends.