The Role of Surface Recombination on the Performance of Perovskite Solar Cells:Effect of Morphology and Crystalline Phase of TiO₂ Contact

Herein, the preparation of 1D TiO2 nanocolumnar films grown by plasma-enhanced chemical vapor deposition is reported as the electron selective layer (ESL) for perovskite solar devices. The impact of the ESL architecture (1D and 3D morphologies) and the nanocrystalline phase (anatase and amorphous) is analyzed. For anatase structures, similar power conversion efficiencies are achieved using an ESL either the 1D nanocolumns or the classical 3D nanoparticle film. However, lower power conversion efficiencies and different optoelectronic properties are found for perovskite devices based on amorphous 1D films. The use of amorphous TiO2 as electron selective contact produces a bump in the reverse scan of the current–voltage curve as well as an additional electronic signal, detected by impedance spectroscopy measurements. The dependence of this additional signal on the optical excitation wavelength used in the IS experiments suggests that it stems from an interfacial process. Calculations using a drift-diffusion model which explicitly considers the selective contacts reproduces qualitatively the main features observed experimentally. These results demonstrate that for a solar cell in which the contact is working properly the open-circuit photovoltage is mainly determined by bulk recombination, whereas the introduction of a “bad contact” shifts the balance to surface recombination.</p

Inverted Current-Voltage Hysteresis in Mixed Perovskite Solar Cells: Polarization, Energy Barriers, and Defect Recombination

Organic-inorganic metal halide perovskite solar cells show hysteresis in their current-voltage curve measured at a certain voltage sweep rate. Coinciding with a slow transient current response, the hysteresis is attributed to a slow voltage-driven (ionic) charge redistribution in the perovskite solar cell. Thus, the electric fi eld profi le and in turn the electron/hole collection effi ciency become dependent on the biasing history. Commonly, a positive prebias is benefi cial for a high power-conversion effi ciency. Fill factor and open-circuit voltage increase because the prebias removes the driving force for charge to pile-up at the electrodes, which screen the electric fi eld. Here, it is shown that the piled-up charge can also be benefi cial. It increases the probability for electron extraction in case of extraction barriers due to an enhanced electric fi eld allowing for tunneling or dipole formation at the perovskite/electrode interface. In that case, an inverted hysteresis is observed, resulting in higher performance metrics for a voltage sweep starting at low prebias. This inverted hysteresis is particularly pronounced in mixed-cation mixed-halide systems which comprise a new generation of perovskite solar cells that makes it possible to reach power-conversion effi ciencies beyond 20%

Single vs mixed organic cation for low temperature processed perovskite solar cells

The present work reports a comparative study between single and mixed organic cation based MAPbI3 and MA0.6FA0.4PbI3 perovskite devices fabricated in conjunction with low temperature processed (<150 °C) ZnO electron transport layers. MA0.6FA0.4PbI3 perovskite devices demonstrate 37% higher power conversion efficiency compared to MAPbI3 perovskite devices developed on the ZnO ETL. In addition, MA0.6FA0.4PbI3 devices exhibit very low photocurrent hysteresis and they are three-fold more stable than conventional MAPbI3 PSCs (perovskite solar cells). An in-depth analysis on the charge transport properties in both fresh and aged devices has been carried out using electrochemical impedance spectroscopy analysis to comprehend the enhanced device stability of the mixed perovskite devices developed on the ZnO ETL. The study also investigates into the interfacial charge transfer characteristics associated with the ZnO/mixed organic cation perovskite interface and concomitant influence on the inherent electronic properties