No abstract

Perovskites have been demonstrated in solar cells with power conversion efficiency well above 20%, which makes them one of the strongest contenders for the next generation photovoltaics. While there are no concerns about their efficiency, very little is known about their stability under illumination and load. Ionic defects and their migration in the perovskite crystal lattice are one of the most alarming sources of degradation, which can potentially prevent the commercialization of perovskite solar cells (PSCs). In this work, we provide direct evidence of electric field-induced ionic defect migration and we isolate their effect on the long-term performance of state-of-the-art devices. Supported by modelling, we demonstrate that ionic defects, migrating on timescales significantly longer (above 10 3 s) than what has so far been explored (from 10 -1 to 10 2 s), abate the initial efficiency by 10-15% after several hours of operation at the maximum power point. Though these losses are not negligible, we prove that the initial efficiency is fully recovered when leaving the device in the dark for a comparable amount of time. We verified this behaviour over several cycles resembling day/night phases, thus probing the stability of PSCs under native working conditions. This unusual behaviour reveals, that research and industrial standards currently in use to assess the performance and the stability of solar cells need to be adjusted for PSCs.Our work paves the way towards much needed new testing protocols and figures of merit specifically designed for PSCs.4

We have developed a charge transport model that explicitly accounts for ion migration. This model has been used to interpret measured current–voltage characteristics that show hysteresis.

Hybrid perovskites represent a new paradigm for photovoltaics, which have the potential to overcome the performance limits of current technologies and achieve low cost and high versatility. However, an efficiency drop is often observed within the first few hundred hours of device operation, which could become an important issue. Here, we demonstrate that the electrode's metal migrating through the hole transporting material (HTM) layer and eventually contacting the perovskite is in part responsible for this early device degradation. We show that depositing the HTM within an insulating mesoporous "buffer layer" comprised of Al2O3 nanoparticles prevents the metal electrode migration while allowing for precise control of the HTM thickness. This enables an improvement in the solar cell fill factor and prevents degradation of the device after 350 h of operation.

The effects of transport layers on perovskite solar cell performance, in particular anomalous hysteresis, are investigated.

The drying process is a crucial step in electrode manufacture that may lead to spatial inhomogeneities in the distribution of the electrode components resulting in impaired cell performance. Binder migration during the drying process, and the ensuing poor binder coverage in certain regions of the electrode, can lead to capacity fade and mechanical failure (e.g. electrode delamination from the current collector). A mathematical model of electrode drying is presented which tracks the evolution of the binder distribution, and is applicable in the relatively high drying rates encountered in industrial electrode manufacture. The model predicts that constant low drying rates lead to a favourable homogeneous binder profiles, whereas constant high drying rates are unfavourable and result in accumulation of binder near the evaporation surface and depletion near the current collector. These results show strong qualitative agreement with experimental observations and provide a cogent explanation for why fast drying conditions result in poorly performing electrodes. Finally, a scheme is detailed for optimisation of a time-varying drying procedure that allows for short drying times whilst simultaneously ensuring a close to homogeneous binder distribution throughout the electrode.

The current decay in response to a sudden change of applied bias up to 1 V has been measured on a methylammonium lead triiodide perovskite solar cell, for temperatures between 258 and 308 K.

Drift-diffusion models that account for the motion of both electronic and ionic charges are important tools for explaining the hysteretic behaviour and guiding the development of metal halide perovskite solar cells. Furnishing numerical solutions to such models for realistic operating conditions is challenging owing to the extreme values of some of the parameters. In particular, those characterising (i) the short Debye lengths (giving rise to rapid changes in the solutions across narrow layers), (ii) the relatively large potential differences across devices and (iii) the disparity in timescales between the motion of the electronic and ionic species give rise to significant stiffness. We present a finite difference scheme with an adaptive time step that is posed on a non-uniform staggered grid that provides second order accuracy in the mesh spacing. The method is able to cope with the stiffness of the system for realistic parameters values whilst providing high accuracy and maintaining modest computational costs. For example, a transient sweep of a current-voltage curve can be computed in only a few minutes on a standard desktop computer.