Consensus statement for stability assessment and reporting for perovskite photovoltaics based on ISOS procedures

Improving the long-term stability of perovskite solar cells is critical to the deployment of this technology. Despite the great emphasis laid on stability-related investigations, publications lack consistency in experimental procedures and parameters reported. It is therefore challenging to reproduce and compare results and thereby develop a deep understanding of degradation mechanisms. Here, we report a consensus between researchers in the field on procedures for testing perovskite solar cell stability, which are based on the International Summit on Organic Photovoltaic Stability (ISOS) protocols. We propose additional procedures to account for properties specific to PSCs such as ion redistribution under electric fields, reversible degradation and to distinguish ambient-induced degradation from other stress factors. These protocols are not intended as a replacement of the existing qualification standards, but rather they aim to unify the stability assessment and to understand failure modes. Finally, we identify key procedural information which we suggest reporting in publications to improve reproducibility and enable large data set analysis. Reliability of stability data for perovskite solar cells is undermined by a lack of consistency in the test conditions and reporting. This Consensus Statement outlines practices for testing and reporting stability tailoring ISOS protocols for perovskite devices.

The Quest for Stability of Perovskite Solar Cells: Understanding Degradation, Improving Lifetimes and Towards Experimental Standards

Konrad Domanski

Perovskite solar cells (PSCs) have attracted a substantial interest owing to a very fast achievement of efficiencies >20 % within only several years of development. However, for any solar cell to become technologically viable, two additional milestones have to be achieved alongside high efficiency: means of industrial production and good operational stability. The former being mostly the focus of the private sector and industrially-oriented research institutes, understanding degradation and improving the stability of PSCs has become one of the major research topics in the field of emerging photovoltaics. This is also the focus of this thesis, where I investigate different degradation mechanisms in PSCs with the aim of extending their long-term stability. This dissertation can be divided into four parts: in the first one, I show how, through a series of prototypes, I designed and built a dedicated setup to investigate stability of PSCs. I also show how I developed its variation to improve by 700 % the efficiency of measuring current-voltage characteristics of our solar cells. Subsequently, I describe the intrinsic instability of, at the time, state-of-the-art PSCs. I show how, along with my colleagues, we managed to achieve breakthrough room temperature stability coupled with record efficiency through incorporation of Cs into perovskite. Next, I describe how and why these solar cells suffer from irreversible degradation if left at elevated temperatures however. This is due to a vulnerability to Au diffusion from the electrode, through Spiro-MeOTAD hole transporting layer into the perovskite. I show how, by incorporation of Cr diffusion barrier I managed to circumvent this problem (albeit at considerable efficiency loss). Subsequently, I describe an alterna-tive solution to the problem by substituting Spiro-MeOTAD with PTAA - a polymeric hole transporting layer. This approach effectively stops Au diffusion without compromising device efficiency, which was at the same time improved by incorporating Rb into the perovskite. Finally, a third approach is presented: replacing the Au electrode with one based on carbon nanotubes. This gives away with using Au and PTAA - both prohibitively expensive materials - and hence paves the way towards facile and inexpensive fabrication of stable PSCs. In the third part, I describe the effects of mobile ions in the perovskite on the behaviour of PSCs. First, I show how they lead to a partial reversibility of losses in aged devices. This has potentially far-reaching consequences for the way stability measurements of PSCs are conducted and how their lifetime is reported. Next, I show how the ionic movement in the perovskite can lead to PSCs with certain architecture to work as high-gain photodetectors. This is enabled by piling up of ions at the interfaces within the devices, which modifies the energy levels within. Finally, in the last chapter I describe systematically, how different factors such as temperature, illumination, atmosphere, load on the device and cycling of the environmental conditions affect the stability of PSCs. Based on this, I discuss the important parameters to control when ageing PSCs, as the first attempt to bring the community to a consensus on how to measure stability of PSCs. This is urgently needed to streamline the efforts to create stable PSCs and to commercialize the technology.

EPFL2018

Exploring pi-extended subporphyrinoids as electron transporting materials in perovskite solar cells

Hiroyuki Kanda, Mohammad Khaja Nazeeruddin

Due to their strong acceptor properties and enhanced charge transport capabilities, non-fullerene electron acceptors based on pi-extended derivatives - subnaphthalocyanines (SubNc) or subphtalocyanine dimers (SubPc(2)) - have been employed in organic photovoltaics in the past as an alternative to the expensive fullerene. However, these promising pi-extended derivatives have not been explored in perovskite solar cell technology. In this work, we implement a vacuum-deposited very thin film of SubNc or SubPc(2) as electron transport layers in perovskite solar cells. In particular, we demonstrate the excellent electron extraction properties of the thin films in contact with perovskite layer. The fabricated perovskite solar cells exhibit enhanced device performances with reduced hysteresis index and improved device stability. Our results validate the use of n-extended subporphyrinoids as promising candidates for perovskite optoelectronics with enhanced stability properties, being essential for further conunercialization of the perovskite technology.

WORLD SCIENTIFIC PUBL CO PTE LTD2022

Revealing fundamentals of charge extraction in photovoltaic devices through potentiostatic photoluminescence imaging

Dmitry Bogachuk, Michael Graetzel, Yuhang Liu, Shaik Mohammed Zakeeruddin

The photocurrent density-voltage (J(V)) curve is the fundamental characteristic to assess opto-electronic devices, in particular solar cells. However, it only yields information on the performance inte-grated over the entire active device area. Here, a method to deter-mine spatially resolved photocurrent images by voltage-dependent photoluminescence microscopy is derived from basic principles. The opportunities and limitations of the approach are studied by the investigation of III-V and perovskite solar cells. This approach allows the real-time assessment of the microscopically resolved local J(V) curve and the steady-state Jsc as well as transient effects. In addi-tion, the measurement contains information on local charge extrac-tion and interfacial recombination. This facilitates the identification of regions of non-ideal charge extraction and enables linking these to the processing conditions. The proposed technique highlights that, combined with potentiostatic measurements, luminescence microscopy can be a powerful tool for the assessment of perfor-mance losses and the improvement of solar cells.

ELSEVIER2022

The Quest for Stability of Perovskite Solar Cells: Understanding Degradation, Improving Lifetimes and Towards Experimental Standards

Konrad Domanski

EPFL2018