A chain is as strong as its weakest link - Stability study of MAPbI(3) under light and temperature

The stability of perovskite solar cells is a key issue for industrial development. One reason for this is the volatile organic methylammonium (MA) cation, which is prone to degas under elevated temperatures from the perovskite. At the same time, small amounts of MA are used for practically all highest performing solar cells. These compositions have also shown relatively promising stabilities. This raises the question of MA stability with respect to different, application-dependent stability requirements. Interestingly, MA stability was mainly studied on thin films that differ from full devices or with architectures which are also prone to degrade. Therefore, the degradation behavior on complete MA containing devices with a relatively stable architecture is required to quantify the long-term stability of MA. This enables to determine at which timescales MA is unstable and which role it can play in future compositions. If MA is indeed unstable at much longer timescales than previously recorded, it also indicates that more severe degradation pathways are currently underappreciated. Here, "weakest link" MAPbI(3) solar cells show stability where devices retained 100% of their initial efficiency over 1000 h of aging under constant illumination and maximum power point tracking at 20 degrees C. At elevated temperatures of 50 and 65 degrees C, the devices retained 100% and 90% of their initial efficiency after 500 h of illumination, respectively. Impressively, at 95 degrees C the MAPbI(3) device retained 85% of its initial efficiency after 500 h under constant illumination, which is some of the best stability data reported to date for MA. Thus, MA-containing devices require further studying. Nevertheless to achieve the necessary industrial lifetimes of more than 25 years, the complete removal of MA is a sensible precaution to systematically avoid any long-term risk factors.

Not All That Glitters Is Gold: Metal-Migration-Induced Degradation in Perovskite Solar Cells

Antonio Abate, Juan-Pablo Correa-Baena, Konrad Domanski, Ulf Anders Hagfeldt, Mohammad Khaja Nazeeruddin, Michael Saliba, Wolfgang Richard Tress

Perovskite solar cells (PSCs) have now achieved efficiencies in excess of 22%, but very little is known about their long-term stability under thermal stress. So far, stability reports have hinted at the importance of substituting the organic components, but little attention has been given to the metal contact. We investigated the stability of state-of-the-art PSCs with efficiencies exceeding 20%. Remarkably, we found that exposing PSCs to a temperature of 70 degrees C is enough to induce gold migration through the hole-transporting layer (HTL), Spiro-MeOTAD, and into the perovskite material, which in turn severely affects the device performance metrics under working conditions. Importantly, we found that the main cause of irreversible degradation is not due to decomposition of the organic and hybrid perovskite layers. By introducing a Cr metal interlayer between the HTL and gold electrode, high-temperature-induced irreversible long-term losses are avoided. This key finding is essential in the quest for achieving high efficiency, long-term stable PSCs which, in order to be commercially viable, need to withstand hard thermal stress tests.

American Chemical Society2016

Window Layers for Silicon Heterojunction Solar Cells

Johannes Peter Seif

Currently, crystalline silicon (c-Si) wafer-based solar cells dominate the photovoltaic market (80-90%). In this thesis we concentrate on silicon heterojunction (SHJ) solar cells that--in contrast to diffused homojunction cells--rely on the application of amorphous silicon (a-Si:H) thin films. Unlike standard homojunction devices, which are typically limited by their highly recombination-active semiconductor-to-metal contacts, SHJ devices exhibit excellent surface passivation enabled by intrinsic and doped a-Si:H films. These a-Si:H layers, however, entail drawbacks for optical performance and carrier transport, two topics that will be addressed in this work. To this end we investigate non-traditional materials for SHJ devices, with the goal of replacing the a-Si:H or the transparent electrodes. These materials include microcrystalline silicon (uc-Si:H) and organic semiconductors for contact formation; amorphous silicon suboxides (a-SiOx:H) for surface passivation; and transparent electrodes applied by atomic layer deposition (ALD) as protective layers against subsequent processing steps. Along with the optical and electrical properties of these materials, we study the impact on device performance associated with their deposition. For this we test the devices under standard testing conditions (25 °C) and at elevated temperatures closer to those encountered in the field. For the investigations on uc-Si:H layers, we vary process parameters (including temperature, pressure, power, excitation frequency and hydrogen dilution) as well as pre-treatments, gas variations and nucleation layers. We assess the suitability of these approaches for SHJ solar cells and apply selected measures in devices. Thereby we demonstrate a gain in short-circuit current density in the range of 0.5-1 mA.cm-2 and good fill factor values of up to 79.2% using either n- and p-type uc-Si:H layers. Furthermore, with the goal of reducing optical losses, we test wide-bandgap a-SiOx:H layers for passivation. In terms of current gain without negative side effects on transport, we argue that these layers are best applied to the electron-collecting contact--put at the front of the device--as their application to the hole-collecting contact introduces a transport barrier for holes. This barrier deteriorates the device performance at 25 °C, but shows a beneficial effect on the temperature coefficient of the device, yielding coefficients as low as -0.1%/°C. In some cases--compared to standard devices--devices with a-SiOx:H layers exhibit superior performance at elevated temperatures, which can be of interest in warmer climates. In parallel to these main topic we also study aluminium-doped zinc oxide (ZnO:Al) layers deposited by ALD as a protective layer against sputter-induced damage and organic semiconductors as transparent electrodes for the hole-collecting contact. In both cases we observe a gain in terms of surface passivation, which indicates that these materials may be beneficial for the contact formation in future device structures. In addition to these material-related investigations, we unravel the temperature-dependence of each individual cell parameter and present a brief comparison of state-of-the-art technologies and their respective temperature dependence.

EPFL2015

Window Layers for Silicon Heterojunction Solar Cells

Johannes Peter Seif

EPFL2015