Reconsidering figures of merit for performance and stability of perovskite photovoltaics

The development of hybrid organic-inorganic halide perovskite solar cells (PSCs) that combine high performance and operational stability is vital for implementing this technology. Recently, reversible improvement and degradation of PSC efficiency have been reported under illumination-darkness cycling. Quantifying the performance and stability of cells exhibiting significant diurnal performance variations is challenging. We report the outdoor stability measurements of two types of devices showing either reversible photo-degradation or reversible efficiency improvement under sunlight. Instead of the initial (or stabilized) efficiency and T as the figures of merit for the performance and stability of such devices, we propose using the value of the energy output generated during the first day of exposure and the time needed to reach its 20% drop, respectively. The latter accounts for both the long-term irreversible degradation and the reversible diurnal efficiency variation and does not depend on the type of process prevailing in a given perovskite cell

Improved chip & component encapsulation by dedicated diffusion barriers to reduce corrosion sensitivity in biological and humid environments

In order to develop an advanced polymer based package technology to obtain small and flexible medical implants, diffusion barriers are essential to protect electronic chips and components for corrosion caused by leaching of body fluids. These barriers need also to stop diffusion of harmful products such as Cu from the device into the body. Furthermore, the barrier layers need to be biocompatible, biostable, compatible with all process steps of the package fabrication and sterilization. Parylene is under investigation as candidate barrier material. WVTR tests are performed to characterize diffusion of water through the Parylene layer. Adhesion tests of Parylene-C on various substrates are performed and if needed adhesion promotion treatments are optimized, since good adhesion is crucial for corrosion prevention. Dedicated corrosion tests have shown that 5um Parylene-C is protecting Cu lines for corrosion at 70 degrees C for at least 170 hours, more testing needs to be performed. Cell culture test have shown that Cu diffusion is not sufficiently stopped by 5um Parylene-C or -N, a thicker Parylene layer or a combination with other barriers might be essential. Also dedicated cell culture protocols with non-adherent cell types will have to be developed to perform reliable tests on these hydrophobic and cytophobic barrier layers

Low-cost electrodes for stable perovskite solar cells

© 2017 Author(s). Cost-effective production of perovskite solar cells on an industrial scale requires the utilization of exclusively inexpensive materials. However, to date, highly efficient and stable perovskite solar cells rely on expensive gold electrodes since other metal electrodes are known to cause degradation of the devices. Finding a low-cost electrode that can replace gold and ensure both efficiency and long-term stability is essential for the success of the perovskite-based solar cell technology. In this work, we systematically compare three types of electrode materials: multi-walled carbon nanotubes (MWCNTs), alternative metals (silver, aluminum, and copper), and transparent oxides [indium tin oxide (ITO)] in terms of efficiency, stability, and cost. We show that multi-walled carbon nanotubes are the only electrode that is both more cost-effective and stable than gold. Devices with multi-walled carbon nanotube electrodes present remarkable shelf-life stability, with no decrease in the efficiency even after 180 h of storage in 77% relative humidity (RH). Furthermore, we demonstrate the potential of devices with multi-walled carbon nanotube electrodes to achieve high efficiencies. These developments are an important step forward to mass produce perovskite photovoltaics in a commercially viable way.status: publishe

Model for the Prediction of the Lifetime and Energy Yield of Methyl Ammonium Lead Iodide Perovskite Solar Cells at Elevated Temperatures

With the realization of highly efficient perovskite solar cells, the long-term stability of these devices is the key challenge hindering their commercialization. In this work, we study the temperature-dependent stability of perovskite solar cells and develop a model capable of predicting the lifetime and energy yield of perovskite solar cells outdoors. This model results from the measurement of the kinetics governing the degradation of perovskite solar cells at elevated temperatures. The individual analysis of all key current-voltage parameters enables the prediction of device performance under thermal stress with high precision. An extrapolation of the device lifetime at various European locations based on historical weather data illustrates the relation between the laboratory data and real-world applications. Finally, the understanding of the degradation mechanisms affecting perovskite solar cells allows the definition and implementation of strategies to enhance the thermal stability of perovskite solar cells.status: publishe

NIR sensors based on photolithographically patterned PbS QD photodiodes for CMOS integration

status: Published onlin

Root-Cause Failure Analysis of Photocurrent Loss in Polythiophene:Fullerene-Based Inverted Solar Cells

Metal oxide transport layers have played a crucial role in recent progress in organic photovoltaic (OPV) device stability. Here, we measure the stability of inverted and encapsulated polythiophene:fullerene cells with MoO₃/Ag/Al composite anode in operational conditions combining solar radiation and 65 °C. Performance loss of over 50% in the first 100 h of the aging is dominated by a drop in the short-circuit current (<i>J</i>_sc). We reveal a concurrent loss in reflectance from 85% to 50% above 650 nm, which is below the optical gap of the used photoactive materials, hence, excluding any major degradation in the bulk of this layer. Correlating the responses of aged devices to a series of test structures comprised of ITO/ZnO cathode, MoO₃/Ag, and MoO₃/Ag/Al anodes and their combinations with the active layer allowed us to identify that the presence of Al causes the reduced reflectance in these devices, independent of the presence of the active layer. Systematic single-stress aging on the test structures further indicates that elevated heat is the cause of the reflectance loss. Cross-section transmission electron microscopy coupled with elemental analysis revealed the unsuspected role of Al; notably, it diffuses through the entire 150 nm thick Ag layer and accumulates at the MoO₃/Ag interface. Moreover, XRD analysis of the aged MoO₃/Ag/Al anode indicates the formation of Ag₂Al alloy. Depth profiling with X-ray photoelectron spectroscopy advanced our understanding by confirming the formation of Ag–Al intermetallic alloy and the presence of oxidized Al only at the MoO₃/Ag interface suggesting a concomitant reduction of MoO₃ to most probably MoO₂. This latter compound is less reflective than MoO₃, which can explain the reduced reflectance in aged devices as proven by optical simulations. On the basis of these results, we could estimate that 20% of the loss in <i>J</i>_sc is ascribed to reduction of MoO₃ triggered by its direct contact with Al

NIR sensors based on photolithographically patterned PbS QD photodiodes for CMOS integration

The integration of infrared sensitive thin-film materials with solution processing capabilities on top of Si substrates is a significant step towards cost-efficient infrared imagers. Colloidal quantum dots based on lead sulfide are very attractive materials for the realization of novel image sensors combining low cost synthesis and processing with deposition over large area and on any substrate. The tunable band gap enables selective detection in wavelengths ranging from the visible up to the short-wave-infrared (SWIR). This work describes the first results of a roadmap that will enable the integration of quantum dot photodiodes (QDPD) on top of a Si based CMOS read-out circuit. Photodiodes using an n-p junction architecture are fabricated on Si substrates, showing low dark current of 30 nA/cm(2) at -1 V reverse bias, EQE above 20% and specific detectivity higher than 10(12) cm Hz(1/2) W-1 at the wavelength of 940 nm. Efficiency is improved by tuning the top contact transparency with optical modeling. Furthermore, photolithographic patterning of the thin-film stack is introduced for the first time, showing the feasibility of pixel pitches down to 40 mu m, opening the way towards high resolution monolithic infrared imagers and the incorporation of infrared sensitive pixels next to visible ones

Spectrally resolved nonlinearity and temperature dependence of perovskite solar cells

An accurate electrical device characterization of hybrid organic-inorganic halide perovskite solar cells (PSCs) is an important prerequisite for further improvement and industrial transfer of this promising photovoltaic technology. In this work, we study the nonlinearity of current versus irradiance as well as the temperature dependence of PSCs in a spectrally resolved manner by highly accurate differential external quantum efficiency (EQE) measurements. We investigate three different types of PSCs fabricated by different research groups. The non linearity of all samples is found to be spectrally invariant, which significantly simplifies spectral mismatch corrections. We demonstrate that misinterpretation of EQE measurements can result in a more than 10% relative error in efficiency measurements, if solar simulators are adjusted to photocurrents determined from differential EQEs. For obtaining an accurate integrated photocurrent from EQEs, we introduce a new, convenient approach that accounts for cell nonlinearities but avoids the time-consuming full analysis of spectrally resolved non linearity. Moreover, for the samples investigated here, it is shown that the differential EQE measured at 0.35 suns bias irradiance represents a reasonably good estimate of the actual EQE at 1 sun. Furthermore, we determine spectrally resolved temperature coefficients (TCs) and show how the band gap blue shift varies with perovskite absorber and temperature

Reconsidering figures of merit for performance and stability of perovskite photovoltaics

The development of hybrid organic-inorganic halide perovskite solar cells (PSCs) that combine high performance and operational stability is vital for implementing this technology. Recently, reversible improvement and degradation of PSC efficiency have been reported under illumination-darkness cycling. Quantifying the performance and stability of cells exhibiting significant diurnal performance variations is challenging. We report the outdoor stability measurements of two types of devices showing either reversible photo-degradation or reversible efficiency improvement under sunlight. Instead of the initial (or stabilized) efficiency and T as the figures of merit for the performance and stability of such devices, we propose using the value of the energy output generated during the first day of exposure and the time needed to reach its 20% drop, respectively. The latter accounts for both the long-term irreversible degradation and the reversible diurnal efficiency variation and does not depend on the type of process prevailing in a given perovskite cell