Andrea R. Bowring

Thermal and Environmental Stability of Semi-Transparent Perovskite Solar Cells for Tandems Enabled by a Solution-Processed Nanoparticle Buffer Layer and Sputtered ITO Electrode

Thermal and environmental stability of metal halide perovskite solar cells remains a major barrier to their commercialization. The industry standard transparent electrode, ITO, has good optoelectronic properties and high stability. We introduce a robust buffer layer by solution-processing AZO nanoparticles, enabling a sputtered amorphous ITO layer without damaging the underlying device. We make both semitransparent cells (12.3%) and mechanically stacked tandems (12.3% + 5.7% = 18.0%) using monocrystalline-silicon solar cells as the bottom cell. We operate the inverted-architecture, semitransparent perovskite solar cell without additional sealing in ambient atmosphere under one-sun equivalent visible illumination and measure a Ts0 lifetime of 124 hours at 100°C.

Hysteresis and transient behavior in current–voltage measurements of hybrid-perovskite absorber solar cells

Hybrid organo-metal halide perovskites are an exciting new class of solar absorber materials and have exhibited a rapid increase in solar cell efficiencies throughout the past two years to over 17% in both meso-structured and thin-film device architectures. We observe slow transient effects causing hysteresis in the current–voltage characterization of these devices that can lead to an over- or underestimation of the solar cell device efficiency. We find that the current–voltage (IV) measurement scan direction, measurement delay time, and light and voltage bias conditions prior to measurement can all have a significant impact upon the shape of the measured IV light curves and the apparent device efficiency. We observe that hysteresis-free light IV curves can be obtained at both extremely fast and slow voltage scan rates but only in the latter case are quasi-steady-state conditions achieved for a valid power conversion efficiency measurement. Hysteretic effects are also observed in devices utilizing alternative selective contacts but differ in magnitude and time scale, suggesting that the contact interfaces have a big effect on transients in perovskite-absorber devices. The transient processes giving rise to hysteresis are consistent with a polarization response of the perovskite absorber that results in changes in the photocurrent extraction efficiency of the device. The strong dependence of the hysteresis on light and voltage biasing conditions in thin film devices for a period of time prior to the measurement suggests that photo-induced ion migration may additionally play an important role in device hysteresis. Based on these observations, we provide recommendations for correct measurement and reporting of IV curves for perovskite solar cell devices.

Cesium Lead Halide Perovskites with Improved Stability for Tandem Solar Cells

A semiconductor that can be processed on a large scale with a bandgap around 1.8 eV could enable the manufacture of highly efficient low cost double-junction solar cells on crystalline Si. Solution-processable organic-inorganic halide perovskites have recently generated considerable excitement as absorbers in single-junction solar cells, and though it is possible to tune the bandgap of (CH3NH3)Pb(BrxI1-x)3 between 2.3 and 1.6 eV by controlling the halide concentration, optical instability due to photoinduced phase segregation limits the voltage that can be extracted from compositions with appropriate bandgaps for tandem applications. Moreover, these materials have been shown to suffer from thermal degradation at temperatures within the processing and operational window. By replacing the volatile methylammonium cation with cesium, it is possible to synthesize a mixed halide absorber material with improved optical and thermal stability, a stabilized photoconversion efficiency of 6.5%, and a bandgap of 1.9 eV.

Semi-transparent polymer solar cells with excellent sub-bandgap transmission for third generation photovoltaics.

Semi-transparent organic photovoltaics are of interest for a variety of photovoltaic applications, including solar windows and hybrid tandem photovoltaics. The figure shows a photograph of our semi-transparent solar cell, which has a power conversion efficiency of 5.0%, with an above bandgap transmission of 34% and a sub-bandgap transmission of 81%.

Towards enabling stable lead halide perovskite solar cells; interplay between structural, environmental, and thermal stability

Metal halide perovskite solar cells are rapidly becoming increasingly competitive with conventional PV technologies. While their efficiencies have been often touted as exceptional, they have received a lot of criticism for an apparent lack of stability. This perspective describes some of the most pressing stability concerns facing perovskite solar cells, and describes some of the recent advances made in this area. We will demonstrate that the solutions to the areas of structural, thermal, and environmental stability are closely linked, and that rational design of the perovskite and careful encapsulation can result in efficient and stable perovskite solar cells. We will conclude with some very promising results, demonstrating perovskite solar cells passing an IEC damp heat stability test.

Interpretation of inverted photocurrent transients in organic lead halide perovskite solar cells: proof of the field screening by mobile ions and determination of the space charge layer widths

In Methyl Ammonium Lead Iodide (MAPI) perovskite solar cells, screening of the built-in field by mobile ions has been proposed as part of the cause of the large hysteresis observed in the current/voltage scans in many cells. We show that photocurrent transients measured immediately (e.g. 100 μs) after a voltage step can provide direct evidence that this field screening exists. Just after a step to forward bias, the photocurrent transients are reversed in sign (i.e. inverted), and the magnitude of the inverted transients can be used to find an upper bound on the width of the space charge layers adjacent to the electrodes. This in turn provides a lower bound on the mobile charge concentration, which we find to be ≳1 × 1017 cm−3. Using a new photocurrent transient experiment, we show that the space charge layer thickness remains approximately constant as a function of bias, as expected for mobile ions in a solid electrolyte. We also discuss additional characteristics of the inverted photocurrent transients that imply either an unusually stable deep trapping, or a photo effect on the mobile ion conductivity.

Thermal Stability of Mixed Cation Metal Halide Perovskites in Air

We study the thermal stability in air of the mixed cation organic-inorganic lead halide perovskites Cs0.17FA0.83Pb(I0.83Br0.17)3 and Cs0.05(MA0.17FA0.83)0.95Pb(I0.83Br0.17)3. For the latter compound, containing both MA+ and FA+ ions, thermal decomposition of the perovskite phase was observed to occur in two stages. The first stage of decomposition occurs at a faster rate compared to the second stage and is only observed at relatively low temperatures (T < 150 °C). For the second stage, we find that both decomposition rate and the activation energy have similar values for Cs0.05(MA0.17FA0.83)0.95Pb(I0.83Br0.17)3 and Cs0.17FA0.83Pb(I0.83Br0.17)3, which suggests that the first stage mainly involves reaction of MA+ and the second stage mainly FA+.

Modeling low-cost hybrid tandem photovoltaics with power conversion efficiencies exceeding 20%

There is a need for photovoltaic technologies with power conversion efficiencies (PCEs) exceeding 20% that can be produced with a module cost under

Transformation from crystalline precursor to perovskite in PbCl 2 -derived MAPbI 3

0.50 per Watt. Here we propose a solution in the form of an organic-inorganic hybrid tandem photovoltaic (HTPV). We have modeled HTPV devices in current-matched and independently operated subcell configurations to explore the potential for an organic top cell to boost the efficiency of inorganic bottom cells. Our modeling shows that with further development of organic photovoltaic technology, an organic top cell may be added to either a Si or CIGS bottom cell of moderate efficiency to achieve tandem efficiencies exceeding 20%.

Thermal and environmental stability of semi-transparent perovskite solar cells for tandems by a solution-processed nanoparticle buffer layer and sputtered ITO electrode

Understanding the formation chemistry of metal halide perovskites is key to optimizing processing conditions and realizing enhanced optoelectronic properties. Here, we reveal the structure of the crystalline precursor in the formation of methylammonium lead iodide (MAPbI3) from the single-step deposition of lead chloride and three equivalents of methylammonium iodide (PbCl2 + 3MAI) (MA = CH3NH3). The as-spun film consists of crystalline MA2PbI3Cl, which is composed of one-dimensional chains of lead halide octahedra, coexisting with disordered MACl. We show that the transformation of precursor into perovskite is not favored in the presence of MACl, and thus the gradual evaporation of MACl acts as a self-regulating mechanism to slow the conversion. We propose the stable precursor phase enables dense film coverage and the slow transformation may lead to improved crystal quality. This enhanced chemical understanding is paramount for the rational control of film deposition and the fabrication of superior optoelectronic devices.The existence of a crystalline precursor is key to perovskite film formation, but the precise chemistry of the precursor and its transformation into perovskite are poorly understood. Here, the authors identify the crystal structure and conversion chemistry of the precursor for PbCl2-derived methylammonium lead iodide perovskites.