Search

The open-circuit voltage (V $_\text{oc}$ ) of CdTe-based photovoltaics may be limited by carrier recombination at interfaces (front or back) or in the absorber layer. Reduction in recombination of a given dominant mechanisms can lead to improved device performance if the remaining mechanisms turn on in a narrow bias range just below the open circuit voltage. In this article, we demonstrate enhanced performance by incorporating solution-processed Cu $_\text{x}$ AlO $_\text{y}$ to form a back-buffer layer in CdSe/CdTe devices. Outstanding minority carrier lifetimes of 656 and 4.2 ns were measured with glass side and film side illumination for device stacks processed with Cu $_\text{x}$ AlO $_\text{y}$ . Devices demonstrated efficiencies of up to 17.4% with V $_\text{oc}$ of 859 mV, FF of 75.6% and J $_\text{sc}$ of 26.9 mAcm−2 while the efficiency of the reference device without the back-buffer layer was 16.5% with V $_\text{oc}$ of 839 mV, FF of 70.6%, and J $_\text{sc}$ of 27.9 mAcm−2.

The calculated levelized cost of electricity for single junction and tandem devices show that when the time for perovskite devices to degrade to 85% of the original output (T85) is only 3 years, tandem device are economically favorable.

Copper (Cu) incorporation is a key process for fabricating efficient CdTe-based thin-film solar cells and has been used in CdTe-based solar cell module manufacturing. Here, we investigate the effects of different Cu precursors on the performance of CdTe-based thin-film solar cells by incorporating Cu using a metallic Cu source (evaporated Cu) and ionic Cu sources (solution-processed cuprous chloride (CuCl) and copper chloride (CuCl2)). We find that ionic Cu precursors offer much better control in Cu diffusion than the metallic Cu precursor, producing better front junction quality, lower back-barrier heights, and better bulk defect property. Finally, outperforming power conversion efficiencies of 17.2 and 17.5% are obtained for devices with cadmium sulfide and zinc magnesium oxide as the front window layers, respectively, which are among the highest reported CdTe solar cells efficiencies. Our results suggest that an ionic Cu precursor is preferred as the dopant to fabricate efficient CdTe thin-film solar cells and modules.

The performance of CdTe solar cells has advanced impressively in recent years with the incorporation of Se. Instabilities associated with light soaking and copper reorganization have been extensively examined for the previous generation of CdS/CdTe solar cells, but instabilities in Cu-doped Se-alloyed CdTe devices remain relatively unexplored. In this work, we fabricated a range of CdSe/CdTe solar cells by sputtering CdSe layers with thicknesses of 100, 120, 150, 180, and 200 nm on transparent oxide-coated glass and then depositing CdTe by close-spaced sublimation. After CdCl

Although back-surface passivation plays an important role in high-efficiency photovoltaics, it has not yet been definitively demonstrated for CdTe. Here, we present a solution-based process, which achieves passivation and improved electrical performance when very small amounts of oxidized Al3+ species are deposited at the back surface of CdTe devices. The open circuit voltage (Voc) is increased and the fill factor (FF) and photoconversion efficiency (PCE) are optimized when the total amount added corresponds to ∼1 monolayer, suggesting that the passivation is surface specific. Addition of further Al3+ species, present in a sparse alumina-like layer, causes the FF and PCE to drop as the interface layer becomes blocking to current flow. The optimized deposit increases the average baseline PCE for both Cu-free devices and devices where Cu is present as a dopant. The greatest improvement is found when the Al3+ species are deposited prior to the CdCl2 activation step and Cu is employed. In this case, the best-cell efficiency was improved from 12.6 to 14.4%. Time-resolved photoluminescence measurements at the back surface and quantum efficiency measurements performed at the maximum power point indicate that the performance enhancement is due to a reduction in the interface recombination current at the back surface.

High-performance perovskite/perovskite tandem solar cells require high-efficiency and stable low-bandgap perovskite subcells. State-of-the-art low-bandgap mixed tin–lead iodide perovskite solar cells exhibit either a high power-conversion efficiency or improved stability, but not both. Here we report a two-step bilayer interdiffusion growth process to simultaneously meet both requirements for formamidinium-based low-bandgap mixed tin–lead iodide perovskite solar cells. The bilayer interdiffusion growth process allows for the formation of high-quality and large-grained perovskite films with only 10 mol% volatile methylammonium. Additionally, one-dimensional pyrrolidinium perovskite was applied to passivate the perovskite film and improve the junction quality, which resulted in a carrier lifetime of 1.1 μs and an open circuit voltage of 0.865 V for our perovskite film and device with a bandgap of 1.28 eV. Our strategies enabled a power-conversion efficiency of 20.4% for low-bandgap perovskite solar cells under AM 1.5G illumination. More importantly, an encapsulated device can retain 92% of its initial efficiency after 450 h of continuous 1 sun illumination. Low-bandgap tin–lead perovskites are key to all-perovskite tandem solar cells but simultaneous improvement in efficiency and stability has proven challenging. Now, Li et al. fabricate tin–lead perovskite cells with reduced methylammonium content that are 20.4% efficient and stable under illumination for 450 h.

Variability of the power generated by photovoltaics due to passing clouds can cause instability in the utility grid. This variability can be mitigated by control strategies which include the integration of energy storage systems. This article proposes a combination of two novel methods, termed here as adaptive moving average control and adaptive state of charge control, to mitigate variability using energy storage. The adaptive moving average control uses an adjustable window size based on a variability metric quantified by a moving standard deviation of the input power, leading to improved mitigation. The adaptive state of charge control maintains flexibility of the storage system by keeping its state of charge close to a reference. This proposed combination reduces storage utilization during times of low variability, and improves the trade-off between the degree of smoothness achieved and the capacity requirement of the storage system. The advantage of the proposed strategy with respect to traditional algorithms is first presented in simulation. The result is further verified experimentally in a field test by using a battery energy storage system controlled by agents written for the Eclipse VOLTTRON platform.

Perovskite solar cells continue to garner significant attention in the field of photovoltaics. As the optoelectronic properties of the absorbers become better understood, attention has turned to more deeply understanding the contribution of charge transport layers for efficient extraction of carriers. Titanium oxide is known to be an effective electron transport layer (ETL) in planar perovskite solar cells, but it is unlikely to result in the best device performance possible. To investigate the importance of band energy alignment between the electron transport layer and perovskite, we employ numerical modeling as a function of conduction band offset between these layers, interface recombination velocity, and ETL doping levels. Our simulations offer insight into the advantages of energy band alignment and allow us to determine a range of surface recombination velocities and ETL doping densities that will allow us to identify novel high performance ETL materials.

Because bifacial solar cells increase the power generated per area, their market share is projected to increase over the next decade. While silicon technologies have implemented bifacial technology...

A key property for high-frequency inductive applications is the magnetic softness, ideally characterized by complete reversibility of the hysteresis curve. Magnetic softness of alloys depends on the dimensional ratio of grain size to the correlation length. The random anisotropy model predicts the optimum magnetic softness if the crystallite size is well below the correlation length. Based on this theory, a common strategy in processing is to obtain extremely fine nanoscale grains via rapidly solidified alloys, followed by a controlled vacuum anneal. As is well known, this conventional approach requires two demanding experimental conditions: 1) an extremely high quench rate, >105 K/s, and 2) a high vacuum anneal near 600 °C to avoid oxidation. Furthermore, the completely amorphous ribbon is extremely elastic and mechanically strong and is not easily made into the fine powder form for net-shaping and 3-D printing of the soft magnets. Therefore, annealing to induce crystallization is a necessary step before using any conventional powdering techniques, such as ball milling. In this article, we report a new processing strategy of making Fe77Ni5.5Co5.5Zr7B4Cu powders directly from the as-spun ribbons without any crystallization annealing step. This is achieved by melt spinning of the alloy at a relatively low quenching rate, resulting in partially crystallized ribbons that deform plastically, a behavior that is in sharp contrast to its amorphous counterpart. The ribbon samples are ball milled into fine powders by tungsten carbide (WC) at various time intervals. Magnetization measurements show extremely low coercive field (Hc) of 0.51 Oe for the powder that is ball milled for 60 min. However, Hc consistently increases with ball milling time, as the samples are strain-hardened, introducing more magneto anisotropy. The coercive fields of these samples are easily recovered during the high-temperature magnetization measurement at 900 K, and an Hc of 3.78 Oe is obtained upon recovery. This strategy paves a new way for straightforwardly making soft magnetic powders while avoiding high quenching rate requirement and the possibility of oxidation during annealing.

Richard E. Smalley, in 2003, defined the Terawatt (TW) Challenge as “Adapting our energy infrastructure to simultaneously address diminishing oil resources and rising levels of atmospheric CO 2.” Smalley, best known for the discovery of C 60, for which he received the 1996 Nobel Prize in Chemistry, continued to address the challenges of anthropomorphic and natural global energy flows until he passed away in 2005. Smalley challenged the world to transform the energy sector. He envisioned electricity transmitted by high-voltage direct current (DC) lines from massively deployed solar plants in sunny areas and remotely sited nuclear plants. He also envisioned using advanced batteries for local storage of energy. To meet the needs of ~10 people in a world with a dwindling oil supply, Smalley asserted that the world would need to transform its fossil-fuel-driven 14-TW (average power) energy used in 2003 to a largely renewable-energy-driven 30–60 TW (average power) in 2050. This would be possible only if solar-electricity costs could be drastically reduced. The challenges associated with this transition have been called the “Terawatt Challenge.” Fifteen years later, solar-module costs have been reduced by tenfold and annual deployment of solar photovoltaic (PV) modules has grown by a factor of 100,from ~1 gigawatt (GW) in 2004 to ~100 GW in 2018, with a total of 500 GW installed worldwide, producing 2% of the planet’s electricity. As global installed solar generating capacity approaches1 TW, we revisit Smalley’s TW challenge to identify what has changed and quantify the TW Challenge for a baseline scenario and for two scenarios designed as upper and lower bounds determined by the degree we implement electrification and storage. In this paper, we show that the energy choices we make today will dramatically affect the magnitude of future global energy requirements.

Metal halide perovskite solar cells have progressed rapidly over the past decade, providing an exceptional opportunity for space photovoltaic (PV) power applications. However, the solar cells to be...

Cadmium telluride (CdTe) is one of the leading photovoltaic technologies with a market share of around 5%. However, there still exist challenges to fabricate a rear contact for efficient transport of photogenerated holes. Here, etching effects of various iodine compounds including elemental iodine (I2), ammonium iodide (NH4I), mixture of elemental iodine and NH4I (I−/I3− etching), and formamidinium iodide were investigated. The treated CdTe surfaces were investigated using Raman spectroscopy, X-ray diffraction (XRD), scanning electron microscopy, and energy-dispersive X-ray spectroscopy. The CdTe devices were completed with or without treatments and tested under simulated AM1.5G solar spectrum to find photoconversion efficiency (PCE). Based on Raman spectra, XRD patterns, and surface morphology, it was shown that treatment with iodine compounds produced Te-rich surface on CdTe films, and temperature-dependent current–voltage characteristics showed reduced back barrier heights, which are essential for the formation of ohmic contact and reduce contact resistance. Based on current–voltage characteristics, the treatment enhanced open-circuit voltage (VOC) up to 841 mV, fill factor (FF) up to 78.2%, and PCE up to 14.0% compared with standard untreated CdTe devices (VOC ∼ 814 mV, FF ∼ 74%, and PCE ∼ 12.7%) with copper/gold back contact.

Motivated by their utility in CdTe-based thin film photovoltaics (PV) devices, an investigation of thin films of the magnesium-zinc oxide (MgxZn1−xO or MZO) alloy system was undertaken applying spectroscopic ellipsometry (SE). Dominant wurtzite phase MZO thin films with Mg contents in the range 0 ≤ x ≤ 0.42 were deposited on room temperature soda lime glass (SLG) substrates by magnetron co-sputtering of MgO and ZnO targets followed by annealing. The complex dielectric functions ε of these films were determined and parameterized over the photon energy range from 0.73 to 6.5 eV using an analytical model consisting of two critical point (CP) oscillators. The CP parameters in this model are expressed as polynomial functions of the best fitting lowest CP energy or bandgap E0 = Eg, which in turn is a quadratic function of x. As functions of x, both the lowest energy CP broadening and the Urbach parameter show minima for x ~ 0.3, which corresponds to a bandgap of 3.65 eV. As a result, it is concluded that for this composition and bandgap, the MZO exhibits either a minimum concentration of defects in the bulk of the crystallites or a maximum in the grain size, an observation consistent with measured X-ray diffraction line broadenings. The parametric expression for ε developed here is expected to be useful in future mapping and through-the-glass SE analyses of partial and complete PV device structures incorporating MZO.

Copper indium diselenide (CuInSe2; CIS) thin film solar cells have been fabricated with absorber layers deposited in a one-stage co-evaporation process at a substrate temperature of T S = 570°C and at different rates, r. The parameters T S and r are the two key variables in a one-stage thermal evaporation process that can be varied for optimization of thin film properties. The role of rate in CIS absorber layer deposition is explored in this study by utilizing Cu and In source calibrations developed previously. These calibrations were obtained by real time spectroscopic ellipsometry analysis, which provided accurate deposition rates in terms of effective thickness (volume per planar area) for Cu and In2Se3 thin films deposited at different Cu and In source temperatures. Good agreement of the measured stoichiometric composition y = [Cu]/[In] and r values with those predicted from the calibration are obtained by assigning film density factors to the Cu and In2Se3 calibration depositions. Solar cells were fabricated from 1.6-1.9 μm thick CIS absorbers with y = 0.91 ± 0.02 deposited at rates in the range of ∽ 3 – 8 A/s. The crystallite size of CIS increases as its deposition rate is reduced, and this appears to give rise to lower nucleation rates for chemical bath deposition of the overlying CdS. An absorber deposition rate for optimum solar cell performance is identified at ~ 3.3 A/s, as reflected in both the open circuit voltage and fill factor. The ultimate goal of this study is to explore and optimize narrow bandgap CIS for use as an absorber in the bottom junction of tandem solar cells.

Thin film solar cells have, in general, three recombination locations that can limit the device performance – the bulk, front interface, and back interface. Unfortunately, it is difficult to determine which of these mechanisms limit any given device. Time resolved photoluminescence measurements provide an estimate for the carrier lifetime and, therefore, bulk recombination. Determining the role of the front and back interface in device performance, though, is more complicated. With this in mind, we investigated the use of current densityvoltage measurements with front and back illumination at varying intensities. Using numerical modeling, we show that recombination information about each interface can be gleaned, and the limiting interface can be determined. The response curve signatures learned from modeling are then applied to measurements of real devices.