On the origin of open-circuit voltage losses in flexible perovskite solar cells.

The possibility to manufacture perovskite solar cells (PSCs) at low temperatures paves the way to flexible and lightweight photovoltaic (PV) devices manufactured via high-throughput roll-to-roll processes. In order to achieve higher power conversion efficiencies, it is necessary to approach the radiative limit via suppression of non-radiative recombination losses. Herein, we performed a systematic voltage loss analysis for a typical low-temperature processed, flexible PSC in configuration using vacuum deposited C as electron transport layer (ETL) and two-step hybrid vacuum-solution deposition for CHNHPbI perovskite absorber.

Planar perovskite solar cells with long-term stability using ionic liquid additives.

Solar cells based on metal halide perovskites are one of the most promising photovoltaic technologies. Over the past few years, the long-term operational stability of such devices has been greatly improved by tuning the composition of the perovskites, optimizing the interfaces within the device structures, and using new encapsulation techniques. However, further improvements are required in order to deliver a longer-lasting technology.

Time-resolved photoluminescence on double graded Cu(In,Ga)Se - Impact of front surface recombination and its temperature dependence.

Time-resolved photoluminescence (TRPL) is applied to determine an effective lifetime of minority charge carriers in semiconductors. Such effective lifetimes include recombination channels in the bulk as well as at the surfaces and interfaces of the device. In the case of Cu(In,Ga)Se absorbers used for solar cell applications, trapping of minority carriers has also been reported to impact the effective minority carrier lifetime.

Bulk and surface recombination properties in thin film semiconductors with different surface treatments from time-resolved photoluminescence measurements.

The knowledge of minority carrier lifetime of a semiconductor is important for the assessment of its quality and design of electronic devices. Time-resolved photoluminescence (TRPL) measurements offer the possibility to extract effective lifetimes in the nanosecond range. However, it is difficult to discriminate between surface and bulk recombination and consequently the bulk properties of the semiconductor cannot be estimated reliably.

Voids and compositional inhomogeneities in Cu(In,Ga)Se thin films: evolution during growth and impact on solar cell performance.

Structural defects such as voids and compositional inhomogeneities may affect the performance of Cu(In,Ga)Se (CIGS) solar cells. We analyzed the morphology and elemental distributions in co-evaporated CIGS thin films at the different stages of the CIGS growth by energy-dispersive x-ray spectroscopy in a transmission electron microscope. Accumulation of Cu-Se phases was found at crevices and at grain boundaries after the Cu-rich intermediate stage of the CIGS deposition sequence.

ALD-Zn Ti O as Window Layer in Cu(In,Ga)Se Solar Cells.

We report on the application of Zn Ti O deposited by atomic layer deposition (ALD) as buffer layer in thin film Cu(In,Ga)Se (CIGS) solar cells to improve the photovoltaic device performance. State-of-the-art CIGS devices employ a CdS/ZnO layer stack sandwiched between the absorber layer and the front contact. Replacing the sputter deposited ZnO with ALD-Zn Ti O allowed a reduction of the CdS layer thickness without adversely affecting open-circuit voltage ( V).

Structural and electronic properties of CdTeSe films and their application in solar cells.

The performance improvement of conventional CdTe solar cells is mainly limited by doping concentration and minority carrier life time. Alloying CdTe with an isovalent element changes its properties, for example its band gap and behaviour of dopants, which has a significant impact on its performance as a solar cell absorber. In this work, the structural, optical, and electronic properties of CdTeSe films are examined for different Se concentrations.

Ni-Al-Cr superalloy as high temperature cathode current collector for advanced thin film Li batteries.

To obtain full advantage of state-of-the-art solid-state lithium-based batteries, produced by sequential deposition of high voltage cathodes and promising oxide-based electrolytes, the current collector must withstand high temperatures (>600 °C) in oxygen atmosphere. This imposes severe restrictions on the choice of materials for the first layer, usually the cathode current collector. It not only must be electrochemically stable at high voltage, but also remain conductive upon deposition and annealing of the subsequent layers without presenting a strong diffusion of its constituent elements into the cathode.

Refractive indices of layers and optical simulations of Cu(In,Ga)Se solar cells.

Cu(In,Ga)Se based solar cells have reached efficiencies close to 23%. Further knowledge-driven improvements require accurate determination of the material properties. Here, we present refractive indices for all layers in Cu(In,Ga)Se solar cells with high efficiency.

Aluminum Chloride-Graphite Batteries with Flexible Current Collectors Prepared from Earth-Abundant Elements.

In the search for low-cost and large-scale stationary storage of electricity, nonaqueous aluminum chloride-graphite batteries (AlCl-GBs) have received much attention due to the high natural abundances of their primary constituents, facile manufacturing, and high energy densities. Much research has focused on the judicious selection of graphite cathode materials, leading to the most notable recent advances in the performance of AlCl-GBs. However, the major obstacle to commercializing this technology is the lack of oxidatively stable, inexpensive current collectors that can operate in chloroaluminate ionic liquids and are composed of earth-abundant elements.

Single-graded CIGS with narrow bandgap for tandem solar cells.

Multi-junction solar cells show the highest photovoltaic energy conversion efficiencies, but the current technologies based on wafers and epitaxial growth of multiple layers are very costly. Therefore, there is a high interest in realizing multi-junction tandem devices based on cost-effective thin film technologies. While the efficiency of such devices has been limited so far because of the rather low efficiency of semitransparent wide bandgap top cells, the recent rise of wide bandgap perovskite solar cells has inspired the development of new thin film tandem solar devices.

Compositionally Graded Absorber for Efficient and Stable Near-Infrared-Transparent Perovskite Solar Cells.

Compositional grading has been widely exploited in highly efficient Cu(In,Ga)Se, CdTe, GaAs, quantum dot solar cells, and this strategy has the potential to improve the performance of emerging perovskite solar cells. However, realizing and maintaining compositionally graded perovskite absorber from solution processing is challenging. Moreover, the operational stability of graded perovskite solar cells under long-term heat/light soaking has not been demonstrated.

Voltage dependent admittance spectroscopy for the detection of near interface defect states for thin film solar cells.

Recently recorded efficiencies of Cu(In,Ga)Se based solar cells were mainly achieved by surface treatment of the absorber that modifies the buffer-absorber interface region. However, only little is known about the electronic properties within this region. In this manuscript voltage dependent admittance spectroscopy is applied to low temperature grown Cu(In,Ga)Se based solar cells to detect near interface defect states in the absorber.

CIGS thin-film solar module processing: case of high-speed laser scribing.

In this paper, we investigate the laser processing of the CIGS thin-film solar cells in the case of the high-speed regime. The modern ultra-short pulsed laser was used exhibiting the pulse repetition rate of 1 MHz. Two main P3 scribing approaches were investigated - ablation of the full layer stack to expose the molybdenum back-contact, and removal of the front-contact only.

Formation of a K-In-Se Surface Species by NaF/KF Postdeposition Treatment of Cu(In,Ga)Se Thin-Film Solar Cell Absorbers.

A NaF/KF postdeposition treatment (PDT) has recently been employed to achieve new record efficiencies of Cu(In,Ga)Se (CIGSe) thin film solar cells. We have used a combination of depth-dependent soft and hard X-ray photoelectron spectroscopy as well as soft X-ray absorption and emission spectroscopy to gain detailed insight into the chemical structure of the CIGSe surface and how it is changed by different PDTs. Alkali-free CIGSe, NaF-PDT CIGSe, and NaF/KF-PDT CIGSe absorbers grown by low-temperature coevaporation have been interrogated.

Surface Passivation for Reliable Measurement of Bulk Electronic Properties of Heterojunction Devices.

Quantum efficiency measurements of state of the art Cu(In,Ga)Se (CIGS) thin film solar cells reveal current losses in the near infrared spectral region. These losses can be ascribed to inadequate optical absorption or poor collection of photogenerated charge carriers. Insight on the limiting mechanism is crucial for the development of more efficient devices.

On the chemical stability of post-lithiated garnet Al-stabilized Li7La3Zr2O12 solid state electrolyte thin films.

Garnet-based Al-doped Li7La3Zr2O12 has the potential to be used as a solid state electrolyte for future lithium microbattery architectures, due to its relatively high Li(+) conductivity and stability against Li. Through this work, a model experiment is presented in which the effect of post-lithiation on phase formation and chemical stability is studied for pulsed laser deposited Al-doped Li7La3Zr2O12 thin films on MgO substrates. We report the implications of the newly suggested post-lithiation route for films with thicknesses between 90 and 380 nm.

Potassium Postdeposition Treatment-Induced Band Gap Widening at Cu(In,Ga)Se₂ Surfaces--Reason for Performance Leap?

Direct and inverse photoemission were used to study the impact of alkali fluoride postdeposition treatments on the chemical and electronic surface structure of Cu(In,Ga)Se2 (CIGSe) thin films used for high-efficiency flexible solar cells. We find a large surface band gap (E(g)(Surf), up to 2.52 eV) for a NaF/KF-postdeposition treated (PDT) absorber significantly increases compared to the CIGSe bulk band gap and to the Eg(Surf) of 1.

Low-temperature-processed efficient semi-transparent planar perovskite solar cells for bifacial and tandem applications.

Semi-transparent perovskite solar cells are highly attractive for a wide range of applications, such as bifacial and tandem solar cells; however, the power conversion efficiency of semi-transparent devices still lags behind due to missing suitable transparent rear electrode or deposition process. Here we report a low-temperature process for efficient semi-transparent planar perovskite solar cells. A hybrid thermal evaporation-spin coating technique is developed to allow the introduction of PCBM in regular device configuration, which facilitates the growth of high-quality absorber, resulting in hysteresis-free devices.

High-Efficiency Polycrystalline Thin Film Tandem Solar Cells.

A promising way to enhance the efficiency of CIGS solar cells is by combining them with perovskite solar cells in tandem devices. However, so far, such tandem devices had limited efficiency due to challenges in developing NIR-transparent perovskite top cells, which allow photons with energy below the perovskite band gap to be transmitted to the bottom cell. Here, a process for the fabrication of NIR-transparent perovskite solar cells is presented, which enables power conversion efficiencies up to 12.

Alkali-templated surface nanopatterning of chalcogenide thin films: a novel approach toward solar cells with enhanced efficiency.

Concepts of localized contacts and junctions through surface passivation layers are already advantageously applied in Si wafer-based photovoltaic technologies. For Cu(In,Ga)Se2 thin film solar cells, such concepts are generally not applied, especially at the heterojunction, because of the lack of a simple method yielding features with the required size and distribution. Here, we show a novel, innovative surface nanopatterning approach to form homogeneously distributed nanostructures (<30 nm) on the faceted, rough surface of polycrystalline chalcogenide thin films.

Local band gap measurements by VEELS of thin film solar cells.

This work presents a systematic study that evaluates the feasibility and reliability of local band gap measurements of Cu(In,Ga)Se2 thin films by valence electron energy-loss spectroscopy (VEELS). The compositional gradients across the Cu(In,Ga)Se2 layer cause variations in the band gap energy, which are experimentally determined using a monochromated scanning transmission electron microscope (STEM). The results reveal the expected band gap variation across the Cu(In,Ga)Se2 layer and therefore confirm the feasibility of local band gap measurements of Cu(In,Ga)Se2 by VEELS.

Unveiling the effects of post-deposition treatment with different alkaline elements on the electronic properties of CIGS thin film solar cells.

Thin film solar cells with a Cu(In,Ga)Se2 (CIGS) absorber layer achieved efficiencies above 20%. In order to achieve such high performance the absorber layer of the device has to be doped with alkaline material. One possibility to incorporate alkaline material is a post deposition treatment (PDT), where a thin layer of NaF and/or KF is deposited onto the completely grown CIGS layer.

Potassium-induced surface modification of Cu(In,Ga)Se2 thin films for high-efficiency solar cells.

Thin-film photovoltaic devices based on chalcopyrite Cu(In,Ga)Se2 (CIGS) absorber layers show excellent light-to-power conversion efficiencies exceeding 20%. This high performance level requires a small amount of alkaline metals incorporated into the CIGS layer, naturally provided by soda lime glass substrates used for processing of champion devices. The use of flexible substrates requires distinct incorporation of the alkaline metals, and so far mainly Na was believed to be the most favourable element, whereas other alkaline metals have resulted in significantly inferior device performance.