The power conversion efficiency of solar cells strongly depends on the open-circuit voltage VOC which, in turn, depends on the recombination activity within the device. A possible source of detrimental charge carrier recombination is band tails. An empirical linear relationship between VOC loss and the Urbach energy of the band tails has been shown in the past. Here we discuss how band tails influence the radiative recombination and the nonradiative recombination in the bulk of the absorber. First, we show through photoluminescence that the band tails can be willfully tuned in state-of-the-art thin-film Cu (In,Ga)Se-2 (CIGSe) absorbers and solar cells on a 20% efficiency level and beyond through the incorporation of alkali atoms. In the second part, we compare our CIGSe results to published results from other solar cell technologies. This comparison reveals that CIGS solar cells follow the previously described empirical trend: an increase in the open-circuit voltage with decreasing band tails. Finally, we model the influence of tail states on the radiative and nonradiative recombination losses: Radiative recombination is increased because carriers thermalize into the tail states and nonradiative recombination of free carriers in the bands is increased because of Shockley-Read-Hall recombination through the tail states. The comparison with experimental data shows that the influence of tail states is even worse than the increase in radiative and SRH recombination predicted by our model. Our results thus suggest that band tails act as one of the main remaining voltage limitations in the majority of state-of-the-art solar cells.

y High specific power, high stowed packing efficiency, low processing cost, and high tolerance against environmental threats (high energy and charged particle radiation) make perovskite solar cell (PSC) a promising candidate for power generation in space. However, vacuum, as encountered in space, causes perovskite outgassing, raising concern for its long-term stability. In this work, we find that PSCs (ITO/SnO2/perovskite/Spiro-MeOTAD/Au) degrade ten times faster upon reducing the pressure from 9 x 10(4) to 5 x 10(3) Pa during operation, due to acceleration of the perovskite transformation and ion migration. Gas permeability of the layers atop perovskite and mobile ion-induced chemical reactions at charge transporting layers and related interfaces are two critical factors. We develop a PSC structure (ITO/PTAA/perovskite/PCBM/ZnO/AZO/[Ni/Al grid]) that effectively mitigates vacuum and illumination-induced degradation pathways, enabling PSCs to realize a low PCE loss rate of 0.007%/h over 1,037 h at the maximum power point under 100 mW cm(-2) illumination at 5 x 10(3) Pa.

A perspective on some strategies to trigger new developments for the next generation of Cu(In,Ga)Se2 solar cells is presented.

Chalcopyrite solar cells achieve efficiencies above 23%. The latest improvements are due to post‐deposition treatments (PDT) with heavy alkalis. This study provides a comprehensive description of the effect of PDT on the chemical and electronic structure of surface and bulk of Cu(In,Ga)Se 2 . Chemical changes at the surface appear similar, independent of absorber or alkali. However, the effect on the surface electronic structure differs with absorber or type of treatment, although the improvement of the solar cell efficiency is the same. Thus, changes at the surface cannot be the only effect of the PDT treatment. The main effect of PDT with heavy alkalis concerns bulk recombination. The reduction in bulk recombination goes along with a reduced density of electronic tail states. Improvements in open‐circuit voltage appear together with reduced band bending at grain boundaries. Heavy alkalis accumulate at grain boundaries and are not detected in the grains. This behavior is understood by the energetics of the formation of single‐phase Cu‐alkali compounds. Thus, the efficiency improvement with heavy alkali PDT can be attributed to reduced band bending at grain boundaries, which reduces tail states and nonradiative recombination and is caused by accumulation of heavy alkalis at grain boundaries.

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 n-i-p configuration using vacuum deposited C-60 as electron transport layer (ETL) and two-step hybrid vacuum-solution deposition for CH3NH3PbI3 perovskite absorber. We identified the ETL/absorber interface as a bottleneck in relation to non-radiative recombination losses, the quasi-Fermi level splitting (QFLS) decreases from similar to 1.23 eV for the bare absorber, just similar to 90 meV below the radiative limit, to similar to 1.10 eV when C-60 is used as ETL. To effectively mitigate these voltage losses, we investigated different interfacial modifications via vacuum deposited interlayers (BCP, B4PyMPM, 3TPYMB, and LiF). An improvement in QFLS of similar to 30-40 meV is observed after interlayer deposition and confirmed by comparable improvements in the open-circuit voltage after implementation of these interfacial modifications in flexible PSCs. Further investigations on absorber/hole transport layer (HTL) interface point out the detrimental role of dopants in Spiro-OMeTAD film (widely employed HTL in the community) as recombination centers upon oxidation and light exposure. [GRAPHICS] .

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. Here we present an approach to constrain systematically the bulk and surface recombination parameters in semiconducting layers and reduces to finding the roots of a mathematical function. This method disentangles the bulk and surface recombination based on TRPL decay times of samples with different surface preparations. The technique is exemplarily applied to a CuInSe2 and a back-graded Cu(In,Ga)Se2 compound semiconductor, and upper and lower bounds for the recombination parameters and the mobility are obtained. Sets of calculated parameters are extracted and used as input for simulations of photoluminescence transients, yielding a good match to experimental data and validating the effectiveness of the methodology. A script for the simulation of TRPL transients is provided.

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)Se2 absorbers used for solar cell applications, trapping of minority carriers has also been reported to impact the effective minority carrier lifetime. Trapping can be indicated by an increased temperature dependence of the experimentally determined photoluminescence decay time when compared to the temperature dependence of Shockley-Read-Hall (SRH) recombination alone and can lead to an overestimation of the minority carrier lifetime. Here, it is shown by technology computer-aided design (TCAD) simulations and by experiment that the intentional double-graded bandgap profile of high efficiency Cu(In,Ga)Se2 absorbers causes a temperature dependence of the PL decay time similar to trapping in case of a recombinative front surface. It is demonstrated that a passivated front surface results in a temperature dependence of the decay time that can be explained without minority carrier trapping and thus enables the assessment of the absorber quality by means of the minority carrier lifetime. Comparison with the absolute PL yield and the quasi-Fermi-level splitting (QFLS) corroborate the conclusion that the measured decay time corresponds to the bulk minority carrier lifetime of 250 ns for the double-graded CIGS absorber under investigation.

We reveal an iodine vapor-induced degradation mechanism in formamidinium–lead-iodide-based perovskite solar cells stressed under combined heat and light illumination.

Flexible, lightweight Cu(In,Ga)Se2 (CIGS) solar cells grown on polymer substrates are a promising technology with fast growing market prospects. However, power conversion efficiencies of solar cells grown at low temperatures (≈450 °C) remain below the efficiencies of cells grown at high temperature on glass substrates. This contribution discusses the impact on cell efficiency of process improvements of low‐temperature CIGS deposition on flexible polyimide and glass substrates. Different strategies for incorporation of alkali elements into CIGS are evaluated based on a large number of depositions. Postdeposition treatment with heavy alkali (here RbF) enables a thickness reduction of the CdS buffer layer and increases the open‐circuit voltage. Na supply during 3rd stage CIGS deposition positively impacts the cell performance. Coevaporation of heavy alkali (e.g., RbF) during capping layer deposition mitigates the adverse shunting associated with high Cu contents, yielding highest efficiencies with near‐stoichiometric absorber compositions. Furthermore, optimization of the deposition sequence results in absorbers with a 1 µm wide notch region with nearly constant bandgap minimum. The improved processes result in a record cell efficiency of 20.8% for CIGS on flexible substrate.

Solar cells based on metal halide perovskites are one of the most promising photovoltaic technologies1–4. Over the past few years, the long-term operational stability of such devices has been greatly improved by tuning the composition of the perovskites5–9, optimizing the interfaces within the device structures10–13, and using new encapsulation techniques14,15. However, further improvements are required in order to deliver a longer-lasting technology. Ion migration in the perovskite active layer—especially under illumination and heat—is arguably the most difficult aspect to mitigate16–18. Here we incorporate ionic liquids into the perovskite film and thence into positive–intrinsic–negative photovoltaic devices, increasing the device efficiency and markedly improving the long-term device stability. Specifically, we observe a degradation in performance of only around five per cent for the most stable encapsulated device under continuous simulated full-spectrum sunlight for more than 1,800 hours at 70 to 75 degrees Celsius, and estimate that the time required for the device to drop to eighty per cent of its peak performance is about 5,200 hours. Our demonstration of long-term operational, stable solar cells under intense conditions is a key step towards a reliable perovskite photovoltaic technology. Addition of an ionic liquid, BMIMBF4, to metal halide perovskite solar cells improves their efficiency and long-term operation under accelerated aging conditions of high temperature and full-spectrum sunlight.

State-of-the-art Cu(In,Ga)Se-2 (CIGS) solar cells are grown with considerably substoichiometric Cu concentrations. The resulting defects, as well as potential improvements through increasing the Cu concentration, have been known in the field for many years. However, so far, cells with high Cu concentrations show decreased photovoltaic parameters. In this work, it is shown that RbF postdeposition treatment of CuInSe2 solar cells allows for capturing the benefits from the improved absorber quality with increasing Cu content. A reduced defect density and an increased doping level for cells with high Cu concentrations close to stoichiometry are demonstrated. Implementing a high mobility front transparent conductive oxide (TCO), the improved absorbers with 1.00 eV bandgap yield a solar cell efficiency of 19.2%, and combined with a perovskite top cell a 4-terminal tandem efficiency of 25.0% are demonstrated, surpassing the record efficiency of both subcell technologies.

Recent improvements in CdTe thin film solar cells have been achieved by using CdTe1−xSex as a part of the absorber layer. This review summarizes the published literature concerning the material properties of CdTe1−xSex and its application in current thin film CdTe photovoltaics. One of the important properties of CdTe1−xSex is its band gap bowing, which facilitates a lowering of the CdTe band gap towards the optimum band gap for highest theoretical efficiency. In practice, a CdTe1−xSex gradient is introduced to the front of CdTe, which induces a band gap gradient and allows for the fabrication of solar cells with enhanced short-circuit current while maintaining a high open-circuit voltage. In some device structures, the addition of CdTe1−xSex also allows for a reduction in CdS thickness or its complete elimination, reducing parasitic absorption of low wavelength photons.

Superconducting YBa2Cu3O7-δ films with different amounts of BaZrO3 nanoinclusions were deposited on SrTiO3(001) by low-fluorine chemical solution deposition with the aim of introducing artificial vortex pinning centres. The Zr concentration over the film thickness could be determined by X-ray photoelectron spectroscopy combined with Ar+ ion etching. The Zr/Y ratio has a constant behaviour in the film's bulk. Zr segregation occurs near the surface and Zr diffusion into the substrate is observed near the interface. Conversely, Sr and Ti from the substrate diffuse into the films. Y3d lineshape analysis and X-Ray Diffraction data pointed out that Ti diffusion causes the formation of Y-Ti-O based spurious phases.

We report on the use of a high bandgap metal-oxide at the front interface of Cu(In,Ga)Se2 (CIGS) solar cells in a point contact concept for reduced interface recombination. Highly resistive HfO2 is applied on the CIGS surface by atomic layer deposition (ALD). Aspects of the surface passivating effect of HfO2 on CIGS were investigated by time-resolved photoluminescence (TRPL), electron beam induced current (EBIC) and capacitance-voltage (C-V) measurements. Two structuring methods for point contact formation are compared, a lithographic top-down and a simple bottom-up approach using NaCl as template. The former method employed a plasma etch step which was found to degrade the performance of solar cells when applied on the CIGS surface. The template method omitted sputtering and allowed patterning of HfO2 up to 10 nm thickness without adversely impacting the opencircuit voltage (VOC). EBIC revealed an improved carrier collection due to the HfO2 coating and a long term stable PL decay was observed. Yet, the point contact concept with HfO2 was not significantly influencing the performance of a CIGS solar cell for the investigated parameter range.

We report a proof-of-concept two-terminal perovskite/Cu(In, Ga)Se2 (CIGS) monolithic thin-film tandem solar cell grown on ultra-thin (30-microns thick), light-weight, and flexible polyimide foil with a steady-state power conversion efficiency of 13.2% and a high open-circuit voltage over 1.75 V under standard test condition.

Garnet Li7La3Zr2O12 (LLZO) is a promising solid-state electrolyte due to its wide electrochemical stability window and high Li-ion conductivity. This electrolyte has potential to be employed in the form of thin films for solid-state batteries, a promising approach in the quest for safer batteries with higher energy densities at lower fabrication costs. In this study, we use a scalable cosputtering process to fabricate LLZO thin films with subsequent postannealing at a temperature of 700 degrees C, significantly below the sintering temperatures employed in ceramic pellet processing. We investigate the roles that Li excess and incorporation of Al play in the film's crystalline phase, microstructure, phase stability, and, ultimately, ionic conductivity. Our results reveal that improving the conductivity of LLZO thin films requires not only the stabilization of the cubic phase but especially the densification of the film and the minimization of the proton exchange degradation mechanism in the presence of moisture and CO2. These issues can be mitigated by effectively controlling the amount of Li and incorporating Al as sintering agent. An ionic conductivity at room temperature of 1.9 X 10(-5) S cm(-1) was achieved with a 400 nm Al-substituted LLZO thin film. Finally, we prove that these LLZO thin films can be successfully deposited and crystallized on a LiCoO2 cathode.

The major obstacle to commercialization of high-voltage Li-ion batteries is the lack of oxidatively stable and inexpensive current collectors that can operate at potentials of up to 5 V vs Li+/Li. In this work, we present titanium nitride as a compelling cathode current collector for high-voltage Li-ion batteries exhibiting higher oxidative stability in LiPF6 and lithium bis(fluorosulfonyl)imide electrolytes than aluminum or stainless steel current collectors. Its high oxidative stability has been assessed with a high-voltage LiMn1.5Ni0.5O4 cathode. TiN/LiMn1.5Ni0.5O4 half cells demonstrated a high Coulombic efficiency of 98.5% at a low C-rate of 0.2 C after 100 cycles.

Knowledge of the absorber bandgap is often needed for assessing the junction quality of a thin film solar cell, for example when computing the open-circuit voltage deficit. The bandgap is typically estimated from the routine measurement of the external quantum efficiency (EQE) of finished devices. However the extraction becomes ambiguous in the case of very thin absorbers, or in presence of bandgap gradients or collection issues of charge carriers. This work reviews several methods for the determination of the bandgap from EQE measurements and discusses their validity conditions. The numerical results are compared based on experimental EQE of several different thin film solar cells such as CuInGaSe2, CuInSe2, CdTe, CuZnSnSe and perovskite, and systematic trends are identified. Numerical simulations are also performed to illustrate the behavior of the different bandgap extraction methods in presence of a bandgap gradient and different magnitudes of the exponential tail states.

We report on the application of ZnxTiyO deposited by atomic layer deposition (ALD) as buffer layer in thin film Cu(In,Ga)Se2 (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-ZnxTiyO allowed a reduction of the CdS layer thickness without adversely affecting open-circuit voltage (VOC). This leads to an increased photocurrent density with a device efficiency of up to 20.8% by minimizing the parasitic absorption losses commonly observed for CdS. ALD was chosen as method to deposit homogeneous layers of ZnxTiyO with varying Ti content with a [Ti]/([Ti] + [Zn]) atomic fraction up to ∼0.35 at a relatively low temperature of 373 K. The Ti content influenced the absorption behavior of the ZnxTiyO layer by increasing the optical bandgap >3.5 eV in the investigated range. Temperature-dependent current–voltage (I–V) measurements of solar cells were performed to investigate the photocurrent blocking behavior observed for high Ti content. Possible conduction band discontinuities introduced by ZnxTiyO are discussed based on X-ray photoelectron spectroscopy (XPS) measurements.

We study the impact of different alkali post‐deposition treatments by thermal admittance spectroscopy and temperature‐dependent current‐voltage (IVT) characteristics of high‐efficiency Cu(In,Ga)Se2 thin‐film solar cells fabricated from low‐temperature and high‐temperature co‐evaporated absorbers. Capacitance steps observed by admittance spectroscopy for all samples agree with the widely observed N1 signature and show a clear correlation to a transport barrier evident from IVT characteristics measured in the dark, indicating that defects are likely not responsible for these capacitance steps. Activation energies extracted from capacitance spectra and IVT characteristics vary considerably between different samples but show no concise correlation to the alkali species used in the post‐deposition treatments. Numerical device simulations show that the transport barrier in our devices might be related to conduction band offsets in the absorber/buffer/window stack.