24 research outputs found
3D Perovskite Passivation with a Benzotriazole-Based 2D Interlayer for High-Efficiency Solar Cells
No full text2H-Benzotriazol-2-ylethylammonium bromide and iodide and its difluorinated derivatives are synthesized and employed as interlayers for passivation of formamidinium lead triiodide (FAPbI3) solar cells. In combination with PbI2 and PbBr2, these benzotriazole derivatives form two-dimensional (2D) Ruddlesden-Popper perovskites (RPPs) as evidenced by their crystal structures and thin film characteristics. When used to passivate n-i-p FAPbI3 solar cells, the power conversion efficiency improves from 20% to close to 22% by enhancing the open-circuit voltage. Quasi-Fermi level splitting experiments and scanning electron microscopy cathodoluminescence hyperspectral imaging reveal that passivation provides a reduced nonradiative recombination at the interface between the perovskite and hole transport layer. Photoluminescence spectroscopy, angle-resolved grazing-incidence wide-angle X-ray scattering, and depth profiling X-ray photoelectron spectroscopy studies of the 2D/three-dimensional (3D) interface between the benzotriazole RPP and FAPbI3 show that a nonuniform layer of 2D perovskites is enough to passivate defects, enhance charge extraction, and decrease nonradiative recombination
Efficient organic solar cells with small energy losses based on a wide-bandgap trialkylsilyl-substituted donor polymer and a non-fullerene acceptor
No full textEfficient organic solar cells based on a blend of PBDS-T as a donor polymer and BTP-eC9 as non-fullerene acceptor are presented and characterized. PBDS-T is an alternating copolymer that comprises easily accessible electron-rich trialkylsilyl-substituted benzodithiophene and electron-deficient benzodithiophene-4,8-dione units and that can be efficiently and reproducibly synthesized in high molecular weights, while keeping good solubility. PBDS-T exhibits a strong absorption between 450 and 700 nm and combines a wide optical bandgap of 1.86 eV, with low-lying energy levels, and a face-on molecular orientation in thin films. Organic solar cells prepared by blending PBDS-T with BTP-eC9 show considerable performance when as-cast films are annealed in solvent vapor and present a high open-circuit voltage of 0.86 V, a low photon-energy loss of 0.53 eV, and an internal quantum efficiency of 93%. The power conversion efficiency reaches 16.4%, which − to the best of our knowledge − is the highest for a conjugated polymer comprising trialkylsilyl side chains in combination with a Y6-based non-fullerene acceptor. Specifically, the trialkylsilyl side-chains of PBDS-T reduce synthetic complexity, result in a low energy loss by ensuring low energetic disorder, and provide competitive device performance
Sub-bandgap Photocurrent Spectra of p-i-n Perovskite Solar Cells with n-Doped Fullerene Electron Transport Layers and Bias Illumination
No full textIn p-i-n perovskite solar cells optical excitation of defect states at the interface between the perovskite and fullerene electron transport layer (ETL) creates a photocurrent responsible for a distinct sub-bandgap external quantum efficiency (EQE). The precise nature of these signals and their impact on cell performance are largely unknown. Here, the effect of n-doping the fullerene on the EQE spectra is studied. The n-doped fullerene is either deposited from solution or by coevaporation. The latter method is used to create undoped-doped fullerene bilayers and investigate the effect of the proximity of the doped region on the EQE spectra. The intensity of the sub-bandgap EQE increases when the ETL is n-doped and also when the device is biased with green light. Using these results, the sub-bandgap EQE signal is attributed to originate from electron trap states in the perovskite with an energy below the conduction band that are filled by excitation with low-energy photons. The trapped electrons give rise to photocurrent when they are collected at a nearby electrode. The enhanced sub-bandgap EQE observed when the ETL is n-doped or bias light is applied, is related to a higher probability to extract trapped electrons under these conditions.</p
Quantifying Non-Radiative Recombination in Passivated Wide-Bandgap Metal Halide Perovskites Using Absolute Photoluminescence Spectroscopy
No full textWide-bandgap (>1.6 eV) mixed-halide perovskites tend to experience notable open-circuit voltage losses in solar cells due to non-radiative recombination. Here, the effects of defects and their passivation on the non-radiative recombination of charge carriers in mixed-halide perovskite solar cells are studied. By determining the quasi-Fermi level splitting via absolute photoluminescence measurements of perovskite layers with and without charge transport layers, bulk and interface contributions are disentangled and compared to the radiative open-circuit voltage. For wide-bandgap perovskites, non-radiative recombination present in the pristine perovskite layers increases with increasing bandgap. The most prominent loss, located at the perovskite – electron transport layer interface (ETL), can be reduced by interface passivation for the different bandgaps studied (1.58 to 1.82 eV) to a level close to that of the intrinsic losses. By combining light-intensity-dependent absolute photoluminescence spectroscopy with sensitive spectral photocurrent measurements it is found that different passivation agents result in a similar decrease of the non-radiative recombination for different bandgaps. This suggests that the gained open-circuit voltage is not due to an improved energy level alignment at the perovskite – ETL interface. Instead, passivation involves eliminating the direct contact between the perovskite semiconductor and the ETL.</p
2D/3D hybrid Cs2AgBiBr6 double perovskite solar cells : improved energy level alignment for higher contact‐selectivity and large open circuit voltage
No full textSince their introduction in 2017, the efficiency of lead-free halide perovskite solar cells based on Cs2AgBiBr6 has not exceeded 3%. The limiting bottlenecks are attributed to a low electron diffusion length, self-trapping events and poor selectivity of the contacts, leading to large non-radiative VOC losses. Here, 2D/3D hybrid double perovskites are introduced for the first time, using phenethyl ammonium as the constituting cation. The resulting solar cells show an increased efficiency of up to 2.5% for the champion cells and 2.03% on average, marking an improvement by 10% compared to the 3D reference on mesoporous TiO2. The effect is mainly due to a VOC improvement by up to 70 mV on average, yielding a maximum VOC of 1.18 V using different concentrations of phenethylammonium bromide. While these are among the highest reported VOC values for Cs2AgBiBr6 solar cells, the effect is attributed to a change in recombination behavior within the full device and a better selectivity at the interface toward the hole transporting material (HTM). This explanation is supported by voltage-dependent external quantum efficiency, as well as photoelectron spectroscopy, revealing a better energy level alignment and thus a better hole-extraction and improved electron blocking at the HTM interface
Origin, Nature, and Location of Defects in PM6:Y6 Organic Solar Cells
No full textTargeted strategies to overcome defects in organic semiconductors require insight into their identity and origin. Here the formation, nature, and location of defects is studied in PM6:Y6 organic solar cells by sensitive EQE measurements. Exposure of the active layer to ambient atmosphere and H2O-saturated compressed air indicates that a trace constituent in ambient air causes the formation of defects. By exposing the active layer to O3-enriched air, O3 is identified as the species creating defects in PM6:Y6 blends. Aging of complete inverted (n–i–p) configuration solar cells in H2O-saturated compressed air also increases the defect response. This is attributed to a reduced band bending at the PM6:Y6 | MoO3 hole-collecting contact, caused by a change in work function of MoO3 interacting with the H2O, which allows more defect states to be filled and available for photoexcitation. By measuring energy resolved-electrochemical impedance spectroscopy and by fabricating semitransparent cells, regular architecture cells, and semitransparent cells with an optical spacer−mirror stack it is found that defects originate predominantly from PM6 and are located near the top electrode, independent of device polarity. Because O3 is omnipresent in ambient atmosphere, albeit in small amounts, it likely causes defects in many organic semiconductors exposed to ambient air
Identifying the Nature and Location of Defects in n–i–p Perovskite Cells with Highly Sensitive Sub-Bandgap Photocurrent Spectroscopy
No full textDefects that exist in perovskite semiconductors and at their interfaces with charge transport layers limit the performance of perovskite solar cells (PSCs). Highly sensitive photocurrent measurements reveal at least two sub-bandgap defect states in n–i–p PSCs that use tin oxide covered with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) as the electron transport layer and tris(4-carbazoyl-9-ylphenyl)amine (TCTA) as the hole transport layer. Semitransparent PSCs with an optical spacer-mirror bilayer on top are used to modulate the interference of light. By varying the thickness of the optical spacer and analyzing the changes in the photocurrent spectra using optical simulations, the defect states that produce photocurrent with sub-bandgap excitation are found to be located near the PCBM-perovskite interface. This conclusion is supported by quasi-Fermi level splitting measurements on perovskite n–i–p half stacks. The observations are explained by an enhanced extraction of trapped electrons from the perovskite at the interface with PCBM.</p
Analysis of the Performance of Narrow-Bandgap Organic Solar Cells Based on a Diketopyrrolopyrrole Polymer and a Nonfullerene Acceptor
No full textThe combination of narrow-bandgap diketopyrrolopyrrole (DPP) polymers and nonfullerene acceptors (NFAs) seems well-matched for solar cells that exclusively absorb in the near infrared but they rarely provide high efficiency. One reason is that processing of the active layer is complicated by the fact that DPP-based polymers are generally only sufficiently soluble in chloroform (CF), while NFAs are preferably processed from halogenated aromatic solvents. By using a ternary solvent system consisting of CF, 1,8-diiodooctane (DIO), and chlorobenzene (CB), the short-circuit current density is increased by 50% in solar cells based on a DPP polymer (PDPP5T) and a NFA (IEICO-4F) compared to the use of CF with DIO only. However, the open-circuit voltage and fill factor are reduced. As a result, the efficiency improves from 3.4 to 4.8% only. The use of CB results in stronger aggregation of IEICO-4F as inferred from two-dimensional grazing-incidence wide-angle X-ray diffraction. Photo- A nd electroluminescence and mobility measurements indicate that the changes in performance can be ascribed to a more aggregated blend film in which charge generation is increased but nonradiative recombination is enhanced because of reduced hole mobility. Hence, while CB is essential to obtain well-ordered domains of IEICO-4F in blends with PDPP5T, the morphology and resulting hole mobility of PDPP5T domains remain suboptimal. The results identify the challenges in processing organic solar cells based on DPP polymers and NFAs as near-infrared absorbing photoactive layers
Revealing defective interfaces in perovskite solar cells from highly sensitive sub-bandgap photocurrent spectroscopy using optical cavities
No full textDefects in perovskite solar cells are known to affect the performance, but their precise nature, location, and role remain to be firmly established. Here, we present highly sensitive measurements of the sub-bandgap photocurrent to investigate defect states in perovskite solar cells. At least two defect states can be identified in p-i-n perovskite solar cells that employ a polytriarylamine hole transport layer and a fullerene electron transport layer. By comparing devices with opaque and semi-transparent back contacts, we demonstrate the large effect of optical interference on the magnitude and peak position in the sub-bandgap external quantum efficiency (EQE) in perovskite solar cells. Optical simulations reveal that defects localized near the interfaces are responsible for the measured photocurrents. Using optical spacers of different lengths and a mirror on top of a semi-transparent device, allows for the precise manipulation of the optical interference. By comparing experimental and simulated EQE spectra, we show that sub-bandgap defects in p-i-n devices are located near the perovskite-fullerene interface
Origin and Energy of Intra‐Gap States in Sensitive Near‐Infrared Organic Photodiodes
No full textTrap states in organic semiconductors are notoriously detrimental to the performance of organic electronics. However, the origin and energetics of trap states remain largely elusive and under debate, especially for bulk-heterojunction (BHJ) photodiodes consisting of electron donor and acceptor materials. Combining three sensitive techniques now enables locating the origin and energy of trap states in six state-of-the-art polymer – non-fullerene acceptor organic photodiodes (OPDs) with noise-based specific detectivities exceeding 1013 Jones. Analyzing the temperature dependence of the reverse-bias dark current density (Jd) identifies intra-gap states in the polymers, lying 0.3−0.4 eV above the energy of the highest occupied molecular orbital, as being responsible for Jd. Sub-bandgap external quantum efficiency spectra of donor-only and acceptor-only diodes confirm that intra-gap states are much more abundant in the polymers. Likewise, responsivity measurements at ultra-low light intensities (10−7 mW cm−2) show trap-mediated charge recombination in BHJ and polymer-only diodes, but not in acceptor-only devices. The results imply that to further improve the specific detectivity of near-infrared OPDs, the intra-gap state energy, and density need to be reduced.</p
