134 research outputs found
A Nitroxide Radical Conjugated Polymer as an Additive to Reduce Nonradiative Energy Loss in Organic Solar Cells
Get PDFNonfullerene-acceptor-based organic solar cells (NFA-OSCs) are now set off to the 20% power conversion efficiency milestone. To achieve this, minimizing all loss channels, including nonradiative photovoltage losses, seems a necessity. Nonradiative recombination, to a great extent, is known to be an inherent material property due to vibrationally induced decay of charge-transfer (CT) states or their back electron transfer to the triplet excitons. Herein, it is shown that the use of a new conjugated nitroxide radical polymer with 2,2,6,6-tetramethyl piperidine-1-oxyl side groups (GDTA) as an additive results in an improvement of the photovoltaic performance of NFA-OSCs based on different active layer materials. Upon the addition of GDTA, the open-circuit voltage (VOC), fill factor (FF), and short-circuit current density (JSC) improve simultaneously. This approach is applied to several material systems including state-of-the-art donor/acceptor pairs showing improvement from 15.8% to 17.6% (in the case of PM6:Y6) and from 17.5% to 18.3% (for PM6:BTP-eC9). Then, the possible reasons behind the observed improvements are discussed. The results point toward the suppression of the CT state to triplet excitons loss channel. This work presents a facile, promising, and generic approach to further improve the performance of NFA-OSCs
Impact of Bimolecular Recombination on the Fill Factor of Fullerene and Nonfullerene-Based Solar Cells: A Comparative Study of Charge Generation and Extraction
Get PDFPower conversion efficiency of nonfullerene organic solar cells have now increased beyond the record of their fullerene-based counterparts. There remain many fundamental questions about their working principles such as charge generation and extraction and losses in these systems. Herein, we present a comparative study of bulk heterojunction solar cells composed of a recently introduced nappthothiadiazole-based polymer (NT812) as the electron donor and two different acceptor molecules, namely PCBM[70] and ITIC. A comparison between photovoltaic performance of these two types of solar cells reveals that the open circuit voltage (Voc) of NT812:ITIC based solar cell is larger but the fill factor (FF) lower than that of NT812:PCBM[70] device. We find the key reason behind this reduced FF in the ITIC-based device to be faster non-geminate recombination relative to NT812:PCBM[70] system, which has previously been assigned to sub-optimum morphology. Nevertheless, NT812:ITIC system is more emissive than NT812:PCBM[70] resulting in smaller non-radiative Voc-loss. Suppression of bimolecular recombination together with reduced non-radiative Voc-loss can be considered as a key optimization route in the development of non-fullerene-based solar cells
Understanding the Role of Order in Y‐Series Non‐Fullerene Solar Cells to Realize High Open‐Circuit Voltages
Get PDFNon-fullerene acceptors (NFAs) as used in state-of-the-art organic solar cells feature highly crystalline layers that go along with low energetic disorder. Here, the crucial role of energetic disorder in blends of the donor polymer PM6 with two Y-series NFAs, Y6, and N4 is studied. By performing temperature-dependent charge transport and recombination studies, a consistent picture of the shape of the density of state distributions for free charges in the two blends is developed, allowing an analytical description of the dependence of the open-circuit voltage VOC on temperature and illumination intensity. Disorder is found to influence the value of the VOC at room temperature, but also its progression with temperature. Here, the PM6:Y6 blend benefits substantially from its narrower state distributions. The analysis also shows that the energy of the equilibrated free charge population is well below the energy of the NFA singlet excitons for both blends and possibly below the energy of the populated charge transfer manifold, indicating a down-hill driving force for free charge formation. It is concluded that energetic disorder of charge-separated states has to be considered in the analysis of the photovoltaic properties, even for the more ordered PM6:Y6 blend
Electron-donating amine-interlayer induced n-type doping of polymer:nonfullerene blends for efficient narrowband near-infrared photo-detection
Get PDFInherently narrowband near-infrared organic photodetectors are highly desired for many applications, including biological imaging and surveillance. However, they suffer from a low photon-to-charge conversion efficiencies and utilize spectral narrowing techniques which strongly rely on the used material or on a nano-photonic device architecture. Here, we demonstrate a general and facile approach towards wavelength-selective near-infrared phtotodetection through intentionally n-doping 500–600 nm-thick nonfullerene blends. We show that an electron-donating amine-interlayer can induce n-doping, resulting in a localized electric field near the anode and selective collection of photo-generated carriers in this region. As only weakly absorbed photons reach this region, the devices have a narrowband response at wavelengths close to the absorption onset of the blends with a high spectral rejection ratio. These spectrally selective photodetectors exhibit zero-bias external quantum efficiencies of ~20–30% at wavelengths of 900–1100 nm, with a full-width-at-half-maximum of ≤50 nm, as well as detectivities of >1012 Jones
Parameterization of Metallic Grids on Transparent Conductive Electrodes for the Scaling of Organic Solar Cells
Get PDFRequirements for Making Thick Junctions of Organic Solar Cells based on Nonfullerene Acceptors
Get PDFSlower carriers limit charge generation in organic semiconductor light-harvesting systems
Get PDFBlends of electron donating and accepting organic semiconductors are widely used as photoactive materials in next generation solar cells and photodetectors. The yield of free charges in these systems is often determined by the separation of interfacial electron-hole pairs, which is expected to depend on the ability of the faster carrier to escape the Coulomb potential. Here we show, by measuring geminate and non-geminate losses and key transport parameters in a series of bulk-heterojunction solar cells, that the charge-generation yield increases with increasing slower carrier mobility. This is in direct contrast with the well-established Braun model where the dissociation rate is proportional to the mobility sum, and recent models that underscore the importance of fullerene aggregation for coherent electron propagation. The behavior is attributed to the restriction of opposite charges to different phases, and to an entropic contribution that favors the joint separation of both charge carriers
Understanding Performance Limiting Interfacial Recombination in pin Perovskite Solar Cells
Get PDFPerovskite semiconductors are an attractive option to overcome the limitations of established silicon based photovoltaic (PV) technologies due to their exceptional opto‐electronic properties and their successful integration into multijunction cells. However, the performance of single‐ and multijunction cells is largely limited by significant nonradiative recombination at the perovskite/organic electron transport layer junctions. In this work, the cause of interfacial recombination at the perovskite/C60 interface is revealed via a combination of photoluminescence, photoelectron spectroscopy, and first‐principle numerical simulations. It is found that the most significant contribution to the total C60‐induced recombination loss occurs within the first monolayer of C60, rather than in the bulk of C60 or at the perovskite surface. The experiments show that the C60 molecules act as deep trap states when in direct contact with the perovskite. It is further demonstrated that by reducing the surface coverage of C60, the radiative efficiency of the bare perovskite layer can be retained. The findings of this work pave the way toward overcoming one of the most critical remaining performance losses in perovskite solar cells
Energetics and Kinetics Requirements for Organic Solar Cells to Break the 20% Power Conversion Efficiency Barrier
Get PDFThe thermodynamic limit for the efficiency of solar cells is predominantly
defined by the energy bandgap of the used semiconductor. In case of organic
solar cells both energetics and kinetics of three different species play role:
excitons, charge transfer states and charge separated states. In this work, we
clarify the effect of the relative energetics and kinetics of these species on
the recombination and generation dynamics. Making use of detailed balance, we
develop an analytical framework describing how the intricate interplay between
the different species influence the photocurrent generation, the recombination,
and the open-circuit voltage in organic solar cells. Furthermore, we clarify
the essential requirements for equilibrium between excitons, CT states and
charge carriers to occur. Finally, we find that the photovoltaic parameters are
not only determined by the relative energy level between the different states
but also by the kinetic rate constants. These findings provide vital insights
into the operation of state-of-art non-fullerene organic solar cells with low
offsets
Sensitivity of Sub-Bandgap External Quantum Efficiency Measurements of Solar Cells under Electrical and Light Bias
Get PDFThe measurement of the external quantum efficiency (EQE) for photocurrent generation at photon energies below the bandgap of semiconductors has always been an important tool for understanding phenomena such as charge photogeneration via tail and trap states. The shape of the subgap EQE can also reveal the subtle but important physics of inter- and intramolecular states that lay at the heart of charge photogeneration in molecular systems such as organic semiconductors. In this work, we examine the influence of optical and electrical noise on the sensitivity of EQE measurements under different electrical and optical bias conditions and demonstrate how to enhance the dynamic range to an unprecedented >100 dB. We identify and study several apparatus-and-device-related factors limiting the sensitivity including: the electrical noise floor of the measurement system; flicker and pick-up noise; probe light source stray light; the photon noise of the light bias source; the electrical noise of the voltage bias source; and the shunt-resistance-limited thermal and electrical shot noise of the device. By understanding and minimizing the influence of these factors we are able to detect EQE signals derived from weak subgap absorption features in both organic and inorganic solar cell systems at photon energies well below their bandgaps
- …
