Correct determination of charge transfer state energy from luminescence spectra in organic solar cells

On the critical competition between singlet exciton decay and free charge generation in non-fullerene based organic solar cells with low energetic offsets

Reducing voltage losses while maintaining high photocurrents is the holy grail of current research on non-fullerene acceptor (NFA) based organic solar cell. Recent focus lies in understanding the various fundamental mechanisms in organic blends with minimal energy offsets - particularly the relationship between ionization energy offset (ΔIE) and free charge generation. Here, we quantitatively probe this relationship in multiple NFA-based blends by mixing Y-series NFAs with PM6 of different molecular weights, covering a broad power conversion efficiency (PCE) range: from 15% down to 1%. Spectroelectrochemistry reveals that a ΔIE of more than 0.3 eV is necessary for efficient photocurrent generation. Bias-dependent time-delayed collection experiments reveal a very pronounced field-dependence of free charge generation for small ΔIE blends, which is mirrored by a strong and simultaneous field-dependence of the quantified photoluminescence from the NFA local singlet exciton (LE). We find that the decay of singlet excitons is the primary competition to free charge generation in low-offset NFA-based organic solar cells, with neither noticeable losses from charge-transfer (CT) decay nor evidence for LE-CT hybridization. In agreement with this conclusion, transient absorption spectroscopy consistently reveals that a smaller ΔIE slows the NFA exciton dissociation into free charges, albeit restorable by an electric field. Our experimental data align with Marcus theory calculations, supported by density functional theory simulations, for zero-field free charge generation and exciton decay efficiencies. We conclude that efficient photocurrent generation generally requires that the CT state is located below the LE, but that this restriction is lifted in systems with a small reorganization energy for charge transfer.

What We have Learnt from PM6:Y6

Over the past three years, remarkable advancements in organic solar cells (OSCs) have emerged, propelled by the introduction of Y6-an innovative A-DA'D-A type small molecule non-fullerene acceptor (NFA). This review provides a critical discussion of the current knowledge about the structural and physical properties of the PM6:Y6 material combination in relation to its photovoltaic performance. The design principles of PM6 and Y6 are discussed, covering charge transfer, transport, and recombination mechanisms. Then, the authors delve into blend morphology and degradation mechanisms before considering commercialization. The current state of the art is presented, while also discussing unresolved contentious issues, such as the blend energetics, the pathways of free charge generation, and the role of triplet states in recombination. As such, this review aims to provide a comprehensive understanding of the PM6:Y6 material combination and its potential for further development in the field of organic solar cells. By addressing both the successes and challenges associated with this system, this review contributes to the ongoing research efforts toward achieving more efficient and stable organic solar cells.

The role of entropy gains in the exciton separation in organic solar cells

In organic solar cell (OSC), the lower dielectric constant of organic semiconductor material induces a strong Coulomb attraction between electron-hole pairs, which leads to a low exciton separation efficiency, especially the charge transfer (CT) state. The CT state formed at the electron-donor (D) and electron-acceptor (A) interface is regarded as an unfavorable property of organic photovoltaic devices. Since the OSC works in a nonzero temperature condition, the entropy effect would be one of the main reasons to overcome the Coulomb energy barrier and must be taken into account. In this review, we review the present understanding of the entropy-driven charge separation and describe how factors such as the dimensionality of the organic semiconductor, energy disorder effect, the morphology of the active layer, and the nonequilibrium effect affect the entropy contribution in compensating the Coulomb dissociation barrier for CT exciton separation and charge generation process. We focus on the investigation of the entropy effect on exciton dissociation mechanism from both theoretical and experimental aspects, which provides pathways for understanding the underlying mechanisms of exciton separation and further enhancing the efficiency of OSCs. This article is protected by copyright. All rights reserved.

Direct Observation of the Charge Transfer States from a Non-Fullerene Organic Solar Cell with a Small Driving Force

For organic solar cells (OSCs), the charge generation mechanism and the recombination loss are heavily linked with charge transfer states (CTS). Measuring the energy of CTS (E_CT) by the most widely used technique, however, has become challenging for the non-fullerene-based OSCs with a small driving force, resulting in difficulty in the understanding of OSC physics. Herein, we present a study of the PM6:Y6 bulk heterojunction. It is demonstrated that electro-absorption can not only reveal the dipolar nature of Y6 but also resolve the morphology-dependent absorption signal of CTS in the sub-bandgap region. The device with the optimum blending weight ratio shows an E_CT of 1.27 eV, which is confirmed by independent measurements. Because of the charge transfer characteristics of Y6, the charge generation at PM6:Y6 interfaces occurs efficiently under a small but non-negligible driving force of 0.14 eV, and the total recombination loss is as low as 0.43 eV.

Analysis of the Performance of Narrow-Bandgap Organic Solar Cells Based on a Diketopyrrolopyrrole Polymer and a Nonfullerene Acceptor

The 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- and 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.

Charge-transfer electronic states in organic solar cells: a TDDFT study

The prediction of new organic photovoltaic materials in organic solar cells (OSCs) must include a precise description of charge-transfer states because they are involved in electron-transfer processes such as charge separation and charge recombination which govern the device efficiency. Also, as the experimental performance of an optoelectronic device is measured for nonequilibrium nanostructures, computational approaches need models that can incorporate morphology effects. Usually, this aspect is treated by molecular dynamics simulation (MDS) methodologies; however, methodologies and formalisms to calculate the electron-transfer processes are still controversial and sometimes do not connect their information with the phase morphologies. In this work we propose a simple and fast characterization of electron-transfer processes to find the rate constants by analysing the distribution of vertical excitation energies of both local excitation (LE) and charge-transfer (CT) states using TD-DFT calculations in the donor-acceptor pair structures which were extracted from MDS. This proposal assumes that conformational changes are prevented and equilibria are not achieved while the electron-transfer events take effect, and thus the only pathway that connects the LE and CT states is their surface crossing point where an ideal distribution might exist. Different density functionals and dialectric models were tested. The results indicate a close relationship between the proposal and experimental data for electron-transfer events, suggesting the application of this method in the rational design of new photovoltaic materials.

The Bulk Heterojunction in Organic Photovoltaic, Photodetector, and Photocatalytic Applications

Organic semiconductors require an energetic offset in order to photogenerate free charge carriers efficiently, owing to their inability to effectively screen charges. This is vitally important in order to achieve high power conversion efficiencies in organic solar cells. Early heterojunction-based solar cells were limited to relatively modest efficiencies (<4%) owing to limitations such as poor exciton dissociation, limited photon harvesting, and high recombination losses. The development of the bulk heterojunction (BHJ) has significantly overcome these issues, resulting in dramatic improvements in organic photovoltaic performance, now exceeding 18% power conversion efficiencies. Here, the design and engineering strategies used to develop the optimal bulk heterojunction for solar-cell, photodetector, and photocatalytic applications are discussed. Additionally, the thermodynamic driving forces in the creation and stability of the bulk heterojunction are presented, along with underlying photophysics in these blends. Finally, new opportunities to apply the knowledge accrued from BHJ solar cells to generate free charges for use in promising new applications are discussed.

Nonequilibrium site distribution governs charge-transfer electroluminescence at disordered organic heterointerfaces

The interface between electron-donating (D) and electron-accepting (A) materials in organic photovoltaic (OPV) devices is commonly probed by charge-transfer (CT) electroluminescence (EL) measurements to estimate the CT energy, which critically relates to device open-circuit voltage. It is generally assumed that during CT-EL injected charges recombine at close-to-equilibrium energies in their respective density of states (DOS). Here, we explicitly quantify that CT-EL instead originates from higher-energy DOS site distributions significantly above DOS equilibrium energies. To demonstrate this, we have developed a quantitative and experimentally calibrated model for CT-EL at organic D/A heterointerfaces, which simultaneously accounts for the charge transport physics in an energetically disordered DOS and the Franck-Condon broadening. The 0-0 CT-EL transition lineshape is numerically calculated using measured energetic disorder values as input to 3-dimensional kinetic Monte Carlo simulations. We account for vibrational CT-EL overtones by selectively measuring the dominant vibrational phonon-mode energy governing CT luminescence at the D/A interface using fluorescence line-narrowing spectroscopy. Our model numerically reproduces the measured CT-EL spectra and their bias dependence and reveals the higher-lying manifold of DOS sites responsible for CT-EL. Lowest-energy CT states are situated ∼180 to 570 meV below the 0-0 CT-EL transition, enabling photogenerated carrier thermalization to these low-lying DOS sites when the OPV device is operated as a solar cell rather than as a light-emitting diode. Nonequilibrium site distribution rationalizes the experimentally observed weak current-density dependence of CT-EL and poses fundamental questions on reciprocity relations relating light emission to photovoltaic action and regarding minimal attainable photovoltaic energy conversion losses in OPV devices.

Local Excitation/Charge-Transfer Hybridization Simultaneously Promotes Charge Generation and Reduces Nonradiative Voltage Loss in Nonfullerene Organic Solar Cells

High power conversion efficiencies in state-of-the-art nonfullerene organic solar cells (NF OSCs) call for elucidation of the underlying working mechanisms of both high photocurrent densities and low nonradiative voltage losses under small energy offsets. Here, to address this fundamental issue, we have assessed the nature of interfacial charge-transfer (CT) states in a representative small-molecule NF OSC (DRTB-T:IT-4F) by time-dependent density functional theory calculations. The calculated results point to the fact that the CT states can borrow considerable oscillator strengths from the energy-close local excitation (LE) states or be fully hybridized with these LE states by molecular aggregation at the donor-acceptor interfaces. The LE/CT hybridization can promote charge generation by direct population of thermalized CT or LE/CT states under illumination. At the same time, the increased oscillator strengths of the lowest CT state will improve the luminescence quantum efficiencies and thus reduce nonradiative voltage losses. Our work suggests that it is crucial to tune the LE/CT hybridization by optimization of the donor and acceptor molecular and interfacial structures to further improve the NF OSC performance.