Spectrally Resolved Exciton Polarizability for Understanding Charge Generation in Organic Bulk Hetero-Junction Diodes

Despite decades of research, the dominant charge generation mechanism in organic bulk heterojunction (BHJ) devices is not completely understood. While the local dielectric environments of the photoexcited molecules are important for exciton dissociation, conventional characterizations cannot separately measure the polarizability of electron-donor and electron-acceptor, respectively, in their blends, making it difficult to decipher the spectrally different charge generation efficiencies in organic BHJ devices. Here, by spectrally resolved electroabsorption spectroscopy, we report extraction of the excited state polarizability for individual donors and acceptors in a series of organic blend films. Regardless of the donor and acceptor, we discovered that larger exciton polarizability is linked to larger π-π coherence length and faster charge transfer across the heterojunction, which fundamentally explains the origin of the higher charge generation efficiency near 100% in the BHJ photodiodes. We also show that the molecular packing of the donor and acceptor influence each other, resulting in a synergetic enhancement in the exciton polarizability.

Ab Initio Study of Charge Separation Dynamics and Pump-Probe Spectroscopy in the P3HT/PCBM Blend

We develop a bottom-up computational method for excited-state dynamics and time-resolved spectroscopy signals in molecular aggregates, on the basis of ab initio excited-state calculations. As an application, we consider the charge separation dynamics and pump-probe spectroscopy in the amorphous P3HT/PCBM blend. To simulate quantum dynamics and time-resolved spectroscopy, the model Hamiltonian for single-excitation and double-excitation manifolds was derived on the basis of fragment-based excited-state calculations within the GW approximation and the Bethe-Salpeter equation. After elucidating the energetics of the electron-hole separation and examining linear absorption spectrum, we investigated the quantum dynamics of exciton and charge carriers in comparison with the pump-probe transient absorption spectra. In particular, we introduced the pump-probe excited-state absorption (ESA) anisotropy as a spectroscopic signature of charge carrier dynamics after exciton dissociation. We found that the charge separation dynamics can be probed by the pump-probe ESA anisotropy dynamics after charge-transfer excitations. The present study provides the fundamental information for understanding the experimental spectroscopy signals, by elucidating the relationship between the excited states, the exciton and charge carrier dynamics, and time-resolved spectroscopy.

Manipulation of Charge Delocalization in a Bulk Heterojunction Material Using a Mid-Infrared Push Pulse

In organic bulk heterojunction materials, charge delocalization has been proposed to play a vital role in the generation of free carriers by effectively reducing the Coulomb attraction via an interfacial charge transfer exciton (CTX). Pump-push-probe (PPP) experiments produced evidence that the excess energy given by a push pulse enhances delocalization, thereby increasing photocurrent. However, previous studies have employed near-infrared push pulses in the range ∼0.4-0.6 eV, which is larger than the binding energy of a typical CTX. This raises the doubt that the push pulse may directly promote dissociation without involving delocalized states. Here, we perform PPP experiments with mid-infrared push pulses at energies that are well below the binding energy of a CTX state (0.12-0.25 eV). We identify three types of CTXs: delocalized, localized, and trapped. The excitation resides over multiple polymer chains in delocalized CTXs, while it is restricted to a single chain (albeit maintaining a degree of intrachain delocalization) in localized CTXs. Trapped CTXs are instead completely localized. The pump pulse generates a "hot" delocalized CTX, which promptly relaxes to a localized CTX and eventually to trapped states. We find that photo-exciting localized CTXs with push pulses resonant to the mid-infrared charge transfer absorption can promote delocalization and, in turn, contribute to the formation of long-lived charge separated states. On the other hand, we found that trapped CTXs are non-responsive to the push pulses. We hypothesize that delocalized states identified in prior studies are only accessible in systems where there is significant interchain electronic coupling or regioregularity that supports either inter- or intrachain polaron delocalization. This, in turn, emphasizes the importance of engineering the micromorphology and energetics of the donor-acceptor interface to exploit the full potential of a material for photovoltaic applications.

Origin of Photovoltaics in Organic Solar Cells at Negligible Energy Level Offsets─An Insight of the Charge Accumulation Effect

Reducing the energy level offset is one of the key elements of low open-circuit voltage loss in organic solar cells. However, the origin of charge separation driving force at negligible energy level offsets still remains unexplained. Herein, from the perspective of built-in potential caused by charge accumulation, we discuss the nonequilibrium energy level displacement as current passing with distinct variable current densities. Due to the different carrier mobilities of electrons and holes in organic semiconductor materials, carriers with high mobility will be rapidly transmitted to the electrode, while those with low mobility will remain in the materials, resulting in the accumulation of corresponding charges. It is suggested that the higher the carrier mobility, the better the efficiency of photovoltaic devices, with the balance of the charge transport.

Short Excited-State Lifetimes Mediate Charge-Recombination Losses in Organic Solar Cell Blends with Low Charge-Transfer Driving Force

A blend of a low-optical-gap diketopyrrolopyrrole polymer and a fullerene derivative, with near-zero driving force for electron transfer, is investigated. Using femtosecond transient absorption and electroabsorption spectroscopy, the charge transfer (CT) and recombination dynamics as well as the early-time transport are quantified. Electron transfer is ultrafast, consistent with a Marcus-Levich-Jortner description. However, significant charge recombination and unusually short excited (S₁ ) and CT state lifetimes (≈14 ps) are observed. At low S₁ -CT offset, a short S₁ lifetime mediates charge recombination because: i) back-transfer from the CT to the S₁ state followed by S₁ recombination occurs and ii) additional S₁ -CT hybridization decreases the CT lifetime. Both effects are confirmed by density functional theory calculations. In addition, relatively slow (tens of picoseconds) dissociation of charges from the CT state is observed, due to low local charge mobility. Simulations using a four-state kinetic model entailing the effects of energetic disorder reveal that the free charge yield can be increased from the observed 12% to 60% by increasing the S₁ and CT lifetimes to 150 ps. Alternatively, decreasing the interfacial CT state disorder while increasing bulk disorder of free charges enhances the yield to 65% in spite of the short lifetimes.

Modified fullerenes as acceptors in bulk heterojunction organic solar cells - a theoretical study

In the present study, electronic structure calculations were used to provide strategies for designing poly(3-hexylthiophene) (P3HT)-fullerene-derivative-based donor-acceptor materials for use in high-efficiency bulk heterojunction organic solar cells (BHJ OSCs). The work systematically analyses the impact of electron-donating and -withdrawing substituents on the opto-electronic properties of the fullerene structures. Parameters relating to the absorption spectra, orbital distributions, and energy ordering of the frontier molecular orbitals (FMO), the interactions between P3HT and the fullerene derivatives, and charge transfer across the interface were investigated. We found that substitution with the electron-withdrawing group NO₂ enhances the electronic coupling between the fullerene and P3HT; however, it reduces the open-circuit voltage (V_OC) of the OSC through lowering the LUMO energy level. Furthermore, the results show that substitution with an electron-withdrawing group (NO₂) and electron-donating group (OCH₃) can improve the power conversion efficiency (PCE) of the OSC, since this slightly improves the photon absorption abilities and charge transfer coupling at the interface without overly compromising V_OC relative to PC₆₁BM. Our study shows that alkyl chain modification in the PC₆₁BM acceptor is a promising strategy for improving the performances of OSCs.

Why ultrafast charge separation occurs in bulk-heterojunction organic solar cells: a multichain tight binding model study

Bulk-heterojunction (BHJ) organic solar cells (OSCs) exhibit ultrafast charge separation (UCS) which enables lower geminate charge recombination and high internal quantum efficiency. Unravelling why UCS occurs in BHJ-OSCs is important for the exploration of devices in future, however it is still far from clear. In this work, we build a multichain tight-binding model to study the conditions for realizing UCS. We propose that two conditions are important: (i) the BHJ-OSC has a morphology with donor and acceptor molecules being individually aggregated; (ii) the ratio of the donor/acceptor interfacial coupling to the internal donor/donor and acceptor/acceptor coupling should be smaller than a threshold. In addition, we suggest that increasing the donor/acceptor energetic offset will boost the UCS efficiency. As a fundamental theoretical analysis on the underlying mechanism of UCS, our work provides design rules for optimizing high-performance BHJ OSCs.

Nonequilibrium Thermodynamics of Charge Separation in Organic Solar Cells

This work presents a novel theoretical description of the nonequilibrium thermodynamics of charge separation in organic solar cells (OSCs). Using stochastic thermodynamics, we take realistic state populations derived from the phonon-assisted dynamics of electron-hole pairs within photoexcited organic bilayers to connect the kinetics with the free energy profile of charge separation. Hereby, we quantify for the first time the difference between nonequilibrium and equilibrium free energy profile. We analyze the impact of energetic disorder and delocalization on free energy, average energy, and entropy. For a high disorder, the free energy profile is well-described as equilibrated. We observe significant deviations from equilibrium for delocalized electron-hole pairs at a small disorder, implying that charge separation in efficient OSCs proceeds via a cold but nonequilibrated pathway. Both a large Gibbs entropy and large initial electron-hole distance provide an efficient charge separation, while a decrease in the free energy barrier does not necessarily enhance charge separation.

Free Charge Carriers in Homo-Sorted π-Stacks of Donor-Acceptor Conjugates

Inspired by the high photoconversion efficiency observed in natural light-harvesting systems, the hierarchical organization of molecular building blocks has gained impetus in the past few decades. Particularly, the molecular arrangement and packing in the active layer of organic solar cells (OSCs) have garnered significant attention due to the decisive role of the nature of donor/acceptor (D/A) heterojunctions in charge carrier generation and ultimately the power conversion efficiency. This review focuses on the recent developments in emergent optoelectronic properties exhibited by self-sorted donor-on-donor/acceptor-on-acceptor arrangement of covalently linked D-A systems, highlighting the ultrafast excited state dynamics of charge transfer and transport. Segregated organization of donors and acceptors promotes the delocalization of photoinduced charges among the stacks, engendering an enhanced charge separation lifetime and percolation pathways with ambipolar conductivity and charge carrier yield. Covalently linking donors and acceptors ensure a sufficient D-A interface and interchromophoric electronic coupling as required for faster charge separation while providing better control over their supramolecular assemblies. The design strategies to attain D-A conjugate assemblies with optimal charge carrier generation efficiency, the scope of their application compared to state-of-the-art OSCs, current challenges, and future opportunities are discussed in the review. An integrated overview of rational design approaches derived from the comprehension of underlying photoinduced processes can pave the way toward superior optoelectronic devices and bring in new possibilities to the avenue of functional supramolecular architectures.

Influence of static disorder of charge transfer state on voltage loss in organic photovoltaics

Spectroscopic measurements of charge transfer (CT) states provide valuable insight into the voltage losses in organic photovoltaics (OPVs). Correct interpretation of CT-state spectra depends on knowledge of the underlying broadening mechanisms, and the relative importance of molecular vibrational broadening and variations in the CT-state energy (static disorder). Here, we present a physical model, that obeys the principle of detailed balance between photon absorption and emission, of the impact of CT-state static disorder on voltage losses in OPVs. We demonstrate that neglect of CT-state disorder in the analysis of spectra may lead to incorrect estimation of voltage losses in OPV devices. We show, using measurements of polymer:non-fullerene blends of different composition, how our model can be used to infer variations in CT-state energy distribution that result from variations in film microstructure. This work highlights the potential impact of static disorder on the characteristics of disordered organic blend devices.

Charge Transfer Excitation and Asymmetric Energy Transfer at the Interface of Pentacene-Perfluoropentacene Heterostacks

High-performance solar cells demand efficient charge-carrier excitation, separation, and extraction. These requirements hold particularly true for molecular photovoltaics, where large exciton binding energies render charge separation challenging at their commonly complex donor-acceptor interface structure. Among others, charge-transfer (CT) states are considered to be important precursors for exciton dissociation and charge separation. However, the general nature of CT excitons and their formation pathways remain unclear. Layered quasiplanar crystalline molecular heterostructures of the prototypical donor-acceptor system pentacene-perfluoropentacene studied at cryogenic temperatures are a paramount model system to gain insights into the underlying physical mechanism. In particular, a detailed experiment-theory analysis on a layered heterojunction featuring perfluoropentacene in its π-stacked polymorph and pentacene in the Siegrist phase indicates that exciton diffusion in unitary films can influence the formation efficiency of CT excitons localized at internal interfaces for these conditions. The correlation of the structural characteristics, that is, the molecular arrangement at the interfaces, with their absorption and photoluminescence excitation spectra is consistent with exciton transfer from pentacene to the CT exciton state only, whereas no transfer of excitons from the perfluoropentacene is detected. Electronic structure calculations of the model systems and investigation of coupling matrix elements between the various electronic states involved suggest hampered exciton diffusion toward the internal interface in the perfluoropentacene films. The asymmetric energy landscape around an idealized internal donor-acceptor interface thus is identified as a reason for asymmetric energy transfer. Thus, long-range effects apparently can influence charge separation in crystalline molecular heterostructures, similar to band gap bowing, which is well established for inorganic pn-junctions.

Deciphering the excited-state dynamics and multicarrier interactions in perovskite core-shell type hetero-nanocrystals

Deciphering and modulating the carrier dynamics of perovskite nanocrystals (Pe-NCs) is crucial for their optoelectronic applications, which remains elusive to date. Herein, we, for the first time, explore the ultrafast dynamics of perovskite core-shell type NCs using CsPbBr₃@ZnS as a model system. According to the transient spectroscopic characterization, a physical picture of the ultrafast dynamics in core-shell Pe-NCs is built. Specifically, we directly observed the "hot" hole transfer from CsPbBr₃ to ZnS and confirmed the formation of charge-transfer state in CsPbBr₃@ZnS NCs. Such ultrafast (<100 fs) hole rearrangement speeds up the carrier cooling and breaks the hot phonon bottleneck effect in Pe-NCs. Moreover, thanks to the charge separation in CsPbBr₃@ZnS NCs, the Auger recombination is largely suppressed and the Auger lifetime is increased nearly 5-fold compared to that of "pure" CsPbBr₃ NCs, which endows CsPbBr₃@ZnS NCs with unique optical gain properties. These results are informative for halide perovskite-based applications, such as photocatalysis, hot-carrier photovoltaics and lasers.

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.

Delocalised kinetic Monte Carlo for simulating delocalisation-enhanced charge and exciton transport in disordered materials

Charge transport is well understood in both highly ordered materials (band conduction) or highly disordered ones (hopping conduction). In moderately disordered materials—including many organic semiconductors—the approximations valid in either extreme break down, making it difficult to accurately model the conduction. In particular, describing wavefunction delocalisation requires a quantum treatment, which is difficult in disordered materials that lack periodicity. Here, we present the first three-dimensional model of partially delocalised charge and exciton transport in materials in the intermediate disorder regime. Our approach is based on polaron-transformed Redfield theory, but overcomes several computational roadblocks by mapping the quantum-mechanical techniques onto kinetic Monte Carlo. Our theory, delocalised kinetic Monte Carlo (dKMC), shows that the fundamental physics of transport in moderately disordered materials is that of charges hopping between partially delocalised electronic states. Our results reveal why standard kinetic Monte Carlo can dramatically underestimate mobilities even in disordered organic semiconductors, where even a little delocalisation can substantially enhance mobilities, as well as showing that three-dimensional calculations capture important delocalisation effects neglected in lower-dimensional approximations.

The first three-dimensional model of transport in moderately disordered materials shows that a little delocalisation can dramatically enhance mobilities.

Charge Energetics and Electronic Level Changes At the Copper(II) Phthalocyanine/Fullerene Junction Upon Photoexcitation

Energy offset at the donor (D)/acceptor (A) interface plays an important role in charge separation in organic photovoltaics. Its magnitude determines the charge separation process under illumination. Extensive studies have been carried out for elucidating the charge transfer (CT) process at different D/A junctions. These works lead to two different views: upon photoexcitation, energies would be (1) consumed in molecular polarization and orientation such that those opposite charges would overcome mutual Coulombic attractive potential at the interface and (2) spent on promoting charges to high-lying delocalized energy states (i.e., hot states), which is necessarily important prior to charge separation. Under these two scheme of studies, the electronic structures and the charge behaviors at the D/A interface should be different under photoexcitation, yet there is so far no direct experimental approach for probing the changes in electronics structures (i.e., Fermi level, vacuum level, frontier molecular orbitals, etc.) upon photoexcitation. Herein, a modified photoelectron spectroscopy (PES) system with an additional solar simulator is used to study the charge distributions and electronic interactions for a standard D/A heterojunction (i.e., copper phthalocyanine (CuPc)/ fullerene (C₆₀)) under photoexcitation. CT states formed as a result of photon energy transfer at the CuPc/C₆₀ junction. Subsequent superpositions of charge transfer and electron polarization effects increase the D/A energy level offsets from 0.75 (ground state measured in the dark) to 1.07 eV (high-lying state measured upon illumination). We showed that there is excess energy consumed for a subtle change in the energy level alignment of the CuPc/C₆₀ junction under illumination, suggesting a new insight for the energy loss mechanism during the photocharge generation processes.

Self-filtering narrowband high performance organic photodetectors enabled by manipulating localized Frenkel exciton dissociation

The high binding energy and low diffusion length of photogenerated Frenkel excitons have long been viewed as major drawbacks of organic semiconductors. Therefore, bulk heterojunction structure has been widely adopted to assist exciton dissociation in organic photon-electron conversion devices. Here, we demonstrate that these intrinsically “poor” properties of Frenkel excitons, in fact, offer great opportunities to achieve self-filtering narrowband organic photodetectors with the help of a hierarchical device structure to intentionally manipulate the dissociation of Frenkel excitons. With this strategy, filter-free narrowband organic photodetector centered at 860 nm with full-width-at-half-maximum of around 50 nm, peak external quantum efficiency around 65% and peak specific detectivity over 10¹³ Jones are obtained, which is one the best performed no-gain type narrowband organic photodetectors ever reported and comparable to commercialized silicon photodetectors. This novel device structure along with its design concept may help create low cost and reliable narrowband organic photodetectors for practical applications.

Narrowband organic photodetectors (OPDs) are attractive for emerging applications. Here, the authors report a simple strategy to produce filter-free narrowband OPDs with outstanding performances by manipulating exciton dissociation with a hierarchical device structure.

Donor-Acceptor Effect of Carbazole-Based Conjugated Polymer Electrets on Photoresponsive Flash Organic Field-Effect Transistor Memories

The molecular structure of polymer electrets is crucial for creating diverse functionalities of organic field-effect transistor (OFET) devices. Herein, a conceptual framework has been applied in this study to design the highly photoresponsive carbazole-based copolymer electret materials for the application of photoresponsive OFET memory. As an electret layer, two 1,8-carbazole-based copolymers were utilized; the copoly(CT) consisted of carbazole as the donor group and thiophene as the π-spacer, whereas the copoly(CBT) was further introduced as an acceptor moiety, benzothiadiazole, for comparison. Both copolymers exhibited efficient visible-light absorption and photoluminescence quenching in the film state, indicating the formation of a considerable number of nonemissive excitons, one of the crucial factors for achieving photoinduced recovery behavior in OFET memories. Compared to copoly(CT) with the pure donor system, faster and more effective photoinduced recovery behavior was discovered in the copoly(CBT) with the conjugated donor-acceptor structure because of the coexistence of the conjugated donor and acceptor groups. Thus, the dissociation of the generated excitons facilitated the stimulating of the unique ambipolar trapping property, resulting in the high-density data storage devices with multilevel current states. In addition, the nonvolatile and durable characteristics demonstrated the feasibility in application of memory and photorecorders.

Can we utilize the higher Frenkel exciton state in biazulene diimides-based non-fullerene acceptors to promote charge separation at the donor/acceptor interface?

The pathway of charge transfer from the Frenkel exciton state of the acceptor to charge transfer states was investigated.

Insights into ultrafast charge-pair dynamics in P3HT:PCBM devices under the influence of static electric fields

Polymer-fullerene blends based on poly(3-hexylthiophene-2,5-diyl) (P3HT) and phenyl-C₆₁-butyric-acid methyl ester (PCBM) have been extensively studied as promising bulk heterojunction materials for organic semiconductor devices with improved performance. In these donor–acceptor systems where the bulk morphology plays a crucial role, the generation and subsequent decay mechanisms of photoexcitation species are still not completely understood. In this work, we use femtosecond transient absorption spectroscopy to investigate P3HT:PCBM diodes under the influence of applied static electric fields in comparison to P3HT:PCBM thin films. At the same time, we try to present a detailed overview about work already done on these donor–acceptor systems. The excited state dynamics obtained at 638 nm from P3HT:PCBM thin films are found to be similar to those observed earlier in neat P3HT films, while those obtained in the P3HT:PCBM devices are affected by field-induced exciton dissociation, resulting not only in comparatively slower decay dynamics, but also in bimolecular deactivation processes. External electric fields are expected to enhance charge generation in the investigated P3HT:PCBM devices by dissociating excitons and loosely bound intermediate species like polaron pairs (PPs) and charge transfer (CT) excitons, which can already dissociate only due to the intrinsic fields at the donor–acceptor interfaces. Our results clearly establish the formation of PP-like transient species different from CT excitons in the P3HT:PCBM devices as a result of a field-induced diffusion-controlled exciton dissociation process. We find that the loosely bound transient species formed in this way also are reduced in part via a bimolecular annihilation process resulting in charge loss in typical donor–acceptor P3HT:PCBM bulk heterojunction semiconductor devices, which is a rather interesting finding important for a better understanding of the performance of these devices.

Electric field effects in P3HT:PCBM solar cell result in polaron-pair-like secondary photoexcitation species showing slower and bimolecular decay characteristics.

Disorder vs Delocalization: Which Is More Advantageous for High-Efficiency Organic Solar Cells?

We investigate the combined influence of energetic disorder and delocalization on electron-hole charge-transfer state separation efficiency in donor-acceptor organic photovoltaic systems using an analytical hopping model and Monte Carlo calculations, coupled with an effective mass model. Whereas energetic disorder increases the separation yield at intermediate and low electric fields for low-efficiency blends with strongly localized carriers, we find that it reduces dramatically the fill factors and power conversion efficiencies in high-efficiency solar cells that require high carrier delocalization within the conjugated segment and high mobility-lifetime product. We further demonstrate that the initial electron-hole distance and thermalization processes play only a minor role in the separation dynamics.

Effect of Ultraviolet Radiation on Organic Photovoltaic Materials and Devices

Organic photovoltaics are a sustainable and cost-effective power-generation technology that may aid the move to zero-emission buildings, carbon neutral cities, and electric vehicles. While state-of-the-art organic photovoltaic devices can be encapsulated to withstand air and moisture, they are currently still susceptible to light-induced degradation, leading to a decline in the long-term efficiency of the devices. In this study, the role of ultraviolet (UV) radiation on a multilayer organic photovoltaic device is systematically uncovered using spectral filtering. By applying long-pass filters to remove different parts of the UV portion of the AM1.5G spectrum, two main photodegradation processes are shown to occur in the organic photovoltaic devices. A UV-activated process is found to cause a significant decrease in the photocurrent across the whole spectrum and is most likely linked to the deterioration of the charge extraction layers. In addition, a photodegradation process caused by UV-filtered sunlight is found to change the micromorphology of the bulk heterojunction material, leading to a reduction in photocurrent at high photon energies. These findings strongly suggest that the fabrication of inherently photostable organic photovoltaic devices will require the replacement of fullerene-based electron transporter materials with alternative organic semiconductors.

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.

Structure-Function Relationship of Organic Semiconductors: Detailed Insights From Time-Resolved EPR Spectroscopy

Organic photovoltaics (OPV) is a promising technology to account for the increasing demand for energy in form of electricity. Whereas the last decades have seen tremendous progress in the field witnessed by the steady increase in efficiency of OPV devices, we still lack proper understanding of fundamental aspects of light-energy conversion, demanding for systematic investigation on a fundamental level. A detailed understanding of the electronic structure of semiconducting polymers and their building blocks is essential to develop efficient materials for organic electronics. Illuminating conjugated polymers not only leads to excited states, but sheds light on some of the most important aspects of device efficiency in organic electronics as well. The interplay between electronic structure, morphology, flexibility, and local ordering, while at the heart of structure-function relationship of organic electronic materials, is still barely understood. (Time-resolved) electron paramagnetic resonance (EPR) spectroscopy is particularly suited to address these questions, allowing one to directly detect paramagnetic states and to reveal their spin-multiplicity, besides its clearly superior spectral resolution compared to optical methods. This article aims at giving a non-specialist audience an overview of what EPR spectroscopy and particularly its time-resolved variant (TREPR) can contribute to unraveling aspects of structure-function relationship in organic semiconductors.

Tailored Interface Energetics for Efficient Charge Separation in Metal Oxide-Polymer Solar Cells

Hybrid organic-inorganic heterointerfaces in solar cells suffer from inefficient charge separation yet the origin of performance limitations are widely unknown. In this work, we focus on the role of metal oxide-polymer interface energetics in a charge generation process. For this purpose, we present novel benzothiadiazole based thiophene oligomers that tailor the surface energetics of the inorganic acceptor TiO₂ systematically. In a simple bilayer structure with the donor polymer poly(3-hexylthiophene) (P3HT), we are able to improve the charge generation process considerably. By means of an electronic characterization of solar cell devices in combination with ultrafast broadband transient absorption spectroscopy, we demonstrate that this remarkable improvement in performance originates from reduced recombination of localized charge transfer states. In this context, fundamental design rules for interlayers are revealed, which assist the charge separation at organic-inorganic interfaces. Beside acting as a physical spacer in between electrons and holes, interlayers should offer (1) a large energy offset to drive exciton dissociation, (2) a push-pull building block to reduce the Coulomb binding energy of charge transfer states and (3) an energy cascade to limit carrier back diffusion towards the interface.

Design rules for minimizing voltage losses in high-efficiency organic solar cells

The open-circuit voltage of organic solar cells is usually lower than the values achieved in inorganic or perovskite photovoltaic devices with comparable bandgaps. Energy losses during charge separation at the donor-acceptor interface and non-radiative recombination are among the main causes of such voltage losses. Here we combine spectroscopic and quantum-chemistry approaches to identify key rules for minimizing voltage losses: (1) a low energy offset between donor and acceptor molecular states and (2) high photoluminescence yield of the low-gap material in the blend. Following these rules, we present a range of existing and new donor-acceptor systems that combine efficient photocurrent generation with electroluminescence yield up to 0.03%, leading to non-radiative voltage losses as small as 0.21 V. This study provides a rationale to explain and further improve the performance of recently demonstrated high-open-circuit-voltage organic solar cells.

Probing the pathways of free charge generation in organic bulk heterojunction solar cells

The fact that organic solar cells perform efficiently despite the low dielectric constant of most photoactive blends initiated a long-standing debate regarding the dominant pathways of free charge formation. Here, we address this issue through the accurate measurement of the activation energy for free charge photogeneration over a wide range of photon energy, using the method of time-delayed collection field. For our prototypical low bandgap polymer:fullerene blends, we find that neither the temperature nor the field dependence of free charge generation depend on the excitation energy, ruling out an appreciable contribution to free charge generation though hot carrier pathways. On the other hand, activation energies are on the order of the room temperature thermal energy for all studied blends. We conclude that charge generation in such devices proceeds through thermalized charge transfer states, and that thermal energy is sufficient to separate most of these states into free charges.

Contradictory models are being debated on the dominant pathways of charge generation in organic solar cells. Here Kurpiers et al. determine the activation energy for this fundamental process and reveal that the main channel is via thermalized charge transfer states instead of hot exciton dissociation.

Nonfullerene Acceptor Molecules for Bulk Heterojunction Organic Solar Cells

The bulk-heterojunction blend of an electron donor and an electron acceptor material is the key component in a solution-processed organic photovoltaic device. In the past decades, a p-type conjugated polymer and an n-type fullerene derivative have been the most commonly used electron donor and electron acceptor, respectively. While most advances of the device performance come from the design of new polymer donors, fullerene derivatives have almost been exclusively used as electron acceptors in organic photovoltaics. Recently, nonfullerene acceptor materials, particularly small molecules and oligomers, have emerged as a promising alternative to replace fullerene derivatives. Compared to fullerenes, these new acceptors are generally synthesized from diversified, low-cost routes based on building block materials with extraordinary chemical, thermal, and photostability. The facile functionalization of these molecules affords excellent tunability to their optoelectronic and electrochemical properties. Within the past five years, there have been over 100 nonfullerene acceptor molecules synthesized, and the power conversion efficiency of nonfullerene organic solar cells has increased dramatically, from ∼2% in 2012 to >13% in 2017. This review summarizes this progress, aiming to describe the molecular design strategy, to provide insight into the structure-property relationship, and to highlight the challenges the field is facing, with emphasis placed on most recent nonfullerene acceptors that demonstrated top-of-the-line photovoltaic performances. We also provide perspectives from a device point of view, wherein topics including ternary blend device, multijunction device, device stability, active layer morphology, and device physics are discussed.

Increases in the Charge Separation Barrier in Organic Solar Cells Due to Delocalization

Because of the low dielectric constant, charges in organic solar cells must overcome a strong Coulomb attraction in order to separate. It has been widely argued that intermolecular delocalization would assist charge separation by increasing the effective initial electron-hole separation in a charge-transfer state, thus decreasing their barrier to separation. Here we show that this is not the case: including more than a small amount of delocalization in models of organic solar cells leads to an increase in the free-energy barrier to charge separation. Therefore, if delocalization were to improve the charge separation efficiency, it would have to do so through nonequilibrium kinetic effects that are not captured by a thermodynamic treatment of the barrier height.

Quantum modeling of ultrafast photoinduced charge separation

Phenomena involving electron transfer are ubiquitous in nature, photosynthesis and enzymes or protein activity being prominent examples. Their deep understanding thus represents a mandatory scientific goal. Moreover, controlling the separation of photogenerated charges is a crucial prerequisite in many applicative contexts, including quantum electronics, photo-electrochemical water splitting, photocatalytic dye degradation, and energy conversion. In particular, photoinduced charge separation is the pivotal step driving the storage of sun light into electrical or chemical energy. If properly mastered, these processes may also allow us to achieve a better command of information storage at the nanoscale, as required for the development of molecular electronics, optical switching, or quantum technologies, amongst others. In this Topical Review we survey recent progress in the understanding of ultrafast charge separation from photoexcited states. We report the state-of-the-art of the observation and theoretical description of charge separation phenomena in the ultrafast regime mainly focusing on molecular- and nano-sized solar energy conversion systems. In particular, we examine different proposed mechanisms driving ultrafast charge dynamics, with particular regard to the role of quantum coherence and electron-nuclear coupling, and link experimental observations to theoretical approaches based either on model Hamiltonians or on first principles simulations.

Ultrafast Long-Range Charge Separation in Nonfullerene Organic Solar Cells

Rapid, long-range charge separation in polymer-fullerene organic solar cells (OSCs) enables electrons and holes to move beyond their Coulomb capture radius and overcome geminate recombination. Understanding the nature of charge generation and recombination mechanisms in efficient, nonfullerene-acceptor-based OSCs are critical to further improve device performance. Here we report charge dynamics in an OSC using a perylene diimide (PDI) dimer acceptor. We use transient absorption spectroscopy to track the time evolution of electroabsorption caused by the dipolar electric field generated between electron-hole pairs as they separate after ionization at the donor-acceptor interface. We show that charges separate rapidly (<1 ps) and that free charge carriers are generated very efficiently (∼90% quantum yield). However, in the PDI-based OSC, external charge extraction is impaired by faster nongeminate decay to the ground state and to lower-lying triplet states.

Lighting up long-range charge-transfer states by a localized plasmonic field

The long-range charge-transfer states in a donor-acceptor system exhibit well separated electron-hole pairs, but are often difficult to achieve by optical means owing to a very small overlap between the wave functions of the donor and acceptor. We have found that the introduction of a spatially confined plasmon can enhance the transition probability to the long-range charge-transfer states as it can effectively break the intrinsic symmetry selection rule imposed on the system. Meanwhile, the intensity borrowed from local excitations could also be selectively promoted, allowing the manipulation of the excited quantum states. In addition, our calculations reveal that the donor and acceptor moieties can be unambiguously visualized in real space by tip-enhanced resonance Raman images. These findings can benefit light-harvesting and also be readily extended to diverse optical processes.

Incoherent Pathways of Charge Separation in Organic and Hybrid Solar Cells

In this work, we investigate the exciton dissociation dynamics occurring at the donor:acceptor interface in organic and hybrid blends employed in the realization of photovoltaic cells. Fundamental differences in the charge separation process are studied with the organic semiconductor polymer poly(3-hexylthiophene) (P3HT) and either [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) or titanium dioxide (TiO₂) acting as the acceptor. By using ultrafast broad-band transient absorption spectroscopy with few-fs temporal resolution, we observe that in both cases the incoherent formation of free charges dominates the charge generation process. From the optical response of the polymer and by tracking the excited-state absorption, we extract pivotal similarities in the incoherent energy pathways that follow the impulsive excitation. On time scales shorter than 200 fs, we observe that the two acceptors display similar dynamics in the exciton delocalization. Significant differences arise only on longer time scales with only an impact on the overall photocarrier generation efficiency.

Effect of Infrared Pulse Excitation on the Bound Charge-Transfer State of Photovoltaic Interfaces

The nature and dynamics of the bound charge-transfer (CT) state in the exciton dissociation process in organic solar cells are of critical importance for the understanding of these devices. It was recently demonstrated that this state can be probed by a new experiment in which an infrared (IR) push-pulse is used to dissociate charges from the bound excited state. Here we proposed a simple quantum dynamics model to simulate the excitation of the IR pulse on the bound CT state with model parameters extracted from quantum chemical calculations. We show that the pulse dissociates the CT state following two different mechanisms: one, fairly expected, is the direct excitation of higher energy CT states leading to charge separation; the other, proposed here for the first time, is a rebound mechanism in which the negative charge is transferred in the opposite direction to form the neutral Frenkel exciton state from where it dissociates.

Intercalated vs Nonintercalated Morphologies in Donor-Acceptor Bulk Heterojunction Solar Cells: PBTTT:Fullerene Charge Generation and Recombination Revisited

In this Letter, we study the role of the donor:acceptor interface nanostructure upon charge separation and recombination in organic photovoltaic devices and blend films, using mixtures of PBTTT and two different fullerene derivatives (PC₇₀BM and ICTA) as models for intercalated and nonintercalated morphologies, respectively. Thermodynamic simulations show that while the completely intercalated system exhibits a large free-energy barrier for charge separation, this barrier is significantly lower in the nonintercalated system and almost vanishes when energetic disorder is included in the model. Despite these differences, both femtosecond-resolved transient absorption spectroscopy (TAS) and time-delayed collection field (TDCF) exhibit extensive first-order losses in both systems, suggesting that geminate pairs are the primary product of photoexcitation. In contrast, the system that comprises a combination of fully intercalated polymer:fullerene areas and fullerene-aggregated domains (1:4 PBTTT:PC₇₀BM) is the only one that shows slow, second-order recombination of free charges, resulting in devices with an overall higher short-circuit current and fill factor. This study therefore provides a novel consideration of the role of the interfacial nanostructure and the nature of bound charges and their impact upon charge generation and recombination.

Impact of interfacial molecular orientation on radiative recombination and charge generation efficiency

A long standing question in organic electronics concerns the effects of molecular orientation at donor/acceptor heterojunctions. Given a well-controlled donor/acceptor bilayer system, we uncover the genuine effects of molecular orientation on charge generation and recombination. These effects are studied through the point of view of photovoltaics—however, the results have important implications on the operation of all optoelectronic devices with donor/acceptor interfaces, such as light emitting diodes and photodetectors. Our findings can be summarized by two points. First, devices with donor molecules face-on to the acceptor interface have a higher charge transfer state energy and less non-radiative recombination, resulting in larger open-circuit voltages and higher radiative efficiencies. Second, devices with donor molecules edge-on to the acceptor interface are more efficient at charge generation, attributed to smaller electronic coupling between the charge transfer states and the ground state, and lower activation energy for charge generation.

Molecular orientation profoundly affects the performance of donor-acceptor heterojunctions, whilst it has remained challenging to investigate the detail. Using a controllable interface, Ran et al. show that the edge-on geometries improve charge generation at the cost of non-radiative recombination loss.

Organic Planar Heterojunctions: From Models for Interfaces in Bulk Heterojunctions to High-Performance Solar Cells

Recent progress regarding planar heterojunctions (PHJs) is reviewed, with respect to the fundamental understanding of the photophysical processes at the donor/acceptor interfaces in organic photovoltaic devices (OPVs). The current state of OPV research is summarized and the advantages of PHJs as models for exploring the relationship between organic interfaces and device characteristics described. The preparation methods and the characterization of PHJ structures to provide key points for the appropriate handling of PHJs. Next, we describe the effects of the donor/acceptor interface on each photoelectric conversion process are reviewed by examining various PHJ systems to clarify what is currently known and not known. Finally, it is discussed how we the knowledge obtained by studies of PHJs can be used to overcome the current limits of OPV efficiency.

Efficient Charge Separation of Cold Charge-Transfer States in Organic Solar Cells Through Incoherent Hopping

We demonstrate that efficient and nearly field-independent charge separation of electron-hole pairs in organic planar heterojunction solar cells can be described by an incoherent hopping mechanism. Using kinetic Monte Carlo simulations that include the effect of on-chain delocalization as well as entropic contributions, we simulate the dissociation of the charge-transfer state in polymer-fullerene bilayer solar cells. The model further explains experimental results of almost field independent charge separation in bilayers of molecular systems with fullerenes and provides important guidelines at the molecular level for maximizing the efficiencies of organic solar cells. Thus, utilizing coherent phenomena is not necessarily required for highly efficient charge separation in organic solar cells.

Direct S0→T Excitation of a Conjugated Polymer Repeat Unit: Unusual Spin-Forbidden Transitions Probed by Time-Resolved Electron Paramagnetic Resonance Spectroscopy

A detailed understanding of the electronic structure of semiconducting polymers and their building blocks is essential to develop efficient materials for organic electronics. (Time-resolved) electron paramagnetic resonance (EPR) is particularly suited to address these questions, allowing one to directly detect paramagnetic states and to reveal their spin-multiplicity, besides its clearly superior resolution compared to optical methods. We present here evidence for a direct S₀→T optical excitation of distinct triplet states in the repeat unit of a conjugated polymer used in organic photovoltaics. These states differ in their electronic structure from those populated via intersystem crossing from excited singlet states. This is an additional and so far unconsidered route to triplet states with potentially high impact on efficiency of organic electronic devices.

Exciton transport in π-conjugated polymers with conjugation defects

Conjugation defects can be divided into energy barriers and energy wells energetically to affect exciton transport.

Light Harvesting for Organic Photovoltaics

The field of organic photovoltaics has developed rapidly over the last 2 decades, and small solar cells with power conversion efficiencies of 13% have been demonstrated. Light absorbed in the organic layers forms tightly bound excitons that are split into free electrons and holes using heterojunctions of electron donor and acceptor materials, which are then extracted at electrodes to give useful electrical power. This review gives a concise description of the fundamental processes in photovoltaic devices, with the main emphasis on the characterization of energy transfer and its role in dictating device architecture, including multilayer planar heterojunctions, and on the factors that impact free carrier generation from dissociated excitons. We briefly discuss harvesting of triplet excitons, which now attracts substantial interest when used in conjunction with singlet fission. Finally, we introduce the techniques used by researchers for characterization and engineering of bulk heterojunctions to realize large photocurrents, and examine the formed morphology in three prototypical blends.

Entropy and Disorder Enable Charge Separation in Organic Solar Cells

Although organic heterojunctions can separate charges with near-unity efficiency and on a subpicosecond time scale, the full details of the charge-separation process remain unclear. In typical models, the Coulomb binding between the electron and the hole can exceed the thermal energy k_BT by an order of magnitude, suggesting that it is impossible for the charges to separate before recombining. Here, we consider the entropic contribution to charge separation in the presence of disorder and find that even modest amounts of disorder have a decisive effect, reducing the charge-separation barrier to about k_BT or eliminating it altogether. Therefore, the charges are usually not thermodynamically bound at all and could separate spontaneously if the kinetics otherwise allowed it. Our conclusion holds despite the worst-case assumption of localized, thermalized carriers and is only strengthened if mechanisms like delocalization or "hot" states are also present.