Efficient Charge Generation Research Articles

Decoding Charge Recombination through Charge Generation in Organic Solar Cells

The in‐depth understanding of charge carrier photogeneration and recombination mechanisms in organic solar cells is still an ongoing effort. In donor:acceptor (bulk) heterojunction organic solar cells, charge photogeneration and recombination are inter‐related via the kinetics of charge transfer states—being singlet or triplet states. Although high‐charge‐photogeneration quantum yields are achieved in many donor:acceptor systems, only very few systems show significantly reduced bimolecular recombination relative to the rate of free carrier encounters, in low‐mobility systems. This is a serious limitation for the industrialization of organic solar cells, in particular when aiming at thick active layers. Herein, a meta‐analysis of the device performance of numerous bulk heterojunction organic solar cells is presented for which field‐dependent photogeneration, charge carrier mobility, and fill factor are determined. Herein, a “spin‐related factor” that is dependent on the ratio of back electron transfer of the triplet charge transfer (CT) states to the decay rate of the singlet CT states is introduced. It is shown that this factor links the recombination reduction factor to charge‐generation efficiency. As a consequence, it is only in the systems with very efficient charge generation and very fast CT dissociation that free carrier recombination is strongly suppressed, regardless of the spin‐related factor.

Enhancing Open-Circuit Voltage and Charge Transportation for Ternary Organic Solar Cells With Energy Cascade Double Nonfullerene Acceptors

An effective double nonfullerene acceptors ternary organic solar cell (OSC) was systemically investigated to improve the performance of OSCs by doping higher lowest unoccupied molecular orbital (LUMO) energy level nonfullerene acceptor ITIC-Th into the PBDB-T-SF: IT-4F active layer. Compared with binary OSCs, the power conversion efficiency of the optimal ternary OSCs with 20 wt% ITIC-Th content is improved from 9.84% to 11.0%, which was attributed to simultaneous improvements in the open-circuit voltage ( V oc) and fill factor with a slightly reduction in short-current density ( J sc). The optimal devices show appropriate cascaded LUMO energy level, small aggregation phases of film morphology and higher hole–electron mobility, which are favorable to charge dissociation and significantly beneficial for efficient charge generation and collection. Moreover, under 20 wt% ITIC-Th, the impedance spectra analysis reveals that the smoother surface of the active layer presents a good Ohmic contact for better charge transportation by establishing more potential pathways.

Photodetection efficiency enhancement of IR sensitive organic photodetectors via triplet energy transfer

Development of the low optical gap organic semiconductors that can emit and sense light in the near to mid IR is an important research area. Photodetection in the IR region comes with its unique challenges. As the optical gap is lowered, often a back electron transfer event from the charge-transfer state of triplet character (3CT) to the molecular triplet state (T1) of photoexcited donor works as a terminal loss channel. Here, we propose a new mechanism for enhancing free charge generation efficiency by attaining a desired control over the spin-dependent recombination pathways in low optical gap polymer-fullerene systems. In carefully designed organic cascades of Pentacene/C60:PCPDTBT: 70-PCBM, we show that 3CT mediated Dexter type triplet energy transfer (TET) process from PCPDTBT to pentacene is instrumental in enhancing external quantum efficiency (EQE) in the IR region of PCPDTBT. Our results show that EQE in the IR region of this multilayer cascade benefits from triplet exciton recycling by at least ˜20%. The proposed TET process is generic in allowing manipulation of spin-dependent recombination pathways in favor of efficient photocurrent generation in low optical gap organic diodes.

Influences of Non-fullerene Acceptor Fluorination on Three-Dimensional Morphology and Photovoltaic Properties of Organic Solar Cells.

Fluorination of conjugated molecules has been established as an effective structural modification strategy to influence properties and has attracted extensive attention in organic solar cells (OSCs). Here, we have investigated optoelectronic and photovoltaic property changes of OSCs made of polymer donors with the non-fullerene acceptors (NFAs) ITIC and IEICO and their fluorinated counterparts IT-4F and IEICO-4F. Device studies show that fluorinated NFAs lead to reduced Voc but increased Jsc and fill-factor (FF), and therefore, the ultimate influence to efficiency depends on the compensation of Voc loss and gains of Jsc and FF. Fluorination lowers energy levels of NFAs, reduces their electronic band gaps, and red-shifts the absorption spectra. The impact of fluorination on the molecular order depends on the specific NFA, and the conversion of ITIC to IT-4F reduces the structural order, which can be reversed after blending with the donor PBDB-T. Contrastingly, IEICO-4F presents stronger π-π stacking after fluorination from IEICO, and this is further strengthened after blending with the donor PTB7-Th. The photovoltaic blends universally present a donor-rich surface region which can promote charge transport and collection toward the anode in inverted OSCs. The fluorination of NFAs, however, reduces the fraction of donors in this donor-rich region, consequently encouraging the intermixing of donor/acceptor for efficient charge generation.

Synthesis of 2,2′-[2,2′-(arenediyl)bis(anthra[2,3-b]thiophene-5,10-diylidene)]tetrapropanedinitriles and their performance as non-fullerene acceptors in organic photovoltaics

Abstract Three novel molecules based on thiophene-fused 11,11,12,12-tetracyano-9,10-anthraquinodimethane (TCAQ) units linked through functionalized π-conjugated arene bridges are synthesized by three-step sequence of Sonagashira coupling, double thioannulation and Knoevenagel condensation. The energy of their frontier orbitals shows that these molecules are suitable for replacing fullerenes as electron acceptors in composites with conjugated donor polymers used in organic photovoltaics (OPV). The composite of polymer donor PCDTBT with fluorene-linked thiophene-fused TCAQs demonstrates good solubility in organic solvents and bulk heterojunction morphology with characteristic donor and acceptor domain size of several tens of nanometers, favourable for photoelectric conversion. This composite also exhibits prominent light-induced EPR signal indicating efficient photoinduced free charge generation. OPV devices based on this composite were fabricated by vacuum-free method. They show photoelectric conversion with maximum PCE of 0.71%.

Measuring Energetic Disorder in Organic Semiconductors Using the Photogenerated Charge-Separation Efficiency

Quantifying energetic disorder in organic semiconductors continues to attract attention because of its significant impact on the transport physics of these technologically important materials. Here, we show that the energetic disorder of organic semiconductors can be determined from the relationship between the internal quantum efficiency of charge generation and the frequency of the incident light. Our results for a number of materials suggest that energetic disorder in organic semiconductors could be greater than previously reported, and we advance ideas as to why this may be the case.

Anatomy of the energetic driving force for charge generation in organic solar cells

Eliminating the excess energetic driving force in organic solar cells leads to a smaller energy loss and higher device performance; hence, it is vital to understand the relation between the interfacial energetics and the photoelectric conversion efficiency. In this study, we systematically investigate 16 combinations of four donor polymers and four acceptors in planar heterojunction. The charge generation efficiency and its electric field dependence correlate with the energy difference between the singlet excited state and the interfacial charge transfer state. The threshold energy difference is 0.2 to 0.3 eV, below which the efficiency starts dropping and the charge generation becomes electric field-dependent. In contrast, the charge generation efficiency does not correlate with the energy difference between the charge transfer and the charge-separated states, indicating that the binding of the charge pairs in the charge transfer state is not the determining factor for the charge generation.

47‐3: AMQLED Display with Highly Efficient Oxide N‐P Charge Generation Junction

We have developed high efficiency tandem quantum‐dot light‐emitting diode (QLED) by using oxide n‐p charge generation junction (CGJ). The tandem QLED with oxide n‐p CGJ exhibits the current efficiency as high as over 120 cd A−1. The oxide n‐p CGJ shows good Ohmic behavior under forward and reverse bias. It is obvious that the reverse current density is nearly equal to the forward current density, which suggest the efficient charge generation in the oxide n‐p CGJ. In this work, we demonstrated a 4‐inch AMQLED display using high efficiency tandem QLED with oxide n‐p CGJ.

P3HT Molecular Weight Determines the Performance of P3HT:O‐IDTBR Solar Cells

Large‐scale production of organic solar modules requires low‐cost and reliable materials with reproducible batch‐to‐batch properties. In case of polymers, their (photo)physical properties depend strongly on the polymers’ molecular weight (MW). Herein, the impact of the MW of the donor polymer poly(3‐hexylthiophene) (P3HT) on the photophysics is studied in blends with a recently developed rhodanine‐endcapped indacenodithiophene nonfullerene acceptor (IDTBR), a bulk heterojunction (BHJ) system that potentially fulfills the aforementioned criteria for large‐scale production. It is found that the power conversion efficiency (PCE) increases when the weight‐average MW is increased from 17 kDa (PCE: 4.0%) to 34 kDa (PCE: 6.6%), whereas a further increase in MW leads to a reduced PCE of 4.4%. It is demonstrated that the charge generation efficiency, as estimated from time‐delayed collection field experiments, varies with the P3HT MW and is the reason for the differences in photocurrent and device performance. These findings provide insight into the fundamental photophysical reasons of the MW dependence of the PCE, which is taken into account when using polymer‐based nonfullerene acceptor blends in solar cell devices and modules.

Bio-inspired hierarchical hetero-architectures of in-situ C-doped g-C3N4 grafted on C, N co-doped ZnO micro-flowers with booming solar photocatalytic activity

The in-situ hierarchical heterojunction photocatalyst consists of C-doped g-C3N4 (CCN) grafted on the C, N co-doped ZnO were successfully realized via simple bio-template hydrothermal approach. The resultants hierarchical heterojunction photocatalyst exhibited excellent UV to the near infrared absorption capability. The electrochemical analysis and photoluminescence spectroscopy revealed that the hierarchical heterojunction photocatalyst possessed excellent charge generation and separation efficiency. The resultant hierarchical heterojunction photocatalyst exhibited remarkable photocatalytic performance in the photodegradation of bisphenol A and photocatalytic hydrogen evolution under simulated solar irradiation. The enhancement of photocatalytic performance was mainly attributed to the combined effect of hierarchical morphology, in-situ doping, and heterojunction formation.

14.7% Efficiency Organic Photovoltaic Cells Enabled by Active Materials with a Large Electrostatic Potential Difference.

Although significant improvements have been achieved for organic photovoltaic cells (OPVs), the top-performing devices still show power conversion efficiencies far behind those of commercialized solar cells. One of the main reasons is the large driving force required for separating electron-hole pairs. Here, we demonstrate an efficiency of 14.7% in the single-junction OPV by using a new polymer donor PTO2 and a nonfullerene acceptor IT-4F. The device possesses an efficient charge generation at a low driving force. Ultrafast transient absorption measurements probe the formation of loosely bound charge pairs with extended lifetime that impedes the recombination of charge carriers in the blend. The theoretical studies reveal that the molecular electrostatic potential (ESP) between PTO2 and IT-4F is large, and the induced intermolecular electric field may assist the charge generation. The results suggest OPVs have the potential for further improvement by judicious modulation of ESP.

Effect of Charge-Transfer State Energy on Charge Generation Efficiency via Singlet Fission in Pentacene-Fullerene Solar Cells.

Singlet fission in pentacene creates two triplet excitons per absorbed photon. In a solar cell, each triplet can generate an electron–hole pair, and hence, external quantum efficiencies exceeding 100% have been reported for pentacene–fullerene solar cells. The energetics of this process are intriguing because the minimum photon energy loss, defined as the energy difference between the (triplet) exciton state and the open-circuit voltage, is less than 0.5 eV and distinctively smaller than that in most organic donor–acceptor solar cells. To investigate the energetics of this process, we analyze the effect of the energy of the lowest unoccupied molecular orbital (LUMO) for different fullerene derivatives. With the LUMO energy becoming less negative, the open-circuit voltage increases and charge generation decreases. For all but one of the fullerenes tested, the charge-transfer state energy is distinctively higher than the pentacene triplet energy, revealing that charge generation via singlet fission is actually endergonic. An elementary Marcus model for the rate of electron transfer provides a qualitative description of the experimental trends, in accordance with an endergonic charge transfer. Considering that charge generation from triplet states is endergonic, involvement of pentacene singlet states, either from direct photoexcitation or via triplet–triplet annihilation, cannot be excluded.

Nonwoven Polyethylene Terephthalate Paper Loaded with Enzyme Coupled Multiwall Carbon Nanotubes for Superior Photocatalytic Activity for Water Remediation

A highly photocatalytic, carbon coated nonwoven polyethylene terephthalate paper (CCPETP) is developed by using the wet-laid method for photodegradation of dye effluents from the textile wastewater treatment. The designed nonwoven PET paper is coated with Multiwall walled carbon nanotubes (MWCNT) catalyst coupled with cationised enzyme agent. Three different types of enzymatic coupling agents were used i.e. Lipase, Glucose oxidase, and Laccase, respectively. Enzymes are generally globular proteins, packed with amino complex N-terminals, oxygenated amines and hydroxyl atoms. The sequence of the amino acids specifies the structure which in turn determines the catalytic activity of the enzyme. The cationization of the enzyme under acidic condition (∼pH 3), detach the amino/oxygen complex structure from their core structure and encapsulate over MWCNT surface. The coupled enzyme dissipates functional oxygenated amines, pyrrolic, pyridinic, graphitic and quaternary type of nitrogen contents. The attachment of functionalized surface groups without forming any defect-rich vacancy promotes efficient charge generation and photocatalytic degradation of methylene blue dye (MB) particulates. Besides, the suggested low-cost porous PET nonwoven paper helps to absorb a large number of dye molecules for efficient dye degradation. The excellent photocatalytic activity of the proposed thin CCPETP photocatalyst is mainly attributed to its specific coupled enzyme, high adsorbing capacity of PET paper, and low recombination of the photogenerated electrons and holes. The optimal loading content of enzyme coupled MWCNT over nonwoven PET paper decolorized ∼99.0% methylene blue (MB) dye in 100 min. The excellent dye degradation efficiency of this low-cost material is attributed its surface characteristics and high absorbing properties. The synergistic effect of cluster active sites and the functional amine/oxygen surface groups promoting the generation of OH ions for MB degradation. The fabricated nonwoven paper is expected for large industrial applications and will provide a generic route towards the fabrication of textile structured photocatalyst.

Large Crystalline Domains and an Enhanced Exciton Diffusion Length Enable Efficient Organic Solar Cells

We studied crystallinity and exciton harvesting in bulk heterojunctions of the semiconducting polymer PffBT4T-2OD and electron acceptor PC71BM that are used to make highly efficient organic solar cells. Grazing incidence wide-angle X-ray scattering shows that the size of crystalline domains of PffBT4T-2OD increases to ∼18 nm in photovoltaic blends upon thermal annealing at 100 °C for 5 min. These domains are larger than the typical exciton diffusion lengths in conjugated polymers. Time-resolved fluorescence measurements show that the exciton diffusion length in PffBT4T-2OD increases from ∼14 to ∼24 nm upon thermal annealing, which enables efficient charge generation in blends with large domains. Solar cells prepared using thermally annealed blends show higher photocurrents, open circuit voltages, and fill factors compared to unannealed blends, which indicates reduced recombination losses. Our results demonstrate the advantages of large crystalline domains in organic photovoltaics, providing exciton diffusion is sufficient.

Favorable Mixing Thermodynamics in Ternary Polymer Blends for Realizing High Efficiency Plastic Solar Cells

AbstractTernary blends with broad spectral absorption have the potential to increase charge generation in organic solar cells but feature additional complexity due to limited intermixing and electronic mismatch. Here, a model system comprising the polymers poly[5,5‐bis(2‐butyloctyl)‐(2,2‐bithiophene)‐4,4‐dicarboxylate‐alt‐5,5‐2,2‐bithiophene] (PDCBT) and PTB7‐Th and PC70BM as an electron accepting unit is presented. The power conversion efficiency (PCE) of the ternary system clearly surpasses the performance of either of the binary systems. The photophysics is governed by a fast energy transfer process from PDCBT to PTB7‐Th, followed by electron transfer at the PTB7‐Th:fullerene interface. The morphological motif in the ternary blend is characterized by polymer fibers. Based on a combination of photophysical analysis, GIWAXS measurements and calculation of the intermolecular parameter, the latter indicating a very favorable molecular affinity between PDCBT and PTB7‐Th, it is proposed that an efficient charge generation mechanism is possible because PTB7‐Th predominantly orients around PDCBT filaments, allowing energy to be effectively relayed from PDCBT to PTB7‐Th. Fullerene can be replaced by a nonfullerene acceptor without sacrifices in charge generation, achieving a PCE above 11%. These results support the idea that thermodynamic mixing and energetics of the polymer–polymer interface are critical design parameter for realizing highly efficient ternary solar cells with variable electron acceptors.

Management of the crystallization in two-dimensional perovskite solar cells with enhanced efficiency within a wide temperature range and high stability

Control over the crystallization in quantum well Ruddlesden-Popper phase halide perovskites is vitally important for the photovoltaic performance. Through managing the molecular stacking in 2-dimentioanl BA2MA3Pb4I13 (n = 4) perovskites based on PTAA hole transporting layers, we achieve enhanced vertical crystal orientations in BA2MA3Pb4I13 polycrystalline films, leading to a champion power conversion efficiency (PCE) of 14.3% (n ≤ 4) with negligible hysteresis in PTAA based p-i-n perovskite solar cells. The enhanced PCE is ascribed to the suppression on change recombination associated with an expedited charge extraction, revealed by transient opto-electrical analyses. Benefitted from the enhanced molecular arrangement revealed by GIWAXS, efficient charge generation at low temperatures (T) is enabled, leading to a negative T-dependence of efficiency in the hot-cast device, showing a peak PCE of 15.0% at 210 K. This trend is likely correlated to the reduced potential barriers in the quantum wells with which the detrapping of photo-carriers is facilitated at smaller thermal energy. Contrastingly, the solar cells with more randomly oriented crystals are found to suffer more from these unfavorable barriers, resulting in decreased PCEs with lowered T. Our findings highlight the opportunity through crystallization management coupled with interface engineering to achieve high efficiency and stable 2D perovskite solar cells within a wide T-window.

Analysis of the charge transfer and separation in electrically doped organic semiconductors by electron spin resonance spectroscopy

Abstract We investigated the charge generation mechanism of electrically doped organic semiconductors (OSs) by electron spin resonance (ESR) analysis. ESR spectroscopy was used to successfully evaluate the radical density of p-doped OSs to estimate the charge transfer efficiency (CTE) of various doped systems. The results showed that the CTE is efficient close to 100% if the dopant molecules are homogenously dispersed and the energy difference (ΔE) between the highest occupied molecular orbital (HOMO) level of the host molecule and lowest unoccupied molecular orbital (LUMO) level of the p-dopant is large. The charge separation efficiency to form free carriers from the radicals is rather low (less than 12% in this study) and is a dominant factor controlling the charge generation efficiency (CGE). An organic dopant molybdenum tris[1,2-bis(trifluoromethyl)ethane-1,2-dithiolene] turns out to be an efficient dopant with the CGE of 9.7% due to high CTE originating from homogenous dispersion of the organic p-dopants and low LUMO level, i.e., large ΔE.

Selective Photooxidation of Amines and Sulfides Triggered by a Superoxide Radical Using a Novel Visible-Light-Responsive Metal-Organic Framework.

Photocatalysis is an efficient and sustainable approach to convert solar energy into chemical energy, simultaneously supplying valuable chemicals. In this study, a novel metal-organic framework (MOF) compound is constructed from anthracene-based organic linkers, which shows visible-light absorption and efficient photoinduced charge generation property. It was applied for triggering photooxidation of benzylamines and sulfides in the presence of environmental benign oxidants of molecular oxygen or hydrogen peroxide. Results show that it is a highly selective photocatalyst for oxidation reactions to produce valuable imines or sulfoxides. We further investigate the underlying mechanism for these photocatalytic reactions by recognizing reactive oxygen species in the reactions. It has been demonstrated that the superoxide radical (O2•-), generated by electron transfer from a photoexcited MOF to oxidants, serves as the main active species for the oxidations. The work demonstrates the great potential of photoactive MOFs for the transformation of organic chemicals into valuable complexes.

Efficient Quaternary Organic Solar Cells with Parallel‐Alloy Morphology

AbstractTwo compatible donors (PBDB‐T and PTB7‐Th) and two miscible acceptors (ITIC and FOIC) are employed to deliver a parallel‐alloy morphology model in non‐fullerene‐based quaternary organic solar cells. PBDB‐T and PTB7‐Th form a parallel link with a slight adjustment of molecular packing into enhanced face‐on crystallites while ITIC disperses into discontinuous FOIC microcrystal regions to form continuous and ordered alloy‐like acceptor phases. Characterization of blend morphology highlights the parallel‐alloy model—enabled by the introduction of PBDB‐T and ITIC, which contributes to improved molecular packing and reduced domain size resulting in efficient charge generation and consistent transport channels. This successful parallel‐alloy quaternary blend morphology demonstrates an enhanced optical absorption, optimized domain size, and nanostructures toward simultaneous improvement in charge transfer and transport. Therefore, a power conversion efficiency of 12.52% is realized for a quaternary device which is 6% higher than the ternary device (PBDB‐T:PTB7‐Th:FOIC) and 12% higher than the binary device (PTB7‐Th:FOIC). Domination of quaternary devices over ternary and binary blends, which is another feasible way to realize highly efficient devices through further investigation of quaternary OSCs, is presented.

Conjugated Block Copolymers as Model Systems to Examine Mechanisms of Charge Generation in Donor–Acceptor Materials

AbstractFully conjugated donor–acceptor block copolymers are established as model systems to elucidate fundamental mechanisms of photocurrent generation in organic photovoltaics. Using analysis of steady‐state photoluminescence quenching, exciton dissociation to a charge transfer state within individual block copolymer chains is quantified. By making a small adjustment to the conjugated backbone, the electronic properties are altered enough to disrupt charge transfer almost entirely. Strong intermolecular coupling of the electron donor is introduced by synthesizing block copolymer nanoparticles. Transient absorption spectroscopy is used to monitor charge generation in block copolymer isolated chains and nanoparticles. While efficient charge transfer is observed in isolated chains, there is no indication of complete charge separation. In the nanoparticles, long‐lived polarons are observed as early as ≈15 ns. Thus, aggregation of electron donors can facilitate efficient charge generation.