Efficient Charge Generation Research Articles

Photophysics and Charge Generation in Low Energy-offset Blends for Organic Solar Cells.

The power conversion efficiency of organic solar cells has seen a huge improvement in recent years with state-of-the-art solar cells showcasing efficiencies of ∼18.5 %, which is approaching the performance of inorganic and hybrid-perovskite solar cell technologies. This improvement can be mainly attributed to the discovery of highly efficient donor:acceptor blends with a near-zero energetic offset between the molecular orbital levels of the donor and the acceptor component. A distinctive feature of the high efficiency, low energy-offset blends is that they exhibit a concomitant increase in the short-circuit density and the open-circuit voltage of the solar cell. High open-circuit voltage results from the reduced photon energy loss in the exciton dissociation step, while a high short-circuit current density can be attributed to an efficient charge generation process. The reasons for the efficient exciton dissociation and subsequent separation of Coulomb bound electron-hole pair at negligible driving force is not well understood and, in this short review, we highlight recent results which shed light on the mechanism of charge generation in low energy-offset blends.

Metal-free four-in-one modification of g-C3N4 for superior photocatalytic CO2 reduction and H2 evolution

Utilization of g-C3N4 as a single photocatalyst material without combination with other semiconductor remains challenging. Herein, we report a facile green method for synthesizing a metal free modified g-C3N4 photocatalyst. The modification process combines four different strategies in a one-pot thermal reaction: non-metal doping, porosity generation, functionalization with amino groups, and thermal oxidation etching. The as-prepared amino-functionalized ultrathin nanoporous boron-doped g-C3N4 exhibited a high specific surface area of 143.2 m2 g−1 which resulted in abundant adsorption sites for CO2 and water molecules. The surface amino groups act as Lewis basic sites to adsorb acidic CO2 molecules, which can also serve as active sites to facilitate hydrogen generation. Besides, the simultaneous use of ammonium chloride as a dynamic gas bubble template along with thermal oxidation etching efficiently boosts the delamination of the g-C3N4 layers to produce ultrathin sheets; this leads to stronger light–matter interactions and efficient charge generation. Consequently, the newly modified g-C3N4 achieved selective gas-phase CO2 reduction into CO with a production yield of 21.95 µmol g−1, in the absence of any cocatalyst. Moreover, a high hydrogen generation rate of 3800 µmol g-1h−1 and prominent apparent quantum yield of 10.6% were recorded. This work opens up a new avenue to explore different rational modifications of g-C3N4 nanosheets for the efficient production of clean energy.

Chemical Design Rules for Non‐Fullerene Acceptors in Organic Solar Cells

AbstractEfficiencies of organic solar cells have practically doubled since the development of non‐fullerene acceptors (NFAs). However, generic chemical design rules for donor‐NFA combinations are still needed. Such rules are proposed by analyzing inhomogeneous electrostatic fields at the donor–acceptor interface. It is shown that an acceptor–donor–acceptor molecular architecture, and molecular alignment parallel to the interface, results in energy level bending that destabilizes the charge transfer state, thus promoting its dissociation into free charges. By analyzing a series of PCE10:NFA solar cells, with NFAs including Y6, IEICO, and ITIC, as well as their halogenated derivatives, it is suggested that the molecular quadrupole moment of ≈75 Debye Å balances the losses in the open circuit voltage and gains in charge generation efficiency.

Light extraction in tandem organic light emitting diodes

Since the invention of organic light emitting diodes (OLEDs), great research efforts have been dedicated to improving their efficiency and lifetime. For high-brightness applications, tandem OLED structures have advantages because of the lower current densities required to achieve high brightness. With the successful development of highly efficient charge generation layers, high brightness tandem OLEDs are used in displays and lighting. However, the major challenge for tandem OLEDs is the low light extraction efficiency, because about 50% of the light is trapped inside the device as waveguide modes. In this Perspective, we first review the recent works done on light extraction, analyze different waveguide mode extraction structures, and then identify the key factors determining the extraction efficiencies in tandem OLEDs.

Light-Induced Charge Accumulation in PTCDI/Pentacene/Ag(111) Heterojunctions

The incorporation of singlet fission (SF) chromophores in solar cells is expected to bring significant increases in the power conversion efficiency thanks to multiexciton generation. However, efficient charge generation in the device is determined by the energy level alignment (ELA) between the active materials, which should favor exciton transport and separation under illumination. By combining ultraviolet photoemission spectroscopy and optical differential reflectance measurements, we determine the ELA in a prototypical SF heterojunction between pentacene (Pc) and perylene-tetracarboxylic-diimide (PTCDI) grown on Ag(111). Time-resolved X-ray photoelectron spectroscopy on such a system reveals light-induced modifications of the ELA; by measuring the transient shift of the core level photoemission lines we observe an accumulation of long-lived holes in the PTCDI within the first hundred picoseconds after the optical pump.

A conjugated donor-acceptor block copolymer enables over 11% efficiency for single-component polymer solar cells

<h2>Summary</h2> Covalently linking donor (D) and acceptor (A) moieties into single material can be an effective strategy to enable efficient exciton dissociation and charge generation. Herein, we synthesized a conjugated D-A block copolymer (PBDB-T-<i>b</i>-PYT), in which a donor block (PBDB-T) was connected to an acceptor block (PYT). Compared with the bulk heterojunction (BHJ) blend mixture containing PBDB-T and PYT with high molecular weights, more efficient exciton dissociation and charge transfer (CT) were observed in the single-component PBDB-T-<i>b</i>-PYT films. Impressively, a single-material organic solar cell (SMOSC) fabricated from PBDB-T-<i>b</i>-PYT alone exhibited a power conversion efficiency (PCE) of 11.32% and a certificated PCE of 10.8%. In addition, higher morphological stability and lower energy loss were also observed in the PBDB-T-<i>b</i>-PYT SMOSC compared with the control PBDB-T:PYT BHJ devices. The results manifest that our newly designed PBDB-T-<i>b</i>-PYT provides a promising design strategy for D-A block copolymers for highly efficient and stable SMOSCs.

Efficient organic solar cells

Solar Cells There is currently considerable interest in the development of organic solar cells (OSCs) because of the emergence of so-called non-fullerene acceptors, which has pushed the power conversion efficiencies of OSCs close to those of inorganic and hybrid semiconductor solar cells. Chen et al. combined experimental and theoretical study of more than 30 non-fullerene OSC devices to demonstrate the key electronic structure parameters of the donor-acceptor blends, establish the relationships between these parameters and the nonradiative voltage losses, and show that these losses can be reduced without sacrificing the charge-generation efficiencies. This work opens the way for the future rational design of high-efficiency OSC materials. Nat. Energy 10.1038/s41560-021-00843-4 (2021).

Small Exciton Binding Energies Enabling Direct Charge Photogeneration Towards Low‐Driving‐Force Organic Solar Cells

AbstractOrganic solar cells (OSCs) with nonfullerene acceptors (NFAs) exhibit efficient charge generation under small interfacial energy offsets, leading to over 18 % efficiency for the single‐junction devices based on the state‐of‐the‐art NFA of Y6. Herein, to reveal the underlying charge generation mechanisms, we have investigated the exciton binding energy (Eb) in Y6 by a joint theoretical and experimental study. The results show that owing to strong charge polarization effects, Y6 has remarkable small Eb of −0.11–0.15 eV, which is even lower than perovskites in many cases. Moreover, it is peculiar that the photoluminescence is enhanced with temperature, and the energy barrier for separating excitons into charges is evidently lower than the thermal energy according to the temperature dependence of photoluminescence, manifesting direct photogeneration of charge carriers enabled by weak Eb in Y6. Thus, charge generation in NFA‐based OSCs shows little dependence on interfacial driving forces.

Small Exciton Binding Energies Enabling Direct Charge Photogeneration Towards Low-Driving-Force Organic Solar Cells.

Organic solar cells (OSCs) with nonfullerene acceptors (NFAs) exhibit efficient charge generation under small interfacial energy offsets, leading to over 18 % efficiency for the single-junction devices based on the state-of-the-art NFA of Y6. Herein, to reveal the underlying charge generation mechanisms, we have investigated the exciton binding energy (Eb ) in Y6 by a joint theoretical and experimental study. The results show that owing to strong charge polarization effects, Y6 has remarkable small Eb of -0.11-0.15 eV, which is even lower than perovskites in many cases. Moreover, it is peculiar that the photoluminescence is enhanced with temperature, and the energy barrier for separating excitons into charges is evidently lower than the thermal energy according to the temperature dependence of photoluminescence, manifesting direct photogeneration of charge carriers enabled by weak Eb in Y6. Thus, charge generation in NFA-based OSCs shows little dependence on interfacial driving forces.

Characterizing the electro-optical properties of polyaniline/poly(vinyl acetate) composite films as-synthesized through chemical route

In this study, the conductive polymer polyaniline/poly(vinyl acetate) PANi/PVAc composite thin films were synthesized by chemical oxidative polymerization method with ammonium per sulfate​ as an oxidizing agent in methanol. The as-synthesized composite were characterized by X-ray diffraction (XRD), Scanning electron microscopy (SEM), ultraviolet–visible (UV–Vis) spectroscopy and photoluminescence (PL) measurement. The temperature dependence of DC electrical conductivity measurements were carried out through a two probe technique. The broad peak was found in the PL spectra of the obtained composite at around 450 nm, suggested that the obtained composite retains more charge generation efficiency. The enhancement in the optical and electrical properties of PANi/PVAc composite makes it as a novel candidate in various optoelectronic applications.

Optimizing side chains on different nitrogen aromatic rings achieving 17% efficiency for organic photovoltaics

Balancing charge generation and low energy loss (Eloss), especially in the wide spectral absorption region is critical to obtain high-performance organic photovoltaics (OPVs). Therefore, Y11-M and Y11-EB are designed and synthesized through modifying alkyl chains on different nitrogen aromatic rings of the reported non-fullerene acceptor Y11. Although all the molecules have almost similar low band-gap (around 1.30 eV), Y11-M and Y11-EB exhibit wider absorption in 410–870 nm region. Eventually, the conventional devices based on Y11-M and Y11-EB possess more efficient charge generation with low Eloss (around 0.44 eV). In addition, outstanding efficiencies of 16.64% and 17.15% with the fill factor of 76.15% and 74.73% are obtained in PM6:Y11-M and PM6:Y11-EB-based devices, both higher than Y11:PM6. The results highlight the importance of rational alkyl chains optimization, and a good structure-property relationship is established as well.

Charge Transfer in Conjugated Polymer Mxene Interface

Optimization of charge generation and charge transport is crucial for the fabrication of highly efficient polymer solar cells. In the present work, using ultrafast pump-probe spectroscopy and atomic force microscopy, we showed the efficient charge generation and transport at polymer blend MXene interface compared to PEDOT:PSS interface. We observed long charge carrier lifetime, strong photoluminescence of donors and acceptors and higher current on PBDB:ITIC thin film prepared on ITO substrate with MXene. Our study provides the guidelines for the fabrication of inexpensive, flexible, and highly flexible polymer solar cells by interface tuning.

Efficient Charge Transport Enables High Efficiency in Dilute Donor Organic Solar Cells.

The donor/acceptor weight ratio is crucial for photovoltaic performance of organic solar cells (OSCs). Here, we systematically investigate the photovoltaic behaviors of PM6:Y6 solar cells with different stoichiometries. It is found that the photovoltaic performance is tolerant to PM6 contents ranging from 10 to 60 wt %. Especially an impressive efficiency over 10% has been achieved in dilute donor solar cells with 10 wt % PM6 enabled by efficient charge generation, electron/hole transport, slow charge recombination, and field-insensitive extraction. This raises the question about the origin of efficient hole transport in such dilute donor structure. By investigating hole mobilities of PM6 diluted in Y6 and insulators, we find that effective hole transport pathway is mainly through PM6 phase in PM6:Y6 blends despite with low PM6 content. The results indicate that a low fraction of polymer donors combines with near-infrared nonfullerene acceptors could achieve high photovoltaic performance, which might be a candidate for semitransparent windows.

Factors That Prevent Spin-Triplet Recombination in Non-fullerene Organic Photovoltaics

Managing the dynamics of spin-triplet electronic states is crucial for achieving high-performance organic photovoltaics. Here we show that the replacement of fullerene with non-fullerene acceptor (NFA) molecules leads to suppression of triplet recombination and thus more efficient charge generation. This indicates that the relaxation of charges to the local triplet exciton state, although energetically allowed, is outcompeted by the thermally activated separation of interfacial charge-transfer excitons (CTEs) in the NFA-based system. By rationalizing our results with Marcus theory, we propose that triplet recombination in the fullerene system is driven by the small energy difference and strong electronic couplings between the CTE state and the lowest-lying triplet exciton state (T1) of fullerene acceptor molecules. In contrast, the large energy difference and small electronic couplings between these states in the NFA-based blends lead to sufficiently slow triplet relaxation rate compared to the charge separation rate (≪1010 s-1), thus preventing triplet recombination.

Photovoltaic cells based on ternary P3HT:PCBM: Ruthenium(II) complex bearing 8-(diphenylphosphino)quinoline active layer

Optical, morphological and photovoltaic properties are investigated for ternary solar cells containing a traditional poly(3-hexylthiophene-2,5-diyl):[6,6]-phenyl-C61-butyric-acid-methyl ester (P3HT:PCBM) bulkheterojunction (BHJ) active layer modified with different concentrations of a novel ruthenium complex [Ru(N-P)2(O-O)], where N-P abbreviates 8-(diphenylphosphino)quinolone and O-O = oxalate dianion. At a low concentration of the Ru-complex (2.5 wt%) the device efficiency is improved by 50% compared with the reference binary devices at ambient conditions. This substantial efficiency enhancement is attributed to the role of the Ru-complex in improving light absorption over a wavelength range of (295–800 nm) in combination with a better matching of the energy levels of the ternary blend system. Moreover, at low concentration, the Ru-complex has a positive impact on the morphology of the active layer in the device. The inclusion of Ru-complex increases the P3HT crystallinity substantially with virtually no effect on the size and orientation of the crystalline lamellae. The enhancement in device efficiency becomes less pronounced with increasing the concentration of the Ru-complex due to the formation of several micron-size domains of [Ru(N-P)2(O-O)] in the ternary active layers. These large domains negatively affect the optical properties and morphology, thus inhibiting efficient charge generation and transport in the corresponding solar cells.

Efficient and Stable Perovskite Solar Cells with a Superhydrophobic Two-Dimensional Capping Layer.

A two-dimensional (2D) perovskite layer is considered to be a desirable defect passivation structure for the lifelong stability of perovskite solar cells (PSCs). However, the efficiency could be compromised behind the traditional PSCs. Herein, we solve this issue by employing a highly hydrophobic organic cation, 2-[4-(trifluoromethyl)phenyl]ethanamine (CF3-PEA), to form a 2D (CF3-PEA)2PbI4 to effectively passivate the 3D MAPbI3 with fewer defects. The new 2D/3D-structured PSCs show reduced charge recombination, an elongated carrier lifetime, efficient charge generation and transport. Those excellent characters lead to a significant enhancement of the efficiency from 17.9% for pristine PSCs to 21.43% for 2D/3D PSCs. Benefiting from the high hydrophobicity of CF3-PEA, the cells show remarkably improved stability by maintaining 83% of the original efficiency exposed to 80% R.H. and 50 °C for 600 h and 87% under 1 sun illumination for 600 h, which makes our PCSs among the most efficient and stable MAPbI3 solar cells.

One-pot synthesis of 3D porous Bi7O9I3/N-doped graphene aerogel with enhanced photocatalytic activity for organic dye degradation in wastewater

The compound Bi7O9I3 has been considered as a promising candidate for organic dye degradation in wastewater, but it has relatively low photocatalytic activity and difficulties in the recycling processes. In this work, a novel floating 3D porous Bi7O9I3/N-doped graphene aerogel (Bi7O9I3/NGA) composite was successfully synthesized through a facile hydrothermal route. The Bi7O9I3/NGA composite exhibited highly enhanced photocatalytic performance toward degrading rhodamine B under visible-light irradiation, which increased 6.0 and 2.3 times compared with the Bi7O9I3 and Bi7O9I3/GA, respectively. The enhancement of photocatalytic degradation activity could be ascribed to the extensively promoted charge generation and migration efficiency, visible light utilization ability and reactive oxygen species production. Besides, the special 3D macroscopic block structure of Bi7O9I3/NGA allowed it to float, making it easy to recycle. The photocatalytic degradation efficiency of Bi7O9I3/NGA composite still could reach up to 92.7% after four consecutive cycles and presented satisfactory stability and reusability. Moreover, a possible photocatalytic degradation mechanism was revealed by radical species trapping and semi-quantitative analyses experiments.

Charge generation efficiency of electrically doped organic semiconductors

Organic semiconductors (OSCs) have been a significant focus of research for electronic devices over the years exclusively due to their superior mechanical and optical properties as opposed to conventional inorganic semiconductors (ISCs) such as silicon or germanium. These unique materials have smoothened the path for developing extremely light-weight and ultrathin electronic devices with built-in flexibility and transparency for a considerably low cost. However, the commercial application of these organic materials is limited by their inferior electrical conductivity and charge carrier mobility compared with inorganic counterparts. This review article presents an overview of the works published on how to control and adjust the electrical conductivity of OSCs, focusing on electrical doping. The main point of this review is related to the principles and fundamental mechanisms of charge generation efficiency (CGE) in OSCs, which are significantly different from those of ISCs. The reported CGE of OSCs, defined as the ratio of the generated charge carriers to the dopants, is found to be in the range of a few percent and is significantly small compared with the ISCs. The origin of this lies in the lower dissociation rate of the charge transfer complexes (CTCs) formed between the tightly bound ionized dopants and generated charges. The CTCs induce localized electron-hole pairs, and the dissociation of such CTCs into free charge carriers requires large activation energies to overcome the strong Coulombic forces exerted on the generated charges by the dopant ions. Therefore, high doping concentrations are required to generate a large number of free charge carriers in doped OSC films. In this review, the CGE of OSCs is discussed in detail from the point of view of CTC formation and charge separation efficiencies.

Direct observation of charge separation in an organic light harvesting system by femtosecond time-resolved XPS

The ultrafast dynamics of photon-to-charge conversion in an organic light-harvesting system is studied by femtosecond time-resolved X-ray photoemission spectroscopy (TR-XPS) at the free-electron laser FLASH. This novel experimental technique provides site-specific information about charge separation and enables the monitoring of free charge carrier generation dynamics on their natural timescale, here applied to the model donor-acceptor system CuPc:C60. A previously unobserved channel for exciton dissociation into mobile charge carriers is identified, providing the first direct, real-time characterization of the timescale and efficiency of charge generation from low-energy charge-transfer states in an organic heterojunction. The findings give strong support to the emerging realization that charge separation even from energetically disfavored excitonic states is contributing significantly, indicating new options for light harvesting in organic heterojunctions.

Additive and High-Temperature Processing Boost the Photovoltaic Performance of Nonfullerene Organic Solar Cells Fabricated with Blade Coating and Nonhalogenated Solvents

Benefitting from narrow band gap nonfullerene acceptors, continually increasing power conversion efficiency (PCE) endows organic solar cells (OSCs) with great potential for commercial application. Fabricating high-performance OSCs with potential for large-scale coating and nonhalogenated solvent processing is a necessity. Herein, we have proposed the use of nonhalogenated solvents combined with high-temperature blade coating to prepare a PM6 (poly[(2,6-(4,8-bis(5-(2-ethylhexyl-3-fluoro)thiophen-2-yl)benzo[1,2-b:4,5-b']dithiophene))-alt-(5,5-(1',3'-di-2-thienyl-5',7'-bis(2-ethylhexyl)benzo[1',2'-c:4',5'-c']dithiophene-4,8-dione)]):Y6 (2,2'-((2Z,2'Z)-((12,13-bis(2-ethylhexyl)-3,9-diundecyl-12,13-dihydro-[1,2,5]thiadiazolo[3,4-e]thieno[2″,3'':4',5']thieno[2',3':4,5]pyrrolo[3,2-g]thieno[2',3':4,5]thieno[3,2-b]indole-2,10-diyl)bis(methanylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene)))blend active layer. The resultant OSCs deliver a PCE of 15.51% when the PM6:Y6 active layer is blade-coated at 90 °C in nonhalogenated o-xylene (o-XY) host solvent containing 1,2-dimethylnaphthalene (DMN) additive. It is found that high-temperature blade coating and nonhalogenated solvent additive DMN can suppress excessive aggregation of Y6 and enhance the crystallinity of PM6 and Y6 by regulating the dynamic process of active layer formation. Finally, an optimized blend morphology with nanofibrous phase separation and enhanced crystallinity are achieved for the PM6:Y6 active layer prepared with high-temperature blade coating and nonhalogenated o-XY:DMN solvents, which not only shortens the film-drying time but also leads to increased charge generation, transport, and collection efficiency. The 1.00 cm2 OSCs prepared with high-temperature blade coating and nonhalogenated solvents exhibit a high PCE of 13.87%. This approach shows great potential for large-area fabrication of OSCs.