Efficient Organic Photovoltaic Devices Research Articles

Electronic properties of donor:acceptor complexes in all-polymer solar cells based on density functional theory

The electronic properties at the donor (D):acceptor (A) interface are a crucial factor in determining the efficiency of organic photovoltaic devices. Here, based on first-principles calculations, the electronic properties of ten configuration complexes composed of D polymer PDPPTPT and A polymer PNDI2OD-TVT were simulated. Results show that the bandgap values of the homo-/heterojunctions decrease with the increase of the number of molecular layers, and that of AAA is close to zero. This indicates that the homogeneous stacking is favorable for charge transport; furthermore, the bandgap of the complexes is affected by the molecular arrangement. Through the differential charge density and Bader charge analysis method, it was found that charge transfer will occur intermolecularly, which promotes the formation of a dipole moment at the D:A interface, and the dipole electric field then helps the dissociation of excitons in the active layer. The amount of charge transfer at the D:A interface in the DDA, DAA and DDAA configurations is about twice that in the DA configuration alone, demonstrating that homogeneous accumulation in complexes can enhance the interface dipole interaction. The comprehensive analysis suggests that homogeneous accumulation is conducive to charge transport, that heterogeneous stacking helps to promote exciton dissociation, and that there should be an optimal ratio. Furthermore, the dipole electric fields formed at the D:A interface exhibit the characteristics of local and non-uniform distribution.

Aggregation Controlled Charge Generation in Fullerene Based Bulk Heterojunction Polymer Solar Cells: Effect of Additive

Optimization of charge generation in polymer blends is crucial for the fabrication of highly efficient polymer solar cells. While the impacts of the polymer chemical structure, energy alignment, and interface on charge generation have been well studied, not much is known about the impact of polymer aggregation on charge generation. Here, we studied the impact of aggregation on charge generation using transient absorption spectroscopy, neutron scattering, and atomic force microscopy. Our measurements indicate that the 1,8-diiodooctane additive can change the aggregation behavior of poly(benzodithiophene-alt-dithienyl difluorobenzotriazole (PBnDT-FTAZ) and phenyl-C61-butyric acid methyl ester (PCBM)polymer blends and impact the charge generation process. Our observations show that the charge generation can be optimized by tuning the aggregation in polymer blends, which can be beneficial for the design of highly efficient fullerene-based organic photovoltaic devices.

Driving Force for Exciton Dissociation in Organic Solar Cells: The Influence of Donor and Acceptor Relative Orientation

The presence of a favorable driving force for electron transfer (ΔG) at the donor/acceptor (D/A) interface is fundamental to overcome the exciton binding energy and improve the efficiency of organic photovoltaic devices (OPVs). Using density functional theory (DFT), time-dependent DFT, and classical molecular dynamics, here we systematically investigate the dependence of ΔG on the molecular orientation between the donor and the acceptor in the dimer. This study was performed by considering the model configuration of D/A species of four donor A–D–A molecules named DRCN­(4–7)­T and C60 acceptor. We identified two configurations that gave the highest negative values of ΔG (which favors exciton dissociation): (i) face-on orientation when the fullerene is located at the central thiophene ring of the DRCN­(4–7)­T molecule. In this situation, the high degree of hybridization between the highest occupied molecular orbital of the acceptor and the lowest unoccupied molecular orbital of the donor stabilizes the energy of the lowest charge transfer state (CT1). (ii) Edge-on orientation when C60 is close to the positive partial charges of the D molecule. In this configuration, the fullerene further polarizes the D molecule which enhances the dimers’ dipole moment. The electrostatic potential produced by this dipole decreases then the energy of the CT1 state. Molecular dynamics calculations performed to simulate the deposition of DRCN5T/C60 blends showed that dimers with a very negative driving force (as identified by our DFT calculations) are indeed present in the film. We also found a tweezers-like effect produced by the lateral alkyl groups attached to the central thiophene ring of the DRCN5T molecule. Those lateral groups can capture the fullerenes and guide them toward the center of the molecule during the solvent evaporation. This process would favor the formation of face-on dimers that tend to have a negative ΔG. The comparison of our data with experimental results measured in DRCN­(4–7)­T suggests that there is a correlation between negative values of ΔG (averaged over some D/A configurations) and the performance of the corresponding solar cell. The fine-tuning of the D/A configuration can make the difference to decrease the germinated recombination and improve the efficiency of OPVs.

Molecular-Level Exploration of the Structure-Function Relations Underlying Interfacial Charge Transfer in the Subphthalocyanine/ C60 Organic Photovoltaic System

The arrangement of organic molecules at the donor-acceptor interface in an organic photovoltaic (OPV) cell can have a strong effect on the generation of charge carriers and thereby cell performance. In this paper, we report the molecular-level exploration of the ensemble of interfacial donor-acceptor pair geometries and the charge-transfer (CT) rates to which they give rise. Our approach combines molecular-dynamics simulations, electronic structure calculations, machine learning, and rate theory. This approach is applied to the boron subphthalocyanine chloride (donor) and ${\mathrm{C}}_{60}$ (acceptor) OPV system. We find that the interface is dominated by a previously unreported donor-acceptor pair edge geometry, which contributes significantly to device performance in a manner that depends on the initial conditions. Quantitative relations between the morphology and CT rates are established, which can be used to advance the design of more efficient OPV devices.

Origin of Low Open-Circuit Voltage in Surfactant-Stabilized Organic-Nanoparticle-Based Solar Cells.

Organic-nanoparticle-based solar cells have drawn great attention due to their eco-friendly and environmentally friendly fabrication procedure. However, these surfactant-stabilized nanoparticles suffer open-circuit voltage loss due to charge trapping and poor extraction rate at the polymer cathode interface. Here, we have investigated the origin of voltage loss and charge trapping in surfactant-stabilized nanoparticle-based devices. Efficient organic photovoltaic (OPV) devices have been fabricated from an aqueous dispersion of poly(3-hexylthiophene-2,5-diyl) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) nanoparticles stabilized by anionic surfactants. AC impedance spectroscopy has been used to understand the charge transport properties in the dark and in operando conditions. We have demonstrated the similarities in the charge transport properties, as well as photocarrier dynamics of the nanoparticle-based OPVs and the bulk heterojunction OPVs despite fundamental differences in their nanostructure morphology. This study emphasizes the possibility of fabricating highly efficient OPVs from organic nanoparticles by reducing surface defects and excess doping of the polymers.

On the effect of pattern substitution and oligo(ethylene oxide) side-chain modification on thiophene-quinoxaline copolymers and their applications in photovoltaic cells

Abstract The molecular design of conjugated polymers for efficient organic photovoltaic devices (OPVs) providing advantageous processing properties, namely solubility in low toxic solvents as ethanol, is an important issue for the progress of OPV technology. In this work, the pattern substitution and chemical nature of the side chain, either n-alkoxy or oligo(ethylene oxide) groups, in alternating thiophene-quinoxaline copolymers is varied and their effect in polymer chain grow, solubility and optical spectra is thoroughly analyzed in light of quantum mechanical calculations. Although the polymer with oligo(ethylene oxide) groups exhibited good solubility in ethanol, it was found that substituting in both meta- and para-positions of the peripheral phenyl rings attached to the quinoxaline groups causes smaller molecular weight, lower solubility in chlorinated solvents and poorer performance in photovoltaic devices. The theoretical studies revealed differences in the most energetically favored conformers for each case of the polymers'substitution pattern that can justify the observed results.

Rational Tuning of Molecular Interaction and Energy Level Alignment Enables High-Performance Organic Photovoltaics.

The performance of organic photovoltaics (OPVs) has rapidly improved over the past years. Recent work in material design has primarily focused on developing near-infrared nonfullerene acceptors with broadening absorption that pair with commercialized donor polymers; in the meanwhile, the influence of the morphology of the blend film and the energy level alignment on the efficiency of charge separation needs to be synthetically considered. Herein, the selection rule of the donor/acceptor blend is demonstrated by rationally considering the molecular interaction and energy level alignment, and highly efficient OPV devices using both-fluorinated or both-nonfluorinated donor/acceptor blends are realized. With the enlarged absorption, ideal morphology, and efficient charge transfer, the devices based on the PBDB-T-F/Y1-4F blend and PBDB-T-F/Y6 exhibit champion power conversion efficiencies as high as 14.8% and 15.9%, respectively.

Homocoupling Defects of a Small Donor Molecule for Organic Photovoltaics: Quantification of the Eutectic State Diagram by Rapid Heat–Cool Differential Scanning Calorimetry

The conjugated small donor molecule p-DTS(FBTTh2)2, leading to efficient bulk heterojunction organic photovoltaic devices with PC71BM, shows a eutectic phase behavior in combination with its homoco...

Stabilizing Silver Window Electrodes for Organic Photovoltaics Using a Mercaptosilane Monolayer

A single layer of the bifunctional molecule 3-mercaptopropyltrimethoxysilane is shown to be remarkably effective at improving the stability of optically thin silver film electrodes towards spontaneous morphological change and oxidation by airborne sulfur. Inclusion of this layer in the novel transparent electrode; WO3 (30 nm) / silver (13 nm) / sol-gel ZnO (27 nm), at the silver / ZnO interface improves the efficiency of organic photovoltaic devices using this electrode by 20%, such that the power conversion efficiency is very close to that achievable using a conventional indium-tin oxide glass electrode; 9.6 % ± 0.2 % vs 10.0 % ± 0.3 %, with the advantage that the silver electrode has a sheet resistance one third that of the ITO glass (4 Ohms sq-1). The mercaptosilane monolayer is also shown to retard silver diffusion into the ZnO layer whilst imparting a favorable 400 meV reduction in electrode work function. In addition to its utility inside the device, this molecular layer is shown to be useful for improving the stability of the silver film electrodes in top-illuminated semi-transparent photovoltaics, since it can be deposited directly onto a completed device from the vapor phase.

Transforming Ionene Polymers into Efficient Cathode Interlayers with Pendent Fullerenes

AbstractA new and highly efficient cathode interlayer material for organic photovoltaics (OPVs) was produced by integrating C60 fullerene monomers into ionene polymers. The power of these novel “C60‐ionenes” for interface modification enables the use of numerous high work‐function metals (e.g., silver, copper, and gold) as the cathode in efficient OPV devices. C60‐ionene boosted power conversion efficiencies (PCEs) of solar cells, fabricated with silver cathodes, from 2.79 % to 10.51 % for devices with a fullerene acceptor in the active layer, and from 3.89 % to 11.04 % for devices with a non‐fullerene acceptor in the active layer, demonstrating the versatility of this interfacial layer. The introduction of fullerene moieties dramatically improved the conductivity of ionene polymers, affording devices with high efficiency by reducing charge accumulation at the cathode/active layer interface. The power of C60‐ionene to improve electron injection and extraction between metal electrodes and organic semiconductors highlights its promise to overcome energy barriers at the hard‐soft materials interface to the benefit of organic electronics.

Transforming Ionene Polymers into Efficient Cathode Interlayers with Pendent Fullerenes.

A new and highly efficient cathode interlayer material for organic photovoltaics (OPVs) was produced by integrating C60 fullerene monomers into ionene polymers. The power of these novel "C60 -ionenes" for interface modification enables the use of numerous high work-function metals (e.g., silver, copper, and gold) as the cathode in efficient OPV devices. C60 -ionene boosted power conversion efficiencies (PCEs) of solar cells, fabricated with silver cathodes, from 2.79 % to 10.51 % for devices with a fullerene acceptor in the active layer, and from 3.89 % to 11.04 % for devices with a non-fullerene acceptor in the active layer, demonstrating the versatility of this interfacial layer. The introduction of fullerene moieties dramatically improved the conductivity of ionene polymers, affording devices with high efficiency by reducing charge accumulation at the cathode/active layer interface. The power of C60 -ionene to improve electron injection and extraction between metal electrodes and organic semiconductors highlights its promise to overcome energy barriers at the hard-soft materials interface to the benefit of organic electronics.

A New Method for the Synthesis of Bromine-Containing Heterocyclic Compounds for Photovoltaic Polymers

With the development and improvement of systems for converting sunlight into electric and thermal energy, more and more work is emerging on the development of the newest and most promising direction in solar energy, namely the creation of solar cells based on photosensitive polymers. Recently the power conversion efficiency of organic photovoltaic (OPV) devices has overcome the barrier of 17%, and thus we can expect a new wave of scientific interest in the development of new, more efficient OPV devices. Unfortunately, during searching for highly efficient chemical structures of OPV polymers, the researchers missed an important point: all photovoltaic polymers consist of aromatic and heteroaromatic «building blocks», which, in turn, are synthesized based on outdated techniques using highly toxic, dangerous for life and environment precursors. The development of «green», environmentally friendly, economically viable methods for the synthesis of photovoltaic polymers and building blocks for their production, will make the energy obtained from OPV truly «green». In this work, we present an alternative, «green» method for synthesizing halogen-containing aromatic and heteroaromatic, expensive building blocks most commonly used in the synthesis of photovoltaic polymers, which can be used to obtain photovoltaic polymers of various structures. We present the original methods for the synthesis of 4,4-dibromo-1,1- biphenyl (1), 4,7-dibromo-2,1,3-benzothiadiazole (2), 2-bromothiophene (3) and 2,5-dibromothiophene (4). All these methods differ from the previously described routes by their simplicity and convenience of their implementation, the absence of corrosive and irritant reagents, good yield and compliance with the principles of «Green Chemistry».

Wide‐Bandgap Small Molecular Acceptors Based on a Weak Electron‐Withdrawing Moiety for Efficient Polymer Solar Cells

Narrow‐bandgap small molecular acceptors (SMAs) with absorption extending into the near‐infrared spectral region such as ITIC derivatives are widely investigated, while the development of their wide‐bandgap counterparts remains largely unexplored. Wide‐bandgap non‐fullerene acceptors (NFAs) are highly desirable and beneficial for constructing efficient device layouts such as ternary blend and tandem solar cells that require multiple light‐harvesting materials with different regions of absorption. In this contribution, the design and synthesis of two wide‐bandgap SMAs (IDT‐TBA and IDDT‐TBA), consisting of a weak electron‐withdrawing moiety (1,3‐diethyl‐2‐thiobarbituric acid, TBA) is presented. Compared to ITIC, this molecular design strategy results in energetically down‐shifted HOMO levels and hence much enlarged bandgaps of 1.91 eV for IDT‐TBA and 1.78 eV for IDDT‐TBA, respectively. Further photovoltaic performance evaluation demonstrates power conversion efficiencies (PCEs) of 6.5% for IDT‐TBA and 7.5% for IDDT‐TBA, respectively, when using PBDB‐T as the electron donor polymer. In addition, time‐delayed collection field (TDCF) experiments suggest that both IDT‐TBA and IDDT‐TBA based cells exhibit field‐independent charge generation with external charge generation efficiencies exceeding 90%, implying negligible geminate recombination losses. The results demonstrate that TBA units are promising and attractive building blocks as weak electron‐withdrawing acceptors to construct wide‐bandgap high‐efficiency SMAs for efficient organic photovoltaic devices.

(Invited) Photocarrier Generation in Single-Absorber Organic Solar Cells

Power Conversion efficiency of organic photovoltaic devices has exceeded 10% by taking advantage of bulk heterojunction (BHJ) structure, where donor and acceptor molecules are microscopically mixed. A remained problem is photovoltage loss from absorbed photon energy. In the BHJ system, LUMO offset is required to promote photodissociation at the donor-acceptor interface, causing serious photovoltage loss. To solve this problem, we are approaching it from single absorber cells, where there is no donor-acceptor interface. Generally, photogeneration efficiency in organic film bulk is quite low because of large exciton binding energy of Frenkel exciton. On the other hand, photogeneration from CT excitons occurs almost spontaneously in the BHJ system. If we could generate charge-transfer (CT) excitons in the single-component film directly, photovoltage loss would be reduced. Recently, we found intramolecular charge-transfer molecules including donor and acceptor units within one molecule show considerable high performance in the single-absorber device. In particular, DTDCPB having triphenyl amine unit (donor) and benzothiadiazole unit (acceptor) was a promising material for the single-absorber system. In this presentation, photogeneration process in the single-component film of CT molecules was investigated from several viewpoints. Molecular design for intramolecular charge-transfer absorption is discussed from the viewpoint of quantum chemical calculation and organic synthesis. The photocarrier dynamics in the single-absorber solar cells is investigated by several techniques such as intensity-modulated photovoltage/photocurrent (IMVS/IMPS) and time delayed collection field (TDCF). Exciton binding energy, that is a key factor for photodissociation in the film bulk, was estimated by Onsager-Braun analysis by using a planar device. Figure 1

Significant impact of monomer curvatures for polymer curved shape composition on backbone orientation and solar cell performances

In the present study, linear or curve-shaped donor units, quinacridone (Qc) and benzo[2,1-b:3,4-b′]dithiophene (BDP), and spacers were introduced for polymerization of four D–A type polymers (PBDPOx-biT, PBDPOx-TT, PQcOx-biT, and PQcOx-TT).The UV–vis absorption spectra of PBDPOx-biT, PQcOx-TT (linear shaped polymer) films were red-shifted compared with the solution absorption, whereas that of a PBDPOx-TT, PQcOx-biT (curve shaped polymer) film was blue-shifted. PBDPOx-biT, PQcOx-TT had high crystallinity. Linear polymers prefer regular and crystalline domains in the film state and lead to more efficient organic photovoltaic (OPV) devices. PQcOx-TT possesses a PCE value of up to 3.4%.

Photodynamics in Metal-Chelating Tetraphenylazadipyrromethene Complexes: Implications for Their Potential Use as Photovoltaic Materials

Recent efforts to improve the photon conversion efficiency in organic photovoltaic devices have shifted toward producing and optimizing electron acceptor materials. In this contribution, the photodynamics of cobalt-, nickel-, and zinc-chelating tetraphenylazadipyrromethene (M(ADP)2) complexes are investigated as part of an evaluation of their potential use as electron acceptor in bulk-heterojunction organic photovoltaic devices using poly(3-hexylthiophene-2,5-diyl) (P3HT) as the electron donor. Steady-state and broad-band transient absorption spectroscopies are used in combination with quantum-chemical calculations in acetonitrile, chloroform, and toluene to investigate their excited-state dynamics and electronic relaxation mechanisms. Zn(ADP)2 shows the longest ground-state repopulation dynamics of the three complexes and the photovoltaic devices that incorporate Zn(ADP)2 exhibit the highest power conversion efficiency. A correlation is observed between the magnitude of the ground-state repopulation life...

The Role of FRET in Non-Fullerene Organic Solar Cells: Implications for Molecular Design

Non-fullerene acceptors (NFAs) have been demonstrated to be promising candidates for highly efficient organic photovoltaic (OPV) devices. The tunability of absorption characteristics of NFAs can be used to make OPVs with complementary donor-acceptor absorption to cover a broad range of the solar spectrum. However, both charge transfer from donor to acceptor moieties and energy (energy) transfer from high-bandgap to low-bandgap materials are possible in such structures. Here, we show that when charge transfer and exciton transfer processes are both present, the coexistence of excitons in both domains can cause a loss mechanism. Charge separation of excitons in a low-bandgap material is hindered due to exciton population in the larger bandgap acceptor domains. Our results further show that excitons in low-bandgap material should have a relatively long lifetime compared to the transfer time of excitons from higher bandgap material in order to contribute to the charge separation. These observations provide significant guidance for design and development of new materials in OPV applications.

Morphological, Chemical, and Electronic Changes of the Conjugated Polymer PTB7 with Thermal Annealing

SummaryThere is considerable interest in improving the performance of organic optoelectronic devices through processing techniques. Here, we study the effect of high-temperature annealing on the properties of the semiconducting polymer PTB7 and PTB7:fullerene blends, of interest as efficient organic photovoltaic (OPV) devices. Annealing to moderate temperature improves the PTB7 morphology and optoelectronic properties. High-temperature annealing also improves morphology but results in poorer optoelectronic properties. This is a result of side chain cleavage that creates by-products that act as trap states, increasing electronic disorder and decreasing mobility. We further observe changes to the PTB7 chemical structure after thermal cleavage that are similar to those following solar irradiation. This implies that side chain cleavage is an important mechanism in device photodegradation, which is a major “burn-in” loss mechanism in OPV. These results lend insight into side chain cleavage as a method of improving optoelectronic properties and suggest strategies for improvement in device photostability.

Theoretical Studies of Singlet Fission: Searching for Materials and Exploring Mechanisms.

In this Review article, a survey is given for theoretical studies in the subject of singlet fission. Singlet fission converts one singlet exciton to two triplet excitons. With the doubled number of excitons and the longer lifetime of the triplets, singlet fission provides an avenue to improve the photoelectric conversion efficiency in organic photovoltaic devices. It has been a subject of intense research in the past decade. Theoretical studies play an essential role in understanding singlet fission. This article presents a Review of theoretical studies in singlet fission since 2006, the year when the research interest in this subject was reignited. Both electronic structure and dynamics studies are covered. Electronic structure studies provide guidelines for designing singlet fission chromophores and insights into the couplings between single- and multi-excitonic states. The latter provides fundamental knowledge for engineering interchromophore conformations to enhance the fission efficiency. Dynamics studies reveal the importance of vibronic couplings in singlet fission.

Impact of Low Molecular Weight Poly(3-hexylthiophene)s as Additives in Organic Photovoltaic Devices.

Despite tremendous progress in using additives to enhance the power conversion efficiency of organic photovoltaic devices, significant challenges remain in controlling the microstructure of the active layer, such as at internal donor-acceptor interfaces. Here, we demonstrate that the addition of low molecular weight poly(3-hexylthiophene)s (low-MW P3HT) to the P3HT/fullerene active layer increases device performance up to 36% over an unmodified control device. Low MW P3HT chains ranging in size from 1.6 to 8.0 kg/mol are blended with 77.5 kg/mol P3HT chains and [6,6]-phenyl C61 butyric acid methyl ester (PCBM) fullerenes while keeping P3HT/PCBM ratio constant. Optimal photovoltaic device performance increases are obtained for each additive when incorporated into the bulk heterojunction blend at loading levels that are dependent upon additive MW. Small-angle X-ray scattering and energy-filtered transmission electron microscopy imaging reveal that domain sizes are approximately invariant at low loading levels of the low-MW P3HT additive, and wide-angle X-ray scattering suggests that P3HT crystallinity is unaffected by these additives. These results suggest that oligomeric P3HTs compatibilize donor-acceptor interfaces at low loading levels but coarsen domain structures at higher loading levels and they are consistent with recent simulations results. Although results are specific to the P3HT/PCBM system, the notion that low molecular weight additives can enhance photovoltaic device performance generally provides a new opportunity for improving device performance and operating lifetimes.