Voltage In Organic Solar Cells Research Articles

Structure-Property Relationships for the Electronic Applications of Bis-Adduct Isomers of Phenyl-C61 Butyric Acid Methyl Ester.

Higher adducts of a fullerene, such as the bis-adduct of PCBM (bis-PCBM), can be used to achieve shallower molecular orbital energy levels than, for example, PCBM or C60. Substituting the bis-adduct for the parent fullerene is useful to increase the open-circuit voltage of organic solar cells or achieve better energy alignment as electron transport layers in, for example, perovskite solar cells. However, bis-PCBM is usually synthesized as a mixture of structural isomers, which can lead to both energetic and morphological disorder, negatively affecting device performance. Here, we present a comprehensive study on the molecular properties of 19 pure bis-isomers of PCBM using a variety of characterization methods, including ultraviolet photoelectron spectroscopy, thermal gravimetric analysis, differential scanning calorimetry, single crystal structure, and (time-dependent) density functional theory calculation. We find that the lowest unoccupied molecular orbital of such bis-isomers can be tuned to be up to 170 meV shallower than PCBM and up to 100 meV shallower than the mixture of unseparated isomers. The isolated bis-isomers also show an electron mobility in organic field-effect transistors of up to 4.5 × 10-2 cm2/(V s), which is an order of magnitude higher than that of the mixture of bis-isomers. These properties enable the fabrication of the highest performing bis-PCBM organic solar cell to date, with the best device showing a power conversion efficiency of 7.2%. Interestingly, we find that the crystallinity of bis-isomers correlates negatively with electron mobility and organic solar cell device performance, which we relate to their molecular symmetry, with a lower symmetry leading to more amorphous bis-isomers, less energetic disorder, and higher dimensional electron transport. This work demonstrates the potential of side chain engineering for optimizing the performance of fullerene-based organic electronic devices.

Intrinsic Photoconductivity Spectral Dependence as a Tool for Prediction of Open-Circuit Voltage in Organic Solar Cells

Organic materials are known for their variety of molecules. Methods to predict the parameters of organic photovoltaic (OPV) cells are required to avoid the time- and resource-consuming processes of manufacturing and testing OPVs. Usually, the open-circuit voltage (Uoc) is estimated as the difference between the ionization energy level of the electron donor molecule (Id) and the electron affinity level of the electron acceptor molecule (EAa). Various measurement methods are used to determine the energy level values of pure materials, which, when combined with energy level shifts due to the donor:acceptor interactions, make these estimations less precise. In this work, photoconductivity measurements were applied to the donor:acceptor films. Near threshold energy, the electron can be directly transferred from the donor to the acceptor molecule. The obtained charge transfer energy (ECT) shows the difference between Id and EAa in the film. This difference was compared to the Uoc value of an OPV made of the same donor:acceptor combination. We show that this approach provides less scattered results and a higher correlation coefficient compared to the Uoc estimation using energy level values.

A DFT study for improving the photovoltaic performance of organic solar cells by designing symmetric non-fullerene acceptors by quantum chemical modification on pre-existed LC81 molecule

Minimizing the energy loss and improving the open circuit voltage of organic solar cells is still a primary concern for scientists working in this field. With the aim to enhance the photovoltaic performance of organic solar cells by minimizing energy loss and improving open circuit voltage, seven new acceptor molecules (LC1-LC7) are presented in this work. These molecules are designed by modifying the terminal acceptors of pre-existed “LC81” molecule based on an indacinodithiophene (IDT) fused core. The end-group modification approach is very fruitful in ameliorating the efficacy and optoelectric behavior of OSCs. The newly developed molecules presented remarkable improvements in performance-related parameters and optoelectronic properties. Among all designed molecules, LC7 exhibited the highest absorption maxima (λmax = 869 nm) with the lowest band-gap (1.79 eV), lowest excitation energy (Ex = 1.42 eV), lowest binding energy, and highest excited state lifetime (0.41 ns). The newly designed molecules LC2, LC3, and LC4 exhibited remarkably improved Voc that was 1.84 eV, 1.82 eV, and 1.79 eV accordingly, compared to the LC81 molecule with Voc of 1.74 eV LC2 molecule showed significant improvement in fill factor compared to the previously presented LC81 molecule. LC2, LC6, and LC7 showed a remarkable reduction in energy loss by showing Eloss values of 0.26 eV, 0.18 eV, and 0.25 eV than LC81 molecule (0.37 eV). These findings validate the supremacy of these developed molecules (especially LC2) as potential components of future OSCs.

Open-circuit voltage of organic solar cells: interfacial roughness makes the difference

Organic photovoltaics (PV) is an energy-harvesting technology that offers many advantages, such as flexibility, low weight and cost, as well as environmentally benign materials and manufacturing techniques. Despite growth of power conversion efficiencies to around 19 % in the last years, organic PVs still lag behind inorganic PV technologies, mainly due to high losses in open-circuit voltage. Understanding and improving open circuit voltage in organic solar cells is challenging, as it is controlled by the properties of a donor-acceptor interface where the optical excitations are separated into charge carriers. Here, we provide an electrostatic model of a rough donor-acceptor interface and test it experimentally on small molecule PV materials systems. The model provides concise relationships between the open-circuit voltage, photovoltaic gap, charge-transfer state energy, and interfacial morphology. In particular, we show that the electrostatic bias generated across the interface reduces the photovoltaic gap. This negative influence on open-circuit voltage can, however, be circumvented by adjusting the morphology of the donor-acceptor interface.

What's Next for Organic Solar Cells? The Frontiers and Challenges

The power conversion efficiency (PCE) of organic solar cells (OSCs) is improved dramatically in recent years and now approaches >19% for single‐junction cells and >20% for tandem cells. Therefore, the practical use of OSCs is becoming a reality. This perspective summarizes the state of the art of OSC characteristics and discusses the challenges that remain in further improving PCE. The short‐circuit current density (J SC) of the state‐of‐the‐art OSCs almost approaches 30 mA cm−2. As the internal quantum efficiencies of these devices exceed 90%, for further improvement in J SC, it is necessary to suppress reflection at interfaces and increase the active layer thickness to absorb as many photons as possible. Further suppression of the nonradiative voltage loss is imperative, as there is still large room for improvement compared to inorganic and perovskite counterparts. Hence, increasing the exciton lifetime and photoluminescence quantum yield of nonfullerene acceptors are pivotal to improving the open‐circuit voltage of OSCs. The fill factors of the latest OSCs approach 80%; however, because the optimized active layer thickness remains ≈100 nm, further suppression of bimolecular charge recombination is needed. Finally, this perspective discusses to what extent PCE can be improved and what can be done to achieve this.

Special Section on “Next Generation Solar Cells”

Molecular photovoltaic devices, in a broad sense, are devices made by using molecules or molecular materials that are capable of converting sunlight into electrical current and voltage. Since the use of low molecular weight organic molecules by Tang,[1] the sensitization of TiO2 with dyes coupled to the use of iodine/iodide red/ox couple by Grätzel and O'Regan,[2] the fabrication of bulk-heterojunction polymer/fullerene-based films by Sariciftci et al.,[3] to the latest breakthrough using hybrid lead based perovskite semiconductor materials by Miyasaka and collaborators,[4] there has been an amazing journey harnessing solar power. The dream for cheap, globally available solar energy is closer than ever with a great deal of excellent research being done in laboratories worldwide. This special collection of research articles in Solar RRL presents a flavor of the research done recently in hot research topics from dye sensitized solar cells, organic solar cells, and perovskite solar cells and serves as an example about how proactive the research in molecular photovoltaic devices is. Photochromic thin films based on Grätzel solar cells represent the ultimate frontier in the application of transparent devices that hold the promise for efficient translucid solar cells for applications in building photovoltaics. The dye design still holds the key for the optimization of the solar cell efficiency, when the primary role of the solar cell is not just the efficiency but also its transparency and its operational stability. In organic solar cells, the design of molecules and organic materials with alike goal, transparency, is also key. Moreover, taking into account the general low open circuit voltage in organic solar cells, it is also crucial to understand and reduce the possible pathways for voltage losses, which leads to better solar cell efficiencies. In fact, the efficiency of organic solar cells has exponentially increased during the last few years due to the synthesis and use of non-fullerene electron acceptor molecules and molecular materials. Further research on non-fullerene electron acceptors, as well as organic doping of semiconductors, will remain a hot topic in the near future with the challenge to match the current efficiency of their perovskite-based counterparts. Perovskite semiconductor materials for photovoltaic applications have been, without doubt, a major achievement in recent research. Applications ranging from energy conversion devices, artificial lighting, sensors and photonic applications, have rapidly appeared in the scientific literature since 2009. Indeed, scientific challenges remain, such as the passivation of defects to boost the solar cell efficiencies and the replacement of lead by a less harmful element, but the gap between laboratory and industrial deployment is closing rapidly. For their industrial application, better materials for grids, low-cost manufacture contacts, and treatments to increase the device stability are goals that will be the focus of present and future intense research. Emilio Palomares studied Biology at the UVEG. After graduating, he joined Prof. Hermenegildo García's group at the UPV where he got his PhD. In 2009, he was awarded an ERC Starting Grant to work on quantum dots for energy conversion devices and an ERC PoC in 2015. In 2019, he was awarded Energy & Environmental Solutions International Chair (E2S) by the Université de Pau et des Pays de l'Adour (UPPA). Later in 2020, he was elected ICIQ Director for the next 5 years. His research is focused on several aspects of light induced electron transfer reactions in supramolecular structures and nanostructured inorganic materials, which has evolved towards the control and improvement of the reactions that govern the efficiency on devices such as molecular solar cells, the creation of new hybrid nanomaterials for hydrogen production, and molecular based sensing devices to detect toxic substance in the environment. He is also involved in promoting science and education in society through chemistry workshops for primary and secondary schools.

Probing the Density of States of Organic and Perovskite Semiconductors By Energy-Resolved Electrochemical Impedance Spectroscopy

The performance of thin-film photovoltaic cells is closely related to the energy alignment of the absorber layers and charge collection layers used for constructing the device and is further influenced by the presence of energetic disorder and defects in the photoactive material. This especially holds for novel organic and perovskite semiconductors, where film processing conditions, such as solvents used, thermal or solvent vapor annealing, and aging of the active materials are known to have an influence on the device performance. Knowledge about the energy of the relevant frontier orbitals such as HOMO and LUMO for organic semiconductors,[1]and valence (VB) and conduction (CB) bands for perovskites is key to designing more efficient materials. In addition, the energetic disorder, such as broadening of the band edges and states that appear in the bandgap are strongly affecting solar cell efficiencies. Knowledge about the density of states close to and inside the bandgap is very relevant in this respect.In this work energy-resolved electrochemical impedance spectroscopy (ER-EIS) as advanced by Nádazdyet al.[2]is applied on various organic bulk-heterojunction and lead-halide perovskite semiconductors to map their energy landscape around and in the band gap. ER-EIS provides a direct electrochemical measurement of the HOMO/VB and LUMO/CB energies, but also resolves band tails and sub-bandgap states with a resolution of up to six orders of magnitude. This enables detection of subtle but important effects that are the consequence of layer processing conditions such as spin coat parameters, solvent combinations, thermal annealing time and temperature, and atmospheric exposure (vacuum, inert, air).We will present experiments that aim to quantify relative changes in the energy landscape due to varying processing conditions and assign physical meaning to these changes (e.g. morphology and (un)intentional doping). The systems under investigation are well-known donor-acceptor bulk-heterojunction (e.g., P3HT:PCBM and PM6:Y6) and triple-cation mixed-halide perovskites (CsFAMA). The results shed new light on largely unknown defect states in the bandgap of solution-processed semiconductors.[1] R. E. M. Willems, C. H. L. Weijtens, X. de Vries, R. Coehoorn, and R. A. J. Janssen, Relating frontier orbital energies from voltammetry and photoelectron spectroscopy to the open-circuit voltage of organic solar cells, Adv. Energy Mater. 2019, 9, 1803677.[2] V. Nádazdy, F. Schauer, and K. Gmucová, Energy resolved electrochemical impedance spectroscopy for electronic structure mapping in organic semiconductors, Appl. Phys. Lett. 2014, 105, 142109.

Achieving 16% Efficiency for Polythiophene Organic Solar Cells with a Cyano‐Substituted Polythiophene

AbstractPolythiophenes (PTs) are promising electron donors in organic solar cells (OSCs) due to their simple structures and excellent synthetic scalability. However, the device performance of PT‐based OSCs is rather poor due mainly to large photon energy losses and an unfavorable active layer morphology. Herein, the authors report a new PT, which is abbreviated as P5TCN‐2F and features cyano‐group substituents for high‐efficiency OSCs. The cyano‐group endows P5TCN‐2F with a deep‐lying highest occupied molecular orbital energy level, which thereby contributed to high open‐circuit voltage in OSCs as a result of reduced non‐radiative recombination energy loss. Moreover, the cyano‐group leads to strong interchain interaction, improved polymer crystallinity, and appropriate miscibility with the prevailing non‐fullerene acceptors. Consequently, P5TCN‐2F offers over 15% power conversion efficiency when blended with various Y‐series non‐fullerene acceptors (Y6, Y6‐BO, eC9, and L8‐BO). Particularly, a champion efficiency of 16.1% is obtained by the P5TCN‐2F:Y6 blend, which is largely higher than that of any previous PT‐based OSCs. Moreover, the average figure of merit of the active layer based on P5TCN‐2F is much superior to that of benzodithiophene‐based polymers. These results suggest the renaissance of PT‐based OSCs and have opened an avenue to access high‐performance materials for the large‐scale production of OSC modules.

Slow Relaxation of Photogenerated Charge Carriers Boosts Open-Circuit Voltage of Organic Solar Cells.

Among the parameters determining the efficiency of an organic solar cell, the open-circuit voltage (VOC) is the one with most room for improvement. Existing models for the description of VOC assume that photogenerated charge carriers are thermalized. Here, we demonstrate that quasi-equilibrium concepts cannot fully describe VOC of disordered organic devices. For two representative donor:acceptor blends, it is shown that VOC is actually 0.1–0.2 V higher than it would be if the system was in thermodynamic equilibrium. Extensive numerical modeling reveals that the excess energy is mainly due to incomplete relaxation in the disorder-broadened density of states. These findings indicate that organic solar cells work as nonequilibrium devices, in which part of the photon excess energy is harvested in the form of an enhanced VOC.

Energetics and Kinetics Requirements for Organic Solar Cells to Break the 20% Power Conversion Efficiency Barrier

The thermodynamic limit for the efficiency of solar cells is predominantly defined by the energy bandgap of the used semiconductor. In case of organic solar cells both energetics and kinetics of three different species play role: excitons, charge transfer states and charge separated states. In this work, we clarify the effect of the relative energetics and kinetics of these species on the recombination and generation dynamics. Making use of detailed balance, we develop an analytical framework describing how the intricate interplay between the different species influence the photocurrent generation, the recombination, and the open-circuit voltage in organic solar cells. Furthermore, we clarify the essential requirements for equilibrium between excitons, CT states and charge carriers to occur. Finally, we find that the photovoltaic parameters are not only determined by the relative energy level between the different states but also by the kinetic rate constants. These findings provide vital insights into the operation of state-of-art non-fullerene organic solar cells with low offsets.

Open-circuit voltage of organic solar cells: Effect of energetically and spatially nonuniform distribution of molecular energy levels in the photoactive layer

Abstract Here, the open-circuit voltage (VOC) of organic solar cells (OSCs) in which the energy levels of the frontier molecular orbitals of the photoactive materials vary depending on the position within the active layer is investigated. The VOC in these devices is critically affected by the energy level of the frontier molecular orbitals that are spatially located near the electrodes (i.e., the highest occupied molecular orbital of the donor near the anode and the lowest unoccupied molecular orbital of the acceptor near the cathode). Other parameters such as the charge generation/recombination and the energy distribution of charge-transfer states appear to exert less influence on the VOC. This work demonstrates that, to maximize the VOC, the spatial distribution of molecular energy levels should be controlled, especially in OSCs that exhibit spatial variation of the energy levels of organic materials within their active layer.

Orbital-Energy Modulation of Tetrabenzoporphyrin-Derived Non-Fullerene Acceptors for Improved Open-Circuit Voltage in Organic Solar Cells.

Tetrabenzoporphyrin (BP) holds attractive characteristics for optoelectronic applications, such as the large π-conjugated framework and high photoabsorption capability. However, its use in organic solar cells (OSCs) has been limited because of the extremely low solubility that hampers direct solution processing and also the high frontier-orbital energies that lead to low open-circuit voltage (VOC). Herein, we examine BP derivatives equipped with multiple strongly electron-withdrawing groups for photovoltaic applications. The derivatives are generated in thin films through a thermal precursor approach, wherein the corresponding bicyclo[2.2.2]octadiene-fused porphyrin derivatives are solution-cast, and then annealed to carry out the in situ retro-Diels-Alder reaction. The frontier-orbital energies of the resulting derivatives are effectively stabilized as compared to pristine BP to such a degree that they afford high VOC of up to 0.94 V when used as a donor or can even work as a new class of nonfullerene acceptor in OSCs. Single-crystal X-ray diffraction analyses demonstrate that the conformation of the BP framework largely varies from being near planar to highly curved depending on its substituents. The morphology of polymer:BP-derivative bulk-heterojunction films prepared by the thermal precursor approach also varies between the BP derivatives. These results can greatly extend the scope of both molecular design and morphology control for utilization of the BP chromophore toward achieving viable organic optoelectronic devices.

Equilibrated Charge Carrier Populations Govern Steady-State Nongeminate Recombination in Disordered Organic Solar Cells.

We employed bias-assisted charge extraction techniques to investigate the transient and steady-state recombination of photogenerated charge carriers in complete devices of a disordered polymer-fullerene blend. Charge recombination is shown to be dispersive, with a significant slowdown of the recombination rate over time, consistent with the results from kinetic Monte Carlo simulations. Surprisingly, our experiments reveal little to no contributions from early time recombination of nonequilibrated charge carriers to the steady-state recombination properties. We conclude that energetic relaxation of photogenerated carriers outpaces any significant nongeminate recombination under application-relevant illumination conditions. With equilibrated charges dominating the steady-state recombination, quasi-equilibrium conceptsappear suited for describing the open-circuit voltage of organic solar cells despite pronounced energetic disorder.

Development of a phenanthrodithiophene‐difluorobenzoxadiazole copolymer exhibiting high open‐circuit voltage in organic solar cells

ABSTRACTA phenanthrodithiophene (PDT)‐difluorobenzoxadiazole (DFBO) copolymer, P‐PDT‐DFBO, was synthesized and characterized. Replacing a thiadiazole with an oxadiazole ring gives the synthesized polymer a highest occupied molecular orbital (HOMO) about 0.1 V lower, and lowest unoccupied molecular orbital energy levels lower than those of its benzothiadiazole (BT) counterpart, due to the more electron‐deficient oxadiazole. Furthermore, since oxadiazole has a larger dipole moment than BT, P‐PDT‐DFBO exhibits greater aggregation strength than previously reported for P‐PDT‐DFBT. The low‐lying HOMO level of P‐PDT‐DFBO gave about 0.1 V higher open‐circuit voltage (Voc), yielding over 0.9 V in a fabricated solar cell. From grazing incidence wide‐angle X‐ray diffraction analysis, P‐PDT‐DFBO formed a favorable face‐on orientation in both neat and blended films, indicating that the incorporation of an oxadiazole moiety can enhance Voc without any orientation change in the solid state. However, a P‐PDT‐DFBO‐based cell exhibited significantly lower Jsc and FF, and thus less power conversion efficiency, not >4.43%, due to its lower hole mobility than P‐PDT‐DFBT. One possible reason for poor performance may be the low crystallinity of P‐PDT‐DFBO in blended film. This may be caused by its strong aggregation tendency, leading to fast crystallization into a semiamorphous structure or to interference with the construction of long‐range ordered structure. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018, 56, 2646–2655

Utilizing magnetic field to study the impact of intramolecular charge transfers on the open-circuit voltage of organic solar cells

Improving the power conversion efficiency of organic solar cells is the essential theme for future application. Open-circuit voltage (Voc) loss of photovoltaic devices is a serious problem preventing performance enhancement. Here, in block copolymer based photovoltaic devices, we study intramolecular charge transfer (intra-CT) states dependent Voc loss (Vloss) through involving multi-beam excitation together with the magnetic field. Along with generating substantial intra-CT states in organic solar cells, the performance of Voc is improved. Magnetic field could increase triplet intra-CT states in copolymer to further decrease Vloss. Furthermore, intra-CTs could result in larger charge dissociation to induce photoluminescence quenching. Overall, we observe that intra-CT states in block copolymer play an important role for the Voc performance in organic solar cells.

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.

Effect of Band Bending and Energy Level Alignment at the Donor/Acceptor Interface on Open-Circuit Voltage in Organic Solar Cells.

Open-circuit voltage ( VOC) is a key parameter governing the power conversion efficiency of organic solar cells. We clarified the effect of band bending in organic semiconductor films on the VOC of phthalocyanine (H2Pc)/fullerene (C60) planar heterojunction (PHJ) OSCs. The VOC was significantly affected by the H2Pc layer thickness, varying from 0.20 V at 2 nm to 0.49 V at 100 nm thickness. The large VOC change is due to the band bending in the H2Pc layer, where the difference in H2Pc layer thickness led to significant changes in the degree of the vacuum-level shift in the C60 layer because of the Fermi level ( EF) difference between the H2Pc and C60 layers. This result demonstrated that a new influential factor affecting the VOC is the position of the EF that is dependent on band bending in organic semiconductor films.

Impact of Triplet Excited States on the Open‐Circuit Voltage of Organic Solar Cells

AbstractThe best organic solar cells (OSCs) achieve comparable peak external quantum efficiencies and fill factors as conventional photovoltaic devices. However, their voltage losses are much higher, in particular those due to nonradiative recombination. To investigate the possible role of triplet states on the donor or acceptor materials in this process, model systems comprising Zn‐ and Cu‐phthalocyanine (Pc), as well as fluorinated versions of these donors, combined with C60 as acceptor are studied. Fluorination allows tuning the energy level alignment between the lowest energy triplet state (T1) and the charge‐transfer (CT) state, while the replacement of Zn by Cu as the central metal in the Pcs leads to a largely enhanced spin–orbit coupling. Only in the latter case, a substantial influence of the triplet state on the nonradiative voltage losses is observed. In contrast, it is found that for a large series of typical OSC materials, the relative energy level alignment between T1 and the CT state does not substantially affect nonradiative voltage losses.

Enhancement in Open-Circuit Voltage in Organic Solar Cells by Using Ladder-Type Nonfullerene Acceptors.

The open-circuit voltage ( Voc) loss has always been a major factor in lowering power conversion efficiencies (PCEs) in bulk heterojunction organic photovoltaic cells (OPVs). A method to improve the Voc is indispensable to achieve high PCEs. In this paper, we investigated a series of perylene diimide-based ladder-type molecules as electron acceptors in nonfullerene OPVs. The D-A ladder-type structures described here lock our π-systems into a planar structure and eliminate bond twisting associated with linear conjugated systems. This enlarges the interface energy gap (Δ EDA), extends electronic delocalization, and hence improves the Voc. More importantly, these devices showed an increase in Voc without compromising either the Jsc or the FF. C5r exhibited a strong intermolecular interaction and a PCE value of 6.1%. Moreover, grazing-incident wide-angle X-ray scattering analysis and atomic force microscopy images suggested that our fused-ring acceptors showed a suitable domain size and uniform blend films, which were not affected by their rigid molecular structures.

Impact of Triplet Excited States on the Open-Circuit Voltage of Organic Solar Cells

Impact of Triplet Excited States on the Open-Circuit Voltage of Organic Solar Cells