Organic Photovoltaic Materials Research Articles

Organic Photovoltaic Acceptors: Crystal Engineering in Organic Photovoltaic Acceptors: A 3D Network Approach (Adv. Energy Mater. 47/2020)

In article number 2002678, Feng He and co-worker summarize the relationship between structure design, packing arrangement and molecular properties of organic photovoltaic acceptors. The concept of a “3D network acceptor” is proposed, with an aim to deepening the understanding of the electronic processes in the active layer and providing guidance for developing new generation organic photovoltaic materials.

A Simple Approach for Unraveling Optoelectronic Processes in Organic Solar Cells under Short‐Circuit Conditions

AbstractThe short‐circuit current (Jsc) of organic solar cells is defined by the interplay of exciton photogeneration in the active layer, geminate and non‐geminate recombination losses and free charge carrier extraction. The method proposed in this work allows the quantification of geminate recombination and the determination of the mobility‐lifetime product (µτ) as a single integrated parameter for charge transport and non‐geminate recombination. Furthermore, the extraction efficiency is quantified based on the obtained µτ product. Only readily available experimental methods (current‐voltage characteristics, external quantum efficiency measurements) are employed, which are coupled with an optical transfer matrix method simulation. The required optical properties of common organic photovoltaic (OPV) materials are provided in this work. The new approach is applied to three OPV systems in inverted or conventional device structures, and the results are juxtaposed against the µτ values obtained by an independent method based on the voltage–capacitance spectroscopy technique. Furthermore, it is demonstrated that the new method can accurately predict the optimal active layer thickness.

Benefits of direct electron detection and PCA for EELS investigation of organic photovoltaics materials

Electron energy-loss spectroscopy (EELS) is a powerful tool for imaging chemical variations at the nanoscale. Here, we investigate a polymer/organic small molecule-blend used as absorber layer in an organic solar cell and employ EELS for distinguishing polymer donor and small molecule acceptor domains in the nanostructured blend based on elemental maps of light elements, such as nitrogen, sulfur or fluorine. Especially for beam sensitive samples, the electron dose needs to be limited, therefore optimized acquisition and data processing strategies are required. We compare data acquired on a post-column energy filter with a direct electron detection camera to data from a conventional CCD camera on the same filter and we investigate the impact of statistical data processing methods (principal components analysis, PCA) on acquired spectra and elemental maps extracted from spectrum images. Our work shows, that the quality of spectra on a direct electron detection camera is far superior to conventional CCD imaging, and thereby allows clear identification of ionization edges and the fine structure of these edges. For the quality of the elemental maps, the application of PCA is essential to allow a clear separation between the donor and acceptor phase in the bulk heterojunction absorber layer of a non-fullerene organic solar cell.

Machine learning property prediction for organic photovoltaic devices

Organic photovoltaic (OPV) materials are promising candidates for cheap, printable solar cells. However, there are a very large number of potential donors and acceptors, making selection of the best materials difficult. Here, we show that machine-learning approaches can leverage computationally expensive DFT calculations to estimate important OPV materials properties quickly and accurately. We generate quantitative relationships between simple and interpretable chemical signature and one-hot descriptors and OPV power conversion efficiency (PCE), open circuit potential (Voc), short circuit density (Jsc), highest occupied molecular orbital (HOMO) energy, lowest unoccupied molecular orbital (LUMO) energy, and the HOMO–LUMO gap. The most robust and predictive models could predict PCE (computed by DFT) with a standard error of ±0.5 for percentage PCE for both the training and test set. This model is useful for pre-screening potential donor and acceptor materials for OPV applications, accelerating design of these devices for green energy applications.

Naphtho[2,3-c]thiophene-4,9-dione based polymers for efficient fullerene solar cells

Designing and synthesizing new organic photovoltaic materials is an inexhaustible driving force for the development of polymer solar cells (PSCs). For an excellent molecular structure with a certain conjugated skeleton, studying its analogues is of considerable significance for fully exploiting its photovoltaic potential. In this work, naphtho[2,3-c]thiophene-4,9-dione (NTD), an analogue of benzo [1,2-c:4,5-c’]dithiophene-4,8-dione (BDD), was designed and synthesized. Based on this building block, alkoxy side chains and alkylthio side chains were grafted on NTD to build NTD-O and NTD-S unit respectively, and then two new D-A conjugated donor polymers, PBNO and PBNS, were synthesized with NTD-O and NTD-S units as acceptor units and asymmetric benzodithiophene unit (asy-BDTBP) as donor units. Later, the photoelectric properties of these two materials in fullerene PSCs were studied. Compared with BDD based benzodithiophene copolymer (asy-BDTBP-BDD), PBNO and PBNS show similar LUMO energy levels, −3.59 eV and −3.63 eV, respectively, PBNO and PBNS show higher HOMO energy levels, which are −5.25 eV and −5.37 eV, respectively. PBNO blended with fullerene acceptor PC71BM exhibited similar power conversion efficiency (PCE) compared to the copolymer asy-BDTBP-BDD (4.64%). The PCE is 4.71% with short-circuit current density (JSC) of 9.85 mA cm−2, open-circuit voltage (VOC) of 0.784 V and fill factor (FF) of 60.98%. Encouragingly, PBNS presented an obviously enhanced PCE in PC71BM system PSCs, and PCE boosted to 7.58% (JSC, 12.40 mA cm−2; VOC, 0.865 V; FF, 70.83%). This work shows that analogues of excellent molecules have huge photovoltaic potential, and that side-chain modification has a significant impact on improving the photovoltaic performance of PSCs.

Charge Separation and Charge Transfer in the Low-Lying Excited States of Pentacene

Pentacene thin films are common constituents of organic photovoltaic materials and a prototypical example of a material that undergoes singlet exciton fission, but significant questions remain rega...

Selenium Heterocyclic Electron Acceptor with Small Urbach Energy for As-Cast High-Performance Organic Solar Cells.

Typical organic photovoltaic materials show high Urbach energies (ca. 25-50 meV), which is considerably higher than those of their inorganic counterparts and limits further improvement in the device efficiency of organic solar cells (OSCs). In this study, we introduce a facile method of selenium substitution to reduce the Urbach energy of organic photovoltaic materials to 20.4 meV (Y6Se), which is the lowest value reported for high-performance organic photovoltaic materials and very close to those (ca. 15 meV) of typical inorganic/hybrid semiconductors, such as crystalline silicon, gallium nitride, and lead-halide perovskite. Next, OSCs based on Y6Se showed 17.7% efficiency, which is among the best results for OSCs and the record efficiency of as-cast single junction OSCs to date.

SEPARAZIONE DI CARICA IN MISCELE DI MATERIALI ORGANICI: VALE LA LEGGE DI COULOMB?

After a brief introduction to organic photovoltaic materials, I discuss the problem of charge separation in these systems. In the best devices, the excitations produced by absorption of a photon can be converted into positive and negative charge carriers (holes and electrons, respectively) with near-100% efficiency. Such efficiencies are apparently at odds with Coulomb’s law, which would lead to change recombination due to their mutual electrostatic attraction. A theoretical model developed within my group overcomes this apparent contradiction, by taking into account quantum mechanical effects such as the delocalization of the charge carriers.

Reducing Energy Loss and Morphology Optimization Manipulated by Molecular Geometry Engineering for Hetero‐junction Organic Solar Cells

Summary of main observation and conclusionMolecular geometry engineering is an effective strategy to control the micromorphology and molecular energy level in organic photovoltaics (OPVs). Two novel copolymers based on alkylsilyl‐ and chloride‐functionalized benzodithiophene (BDT) were designed and synthesized for wide bandgap copolymer donor materials in OPVs. It was found that the two copolymers exhibited distinctly different properties in active layer when blended with fullerene‐free acceptor IT‐4F. The chloride‐functionalized copolymer PBDTCl‐TZ with deeper molecular energy level and better coplanar structure induced more ordered aggregation in blend film. Thus, the device based on PBDTCl‐TZ exhibits better energy alignment with IT‐4F and smaller radiative recombination. Furthermore, the non‐radiative recombination of PBDTCl‐TZ:IT‐4F based device is about 45 mV lower than the PBDTSi‐TZ:IT‐4F based device, contributing to a lower energy loss (Eloss), and a higher open‐circut voltage (VOC). As a result, the devices based on the blend of PBDTCl‐TZ:IT‐4F exhibit a high power conversion efficiency (PCE) of up to 12.2% with a high VOC of 0.837 V, higher than that of PBDTSi‐TZ:IT‐4F, of which the PCE is 11.2% with a VOC of 0.781 V.

Organic Semiconductors for Vacuum-Deposited Planar Heterojunction Solar Cells.

Relative to widely used solution-processed bulk heterojunction organic solar cells (OSCs), planar heterojunction (PHJ) OSCs by vacuum-depositing active layers sequentially avoid tedious control of the blend film morphology, and it is easy to understand the physical process at the donor/acceptor interface. Here we summarize the developments of electron donor and acceptor materials for vacuum-deposited PHJ OSCs in the past decades and discuss the relationship between molecular structure and device performance. Finally, the challenges and prospects for the development of vacuum-deposited PHJ OSCs are also proposed. In addition to some basic requirements for high performance organic photovoltaic materials, such as broad and strong absorption, matched energy levels between donors and acceptors, and high charge carrier mobility, we suggest that extending the exciton diffusion length of organic photovoltaic materials should boost PHJ OSCs gradually become another option for organic photovoltaic application.

Application of organic photovoltaic materials (OPV) as greenhouse roof structures: A review

Organic Photovoltaic (OPV), as a third-generation PV technology, is becoming an appropriate substance used for greenhouse roofing structures. Semi-transparent OPV has a variety of merits such as low weight, flexibility, low environment impact and short energy payback time. Besides, it can harness larger amounts of sunlight as means of a very strong light absorbent material. This study shares some of the latest research which examines the feasibility of using semi-transparent, flexible organic photovoltaic (OPV) modules as greenhouse shading material. By using such modules, it may be possible to utilize the existing greenhouse-based agricultural areas for electricity production. The concept projects OPV modules to shade greenhouses and reduces excess solar energy which may result in reducing internal surrounding heat thus helps to mitigate the control environment. This will furthermore control plant heat stress which is one of the most important factors for plant growth. Some conclusion on the quality and quantity of plants with respect to the energy consumption in the greenhouse are also discussed.

Highly efficient near-infrared hybrid perovskite solar cells by integrating with a novel organic bulk-heterojunction

Abstract Extending photoelectric response to near-infrared region (NIR) has become an urgent subject for the research of perovskite solar cells (PSCs). However, it is still a challenge due to the shortage of matching NIR photovoltaic materials and device structure. The rapid development of NIR organic photovoltaic materials and devices (OPVs) in recent years offers new opportunity for developing such PSCs. Herein, to broaden the photoresponse of PSCs, a novel PBDB-TF: BTP-4Cl bulk-heterojunction (BHJ) organic layer was successfully integrated on the PSCs, which extended the photoresponse of the device to 950 nm. And more, to boost the utilization of NIR light, the Au nanorods (Au NRs) were introduced into the organic BHJ layer through localized surface plasmon effect (LSPR). To further improve the device stability and satisfy solution competition, the MoO3 was fabricated as hole transport layer substituting traditional Spiro-OMeTAD. After adopting such a device, the power conversion efficiency (PCE) was increased significantly from 16.67% to 21.55%, which was the optimum among the reported organic/perovskite hybrid solar cells and the devices employing MoO3 as the hole transport layer. The illumination and humidity stability of the perovskite/organic/noble metal integrated solar cells were also significantly improved. The devices under standard sunlight irradiation for 1000 h maintained more than 70% of initial PCE. This work demonstrates that the perovskite/organic/noble metal integrated structure is a novel and powerful approach to obtain efficient and stable NIR-harvesting PSCs.

A Universal Method to Enhance Flexibility and Stability of Organic Solar Cells by Constructing Insulating Matrices in Active Layers

AbstractWith the rapid development of power conversion efficiency (PCE), flexibility–stability of organic solar cells (OSCs) are becoming one of the primary barriers for commercialization. This work shows that insulating poly(aryl ether) (PAE) resins have highly twisted‐stiff backbones without any side chains, which possess excellent mechanical stability, thermal stability, and good compatibility with organic photovoltaic materials. After introducing 5 wt% PAE resin as supporting matrices into the bulk heterojunction (BHJ) layer, the device yields a high PCE of 16.13%. Importantly, the devices show impressive flexibility and improved stability with passivated morphology, such as PM6/Y6‐based devices with 30 wt% PAE retains the PCE of 15.17% and exhibits enhanced 4.4‐fold elongation at break (25.07%). This is the recorded stretchability of the BHJ layer for OSCs with PCE > 8%, and morphological changes during tensile deformation are first investigated by in situ wide‐angle X‐ray scattering measurements. The PAE matrices strategy exhibits good universality in the other four photovoltaic systems. These results demonstrate that heat‐resistant PAE resins serve as supporting matrices with a tunneling effect into OSCs without sacrificing photovoltaic performance and simultaneously improve the flexibility and stability of devices, which can play an important role in promoting the development of stable and wearable electronics.

A Genetic Algorithm Approach to Design Principles for Organic Photovoltaic Materials

AbstractThe increase in the efficiency of organic photovoltaic (OPV) devices relies on understanding the underlying science of several interconnected physical mechanisms that prevent the success of 1D optimization strategies. Here, a combination of kinetic Monte Carlo simulations of exciton dynamics with a genetic algorithm to automatically optimize the external quantum efficiency of donor–acceptor interfaces under different scenarios is employed. Simulations include phenomena from light absorption to exciton diffusion, dissociation, radiative recombination, and internal conversion, thus modeling the main physical processes that define the overall efficiency of an OPV up to charge separation. It is shown that when internal conversion is kept in check, the combination of optimal transition dipole moments and absorption energies points at low bandgap polymers as the most promising candidates for donor materials. However, when non‐radiative deexcitation mechanisms are stronger, the optimization strategy shifts toward higher bandgaps, focusing rather on increasing the fluorescence quantum yield of the donor. Finally, the approach shows that adjusting the energy levels of the acceptor so that exciton transfers across the interface become negligible produces important gains in efficiency and at the same time reduces the system's dependence on large electronic couplings. The findings indicate pathways for engineering highly efficient organic interfaces.

Rational design of naphthalimide based small molecules non-fullerene acceptors for organic solar cells

To explore optical electronic and charge transfer properties, a series of D-π-A type of molecules with central core of 9,9-dimethyl-9H-fluorene as donor linked by 6-fluoro-4-(prop-1-yn-1-yl)benzo[1,2,5]thiadiazole as π-bridge to variable end group acceptor materials have been designed for organic solar cells (OSCs). Optoelectronic properties of designed molecules M1-M4 with similar central core and π-bridge but with different end groups were compared with R as representative of our system with 1,8-naphthalimide as end group. These optoelectronic properties are influenced by different end groups. Lower band gaps and longer wavelength of absorption have been observed for molecules by analyzing their frontier molecular orbitals. Furthermore, the computed reorganization energies for designed molecules are also comparable to the R so these molecules can be used as electron and hole transport materials. Among designed molecules M3 has higher wavelength of absorption along with minimum band gap and suitable distribution pattern of HOMO and LUMO during transition. The results presented display that varying the end groups is a highly promising approach in order to develop a series of D-π-A type of materials for organic photovoltaics. So, our computed results display that these designed molecules can be used as an excellent candidates for OSCs.

Layer-Dependent Quasiparticle Electronic Structure of the P3HT:PCBM Interface from a First-Principles Substrate Screening GW Approach

A prototypical organic photovoltaic material is a heterojunction composed of the blend of regioregular poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C$_{61}$-butyric acid methyl ester (PCBM). Microscopic understanding of the energy conversion mechanism in this system involves the relationship between the electronic structure and the atomistic geometry of P3HT:PCBM interfaces. In this work, the effect of the number of P3HT layers on the electronic structure of the P3HT:PCBM interface is studied by means of first-principles $GW$. We apply the substrate screening approach to accelerate such calculations and to better understand the many-body dielectric screening at the interface. The quasiparticle band gap of the entire interface is found to decrease as the number of P3HT layers increases. The gaps of the individual components of the interface are found to be smaller than their isolated counterparts, with strong dependence on the number of P3HT layers. Importantly, when comparing the system of P3HT:PCBM - where a single interface is present - and the system of P3HT:PCBM:P3HT, where an interface is formed on either side of PCBM, we find that the two systems exhibit very different quasiparticle energy level alignments. We discuss possible implications of our findings in related experiments. The observed trends in layer-dependent quasiparticle electronic structure of P3HT:PCBM interfaces provide computational insight into energy conversion pathways in these materials.

Analytical Modeling Based on Modified Effective Medium Theories for Optical Properties of Photovoltaic Material-Incorporated Plasmonic Nanoparticles

Theoretical guidance on the optical properties of plasmonic nanoparticles (NPs) is of significant importance in tremendous numbers of fields like photovoltaics. The incorporation of plasmonic NPs into photovoltaic material can promote optical absorption either via the excitation of localized surface plasmon resonance (LSPR) modes or due to multiple light scattering. Since most fabrication techniques for the incorporation of NPs into photovoltaic material result in a random array of NPs with various sizes, numerical simulations based on solving the Maxwell equations are computationally expensive and prohibitively slow for this large number of NPs. Therefore, in this paper, based on modified effective medium theories, taking into account finite size of NPs, size dispersion for NPs, extrinsic dynamic effect, and intrinsic confinement effect, fast and cost-effective analytical modeling, considering both LSPR and scattering effects, is presented to obtain the optical properties of photovoltaic material incorporated by spherical NPs with nonuniform size and random distribution. Then, by means of presented analytical modeling, considering reasonably low and high volume fractions of NPs in addition to small and large size of NPs, the effect of different parameters of embedded NPs into organic and inorganic photovoltaic materials is explored.

N-Type Molecular Photovoltaic Materials: Design Strategies and Device Applications.

The use of photovoltaic technologies has been regarded as a promising approach for converting solar energy to electricity and mitigating the energy crisis, and among these, organic photovoltaics (OPVs) have attracted broad interest because of their solution processability, flexibility, light weight, and potential for large-area processing. The development of OPV materials, especially electron acceptors, has been one of the focuses in recent years. Compared with fullerene derivates, n-type non-fullerene molecules have some unique merits, such as synthetic simplicity, high tunability of the absorption and energy levels, and small energy loss. In the last 5 years, organic solar cells based on n-type non-fullerene molecules have achieved a significant breakthrough in the power conversion efficiency from approximately 4% to over 17%, which is superior to those of fullerene-based solar cells; meanwhile, n-type non-fullerene molecules have created brand new opportunities for the application of OPVs in some special situations. This Perspective analyzes the key design strategies of high-performance n-type molecular photovoltaic materials and highlights instructive examples of their various applications, including in ternary and tandem solar cells, high-efficiency semitransparent solar cells for power-generating building facades and windows, and indoor photovoltaics for driving low-power-consumption devices. Moreover, to accelerate the pace toward commercialization of OPVs, the existing challenges and future directions are also reviewed from the perspectives of efficiency, stability, and large-area fabrication.

The effect of different aromatic conjugated bridges on optoelectronic properties of diketopyrrolopyrrole-based donor materials for organic photovoltaics.

A series of twelve Acceptor-π-Donor-π-Acceptor (A-π-D-π-A) topology-based donor molecules, where diketopyrrolopyrrole (DPP) as donor core unit is connected through furan which acts as conjugated π-bridge (CB) to aromatic derivatives (Ar) as acceptor units, have been investigated by making substitutions in acceptor units by using density functional theory(DFT) and time-dependent density functional theory (TD-DFT) for organic solar cell applications. The comparative study of optoelectronic properties indicates that thiadiazole with pyridine units containing molecules (M6b) exhibit lower energy of highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels than those of oxadiazole and pyridine containing units (M6b). Among our investigated donors, the smallest Eg of 1.60eV was observed for both M6a and M6b with distinctive broad absorption at 843 and 857nm, respectively. Overall, smaller electron transfer (λe) values in contrast to hole transfer (λh) demonstrate that these donor compounds would be best for λe. The calculated open circuit voltage (Voc) is 2.45 and 2.17eV, regarding bisPCBM and PC60BM (phenyl-C61-butyric acid methyl ester) acceptors. Thus, these theoretical calculations not only endorse the deep consideration between the chemical structures and optoelectronic characteristics of the donor-acceptor systems but also suggest appropriate materials for high-performance Organic Photovoltaics (OPV). Graphical abstract.

Constructing highly efficient all-inorganic perovskite solar cells with efficiency exceeding 17% by using dopant-free polymeric electron-donor materials

Abstract Development of all-inorganic perovskite solar cells (PSCs) is highly desirable due to its excellent thermal stability and great potential for tandem solar cells. One main bottleneck that restricts its further development is the absence of the ideal hole-transporting materials which is critical to device performance and stability. Herein, we construct a brand-new architecture of all-inorganic PSCs via employing a series of doping-free polymeric organic photovoltaic donor materials as hole transport materials including J71, PBDB-T, PM6 with suitable energy level and carrier transport ability. The mechanism in terms of film molecular microscopic structure, electrical transport dynamics, electrical potential distribution were systematically investigated by grazing incidence wide-angle X-ray scattering (GIWAXS), conductive atomic force microscope (C-AFM) and kelvin probe force microscopy (KPFM) measurements. This work not only proposes a series of new dopant-free polymeric hole transporting materials for all-inorganic PSCs, but also demonstrates the fabrication of high efficiency devices with power conversion efficiencies over 17%, a record of all-inorganic PSCs based on dopant-free polymeric materials to our knowledge.