Heterojunction Organic Photovoltaics Research Articles

Non-fullerene organic photovoltaics based on thienopyrroledione comprising random copolymers; effect of alkyl chains

Two new random donor-acceptor (D-A) copolymers, signed as P1 and P2, were designed and synthesized. Electrochemical and spectroelectrochemical measurements were performed to investigate absorption, energy levels, electronic and optical band gaps for comparison. The polymers were used as donor polymers in the active layer to fabricate non-fullerene, bulk heterojunction (BHJ) organic photovoltaics (OPVs). Investigations were carried out through the conventional BHJ structure; ITO/PEDOT: PSS/Active Layer/LiF/Al, where active layer consists of 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2′,3′-d’]-s-indaceno[1,2-b:5,6-b’]dithiophene (ITIC) as the acceptor and thienopyrroledione containing donors. The device based on P1:ITIC(1:1) blend with a thickness of 161 nm gave the best performance with a power conversion efficiency (PCE) of 7.94%, an open-circuit voltage (VOC) of 0.86 V, a short-current density (JSC) of 18.45 mA cm−2 and a fill factor (FF) of 50.12%. The highest PCE obtained from P2 based organic solar cell is 1.96%. P2 exhibited low solubility attributed to the lack of alkyl groups enhancing polymer solubility, electronic properties, and photovoltaic performances. The research outputs exhibit that introduction of alkyl chains on the polymer backbone can enhance device performance.

Continuum Approach for Studying Morphological Deformations of Multiple-Phase Organic Photovoltaic Cells

Optimizing the properties of the mosaic morphology of bulk heterojunction (BHJ) organic photovoltaics (OPV) is not only challenging technologically but also intriguing from the mechanistic point of view. Among the recent breakthroughs in studies of BHJ morphology, is the identification and utilization of a three-phase (donor/mixed/acceptor) BHJ, where the (intermediate) mixed-phase can inhibit morphological changes, such as phase separation. Using a continuum approach, we develop and analyze a model that undertakes the coupling between the spatiotemporal evolution of the material and charge dynamics along with charge transfer at the device electrodes. Starting from an ideal BHJ represented by stripes, we reveal and distinguish between generic mechanisms that alter the evolution of stripes: the bending (zigzag mode) and the pinching (cross-roll mode) of the donor/acceptor domains. We then emphasize that the latter instability mode, is notorious as it leads to the formation of disconnected domains and hence to loss of charge flux. Consequently, the analysis implies that donor-acceptor mixtures with higher mixing energy are more likely to develop pinching under charge-flux boundary conditions. We believe that these results provide a qualitative roadmap for morphological BHJ optimization, using mixed-phase composition and moreover, open new vistas to application involving three phase morphologies, such as ion-intercalated rechargeable batteries.

Effects of the photoactive layer properties and current transmission mechanism on optical and electrical characteristics of organic photovoltaic

The P3HT:PCBM is an important organic material as an active layer for bulk heterojunction (BHJ) organic photovoltaics (OPVs). However, in terms of photophysical properties, blend ratios of both materials and device design is an important parameter for OPVs devices. In this study, the P3HT:PCBM photoactive layer, blended in different proportions by mass (1:1, 1:0.8, 1:0.6) was coated on the PEDOT:PSS surface using the spin coater method for different devices. Finally, Al have been evaporated on the photoactive layer and the device structure completed. In other examples, 2 nm lithium fluoride (LiF) as buffer layer evaporated on the active layer for each blending ratio before metal. Optical absorption, AFM, XRD, Raman and XPS measurements were taken for each active layer and it was observed that the surface morphology and absorption characteristic of the active layer changed according to the mixing ratios. This change in active layer properties directly affected the device parameters. Electrical characterization of the six different devices performed with current density-voltage (J–V) measurement and by used a solar simulator under 100 mW/cm2 irradiation intensity. Depending on the increase in short circuit current density (Jsc) and open current voltage (Voc) values in the devices, the power conversion efficiency (PCE) of the OPVs obtained varied between 0.86% and 2.74%. This change can be attributed to the fact that the P3HT:PCBM photoactive layer forms a smoother surface morphology depending on the PCBM ratio and the series resistance decreases. Another reason for this change in PCE can be explained by the effective electron injection of the LiF layer, LiF/Al layer with lower work function and less carrier deposition at its metal-active layer interfaces. It result has been obtained that the active layer with a ratio of 1:0.8 has a more homogeneous surface morphology and the series resistance decreases, the presence of the LiF layer in the device structures increases the Jsc and Voc, accordingly the PCE increases to 2.74%. These results showed that BHJ OPV with high PCE can be obtained with the ideal mixing ratio and appropriate device design.

Towards Improving the Efficiency of Organic Solar Cells by Coarse-Grained Atomistic Modeling of Processing Dependent Morphologies

Solar energy conversion to electricity using organic semiconductor materials is a complex process due to the underlying transport physics of electrons and photons. Experimental characterizations of the 3-D morphology of bulk heterojunction organic photovoltaics are challenging; the poor contrast of the reconstructed morphology due to weak electronic scattering of organic molecules is a major impediment for microstructural imaging by electron microscopy. Thus, enhancing the power-conversion efficiency (PCE) of organic solar cells requires predictive design of both material and processing parameters. To this end, large-scale coarse-grained molecular simulations are needed to probe the morphology of the nanostructures, to develop process-structure-performance correlations that assist in fundamental understanding of the physical mechanisms, and to simultaneously aid in selection and optimization of design parameters. Here, we summarize the outcomes from high-performance coarse-grained molecular dynamics simulations that are employed to mimic solvent evaporation and thermal annealing of typical bulk heterojunction solar cell active layers. The latter consist of a blend of electron-donor and electron-acceptor materials. We extensively explore the dependence of PCE and the thermo-mechanical stability of the blend morphology on different solution processing conditions, and correlate the identified parameters with the dominant design variables and the microstructure. The simulations reveal that the composition of constituent donor and acceptor materials, respective molecular weights, polydispersity of donor polymer chains, and the thermal annealing temperature are the major parameters that significantly impact the morphology, thermo-mechanical stability, and subsequently the PCE of organic photovoltaics.

Bending and pinching of three-phase stripes: From secondary instabilities to morphological deformations in organic photovoltaics.

Optimizing the properties of the mosaic nanoscale morphology of bulk heterojunction (BHJ) organic photovoltaics (OPV) is not only challenging technologically but also intriguing from the mechanistic point of view. Among the recent breakthroughs is the identification and utilization of a three-phase (donor-mixed-acceptor) BHJ, where the (intermediate) mixed phase can inhibit mesoscale morphological changes, such as phase separation. Using a mean-field approach, we reveal and distinguish between generic mechanisms that alter, through transverse instabilities, the evolution of stripes: the bending (zigzag mode) and the pinching (cross-roll mode) of the donor-acceptor domains. The results are summarized in a parameter plane spanned by the mixing energy and illumination, and show that donor-acceptor mixtures with higher mixing energy are more likely to develop pinching under charge-flux boundary conditions. The latter is notorious as it leads to the formation of disconnected domains and hence to loss of charge flux. We believe that these results provide a qualitative road map for BHJ optimization, using mixed-phase composition and, therefore, an essential step toward long-lasting OPV. More broadly, the results are also of relevance to study the coexistence of multiple-phase domains in material science, such as in ion-intercalated rechargeable batteries.

Benzodithiophene-based wide-bandgap small-molecule donors for organic photovoltaics with large open-circuit voltages

Abstract We have developed new small-molecule electron donors, SMBDT-S and SMBDT-SF, based on benzo[1,2-b:4,5-b’]dithiophene (BDT) with thiophene side chains as a core unit and rhodanine end-capping units to enhance the open-circuit voltage (Voc), which is one of the factors limiting performance of small-molecule organic photovoltaics (OPVs). The SMBDT-S and SMBDT-SF donor molecules exhibited excellent thermal stability and high crystallinity. SMBDT-S was found to have a wide bandgap of 1.85 eV with the highest occupied molecular orbital (HOMO) energy level of − 5.56 eV. The fluorination in SMBDT-SF led to the same bandgap of 1.86 eV with an even lower-lying HOMO energy level of − 5.72 eV. Both SMBDT-S and SMBDT-SF have large thin-film absorption coefficients of (1.4–1.5) × 105 cm−1. Bulk heterojunction OPV devices based on pairing SMBDT-S and SMBDT-SF respectively with PC71BM electron acceptor showed Voc as high as 1.18 eV, which is among the highest Voc reported for all-small-molecule solar cells. These results demonstrate that introducing fluorine atoms and alkylthio side chains are effective strategies to downshift the HOMO energy level and enhance the Voc in OPVs.

Initial Engineering and Outdoor Stability Assessment of "Gray/Black" Fullerene-Free Organic Photovoltaics Based on Only Two Complementary Absorbing Materials: A Tetrabenzotriazacorrole and a Subphthalocyanine.

Broad absorption is a desired characteristic of materials employed in the photoactive layers of organic photovoltaic (OPV) devices. Here, we have identified tetrabenzotriazacorroles (Tbcs) as complementary absorbing chromophores and electron donors to the promising nonfullerene acceptors boron subphthalocyanines (BsubPcs). These two materials, which can be utilized as donor–acceptor pairs within fullerene-free OPVs, yield spectral coverage over the entire visible range of 300–750 nm. Oxy phosphorus Tbc derivative (POTbc) was employed as an electron donor and paired initially with multiple BsubPc derivatives having a distribution of highest occupied molecular orbital/lowest unoccupied molecular orbital energy levels in planar heterojunction OPVs. These devices were “gray/black” due to the broad absorption across the visible spectrum. Upon screening, the partially halogenated chloro hexachloro BsubPc (Cl–Cl6BsubPc) showed the greatest promise for coupling with POTbc. The thickness ratio and total thickness of the active layer were then probed in order to identify the optical and electrical limitations on the POTbc/Cl–Cl6BsubPc-based OPV device. A maximum power conversion efficiency (PCE) of 2.13% was achieved at 60 nm total thickness of the active layer and 1 to 3 (POTbc to Cl–Cl6BsubPc) thickness ratio. Outdoor stability of the champion device was evaluated using protocols established by International Summits on OPV Stability and was found to be on par with an α-sexithiophene/Cl–Cl6BsubPc baseline OPV.

Investigation of Hierarchical Structure Formation in Merocyanine Photovoltaics

Author(s): Murrey, TL; Hertel, D; Nowak, J; Bruker, R; Limbock, T; Neudorfl, J; Ruth, S; Schelter, J; Olthof, S; Radulescu, A; Moule, AJ; Meerholz, K | Abstract: Merocyanines (MCs) are a versatile class of small-molecule dyes. Their optoelectronic properties are easily tunable by chemically controlling their donor-acceptor strength, and their structural properties can be tuned by simple side-chain substitution. This manuscript demonstrates a novel series of MCs featuring an indoline donor with varying hydrocarbon side-chain length (from 6 to 12 carbons) and a tert-butyl-thiazole acceptor, labeled InTBT. Bulk heterojunction organic photovoltaics are fabricated with a [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) acceptor and characterized. Films composed of I8TBT:PCBM and I9TBT:PCBM produced the highest power conversion efficiency of 4.5%, which suggests that the morphology is optimized by controlling the side-chain length. Hierarchical structure formation in InTBT:PCBM films is studied using grazing incidence X-ray diffraction (GIXRD), small-angle neutron scattering (SANS), and atomic force microscopy (AFM). When mixed with PCBM, InTBTs with ≤8 side-chain carbons form pure crystalline domains, while InTBTs with ≥9 side-chain carbons mix well with PCBM. SANS demonstrates that increasing side-chain length increases the InTBT-rich domain size. In addition, a branched hexyl-dodecyl side-chain IHDTBT:PCBM film was studied and found to exhibit the worst-performance organic photovoltaic (OPV) device. The large-branched side chain inhibited mixing between IHDTBT and PCBM resulting in large segregated phases.

Ultrafast Charge Dynamics in Dilute-Donor versus Highly Intermixed TAPC:C60 Organic Solar Cell Blends.

Elucidating the interplay between film morphology, photophysics, and device performance of bulk heterojunction (BHJ) organic photovoltaics remains challenging. Here, we use the well-defined morphology of vapor-deposited di-[4-(N,N-di-p-tolyl-amino)-phenyl]cyclohexane (TAPC):C60 blends to address charge generation and recombination by transient ultrafast spectroscopy. We gain relevant new insights to the functioning of dilute-donor (5% TAPC) fullerene-based BHJs compared to molecularly intermixed systems (50% TAPC). First, we show that intermolecular charge-transfer (CT) excitons in the C60 clusters of dilute BHJs rapidly localize to Frenkel excitons prior to dissociating at the donor:acceptor interface. Thus, both Frenkel and CT excitons generate photocurrent over the entire fullerene absorption range. Second, we selectively monitor interfacial and bulk C60 clusters via their electro-absorption, demonstrating an energetic gradient that assists free charge generation. Third, we identify a fast (<1 ns) recombination channel, whereby free electrons recombine with trapped holes on isolated TAPC molecules. This can harm the performance of dilute solar cells, unless the electrons are rapidly extracted in efficient devices.

Electric field effects on organic photovoltaic heterojunction interfaces: The model case of pentacene/C60

The applied electric field on organic heterojunction interface can significantly affect organic photovoltaic (OPV) performance. Here, to explore electric field effects on OPV heterojunction interface, the pentacene/C60 complexes with face-on and edge-on configurations were constructed as model system, based upon quantum chemistry calculations that take into account of solid polarization effects and apply density functional with optimally tuned range separation parameters, we systematically studied the electric fields effects on the complexes’ geometries, electronic structures, excitation and spectral properties, as well as the rate constants of charge transfer (CT), exciton dissociation (ED) and charge recombination (CR) processes. The electric field can effectively modify interface electronic structures and energy level alignments, and the increasing of electric field that is perpendicular to interface can increase CT energetic loss, reduce the lowest CT excitation energy and the open-circuit voltage. The introducing electric field can modulate the kinetics of electron processes at heterojunction interface.

Porphyrins-based Materials as Efficient Donor in Organic Solar Cells

Twenty years after the discovery of bulk heterojunction solar cells (BHJSCs), porphyrins were underrepresented in the field of organic solar cells (OCSs)1 despite their high epsilon of 300 000 M-1cm-1 and their electron rich aromatic core, which make them a good electron donors. In 2012, a power conversion efficiency (PCE) of 8.08% for a BHJSC built with a small molecule containing a porphyrin core was reported.2 Then in 2017, a PCE of 6.44 % was reported for BHJSC constructed with a polymer containing a porphyrin inside the conjugated chain.3 These advances brought porphyrins at the center of attention in the field of OSCs. In 2019, using a planar non-fullerene acceptor and a porphyrin based-polymer, a 9.16% PCE was reported; a record for this type of dye.4 Both a high Voc of 1.01 V and a low Eloss of 0.45 eV, two BHJSC’s characteristics, make the porphyrin dyes competing with the most efficient donors of the field. Mechanistically speaking, like all donors porphyrin-based donors form interfacial charge transfer (CT) species with the acceptors where upon excitation, a charge separated (CS) state is produced efficiently. However, detailed investigations of the CT signature and photophysical traits of these species simply do exit due to the bulk nature of the materials. We designed a supramolecular porphyrin-graphene nanoribbon (GNR) model taking advantage of π-stacking effects between GNRs and anchoring pyrene groups. Upon selective excitation of the GNRs, the GNRs form excitons for which 95% are involved in an energy transfer with the π-stacked porphyrin-GNR CT species very efficiently (35%in 759 fs; 65% < 135 fs), then an electron transfer (charge separation) occurs from these excited CT species to the GNRs in the ps time scale. 1. Bucher, L.; Desbois, N.; Harvey, P.; Sharma, G.; Gros. C.; Porphyrins and BODIPY as Building Blocks for Efficient Donor Materials in Bulk Heterojunction Solar Cells. Solar RRL 2017, 1, 1700127.2. Huang, Y.; Li, L.; Peng, X.; Peng, J.; Cao, Y.; Solution processed small molecule bulk heterojunction organic photovoltaics based on a conjugated donor–acceptor porphyrin. J. Mater. Chem. 2012, 22, 21841-21844.3. Bucher, L.; Tanguy, L.; Fortin, D.; Desbois, N.; Harvey, P.; Sharma, G.; Gros, C.; A Very Low Band Gap Diketopyrrolopyrrole–Porphyrin Conjugated Polymer. Chempluschem 2017, 82, 625-630.4. Tanguy, L.; Malhotra, P.; Singh, S.; Brisard, G.; Sharma, G.; Harvey, P.; A 9.16% Power Conversion Efficiency Organic Solar Cell with a Porphyrin Conjugated Polymer Using a Nonfullerene Acceptor. ACS Appl. Mater. Interfaces 2019, 11, 31, 28078-28087. Figure 1

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.

Fusion of thienyl into the backbone of electron acceptor in organic photovoltaic heterojunctions: a comparative study of BTPT-4F and BTPTT-4F

The fusion of thiophene into a non-fullerene acceptor backbone causes significant suppression of the charge recombination rate at the donor/acceptor heterojunction interface.

Interface configuration effects on excitation, exciton dissociation, and charge recombination in organic photovoltaic heterojunction

AbstractThe morphology of donor‐acceptor heterojunction interface significantly affects the electron/hole processes in organic solar cells, including charge transfer (CT), exciton dissociation (ED), and charge recombination (CR). Here, to investigate interface molecular configuration effects, the donor‐acceptor complexes with face‐on, edge‐on, and end‐on configurations were constructed as model systems for the p‐SIDT(FBTTh2)2/C60 heterojunction. The geometries, electronic structures, and excitation properties of monomers and the complexes with three configurations were studied based on density functional theory (DFT) and time‐dependent DFT calculations with optimally tuned range separation parameters and solid polarization effects. In terms of Marcus theory, the rate constants of ED and CR processes were analyzed. The results show that most of the excited states for p‐SIDT(FBTTh2)2 exhibit an intramolecular CT character, and the similarity of the excitation characters (CT and local excitation) and energies among three complexes with different configurations indicate that the electronic structure and excitation properties are insensitive to the interfacial molecular configurations. However, the rates of ED and CR processes heavily depend on it. These results underline the importance of controlling molecular configuration and then the morphology at the heterojunction interface in organic solar cells.

Analysis of bulk heterojunction organic solar cell blends by solid-state NMR relaxometry and sensitive external quantum efficiency – Impact of polymer side chain variation on nanoscale morphology

Abstract A significant number of organic electronic devices rely on blends of electron-donating and electron-accepting molecules. In bulk heterojunction organic photovoltaics, the nanoscopic phase behavior of the two individual components within the photoactive layer has a major impact on the charge separation and charge transport properties. For polymer:fullerene solar cells, it has been hypothesized that an increased accessibility of the electron-deficient monomer unit in push-pull type low bandgap polymers allows for fullerene ‘docking’. The close proximity of electron donor and acceptor molecules enables more efficient charge transfer, which is beneficial for the device efficiency. With this in mind, we synthesized a series of PBDTTPD [poly(benzodithiophene-thienopyrroledione)] low bandgap copolymers with varying side chains. Solar cells were fabricated for all polymers and the device characteristics were compared. The combination of proton wideline solid-state NMR (ssNMR) relaxometry and sensitive external quantum efficiency (sEQE) measurements was shown to provide essential information on donor-acceptor interactions and phase separation in bulk heterojunction organic photovoltaics. The reduced charge transfer state absorption and the observed phase separation of crystalline PC71BM domains for the polymers containing the most accessible methyl-TPD unit indicate a diminished contact between donor and acceptor, leading to a loss in performance.

Entropy promotes charge separation in bulk heterojunction organic photovoltaics

Charge separation is one of the crucial processes in the photoelectric conversion of organic photovoltaics. It is considered that organic photovoltaics have low performances because of strong Coulomb attraction. In this work, an entropy-driven charge-separation mechanism was elucidated using a graph algorithm and dynamic Monte Carlo simulations. Helmholtz energies were decreased by entropies, and had a barrier (240–270 meV) at an electron-hole distance of 6 nm; separation efficiencies at interfaces of large donor: small acceptor domains were lower than those at large donor: large acceptor domains, which can be attributed to their higher barrier-separation was promoted by a decreased barrier.

Electrochemical Advances in Boron Subnaphthalocyanines (BsubNcs) for Organic Photovoltaic Applications: The Ambient Stability of Bsubnc Mixed Alloys; Their Separation and Electrochemical Properties; And Chemical Progresses Towards Pure Bsubncs to Yield Their Individual Electrochemical Evaluation

For some time, our group has been focused on the design, synthesis and application of derivatives of boron subphthalocyanines (BsubPcs). Our focal point has been and continues to be equally between the basic chemistry of BsubPcs and their application in organic electronic devices with a focus on two specific device applications, organic photovoltaics (OPVs)/solar cells[1,2] and organic light emitting diodes (OLEDs). More recently, we have taken an equal approach to derivatives similar to BsubPcs, boron subnaphthalocyanines (BsubNcs); BsubNcs being unique and beneficial materials for OPV application.[3] We have recently shown that BsubNcs actually become randomly chlorinated during their synthetic preparation and actually then form a mixed alloy composition of chlorinated materials, which we have designated as Cl-ClnBsubNcs.[4] The mixed alloy composition is unique, and has been determined to be a mixture of 24 (more or less) chlorinated BsubNcs that despite being a mixture form single crystals. The formation of single crystals is enabled by the chlorine atoms occupying vacancies within the solid state structure, the vacancies being the so-called “bay position” of the BsubNcs structure. After establishing the correlation of electrochemical characteristics and OPV performance for Cl-ClnBsubNcs, we have recently shown that phenoxylated versions of ClnBsubNc are also a mixed alloy and applicable in BHJ OPVs also as electron accepting materials.[5] We have also shown that Cl-ClnBsubNcs when applied as electron donating materials in OPVs have the same performance correlations to the amount of chlorination present.[6] We have also shown that BsubNcs are equally stable within an OPV in the ambient environment compared with BsubPcs.[7] This accumulation of properties led us to consider how to separate the alloyed mixtures and how to prepare pure BsubNcs. During this presentation I will describe a chemical pathway to enable the separation of the mixed alloys of BsubNcs. We found that not all 24 (more or less) isomers can be separated, yet this process has enabled a more detailed electrochemical characterization and correlation to amount of halogenation. I will also describe a chemical pathway to pure BsubNcs, avoiding halogenation/chlorination in the bay position. This pathway enables a detailed characterization of a pure BsubNc including its electrochemical properties. Finally, if completed by the presentation time, I will also describe a chemical pathway to enable complete halogenation of the bay-position of a BsubNc. This pathway then enabled the characterization and therefore the impact on the electrochemical and other physical properties of complete bay-position halogenation. Co-authors/investigators will be identified during this presentation. References. [1] “Outdoor Performance and Stability of Boron Subphthalocyanines Applied as Electron Acceptors in Fullerene-Free Organic Photovoltaics.” Josey, D.; et al, ACS Energy Lett., 2017, 2 (3), 726–732. DOI: 10.1021/acsenergylett.6b00716. [2] “Boron Subphthalocyanines as Electron Donors in Outdoor Lifetime Monitored Organic Photovoltaic Cells.” Garner, R.K.; et al, Solar Energy Materials and Solar Cells, 2018 176, 331-335. DOI: 10.1016/j.solmat.2017.10.018 [3] “8.4% efficient fullerene-free organic solar cells exploiting long-range exciton energy transfer” Cnops, K.; et al., Nature Comm., 5, Article number: 3406, DOI:10.1038/ncomms4406. [4] “The mixed and alloyed chemical composition of chloro-(chloro)n-boron subnaphthalocyanines dictates their physical properties and performance in organic photovoltaics.” Dang, J.D.; et al, J. Mat. Chem. A., 2016, 4, 9566-9577. [5] “Phenoxy-(chloro)n-boron subnaphthalocyanines; alloyed mixture, electron-accepting functionality, enhanced solubility for bulk heterojunction organic photovoltaics” Dang, J.D.; et al, ACS Omega, 2018, 3(2), 2093–2103. DOI: 10.1021/acsomega.7b01892. [6] “The Mixed Alloyed Chemical Composition of Chloro-(chloro)n-Boron Subnaphthalocyanines Dictates Their Performance as Electron-Donating and Hole-Transporting Materials in Organic Photovoltaics” Garner, R.K.; et al, ACS Appl. Energy Materials, 2017, 1(3), 1029-1036. DOI: 10.1021/acsaem.7b00180. [7] "Outdoor Stability of Chloro-(Chloro)n-Boron Subnaphthalocyanine and Chloro-Boron Subphthalocyanine as Electron Acceptors in Bilayer and Trilayer Organic Photovoltaics" Josey, D.; et al, ACS Applied Energy Materials, 2018, submitted. Figure 1

Pattern formation aspects of electrically charged tri-stable media with implications to bulk heterojunction in organic photovoltaics

A common thread in designing electrochemically based renewable energy devices comprises materials that exploit nano-scale morphologies, e.g., supercapacitors, batteries, fuel cells, and bulk heterojunction organic photovoltaics. In these devices, however, Coulomb forces often influence the fine nano-details of the morphological structure of active layers leading to a notorious decrease in performance. By focusing on bulk heterojunction organic photovoltaics as a case model, a self-consistent mean-field framework that combines binary (bi-stable) and ternary (tri-stable) morphologies with electrokinetics is presented and analyzed, i.e., undertaking the coupling between the spatiotemporal evolution of the material and charge dynamics along with charge transfer at the device electrodes. Particularly, it is shown that tri-stable composition may stabilize stripe morphology that is ideal bulk heterojuction. Moreover, since the results rely on generic principles they are expected to be applicable to a broad range of electrically charged amphiphilic-type mixtures, such as emulsions, polyelectrolytes, and ionic liquids.

The Twisted Benzo[ghi]-Perylenetriimide Dimer as a 3D Electron Acceptor for Fullerene-Free Organic Photovoltaics.

A series of twisted N,N-linked benzo[ghi]-perylenetriimide dimers (t-BPTI) with various lengths of the α-branched alkyl side chain at the six-membered imide ring position was designed, synthesized, and characterized. These compounds showed the low-lying LUMO energy level of -3.78 eV, which was similar to that of PC61 BM (-3.71 eV), but with intensive optical absorption in the range 350-500 nm. The twisted molecular geometry with two nearly perpendicular BPTI planes achieved a favorable nanoscale phase separation by relieving the self-aggregation of rigid BPTI units in blend films. The acceptor t-BPTI-3 unit with the longest alkyl side chains has been demonstrated to be an efficient electron acceptor in solution-processed bulk heterojunction organic photovoltaics (OPV), giving a power conversion efficiency of 3.68 % when using conjugated polymer PTB7-Th as the donor and without additional treatments.

High-Performance Near-Infrared Absorbing n-Type Porphyrin Acceptor for Organic Solar Cells.

While the outstanding charge transport and sunlight-harvesting properties of porphyrin molecules are highly attractive as active materials for organic photovoltaic (OPV) devices, the development of n-type porphyrin-based electron acceptors has been challenging. In this work, we developed a high-performance porphyrin-based electron acceptor for OPVs by substitution of four naphthalene diimide (NDI) units at the perimeter of a Zn-porphyrin (PZn) core using ethyne linkage. Effective π-conjugation between four NDI wings and the PZn core significantly broadened Q-band absorption to the near infrared region, thereby achieving the narrow band gap of 1.33 eV. Employing a windmill-structured tetra-NDI substituted PZn-based acceptor ( PZn-TNDI) and mid-band gap polymer donor (PTB7-Th), the bulk heterojunction OPV devices achieved a power conversion efficiency (PCE) of 8.15% with an energy loss of 0.61 eV. The PCE of our PZn-TNDI-based device was the highest among the reported OPVs using porphyrin-based acceptors. Notably, the amorphous characteristic of PZn-TNDI enabled optimization of the device performance without using any additive, which should make industrial fabrication simpler and cheaper.