Polymer Fullerene Bulk Heterojunction Research Articles

Enhancement of resistive switching behavior of organic resistive random access memory devices through UV-Ozone treatment

We fabricated organic resistive random-access memory (RRAM) devices using a low-cost solution-process method. All the processes were performed at temperatures below 135 °C under ambient atmospheric conditions. The RRAM resistive switching layer was formed from a polymer-fullerene bulk heterojunction using poly(3-hexylthiophene-2,5-diyl) (P3HT) and (6,6)-phenyl C61 butyric acid methyl ester (PCBM). The fabricated organic RRAM device exhibited typical nonvolatile bipolar resistive switching behavior with an ON/OFF ratio of ∼40, but it provided a low endurance of 27 cycles. Therefore, for enhanced stability, simple UV–Ozone (UVO) treatment was applied to the P3HT:PCBM organic bulk heterojunction layer. The organic RRAM device with UVO treatment exhibited an enhanced performance with an ON/OFF ratio of ∼400 and an endurance of 47 cycles. In addition, complementary resistive switching behavior was observed. The conduction mechanisms of the organic RRAM device were investigated by fitting the measured I–V data to numerical equations, and Schottky emission and Ohmic conduction were the main conduction mechanisms for the high-resistance and low-resistance states for the RRAM device with or without UVO treatment.

Molecular Insights into the Mechanical Properties of Polymer–Fullerene Bulk Heterojunctions for Organic Photovoltaic Applications

We investigate the mechanical properties of $\pi$-conjugated polymeric materials composed of regioregular poly(3-hexylthiophene) (P3HT) and fullerene C$_{60}$ using coarse-grained molecular dynamics simulations. Specifically, we perform tensile simulations of P3HT:C$_{60}$ composites with varied degrees of polymerization and C$_{60}$ mass fractions to obtain their stress-strain responses. Decomposition of stress tensor into kinetic energy and virial contributions indicates that the tensile moduli of the pure P3HT samples are greatly dependent on non-bonded interactions and on bonded interactions associated with bond-stretching, while the addition of C$_{60}$ leads to an increase in the tensile modulus originating from enhanced non-bonded interactions associated with C$_{60}$. Additionally, the tensile strength of the P3HT:C$_{60}$ samples correlates well with molecular chain entanglements, which are characterized by the average number of kinks per chain obtained from primitive path analysis. We also find that the upper and lower yield points characterizing strain softening become more pronounced with increasing C$_{60}$ mass fraction. Persistent homology analysis indicates that the emergence of the yield points correlates well with the coalescence of microvoids in the course of tensile deformation, resulting in the generation of larger voids. These results provide a fundamental understanding of the molecular determinants of the mechanical properties of $\pi$-conjugated polymer-fullerene composites, which can also help to interpret and predict the mechanical properties of other polymer composites containing fullerene.

Molecular Dynamics Investigation of the Mechanical Properties of Polymer–Fullerene Bulk Heterojunctions

Molecular Dynamics Investigation of the Mechanical Properties of Polymer–Fullerene Bulk Heterojunctions

Heteroleptic Tin-Antimony Sulfoiodide for Stable and Lead-free Solar Cells

Summary The quaternary chalcogenide halides of group IV and V elements have attracted much attention due to their interesting semiconducting properties as well as a suitable band gap for solar cells. Here, for the first time, we report on solar cells using tin-antimony sulfoiodide (Sn2SbS2I3). Sn2SbS2I3 solar cells were fabricated using a chemical single-step deposition process with a solution containing a SbCl3-thiourea complex and SnI2 with the configuration of TiO2 and poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]dithiophene)-alt-4,7-(2,1,3-enzothiadiazole)] as the electron- and hole-transporting layers, respectively. The best-performing cell exhibits a power conversion efficiency of 4.04% under the illumination of standard AM 1.5G conditions (100 mW cm−2). These unencapsulated cells exhibited good stabilities at 80% relative humidity, 85°C in air, and under illumination, respectively. These results provide guidelines for fabrication of lead-free heteroleptic perovskite solar cells by hosting divalent or combinations of monovalent and trivalent metal cations.

Charge carrier dynamics in PffBT4T-2OD: PCBM organic solar cells

Abstract We investigate the charge carrier dynamics of inverted organic solar cells (OSCs) based on PffBT4T-2OD: PCBM and PTB7: PCBM – the two leading systems among the OSCs based on polymer-fullerene bulk-heterojunction – to elucidate the origin of their performance difference. Transient absorption spectroscopy (TAS) and photo-electrochemical impedance spectroscopy (photo-EIS) were employed to unravel the photo-physics that govern the cell operation of these two highly efficient bulk heterojunction OSCs. While photo-EIS indicates that the two systems under study exhibit similar behavior in terms of recombination, TAS results reveal that PffBT4T-2OD: PCBM systems not only have higher charge generation rate but also more efficient charge transfer than PTB7: PCBM systems, leading to the power conversion efficiency of PffBT4T-2OD: PCBM-based OSCs (9.16%) that is higher than that of PTB7: PCBM-based OSCs (6.44%).

Characterization of Interfacial Structure in Polymer-Fullerene Bulk Heterojunctions via ^{13}C {^{2}H} Rotational Echo Double Resonance NMR.

We introduce a new application of solid state NMR measurements towards characterizing the donor-acceptor interfaces within bulk heterojunction (BHJ) films. Rotational echo double resonance (REDOR) is used to measure dipolar couplings between ^{13}C nuclei on the acceptor phenyl-C_{61}-butyric acid methyl ester (PCBM) fullerene cage, which is ≈18% isotopically enriched with ^{13}C, and beta hydrogens on the donor poly(3-hexyl thiophene) (P3HT) main chain, which are >95% isotopically enriched with ^{2}H. The ^{13}C-^{2}H dipolar couplings are used for constraining possible models of molecular packing in the amorphous mixed phase of a P3HT/PCBM BHJ. The films studied are highly mixed (>80%) and have a maximum length scale of composition nonuniformity of ≈6 nm in the mixed phase, as demonstrated by ^{1}H spin diffusion NMR and supported by TEM. The REDOR results show that despite the lack of phase separation at length scales greater than ≈6 nm, neat P3HT and PCBM clusters exist on ≈3 nm size scales, and, for the average PCBM molecule, the number of nearest neighbors P3HTs is two.

Mixed Domains Enhance Charge Generation and Extraction in Bulk‐Heterojunction Solar Cells with Small‐Molecule Donors

AbstractThe interplay between nanomorphology and efficiency of polymer‐fullerene bulk‐heterojunction (BHJ) solar cells has been the subject of intense research, but the generality of these concepts for small‐molecule (SM) BHJs remains unclear. Here, the relation between performance; charge generation, recombination, and extraction dynamics; and nanomorphology achievable with two SM donors benzo[1,2‐b:4,5‐b]dithiophene‐pyrido[3,4‐b]‐pyrazine BDT(PPTh2)2, namely SM1 and SM2, differing by their side‐chains, are examined as a function of solution additive composition. The results show that the additive 1,8‐diiodooctane acts as a plasticizer in the blends, increases domain size, and promotes ordering/crystallinity. Surprisingly, the system with high domain purity (SM1) exhibits both poor exciton harvesting and severe charge trapping, alleviated only slightly with increased crystallinity. In contrast, the system consisting of mixed domains and lower crystallinity (SM2) shows both excellent exciton harvesting and low charge recombination losses. Importantly, the onset of large, pure crystallites in the latter (SM2) system reduces efficiency, pointing to possible differences in the ideal morphologies for SM‐based BHJ solar cells compared with polymer‐fullerene devices. In polymer‐based systems, tie chains between pure polymer crystals establish a continuous charge transport network, whereas SM‐based active layers may in some cases require mixed domains that enable both aggregation and charge percolation to the electrodes.

Determining Material-Specific Morphology of Bulk-Heterojunction Organic Solar Cells Using AFM Phase Imaging

A technique for determining the material-specific morphology of a polymer–fullerene blend is presented. This technique is applied to solution processed bulk-heterojunction organic solar cells with different weight ratios of polymer–fullerene blend using the PTB7:PCBM material system. Optical and electrical characterizations show that the light absorption increases for larger polymer (PTB7) content, while the fill factor of the fabricated solar cells is improved for larger fullerene (PCBM) content. The material-specific morphologies of polymer–fullerene bulk-heterojunctions are measured by employing AFM phase imaging. The measured AFM phase images reveal that fullerene material forms flake-like clusters which are embedded in the polymer network. The size of the flakes is increasing with larger content of fullerene material. By correlating the optical and electrical properties with the measured bulk-heterojunction morphologies, the relation between bulk-heterojunction structure and solar cell performances i...

Mechanical Properties of Polymer–Fullerene Bulk Heterojunction Films: Role of Nanomorphology of Composite Films

This paper reports the tensile properties and fracture mechanism of PTB7:PC71BM bulk heterojunction (BHJ) films as a function of composition mixing ratio. An increased concentration of fullerene makes the BHJ films stiffer and more brittle, and fracture occurs along aggregated fullerene domain boundaries. The tensile strength is maximized at a polymer–fullerene content ratio of 1:1. Furthermore, an additive, 1,8-diiodoctane (DIO), in the films induces fine nanomorphology, which increases the stiffness and strength and reduces the ductility of the films further. This is especially true under a high PC71BM load due to the expanded interfacial surface areas between the PC71BM and PTB7 polymer domains. The photovoltaic performance of the BHJ films on polydimethylsiloxane (PDMS) substrates after tensile stretching cycles is also examined in detail.

Dielectric Screening To Reduce Charge Transfer State Binding Energy in Organic Bulk Heterojunction Photovoltaics

Reaching high efficiencies in organic photovoltaics is often limited by dissociating charge transfer states at electron donor–acceptor interfaces due to their large binding energies and rapid geminate recombination. By adding a high-permittivity small molecule, camphoric anhydride, at 20 wt % to a polymer–fullerene bulk heterojunction photovoltaic device, we increased the film permittivity, reduced the charge transfer state energy, and increased the power conversion efficiency by 75%. At higher concentrations, the camphoric anhydride begins to crystallize and phase separate from the bulk heterojunction, causing the permittivity and power conversion efficiency to decrease. Adding camphoric anhydride in low concentrations is an effective strategy to increase permittivity and reduce recombination at the donor–acceptor interface and as a result increase organic photovoltaic efficiency in bulk heterojunction devices.

Grazing incidence resonant soft X-ray scattering for analysis of multi-component polymer-fullerene blend thin films

Grazing incidence small angle X-ray scattering (GISAXS) methods are frequently very successfully utilized for morphological investigations of thin polymer blend films used in organic photovoltaics. However, conventional GISAXS does no longer provide material sensitivity when the blend consists of more than two components due to the contrast conditions. In case of multi-component blends the use of grazing incidence resonant soft X-ray scattering (GI-RSoXS) can overcome such problems. In this work we exemplarily apply GI-RSoXS to investigate simultaneously the vertical and lateral morphology of polymer-fullerene bulk heterojunction layers made from poly[2,1,3-benzothiadiazole-4,7-diyl[4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b’]dithiophene-2,6-diyl]] (PCPDTBT) and [6,6]-penyl-C 61 butyric acid methyl ester (PCBM) with added photosensitizer perylene diimide (PDI) with and without use of the solvent additive 1,8-octanedithiol (ODT). The investigation reveals that films without solvent additive ODT tend to show only vertical phase separation while films prepared with solvent additive also phase separate laterally. PDI seems to assist lateral phase separation between PCPDTBT and PCBM by expelling PCBM from the amorphous polymer matrix.

Improved efficiency of polymer-fullerene bulk heterojunction solar cells by the addition of Cu(II)-porphyrin-oligothiophene conjugates

Ternary organic photovoltaic devices were prepared through the addition of small amounts of metalloporphyin-terthiophenes to two established low bandgap polymer:fullerene blends. The methodology afforded a PCDTBT:PC71BM-based device that displayed an initial power conversion efficiency of 5.1%, an increase of 16% when compared to the binary blend. Among the range of metalloporphyrins considered in this study, the Cu(II)-porphyrin derivatives were found to result in the most significant efficiency improvements.

Criteria for validating polaron pair dissociation in polymer-fullerene bulk heterojunction solar cells

The dissociation of polaron pairs into free charge carriers in organic bulk heterojunction solar cells is a fundamental step in generating photocurrent and is still in debate. In this study, we propose two simple criteria that can be used to test the validity of any polaron pair dissociation model for polymer-fullerene bulk heterojunction solar cells. The first criterion states that the ratio of the bimolecular recombination current density to the maximum photocurrent density should increase as a function of applied voltage. The second criterion states that the ratio of the bimolecular recombination current density to the maximum photocurrent density at short circuit should not be larger than 1. We apply these criteria to test the validity of the widely used Onsager-Braun model by using the experimental current-voltage data of poly[2-methoxy-5-(3′-7′-dimethyloctyloxy)-p-phenylene vinylene] (OC1C10-PPV) and [6,6]-phenyl C61-butyric acid methylester (PCBM) based solar cells. We find that our numerical analysis is not suitable to employ these criteria. Our analytical analysis, on the other hand, clearly demonstrates that the Onsager-Braun model simply cannot fulfill the first criteria. The reason is because the polaron pair dissociation given by the Onsager-Braun model is too strongly influenced by the electric field (i.e., decreases too rapidly as the electric field decreases). The analysis provides a further evidence against the widely used Onsager-Braun model. The proposed criteria can help us to determine the correct model for polaron pair dissociation by serving as a guideline on how strongly the electric field is allowed to influence the polaron pair dissociation.

Naphthodithieno[3,2-b]thiophene-based donor-acceptor copolymers: Synthesis, characterization, and their photovoltaic and charge transport properties

Herein, we report the synthesis, characterization, and their photovoltaic and field–effect properties of two dialkoxyl-substituted naphthodithieno[3,2-b]thiophene (NDTT)-based copolymers, namely P1 and P2. The NDTT-based copolymers exhibit broad absorption profiles throughout the visible spectrum up to ca. 800 nm. The copolymers have HOMO/LUMO energy levels of ca. –5.14 eV/–3.52 eV. The dialkoxyl-substituted NDTT units endow the copolymers with enhanced solution processability, and crystalline stacking thin films with small π–π distances of down to 3.59 Å. Polymer-fullerene bulk heterojunction solar cells that fabricated from a blend of P2/PC71BM using diiodooctane as a solvent additive, afforded the highest power conversion efficiency of 3.54%. Solution-processed polymer field-effect transistors based on P1 with bottom-gate bottom-contact geometry exhibited promising hole mobilities of 0.070 cm2 V−1 s−1.

Cathode deposition, paramagnetic defect formation and performance degradation in polymer–fullerene solar cells

Although changes in morphology and defect distribution at the cathode/active layer interface are presumed to limit the efficiency of sputtered cathodes for organic solar cells, little information is available on the nature of defects involved and on possible strategies to mitigate these effects. We demonstrate the sputter-induced formation of paramagnetic centers associated to the change in conformation and amorphization of P3HT in the proximity of the cathodes, which strongly affects the performance of polymer–fullerene bulk heterojunction photovoltaics. Similarity in g-values and X-ray diffraction analysis suggest that defects produced at the cathode/organic interface by sputtering are of the same nature as those produced by thermal annealing at 150–180°C, and that thin calcium layers thermally evaporated prior to cathode sputtering act as thermal sinks mitigating the defect formation by impinging sputtered ions.

High-Performance Inverted Organic Photovoltaics Without Hole-Selective Contact.

A detailed investigation of the functionality of inverted organic photovoltaics (OPVs) using bare Ag contacts as the top electrode is presented. The inverted OPVs without a hole-transporting layer (HTL) exhibit a significant gain in hole-carrier selectivity and power-conversion efficiency (PCE) after exposure in ambient conditions. Inverted OPVs comprised of ITO–ZnO–poly(3-hexylthiophene-2,5-diyl)/phenyl-C61-butyric acid methyl ester (P3HT/PCBM)–Ag demonstrate over 3.5% power conversion efficiency only if the devices are exposed in air for over 4 days. As concluded through a series of measurements, the oxygen presence is essential to obtaining fully operational solar cell devices without HTL. Moreover, accelerated stability tests under damp heat conditions (RH = 85% and T = 65 °C) performed to nonencapsulated OPVs demonstrate that HTL-free inverted OPVs exhibit comparable stability to the reference inverted OPVs. Importantly, it is shown that bare Ag top electrodes can be efficiently used in inverted OPVs using various high-performance polymer–fullerene bulk heterojunction material systems demonstrating 6.5% power-conversion efficiencies.

Transparent conductive ZnO layers on polymer substrates: Thin film deposition and application in organic solar cells

Aluminum doped ZnO (AZO) and pure ZnO thin films are grown on polymer substrates by pulsed-laser deposition and the optical, electrical, and structural film properties are investigated. Laser fluence, substrate temperature, and oxygen pressure are varied to obtain transparent, conductive, and stoichiometric AZO layers on polyethylene terephthalate (PET) that are free of cracks. At low fluence (1J/cm2) and low pressure (10−3mbar), AZO/PET samples of high optical transmission in the visible range, low electrical sheet resistance, and high figure of merit (FOM) are produced. AZO films on fluorinated ethylene propylene have low FOM. The AZO films on PET substrates are used as electron transport layer in inverted organic solar cell devices employing P3HT:PCBM as photovoltaic polymer-fullerene bulk heterojunction.

A model for studying the performance of P3HT:PCBM organic bulk heterojunction solar cells

In the study of bulk heterojunction (BHJ) solar cells based on poly(3-hexylthiophene) (P3HT) and a methanofullerene derivative (PCBM), P3HT/PCBM device performance is strongly depending on the thickness of active region, materials in use and fabrication methods. In such devices, optoelectronic behaviors such as charge carrier generation and recombination, photocurrent generation and charge transport mechanism are different in devices of different fabrication methods. An electrical model accounting for study of the performance of P3HT:PCBM organic bulk heterojunction solar cells is developed to achieve the device parameter for high performance of the polymer-fullerene bulk heterojunction solar cells. In this model, by solving the drift–diffusion equations by considering the boundary condition and uniform potential energy, the effects of the active region thickness, potential energy and photocurrent generation on the performance of P3HT:PCBM bulk heterojunction solar cell have been studied. Simulated current–voltage characteristics as a function of active layer thickness reveal relatively good agreement between the model's predictions and published modeling and experimental reports.

(Invited) Development of Tailor Made Donor-Acceptor Pairs for Fullerene-Free Organic Solar Cells

Organic solar cells (OCS) are now an established clean energy technology.1 These photovoltaic devices can be printed at low temperatures onto large area, flexible substrates, and can be made semi-transparent, making them a low-cost alternative to traditional inorganic devices, and can be utilized in many niche applications. Current state-of-the art relies on a polymer-fullerene bulk-heterojunction active layer, with power conversion efficiencies reaching 10%.2 While promising, certain limitations have prevented large scale commercialization. In an effort to advance OCS, the development of low-cost and sustainable active layer materials that can be mass-produced and yield high performance devices is essential. One area of our research at Dalhouise University centers on the development of small molecue based organic p-conjugated materials to replace the common fullerene acceptors.3 This talk will highlight our materials design strategy and discuss key thin-film characterization data and solar cell performance. (1) Su, Y.-W.; Lan, S.-C.; Wei, K.-H. Mater. Today 2012, 15, 554. (2) Li, G.; Zhu, R.; Yang, Y. Nat. Photonics 2012, 6, 153. (3) Eftaiha, A. F.; Hill, I. G.; Sun, J.-P.; Welch, G. C. J. Mater. Chem. A 2014, 2, 1201.

Photoinduced Dynamics of Charge Separation: From Photosynthesis to Polymer-Fullerene Bulk Heterojunctions.

Understanding charge separation and charge transport is crucial for improving the efficiency of organic solar cells. Their active media are based on organic molecules and polymers, serving as both light-absorbing and transport layers. The charge-transfer (CT) states play an important role, being intermediate for free carrier generation and charge recombination. Here, we use light-induced electron paramagnetic resonance spectroscopy to study the CT dynamics in blends of the polymers P3HT, PCDTBT, and PTB7 with the fullerene derivative C60-PCBM. Time-resolved EPR measurements show strong spin-polarization patterns for all polymer-fullerene blends, confirming predominant generation of singlet CT states and partial orientation ordering near the donor-acceptor interface. These observations allow a comparison with charge separation processes in molecular donor-acceptor systems and in natural and artificial photosynthetic assemblies, and thus the elucidation of the initial steps of sequential CT in organic photovoltaic materials.