Fullerene Bulk Heterojunction Research Articles

Optimized phase separation in low-bandgap polymer:fullerene bulk heterojunction solar cells with criteria of solvent additives

We investigate a correlation between the type of solvent additives (SAs) with specific criteria such as aromatic additives (AAs) and non-aromatic additives (NAAs) and phase separation in the bulk heterojunction (BHJ) films comprising low-band gap polymer and fullerene derivatives. When AAs are used as SAs, the geometrical structures (π-π and lamellar stacking) of aggregated polymer chains do not significantly change. However, NAAs increase the lamellar stacking distance through a strong interaction with non-aromatic segments of polymers. Therefore, a well-phase separated BHJ morphology with the finer fibrils is developed, thereby leading to balanced charge mobilities and a reduced charge recombination in BHJ solar cells. Finally, the optimized solar cell exhibits a high power conversion efficiency of 7.9%.

Understanding How Polymer Properties Control OPV Device Performance: Regioregularity, Swelling, and Morphology Optimization Using Random Poly(3-butylthiophene-co-3-octylthiophene) Polymers

The performance of polymer:fullerene bulk heterojunction (BHJ) photovoltaics is highly sensitive to the morphology of the polymer within the active layer. To tune this morphology, we constructed both blend-cast and sequentially processed BHJ devices from the fullerene derivative [6,6]-phenyl-C60-butyric acid methyl ester (PCBM), in combination with a series of random poly(3-butylthiophene-co-3-octylthiophene)s with different fractions of each monomer, with the goal of controllably varying the average polymer side-chain length. What we found, however, was that the most important parameter for predicting device performance across this series of polymers was the regioregularity of the particular synthetic batch of polymer used, not the average side-chain length. Moreover, we found that regioregularity affected device performance in different ways depending on the processing route: lower regioregularity led to improved performance for sequentially processed devices, but was detrimental to the performance of b...

Impact of Fullerene Structure on Nanoscale Morphology and Miscibility and Correlation of Performance on Small Molecules: Fullerene Solar Cell

This manuscript reports the impact of fullerene structure on the morphology and miscibility of small molecules via a fullerene bulk heterojunction solar cell. The small angle neutron scattering and neutron reflectometry measurements were analyzed to provide quantifiable measures of the morphology of the resultant mixtures, offering miscibility, domain sizes, interfacial area between the small molecule and fullerene, and depth profiles in the mixtures. These results indicate that the bis-adduct fullerenes exhibit lower miscibility in small molecules. Correlation of miscibility and morphology to photovoltaic properties indicates that small molecule/fullerene miscibility is crucial to rationally optimize the design of fullerenes for use in small molecule organic photovoltaics. A higher open circuit voltage was obtained for bis-adduct fullerene devices which, however, does not translate to an increased power conversion efficiency. This decrease in performance is associated with the lower miscibility of bis-fu...

Solution-processable MoOx nanocrystals enable highly efficient reflective and semitransparent polymer solar cells

Solution-manufacturing of organic solar cells with best-in-class power conversion efficiency (PCE) will require all layers to be solution-coated without compromising solar cell performance. To date, the hole transporting layer (HTL) deposited on top of the organic bulk heterojunction layer in the inverted architecture is most commonly an ultrathin (<10nm) metal oxide layer prepared by vacuum-deposition. Here, we show that an alcohol-based nanocrystalline MoOx suspension with carefully controlled nanocrystal (NC) size can yield state of the art reflective and semitransparent solar cells. Using NCs smaller than the target HTL thickness (∼10nm) can yield compact, pinhole-free films which result in highly efficient polymer:fullerene bulk heterojunction (BHJ) solar cells with PCE=9.5%. The solution processed HTL is shown to achieve performance parity with vacuum-evaporated HTLs for several polymer:fullerene combinations and is even shown to work as hole injection layer in polymer light emitting diodes (PLED). We also demonstrate that larger MoOx NCs (30–50nm) successfully composite MoOx with Ag nanowires (NW) to form a highly conducting, transparent top anode with exceptional contact properties. This yields state-of-the-art semitransparent polymer: fullerene solar cells with PCE of 6.5% and overall transmission >30%. The remarkable performance of reflective and semitransparent OPVs is due to the uncommonly high fill factors achieved using a carefully designed strategy for implementation of MoOx nanocrystals as HTL materials.

Solvent Annealing Effects in Dithieno[3,2-b:2′,3′-d]pyrrole–5,6-Difluorobenzo[c][1,2,5]thiadiazole Small Molecule Donors for Bulk-Heterojunction Solar Cells

Low-bandgap small molecule (SM) donors that can be solution-processed with fullerene acceptors (e.g., PC61/71BM) are proving to be particularly promising in bulk-heterojunction (BHJ) solar cells. Compared to their π-conjugated polymer counterparts, SM donors are well-defined (monodisperse) and more synthetically modular, with relatively wide ranges of bandgaps that can be achieved in stepwise couplings of various donor and acceptor motifs. However, the optimization of SM–fullerene morphologies and BHJ device efficiencies relies more specifically on the use of processing additives, postprocessing thermal, or solvent vapor annealing (SVA) approaches, and achieving adequate interpenetrating networks and structural order in BHJ thin films can be challenging. In this report, we examine the correlated effects of molecular structure and postprocessing SVA on the BHJ solar cell performance of a set of π-extended SM donors composed of dithieno[3,2-b:2′,3′-d]pyrrole (DTP) and 5,6-difluorobenzo[c][1,2,5]thiadiazole ...

Direct characterization of the energy level alignments and molecular components in an organic hetero-junction by integrated photoemission spectroscopy and reflection electron energy loss spectroscopy analysis

A novel, direct method for the characterization of the energy level alignments at bulk-heterojunction (BHJ)/electrode interfaces on the basis of electronic spectroscopy measurements is proposed. The home-made in situ photoemission system is used to perform x-ray/ultraviolet photoemission spectroscopy (XPS/UPS), reflection electron energy loss spectroscopy (REELS) and inverse photoemission spectroscopy of organic-semiconductors (OSCs) deposited onto a Au substrate. Through this analysis system, we are able to obtain the electronic structures of a boron subphthalocyanine chloride:fullerene (SubPC:C60) BHJ and those of the separate OSC/electrode structures (SubPC/Au and C60/Au). Morphology and chemical composition analyses confirm that the original SubPC and C60 electronic structures remain unchanged in the electrodes prepared. Using this technique, we ascertain that the position and area of the nearest peak to the Fermi energy (EF = 0 eV) in the UPS (REELS) spectra of SubPC:C60 BHJ provide information on the highest occupied molecular orbital level (optical band gap) and combination ratio of the materials, respectively. Thus, extracting the adjusted spectrum from the corresponding SubPC:C60 BHJ UPS (REELS) spectrum reveals its electronic structure, equivalent to that of the C60 materials. This novel analytical approach allows complete energy-level determination for each combination ratio by separating its electronic structure information from the BHJ spectrum.

Influence of Physical Load on the Stability of Organic Solar Cells with Polymer : Fullerene Bulk Heterojunction Nanolayers

Influence of Physical Load on the Stability of Organic Solar Cells with Polymer : Fullerene Bulk Heterojunction Nanolayers

High Detectivity Organic Photodetectors With Molybdenum (VI) Oxide and C60Layers

This paper proposes high-sensitive polymer/ fullerene bulk-heterojunction organic photodetectors (OPDs). A typical polymeric hole transport layer, (poly(3,4-ethylenedioxythophene):poly(styrenesulfonate), has substitutes with a molybdenum (VI) oxide (MoO3) layer. A thin C 60 hole blocking layer was also introduced to reduce the dark current performance of the OPD. Optical interference simulation also accomplished along with the generalized transfer matrix method to show the behavior of wavelengths in OPDs. As a result, the lowest dark current density of 14 pA/cm2, and 40% above EQE, has achieved in the visible range. In regards to the photodetector characteristics, a reasonable responsivity of 0.21 A/W and high detectivity of $2.4 \times 10^{12}$ cmHz $^{1/2}/\text{W}$ were also obtained.

Structure-induced resonant tail-state regime absorption in polymer: fullerene bulk-heterojunction solar cells

In this work, we present resonant tail-state regime absorption enhanced organic photovoltaics. We combine periodically structured ${\mathrm{TiO}}_{2}$ bottom electrodes with P3HT-PCBM bulk-heterojunction solar cells in an inverted device configuration. The wavelength-scale patterns are transferred to the electron-selective bottom electrodes via direct laser interference patterning, a fast method compatible with roll-to-roll processing. Spectroscopic and optoelectronic device measurements suggest polarization-dependent absorption enhancement along with photocurrent generation unambiguously originating from the population of tail states. We discuss the effects underlying these absorption patterns with the help of electromagnetic simulations using the discontinuous Galerkin time domain method. For this, we focus on the total absorption spectra along with spatially resolved power loss densities. Our simulations stress the tunability of the absorption resonances towards arbitrary wavelength regions.

Dramatically enhanced performances and ideally controlled nano-morphology via co-solvent processing in low bandgap polymer solar cells

The device performance of photovoltaics with a polymer:fullerene bulk heterojunction (BHJ) structure, consisting of DT-PDPP2T-TT donor polymer and poly(3-hexylthiophene):[6,6]phenyl-C61-butyric acid methyl ester (PC61BM) acceptor compound, was investigated as a function of co-solvent composition. An enhancement of the photocurrent density and fill factor is observed in diodes made by spin-coating with chloroform mixed with ortho-dichlorobenzene, which allows a significantly higher device efficiency of 5.55% compared to diodes made from neat chloroform (efficiency = 3.61%). To clarify the role of the co-solvent, we investigated the nanoscale morphology with AFM, TEM and 2D-GIWAXS techniques and also the free-charge carrier mobility via space-charge limited current theory. We obtained the result that, under such supersaturated conditions, co-solvents induce increased polymer crystalline aggregation into a 3D phase structure and boost charge-carrier transport characteristics. This provides a rational basis for the development of ideally-controlled BHJ films that yield efficient DT-PDPP2T-TT:PCBM solar cells. Therefore, carefully selecting solvent mixtures provides an approach toward efficient low bandgap polymer solar cells.

Copper oxide hole transport materials for heterojunction solar cell applications

In this study, polymer:fullerene bulk-heterojunction hybrid solar cells with the structure, fluorine-doped tin oxide (FTO)/Cu2O/poly (3-hexylthiophene): 6,6-phenyl-C61-butyric acid methyl ester (P3HT:PCBM):TiO2/Ca/Al, were fabricated. The effect of a p-type Cu2O thin layer deposited between the active layer (P3HT:PCBM) and the FTO layer on cell performance was investigated; the p-type Cu2O thin layer was deposited through thermal evaporation. The results showed that the Cu2O interfacial layer increased the efficiency and stability of the hybrid solar cells. The optimal performance was obtained for the solar cell structure FTO/Cu2O (10nm)/P3HT:PCBM:TiO2 (15wt.%)/Ca/Al (best cell structure), which had the following specifications: open-circuit voltage=0.58V, short circuit current density=12.14mA/cm2, fill factor=58.63%, and η=4.13%. The Cu2O layer prevented the rapid degradation of the interface, thereby increasing the lifetime of the hybrid solar cells.

(Invited) The Effect of Increased Dielectric Screening on Bimolecular Recombination of Photogenerated Charges in Polymer:Fullerene Bulk Heterojunctions

Bimolecular recombination studies reveal a one order magnitude slower bimolecular recombination when normalized to charge mobility in a polymer:fullerene bulk heterojunction solar cell with a dielectric constant of seven. The techniques used were charge extraction using nanosecond switch combined with charge extraction by linearly increasing voltage and integral mode time of flight measurements. The measured high dielectric constant in the polymer:fullerene blend is due to the exceptionally high dielectric constant (16.7) of the conjugated polymer donor component employed. This value, indicative of increased dielectric screening of charges compared to most organic solar cell materials with a typical dielectric constant around 4,is one of the highest reported to date. The slower bimolecular recombination mentioned above is compared to well established poly(3-hexylthiophene) and low badngap polymer PCPDTBT:fullerene heterojunctions, and is explained by the smaller coulomb capture radius in a diffusion controlled recombination model. Longer charge carrier lifetimes together with a larger charge mobility in the high dielectric constant bulk heterojunctions leads to larger fill factors (0.6) and excellent photovoltaic performance in solar cells using large active layer thicknesses approaching 300 nm. Ultimately, increasing the dielectic constant of organic photoactive laterials to and beyond 10 is expected to bridge the performance gap between inorganic and organic solar cells.

Organic Position-Sensitive Detectors Based on ZnO:Al and CuPc:C60.

Organic position-sensitive detector (OPSD) based on copper phthalocyanine CuPc:fullerene C60 bulk-heterojunction with an inverted structure have been fabricated using aluminum doped ZnO (ZnO:Al) as a resistive layer, which is prepared by sol-gel method. The resistance length of the one-dimensional PSD is fixed at 5 mm, and the Ag common electrode is fabricated by vacuum evaporation within the 100-µm width. The current density-voltage characteristics with different structures of photodetector, the influence of ZnO:Al resistivity on the thickness and the position characteristics of PSDs are investigated. The experimental results indicate that the architecture, which uses an inverted structure, increases sensitivity under red light illumination compared to the conventional structure. In addition, the thickness of the ZnO:Al has influence on the position characteristics. The resistivity of ZnO:A film with Al doping concentration of 2 mol% prepared in this study is around 150 Ωcm and it increases from less than approximately 400 nm-thickness. These characteristics seem to be correlated with the properties of ZnO:AI resistive layer. For a device with a 620 nm-thick ZnO:Al layer, the measured position values obtained from the output photocurrent agree with the actual position values under red laser light illumination. CuPc:C60 OPSD with an inverted structure exhibits red light sensitivity, high incident-photon-to-current conversion efficiency of above 80% at -3 V and linearity error of 5.9% at -2 V.

π-Bridge-Independent 2-(Benzo[c][1,2,5]thiadiazol-4-ylmethylene)malononitrile-Substituted Nonfullerene Acceptors for Efficient Bulk Heterojunction Solar Cells

Molecular acceptors are promising alternatives to fullerenes (e.g., PC61/71BM) in the fabrication of high-efficiency bulk-heterojunction (BHJ) solar cells. While solution-processed polymer–fullerene BHJ devices have recently met the 10% efficiency threshold, molecular acceptors have yet to prove comparably efficient with polymer donors. At this point in time, it is important to forge a better understanding of the design parameters that directly impact small-molecule (SM) acceptor performance in BHJ solar cells. In this report, we show that 2-(benzo[c][1,2,5]thiadiazol-4-ylmethylene)malononitrile (BM)-terminated SM acceptors can achieve efficiencies as high as 5.3% in BHJ solar cells with the polymer donor PCE10. Through systematic device optimization and characterization studies, we find that the nonfullerene analogues (FBM, CBM, and CDTBM) all perform comparably well, independent of the molecular structure and electronics of the π-bridge that links the two electron-deficient BM end groups. With estimated...

Relationship between photostability and nanostructures in DTS(FBTTh2)2:fullerene bulk-heterojunction films

Successful fabrication and operation of highly stable organic photovoltaic (OPV) cells demand improved durability of the semiconductor materials used in the active layer. In this study, we performed photooxidation studies on DTS(FBTTh2)2:fullerene bulk-heterojunction (BHJ) films, one of the promising small-molecule materials for OPV cells, by using two-dimensional (2D) grazing-incidence wide-angle X-ray scattering (GIWAXS), absorption spectroscopy, and atomic force microscopy (AFM). The results of 2D-GIWAXS and absorption spectroscopic analyses indicate that increasing the crystallinity of DTS(FBTTh2)2:fullerene BHJ films improved their photostability. The AFM results suggest that the roughness of the original DTS(FBTTh2)2:fullerene films was also related to their photooxidation rates. The results obtained in this work show that changing the fullerene derivative species and controlling the nanostructures of the thin films are both important criteria for improving the photostability of OPV cells.

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.

Controlling additive behavior to reveal an alternative morphology formation mechanism in polymer : fullerene bulk-heterojunctions

The high solubility of DPP-TT-T in 1,8-diiodooctane leads the microstructure formation towards thermodynamic equilibrium.

Insights into the nanoscale lateral and vertical phase separation in organic bulk heterojunctions via scanning probe microscopy.

Solution processed polymer (donor) and fullerene (acceptor) bulk heterojunctions are widely used as the photo active layer in organic solar cells. Intimate mixing of these two materials is essential for efficient charge separation and transport. Identifying relative positions of acceptor and donor rich regions in the bulk heterojunction with nanometer scale precision is crucial in understanding intricate details of operation. In this work, a combination of Ar(+)2000 gas cluster ion beam and scanning probe microscopy is used to examine the lateral and vertical phase separation within regio-regular poly(3-hexylthiophene)(P3HT):phenyl-C60-butyric acid methyl ester (PCBM) bulk heterojunction. While the Ar(+)2000 gas cluster ion beam is used as a sputter tool to expose the underneath layers, scanning probe microscopy techniques are used to obtain two-dimensional (2D) electrical maps (with sub-2 nm lateral resolution). The electrical mapping is decoded to chemical composition, essentially producing lateral and vertical maps of phase separation. Thermal stress causes large PCBM-rich hillocks to form, and consequently affecting the balance of P3HT:PCBM heterojunctions, hence a negative impact on the efficiency of the solar cell. We further developed a method to analyze the efficiency of exciton dissociation based on the current maps and a loss of 20% in efficiency is observed for thermally degraded samples compared to fresh un-annealed samples.

Locally resolved large scale phase separation in polymer:fullerene blends

Large scale phase separation in polymer:fullerene bulk heterojunction solar cells investigated by electroluminescence imaging (ELI) and light-beam induced current (LBIC) measurements.

Beyond PCBM: methoxylated 1,4-bisbenzyl[60]fullerene adducts for efficient organic solar cells

A novel methoxylated 1,4-bisbenzyl fullerene adduct gives better performance in polymer:fullerene bulk heterojunction photovoltaic devices than traditional PCBM, and it is easily synthesized.