Bulk Heterojunction Organic Photovoltaic Cells Research Articles

Highly transparent ZTO/Ag/ZTO multilayer electrode deposited by inline sputtering process for organic photovoltaic cells

Indium-free ZTO/Ag/ZTO multilayer electrodes were deposited on glass by RF sputtering of zinc tin oxide (ZTO) and pulsed DC sputtering of silver (Ag) at room temperature in the specially designed Inline sputtering system and evaluated as transparent electrode of organic photovoltaic cells (OPVs). The dependence of the electrical and optical properties of ZTO films on the O2/Ar + O2 ratio was investigated. We fabricated the ZTO/Ag/ZTO multilayer electrode with a sheet resistance of 8.38–10.18 Ω sq−1 and average transmittance of 84.4–85.5% in the visible region (λ = 380–770 nm), which is compatible with commercial indium tin oxide (ITO). As an application, organic solar cells fabricated on the ZTO/Ag/ZTO multilayer electrodes with the conventional device architecture were compared with those on commercial ITO electrodes. The bulk heterojunction OPVs fabricated on the multilayer electrodes exhibited a short-circuit current (JSC) of 7.06 mA cm−2, a fill factor (FF) of 0.62, an open-circuit voltage of (VOC) of 0.64 V, and a power-conversion efficiency of 2.80%. This indicates that ZTO/Ag/ZTO multilayer electrode is a promising electrode scheme for bulk heterojunction organic photovoltaic cells (BHJ-OPVs).

Nanomorphology Evolution of P3HT/PCBM Blends during Solution-Processing from Coarse-Grained Molecular Simulations

Solvent screening is a critical aspect of the nanomorphological control of low-cost, all-solution-processed bulk heterojunction organic photovoltaic cells. In order to reveal the correlations between solvent/solvent mixtures and the bulk heterojunction nanomorphologies during solution-processing, we constructed a multiscale, coarse-grained molecular simulation model for ternary solvent/P3HT/PCBM mixtures to systematically investigate the nanomorphologies of P3HT/PCBM blends during solution-processing in solutions over a wide range of P3HT/PCBM solubilities and solvent evaporation rates experienced during spin-casting processes. The resulting bulk heterojunction layer morphologies of the dried films were in good agreement with available experimental results from as-spun films, which validates our coarse-grained model. Our simulations indicated that the bulk heterojunction morphologies formed in solution involve a complicated interplay among the affinities of the solvent, P3HT, and PCBM in the ternary syste...

Non-destructive optical characterization of phase separation in bulk heterojunction organic photovoltaic cells

The dithiophene donor–acceptor copolymers that are bridged either with carbon (C-PCPDTBT) or silicon atoms (Si-PCPDTBT) belong to a promising family of materials for use in photoactive layers for organic photovoltaic cells (OPVs). In this work, we implement the non-destructive Spectroscopic Ellipsometry technique in the near infrared to the far ultraviolet spectral region in combination with advanced theoretical modeling to investigate the vertical distribution of the C-PCPDTBT and Si-PCPDTBT polymer and fullerene ([6,6]-phenyl C71-butyric acid methyl ester – PC70BM) phases in the blend, as well as the effect of the polymer-to-fullerene ratio on the distribution mechanism. It was found that the C-PCPDTBT:PC70BM blends have donor-enriched top regions and acceptor-enriched bottom regions, whereas the donor and acceptor phases are more homogeneously intermixed in the Si-PCPDTBT:PC70BM blends. We suggest that the chemical incompatibility of the two phases as expressed by the difference in their surface energy, may be a key element in promoting the segregation of the lower surface phase to the top region of the photoactive layer. We found that the increase of the photoactive layer thickness reduces the polymer enrichment at the cathode, producing a more homogeneous phase distribution of donor and acceptor in the bulk that leads to the increase of the OPV efficiency.

Continuous synthesis of zinc oxide nanoparticles in a microfluidic system for photovoltaic application

This study describes the synthesis of zinc oxide nanoparticles (ZnO NPs) using a microfluidic system. A continuous and efficient synthetic process was developed based on a microfluidic reactor in which was implemented a time pulsed mixing method that had been optimized using numerical simulations and experimental methods. Numerical simulations revealed that efficient mixing conditions could be obtained over the frequency range 5-15 Hz. This system used ethanol solutions containing 30 mM sodium hydroxide (NaOH) or 10 mM dehydrated zinc acetate (Zn(OAc)2) under 5 Hz pulsed conditions, which provided the optimal mixing performance conditions. The ZnO NPs prepared using the microfluidic synthetic system or batch-processed system were validated by several analytical methods, including transmission electron microscopy (TEM), energy dispersive X-ray spectrometry (EDS), X-ray diffraction (XRD), UV/VIS NIR and zeta (ζ) potential analysis. Bulk-heterojunction organic photovoltaic cells were fabricated with the synthesized ZnO NPs to investigate the practicability and compared with batch-process synthesized ZnO NPs. The results showed that microfluidic synthesized ZnO NPs had good preservability and stability in working solution and the synthetic microfluidic system provided a low-cost, environmentally friendly approach to the continuous production of ZnO NPs.

Near-Infrared Absorbing Thienoisoindigo-Based Copolymers for Organic Photovoltaics

We present bulk heterojunction organic photovoltaic (OPV) cells of novel electron accepting moiety, dithienoketopyrrole-based π-conjugated copolymers. An analog of isoindigo, thus so-called thienoisoindigo, was coupled to benzodithiophene, cyclopentadithiophene, and fluorene, affording a low bandgap and an appropriate LUMO level (−3.8 eV) for n-type methanofullerene (PCBM). A laser and Xe-flash-time-resolved microwave conductivity was extensively applied for guiding the processing conditions (p/n blend ratio, solvent, and thermal annealing), giving clearly defined optimized parameters. OPV performances are discussed in-depth in conjunction with atomic force microscope, grazing incidence X-ray diffraction, modulation of light intensity, field-effect transistor, and femtosecond transient absorption spectroscopy. We found that excessively shortened singlet exciton lifetime of polymer (<1 ps) competes with the photocurrent generation, which may arise from its near-infrared absorbing nature. The maximized power conversion efficiency was 1.4%, and there is still room for boosting the performance by rational design of counter donor unit and solubilizing substituents.

Determining the optimum morphology in high-performance polymer-fullerene organic photovoltaic cells

The morphology of bulk heterojunction organic photovoltaic cells controls many of the performance characteristics of devices. However, measuring this morphology is challenging because of the small length-scales and low contrast between organic materials. Here we use nanoscale photocurrent mapping, ultrafast fluorescence and exciton diffusion to observe the detailed morphology of a high-performance blend of PTB7:PC71BM. We show that optimized blends consist of elongated fullerene-rich and polymer-rich fibre-like domains, which are 10–50 nm wide and 200–400 nm long. These elongated domains provide a concentration gradient for directional charge diffusion that helps in the extraction of charge pairs with 80% efficiency. In contrast, blends with agglomerated fullerene domains show a much lower efficiency of charge extraction of ~45%, which is attributed to poor electron and hole transport. Our results show that the formation of narrow and elongated domains is desirable for efficient bulk heterojunction solar cells.

Low-boiling-point solvent additives can also enable morphological control in polymer solar cells

Processing organic photovoltaic (OPV) blend solutions with high-boiling-point solvent additives has recently been used for morphological control in bulk-heterojunction OPV cells. Here we show that even low-boiling-point solvents can be effective additives. When P3HT:PCBM OPV cells were processed with a low-boiling-point solvent tetrahydrafuran as an additive in parent solvent o-dichlorobenzene, charge extraction increased leading to fill factors as high as 69.5%, without low work-function cathodes, electrode buffer layers or thermal treatment. This was attributed to PCBM demixing from P3HT domains and better vertical phase separation, as indicated by photoluminescence lifetimes, hole mobilities, and shunt leakage currents. Dependence on solvent parameters and applicability beyond P3HT system was also investigated.

Insertion of fullerene layer for bulk heterojunction organic photovoltaic cell fabricated by electrospray deposition method

AbstractAn improved power conversion efficiency (PCE) of bulk heterojunction organic photovoltaic cell (OPV) was achieved by inserting an n‐type [6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM) layer between the active layer and a metal electrode. The controlled substrate temperature was found to be a useful parameter for the multilayer structure (active layer/ PCBM) by the electrospray deposition method. Under optimized substrate temperature during the PCBM deposition, a multilayer structure could be formed, and the PCE was improved up to 1.94%.magnified image(© 2013 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Quantifying space charge accumulation in organic bulk heterojunctions by nonlinear optical microscopy

In this work, we apply electric field induced second harmonic generation microscopy to directly observe and quantify space charge accumulation in operating organic bulk heterojunction photovoltaic cells comprised of poly(4,4-dioctyldithieno(3,2-b:2′,3′-d)silole)-2,6-diyl-alt-(2,1,3-benzothiadiazole)-4,7-diyl) (PSBTBT) mixed with phenyl-C61-butyric acid methyl ester (PCBM). We adjust the effective electron and hole mobility within these blends by altering the relative composition of PSBTBT and PCBM, and observe dramatic shifts in space charge accumulation. The PSBTBT rich device shows strong electron accumulation (2.8×1014e−/cm3) and the PCBM rich device shows strong hole accumulation (5.5×1014h+/cm3).

Control of Interface Order by Inverse Quasi-Epitaxial Growth of Squaraine/Fullerene Thin Film Photovoltaics

It has been proposed that interface morphology affects the recombination rate for electrons and holes at donor-acceptor heterojunctions in thin film organic photovoltaic cells. The optimal morphology is one where there is disorder at the heterointerface and order in the bulk of the thin films, maximizing both the short circuit current and open circuit voltage. We show that an amorphous, buried functionalized molecular squaraine donor layer can undergo an "inverted" quasi-epitaxial growth during postdeposition processing, whereby crystallization is seeded by a subsequently deposited self-assembled nanocrystalline acceptor C60 cap layer. We call this apparently unprecedented growth process from a buried interface "inverse quasi-epitaxy" where the crystallites of these "soft" van der Waals bonded materials are only approximately aligned to those of the cap. The resulting crystalline interface hastens charge recombination, thereby reducing the open circuit voltage in an organic photovoltaic cell. The lattice registration also facilitates interdiffusion of the squaraine donor and C60 acceptor, which dramatically improves the short circuit current. By controlling the extent to which this crystallization occurs, the voltage losses can be minimized, resulting in power conversion efficiencies of ηP = 5.4 ± 0.3% for single-junction and ηP = 8.3 ± 0.4% for tandem small-molecule photovoltaics. This is a general phenomenon with implications for all organic donor-acceptor junctions. That is, epitaxial relationships typically result in a reduction in open circuit voltage that must be avoided in both bilayer and bulk heterojunction organic photovoltaic cells.

Systematic Investigation of Organic Photovoltaic Cell Charge Injection/Performance Modulation by Dipolar Organosilane Interfacial Layers

With the goal of investigating and enhancing anode performance in bulk-heterojunction (BHJ) organic photovoltaic (OPV) cells, the glass/tin-doped indium oxide (ITO) anodes are modified with a series of robust silane-tethered bis(fluoroaryl)amines to form self-assembled interfacial layers (IFLs). The modified ITO anodes are characterized by contact angle measurements, X-ray reflectivity, ultraviolet photoelectron spectroscopy, X-ray photoelectron spectroscopy, grazing incidence X-ray diffraction, atomic force microscopy, and cyclic voltammetry. These techniques reveal the presence of hydrophobic amorphous monolayers of 6.68 to 9.76 Å thickness, and modified anode work functions ranging from 4.66 to 5.27 eV. Two series of glass/ITO/IFL/active layer/LiF/Al BHJ OPVs are fabricated with the active layer = poly(3-hexylthiophene):phenyl-C71-butyric acid methyl ester (P3HT:PC71BM) or poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)-carbonyl]thi-eno[3,4-b]thiophenediyl]]:phenyl-C71-butyric acid methyl ester (PTB7:PC71BM). OPV analysis under AM 1.5G conditions reveals significant performance enhancement versus unmodified glass/ITO anodes. Strong positive correlations between the electrochemically derived heterogeneous electron transport rate constants (ks) and the device open circuit voltage (Voc), short circuit current (Jsc), hence OPV power conversion efficiency (PCE), are observed for these modified anodes. Furthermore, the strong functional dependence of the device response on ks increases as greater densities of charge carriers are generated in the BHJ OPV active layer, and is attributable to enhanced anode carrier extraction in the case of high-ks IFLs.

Significant Efficiency Enhancement of Bulk Heterojunction Organic Photovoltaics Using Solution-Processable Interfacial Bilayers

A solution-processable interfacial bilayer is found to be able to significantly increase a short-circuit current and thus the overall performance of bulk heterojunction organic photovoltaic cells. The interfacial bilayer is comprised of surface-modified single-layer graphene oxide and a donor polymer. The use of the interfacial bilayer can minimize exciton quenching and inverse current and does not alter the energy level of photovoltaic devices, which is also applicable to other organic optoelectronic diode devices.

Investigation of the Role of the Acceptor Molecule in Bulk Heterojunction Photovoltaic Cells Using Impedance Spectroscopy

An investigation of the recombination kinetics and the density of states distribution in bulk heterojunction organic photovoltaic cells was undertaken using impedance spectroscopy measurements under varying levels of illumina- tions. Three organic polymer solar cells were investigated which differed only by the p-type polymer used in the active layers while the device architecture and acceptor molecule (PC70BM) were kept the same. We found that the density of states profiles for the three cells are similar and can be superimposed by a horizontal shift due to a difference between the polymers' HOMO and the PC70BM's LUMO levels. The recombination mechanisms for the three cells showed the same behavior, suggesting an important role of the acceptor in this process; however, large differences in the absolute value of carrier lifetime τeff related to the choice of the polymer are evident, indicating that the recombination kinetics depends on a combination of the properties of both materials and the morphology of their blend.

POSS-Enhanced Phase Separation in Air-Processed P3HT:PCBM Bulk Heterojunction Photovoltaic Systems

Nanoparticles have been shown in some cases to improve phase separation and morphology in bulk heterojunction organic photovoltaic cells. In this study, the effect of incorporation of polyhedral oligomeric silsesquioxane (POSS) molecules of different structures in air processed poly(3-hexylthiophene-2,5-diyl) (P3HT) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM) films and photovoltaic cells was evaluated. Morphology and composition of the nanoscalephase-separated domains were determined via conductive atomic force microscopy in conjunction with nanomechanical mapping and Raman imaging. UV-vis and fluorescence spectroscopy analysis of the films was performed at different stages of the process and with different levels of solvent vapor and thermal annealing. It was found that POSS molecules of selected structures provided enhancement in morphology control in films, translating to improvements in fill factor and power conversion efficiency of laboratory-scale OPV cells. The findings indicate the potential for further improvements in solar cell performance with specifically tailored POSS/polymer phase-separated systems.

Full donor-type conjugated polymers consisting of alkoxy- or alkylselenophene-substituted benzodithiophene and thiophene units for organic photovoltaic devices

Full donor-type conjugated polymers containing benzodithiophene and thiophene derivative units were synthesized as electron donors for organic photovoltaic devices. The alkoxy-substituted benzo[1,2-b:4,5-b′]dithiophene (BDT) monomer, 2,6-bis(trimethyltin)-4,8-di(2-ethylhexyloxyl)benzo[1,2-b:4,5-b′]dithiophene, was polymerized with 2,5-dibromothiophene through a Pd(0)-catalyzed Stille coupling reaction. To enhance the interchain interactions between polymers chains, an alkylselenophene-substituted BDT derivative was newly synthesized, and copolymerized with the same counter monomer parts. The two newly synthesized polymers were characterized for use in organic photovoltaic devices as electron donors. Measured optical band gap energies of the polymers were 2.10 and 1.96eV, depending on polymer structure. Field-effect transistors were fabricated using the polymers to measure their hole mobilities, which ranged from 10−3 to 10−5cm2V−1s−1 depending on the polymer structure. Bulk heterojunction organic photovoltaic cells were fabricated using conjugated polymers as electron donors and a [6,6]-phenyl C71-butyric acid methyl ester (PC71BM) as an electron acceptor. One fabricated device showed a power conversion efficiency of 2.73%, an open-circuit voltage of 0.72V, a short-circuit current of 7.73mAcm−2, and a fill factor of 0.46, under air mass (AM) 1.5 global (1.5G) illumination conditions (100mWcm−2).

Effects of Intramolecular Donor–Acceptor Interactions on Bimolecular Recombination in Small-Molecule Organic Photovoltaic Cells

To enhance the device performance of bulk heterojunction organic photovoltaic (OPV) cells, recombination processes must be suppressed. Here, we prepared OPV cells using donor materials composed of different electron-rich donor and electron-deficient acceptor units and the acceptor material 6,6-phenyl-C61-butyric acid methyl ester. The effects of the polarization of the donor materials which was induced by the intramolecular donor–acceptor (D–A) interactions on the bimolecular recombination constant of each device were studied using a time-of-flight method. A large D–A interaction decreased the bimolecular recombination constant, increasing the fill factor of the device. An active layer with a large dielectric constant led to a small Coulomb capture radius between the radical cations and anions. Overall, a larger D–A interaction enhanced the device performance of small-molecule OPV cells.

Bulk-heterojunction organic photovoltaic cells fabricated using a high-viscosity solution of poly(3-hexylthiophene) with extremely high molecular weight

In the present work, we demonstrated that the amount of both donor and acceptor materials in the fabrication of BHJ OPVs could be strongly reduced using a high viscosity P3HT solution with the Mw of 680 900. In addition, only slight fluctuations were observed in PCEs of around 3.8% for all OPVs based on P3HT for Mw ranging from 46 600 to 680 900.

Fullerene nanowires as a versatile platform for organic electronics

The development of organic semiconducting nanowires that act as charge carrier transport pathways in flexible and lightweight nanoelectronics is a major scientific challenge. We report on the fabrication of fullerene nanowires that is universally applicable to its derivatives (pristine C60, methanofullerenes of C61 and C71, and indene C60 bis-adduct), realized by the single particle nanofabrication technique (SPNT). Nanowires with radii of 8–11 nm were formed via a chain polymerization reaction induced by a high-energy ion beam. Fabrication of a poly(3-hexylthiophene) (P3HT): [6,6]-phenyl C61 butyric acid methyl ester (PC61BM) bulk heterojunction organic photovoltaic cell including PC61BM nanowires with precisely-controlled length and density demonstrates how application of this methodology can improve the power conversion efficiency of these inverted cells. The proposed technique provides a versatile platform for the fabrication of continuous and uniform n-type fullerene nanowires towards a wide range of organic electronics applications.

High performance organic photovoltaic cells with blade-coated active layers

High performance bulk heterojunction organic photovoltaic (OPV) cells with blade-coated active layers of POD2T-DTBT blended with [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) are demonstrated in this work. The blade-coated cells exhibit power conversion efficiencies of up to 6.74%, which is comparable to those of conventional spincoated cells but obtained with just a fraction of the material amount used for spincoating. The choice of solvent and the process conditions are manipulated to achieve dense, well-separated morphology which increases the interfacial area for charge dissociation in the blend film. The blade coating technique can be applied in the fabrication of large area, high performance OPV cells with highly efficient materials usage.

New Semiconductors Based on 2,2′-Ethyne-1,2-diylbis[3-(alk-1-yn-1-yl)thiophene] for Organic Opto-Electronics

We report the synthesis and properties of three novel acetylenic materials based on the new electron-rich building block, 2,2′-ethyne-1,2-diylbis[3-(alk-1-yn-1-yl)thiophene] (EBT). The synthesis of this new nonacene core is efficient and straightforward, and variation among n-hexyl, n-tetradecyl, and 2-ethylhexyl substituents substantially impacts the materials properties. Appending 2-ethylhexyl substituted diketopyrrolopyrrole (DPP) units to either terminus of the EBT core yields a series of low band gap molecules that are characterized in detail by a range of experimental microstructure and electronic structure probes and by density functional theory (DFT) computation. Detailed morphology/microstructure characterization of spin-cast films by X-ray diffraction and AFM reveals instructive microstructure and electronic/photovoltaic response relationships in both organic field-effect transistors and bulk-heterojunction organic photovoltaic cells. Thus, the former devices exhibit hole mobilities (μh) as larg...