Performance Of Bulk Heterojunction Research Articles

Importance of the Donor:Fullerene Intermolecular Arrangement for High-Efficiency Organic Photovoltaics

The performance of organic photovoltaic (OPV) material systems are hypothesized to depend strongly on the intermolecular arrangements at the donor:fullerene interfaces. A review of some of the most efficient polymers utilized in polymer:fullerene PV devices, combined with an analysis of reported polymer donor materials wherein the same conjugated backbone was used with varying alkyl substituents, supports this hypothesis. Specifically, the literature shows that higher-performing donor-acceptor type polymers generally have acceptor moieties that are sterically accessible for interactions with the fullerene derivative, whereas the corresponding donor moieties tend to have branched alkyl substituents that sterically hinder interactions with the fullerene. To further explore the idea that the most beneficial polymer:fullerene arrangement involves the fullerene docking with the acceptor moiety, a family of benzo[1,2-b:4,5-b']dithiophene-thieno[3,4-c]pyrrole-4,6-dione polymers (PBDTTPD derivatives) was synthesized and tested in a variety of PV device types with vastly different aggregation states of the polymer. In agreement with our hypothesis, the PBDTTPD derivative with a more sterically accessible acceptor moiety and a more sterically hindered donor moiety shows the highest performance in bulk-heterojunction, bilayer, and low-polymer concentration PV devices where fullerene derivatives serve as the electron-accepting materials. Furthermore, external quantum efficiency measurements of the charge-transfer state and solid-state two-dimensional (2D) (13)C{(1)H} heteronuclear correlation (HETCOR) NMR analyses support that a specific polymer:fullerene arrangement is present for the highest performing PBDTTPD derivative, in which the fullerene is in closer proximity to the acceptor moiety of the polymer. This work demonstrates that the polymer:fullerene arrangement and resulting intermolecular interactions may be key factors in determining the performance of OPV material systems.

Highly efficient thieno[3,4-c]pyrrole-4,6-dione-based solar cells processed from non-chlorinated solvent

To obtain high performance bulk heterojunction organic solar cells, the selection of solvents to prepare the donor/acceptor blend is as important as the choice of the donor/acceptor materials themselves. State-of-the-art lab-scale polymer solar cells have evolved around chlorinated solvents such as chloroform, chlorobenzene and o-dichlorobenzene. However, for large scale applications, benign processing solvents may become inevitable. In this work, we used a mixture of Xylenes (a chlorine-free solvent), methyl naphthalene (MeN) and 1,8-diiodoctane (DIO) to modulate the nano-scale morphology of poly(4,4-bis(2-ethylhexyl)-dithieno[3,2-b:2′,3′-d]silole-alt-1,3-(5-octylthieno[3,4-c]pyrrole-4,6-dione) (PDTSTPD)/PCBM blend, one of the most efficient active layer in polymeric solar cell. Power conversion efficiencies up to 5.5% (with PC61BM) and 6.2% (with PC71BM) were obtained for photovoltaic devices with an active area of 1.0 cm2.

Recent Approaches to Controlling the Nanoscale Morphology of Polymer-Based Bulk-Heterojunction Solar Cells

The need for clean, inexpensive and renewable energy has increasingly turned research attention towards polymer photovoltaic cells. However, the performance efficiency of these devices is still low in comparison with silicon-based devices. The recent introduction of new materials and processing techniques has resulted in a remarkable increase in power-conversion efficiency, with a value above 10%. Controlling the interpenetrating network morphology is a key factor in obtaining devices with improved performance. This review focuses on the influence of controlled nanoscale morphology on the overall performance of bulk-heterojunction (BHJ) photovoltaic cells. Strategies such as the use of solvents, solvent annealing, polymer nanowires (NWs), and donor–acceptor (D–A) blend ratios employed to control the active-layer morphologies are all discussed.

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 Phase Separation in Bulk Heterojunction Solar Cells via Supramolecular Chemistry

In this work, we have prepared two donor–acceptor–donor (D-A-D) π-conjugated oligomers to investigate the effect of phase separation on the performance of bulk heterojunction (BHJ) solar cells. These charge transfer low band gap π-conjugated oligomers (TTB and NMeTTB) were synthesized by Knoevenagel condensation of terthiophenecarbaldehyde and barbiturate appended pyran derivative. The thin film morphology of both the oligomers and along with electron acceptor [6,6]-phenyl-C60-butyric acid methyl ester (PC61BM) was investigated by atomic force microscopy (AFM) and transmission electron microscopy (TEM). The blend of NMeTTB and PC61BM thin film yield highly ordered thin film, whereas there was clear phase separation between TTB and PC61BM in thin film.The BHJ solar cell was fabricated using a blend of NMeTTB and TTB with PC61BM acceptor in 1:1 ratio as active layer, and a power conversion efficiency of 1.8% was obtained. This device characteristic was compared with device having TTB:PC61BM as active layer,...

Enhanced performance of polymer solar cells using solution-processed tetra-n-alkyl ammonium bromides as electron extraction layers

We investigate solution-processed tetra-n-alkyl ammonium bromides (TAABs) as electron extraction layers (EELs) in bulk heterojunction (BHJ) solar cells based on poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61 butyric acid methyl ester (PCBM). The EELs of TAABs lead to simultaneous increases in open-circuit voltage (VOC), short-circuit current density, and fill factor, enhancing the power conversion efficiency of BHJ solar cells from 2.38% to 4.02–4.19%. It is interesting that the same increase in VOC of about 0.14 V is obtained for devices with Al, Ag, and Au cathodes by using the EEL of TOAB. The self-assembly of TOAB into the lamellar structure stacked upright atop P3HT:PCBM is corroborated by synchrotron X-ray diffraction. The surface morphologies of TAABs atop P3HT:PCBM, explored by atomic force microscopy, show that there are ultrathin layers of TAABs in the regions beside large island clusters. The underlying mechanism is inferred that the TAAB molecules introduce anisotropic dipoles toward P3HT:PCBM which significantly elevate the vacuum level of the metal cathode for the efficient electron extraction. Our results provide a simple method to fabricate high performance BHJ solar cells by solution processing.

Effect of Incorporated Nitrogens on the Planarity and Photovoltaic Performance of Donor–Acceptor Copolymers

Systematic control of the chemical structure of conjugated polymers is critically important to elucidate the relationship between the conjugated polymer structures and properties and to optimize their performance in bulk heterojunction (BHJ) polymer solar cell (PSC) devices. Herein, we synthesized three new copolymers, i.e., P0, P1, and P2; these copolymers contain the same benzodithiophene donor unit but have different acceptor units with different numbers of nitrogen atoms in the range of 0–2. The effects of the introduced nitrogen atoms on the structural, optical, electrical, and photovoltaic properties of the conjugated polymers were investigated; the structural properties of the polymers, in particular, were studied using both experimental (grazing-incidence X-ray scattering (GIXS) measurements) and computational methods (molecular simulation). As the number of introduced nitrogen atoms increased, the planarity of the main chain conformation increased in the order of P0 < P1 < P2. Additionally, the P0, P1, and P2 polymers showed increased interlayer domain spacings of 1.61, 1.72, and 1.78 nm, respectively, with increased intermolecular ordering. These results were in excellent agreement with the simulation results. In addition, the enhanced planarity resulted in a red-shifting at the onset of absorption in the polymer film from 544 to 585 nm, a downshift in the lowest unoccupied molecular orbital (LUMO) energy level from −3.02 to −3.26 eV, and an increase in the hole mobility from 2.33 × 10–6 to 3.78 × 10–5 cm2/(V s). As a result, we observed dramatically enhanced performance of the PSCs in the order of P0 < P1 < P2. For example, the P2:PC61BM device exhibited a 3.5-fold improvement in power conversion efficiency (PCE) compared to that of P0:PC61BM. The further optimization of P2 with PC71BM showed the PCE of 3.22%.

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.

Effects of Solubilizing Group Modification in Fullerene Bis-Adducts on Normal and Inverted Type Polymer Solar Cells

Structural control of solubilizing side groups in fullerene-based electron acceptors is critically important to optimize their performance in bulk heterojunction (BHJ)-type polymer solar cell (PSC) devices. The structural changes of fullerene derivatives affect not only their optical and electrochemical properties but also their solubility and miscibility with electron donor polymers. Herein, we synthesized a series of o-xylenyl C60 bis-adduct (OXCBA) derivatives with different solubilizing side groups to systematically investigate the effects of fullerene derivative structures on the photovoltaic properties of PSCs. The xylenyl side groups on the OXCBA were modified to produce several different OXCBA derivatives in which the xylenyl groups were functionalized with fluorine (FXCBA), nitro (NXCBA), methoxy and bromine (BMXCBA), and phenyl groups (ACBA). End group modifications of OXCBA dramatically affect photovoltaic performance in blend films with poly(3-hexylthiophene) (P3HT), resulting in power convers...

A Supramolecular “Double‐Cable” Structure with a 12944 Helix in a Columnar Porphyrin‐C60 Dyad and its Application in Polymer Solar Cells

AbstractA novel porphyrin‐C60 dyad (PCD1) is designed and synthesized to investigate and manipulate the supramolecular structure where geometrically isotropic [such as [60]fullerene (C60)] and anisotropic [such as porphyrin (Por)] units coexist. It is observed that PCD1 possesses an enantiomeric phase behavior. The melting temperature of the stable PCD1 thermotropic phase is 160 °C with a latent heat (ΔH) of 18.5 kJ mol−1. The phase formation is majorly driven by the cooperative intermolecular Por–Por and C60–C60 interactions. Structural analysis reveals that this stable phase possesses a supramolecular “double‐cable” structure with one p‐type Por core columnar channel and three helical n‐type C60 peripheral channels. These “double‐cable” columns further pack into a hexagonal lattice with a = b = 4.65 nm, c = 41.3 nm, α = β = 90°, and γ = 120°. The column repeat unit is determined to possess a 12944 helix. With both donor (D; Pro) and acceptor (A; C60) units having their own connecting channels as well as the large D/A interface within the supramolecular “double‐cable” structure, PCD1 has photogenerated carriers with longer lifetimes compared to the conventional electron acceptor [6,6]‐phenyl‐C61‐butyric acid methyl ester. A phase‐separated columnar morphology is observed in a bulk‐heterojunction (BHJ) material made by the physical blend of a low band‐gap conjugated polymer, [poly[2,6‐(4,4‐bis‐(2‐ethylhexyl)‐4H‐cyclopenta [2,1‐b;3,4‐b′]‐dithiophene)‐alt‐4,7‐(2,1,3‐benzothia‐diazole)] (PCPDTBT), and PCD1. With a specific phase structure in the solid state and in the blend, PCD1 is shown to be a promising candidate as a new electron acceptor in high performance BHJ polymer solar cells.

Poly(3-hexylthiophene): synthetic methodologies and properties in bulk heterojunction solar cells

Poly(3-hexylthiophene) (P3HT) remains of significant importance as a prototypical benchmark hole conductor material in Organic Photovoltaics (OPVs). In this review we discuss synthetic strategies to P3HT, particularly focusing on those leading to the regioregular form and discussing key physical and morphological properties. Finally, a survey and a brief discussion of P3HT performance in bulk-heterojunction (BHJ) OPVs are also provided.

Effect of trichlorobenzene additive on the performance and morphology of polyfluorene and fullerene derivative bulk heterojunction solar cells

The influence of additive, namely 1,3,5-trichlorobenzene (TCB), on the morphology and performance of bulk heterojunction (BHJ) organic solar cells was studied based on a 1:2 (w/w) blend of benzothiadiazole/thiophene-based copolymers (PFTBzTT) to [6,6]-phenyl-C61 butyric acid methyl ester (PCBM). The active layers were deposited by spin-coating from solutions using chloroform, with different additive concentrations from 0 to 36 mg/ml. The addition of solvent additive into the polymer solution was able to improve the performance of BHJ solar cells. The maximum power conversion efficiency (%PCE) of 0.85% was obtained for a cell with the TCB concentration of 12 mg/ml after annealing at 180°C for 20 min. From atomic force microscopy (AFM) images, the films processed without TCB appear smoother than those with the TCB additive. A large extent of segregation was also observed in the films processed with a high TCB concentration. The optical images of the thin films suggest that the optimum concentrations of the additive cause the polymer self-organisation, resulting in some aggregation of large PCBM crystals. © 2011 Canadian Society for Chemical Engineering

Conjugated polymers based on quinoxaline for polymer solar cells

Quinoxaline-based π-conjugated donor polymers with different substituent groups (phenyl, p-fluorophenyl, p-methoxyphenyl, methyl, or hydrogen) on the quinoxaline unit were synthesized and characterized, focusing on the effects of these substituent groups on the performance of bulk heterojunction (BHJ)-based polymer solar cells fabricated with phenyl C71 butyric acid methyl ester ([70]PCBM) as an acceptor. Polymers with phenyl groups or p-methoxyphenyl groups showed nanometer-scale phase separation or uniform morphology in BHJ film, together with high field effect transistor (FET) hole mobility. On the other hand, polymers with methyl or hydrogen and a polymer with fluorine exhibited micrometer-scale domains in BHJ and low hole mobility. It is suggested that the mostly steric hindrance of the substituents regulate the polymer's aggregation properties and compatibility with [70]PCBM, which probably affects the hole transport properties and BHJ morphologies. We also investigated the effects of substituent groups on highest occupied molecular orbital energy level and optical band gap of the polymers.

High‐Performance Solar Cells using a Solution‐Processed Small Molecule Containing Benzodithiophene Unit

A conjugated small molecule containing a benzodithiophene unit shows high performance in bulk heterojunction (BHJ) solar cells. Using the simple solution spinning process, a high PCE is achieved by employing this molecule as the donor in BHJ solar cells.

Photovoltaic performance of bifunctional low band gap conjugated copolymer

We have investigated the bifunctional properties using ambipolar low band gap conjugated copolymer in organic bulk-heterojunction (BHJ) solar cells. Ambipolar copolymer, poly[3,6-dithien-2-yl-2,5-di(2-decyltetradecanyl)-pyrrolo[3,4- c]pyrrole-1,4-dione-5′,5′’-diyl- alt-benzo-2,1,3-thiadiazol-4,7-diyl] (PDTDPP- alt-BTZ), successfully works as the bifunctional material acting as both donor or acceptor. However, because of fast recombination nature and broken charge balance of hole and electron, solar cell performances were significantly limited. The results indicate that having distinct role of each component might be better for the high performance BHJ solar cells rather than bifunctional property based on good charge transport properties of both hole and electrons.

High performance bulk-heterojunction organic solar cells fabricated with non-halogenated solvent processing

High efficiency bulk heterojunction (BHJ) organic solar cells based upon poly(3-hexylthiophene) (P3HT)-[6,6]-phenyl-C 61-butyric acid methyl ester (PCBM) are typically solution processed using halogenated solvents like dichlorobenzene (DCB). In this report, we tune the quality of solvent systems using more environmentally friendly, halogen-free solvents to mimic that of DCB based on Hansen solubility parameters (HSPs). A mixture of acetophenone (AP) and mesitylene (MS) can nearly match the HSPs for DCB. Solar cells fabricated using this solvent mixture can exhibit power conversion efficiency (PCE) comparable to that from DCB with a similar surface morphology. It is critical to control the drying process in the mixed solvent system due to significant volatility difference between AP and MS in order to achieve comparable morphology and PCE. This report illustrates a route to fabricate high efficiency BHJ solar cells from halogen-free solvent systems that could eliminate potential hurdles for manufacturing scale-up of organic BHJ solar cells.

Using Bulk Heterojunction Field Effect Measurements to Understand Charge Transport in Solar Cell Materials

AbstractAmbipolar organic thin-film transistors (OTFTs) have been used to study the transport of charge carriers in bulk heterojunction (BHJ) organic photovoltaic devices. Active layers of phase separated blend of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM), have been chosen due to their use in performance BHJ organic photovoltaic devices as well as ease of device fabrication. A method for determining recombination rate after exciton dissociation and measurement of excess carrier lifetime has been reported by studying drain current behavior which yields carrier mobility, conductivity, and carrier concentration both in dark and AM1.5g illumination. Channel-length dependent measurements of the photocurrent show that significant recombination of separated charge carriers begins to occur at lengths greater than 10 μm. A recombination rate of cm-3 s-1 and a carrier lifetime of ≥ 8.8 ms has been calculated.