Conjugated polymer-functionalized carbon nanotubes enhance the photovoltaic properties of polymer solar cells

In this study, we fabricated a series of polymer solar cells (PSCs) incorporating blends of the maleimide?Cthiophene copolymer PTM10, multi-walled carbon nanotubes (MWCNTs) functionalized with the 2-hydroxyethyl?Cpresenting maleimide?Cthiophene copolymer PTM21-OH (PTM21-CNT), and the fullerene derivative [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) in various weight ratios. PTM21-CNT behaved as an efficient compatibilizer for PTM10 and PCBM and as a charge transport assister when incorporated in the photoactive layers of the PSCs. The energy levels of the lowest unoccupied molecular orbitals and highest occupied molecular orbitals in thin films of PTM10/PTM21-CNT/PCBM blends decreased upon increasing the PTM21-CNT content. The photovoltaic performance of PSCs incorporating the PTM10/PTM21-CNT/PCBM blends increased upon increasing the PTM21-CNT content, presumably because of the high charge-transporting capacity of the MWCNTs. The highest short-circuit current density and photo-energy conversion efficiency were enhanced by approximately 26% and 27%, respectively, relative to those of the PSC without the incorporation PTM21-CNT.     

Effect of multiple adduct fullerenes on charge generation and transport in photovoltaic blends with poly(3‐hexylthiophene‐2,5‐diyl)

The effect of replacing [6,6]‐phenyl‐C₆₁ butyric acid methyl ester (PCBM) by its multiadduct analogs (bis‐PCBM and tris‐PCBM) in bulk heterojunction organic solar cells with poly(3‐hexylthiophene‐2,5‐diyl) (P3HT) is studied in terms of blend film microstructure, photophysics, electron transport properties, and device performance. Although the power conversion efficiency of the blend with bis‐PCBM is similar to the blend with PCBM, the performance of the devices with tris‐PCBM is considerably lower as a result of small photocurrent. Despite the lower electron affinity of the fullerene multiadducts, μs‐ms transient absorption measurements show that the charge generation efficiency is similar for all three fullerenes. The annealed blend films with multiadducts show a lower degree of fullerene aggregation and lower P3HT crystallinity than the annealed blend films with PCBM. We conclude that the reduction in performance is due largely to poorer electron transport in the blend films from higher adducts, due to the poorer fullerene network formation as well as the slower electron transport within the fullerene phase, confirmed here by field effect transistor measurements. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2010     

Polymers for application in organic solar cells: Bithiophene can work better than thienothiophene when coupled to benzodithiophene

The synthesis and characterization of two low band gap copolymers ( P1 and P2 ) incorporating benzo[1,2‐b:4,5‐b']dithiophene unit substituted with octylsulfanylthienyl groups (OSBT) are here reported. These materials, designed to be employed in polymer solar cells (PSCs), were obtained from alternating OSBT and bithiophene ( P1 ) or thienothiophene ( P2 ) units. Their structural electrochemical and photophysical properties were investigated. They are thermally stable and soluble in organic solvents from which they easily form films. They also form π‐stacks in solution, in film and display a moderate solvatochromism. These polymers were tested with [70]PCBM in bulk‐heterojunction (BHJ) PSCs where they act as donor materials and [70]PCBM is the electron acceptor. The best device, obtained using a 1:3 weight ratio for the P1 :[70]PCBM blend, shows a PCE around 1.5%. A broad response from 350 to 700 nm is also observed in the external quantum efficiency (EQE) curves, wider for P1 with respect to P2 . © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 1603–1614     

Formation of a porphyrin/fullerene light-harvesting system from self-assembly of porphyrin-end-functionalized polycyclohexane and [6,6]-phenyl-C61-butyric acid methyl ester: Effects of microphase separation and π–π interactions

Thin films composed of zinc tetraphenylporphyrin-end-functionalized polycyclohexane (ZnTPP-PCHE) and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) blends were prepared to investigate their potential as light-harvesting systems within polymer solar cells (PSCs). Both the microphase separation characteristics and the extent of π–π interactions in these ZnTPP-PCHE/PCBM blends had significant effects on the optical properties of the films. The extent to which the absorption bands of the films overlapped the terrestrial solar spectrum was increased considerably when the blends formed a co-continuous structure, even though films of pure ZnTPP-PCHE or PCBM exhibited only weak absorption over that wavelength region. We conclude that these polymer blend films may be considered as viable candidates for light-harvesting systems within PSCs. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2013     

Porphyrin–diindenothieno[2,3‐b]thiophene alternating copolymer—a blue‐light harvester in ternary‐blend polymer solar cells

Porphyrin, despite chosen by Nature as light harvesting units, hasn't revealed its full potentials as a structural unit in porphyrin‐incorporated polymers (PPors). A novel PPor was synthesized to investigate the origins of the low performances of PPor‐based polymer solar cells (PSCs). The polymer features broad absorption in the blue‐light region, because the diindenothieno[2,3‐b]thiophene (DITT) unit extended the conjugation in the polymer backbone. PPor‐DITT/PC₇₁BM based PSCs have a high V_oc (0.79 V). Their low J_sc and fill factor (FF) were attributed to the un‐optimized morphology, as indicated by the photoluminescence quenching and atomic force microscopy (AFM) experiments. Using PPor‐DITT as a blue‐light harvesting dopant in an amorphous host leverage the strong 400–550 nm absorption of PPor‐DITT and circumvent the difficulties in reaching optimized morphology in the PPor/PCBM thin films. An addition of 2 wt % of PPor‐DITT in ternary‐blend PSCs resulted in a 10 % increase of external quantum efficiency (EQE) in the blue‐light region. However, in a crystalline host, the dopant decreased the crystallinity of the host and led to large drops in FF and power conversion efficiencies (PCEs). The study provides an alternative route and expands the application of PPors in PSCs as a blue‐light harvester in ternary‐blend PSCs using amorphous polymers as host. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Effects of amorphous poly(3‐hexylthiophene) on active‐layer structure and solar cells performance

A key challenge to the development of polymer‐based organic solar cells is the issue of long‐term stability, which is mainly caused by the unstable time‐dependent morphology of active layers. In this study, poly(3‐hexylthiophene) (P3HT)/[6,6]‐phenyl C60‐butyric acid methyl ester (PCBM) blend is used as a model system to demonstrate that the long‐term stability of power conversion efficiency can be significantly improved by the addition of a small amount of amorphous regiorandom P3HT into semicrystalline regioregular one. The optical properties measured by UV–vis absorption and photoluminescence reveal that regiorandom P3HT can intimately mix with PCBM and prevent the segregation of PCBM. In addition, X‐ray scattering techniques were adopted to evidence the retardation of phase separation between P3HT and PCBM when regiorandom P3HT is added, which is further confirmed by optical microscopy that shows a reduction of large PCBM crystals after annealing at high temperature in the presence of regiorandom P3HT. The improvement of the long‐term stability is attributed to the capability of amorphous P3HT to be thermodynamically miscible with PCBM, which allows the active layer to form a more stable structure that evolves slower and hence decelerates the device decay. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 975–985     

Stabilization of the film morphology in polymer: Fullerene heterojunction solar cells with photocrosslinkable bromine‐functionalized low‐bandgap copolymers

Novel bromine‐functionalized photocrosslinkable low‐bandgap copolymers, PBDTTT‐Br25 and PBDTTT‐Br50, are synthesized via Stille cross‐coupling polymerization for the purpose of stabilizing the film morphology in polymer solar cells (PSCs). Photocrosslinking of PBDTTT‐Br25 and PBDTTT‐Br50 copolymers dramatically improves the solvent resistance of the active layer without disrupting the molecular ordering and charge transport, which is confirmed by the insolubility of the films washed by organic solvents and by their thermal behavior. As a result, the formation of large aggregations of fullerene is suppressed in polymer:fullerene blend films even after prolonged thermal annealing, and the stability of the device is enhanced when compared with cells based on noncrosslinkable PBDTTT. The power conversion efficiency of the PSCs based on PBDTTT‐Br25 and PBDTTT‐Br50 reaches 5.17% and 4.48%, respectively, which is improved obviously in comparison with that (4.26%) of the PSCs based on the control polymer PBDTTT. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 3123–3131     

Enhanced properties of photovoltaic devices tailored with novel supramolecular structures based on reduced graphene oxide nanosheets grafted/functionalized with thiophenic materials

For verifying the influence of donor–acceptor supramolecules on photovoltaic properties, different hybrids were designed and used in organic solar cells. In this respect, reduced graphene oxide (rGO) was functionalization with 2‐thiophene acetic acid (rGO‐f‐TAA) and grafted with poly(3‐dodecylthiophene) (rGO‐g‐PDDT) and poly(3‐thiophene ethanol) (rGO‐g‐PTEt) to manipulate orientation of poly(3‐hexylthiophene) (P3HT) assemblies. Face‐on, edge‐on, and flat‐on orientations were detected for assembled P3HTs on rGO and its functionalized and grafted derivatives, respectively. Alteration of P3HT orientation from face‐on to flat‐on enhanced current density (J _sc), fill factor (FF), and power conversion efficiency (PCE) and thus J _sc = 7.11 mA cm^?2, FF = 47%, and PCE = 2.14% were acquired. By adding phenyl‐C71‐butyric acid methyl ester (PC71BM) to active layers composed of pre‐designed P3HT/rGO, P3HT/rGO‐f‐TAA, P3HT/rGO‐g‐PDDT, and P3HT/rGO‐g‐PTEt hybrids, photovoltaic characteristics further improved, demonstrating that supramolecules appropriately mediated in P3HT:PC71BM solar cells. Phase separation was more intensified in best‐performing photovoltaic systems. Larger P3HT crystals assembled onto grafted rGOs (95–143 nm) may have acted as convenient templates for the larger and more intensified phase separation in P3HT:PCBM films. The best performances were reached for P3HT:P3HT/rGO‐g‐PDDT:PCBM (J _sc = 9.45 mA cm^?2, FF = 54%, and PCE = 3.16%) and P3HT:P3HT/rGO‐g‐PTEt:PCBM (J _sc = 9.32 mA cm^?2, FF = 53%, and PCE = 3.11%) photovoltaic systems. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55 , 1877–1889     

Comparison of two types of vertically aligned ZnO NRs for highly efficient polymer solar cells

Vertically aligned ZnO nanorods (NR) are prepared by two different syntheses methods and applied on polymer solar cells (PSCs). The ZnO electrodes work as the electron transport layer with the P3HT:PCBM blend acting as the active material. Several organic blend solution conditions are optimized: concentration, solvent, and deposition speed. The effect of different NR electrode morphologies is analyzed on the solar cell performance and characterized by current–voltage curves and IPCE analyses. The photovoltaic performance of the solar cells was observed to be influenced by many factors, among them infiltration of the organic P3HT:PCBM blend within the ZnO NR layer. The infiltration of the active layer was monitored by cross section SEM and energy dispersive X-ray spectroscopy analyses. Our results show that higher power conversion efficiencies are achieved when shorter NRs lengths are applied. The best power conversion efficiency obtained was 2.0% for a 400 nm ZnO NR electrode. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2013     

Synthesis of all‐conjugated poly(3‐hexylthiophene)‐block‐ poly(3‐(4′‐(3″,7″‐dimethyloctyloxy)‐3′‐pyridinyl)thiophene) and its blend for photovoltaic applications

New all‐conjugated block copolythiophene, poly(3‐hexylthiophene)‐block‐poly(3‐(4′‐(3″,7″‐dimethyloctyloxy)‐3′‐pyridinyl)thiophene) (P3HT‐b‐P3PyT) was successfully prepared by Grignard metathesis polymerization. The supramolecular interaction between [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PCBM) and P3PyT was proposed to control the aggregated size of PCBM and long‐term thermal stability of the photovoltaic cell, as evidenced by differential scanning calorimetry (DSC), transmission electron microscopy (TEM), and optical microscopy. The effect of different solvents on the electronic and optoelectronic properties was studied, including chloroform (CL), dichlorobenzene (DCB), and mixed solvent of CL/DCB. The optimized bulk heterojunction solar cell devices using the P3HT‐b‐P3PyT/PCBM blend showed a power conversion efficiency of 2.12%, comparable to that of P3HT/PCBM device despite the fact that former had a lower crystallinity or absorption coefficient. Furthermore, P3HT‐b‐P3PyT could be also used as a surfactant to enhance the long‐term thermal stability of P3HT/PCBM‐based solar cells by limiting the aggregated size of PCBM. This study represents a new supramolecular approach to design all‐conjugated block copolymers for high‐performance photovoltaic devices. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011.     

Impact of side‐chain length on the phase structures of P3ATs and P3AT:PCBM films as revealed by SSNMR and FTIR

It is known that poly(3‐alkylthiophene) (P3AT) side‐chain length notably influences the photovoltaic performances of relating devices. However, comprehensively study on its impact on the structures of P3ATs and their blends with [6, 6]‐phenyl‐C₆₁ butyric acid methyl ester (PCBM) is insufficient. By using solid‐state NMR and FTIR techniques, four P3ATs and their PCBM blends are investigated in this work, focusing on the phase structures as modulated by side‐chain length. Recently, we revealed multiple crystalline main‐chain packings of packing a and b together with a mesophase in poly(3‐butylthiophene) (P3BT) films (DOI: 10.1021/acs.macromol.6b01828). Here, the semicrystalline structures are investigated on poly(3‐hexylthiophene) (P3HT), poly(3‐octylthiophene) (P3OT), and poly(3‐dodecylthiophene) (P3DDT) with traditional form I modification, where packing a and the amorphous phase are probed. Furthermore, crystallized side chain within packing a is detected in both P3OT and P3DDT films, which shows a FTIR absorption at 806 cm⁻¹. Structural studies are also conducted on P3AT:PCBM blends. Compared with the pure P3ATs, the polymer crystallinities of the blends show reduction of about 40% for P3OT and P3DDT, whereas only about 10% for P3HT. Moreover, in P3BT:PCBM and P3HT:PCBM, the crystalline polymers and PCBM are phase separated, while in P3OT:PCBM and P3DDT:PCBM, blend components are mostly miscible. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018 , 56, 751–761     

Characterization and Distribution of Poly(3‐hexylthiophene) Phases in an Annealed Blend Film

The characteristic absorption spectra of three kinds of phases, the isolated, ordered, and disordered phases, in a solvent‐vapor annealed poly(3‐hexylthiophene)/[6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (P3HT/PCBM) blend film were studied by means of spectroelectrochemistry (SEC) and time‐resolved absorption spectroscopy (TAS). The results reveal that the content of three phases are 12 % isolated, 37 % ordered, and 51 % disordered for the annealed P3HT neat film, and 25 % isolated, 31 % ordered, and 44 % disordered for the annealed P3HT/PCBM blend film. The vertical distribution of the different phases in the blend film was studied by SEC, and the results show that the ordered and isolated phases are mainly distributed in the top and in the bottom of the annealed films, respectively, while the disordered phase is mainly distributed in the middle and the bottom of the films.     

Synthesis and Photovoltaic Properties of Ester Group Functionalized Polythiophene Derivatives

To increase the open circuit voltage (V_OC) of polymer solar cells (PSCs) based on polythiophene, two new ester group functionalized polythiophene derivatives, PCTDT and PCTBDT, were designed and synthesized via alternating copolymerization of thiophene‐3‐carboxylate (CT) with the 2,2′‐bithiophene (DT) and benzodithiophene (BDT) units, respectively. The resulting copolymers exhibited broad and strong absorptions in the visible region, which was similar to that of the commonly used poly(3‐hexylthiophene) (P3HT). Through cyclic voltammetry measurements, it was found that both copolymers showed lower HOMO energy levels (−5.27 eV for PCTDT and −5.36 eV for PCTBDT) than that of P3HT (−5.03 eV), indicating that the HOMO energy level could be efficiently reduced by introducing the ester group into the polymer side chain. Photovoltaic properties of the copolymers blended with [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PCBM) as electron acceptor were investigated. The obtained two devices possessed both relatively large short circuit current (I_SC) and higher V_OC than that of P3HT:PCBM blend. For PCTBDT:PCBM blend, a power conversion efficiency (PCE) up to 2.32%, an I_SC of 6.94 mA · cm⁻², and a V_OC of 0.80 V were observed while PCTDT:PCBM system demonstrated a PCE of 1.75% with a V_OC of 0.68 V.

     

Polynorbornene dicarboximide/amine functionalized graphene hybrids for potential oxygen barrier films

In this work, we prepared polynorbornene dicarboximide (PND)/amine functionalized graphene (AFG) hybrids in N,N‐dimethylacetamide (DMAc). The AFG was synthesized by modifying graphene oxides (GO) with amine groups. The AFG sheets were well‐dispersed in DMAc and randomly distributed throughout the PND matrix in the hybrid films, which enhanced the mechanical and oxygen barrier properties of the PND/AFG hybrid films. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Regioregular poly(3‐alkanoylthiophene): Synthesis and electrochemical,photophysical, charge transport,and photovoltaic properties

Head‐to‐tail regioregular poly(3‐heptanoylthiophene) (PHOT) was synthesized by Ni‐catalyzed polycondensation of the 2,2‐dimethyl‐1,3‐propanediol‐protected Grignard monomer followed by deprotection. Cyclic voltammetric (CV) study demonstrates that the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of PHOT are 0.5 eV lower in energy than those of the head‐to‐tail poly(3‐hexylthiophene) (HT‐P3HT). Their optical band gaps are essentially the same. Incomplete photoluminescence (PL) quenching was observed in thin films of the 1:1 blend of PHOT and HT‐P3HT. PHOT displayed a glass transition at ～269 °C and decomposed at ～300 °C according to differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). Wide‐angle X‐ray diffraction (WAXD) study showed that PHOT exists in a not highly ordered state in solid films especially in the π‐stacking direction. Only p‐channel activity was observed in field‐effect transistors (FETs) for PHOT. The hole mobility was on the order of 10^?4 cm² V^?1 s^?1. Photovoltaic devices with an active layer of 1:1 blend of PHOT and PC₇₁BM had a power conversion efficiency (PCE) of ～0.5%. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2010     

Effects of Thermal Annealing Upon the Nanomorphology of Poly(3‐hexylselenophene)‐PCBM Blends

Grazing incidence X‐ray diffraction (GI‐XRD) is used to characterize the crystallographic dynamics of low molecular weight (LMW) and high molecular weight (HMW) poly(3‐hexylselenophene) (P3HS) films and blend films of P3HS with [6‐6‐]‐phenyl‐C₆₁‐butyric acid methyl ester (PCBM) as a function of ‘step‐by‐step’ thermal annealing, from room temperature to 250 °C. The temperature‐dependent GIXRD data show how the melting point of P3HS crystallites is decreased by the presence of PCBM. P3HS crystallite domain sizes dramatically increase upon annealing to the P3HS melting temperature. The formation of well‐oriented HMW P3HS crystallites with the (100) plane parallel to the substrate (edge‐on orientation), when cooled from melt, are observed. We compare the behaviour of P3HS pure and blend films with that of poly(3‐hexyl)thiophene (P3HT) pure and PCBM blended films and suggest that the similar temperature dependent behaviour we observe may be a common to polythiophene and related polymers and their blends.

     

In situ thermo-optical studies of polymer:fullerene blend films

Optical properties of a blend thin film (1:1 wt) of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) exposed to a stepwise heating and cooling, have been reported and compared with the properties of pure PCBM and P3HT films. The UV–Vis(T) absorption measurements were performed in situ, during annealing and cooling runs, at the precisely defined temperatures, in a range of 20–210 °C. It was demonstrated that this new method allows to observe the changes of absorption coefficient spectra and absorption edge parameters: the energy gap (E_G) and the Urbach energy (E_U), connected with the length of conjugation and structural disorder of thin film, respectively. Several stages, during annealing/cooling runs, were distinguished for the P3HT:PCBM blend film and related to the following processes, as an increase of P3HT crystallinity in the blend, the orderly stacking of polymer chains, thermally induced structural defects and the phase separation, caused by an aggregation of PCBM in the polymer matrix. These changes were also observed on the P3HT:PCBM film surface, by means to the microscopic studies.     

Alternating copolymers of electron‐rich arylamine and electron‐deficient 2,1,3‐benzothiadiazole: Synthesis,characterization and photovoltaic properties

A series of alternating copolymers of electron‐rich arylamine and electron‐deficient 2,1,3‐benzothiadiazole (BT), PV‐BT, DP‐BT, and TP‐BT, were synthesized by Heck coupling reaction. UV–vis absorption and fluorescence spectra show that the copolymerization of electron‐rich diphenylamine (DP), triphenylamine (TP), MEH‐PV (PV), and electron‐deficient BT results in low‐bandgap conjugated polymers. Within the three copolymers of PV‐BT, DP‐BT, and TP‐BT, TP‐BT possesses the highest hole mobility of 4.68 × 10^{? 5} cm²/V, as determined from the space charge limited current (SCLC) model. The bulk heterojunction‐typed polymer solar cells (PSCs) were fabricated with the blend of the copolymers and PCBM as the photosensitive layer. The power conversion efficiencies (PCE) of the PSCs based on PV‐BT, DP‐BT, and TP‐BT reached 0.26%, 0.39%, and 0.52%, respectively, under the illumination of AM 1.5, 100 mW/cm². The results indicate that TP‐BT is a promising photovoltaic polymer for PSCs. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 3861–3871, 2007     

4,9‐Dihydro‐s‐indaceno[1,2‐b:5,6‐b′]dithiophene‐4,9‐dione functionalized copolymers for organic photovoltaic devices

The synthesis of conjugated polymers 1 – 5 functionalized with 4,9‐dihydro‐s‐indaceno[1,2‐b:5,6‐b′]dithiophene‐4,9‐dione in the backbone is reported and their use in the construction of organic solar cells is demonstrated. Increasing the molar ratio of 2,7‐dibromo‐3,8‐dihexyl‐4,9‐dihydro‐s‐indaceno[1,2‐b:5,6‐b′]dithiophene‐4,9‐dione, relative to 4,4′‐dihexyl‐5,5′‐dibromo‐2,2′‐bithiophene, in the copolymer synthesis significantly lowers the solubility of these polymers. The incorporation of highly conjugated 3,8‐dihexyl‐4,9‐dihydro‐s‐indaceno[1,2‐b:5,6‐b′]dithiophene‐4,9‐dione unit into the polymer backbone has been confirmed by UV–vis absorption. The observation of decreasing quantum yield for the emission in the order of 1 , 2 , 3 is consistent with copolymers with different comonomer content. The power conversion efficiencies of solar cells using blends of these polymers with PCBM ([6,6]‐phenyl C₆₁‐butyric acid methyl ester) were determined to be 0.11% for polymer 1 , 0.33% for 2 , and 0.26% for 3 , respectively. Under identical white light illumination, the power conversion efficiency of the device based on polymer 2 /PCBM as the active layer was three times higher compared to that of device based on polymer 1 /PCBM. Owing to the limited solubility and poor film‐forming ability of polymer 3 , the power conversion efficiency of solar cell based on 3 /PCBM blend is lower than that of 2 /PCBM blend, but is still larger than that of 1 /PCBM blend. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 2680–2688, 2008     

Transient Absorption Spectroscopy Studies on Polythiophene–Fullerene Bulk Heterojunction Organic Blend Films Sensitized with a Low‐Bandgap Polymer

Recently, the concept of near‐infrared sensitization is successfully employed to increase the light harvesting in large‐bandgap polymer‐based solar cells. To gain deeper insights into the operation mechanism of ternary organic solar cells, a comprehensive understanding of charge transfer–charge transport in ternary blends is a necessity. Herein, P3HT:PCPDTBT:PCBM ternary blend films are investigated by transient absorption spectroscopy. Hole transfer from PCPDTBT‐positive polarons to P3HT in the P3HT:PCPDTBT:PCBM 0.9:0.1:1 blend film can be visualized. This process evolves within 140 ps and is discussed with respect to the proposed charge‐generation mechanisms.