High‐Resolution Spectroscopic Mapping of the Chemical Contrast from Nanometer Domains in P3HT:PCBM Organic Blend Films for Solar‐Cell Applications

A high‐resolution near‐field spectroscopic mapping technique is successfully applied to investigate the influence of thermal annealing on the morphology of a poly(3‐hexylthiophene) and [6,6]‐penyl‐C₆₁ butyric acid methyl ester (P3HT:PCBM) blend film. Based on the simultaneously recorded morphological and spectroscopic information, the interplay among the blend film morphology, the local P3HT:PCBM molecular distribution, and the P3HT photoluminescence (PL) quenching efficiency are systematically discussed. The PL and Raman signals of the electron donor (P3HT) and acceptor (PCBM) are probed at an optical resolution of approximately 10 nm, which allows the chemical nature of the different domains to be identified directly. In addition, the local PL quenching efficiency, which is related to the electron transfer from P3HT to PCBM, is quantitatively revealed. From these experimental results, it is proposed that high‐resolution near‐field spectroscopic imaging is capable of mapping the local chemical composition and photophysics of the P3HT:PCBM blends on a scale of a few nanometers.     

Charge‐Separation Dynamics in Inorganic–Organic Ternary Blends for Efficient Infrared Photodiodes

Knowledge about the working mechanism of the PbS:P3HT:PCBM [P3HT=poly(3‐hexylthiophene), PCBM=[6,6]‐phenyl‐C₆₁ ‐butyric acid methyl ester] hybrid blend used for efficient near‐infrared photodiodes is obtained from time‐resolved photoluminescence (PL) studies. To understand the role of each component in the heterojunction, the PL dynamics of the ternary (PbS:P3HT:PCBM) blend and the binary (PbS:P3HT, PbS:PCBM and P3HT:PCBM) blends are compared with the PL of the pristine PbS nanocrystals (NCs) and P3HT. In the ternary blend the efficiency of the charge transfer is significantly enhanced compared to the one of PbS:P3HT and PbS:PCBM blends, indicating that both hole and electron transfer from excited NCs to the polymer and fullerene occur. The hole transfer towards the P3HT determines the equilibration of their population in the NCs after the electron transfer towards PCBM, allowing their re‐excitation and new charge transfer process.     

Effects of incorporation of copper sulfide nanocrystals on the performance of P3HT: PCBM based inverted solar cells

It has been reported that performance of bulk heterojunction organic solar cells can be improved by incorporation of an additive like metal and semiconducting nanoparticles in the active layer. Here in, we have synthesized Cu₂S nanocrystals (NCs) by chemical route and studied its dispersion in poly (3-hexylthiophene) [6, 6]-phenyl C61-butyric acid methyl ester (P3HT: PCBM) matrix. Variation in the performance parameters with change in the concentration of Cu₂S NCs into the P3HT: PCBM matrix has also been studied and it was found that the inverted geometry device with concentration of 20 wt% of Cu₂S NCs and having the structure ITO/ZnO (NPs)/P3HT: PCBM:Cu₂S NCs/MoO₃/Al has shown maximum efficiency of 3.39% which is more than 100% increase in comparison with devices without Cu₂S NCs. Photoluminescence measurements studies unveiled that the incorporation of Cu₂S NCs into a P3HT: PCBM matrix has helped in quenching photoluminescence which suggests more effective exciton dissociation at the interfaces between the P3HT and PCBM domains. The Nyquist plots obtained from impedance spectroscopy at 1 V bias in the dark has suggested the effective lifetime and global mobilities for P3HT: PCBM as 0.267 ms and 1.17 × 10⁻³ cm²/V-S and for P3HT: PCBM:Cu₂S NCs (20 wt%) systems as 0.156 ms and 2.02 × 10⁻³ cm²/V-S respectively. Based on observed photoluminescence quenching, calculated effective lifetime and global mobility, we have tried to explain the possible reason for improvement in the efficiency with the very well dispersion of Cu₂S NCs into the P3HT: PCBM matrix.     

Charge transport properties and microstructure of polythiophene/polyfluorene blends

We present a combined charge transport and X-ray diffraction study of blends based on regioregular poly(3-hexylthiophene) (P3HT) and the polyfluorene co-polymer poly((9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis(3-hexylthien-5-yl)-2,1,3-benzothiadiazole]-2′,2′′-diyl) (F8TBT) that are used in efficient all-polymer solar cells. Hole mobility is observed to increase by nearly two orders of magnitude from less than 10^?7 cm² V^?1 s^?1 for as spin-coated blends to 6 × 10^?6 cm² V^?1 s^?1 for blends annealed at 453 K at a field of 2.7 × 10⁵ V/cm, but still significantly below the time-of-flight mobility of unblended P3HT of 1.7 × 10^?4 cm² V^?1 s^?1. The hole mobility of the blends also show a strong negative electric-field dependence, compared with a relatively flat electric-field dependence of unblended P3HT, suggestive of increased spatial disorder in the blends. X-ray diffraction measurements reveal that P3HT/F8TBT blends show a phase separation of the two components with a crystalline part attributed to P3HT and an amorphous part attributed to F8TBT. In as-spun and mildly annealed blends, the measured d-values and relative intensities of the 100, 200 and 300 P3HT peaks are noticeably different to unblended P3HT indicating an incorporation of F8TBT in P3HT crystallites that distorts the crystal structure. At higher anneal temperatures the blend d-values approach that of unblended P3HT suggesting a well separated blend with pure P3HT crystallites. P3HT crystallite size in the blend is also observed to increase with annealing from 3.3 to 6.1 nm, however similar changes in crystallite size are observed in unblended P3HT films with annealing. The lower mobility of P3HT/F8TBT blends is attributed not only to increased P3HT structural disorder in the blend, but also due to the blend morphology (increased spatial disorder). Changes in hole mobility with annealing are interpreted in terms of the need to form percolation networks of P3HT crystallites within an F8TBT matrix, with a possible contribution due to the intercalation of F8TBT in P3HT crystallites acting as defects in the as-prepared state.     

Effects of carbon chain on hole-transport properties in naphtho[2,1-b:6,5-b′]difuran derivatives: Remarkable anisotropic mobilities

Theoretical investigations of hole-transport properties in two naphtho[2,1-b:6,5-b′]difuran derivatives as novel p-type organic semiconductor based on the Marcus–Hush theory combining quantum mechanics are carried out. This work focuses on the effects of carbon chain on molecular orbitals, partial charge difference, ionization potential, internal energy relaxation, and hole-transport behaviors. Through computational modeling, we are shedding light on the favorable function of C8-DPNDF single crystal as p-type organic material. With the introduction of octyl group, C8-DPNDF single crystal possesses high hole-transfer mobilites (1.589 cm² V⁻¹ s⁻¹) and remarkable anisotropic behavior. The simulated anisotropic mobility curve of C8-DPNDF demonstrates the maximum value of the mobility appears when the measuring conducting channel is along the b-axis of the single crystal. The adiabatic ionization potential (AIP) and vertical ionization potential (VIP) of C8-DPNDF are about 6.312 and 6.399 eV, that is, slightly smaller than those of DPNDF. The relatively small IP values can ensure effective hole injection from the source electrode. The data obtained from the present work can be used to prove that C8-DPNDF molecule has the potential to develop into high-efficient p-type organic semiconductor materials, whose hole-transport mobility can be further improved when the measuring transistor channel is controlled carefully.     

Electromodulation of monomer and excimer phosphorescence in vacuum-evaporated films of platinum (II) complexes of 1,3-di(2-pyridyl)benzenes

Electric field-modulated photoluminescence (EML) measurements are presented for vacuum-evaporated films of cyclometallated Pt (II) complexes of 1,3-di(2-pyridyl) benzenes used as triplet emitters in organic light-emitting diodes (OLEDs). The excimer phosphorescence is quenched by the external electric field of 2.5 MV/cm up to 25% but the same effect on monomer phosphorescence is one order of magnitude smaller. The higher quenching effect for triplet excimers than triplet monomers in solid films of Pt complexes is rationalized assuming excimers to be populated within excimer-active domains of the films through an intermediate stage of geminate (e–h) pairs derived from dissociated monomer excitons. The EML data for excimers are successfully described in the framework of Sano–Tachiya–Noolandi–Hong (STNH) theory of geminate (e–h) pair recombination where the final recombination step (e–h capture) proceeds on a sphere of finite radius (a) with a finite speed. The conventional Onsager theory (a = 0) is sufficient to explain the EML quenching effect for monomers. The results are important for explaining the decrease of electroluminescence quantum efficiency observed in OLEDs working under high electric fields.     

Bis(9,9-dihexyl-9H-fluorene-7-yl)amine (BDFA) as a new donor for porphyrin-sensitized solar cells

Two novel zinc porphyrin dyes (coded as ZZX-N1 and ZZX-N2) with a bis(9,9-dihexyl-9H-fluorene-7-yl)amine (BDFA) as an electron donor and a benzoic acid as an acceptor were designed and synthesized in a donor–π–acceptor configuration for dye-sensitized solar cells. ZZX-N1 with two small 3,5-di-tert-butylphenyl groups on the meso positions of porphyrin ring and ZZX-N2 with two large 2,6-dioctoxylphenyl groups displayed similar absorption spectra with an onset ∼700 nm, and ∼28 nm red-shift was observed when compared to YD2 because of stronger electron donating ability of bis(9,9-dihexyl-9H-fluorene-7-yl)amine (BDFA) than bis(4-hexylphenyl)amine in YD2. ZZX-N1-sensitized cells exhibited higher energy conversion efficiency than ZZX-N2-sensitized device (5.78% vs. 3.61%). The electrochemical impedance study showed higher electron recombination resistance in the interface of TiO₂/dye/electrolyte in ZZX-N1-sensitized cell than in ZZX-N2-sensitized cell. The transient decay measurements showed the longer electron lifetime of former than the later. The density functional theory calculations suggested that this could be due to small voids among ZZX-N1 dye molecules on the TiO₂ surface, preventing the charge recombination with the redox couple. The results demonstrated the potential of BDFA as an excellent functional group for high efficiency solar cells.     

Electrical characterization of poly(amide-imide) for application in organic field effect devices

The admittance spectra and current–voltage (I–V) characteristics are reported of metal–insulator–metal (MIM) and metal–insulator–semiconductor (MIS) capacitors employing cross-linked poly(amide–imide) (c-PAI) as the insulator and poly(3-hexylthiophene) (P3HT) as the active semiconductor. The capacitance of the MIM devices are constant in the frequency range from 10 Hz to 100 kHz, with tan δ values as low as 7 × 10⁻³ over most of the range. Except at the lowest voltages, the I–V characteristics are well-described by the Schottky equation for thermal emission of electrons from the electrodes into the insulator. The admittance spectra of the MIS devices displayed a classic Maxwell–Wagner frequency response from which the transverse bulk hole mobility was estimated to be ∼2 × 10⁻⁵ cm² V⁻¹s⁻¹ or ∼5 × 10⁻⁸ cm² V⁻¹s⁻¹ depending on whether or not the surface of the insulator had been treated with hexamethyldisilazane (HMDS) prior to deposition of the P3HT. From the maximum loss observed in admittance-voltage plots, the interface trap density was estimated to be ∼5 × 10¹⁰ cm⁻² eV⁻¹ or ∼9 × 10¹⁰ cm⁻² eV⁻¹ again depending whether or not the insulator was treated with HMDS. We conclude, therefore, that HMDS plays a useful role in promoting order in the P3HT film as well as reducing the density of interface trap states. Although interposing the P3HT layer between the insulator and the gold electrode degrades the insulating properties of the c-PAI, nevertheless, they remain sufficiently good for use in organic electronic devices.     

High resolution electrical characterisation of organic photovoltaic blends

In this work, electrostatic force microscopy (EFM) and conductive atomic force microscopy (C-AFM) are applied to perform high-resolution electrical characterisation of organic photovoltaic films. These films are composed of the C₆₀-derivative PCBM blended with hole conductive conjugated polymers PPV derivatives or P3HT. It is demonstrated that both EFM and C-AFM are able to electrically evidence phase separation in the blends, suggesting in addition higher density of carriers along interfaces. Correlation between the EFM contrast and the photovoltaic properties of the blends was observed. Local spectroscopy (I-V curves) completes the C-AFM investigations, analysing charge transport mechanisms in the P3HT:PCBM blend. Significant modifications of the local electrical properties of P3HT are shown to occur upon blending. Space charge limited current is evidenced in the blend and a hole mobility of 1.7 × 10⁻² cm² V⁻¹ s⁻¹ is determined for P3HT.     

Photo‐induced Charge Transfer and Relaxation of Persistent Charge Carriers in Polymer/Nanocrystal Composites for Applications in Hybrid Solar Cells

The photo‐induced charge transfer and the dynamics of persistent charge carriers in blends of semiconducting polymers and nanocrystals are investigated. Regioregular poly(3‐hexylthiophene) (P3HT) is used as the electron donor material, while the acceptor moiety is established by CdSe nanocrystals (nc‐CdSe) prepared via colloidal synthesis. As a reference system, organic blends of P3HT and [6,6]‐phenyl C₆₁‐butyric acid methyl ester (PCBM) are studied as well. The light‐induced charge transfer between P3HT and the acceptor materials is studied by photoluminescence (PL), photo‐induced absorption (PIA) and light‐induced electron spin resonance spectroscopy (LESR). Compared to neat P3HT samples, both systems show an intensified formation of polarons in the polymer upon photo‐excitation, pointing out successful separation of photogenerated charge carriers. Additionally, relaxation of the persistent charge carriers is investigated, and significant differences are found between the hybrid composite and the purely organic system. While relaxation, reflected in the transient signal decay of the polaron signal, is fast in the organic system, the hybrid blends exhibit long‐term persistence. The appearance of a second, slow recombination channel indicates the existence of deep trap states in the hybrid system, which leads to the capture of a large fraction of charge carriers. A change of polymer conformation due to the presence of nc‐CdSe is revealed by low temperature LESR measurements and microwave saturation techniques. The impact of the different recombination behavior on the photovoltaic efficiency of both systems is discussed.     

Analysis of exciton annihilation in high-efficiency sky-blue organic light-emitting diodes with thermally activated delayed fluorescence

We study external quantum efficiency (η_EQE) roll-off in organic light-emitting diodes (OLEDs) using thermally-activated delayed fluorescence (TADF) of 4,5-di (9H-carbazol-9-yl) phthalonitrile (2CzPN). Using 2CzPN intramolecular rate constants from optical analyses, we construct an exciton quenching model incorporating intersystem crossing and reverse intersystem crossing. The model indicates that singlet–triplet annihilation and triplet–triplet annihilation dominate η_EQE roll-off because of the relatively long 2CzPN triplet lifetime of 273 μs. This work yields a method to relax the exciton quenching process in TADF based OLEDs.     

Electrical and photoelectrical properties of P3HT/n-Si hybrid organic–inorganic heterojunction solar cells

A detail analysis of electrical and photoelectrical properties of hybrid organic–inorganic heterojunction solar cells poly(3-hexylthiophene) (P3HT)/n-Si, fabricated by spin-coating of the polymeric thin film onto oxide passivated Si(1 0 0) surface, was carried out within the temperature ranging from 283 to 333 K. The dominating current transport mechanisms were established to be the multistep tunnel-recombination and space charge limited current at forward bias and leakage current through the shunt resistance at reverse bias. A simple approach was developed and successfully applied for the correct analysis of the high frequency C–V characteristics of hybrid heterojunction solar cells. The P3HT/n-Si solar cell under investigation possessed the following photoelectric parameters: J_sc = 16.25 mA/cm², V_oc = 0.456 V, FF = 0.45, η = 3.32% at 100 mW/cm² AM 1.5 illumination. The light dependence of the current transport mechanisms through the P3HT/n-Si hybrid solar cells is presented quantitatively and discussed in detail.     

Composition tuning of a mixture of thienothiophene-based polymer (PTB7) and PC70BM using a novel additive,tetrabromothiophene (Br-ADD)

A novel additive, tetrabromothiophene (Br-ADD), is shown to improve the properties of bulk heterojunction consisting of poly(3-hexylthiophene: -phenyl-C₆₀ (P3HT:PC₆₀BM) and a thienothiophene-based polymer: C₇₀-butyric acid methyl ester (PTB7:PC₇₀BM). The hydrophobic Br-ADD selectively interacts with the fused heteroaromatic rings on the polymers to increase the polymer surface energy, which improves the bulk heterojunction and photovoltaic performances of the resulting polymer solar cells (23.5% for P3HT and 12.3% for PTB7). The selective interactions between Br-ADD and the polymer and the properties of the bulk heterojunctions are characterized using UV–vis spectroscopy, photoluminescence (PL), atomic force microscopy (AFM), secondary ion mass spectroscopy (SIMS), transmission electron microscopy (TEM), and J–V measurements.     

Performance improvement of low bandgap polymer bulk heterojunction solar cells by incorporating P3HT

This work demonstrates a significant improvement of device performance by incorporating the polymer poly(3-hexylthiophene-2,5-diyl) (P3HT) into a low bandgap polymer poly[2,1,3-benzothiadiazole-4,7-diyl[4,4-bis(2-ethylhexyl)-4H-cyclopenta [2,1-b:3,4-b′]dithiophene-siloe 2,6-diyl]] (Si-PCPDTBT) and [6,6]-phenyl C71 butyric acid methyl ester (PC₇₁BM) host system, to form a ternary blend bulk heterojunction solar cell. The P3HT concentration was varied from 1 to 5 wt% in the host system. P3HT functions as a morphology control agent in this ternary system. A small weight percentage of P3HT can enhance the light absorption, polymer phase separation, exciton separation and charge carrier mobilities. These results are supported by UV–vis spectroscopy, X-ray diffraction, photoluminescence analysis and other characterisation methods. The highest average power conversion efficiency improvement of 10% was achieved by adding 1 wt% P3HT to the host system. This study reveals a promising way to achieve high efficiency solar cells using a low bandgap polymer.     

Role of electron blocking and trapping layers in transport characterization of a photovoltaic polymer poly(3-hexylthiophene)

Hole injection and transport in films (300-350 nm) of poly(3-hexylthiophene) (P3HT) were investigated by dark-injection space-charge-limited current (DI-SCLC) technique. For samples with a nominally hole-only configuration of anode/P3HT/Au, the DI current transients depart significantly from the theory, and the signals cannot be used for reliable carrier mobility extraction. The origin of the departure can be attributed to electron leakage from the Au cathode. We outline a means of suppressing electron leakage by inserting an interlayer between the P3HT and the cathode. This interlayer has dual functions of blocking and trapping electrons. Using this interlayer, we obtain well-defined DI-SCLC signals for reliable carrier mobility determination. With a suitable interlayer to suppress undesirable carrier injection and transport, DI-SCLC technique should find broad applications in the transport characterization of narrow gap photovoltaic polymers.     

Photoconductivity of a Low‐Bandgap Conjugated Polymer

The photoconductive properties of a novel low‐bandgap 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‐benzothiadiazole)], PCPDTBT, with an optical energy gap of E_g ～ 1.5 eV, have been studied. The results of photoluminescence and photoconductivity measurements indicate efficient electron transfer from PCPDTBT to PCBM ([6,6]‐phenyl‐C61 butyric acid methyl ester, a fullerene derivative), where PCPDTBT acts as the electron donor and PCBM as the electron acceptor. Electron‐transfer facilitates charge separation and results in prolonged carrier lifetime, as observed by fast (t > 100 ps) transient photoconductivity measurements. The photoresponsivities of PCPDTBT and PCPDTBT:PCBM are comparable to those of poly(3‐hexylthiophene), P3HT, and P3HT:PCBM, respectively. Moreover, the spectral sensitivity of PCPDTBT:PCBM extends significantly deeper into the infrared, to 900 nm, than that of P3HT. The potential of PCPDTBT as a material for high‐efficiency polymer solar cells is discussed.     

Effect of CdSe nanoparticles incorporation on the performance of P3OT organic photovoltaic cells

Photoluminescence and photovoltaic properties of P3OT:%CdSe nanocomposite films are investigated as a function of the mass concentration (wt%) of the CdSe nanoparticles (NPs) incorporated in the films. The incorporation of CdSe NPs produces a quenching of the photoluminescence and improves the performance of the nanocomposite solar cells. These effects are explained in terms of exciton dissociation and charge separation occurring at P3OT/CdSe interfaces within the Förster formalism, involving non-radiative energy transfer from the donor (P3OT) to the acceptor (CdSe NPs). An exciton quenching rate constant of 1.4×10⁻¹⁰ cm³ s⁻¹ is determined using the Stern–Volmer equation. In addition, scanning electron microscopy (SEM) images reveal that surface morphology is changed by CdSe NPs incorporation, in agreement with FTIR spectra. The current density–voltage (J–V) characteristics of ITO/P3OT:%CdSe/Al photovoltaic cells performed for different CdSe concentrations are also reported and indicate a significant improvement of the photovoltaic parameters cells, particularly, the conversion efficiency becomes 20 times greater than that of the cell based on pure polymer.     

Comparative study of spectral and morphological properties of blends of P3HT with PCBM and ICBA

We report a comparative study on spectral and morphological properties of two blend systems for polymer solar cells: the donor material poly(3-hexylthiophene) (P3HT) in combination with the acceptor material of either [6,6]-phenyl-C₆₁ butyric acid methyl ester (PCBM) or indene-C₆₀ bisadduct (ICBA) that was reported to enhance efficiencies of polymer solar cells. Optical microscopy and grazing incidence X-ray scattering reveal the stronger tendency of PCBM to from larger and more ordered domains/grains than ICBA either in pure or blend films. Compared to PCBM, the presence of ICBA also substantially perturbs the organization and longer-range ordering of P3HT in increasing the ICBA ratio in blends. With larger and more ordered phase-separated domains, the P3HT/PCBM blend films exhibit significant optical scattering at higher PCBM ratios. Yet, such optical scattering is not significant for P3HT/ICBA blends (even with high ICBA ratios). Overall, results here suggest the reported higher efficiencies of P3HT/ICBA solar cells (vs. P3HT/PCBM cells) cannot be attributed to larger and/or more ordered phase-separated donor–acceptor domains and other characteristics play more important roles in this case.     

Quantitative estimation of exciton quenching strength at interface of charge injection layers and organic semiconductor

The performance of polymer light emitting diodes (PLEDs) degrades due to exciton quenching at the interface with charge injection layers and electrodes. We investigate the photo-physics of singlet excitons in Poly (9, 9-dioctylfluorene-alt-benzothiadiazole) (F8BT) conjugated polymer interfaced with various commonly used hole and electron injection layers. Absolute, steady-state and transient photoluminescence (PL) studies are carried out on pristine F8BT film and films with injection layer/F8BT to understand the role of injection layers on exciton quenching. Exciton quenching by the charge injection layers is treated by accounting for both exciton diffusion and the non-radiative transfer of energy to the charge injection layer. The non-radiative transfer of energy is modelled using dipole-dipole interaction theory coupled with diffusion of excitons, from which we obtain the exciton capture radius (x₀) in the range of 1–7 nm. We also correlate x₀ with PL decay time (τ) using the relation τ α 1/x₀³. The steady-state PL yield for each case also shows correlation with the PL decay lifetime. This study provides interesting insight on the selection criterion for injection layer to be used in PLEDs for minimizing optical losses while preserving the electronic injection properties.     

Bulk heterojunction formation with induced concentration gradient from a bilayer structure of P3HT:CdSe/ZnS quantum dots using inter-diffusion process for developing high efficiency solar cell

Bulk heterojunction (BHJ) solar cells consisting of poly(3-hexylthiophene) (P3HT) as donor and cadmium selenide/zinc sulphide (CdSe/ZnS) core shell quantum dots (QDs) as acceptor have been developed. Starting from the bilayer of P3HT/QD structure a BHJ is induced using the process of thermal inter-diffusion. The absorption measurements on the bilayer structure show that the absorption coefficient increases and the absorption spectrum becomes broader in the annealed device. Also, the photoluminescence of the annealed device is found to decrease by an order of magnitude showing a significant transfer of electrons to the QDs. With this approach and under broadband white light with an irradiance of 8.19 mW/cm², we have been able to achieve a power conversion efficiency of 5.1% and fill factor 0.45 for this solar cell.