Time‐Dependent Morphology Evolution by Annealing Processes on Polymer:Fullerene Blend Solar Cells

Changes in the nanoscale morphologies of the blend films of poly (3‐hexylthiophene) (P3HT) and [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PCBM), for high‐performance bulk‐heterojunction (BHJ) solar cells, are compared and investigated for two annealing treatments with different morphology evolution time scales, having special consideration for the diffusion and aggregation of PCBM molecules. An annealing condition with relatively fast diffusion and aggregation of the PCBM molecules during P3HT crystallization results in poor BHJ morphology because of prevention of the formation of the more elongated P3HT crystals. However, an annealing condition, accelerating PCBM diffusion after the formation of a well‐ordered morphology, results in a relatively stable morphology with less destruction of crystalline P3HT. Based on these results, an effective strategy for determining an optimized annealing treatment is suggested that considers the effect of relative kinetics on the crystallization of the components for a blend film with a new BHJ materials pair, upon which BHJ solar cells are based.     

Electrostatic Self‐Assembly Conjugated Polyelectrolyte‐Surfactant Complex as an Interlayer for High Performance Polymer Solar Cells

A simple method is demonstrated to improve the film‐forming properties and air stability of a conjugated polyelectrolyte (CPE) without complicated synthesis of new chemical structures. An anionic surfactant, sodium dodecybenzenesulfonate (SDS), is mixed with cationic CPEs. The electrostatic attraction between these two oppositely‐charged materials provides the driving force to form a stable CPE‐surfactant complex. Compared with a pure CPE, this electrostatic complex is not only compatible with highly hydrophobic bulk‐heterojunction (BHJ) films, e.g. poly(3‐hexylthiophene):[6,6]‐phenyl C₆₁ butyric acid methyl ester (P3HT:PCBM), but also works well with other low bandgap polymer‐based BHJ films. Using this complex as a cathode interface layer, a high power conversion efficiency of 4% can be obtained in P3HT:PCBM solar cells together with improved stability in air. Moreover, ～20% performance enhancement can also be achieved when the complex is used as an interlayer to replace calcium metal for low bandgap polymer‐based BHJ systems.     

Effects of Thermal Annealing Upon the Morphology of Polymer–Fullerene Blends

Grazing incidence X‐ray scattering (GIXS) is used to characterize the morphology of poly(3‐hexylthiophene) (P3HT)–phenyl‐C61‐butyric acid methyl ester (PCBM) thin film bulk heterojunction (BHJ) blends as a function of thermal annealing temperature, from room temperature to 220 °C. A custom‐built heating chamber for in situ GIXS studies allows for the morphological characterization of thin films at elevated temperatures. Films annealed with a thermal gradient allow for the rapid investigation of the morphology over a range of temperatures that corroborate the results of the in situ experiments. Using these techniques the following are observed: the melting points of each component; an increase in the P3HT coherence length with annealing below the P3HT melting temperature; the formation of well‐oriented P3HT crystallites with the (100) plane parallel to the substrate, when cooled from the melt; and the cold crystallization of PCBM associated with the PCBM glass transition temperature. The incorporation of these materials into BHJ blends affects the nature of these transitions as a function of blend ratio. These results provide a deeper understanding of the physics of how thermal annealing affects the morphology of polymer–fullerene BHJ blends and provides tools to manipulate the blend morphology in order to develop high‐performance organic solar cell devices.     

Effect of Trace Solvent on the Morphology of P3HT:PCBM Bulk Heterojunction Solar Cells

The performance of bulk‐heterojunction (BHJ) solar cells is strongly correlated with the nanoscale structure of the active layer. Various processing techniques have been explored to improve the nanoscale morphology of the BHJ layer, e.g., by varying the casting solvent, thermal annealing, solvent annealing, and solvent additives. This paper highlights the role of residual solvent in the “dried” BHJ layer, and the effect of residual solvents on PCBM diffusion and ultimately the stability of the morphology. We show that solvent is retained within the BHJ film despite prolonged heat treatment, leading to extensive phase separation, as demonstrated by the growth in the size and quantity of PCBM agglomerates. The addition of a small volume fraction of nitrobenzene to the casting solution inhibits the diffusion of PCBM in the dry film, resulting in smaller PCBM agglomerates, and improves the fill factor of the BHJ device to 0.61 without further tempering. The addition of nitrobenzene also increases the P3HT crystalline content, while increasing the onset temperature for melting of P3HT side chains and backbone. The melting temperature for PCBM is also higher with the nitrobenzene additive present.     

Effect of Alkyl Side‐Chain Length on Photovoltaic Properties of Poly(3‐alkylthiophene)/PCBM Bulk Heterojunctions

The morphological, bipolar charge‐carrier transport, and photovoltaic characteristics of poly(3‐alkylthiophene) (P3AT):[6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM) blends are studied as a function of alkyl side‐chain length m, where m equals the number of alkyl carbon atoms. The P3ATs studied are poly(3‐butylthiophene) (P3BT, m = 4), poly(3‐pentylthiophene) (P3PT, m = 5), and poly(3‐hexylthiophene) (P3HT, m = 6). Solar cells with these blends deliver similar order of photo‐current yield (exceeding 10 mA cm^?2) irrespective of side‐chain length. Power conversion efficiencies of 3.2, 4.3, and 4.6% are within reach using solar cells with active layers of P3BT:PCBM (1:0.8), P3PT:PCBM (1:1), and P3HT:PCBM (1:1), respectively. A difference in fill factor values is found to be the main source of efficiency difference. Morphological studies reveal an increase in the degree of phase separation with increasing alkyl chain length. Moreover, while P3PT:PCBM and P3HT:PCBM films have similar hole mobility, measured by hole‐only diodes, the hole mobility in P3BT:PCBM lowers by nearly a factor of four. Bipolar measurements made by field‐effect transistor showed a decrease in the hole mobility and an increase in the electron mobility with increasing alkyl chain length. Balanced charge transport is only achieved in the P3HT:PCBM blend. This, together with better processing properties, explains the superior properties of P3HT as a solar cell material. P3PT is proved to be a potentially competitive material. The optoelectronic and charge transport properties observed in the different P3AT:PCBM bulk heterojunction (BHJ) blends provide useful information for understanding the physics of BHJ films and the working principles of the corresponding solar cells.     

Three‐Dimensional Bulk Heterojunction Morphology for Achieving High Internal Quantum Efficiency in Polymer Solar Cells

Here, an investigation of three‐dimensional (3D) morphologies for bulk heterojunction (BHJ) films based on regioregular poly(3‐hexylthiophene) (P3HT) and [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PCBM) is reported. Based on the results, it is demonstrated that optimized post‐treatment, such as solvent annealing, forces the PCBM molecules to migrate or diffuse toward the top surface of the BHJ composite films, which induces a new vertical component distribution favorable for enhancing the internal quantum efficiency (η_IQE ) of the devices. To investigate the 3D BHJ morphology, novel time‐of‐flight secondary‐ion mass spectroscopy studies are employed along with conventional methods, such as UV‐vis absorption, X‐ray diffraction, and high‐resolution transmission electron microscopy studies. The η_IQE of the devices are also compared after solvent annealing for different times, which clearly shows the effect of the vertical component distribution on the performance of BHJ polymer solar cells. In addition, the fabrication of high‐performance P3HT:PCBM solar cells using the optimized solvent‐annealing method is reported, and these cells show a mean power‐conversion efficiency of 4.12% under AM 1.5G illumination conditions at an intensity of 100 mW cm^?2.     

The effect of charge extraction layers on the photo-stability of vacuum-deposited versus solution-coated organic solar cells

Organic solar cells (OSCs) are studied for their photo-stability in inert atmosphere. Polymer solar cells with a bulk heterojunction (BHJ) of poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) are contrasted with small molecule solar cells with a BHJ of chloroindium phthalocyanine (ClInPc) and C₆₀-fullerene. A series of charge extraction layers at the hole and electron collecting contacts are examined for their role in OSC performance and stability. The inter-compatibilities of these extraction layers in vacuum-deposited small molecule OSCs (SM-OSCs) versus solution-coated polymer OSCs (P-OSCs) are explored. Through photo-stability studies, we show that interfacial extraction layers are necessary to avoid contact photo-degradation, which otherwise leads to strong reductions in OSC efficiencies. We also highlight certain extraction layer combinations that result in strong inter-electrode degradation, and we discuss incompatibilities in extraction layers among SM-OSCs versus P-OSCs. Our results suggest that the presence of excitons at the organic-electrode interface likely plays a critical role in contact photo-degradation. By minimizing contact photo-degradation, which dominates the majority of short-term OSC degradation, a new avenue for studying OSC stability behavior and opportunities to focus on other losses in OSCs become possible.     

Precise Structural Development and its Correlation to Function in Conjugated Polymer: Fullerene Thin Films by Controlled Solvent Annealing

The impact of controlled solvent vapor exposure on the morphology, structural evolution, and function of solvent‐processed poly(3‐hexylthiophene):[6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (P3HT:PCBM) bilayers is presented. Grazing incident wide angle X‐ray scattering (GIWAXS) shows that the crystallization of P3HT increases with solvent exposure, while neutron reflectivity shows that P3HT simultaneously diffuses into PCBM, indicating that an initial bilayer structure evolves into a bulk heterojunction structure. Small angle neutron scattering (SANS) shows the agglomeration of PCBM and the formation of a PCBM pure phase when solvent annealing for 90 min. The structural evolution can be described as occurring in two stages: the first stage combines the enhanced crystallization of P3HT and diffusion of PCBM into P3HT, while the second stage entails the agglomeration of PCBM and formation of a PCBM pure phase. The phase separation of PCBM from P3HT is not driven by P3HT crystallinity, but is due to the concentration of PCBM exceeding the miscibility limit of PCBM in P3HT. Correlation of the morphology to photovoltaic activity shows that device performance significantly improves with solvent annealing for 90 min, indicating that both sufficient P3HT crystallization and formation of a PCBM pure phase are crucial in the optimization of the morphology of the active layer.     

Photocrosslinkable Polythiophenes for Efficient,Thermally Stable,Organic Photovoltaics

Photocrosslinkable bromine‐functionalized poly(3‐hexylthiophene) (P3HT‐Br) copolymers designed for application in solution‐processed organic photovoltaics are prepared by copolymerization of 2‐bromo‐3‐(6‐bromohexyl) thiophene and 2‐bromo‐3‐hexylthiophene. The monomer ratio is carefully controlled to achieve a UV photocrosslinkable layer while retaining the π–π stacking feature of the conjugated polymers. The new materials are used as electron donors in both bulk heterojunction (BHJ) and bilayer type photovoltaic devices. Unlike devices prepared from either P3HT:PCBM blend or P3HT‐Br:PCBM blend without UV treatment, photocrosslinked P3HT‐Br:PCBM devices are stable even when annealed for two days at the elevated temperature of 150 °C as the nanophase separated morphology of the bulk heterojunction is stabilized as confirmed by optical microscopy and grazing incidence wide angle X‐ray scattering (GIWAXS). When applied to solution‐processed bilayer devices, the photocrosslinkable materials show high power conversion efficiencies (～2%) and excellent thermal stability (3 days at 150 °C). Such performance, one of the highest obtained for a bilayer device fabricated by solution processing, is achieved as crosslinking does not disturb the π–π stacking of the polymer as confirmed by GIWAXS measurements. These novel photocrosslinkable materials provide ready access to efficient bilayer devices thus enabling the fundamental study of photophysical characteristics, charge generation, and transport across a well‐defined interface.     

Correlating dilute solvent interactions to morphology and OPV device performance

Solvent additives have been explored as a reliable way to control the morphology in bulk-heterojunction (BHJ) layers for improved device performance. We show that the choice of solvent additives has direct implications on morphological evolution, i.e. poly(3-hexylthiophene) (P3HT): [6,6]-phenyl C61-butyric acid methyl ester (PCBM) BHJ films processed with a small amount of 1,8-diiodooctane or 1-chloronaphthalene have more crystalline PCBM domains compared to crystalline P3HT domains, while the opposite is true for films cast with nitrobenzene additive and films cast purely from chlorobenzene. The BHJ film cross-links when annealed at 300 °C in the presence of 1,8-diiodooctane. Cross-linking is found to occur even in pristine P3HT and PCBM films annealed under similar conditions. NMR spectroscopy is presented as a viable technique for quantitative analysis of the amount of solvents left in the BHJ films before and after heat treatment. Despite differences in the ways the additives affect the morphology of the BHJ layer, device performance remained stable over 300 h for all additives tested.     

Effect of SWNT Defects on the Electron Transfer Properties in P3HT/SWNT Hybrid Materials

Poly(3‐hexylthiophene) (P3HT) hybrids with single‐walled carbon nanotubes (SWNTs) were prepared using a series of SWNTs with various defect contents on their surfaces. The hybrids were synthesized by exploiting the π–π interaction between P3HT and the SWNTs, resulting in efficient dispersion of the carbon nanotubes in the P3HT solution. UV‐visible and photoluminescence (PL) spectra showed that the carbon nanotubes quench the PL of P3HT in the hybrids, indicating that electron transfer occurs from photo‐excited P3HT to the SWNTs. This electron transfer from P3HT to carbon nanotubes was disrupted by the presence of defects on the SWNT surfaces. However, the PL lifetime of P3HT in the hybrids was found to be the same as that of pure P3HT in solution, indicating the formation of a ground‐state non‐fluorescent complex of P3HT/SWNTs.     

Abrupt Morphology Change upon Thermal Annealing in Poly(3‐Hexylthiophene)/Soluble Fullerene Blend Films for Polymer Solar Cells

The in situ morphology change upon thermal annealing in bulk heterojunction blend films of regioregular poly(3‐hexylthiophene) (P3HT) and 1‐(3‐methoxycarbonyl)‐propyl‐1‐phenyl‐(6,6)C₆₁ (PCBM) is measured by a grazing incidence X‐ray diffraction (GIXD) method using a synchrotron radiation source. The results show that the film morphology—including the size and population of P3HT crystallites—abruptly changes at 140 °C between 5 and 30 min and is then stable up to 120 min. This trend is almost in good agreement with the performance change of polymer solar cells fabricated under the same conditions. The certain morphology change after 5 min annealing at 140 °C is assigned to the on‐going thermal transition of P3HT molecules in the presence of PCBM transition. Field‐emission scanning electron microscopy measurements show that the crack‐like surface of blend films becomes smaller after a very short annealing time, but does not change further with increasing annealing time. These findings indicate that the stability of P3HT:PCBM solar cells cannot be secured by short‐time annealing owing to the unsettled morphology, even though the resulting efficiency is high.     

Optical,electrical and mechanical properties of indium tin oxide on polyethylene terephthalate substrates: Application in bulk-heterojunction polymer solar cells

We investigated optical, electrical and mechanical properties of indium tin oxide (ITO) on flexible polyethylene terephthalate (PET) substrate, considering bulk-heterojunction (BHJ) polymer solar cells applications. Encapsulation of flexible solar cells with the architecture PET/ITO/PEDOT:PSS/P3HT:PCBM (or P3HT:PCBM:AZ-NDI-4)/Al was done by direct brush-painting with nail enamel. Active cell layer blends of [6,6]-phenyl C₆₁ butyric acid methyl ester (PCBM) with regioregular or regiorandom poly(3-hexylthiophene-2,5-diyl) (P3HT) were applied. Additionally for this role the mixture of regioregular P3HT:PCBM with naphthalene diimide–imine with four thiophene rings AZ-NDI-4 was tested. Obtained photovoltaic (PV) and optical (UV–vis) results of the flexible polymer solar cells were compared with the same architecture of devices on the glass/ITO substrate.     

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.     

Elaboration of P3HT/CNT/PCBM Composites for Organic Photovoltaic Cells

This Full Paper focuses on the preparation of single‐walled or multi‐walled carbon nanotube solutions with regioregular poly(3‐hexylthiophene) (P3HT) and a fullerene derivative 1‐(3‐methoxycarbonyl) propyl‐1‐phenyl[6,6]C₆₁ (PCBM) using a high dissolution and concentration method to exactly control the ratio of carbon nanotubes (CNTs) to the P3HT/PCBM mixture and disperse the CNTs homogeneously throughout the matrix. The CNT/P3HT/PCBM composites are deposed using a spin‐coating technique and characterized by absorption and fluorescence spectroscopy and by atomic force microscopy to underline the structure and the charge transfer between the CNTs and P3HT. The performance of photovoltaic devices obtained using these composites as a photoactive layer mainly show an increase of the short circuit current and a slight decrease of the open circuit voltage which generally leads to an improvement of the solar cell performances to an optimum CNT percentage. The best results are obtained with a P3HT/PCBM (1 : 1) mixture with 0.1 wt % multi‐walled carbon nanotubes with an open circuit voltage (V_oc) of 0.57 V, a current density at the short‐circuit (I_sc) of 9.3 mA cm^–2 and a fill factor of 38.4 %, which leads to a power conversion efficiency of 2.0 % (irradiance of 100 mW cm^–2 spectroscopically distributed following AM1.5).     

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.     

Stabilization of poly(3-hexylthiophene)/PCBM morphology by hydroxyl group end-functionalized P3HT and its application to polymer solar cells

A new concept to stabilize the morphology of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) blend through H-bond formation by using a hydroxyl group end-functionalized P3HT (HOC-P3HT-COH) as a compatibilizer is presented. Domain size of the PCBM crystals in the annealed P3HT/PCBM film is diminished with addition of HOC-P3HT-COH. Surface roughness of the P3HT/PCBM film also becomes smoother with addition of HOC-P3HT-COH. Thermal stability of solar cell device is improved significantly through the H-bond formation between HOC-P3HT-COH and PCBM. A high performance and thermal stable polymer solar cell with 4.06% power conversion efficiency under AM1.5G irradiation is fabricated with 5% HOC-P3HT-COH in P3HT/PCBM layer.     

Solvent‐Induced Morphology in Polymer‐Based Systems for Organic Photovoltaics

Studies on the influence of four different solvents on the morphology and photovoltaic performance of bulk‐heterojunction films made of poly(3‐hexylthiophene) (P3HT) and [6,6]‐phenyl‐C₆₁ butyric acid methyl ester (PCBM) via spin‐coating for photovoltaic applications are reported. Solvent‐dependent PCBM cluster formation and P3HT crystallization during thermal annealing are investigated with optical microscopy and grazing‐incidence wide‐angle X‐ray scattering (GIWAXS) and are found to be insufficient to explain the differences in device performance. A combination of atomic force microscopy (AFM), X‐ray reflectivity (XRR), and grazing‐incidence small‐angle X‐ray scattering (GISAXS) investigations results in detailed knowledge of the inner film morphology of P3HT:PCBM films. Vertical and lateral phase separation occurs during spin‐coating and annealing, depending on the solvent used. The findings are summarized in schematics and compared with the IV characteristics. The main influence on the photovoltaic performance arises from the vertical material composition and the existence of lateral phase separation fitting to the exciton diffusion length. Absorption and photoluminescence measurements complement the structural analysis.     

A Simple and Effective Modification of PCBM for Use as an Electron Acceptor in Efficient Bulk Heterojunction Solar Cells

[6,6]‐phenyl‐C‐61‐butyric acid methyl ester (PCBM) and poly(3‐hexylthiophene) (P3HT) are the most widely used acceptor and donor materials, respectively, in polymer solar cells (PSCs). However, the low LUMO (lowest unoccupied molecular orbital) energy level of PCBM limits the open circuit voltage (V_oc) of the PSCs based on P3HT. Herein a simple, low‐cost and effective approach of modifying PCBM and improving its absorption is reported which can be extended to all fullerene derivatives with an ester structure. In particular, PCBM is hydrolyzed to carboxylic acid and then converted to the corresponding carbonyl chloride. The latter is condensed with 4‐nitro‐4’‐hydroxy‐α‐cyanostilbene to afford the modified fullerene F . It is more soluble than PCBM in common organic solvents due to the increase of the organic moiety. Both solutions and thin films of F show stronger absorption than PCBM in the range of 250–900 nm. The electrochemical properties and electronic energy levels of F and PCBM are measured by cyclic voltammetry. The LUMO energy level of F is 0.25 eV higher than that of PCBM. The PSCs based on P3HT with F as an acceptor shows a higher V_oc of 0.86 V and a short circuit current (J_sc) of 8.5 mA cm^?2, resulting in a power conversion efficiency (PCE) of 4.23%, while the PSC based on P3HT:PCBM shows a PCE of about 2.93% under the same conditions. The results indicate that the modified PCBM, i.e., F , is an excellent acceptor for PSC based on bulk heterojunction active layers. A maximum overall PCE of 5.25% is achieved with the PSC based on the P3HT: F blend deposited from a mixture of solvents (chloroform/acetone) and subsequent thermal annealing at 120 °C.     

Measuring Domain Sizes and Compositional Heterogeneities in P3HT‐PCBM Bulk Heterojunction Thin Films with 1H Spin Diffusion NMR Spectroscopy

The application of ¹H spin diffusion nuclear magnetic resonance (NMR) is expanded to polymer‐fullerene blends for bulk heterojunction (BHJ) organic photovoltaics (OPV) by developing a new experimental methodology for measuring the thin films used in poly‐3‐hexylthiophene–phenyl C61‐butyric acid methyl ester (P3HT‐PCBM) OPV devices and by creating an analysis framework for estimating domain size distributions. It is shown that variations in common P3HT‐PCBM BHJ processing parameters such as spin‐coating speed and thermal annealing can significantly affect domain size distributions, which in turn affect power conversion efficiency. ¹H spin diffusion NMR analysis reveals that films spin‐cast at fast speeds in dichlorobenzene are primarily composed of small (<10 nm) domains of each component; these devices exhibit low power conversion efficiencies (η = 0.4%). Fast‐cast films improve substantially by thermal annealing, which causes nanometer‐scale coarsening leading to higher efficiency (η = 2.2%). Films spin‐cast at slow speeds and then slowly dried exhibit larger domains and even higher efficiencies (η = 2.6%), but do not benefit from thermal annealing. The ¹H spin diffusion NMR results show that a significant population of domains tens of nanometers in size is a common characteristic of samples with higher efficiencies.