PCBM Thin Films Research Articles

On the Use of Reflection Polarized Optical Microscopy for Rapid Comparison of Crystallinity and Phase Segregation of P3HT:PCBM Thin Films.

Rapid, nondestructive characterization techniques for evaluating the degree of crystallinity and phase segregation of organic semiconductor blend thin films are highly desired for in-line, automated optoelectronic device fabrication facilities. Here, it is demonstrated that reflection polarized optical microscopy (POM), a simple technique capable of imaging local anisotropy of materials, is capable of determining the relative degree of crystallinity and phase segregationof thin films of polymer:fullerene blends. While previous works on POM of organic semiconductors have largely employed the transmission geometry, it is demonstrated that reflection POM provides 3× greater contrast. The optimal configuration is described to maximize contrast from POM images of polymer:fullerene films, which requires Köhler illumination and slightly uncrossed polarizers, with an uncrossing angle of ±3°. It is quantitatively demonstrated that contrast in POM images directly correlates with 1) the degree of polymer crystallinity and 2) the degree of phase segregation between polymer and fullerene domains. The origin of the bright and dark domains in POM is identified as arising from symmetry-broken liquid crystalline phases (i.e., dark conglomerates), and it is proven that they have no correlation with surface topography. The use of reflection POM as a rapid diagnostic tool for automated device fabrication facilities is discussed.

Characterization of the in-situ degradation process of P3HT:PCBM based on hyperspectral and neural networks

In situ online observation of surface morphology during degradation processes is of paramount importance for exploring the stability of organic photovoltaic materials. In this study, we designed an in situ online characterization system based on hyperspectral and neural network technologies, and observed the degradation processes of P3HT:PCBM thin film materials. The system is capable of collecting hyperspectral image data from 101 channels within the 400–700 nm wavelength range for characterizing detailed surface features of materials. Additionally, to automate the processing of hyperspectral image data, we designed a spectral image segmentation algorithm based on neural networks and proposed a foreground attention mechanism to improve the segmentation accuracy of the algorithm. The experimental results indicate that the system can achieve high spectral characterization of P3HT:PCBM thin film materials and automate image data processing through artificial intelligence algorithms, with an image segmentation accuracy of 99.62 %. Furthermore, owing to the higher spectral resolution of this system and its computer-assisted analysis capabilities for material image data, not only are the in-situ variations in size, density, and formation rate of aggregates formed during the thermal degradation process of P3HT:PCBM thin film materials experimentally analyzed, but also the fluorescence changes at the edges of aggregates during the photodegradation process are revealed. The reliable code can be found at the following link: https://github.com/HyperSystemAndImageProc/IONFMDP-UHHNN.

THE OPTIMIZATION OF P3HT:PCBM THIN FILM THICKNESS FOR ORGANIC SOLAR CELLS

This study presents the complex relationship between film thickness and the properties of P3HT:PCBM thin films, looking at how different thicknesses affected their structural and optical properties. Poly(3, hexylthiophene) (P3HT) and poly(3, hexylthiophene) (PCMB) were blended in a 0.6:1 ratio and deposited using spin coating techniques. The research involve investigating the resulting changes in surface topography, optical properties, and electrical behaviour by systematically adjusting the film thickness through multiple coating repetitions ranging from one to three repetitions. Profilometric analysis revealed a direct relationship between film thickness and surface roughness, highlighting the importance of thickness in interfacial interactions within solar cells. Optical studies revealed fascinating trends in absorbance, transmittance, absorption coefficient, and extinction coefficient, highlighting the films' behaviour at various wavelengths. Surprisingly, regardless of thickness, the optical band gap remained constant, providing valuable insights for optimizing photon energy conversion. Measurements of the Hall effect revealed the effect of thickness on carrier mobility, concentration, conductivity, resistivity, and Hall voltage. Thinner films had higher mobility but lower carrier concentration, revealing a delicate balance critical for optimizing charge transport. The comprehensive analysis of the study sheds light on the complex interplay between film thickness and various properties, providing useful guidance for designing efficient P3HT:PCBM organic solar cells. This study's findings highlight the importance of precise control over film dimensions in improving the performance and efficiency of organic photovoltaic devices. This investigation paves the way for tailored approaches in the fabrication of next-generation organic solar cells, pushing the limits of renewable energy technology.

Investigation of the electrical, optical and photophysical properties of PTB7:PCBM-thin films

Investigation of the electrical, optical and photophysical properties of PTB7:PCBM-thin films

An additive manufacturing approach based on electrohydrodynamic printing to fabricate P3HT:PCBM thin films

Additive manufacturing (AM) enables the production of high value and high performance components with applications from aerospace to biomedical fields. We report here on the fabrication of poly(3-hexylthiophene): phenyl-C61-butyric acid methyl ester (P3HT:PCBM) thin films through the electrohydrodynamic atomization (EHDA) process and its integration as absorber layer for organic solar cells. Prior to the film fabrication, the optimization of the process was carried out by developing the operating envelope for the P3HT:PCBM ink to determine the optimal flow rate and the appropriate applied voltage to achieve a stable-cone deposition mode. The EHDA printed thin-film’s topography, morphology and optical properties were systematically analyzed. The root-mean-square roughness was found to vary significantly with the annealing temperature and the flow rate and ranged from 1.938 to 3.345 nm. The estimated film mass and thickness were found between 3.235 and 23.471 mg and 597.5 nm to 1.60 µm, respectively. The films exhibited a broad visible absorption spectrum ranging from ~ 340 to ~ 600 nm, with a maximum peak λmax located at ~ 500 nm. As the annealing temperature and the flow rate were increased, discernible alterations in the PCBM clusters were consequently observed in the blends of the film and the size of the PCBM clusters has decreased by 3% while the distance between them was highly reduced by as much as 82%.

Effect of structure on semiconducting properties of a small molecule n-type organic semiconductor: Phenyl-C61-butyric acid methyl ester

The fullerene-based derivative phenyl-C61-butyric acid methyl ester (PCBM) has attracted attention as an n-type transistor channel and an organic photovoltaic material due to its solution processability, stability, and high electron mobility. However, disorder in solution-processed PCBM thin films makes understanding of charge carrier transport mechanisms difficult. We have fabricated PCBM-based field effect transistors and improved device performance by reducing electron trapping at the dielectric–semiconductor interface and improving charge carrier injection. Single-wall carbon nanotubes were successfully integrated into conventional gold electrodes to improve injection efficiency. Meyer–Neldel behavior has been investigated in a study of charge carrier transport in PCBM thin films. Crystallization of PCBM was observed during the measurement. We find that charge carriers hop between molecules in mostly amorphous regions from 145 to 240 K. The transport is facilitated by the formation of more crystallized regions when samples are heated as high as 240 K.

Effect of PTAA and PCBM concentrations on the electrical response of bilayer heterojunction PTAA/PCBM Diode

Poly[bis(4-phenyl) (2,4,6-trimethylphenylamine] (PTAA) is a new promising hole transport material, and [6,6]-Phenyl C61 butyric acid methyl ester (PCBM) is a suitable electron acceptor material. This study deposited thin-film PTAA and PCBM at different spin coatings ranging from 1000 rpm to 5000 rpm. Ultraviolet–visible (UV–vis) spectrometer, X-ray diffraction (XRD) machine, and scanning electron microscope (SEM) techniques were used to determine the optical properties and structural properties, respectively. Thermal annealing with temperatures of 80 °C, 100 °C, 120 °C, and 150 °C was introduced to improve the properties of the PTAA and PCBM thin film. The results of SEM images for PTAA annealed at 150 °C appeared to darken and melt, whereas PCBM annealed at 120 °C and 150 °C had granule properties. The ideality factors for bilayer heterojunction diodes and bulk heterojunction PTAA/PCBM diodes with increased layers were 5.8–60.2 and 30.8–77.2, respectively. The optimum spin rate for the bilayer heterojunction PTAA/PCBM was 5000 rpm. The most appropriate diode behavior of the bilayer was at 5.0/1.0 wt%, 1.0/5.0 wt%, and 3.0/3.0 wt% of PTAA/PCBM, with the calculated ideality factors of 5.8, 6.2, and 15.45 with turn-on voltage of 1.57 V, 1.70 V, and 1.82 V, respectively.

Improving the photovoltaic performance of inverted perovskite solar cells via manipulating the molecular packing structure of PCBM

In this study, the molecular packing structure of solution-processed phenyl-C61-butyric acid methyl ester (PCBM) thin film was manipulated by varying the volume ratio of chlorobenzene (CB) to bromobenzene (BrB) from 100:0 to 50:50, which largely influences the device performance of the PCBM/perovskite heterojunction solar cells. Absorbance spectra, photoluminescence spectra, atomic force microscopic images and contact angle images were used to investigate the molecular packing structure effects of the PCBM thin films on the device performance of the inverted perovskite solar cells. Our experimental results show that the formation of PCBM aggregates and the contact quality at the PCBM/perovksite interface significantly influence the open-circuit voltage, short-circuit current density and fill factor of the resultant solar cells simultaneously. It is noted that the PCE of the encapsulated inverted CH3NH3PbI3 (MAPbI3) solar cells exhibited a stable and high power conversion efficiency of 18%.

Improved performance of PCBM/MAPbI3 heterojunction photovoltaic cells with the treatment of a saturated BCP/IPA solution

The interfacial contact quality between the hydrophilic MAPbI 3 and hydrophobic PCBM thin films are improved by using the treatment of a saturated bathocuproine/isopropanol (BCP/IPA) solution, which largely increases the power conversion efficiency (PCE) from 12.96% to 18.67%. The atomic force microscopic images, water-droplet contact angle images, viscosity measurement results and near-infrared transmittance spectra show that the increased open-circuit voltage (V OC ) is related to the formation of a denser PCBM thin film and the better contact quality of the PCBM/MAPbI 3 heterojunction. Besides, the power-dependent current density-voltage (J-V) curves show that the lower defect density in the solar cells results in the higher V OC . In other words, the treatment of a saturated BCP/IPA solution can decrease the defect density of the PCBM/MAPbI 3 heterojunction and thereby boosting the photovoltaic performance and extending the device lifespan. • The saturated BCP/IPA solution treatment increases the density of the PCBM thin film. • The saturated BCP/IPA solution treatment strengthens the contact quality at PCBM/MAPbI 3 interface. • The saturated BCP/IPA solution treatment increases the hydrophobicity of PCBM/MAPbI 3 heterojunction. • The saturated BCP/IPA solution treatment improves the PCE of photovoltaic cells from 12.69% to 18.67%.

On the role of solution-processed bathocuproine in high-efficiency inverted perovskite solar cells

The s-shaped J-V curve of perovskite solar cells can be eliminated using the solvent penetration (SP) process with a PCBM spacer. The experimental results show that the viscosity of the solvent used in the SP process influences the shunt and series resistances simultaneously. In addition, the fill factor of PEDOT:PSS based perovskite solar cells can be largely increased from 40.3% to 70.7% when the bathocuproine (BCP) molecules are dissolved in IPA solvent during the SP process, which significantly increases the power conversion efficiency (PCE) by 98.9%. Besides, the PCE of perovskite solar cells increases from 13.56% to 20.29% when the PEDOT:PSS film is replaced by a P3CT-Na thin film. It is noted that the absorbance spectra, photoluminescence (PL) spectra, time-resolved PL, Raman spectra and X-ray diffraction patterns demonstrate that the BCP molecules penetrate into the PCBM thin film and the PCBM/perovskite interface during the SP process, which increases the electron mobility of the PCBM thin film and passivates the interfacial MA cations in perovskite thin film, respectively. In other words, the solution-processed BCP small molecules are vertically distributed across the PCBM thin film. Our results help completely understand the entire roles of the solution-processed BCP molecules in the high-performance inverted-type perovskite solar cells.

Interface engineering of electrochemically deposited ZnO nanorods as electron transport layer in polymer solar cells using organic dyes

The electron transporting layer (ETL) plays a vital role on determining the charge extraction, charge transfer and consequently the photovoltaic performance of polymer solar cells (PSCs). Methylene Blue (MB) and Rhodamine 6G (Rh6G) were applied as modifying agents to reduce the inherent incompatibility between the ZnO NRDs and the P3HT:PCBM based active layer. ZnO NRDs were electrochemically grown on the seed layer-coated ITO electrode using a constant potential and they were modified with MB and Rh6G dyes by a dip-coting method. The optical properties, structural defects and morphology of the unmodified and dye-modified ZnO NRDs were compared. The rate of charge recombination and electron transfer as the most important factors influencing the photovoltaic performance of PSCs was evaluated using different techniques including PL-decay, EIS and AFM. Rh6G appeared to act as a more appropriate modifying agent, due to its larger π-conjugated network, which was beneficial to the charge transfer at the interfaces and decreasing the energy loss within the fabricated PSCs. The surface morphology of the P3HT:PCBM thin film layer appeared to be smoother in the Rh6G-modified samples. The highest charge mobility of 4.56 × 10−4 cm2/V was observed for the Rh6G-modified devices, which was about 120% and 54% higher than that obtained for the unmodified and MB-modified samples, respectively. The fabricated MB-and Rh6G-modified devices illustrated PCE of 3.65 and 5.1%, respectively, which were considerably higher than that achieved with the unmodified ZnO NRDs based devices prepared in the current work and reported previously.

Machine learned metaheuristic optimization of the bulk heterojunction morphology in P3HT:PCBM thin films

We discuss results from a machine learned (ML) metaheuristic cuckoo search (CS) optimization technique that is coupled with coarse-grained molecular dynamics (CGMD) simulations to solve a materials and processing design problem for organic photovoltaic (OPV) devices. The method is employed to optimize the composition of donor and acceptor materials, and the thermal annealing temperature during the morphological evolution of a polymer blend active layer composed of poly-(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM), for an increased power conversion efficiency (PCE). The optimal solutions, which are in qualitative agreement with earlier experiments, identify correlation between the design variables that contributes to an enhanced material performance. The framework is extended to multi-objective design (MOCS-CGMD) to attain a Pareto optimality for the blend morphology, and enhance concurrently the exciton diffusion to charge transport probability and the ultimate tensile strength of the material. The predictions reveal that a higher annealing temperature enhances the exciton diffusion to charge transport probability, while a PCBM weight fraction between 0.4 and 0.6 increases the tensile strength of the underlying blend morphology.

On the Growth of Evaporated NaF on Phenyl-C61-butyric Acid Methyl Ester and Poly(3-hexylthiophene)

NaF has been considered as material for the electron extraction layer in organic photovoltaic (OPV) devices. Thin layers of NaF were evaporated onto PCBM and P3HT thin films. The chemical state of ...

Origins of the s-shape characteristic in J–V curve of inverted-type perovskite solar cells

Fullerene derivative thin films have been widely used in inverted-type perovskite solar cells as the electron transport layer (ETL) and hole blocking layer. However, the smooth contact at the interface between the hydrophobic [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) and hydrophilic CH3NH3PbI3 (MAPbI3) thin film has not yet been completely understood. The contact at the PCBM/MAPbI3 interface strongly influences the photovoltaic performance. The photovoltaic devices were characterized by measuring the light intensity-dependent current density–voltage (J–V) curves and impedance spectra, which show that the contact at the PCBM/MAPbI3 interface simultaneously influences the shunt resistance (carrier recombination) and series resistance (interfacial contact). In addition, x-ray diffraction patterns, atomic force microscopic images, absorbance spectra and photoluminescence spectra were used to explore the contact at the PCBM/MAPbI3 interface. The experimental results show that the flat MAPbI3 thin film cannot be completely covered by a PCBM thin film and thereby results in the s-shape characteristic in the J–V curve of the resultant solar cells. The s-shaped J–V curve can be suppressed by increasing the crystallinity and surface roughness of the MAPbI3 thin film. With the use of an interface modification layer in between the PCBM thin film and Ag cathode, the power conversion efficiency of MAPbI3 solar cells can be increased from 10.50% to 13.71%.

Comparative solvent quality dependent crystallization in solvent vapor annealing of P3HT:PCBM thin films by in-situ GIWAXS

We present a novel in-depth study correlating multiple solvent properties (boiling point, vapor pressure, and solubilities of P3HT and PCBM) on P3HT and PCBM crystal growth kinetics in blend thin films via in-situ GIWAXS in conjunction with multiple substrate energies. Solvent vapor annealed (SVA) P3HT:PCBM films in a closed chamber, saturated with different solvents (o-dichlorobenzene (DCB), o-xylene, tetrahydrothiophene (THT), toluene, benzene, tetrahydrofuran (THF), and carbon disulfide (CS2)) were studied in real time. We observed that structure growth and dissolution kinetics of the both P3HT and PCBM crystals largely depend on physical properties of solvent and relative solubilities of P3HT and PCBM. The film morphology of the annealed films resulted in PCBM enrichment and occasional crystallization occurred at the solvent vapor saturated air interfaces. We observed strikingly different PCBM crystallization upon annealing of the films when cast from THF resulting in highly ordered PCBM crystals at the film surface. THF has high vapor pressure and relatively low solubilities for both P3HT and PCBM. At the other extreme CS2 produced the lowest crystallinity due to its very high vapor pressure such that the solvent evaporates before significant crystallization can occur. We characterized all the annealed samples using AFM, UV-Vis spectroscopy, and XPS. AFM probed topographic structures with coarsening of both the P3HT and PCBM domains at air surface by solvent annealing. We observe via UV-vis spectroscopy that, in general, intrachain electronic delocalization and interchain electron transitional jump due to π-π stacking increases in the post SVA films. This indicates that the chains are better organized with increased conjugation lengths. SVA has a substantial effect on P3HT monomer to PCBM ratio as determined by XPS at the polymer-air interface. In all cases, relative to the P3HT monomer:PCBM ratio of as-cast films from DCB, the air surfaces became more enriched by PCBM with SVA.

Stability of Polymer:PCBM Thin Films under Competitive Illumination and Thermal Stress

AbstractThe combined effects of illumination and thermal annealing on the morphological stability and photodimerization in polymer/fullerene thin films are examined. While illumination is known to cause fullerene dimerization and thermal stress their dedimerization, the operation of solar cells involves exposure to both. The competitive outcome of these factors with blends of phenyl‐C61‐butyric acid methyl ester (PCBM) and polystyrene (PS), supported on PEDOT:PSS is quantified. UV–vis spectroscopy is employed to quantify dimerization, time‐resolved neutron reflectivity to resolve the vertical composition stratification, and atomic force microscopy for demixing and coarsening in thin films. At the conventional thermal stress test temperature of 85 °C (and even up to the PS glass transition), photodimerization dominates, resulting in relative morphological stability. Prior illumination is found to result in improved stability upon high temperature annealing, compatible with the need for dedimerization to occur prior to structural relaxation. Modeling of the PCBM surface segregation data suggests that only PCBM monomers are able to diffuse and that illumination provides an effective means to control dimer population, and thus immobile fullerene fraction, in the timescales probed. The results provide a framework for understanding of the stability of organic solar cells under operating conditions.

Improvement of polymer:fullerene bulk heterojunction morphology via temperature and anti-solvent effect

Morphology of polymer:fullerene bulk heterojunctions (BHJ) displays beside suitable energy level alignment between donor and acceptor as well as a sufficient absorption one of the most crucial impacts on organic photovoltaic device efficiency and stability. In particular, the structural order of the polymer phase impacts the charge separation efficiency and transport. In this work we present a comprehensive study on the control of the polymeric order initiated already in solution by using fractions of anti-solvent isopropanol (IPA), and via adjusting the processing temperature for common solutions of an anthracene-containing poly(p-phenylene-ethynylene)-alt-poly(p-phenylene-vinylene) bearing statistically either linear (octyloxy) or branched (2-ethylhexyloxy) side-chains (AnE-PVstat) and phenyl-C61-butyric acid methyl ester (PCBM). Detailed analysis of photophysical data, absorption, and photoluminescence (PL) spectra, of deposited AnE-PVstat:PCBM thin films suggest the formation of J-aggregates for the copolymer to be in strong correlation with the processing formulation (presence of non-solvent IPA) and post-treatment parameters (annealing temperature). The findings for our system indicate that the addition of IPA increases the intermolecular interaction between the polymer chains, consistent with enhanced charge carrier (hole) mobility (μh) in the final film reaching values as high as 10−3 cm2 V−1 s−1. The subtle control of polymer aggregation led to an overall variation in the solar cell device efficiency by about 50%, with optimal processing settings of IPA additive and a solution temperature of 15 °C.

Additive assisted morphological optimization of photoactive layer in polymer solar cells

Herein, we clearly demonstrate that the inclusion of the organic solid additive (2,3-dihydroxypyridine, DOH) into the poly(diketopyrrolopyrrole-terthiophene) (PDPP3T) and phenyl-C61-butyric acid methyl ester (PCBM) (PDPP3T:PCBM) photoactive blend system alters the nanoscale morphology and enhances the photovoltaic (PV) performance of the inverted bulk heterojunction (BHJ) based polymer solar cells (PSCs). The influence of DOH additive casted PDPP3T: PCBM thin films on the optical, morphology and structural features is evaluated and correlated with PV characteristics. Topography images through atomic force microscopy reveal that the incorporation of DOH into PDPP3T: PCBM induces a finer nanoscale phase segregation between polymer and fullerene domains with fibrillar morphology. The PV performance of the DOH processed PDPP3T: PCBM devices is evaluated by current-voltage (J-V) characteristics and compared with pristine and 1, 8-diiodooctane (DIO) modified PDPP3T: PCBM devices. Interestingly, the incorporation of DOH (0.5 wt%) into PDPP3T: PCBM device witnessed the best PCE of 6.36%, which is significantly higher (69.1%) than that of the reference (3.76%). This significant enhancement in the performance of the device is mainly attributed to a dramatic increase in the short-circuit current density and fill-factor due to the improved bicontinuous interpenetrated phase separation and balanced charge transport. In addition, the measurements on photo-induced charge carrier extraction by linearly increasing voltage reveal that DOH incorporated devices exhibit higher mobility and charge carrier density as compared to those of pristine modified BHJ PSCs. The presence of vicinal functional groups in DOH contributes to the possible molecular level interactions with the blend components and accounts for the morphology and device performance enhancements.

A dendritic oligoarylamine-substituted benzimidazole derivative as a useful n-type dopant

Introduction of a dendritic oligoarylamine group at the 2-position of 1,3-dimethyl-2,3-dihydro-1H-benzimidazole (N3-DMBI-H) has significantly improved the chemical stability in solution phase. In addition to the preeminent solution-processability of N3-DMBI-H, the n-type doping ability has also been confirmed with the electrical characteristics of the N3-DMBI-H-doped PCBM thin films.

Optimization of effective absorption enhancement of paired-strips gold nanoantennas arrays in organic thin-films

This study sought to optimize the dimensional characteristics of paired-strips gold nanoantennas embedded in a P3HT: PCBM thin-film by taking into account the tradeoff between the size of the nanostructures and absorber layer as well as the gaps between nanoparticles, to maximize the effective absorption enhancement. The average enhancement behavior within the working region was discussed using integral analysis, which is important for overall enhancement. The discussion would focus on comparing the bands’ features of paired-strips nanoantennas embedded in a dielectric thin-film, and in air. By the average absorption 3D slices plots, in which the dimension width, height, and gap are changed with a fixed wavelength; the optimized dimension of paired-strips nanoantennas could be realized. Fixing the period (400 nm) of paired-strips nanoantennas embedded in P3HT:PCBM thin-films (120 nm in thickness) enhanced absorption by 9.8 times.