Bottom panel: plot of local composition for linear polymers as a function of distance from the interface. Insets report the average number of polymers per cluster as a function of distance from the interface.
Rationale for Research
Then, exciton dissociation at the interface of the two polymer domains creates charges that contribute to conductivity. However, the optimal blend morphology conditions that can increase the power conversion efficiency (PCE) of a solar cell remain elusive, also due to the difficulty in observing the molecular structure of the blend using standard experimental techniques, e.g. X-ray diffraction [6, 7], atomic force microscopy [8, 9], optical spectroscopy [7] and edge X-ray absorption fine structure spectroscopy [10].
Aims and Objectives
Our goal will be to study the mechanisms and efficiency of device stability under these conditions. The relationship between the power conversion efficiency (PCE) of the device and the different dopant-induced nanomorphologies and transport properties of the device will also be investigated.
Overview
We observed a collapse of the effective sizes of both polymer species, as measured by their radii of gyration, as we approached the interface. However, in this regime the energy analysis of both polymer species as a function of distance from the interface has been particularly useful.
Computational models for topologically-different poly- mer blends
In some cases this is an undesirable effect and in general it is difficult to estimate the surface composition of the species. Wu and Friedickson [42] gave a different view on the surface enrichment of one of the surface species using some qualitative arguments based on a scaled theory.
Computational models for conducting polymer blends
They were also able to reproduce some relevant structural quantities, such as the radial distribution function of P3HT monomers, PCBM monomers and the mixture of the two monomers. One of the limitations of CG simulations is of course related to the ability to reliably estimate dynamic quantities, e.g.
![Figure 2.1: Chemical structures of P3HT and C 60 with CG sites depicted and labeled [56].](https://thumb-ap.123doks.com/thumbv2/pubpdfnet/10641344.0/29.892.319.569.130.401/figure-chemical-structures-p3ht-cg-sites-depicted-labeled.webp)
Experimental approach
- Optical properties of organic photovoltaic materials
- Device parameters of a solar cell
- Equivalent electrical circuit models for OTFSC devices
- Effect of morphology on the performance of the cell
Among all the physical parameters listed above, which affected the performance of the organic solar cell. The effect of additive solvents can also be observed in the surface morphology of the films, which is strongly related to the performance of the device.
![Figure 2.3: Schematic currentvoltage characteristics of bulk heterojunction solar cells in (a) linear and (b) semilogarithmic representation [64].](https://thumb-ap.123doks.com/thumbv2/pubpdfnet/10641344.0/32.892.257.645.625.808/figure-schematic-currentvoltage-characteristics-heterojunction-linear-semilogarithmic-representation.webp)
Introduction
Finally, we will draw some conclusions which will be useful to introduce the main chapter following this one, where the most important results of this research study will be presented and discussed in a complete and exhaustive manner.
Model and Methods
We approach the initial particle before we reach the rate of polymerization of the cyclic polymer. The wall potential energy is shifted so that the wall-bead interaction energy is zero at the shear distance.
Linear Polymers
Rling is almost the same for the final configuration compared to the initial one over all the distances from the bottom edge of the box. In the initial configuration, the linear chains approaching the wall are in the range between z≈50.18σ and z ≈57.34σ from the bottom edge of the simulation box. In the final configuration, the linear chains approaching the wall are between z≈57.35σ and z≈64.51σ from the lower edge of the simulation box.
Cyclic Polymers
The same calculation of the mean radius of gyration and the variance of the radius of gyration as a function of the distance from the bottom edge of the simulation box is made for cyclic polymers. In the initial configuration, cyclic polymers coming closer to the wall are in the region between z≈51.04σ and z≈56.14σ from the bottom edge of the simulation box. For the final configuration, the estimated range of cyclic polymers coming closer to the wall is between z≈61.25σ and z≈66.35σ from the bottom edge of the simulation box.
Linear-Cyclic mixtures
We can then conclude that linear polymers do not seem to diffuse towards the center of the box (as in the pure case) due to linear–cyclic polymer interactions. As shown in Table 3.5, the size distribution of cyclic polymers in the final configuration reached by the polymer melt is almost the same as in the initial one. In the final configuration, the average positions of the cyclic polymer beads become between z ≈49.62σ and z≈54.81σ over the entire z range, indicating that most of the cyclic polymers in the melt move away from the wall.
Conclusions
Introduction
The experimental evidence of the relevance of entropy in determining the surface excess at the air-polymer film interface was recently provided by Lee and co-workers [13]. We generated initial configurations for binary mixtures of linear and cyclic polymers of Nb grains per polymer and the mixture is exposed to void at low and high z values. Our goal was to understand the possible microscopic mechanisms underlying polymer enhancement at the interface, and to achieve this goal we wanted to understand the role of entropic factors, such as architecture, molecular weight and the position of chain ends of linear polymers, in the determine which of the two polymer species will be preferentially adsorbed at the interface.
Model and Methods
Starting from that particle, the position of the next one is generated at a distance equal to dFENE with probability p = 1−NN. In the probability formulap, N is the actual number of growing cyclic polymer particles and Nb is the degree of polymerization. In this case, the algorithm will choose the position of the next particle by placing it along a random direction (as we did for the linear polymer beads).
The Low Cyclic Composition Regime
Density and Local Composition Profiles
Figures 4.2-4.3 show the local density for two types of chains as a function of distance from the interface. The top panel of Figure 4.6 shows a histogram of the average number density ρ=N/V for Nb= 10,100 and for two polymer species as a function of distance from the interface. The densities of the two species far from the interface are in the ratio 30 : 70, as expected for casec = 0.7.
Radius of Gyration
Right panel: same as left panel for the transverse component of RTg as a function of distance from the interface. Cyclic polymers appear to have a slightly smaller radius of gyration than linear polymers at the interface. However, linear polymers show a noticeably larger drop in radius of gyration at the interface than cyclic polymers.
Instantaneous Interface
In Figure 4.12 we report how the exponentν varies for the system sizes we have considered as a function of the distance from the interface. The proof is that the percentage of middle beads at the interface is always less than the percentage of chain ends. This configuration is compatible with the geometry sketched in Figure 4.16; (a), where intermediate beads of linear polymers tend to remain closer to the interface.
Diffusion coefficients
The parallel components of the diffusion coefficient as a function of distance from the interface are reported in Figures 4.22-4.23. In general, we see that the diffusion of the two types of polymer increases as they approach the interface from the bulk. Furthermore, since cyclic polymers tend to increase their size slightly along directions parallel to the interface as the interface is approached, the higher mobility increase of linear polymers reported in Figure 4.22 is also better understood.
Interaction Energies of polymer species
ALL refers to the energy per bead as calculated, regardless of the nature of the bead. The energies per grain for the two polymer types are shown in the figures as a function of the distance from the interface. In Figure 4.26 we show the total energy per grain of the two polymer types as a function of distance from the interface for Nb = 10 (left panel) and Nb = 100 (right panel).
Percolation
If we consider the long chain case, the first thing we notice is that the total energy per bead of the two polymer species now becomes very similar regardless of the distance from the interface (see the right panel of Figure 4.26). In the case of cyclic polymers, the percolation distance is below 4σ and for both Nb = 10.20 polymerization stages, less than 5% of clusters of nearly two polymers reach this distance. For cyclic polymers, we still observe a monotonous increase of the percolation distance up to 10σ and an average number of polymers per cluster equal to 3.
The equimolar regime
- Density and Local Composition Profiles
- Radius of Gyration
- Instantaneous Interface
- Diffusion coefficients
- Interaction Energies of polymer species
- Percolation
In this subsection, we again monitored the radius of gyration Rg (recall that this quantity is related to polymer flexibility and conformational entropy), as a function of distance from the interface. In summary, in the long chain case, the entropy competition due to chain ends (increasing for linear chains) and that due to polymer conformation (decreasing for linear chains) favors the enrichment of cyclic polymers at the interface. We keep the same notation adopted in the case of the higher linear chain composition and report the total and self-energies per bead for the two types of polymer in Figure (4.37) as a function of distance from the interface.
Introduction
For example, in P3HT:PCBM mixture active layer, the crystallization of the P3HT phase can be inhibited by the clustering of fullerene in the mixture annealing. Similarly, the processing solvent addition was found to improve the morphology of the active layer based on PCPDTBT:PC71BM by enhancing the aggregation of the polymer[102]. It is clear that the performance of these devices is critically dependent on the morphology of the donor and acceptor mixture active layer.
Material and Methods
However, the mechanism of host solvent interactions with DIO, CN, and DMSO in the photoactive environment and its effectiveness on device stability remained unclear. Photoactive layers were spin-coated from the solution at 1200 rpm onto the PEDOT:PSS layer. The optical absorption of the films was measured using UV/VIS dual beam spectrophotometer (Model: UV-1601).
Solar cell parameters
Space Charge limited current
As the applied voltage increases, more charges are injected from the electrodes and the current in the device increases sharply. Finally, at a high applied voltage the current in the device slowly reaches maximum at a slower rate. This region is called space-charge limited current (SCLC), which is limited by the maximum amount of charge available in the medium.
Results and Discussion
- Optical absorption properties of the devices
- Electrical properties of the devices
- Comparison on the effect of various additives on performance of OTFSCOTFSC
- Charge transport properties on solvent mixtures
The absorption spectra reflect the influence of the processing solvent on the device performance and active layer morphology, with significant changes seen in the DIO-prepared device. These low PCE results may be due to poor miscibility of the mixture in the active layer. The DMSO-based device has very low mobility compared to others, which contributed to the poor performance of the device.
Surface Morphologies of the Photoactive layers
The SEM image of the P3HT:PCBM film prepared by the addition of DMSO solvents shows heterogeneous nanostructures with the formation of clusters on the surface of the film. The surface morphology confirmed the existence of an interwoven network of P3HT and PCBM in the photoactive layer. Moreover, the increase in the molecular weight of the polymer chains ultimately results in the enrichment of cyclic chains on the surface, which agrees with the theory for the long chain limit.
![Table 2.1: Temperatures and Mixture Ratios of Atomistic Systems (pressure = 1 atm in all cases) [56]](https://thumb-ap.123doks.com/thumbv2/pubpdfnet/10641344.0/29.892.189.707.550.700/table-temperatures-mixture-ratios-atomistic-systems-pressure-cases.webp)
