Organic Solar Cells: Materials, Devices, Interfaces, and Modeling


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Recently, organic solar cells have gained extensive attention as a next-generation photovoltaic technology due to their light weight, mechanical flexibility, and solution-based cost-effective processing. Organic Solar Cells: Materials, Devices, Interfaces, and Modeling provides an in-depth understanding of the current state of the art of organic solar cell technology.

Encompassing the full spectrum of organic solar cell materials, modeling and simulation, and device physics and engineering, this comprehensive text:. Organic Solar Cells: Materials, Devices, Interfaces, and Modeling makes an ideal reference for scientists and engineers as well as researchers and students entering the field from broad disciplines including chemistry, material science and engineering, physics, nanotechnology, nanoscience, and electrical engineering.

ISBN 10: 1482229838

Search all titles. Search all titles Search all collections. Your Account Logout. Therefore, the detected signals are generated only at interfaces, and the SFG vibrational spectroscopy could in principle identify and quantify the surface composition, thus shedding light on interfacial segregation of a component. This issue of interfacial segregation of additives has already been extensively investigated by SFG spectroscopy for non-conjugated polymers Lu et al.

It has been reported that the charge transport in the blend depends directly on the effective vertical as well as the lateral phase segregation of the polymer and fullerene domains within the blend Quiles et al. The charge extraction process is improved when the donor polymer domains segregate toward the anode interface and the acceptor fullerene domains segregate toward the cathode interface Heutz et al.

Such combination of vertical and lateral phase segregated morphology favors the charge transport across the blend and across the interfaces, thus improving the device performance. Apart from improved charge extraction, the inverted device structure allows for the replacement of both the low work function metal cathode and the PEDOT:PSS HTL, which are responsible for the degradation of the device, and thus improving the device lifetime Sahin et al.

For example, it has been shown that P3HT does not block the electron conduction but PCBM is known to significantly block the hole conduction through the blend Germack et al. Nevertheless, this was not observed Wang H. With such reverse interfacial segregation of the active layer, it was expected that the device would exhibit high series resistance due to the hole blocking by PCBM at the anode, leading to a reduced device performance. The degree of the vertical phase segregation depends on the nature of the substrate and spin coating conditions, and can also be controlled using thermal annealing or by changes in the blend-substrate interface by the modification of the substrate surface energy Quiles et al.

This is shown in Figure 5 , where the depth profile of PCBM in the blend was measured by ellipsometry, and investigated under several fabrication conditions. In contrast, it is larger for the blend spin-coated on fused silica substrates and is enhanced by thermal annealing Figure 5A. In this latter case, the depth profiles suggest that the free surface is enriched of PCBM clusters, a fact that is confirmed by in situ optical microscopy during thermal annealing. Furthermore, as can be seen in Figure 5D , the deposition of a molecular self-assembled monolayer of hexamethyldisilazane on the substrate modifies its surface energy, leading to a change in the direction of compositional gradient of PCBM in the blend, with the film enriched of P3HT domains toward the substrate while PCBM is displaced toward the free surface Quiles et al.

Reprinted by permission from Macmillan Publishers Ltd Quiles et al. One study has used NR for analyzing the compositional profiles of the layers along the depth of the device to reveal the influence of the different capping metal electrodes as well as the addition of the solvent additive nitrobenzene on the thermal stability of the device Mauger et al. The same study has also further reported that changes in the compositional profiles due to the thermal annealing can be due to a specific charge transfer or doping process occurring between the metal electrodes and PCBM due to differences in their Fermi Levels Mauger et al.

It has been shown that doping and charge transfer occurs more strongly with Ca than Al, leading to differences in the vertical phase segregation and the compositional profiles depending upon the type of the electrode Mauger et al. In contrast, it was found that the solvent additive can lead to a significant reduction in both the vertical phase segregation and the concentration of PCBM at the interface. The interfacial layers may influence both the energy level alignment and the vertical phase segregation within the device, which affect the device performance.

The study has demonstrated by employing ultraviolet photoelectron spectroscopy UPS that the observed difference in the device performance for each of these three donor polymers is not due to a hole extraction barrier for the PTB7 device Oehzelt et al. Such inverted polymer solar cell devices have shown an improvement in the efficiency from 7. The concept of barriers for injection and extraction of charge carriers, or simply, the energy barriers, arises from the difference in chemical potential or Fermi energy—E F between two dissimilar materials when they are joined together.

Upon contact, electron transfer will occur at the interface until the chemical potential becomes the same throughout the materials. In Figure 6A is shown the case of a metal whose Fermi level before contact is below that of the organic semiconductor. After joining them, electron transfer from the organic material to the metal will occur, until the Fermi level is the same throughout the materials.

Similarly, if the Fermi level of the metal before contact is higher than that of the semiconductor, there will be electron transfer to the semiconductor LUMO, as illustrated in Figure 6B. A Fermi level of metal is below that for the organic semiconductor, before contact. B Same as A , but for the Fermi level of metal below that for the organic semiconductor.

Organic Solar Cells - Materials Devices Interfaces And Modeling Hardcover

This difference in the energy levels has been termed as an energy barrier if a given carrier must have its energy increased from one material to another, as it crosses the interface. Thus, an energy barrier would depend on the energy levels of the materials, on which carrier is being considered and also on which direction it is being transported. This energy difference may either allow or prevent the movement of charge carriers across the interface depending upon the magnitude of the energy difference and the direction of the movement of the charge carriers.

Therefore, in some cases there could be a barrier for carrier injection, but not for extraction across the same interface. Several studies have investigated in different ways the influence of the injection and extraction barriers on the device performance Pfeiffer et al. The effect of the energy barriers in general is to hinder the transfer of the charge carriers across the interfaces, significantly affecting the electrical characteristics of the devices. The physical mechanisms which are related to this charge transfer process and its effect on the J-V curves have been explained in these studies.

These FHJ solar cells consist of the separate layers of donor polymer and acceptor molecule, in contrast with BHJ solar cells where a layer of the blend of the donor polymer and acceptor molecule is deposited to form the nanostructured active layer of the device. The n-type materials form ohmic contacts with the metals having lower work functions, while they yield rectifying contacts with metals having higher work functions. In case of the p-type materials, the opposite effect is exhibited. Photovoltaic response was observed, albeit with a much lower V oc than expected from the materials Fermi levels.


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The authors explained this by the several pathways for charge recombination in the junction wiggly arrows labeled as 3 in Figure 7 , which suppressed free carrier concentrations that were not enough to cancel the built-in field V bi in the device. Reproduced from Hiller et al. As the barriers affect the device performance, it is important to identify the types of barriers present within the device so that a suitable selection of the materials can be done for improving the device performance.

The study employed intentional introduction of the barriers with known type and magnitude for the different device configurations shown in Figures 8A,B , corresponding to extraction and injection barriers.


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In case of extraction barriers for holes, the current-voltage J—V characteristics were measured under different light intensities ranging from 0. A normalized photocurrent data was plotted, allowing a comparison of the strength of the S-kink, which is more pronounced for the highest intensities. As can be seen in Figure 8A , there are points of intersection near V oc of the normalized J-V curves, as V oc increases with light intensity. They are characteristic of the presence of an extraction barrier, as confirmed by numerical simulations of the J-V curves.

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This leads to higher recombination at the higher light intensities and thus more pronounced S-kinks. Normalized J—V curves for a series of illumination intensities. Similarly, in case of injection barriers, the J-V characteristics were measured again under different light intensities ranging from 0.

These results are characteristics of the presence of an injection barrier in the device. It has been explained that for an injection barrier, there is a low built-in potential V bi as compared to the E DA gap.

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As shown in Figure 8B , for the devices with planar heterojunction architecture, this induces a possibility of higher V oc as compared to V bi , with the current close to V oc being completely diffusion-driven against the field. For higher light intensities, this competition between the diffusion and the drift is seen in the form of S-kinks. Some earlier studies have reported that the open circuit voltage may be dependent or independent of the choice of the electrodes Brabec et al. This mechanism is called Fermi level pinning Brabec et al.

However, it was further shown that such Fermi level pinning is valid only in the cases of ohmic contacts, so that for non-ohmic contacts V oc depends upon the metal electrode work functions Mihailetchi et al. Another more recent investigation has reported the effects of injection and extraction barriers on the effective charge carrier mobilities, generation of S-kink and the influence of these barriers on the performance of the both FHJ and BHJ solar cells Tress et al.

In C , the data for one BHJ dash-dotted is also shown.

Reproduced with permission from Tress et al. Furthermore, for FHJs there is the generation of an S-kink irrespective of the presence of the injection and the extraction barriers Figures 9A—D. The reason for the generation of the S-kink in the solar cell J-V curve is related to the direction of the electric field in the region of S-kink, which in turn depends upon the type of barrier present in the device. It has been found that in case of the FHJs, in the region closer to V oc the electric field is reversed and the photocurrent is mainly driven by diffusion. Indeed, earlier studies have also reported such importance of the diffusion gradients for FHJs Gregg and Hanna, However, in case of the BHJs, the diffusion gradient for the movement of charge carriers toward the respective electrodes is not present Tress et al.

As a result, the built in potential is higher in case of the BHJs without barriers, as compared to the device with the barriers and same donor HOMO levels. After the identification of the actual energy barriers present in the device, it is thus important to address the problem of the misaligned energy levels across the interfaces to facilitate charge transfer and improve the device efficiency. Several studies have reported different approaches to improve the performance of the devices through proper alignment of the energy barriers Hau et al. The two major strategies used so far are described in the sections below: i addition of an interlayer that converts a large barrier into two smaller barriers, thus reducing their net effect on the charge transport throughout the device structure; ii use of molecularly thin dipole layers at an interface to shift the energy levels and reduce the effective barrier height.

Earlier attempts to address the issue of the misalignment of the energy barriers have included methods to rationally design the materials of the active layers like donor polymers and the acceptor fullerene in order to facilitate an efficient charge transfer across the interfaces and thus an improved device performance Small et al. However, the introduction of an interlayer between the materials that present an energy barrier, with an intermediate energy level, converts a large barrier into two smaller ones, thus reducing their net effect on the charge transport throughout the device structure.

A few examples of using such strategy are described below. The first ones involve the introduction of a cross-linkable n-type acceptor fullerene, incorporation of vertically aligned, cross-linked fullerene nanorods and addition of a combination of Indene-C 60 Bis-Adduct ICBA and cross-linked fullerene interlayer Cheng et al.

The cross linking of the fullerene acceptor functionalized with a dendron containing two styryl groups as thermal cross-linkers PCBSD, [6,6]-phenyl-Cbutyric styryl dendron ester yields a robust fullerene interlayer to be used between the metal-oxide buffer layers and the active layer for efficient charge extraction in an inverted solar cell Hsieh et al. This prevents interfacial erosion of the fullerene layer against the organic solvent when depositing the active layer. With the addition of such interlayer, initial improvements in the efficiency from 3. Further studies have shown application of an interlayer of vertically aligned and interpenetrating network of cross-linked fullerene nanorods of PCBSD below the active layer of the inverted device Chang et al.

Such vertically aligned network of nanorods interlayer provides a larger interfacial area and better aligned energy levels across the interfaces, leading to better charge transport so that improvement in the efficiency from 6.

Organic Solar Cells: Materials, Devices, Interfaces, and Modeling Organic Solar Cells: Materials, Devices, Interfaces, and Modeling
Organic Solar Cells: Materials, Devices, Interfaces, and Modeling Organic Solar Cells: Materials, Devices, Interfaces, and Modeling
Organic Solar Cells: Materials, Devices, Interfaces, and Modeling Organic Solar Cells: Materials, Devices, Interfaces, and Modeling
Organic Solar Cells: Materials, Devices, Interfaces, and Modeling Organic Solar Cells: Materials, Devices, Interfaces, and Modeling
Organic Solar Cells: Materials, Devices, Interfaces, and Modeling Organic Solar Cells: Materials, Devices, Interfaces, and Modeling
Organic Solar Cells: Materials, Devices, Interfaces, and Modeling Organic Solar Cells: Materials, Devices, Interfaces, and Modeling
Organic Solar Cells: Materials, Devices, Interfaces, and Modeling Organic Solar Cells: Materials, Devices, Interfaces, and Modeling
Organic Solar Cells: Materials, Devices, Interfaces, and Modeling Organic Solar Cells: Materials, Devices, Interfaces, and Modeling
Organic Solar Cells: Materials, Devices, Interfaces, and Modeling

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