Exciton Recombination In The Fullerene Phase Of Bulk Heterojunction Organic Solar Cells
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| Author | : George Frederick Burkhard |
| Publisher | : Stanford University |
| Total Pages | : 113 |
| Release | : 2011 |
| Genre | : |
| ISBN | : |
Finding alternatives to fossil fuel energy sources is necessary to stem global warming, to provide economic and political independence, and to keep up with increasing energy demand. Because of their low cost, flexibility, and because the material resources needed to make them are abundant, organic polymer solar cells are an attractive alternative to conventional solar technology. Organic solar technology has been developing rapidly; however, with the best power conversion efficiencies at ~8%, much improvement is needed before it can be competitive with established solar technologies. Poly-3-hexylthiophene:[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) solar cells are the most studied type of organic solar cell. Nevertheless, their loss mechanisms are still not fully understood. In this work, we study excitonic losses in the PCBM phase of the blend. We develop a way to accurately measure internal quantum efficiencies (IQEs) and use this technique to characterize P3HT:PCBM devices. We observe spectral dependence of the IQE and conclude that a majority of excitons generated in the PCBM are lost to Auger recombination with polarons that are trapped in that phase. We also provide evidence that this process may happen in other materials and may be a critical factor in limiting exciton diffusion in organic semiconductors.
| Author | : George Frederick Burkhard |
| Publisher | : |
| Total Pages | : |
| Release | : 2011 |
| Genre | : |
| ISBN | : |
Finding alternatives to fossil fuel energy sources is necessary to stem global warming, to provide economic and political independence, and to keep up with increasing energy demand. Because of their low cost, flexibility, and because the material resources needed to make them are abundant, organic polymer solar cells are an attractive alternative to conventional solar technology. Organic solar technology has been developing rapidly; however, with the best power conversion efficiencies at ~8%, much improvement is needed before it can be competitive with established solar technologies. Poly-3-hexylthiophene:[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) solar cells are the most studied type of organic solar cell. Nevertheless, their loss mechanisms are still not fully understood. In this work, we study excitonic losses in the PCBM phase of the blend. We develop a way to accurately measure internal quantum efficiencies (IQEs) and use this technique to characterize P3HT:PCBM devices. We observe spectral dependence of the IQE and conclude that a majority of excitons generated in the PCBM are lost to Auger recombination with polarons that are trapped in that phase. We also provide evidence that this process may happen in other materials and may be a critical factor in limiting exciton diffusion in organic semiconductors.
| Author | : Hemant M. Shah |
| Publisher | : |
| Total Pages | : 260 |
| Release | : 2012 |
| Genre | : Exciton theory |
| ISBN | : |
Characterization of the optical and electrical properties of organic solar cell materials is of prime importance to organic solar cell design. This thesis describes the use of capacitive photocurrent measurements to study the exciton generation and dissociation mechanisms of organic solar cell materials. The emphasis is on the study of the methanofullerene derivatives (e.g., PCBM) which act as the electron acceptor material. This is because much work has already been done studying electron donating polymers used in organic solar cells (in an effort to enhance their absorbance coefficient), but less information is available on the acceptor material. In the blend films of MDMO-PPV: PCBM, the charge generation rate in PCBM was discovered to be much higher than would be expected from the absorbance cross-section. This observation led to design of a photovoltage bleaching experiment to examine the charge generation mechanism. Here the effect of illumination by a tunable light source on the open circuit photovoltage of a MDMO-PPV: PCBM bulk heterojunction solar cell was measured. Illumination of light at the PCBM ground state singlet exciton causes a sharp decrease in the photovoltage, while illumination at the ground state MDMO-PPV exciton shows no change. A direct pathway of recombination of above gap generated charge carriers was identified. Photovoltage bleaching results suggests that excitation at the PCBM ground singlet state exciton increases the recombination rate of higher energy excitations, either by acting as a recombination center or by forcing higher energy carriers into short lived states that recombine before reaching the contacts. The fact that the photovoltage bleaching correlates with the ground state PCBM singlet exciton suggests that charge dissociation from PCBM preferentially generates long-lived localized states. Capacitive photocurrent measurements were then performed on isolated methanofullerene derivatives, with the polymer donor material absent. Several low energy transitions were resolved in the optical spectroscopy of methanofullerene derivatives. These low energy states lay below the optical energy band-gap of these materials, so that their presence was unexpected. It was determined that the low energy states overlapped with the plasmon state of the highly conducting substrates (Indium Tin Oxide) which were in close proximity with the PCBM. Plasmon states in ITO have been observed previously, but the results presented in this thesis are unique in that this is the first evidence of charge transfer from the plasmon state of ITO to a high electron affinity fullerene derivative. The results show the evidence of charge transfer from PCBM to ITO over a broad wavelength range of 400 - 2400 nm (3 eV - 0.5 eV). Few materials have been observed to have absorbance and charge transfer over such a large range of energies in the infra-red regime. These results open a new direction for development of organic solar cell design with higher power conversion efficiencies.
| Author | : Steven Hawks |
| Publisher | : |
| Total Pages | : 241 |
| Release | : 2015 |
| Genre | : |
| ISBN | : |
My thesis focuses on improving and understanding a relatively new type of solar cell materials system: polymer:fullerene bulk-heterojunction (BHJ) blends. These mixtures have drawn significant interest because they are made from low-cost organic molecules that can be cast from solution, which makes them a potential cheap alternative to traditional solar cell materials like silicon. The drawback, though, is that they are not as efficient at converting sunlight into electricity. My thesis focuses on this issue, and examines the loss processes holding back the efficiency in polymer:fullerene blends as well as investigates new processing methods for overcoming the efficiency limitations. The first chapter introduces the subject of solar cells, and polymer:fullerene solar cells in particular. The second chapter presents a case study on recombination in the high-performance PBDTTT polymer family, wherein we discovered that nongeminate recombination of an anti-Langevin origin was the dominant loss process that ultimately limited the cell efficiency. Electroluminescence measurements revealed that an electron back-transfer process was prevalent in active layers with insufficient PC$_{71}$BM content. This work ultimately made strong headway in understanding what factors limited the relatively unexplored but highly efficient PBDTTT family of polymers. In the next chapter, I further explore the recombination mechanisms in polymer:fullerene BHJs by examining the dark diode ideality factor as a function of temperature in several polymer:fullerene materials systems. By re-deriving the diode law for a polymer:fullerene device with Shockley-Read-Hall recombination, we were able to confirm that trap-assisted recombination through an exponential band-tail of localized states is the dominant recombination process in many polymer:fullerene active layers. In the third chapter, I present a generalized theoretical framework for understanding current transients in planar semiconductor devices, like those discussed above. My analysis reveals that the apparent free-carrier concentration obtained via the usual integral approach is altered by a non-trivial factor of two, sometimes leading to misinterpretations of the charge densities and overall device physics. This new perspective could have far-reaching effects on semiconductor research and technology. Finally, in the last two chapters, I discuss the device physics associated with a relatively novel method for fabricating nanoscale polymer:fullerene BHJs: solution sequential processing (SqP). In particular, I compare recombination in SqP vs. traditionally processed blend-cast devices, and demonstrate that SqP is a more scalable method for making BHJ solar cells. In the final chapter, I examine an unexpected discovery that occurred while working on the content in Chapter 5. Specifically, Chapter 6 examines electrode metal penetration in the SqP quasi-bilayer active layer architecture. Therein, we unexpectedly found that evaporated metal can readily penetrate into fullerene-rich layers, up to $\sim$70 nm or more. The details and consequences of this surprising occurrence are discussed in detail.
| Author | : Armantas Melianas |
| Publisher | : Linköping University Electronic Press |
| Total Pages | : 101 |
| Release | : 2017-04-18 |
| Genre | : |
| ISBN | : 9176855635 |
Organic photovoltaic (OPV) devices based on semiconducting polymers and small molecules allow for a low cost alternative to inorganic solar cells. Recent developments show power conversion efficiencies as high as 10-12%, highlighting the potential of this technology. Nevertheless, further improvements are necessary to achieve commercialization. To a large extent the performance of these devices is dictated by their ability to extract the photo-generated charge, which is related to the charge carrier mobility. Various time-resolved and steady-state techniques are available to probe the charge carrier mobility in OPVs but often lead to different mobility values for one and the same system. Despite such conflicting observations it is generally assumed that charge transport in OPV devices can be described by well-defined charge carrier mobilities, typically obtained using a single steady-state technique. This thesis shows that the relevance of such well-defined mobilities for the charge separation and extraction processes is very limited. Although different transient techniques probe different time scales after photogeneration, they are mutually consistent as they probe the same physical mechanism governing charge motion – gradual thermalization of the photo-generated carriers in the disorder broadened density of states (DOS). The photo-generated carriers gradually lose their excess energy during transport to the extracting electrodes, but not immediately. Typically not all excess energy is dissipated as the photo-generated carriers tend to be extracted from the OPV device before reaching quasi-equilibrium. Carrier motion is governed by thermalization, leading to a time-dependent carrier mobility that is significantly higher than the steady-state mobility. This picture is confirmed by several transient techniques: Time-resolved Terahertz Spectroscopy (TRTS), Time-resolved Microwave Conductance (TRMC) combined with Transient Absorption (TA), electrical extraction of photo-induced charges (photo-CELIV). The connection between transient and steady-state mobility measurements (space-charge limited conductivity, SCLC) is described. Unification of transient opto-electric techniques to probe charge motion in OPVs is presented. Using transient experiments the distribution of extraction times of photo-generated charges in an operating OPV device has been determined and found to be strongly dispersive, spanning several decades in time. In view of the strong dispersion in extraction times the relevance of even a well-defined time-dependent mean mobility is limited. In OPVs a continuous ‘percolating’ donor network is often considered necessary for efficient hole extraction, whereas if the network is discontinuous, hole transport is thought to deteriorate significantly, limiting device performance. Here, it is shown that even highly diluted donor sites (5.7-10 %) in a buckminsterfullerene (C60) matrix enable reasonably efficient hole transport. Using transient measurements it is demonstrated that hole transport between isolated donor sites can occur by long-range hole tunneling (over distances of ~4 nm) through several C60 molecules – even a discontinuous donor network enables hole transport
| Author | : Lukas Schmidt-Mende |
| Publisher | : Walter de Gruyter GmbH & Co KG |
| Total Pages | : 518 |
| Release | : 2016-05-24 |
| Genre | : Technology & Engineering |
| ISBN | : 3110388510 |
With the increasing world-energy demand there is a growing necessity for clean and renewable energy. The sun being one of the most abundant potential sources accounts for less than 1% of the global energy supply. The market for solar cells is one of the most strongly increasing markets, even though the prize of conventional solar cells is still quite high. New emerging technologies, such as organic and hybrid solar cells have the potential to decrease the price of solar energy drastically. This book offers an introduction to these new types of solar cells and discusses fabrication, different architectures and their device physics on the bases of the author's teaching course on a master degree level. A comparison with conventional solar cells will be given and the specialties of organic solar cells emphasized.
| Author | : Michael C. Heiber |
| Publisher | : |
| Total Pages | : 144 |
| Release | : 2012 |
| Genre | : Exciton theory |
| ISBN | : |
The efficient exciton dissociation and subsequent charge separation occurring in optimized polymer:fullerene devices has been difficult to understand and simulate. Both exciton delocalization and hot charge separation are proposed to facilitate charge separation. To test these theories, both conceptual models were implemented into a dynamic Monte Carlo (DMC) simulation and tested using a simple bilayer device architecture. Using experimentally determined parameters, exciton delocalization accounted for a significant reduction in geminate recombination when compared to the traditional, bound polaron pair model. In addition, the hot charge separation process was able to further reduce the geminate recombination, but only when the hot charge mobility was several orders of magnitude larger than the standard charge mobility. Given the apparent importance of exciton delocalization, the magnitude of exciton delocalization in regioregular P3HT was investigated further. By fitting exciton delocalization models to previously published experimental data, two separate estimates for the magnitude of exciton delocalization were extracted. A simple lattice model was fit to exciton-exciton annihilation behavior in P3HT films and a detailed DMC model was fit to exciton dissociation dynamics of P3HT:PCBM blend films, leading to exciton delocalization radius estimates of 1.6 [plus or minus] 0.25 nm and 1.9 [plus or minus] 0.6 nm, respectively, which were significantly smaller than previously published values. With such a low magnitude of exciton delocalization, the first exciton dissociation model proposed predicted a large magnitude of geminate recombination. By including charge delocalization, it was hypothesized that formation of a bound polaron pair would be prohibited, resulting in a significant reduction in geminate recombination. This concept was implemented using a simple spherical bead model for both exciton and charge delocalization. To test this model in a more experimentally relevant device, a model bulk heterojunction device was used. In addition, test conditions were expanded to investigate a range of electric fields. A massive reduction in geminate recombination was observed simply by increasing the delocalization radius from 0.5 to 1.5 nm. As a result, it was concluded that both exciton and charge delocalization are dominant factors controlling the exciton dissociation dynamics and the geminate recombination behavior in P3HT:PCBM bulk heterojunction solar cells.
| Author | : Christoph Brabec |
| Publisher | : John Wiley & Sons |
| Total Pages | : 597 |
| Release | : 2011-09-22 |
| Genre | : Technology & Engineering |
| ISBN | : 3527623205 |
Providing complementary viewpoints from academia as well as technology companies, this book covers the three most important aspects of successful device design: materials, device physics, and manufacturing technologies. It also offers an insight into commercialization concerns, such as packaging technologies, system integration, reel-to-reel large scale manufacturing issues and production costs. With an introduction by Nobel Laureate Alan Heeger.
| Author | : Steve Albrecht |
| Publisher | : |
| Total Pages | : 140 |
| Release | : 2015 |
| Genre | : |
| ISBN | : |
A dramatic efficiency improvement of bulk heterojunction solar cells based on electron-donating conjugated polymers in combination with soluble fullerene derivatives has been achieved over the past years. Certified and reported power conversion efficiencies now reach over 9% for single junctions and exceed the 10% benchmark for tandem solar cells. This trend brightens the vision of organic photovoltaics becoming competitive with inorganic solar cells including the realization of low-cost and large-area organic photovoltaics. For the best performing organic materials systems, the yield of charge generation can be very efficient. However, a detailed understanding of the free charge carrier generation mechanisms at the donor acceptor interface and the energy loss associated with it needs to be established. Moreover, organic solar cells are limited by the competition between charge extraction and free charge recombination, accounting for further efficiency losses. A conclusive picture and the development of precise methodologies for inves...
| Author | : Lukas Schmidt-Mende |
| Publisher | : Walter de Gruyter GmbH & Co KG |
| Total Pages | : 494 |
| Release | : 2022-08-01 |
| Genre | : Technology & Engineering |
| ISBN | : 3110731525 |
With the increasing world-energy demand there is a growing necessity for clean and renewable energy. This book offers an introduction to novel types of solar cells, which are processed from solution. We discuss fabrication, different architectures and their device physics of these solar cells on the bases of the author‘s teaching course on a master degree level. A comparison with conventional solar cells is given and the specialties of organic, hybrid and perovskite solar cells are emphasized.
