Nano Crystals

While previous methods of quantum dot creation relied on expensive molecular beam epitaxy processes, fabrication using colloidal synthesis allows for a more cost-effective manufacture. A thin film of nanocrystals is obtained by a process known as “spin-coating”. This involves placing an amount of the quantum dot solution onto a flat substrate, which is then rotated very quickly. The solution spreads out uniformly, and the substrate is spun until the required thickness is achieved.

Quantum dot based photovoltaic cells based around dye-sensitised colloidal TiO₂ films were investigated in 1991 ^[1] and were found to exhibit promising efficiency of converting incident light energy to electrical energy, and were found to be incredibly encouraging due to the low cost of materials in the search for more commercially viable/affordable renewable energy sources. A single-nanocrystal (channel) architecture in which an array of single particles between the electrodes, each separated by ~1 exciton diffusion length, was proposed to improve the device efficiency (figure below) ^[2]and research on this type of solar cell is being conducted by groups at Stanford, Berkeley and the University of Tokyo.

Although research is still in its infancy and is ongoing, in the future quantum dot based photovoltaics may offer advantages such as mechanical flexibility (quantum dot-polymer composite photovoltaics ^[3]) as well as low cost, clean power generation ^[4] and an efficiency of 65%.^[5].

Recent research in experimenting with lead selenide (PbSe) semiconductor, as well as with cadmium telluride (CdTe), which has already been well established in the production of “classic” solar cells. Other materials are being researched as well. These materials are unlikely to have an impact in generating clean energy on a widespread basis, however, due to the toxicity of lead and cadmium.

Polymer solar cell

Polymer solar cells are a type of organic solar cell: they produce electricity from sunlight. A relatively novel technology, they are being researched by universities, national laboratories and several companies around the world.

Currently, many solar cells in the world are made from a refined, highly purified silicon crystal, similar to those used in the manufacture of integrated circuits and computer chips. The high cost of these silicon solar cells and their complex production process has generated interest in developing alternative photovoltaic technologies.

Compared to silicon-based devices, polymer solar cells are lightweight (which is important for small autonomous sensors), disposable, inexpensive to fabricate, flexible, customizable on the molecular level, and have lower potential for negative environmental impact. An example device is shown in Fig. 1.

The following discussion is based primarily on Mayer et al.’s review, cited below. Organic photovoltaics are comprised of electron donor and electron acceptor materials rather than semiconductor p-n junctions. The molecules forming the electron donor region of organic PV cells, where exciton electron-hole pairs are generated, are generally conjugated polymers possessing delocalized π electrons that result from carbon p orbital hybridization. These π electrons can be excited by light in or near the visible part of the spectrum from the molecule’s highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO), denoted by a π -π* transition. The energy gap between these orbitals determines which wavelengths of light can be absorbed.

Unlike in an inorganic crystalline PV material, with its band structure and delocalized electrons, excitons in organic photovoltaics are strongly bound with an energy between 0.1 and 1.4eV. This strong binding occurs because electronic wave functions in organic molecules are more localized, and electrostatic attraction can thus keep the electron and hole together as an exciton. The electron and hole can be dissociated by providing an interface across which the chemical potential of electrons decreases. The material that absorbed the photon is the donor, and the material acquiring the electron is called the acceptor. In Fig. 2, the polymer chain is the donor and the fullerene is the acceptor. After dissociation has taken place, the electron and hole may still be joined as a geminate pair and an electric field is then required to separate them.

After exciton dissociation, the electron and hole must be collected at contacts. However, charge carrier mobility now begins to play a major role: if mobility is not sufficiently high, the carriers will not reach the contacts, and will instead recombine at trap sites or remain in the device as undesirable space charges that oppose the drift of new carriers. The latter problem can occur if electron and hole mobilities are highly imbalanced, such that one species is much more mobile than the other. In that case, space-charge limited photocurrent (SCLP) hampers device performance.

As an example of the processes involved in device operation, organic photovoltaics can be fabricated with an active polymer and a fullerene-based electron acceptor. The illumination of this system by visible light leads to electron transfer from the polymer chain to a fullerene molecule. As a result, the formation of a photo-induced quasiparticle, or polaron (P⁺), occurs on the polymer chain and the fullerene becomes an ion-radical C₆₀^– Polarons are highly mobile along the length of the polymer chain and can diffuse away. Both the polaron and ion-radical possess spin S= ½, so the charge photoinduction and separation processes can be controlled by the Electron Paramagnetic Resonance method.

Architectures

This section is derived largely from Mayer’s review, referenced below. The simplest architecture that may be used for an organic PV device is a planar heterojunction, shown in Fig. 1. A film of active polymer (donor) and a film of electron acceptor are sandwiched between contacts in a planar configuration. Excitons created in the donor region may diffuse to the junction and separate, with the hole remaining behind and the electron passing into the acceptor. However, planar heterojunctions are inherently inefficient; because charge carriers have diffusion lengths of just 3-10nm in typical organic semiconductors, planar cells must be thin to enable successful diffusion to contacts, but the thinner the cell, the less light it can absorb.

Bulk heterojunctions (BHJs) address this shortcoming. In a BHJ, the electron donor and acceptor materials are blended together and cast as a mixture that then phase-separates. Regions of each material in the device are separated by only several nanometers, a distance optimized for carrier diffusion. Although devices based on BHJs are a significant improvement over planar designs, BHJs require sensitive control over materials morphology on the nanoscale. A great number of variables, including choice of materials, solvents, and the donor-acceptor weight ratio can dramatically affect the BHJ structure that results. These factors can make rationally optimizing BHJs difficult.

The next logical step beyond BHJs are ordered nanomaterials for solar cells, or ordered heterojunctions (OHJs). This paradigm eliminates much of the variability associated with BHJs. OHJs are generally hybrids of ordered inorganic materials and organic active regions. For example, a photovoltaic polymer can be deposited into pores in a ceramic such as TiO₂. Holes still must diffuse along the length of the pore through the polymer to a contact, so OHJs do have thickness limitations. Mitigating the hole mobility bottleneck will thus be key to further enhancing OHJ device performance, but controlling morphology inside the confines of the pores is challenging.

 Engineers at the University of California, San Diego (UCSD) have employed “nanowires” to boost the efficiency of organic solar cells ^[1].

Conclusion

At the moment, an open question is to what degree polymer solar cells can commercially compete with silicon solar cells. The silicon solar cell industry has the important industrial advantage of being able to leverage the infrastructure developed for the computer industry. Besides, the present efficiency of polymer solar cells lies near 5 percent, much below the value for silicon cells. Polymer solar cells also suffer from environmental degradation. Good protective coatings are still to be developed.

Still, organic PV devices show great promise for decreasing the cost of solar energy to the point where it may become widespread in the decades ahead. While great progress has been made in the last ten years with respect to understanding the chemistry, physics, and materials science of organic photovoltaics, work remains to be done to further improve their performance. Specifically, novel nanostructures must be optimized to promote charge carrier diffusion; transport must be enhanced through control of order and morphology; and interface engineering must be applied to the problem of charge transfer across interfaces. Novel molecular chemistries and materials offer hope for revolutionary, as opposed to evolutionary, breakthroughs in device efficiencies in the future.

Hybrid photovoltaic cells are a mix of two solar cell technologies^[1].

They comprise dye-sensitized titanium dioxide coated and sintered on a transparent semi-conducting oxide, and a p-type, polymeric conductor, such as PEDOT or PEDOT-TMA,^[2][3] which carries electrons from the counter electrode to the oxidized dye. Since the one polymer replaces the multi-component electrolyte the cells are expected to be far simpler to make reproducibly and should afford the same or similar form factors as the polymer solar cells. This technology, like that of the polymer cell, has not yet advanced to the performance level of that of the dye-sensitized solar cell technology. The efficiency values are in the single digits range. One of the causes of low performance is incomplete filling of the small cavities in the titanium dioxide nanoparticles.

Organic photovoltaic cells are solar cells made mostly of organic molecules. Specifically, the active layer of the device is made of organic material.

Many scientists and engineers believe organic solar cells will provide a cheaper alternative to traditional inorganic cells, since it is thought that economies of scale due to large-scale production of organic polymers will turn out to be less expensive than the current costs for fabrication of silicon or other inorganic materials. However, organic solar cells have much lower efficiencies than traditional technologies. Organic solar cells are considered to be a third generation technology.

There are three main types of organic photovoltaic technologies: 1) Molecular OPV, 2) Polymer OPV, and 3) Hybrid OPV. The main differences between these three technologies are the fabrication methods employed and the types of materials that are used. ^[1]

Molecular OPV

Molecular photovoltaic devices are typically fabricated by sublimating successive layers of electron and hole transporting materials under vacuum. Common materials include PTCBI, PTCDA, Me-PTCDI, Pe-PTCDI, H2Pc, MPc where M stands for (Zn, Cu), TPyP, TPD, CBP, C60, and PCBM.

Polymer OPV

Polymer photovoltaic devices are typically made by solution processing blends of two conjugated polymers or a conjugated polymer with a molecular sensitizer. The most common materials are PPV – Poly(p-phenylene vinylene), polyfluorenes, or polythiophenes. Polymer solar cells are the most heavily researched of all OPV technologies because they are the most promising when it comes to low cost. In general, it is thought that solution processing will be the most cost effective way to fabricate solar cells.

Hybrid OPV

Hybrid photovoltaic devices make use of both organic and inorganic materials. For example, research has been done on polymer-nanocrystal blended active layers, including the use of quantum dots. Research has also been done on the use of metals such as TiO₂. These technologies have not yet surpassed the best polymer OPV technology, but they are promising.

References

Sun, Sam-Shajing & Sariciftci, Niyazi Serdar, (2005). Organic Photovoltaics: Mechanisms, Materials, and Devices. Boca Raton, Fl: CRC Press.