Organic photovoltaics
Global energy demand is one of the great concerns of today. The amount of usable solar energy is huge. In fact, it is the largest of all alternatives and power sources, which is why it attracts so much interest in many fields of research. Photovoltaics is a discipline that provides access to renewable and clean electrical energy through devices that can be installed and adapted according to consumption needs. However, the photocells that prevail in the market are limited in investment and manufacturing. Organic solar cells exhibit several advantages over conventional solid-state technologies, such as light weight, flexibility, semi-transparency, large area coverage and low cost/waste of materials. The development of interfaces that fulfill the role of photogeneration and charge transport in an organic solar cell implies the constant search for new strategies that allow improvement in the system architecture and in its net performance for the conversion of solar energy into electricity. The nature of the materials used in the photoactive film of the device in question lies in their electron donating and accepting properties and in their ability to transport charge carriers resulting from hole-electron pair dissociation.
Ternary organic solar cell structure. Source: Ossila
Bulk heterojunctions
Bulk heterojunctions (BHJs) have represented, for some years, the most successful option in terms of overall photovoltaic performance of the photocell. A BHJ has a structure such that the mixture of materials with donation and electronic acceptance characteristics facilitates and increases the effective interactions between both and at the same time decreases the interface that distinguishes them at the nanometric/mesoscopic scale. It is important to highlight that in organic and polymeric compounds the phenomena that occur along the interface govern the excitonic separation and the subsequent transport by the corresponding surface. The tendency and the custom has been, then, the elaboration of donor/acceptor binary mixtures.
The role of the third party in the device
The incorporation of a third material in the heterojunction can represent, in principle, a drastic change in the configuration of the device itself, as well as the arrangement of interface sections, substrates and contacts. Some of the benefits of introducing this new component to the usual donor/acceptor combination are the increase in stability with respect to its binary counterparts via morphological ordering, the extension of the optical absorption characteristics due to the modification of the energy levels of the semiconductors involved and the capture of light in broader regions of the solar spectrum (which stimulates photogeneration and the consequent obtaining of a higher short-circuit current density, Jsc), a superior character for charge transport dissociated, the effective breaking of the hole-electron bound pair and, of course, the improvement in efficiency.
What happens when having two electron acceptors and a donor moiety (D-A-A) or two electron donors and an acceptor (D-D-A) in the heterojunction is to facilitate the interactions characteristic of charge transfer between both subunits of the optically active film, such as donor/acceptor coupling through from the improvement in the distribution of the crystalline structure, a greater coverage as far as the absorption of electromagnetic radiation is concerned, the better solubility of the mixture with the solvent used to prepare the photoactive layer and the miscibility between the components.
Interestingly, ternary organic solar cells have a history that already exceeds a decade in the research field (actually, since 2009), but the specific application of the techniques and principles linked to this configuration is relatively recent. The reason for this time lag is mainly due to the transition from fullerene-type acceptors to small molecules and non-fullerene acceptors (NFAs). The main limitation of fullerenes is the absorption of light, which in turn affects the photocurrent and the use of the incident power. Since the advent of NFAs, the possible options for ternary organic solar cells have diversified remarkably. The opportunities for synthesis and combination are enormous today.
Let's talk about efficiency
In an ideal case, the enhancement in power conversion efficiency (PCE) is estimated to be 35% for a device that has a wide range of light absorption ranging from 200 nm to 400 nm in wavelength (where, in most cases, deficiencies are shown despite the large flux of photons that can be exploited), that is, the insertion of a third component with absorptive sensitivity in that range could increase the PCE of the photocell to beyond of 25% (considering that the current record is close to 19% for mixtures of two components). This estimation reveals the importance of the compatibility between the spectra of the participating molecules in order to maximize the assimilation of radiation and the great benefit that the third donor or acceptor subunit brings.
How to choose the third component?
The choice of the third component of the solar photocell is not such a simple matter, much less trivial. It depends, in the first place, on the very identity of the molecule to be used. That is, if it is a primarily electron-donating semiconductor or if, on the contrary, an electron acceptor character dominates in it. In addition, the third component must have good light absorption regardless of whether it is a donor or an acceptor, since the reason for its use lies precisely in the expansion of the capacity to capture the available radiation. Another point to consider is the nature of this third component in the processing of the mixture, since it can be another organic semiconductor, an inorganic fragment, a polymer, a dye, a quantum dot (particles with semiconducting properties) and so on. This implies that the morphological and electronic complement with the two original systems must be carried out in a suitable way.
In summary, the central objective of ternary organic solar cells is to take advantage of the absorption, transport, thermal stability, solubility and versatility properties of a new material to overcome or mitigate the deficiencies exhibited by the original semiconductors without sacrificing mobility or aspects related to the D-A behavior of the interpenetrating regions of the heterojunction. Future views indicate that the trend in the use of this alternative to cause “energy cascades” that accelerate the injection and transport of charge as well as to expand the characteristics of optical absorption will continue in the coming years. And why not think of a fourth?
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