Non-fullerene acceptor

Advantages Fullerene acceptors (FAs) have been used extensively in OSCs. This is rationalized by several characteristics of fullerenes. The three-dimensional character causes them to be suitable materials for bulk heterojunction structures. Additionally, its electronic configuration (delocalized LUMOs) allows for efficient percolation and high electron mobility. Another consequence is that they are easily coupled to compatible donor polymers. However, fullerene acceptor organic solar cells (FA-OSCs) encounter a limited efficiency. The energy levels in fullerene compounds are relatively constant and difficult to alter. Moreover, they employ weak absorption in the visible spectrum and the near-infrared spectrum and low thermal instability and photochemical instability. The acceptors need to be purified extensively, adding to the economical and temporal disadvantages of using FAs. reaching a higher value than its FA-based counterpart. Disadvantages One of the downsides of using SMAs is the fact that, under atmospheric conditions, they tend to engage in disordered (anisotropic) states as a result of their planar structures. They are often planar as aromaticity is required for sufficient electron mobility. The lack of order may diminish electron transport and effective extraction routes that lead to induced current. This makes them less compatible for bulk heterojunction blends than FAs. Another downside to research on SMA usage is the profound scala of possibilities of donor-acceptor pairs that scientists are challenged to induce. ==Physics==

The mechanism of current induction in organic solar cells involves a charge transfer. After electromagnetic absorption and exciton formation in the electron donor polymer, the excited electron is moved towards the acceptor conduction band (LUMO) as a result of the lower energy value than the donor LUMO. This process is called a charge separation, and the corresponding energy value E_{CS} satisfies E_{CS} = E^A_{LUMO} - E^D_{HOMO} where CS denotes charge separation, A denotes the acceptor and D denotes the donor molecule. Along with the Coulombic potential that needs to be surpassed, the maximum energy obtained from the process is defined as the Charge Transfer energy, E_{CT}. The difference between the optical excitation energy (the optical band gap energy, E^{opt}_g) and the charge transfer energy is the driving force of the system. An advantage of NF-OSCs over current fullerene-based OSCs is that the SMAs used are relatively compatible with donors, as a result of their electronic affinity tunability. Their compatibility originates from their LUMO-energy value similarity. The driving force is minimized to solely Coulombic contributions (V_{loss}, which depends explicitly on the value of the driving force, along with radiative and non-radiative losses during the current induction process. Thus, for NF-OSCs, E_{loss} = qV_{loss}, with q the electron's charge, is minimized, leading to a higher useful energy output. The result is a high open-circuit voltage V_{OC} of the solar cell compared to fullerene counterparts, with reports of values as high as 1.1V. However, the diminished charge separation energy cost negatively influences the tendency of excited electrons in the donor conduction band to transport to the acceptor LUMO as it is less preferred energetically. This gives rise to the fact that electrons induced in the current are more energetic, but fewer electrons are induced. This means that the short-circuit current density J_{SC} and the fill factor (FF) are decreased. In terms of the PCE, the higher open-circuit voltage is compensated by the lower short-circuit current density and fill factor. Researchers showed that ultrafast charge separation is possible with negligible driving force. In fact, the electrical external quantum efficiency EQE_{EL} is highest for donor-acceptor blends with lowest driving force. ==Types==

One of the main advantages of the non-fullerene acceptors is their ability to be tuned and customized by chemical modification. This in contrary to fullerene acceptors. It also immediately creates a bottleneck because of the huge amount of possibilities there are which could be applied as an SMA. A wide variety of SMAs are tested to be a successful acceptor, but two classes of SMAs have proven to give the best results concerning Power Conversion Efficiency (PCE) and have made the greatest attribute to the recent development in NFA-OSCs. Rylene diimides Rylene diimides are, as said, one of the two main subclasses which are a basis for acceptor-molecules in modern NFA-OSCs. Rylene diimides are industrial dyes and can be divided into, once again, two subclasses: Perylene Diimides (PDIs) and Naphthalene Diimides (NDIs). Rylene diimides consist of a planar rylene framework and numerous constructions can be made by attaching certain subgroups and by using more PDI molecules in one acceptor. The mono-PDI molecule is shown in the figure on the right. Rylene diimides are considered good acceptors because of their favourable properties. Rylene diimides usually have high electron mobility values \mu_e due to intermolecular π-stacking. ==Future development==

In current research, rylene diimides (for small band-gap energy donors) and FREAs (for large band-gap energy donors) have shown the most potential for becoming commercially viable solar cell materials for bulk heterojunction blend cells. Wide band gap donors are known to enhance voltage and diminish current density, but in combination with FREAs both values can be relatively high. There are still a lot of improvements to be made before an NFA-OSC can be commercially profitable. First of all, the PCE should be increased to at least 15% since this is the minimal value for commercial application. As said, PCEs already have exceeded 13% so recent development is on the right track. PCEs can be increased by designing even better NFAs, for instance, on the level of electron mobility NFAs still can increase a lot compared to FAs ( 3.3 \cdot 10^{-4} cm^2 V^{-1} s^{-1} for the best NFAs compared to 7.0 \cdot 10^{-4} cm^2 V^{-1} s^{-1} for the best FAs). Improvements can also be made in the following aspects: better donor matching, tandem constructions, BHJ morphology and domain purity of the donor and acceptor. Besides these theoretical research aspects, implementation in a life size commercial solar cell also brings a lot of challenges, such as easy and sustainable device fabrication methods and long-term stability of the organic compounds. Studies also show that with upscaling, the PCE in general drops. On all of these areas, NFA-OSCs show great potential but it will take a lot of research before a solid non-fullerene acceptor-organic solar cell can compete with inorganic solar cells. == See also ==