Organic solar cells (OSC) are an exciting next-generation option for photovoltaics. The main advantage is that they’re cheap, easily processed using solution-based methods, which opens up many innovative applications – printable cells, even solar paint!
The main issue holding OSCs back is efficiency – the 10% threshold has not yet been reached, and most are in the 4-5% range. Although there are several reasons for this, one of the most important is the metal used as the negative electrode. Aluminum is cheap and commonly used, but oxygen easily oxidizes it, create aluminum oxide and eliminating the desired conductive properties. Other metals like gold and silver are more ‘noble’ in that they don’t react as easily, however the high work functions of these metals reduces the possible open-circuit voltage (Voc), short-circuit current (Jsc), fill factor, and resulting efficiency.
Material scientists have solved this problem by adding a ‘buffer layer’ between the active absorbing layer and the conducting cathode that essentially changes the work function to maximize Voc. However, an optimal material for these buffer layers hasn’t yet been found, as potential options either are also easily oxidized (Ca), adhere poorly to the absorbing layer (polar organics), or don’t fit in well with solution-based processing (inorganics).
However, a recent Science Reports article demonstrates the use of a new organic buffer layer made of a functionalized fullerene that can be used in conjunction with any variety of metal cathode. The new cell created using this layer provides an efficiency of 8.5%, very high for state-of-the-art OSCs! Below you can see the basic cell design:In the middle, you’ll see the mix of blue and brown regions that represent the electron- and donor- accepting regions of the absorption layer. Electrons and holes migrate toward the cathode and anode, respectively, and the green buffer layer indicates the new material used in this study. On the left are diagrams for the two materials that both showed efficiency improvements when used as the buffer layer. Both molecules are fullerene molecules, basically a 60-atom ball of carbon atoms connected in a hexagonal grid, connected to either a tertiary amine (left) or a sulfobetaine (right). Fullerenes perfect for this application because: 1) they are electron acceptors, so naturally encourage electrons to move toward the cathode, and 2) since they are made of aromatic carbons, they induce pi-pi interactions with the absorbing layer that make them want to stick!
The authors tried both types of materials for the buffer layers and compared the current-voltage (I-V) curves to a cell using a plain old silver cathode (I-V curves are the bread and butter of looking at solar cell performance. Scientists look for as close to a rectangular shape as possible and a high Voc – the value of the voltage when the current density is 0 – you’ll see below in a second). Here are the curves:A couple things to point out:
1) With the silver cathode alone (gray), Voc is about 0.4 V – see where the gray line hits 0 current density?
2) Both the C60 buffer layers improve Voc to almost 0.8 V! You’ll see the whole curve is more rectangular and more importantly contains more area within it (think about filling up the area between the curve and the horizontal black line at zero current density). This will correspond to more overall power out of the cell and a better efficiency.
3) Both the C60-N (tertiary amine) and C60-SB (betaine) perform basically the same, with the C60-SB getting a slightly higher Voc. This suggests the choice of functionalization may be flexible and the C60 is the most important part of the layer.
So why does this type of buffer layer work? Well, above and beyond the fullerene advantages for electron accepting and adhesion, these organic molecules apparently create an interfacial dipole moment that decreases the work function of the molecule-metal combination. Work functions are the energy required to excite a bound electron from the metal to the vacuum, where it would be free. Decreasing this value with the interfacial dipole increases the natural electrostatic field in the cell created across the electron donor and acceptor and thus increases the highest possible voltage. This design allows the use of almost any metal electrode, since the work function is now dictated by the buffer layer!
Slowly but surely, the efficiency of these organic solar cells are rising to fully take advantage of their low costs of production. We should see some pretty ingenious applications in the near future! Solar cell spray paint, anyone?
Page, Z., Liu, Y., Duzhko, V., Russell, T., & Emrick, T. (2014). Fulleropyrrolidine interlayers: Tailoring electrodes to raise organic solar cell efficiency Science DOI: 10.1126/science.1255826