Important materials for the active layer in organic solar cellsJon Eggert Carlé
The active layer consists of two materials, a donor and an acceptor. In this section key materials in the two categories will be presented.
The donor in the solar cell is the light absorbing material, a polymer or small molecule, in which an electron is excited and transferred to the acceptor. The most common light absorbing materials are presented here.
P3HT. Poly(3-hexylthiophene), P3HT, see Figure 1, is to date the most studied polymer for polymer solar cells. The efficiency of a P3HT/PCBM solar cell is typically 4-5 %, which is close to the optimal performance for this systemDOI:10.1063/1.1521244DOI:10.1002/adfm.200390011DOI:10.1002/adfm.200500211DOI:10.1021/ol051950y.
Synthesis of P3HT is short 3-4 steps are needed depending on the synthetic method chosen. A few strategies are: RiekeDOI:10.1021/ja00106a027, McCulloughMccullough RD, Williams SP, Tristram-Nagle S, Jayaraman M (1995) Synth Met 69:279, Grignard metathesis (GRIM)Loewe, R. S., Khersonsky, S. M. and McCullough, R. D. (1999), A Simple Method to Prepare Head-to-Tail Coupled, Regioregular Poly(3-alkylthiophenes) Using Grignard Metathesis. Adv. Mater., 11: 250–253, direct arylation polymerization (DARP)DOI:10.1002/pola.26175 or Stille cross couplingDOI:10.1039/A706583C. Highly regioregular P3HT is relatively easy to synthesize and process, relatively stable and cheap/affordableDOI:10.1002/adma.201100792. The challenge with P3HT is the poor overlap with the solar spectrum and thus to increase the efficiency of the polymer solar cells, researchers have the past decade studied another type of polymer materials called low band gap polymers.
Low band gap polymers. Low band gap polymers absorb light with wavelengths longer than 650 nm corresponding to a band gap lower than 2 eV. This means that they have a better overlap with the solar spectrum and can potentially absorb more photons than P3HT and thereby increase the current and efficiency of the solar cellDOI:10.1016/j.solmat.2007.01.015. In Figure 2 some low band gap polymers are shown. The low band gap polymers normally consist of an electron rich motif, a donor unit, and an electron poor motif, an acceptor unit. This donor-acceptor approach creates a partial charge separation at the backbone of the polymer, which results in a lowering of its band gapDOI:van Mullekom HAM, Vekemans JAJM, Havinga EE, Meijer EW (2001) Mater Sci Eng , R 32:1. The device performance has been increased to 7-10% by the use of low band gap polymers in the active layerDOI:10.1038/nphoton.2009.192DOI:10.1038/ncomms2411DOI:10.1021/ma4003165.
The acceptor is the material which accepts the electrons from the donor and thus ensures charge separation. The most important acceptors for organic solar cells will be presented here.
Fullerene. The most applied fullerene is the soluble PC61BM (phenyl-C61-butyric acid methyl ester) which is soluble in most organic solvents. In the early days of organic solar cells the C60 fullerene was evaporated onto the polymer layer and the active layer of the solar cell was a bilayer. With the PC61BM it was possible to blend the polymer and the acceptor in solution before spin coating and the active layer was now a bulk heterojunctionDOI:10.1126/science.270.5243.1789. In Figure 3 the C60 and the PC61BM are shown. PCBM is synthesized in 4 steps from C60DOI:10.1039/C0CP01178A.
In addition to PC61BM there are many other types of fullerene, some examples are shown in Figure 4, most importantly is PC71BM which has the same functional group but is based on C70. Researchers have studied several derivate of fullerenes in order to tune the energy levels of the compounds, e.g. the LUMO level of ICBA is 0.17 eV higher compared to PC61BM, this means that the VOC and thus the efficiency is increased when using ICBA instead of PC61BM in a P3HT based solar cellDOI:10.1002/asia.201300600.
Other types of acceptors. Since 2010 researchers have studied other small molecules as acceptors in order to apply these in high performing organic solar cells. This amongst others includes derivates of perylene and pentaceneDOI:10.1021/cm1023019DOI:10.1002/adma.200903628DOI:10.1051/epjpv/2013020. Polymers have also been studied as acceptors in order to prepare all-polymer solar cellsDOI:10.1016/j.mattod.2013.04.005DOI:10.1002/9783527648689.ch14. The advantage is that it is easier to tune the band gap of the polymers and thus ensure efficient charge transport between the donor and acceptor in the active layer. Some examples of small molecules and polymers used as acceptors in organic solar cells can be seen in Figure 5.