Introduction to Organic Solar Cells - Organic Solar Cells: Materials and Device Physics - page 4

Fig. 1.2 AM1.5G reference
solar spectrum
wavelength range where the inherent absorption of the material is weak, various
light trapping techniques such as gratings [ 16 , 17 ], folded cells [ 18 ], and lens
concentrators [ 19 ] have been suggested. A main concern for organic materials is
the commonly large band gap and small absorption range which lead to low
absorption efficiency of photons in the long wavelength region. With a LUMO-
HOMO difference of 1.1 eV, it is reported that *77 % of solar light is absorbed
[ 20 ]. In contrast, the benchmark material for OSCs, P3HT, has a band gap of
*1.9 eV, while most organic materials have band gaps of *2eV[ 21 ]. Figure 1.2
shows the AM1.5G reference solar spectrum. Compared to common materials
which do not absorb in the region [700 nm, it is evident that a large portion of
energy can be harvested in the long wavelength regions ([700 nm) and the
absorption efficiency can be enhanced by using low band gap materials.
1.3.2 Exciton Diffusion and Dissociation
With an exciton generated, the next question is how to separate the bound electron-
hole pair to generate free charges which eventually leads to electricity generation.
An ingenious solution was provided by Tang et al., who demonstrated that using two
different organic materials with correctly aligned band levels can result in efficient
solar cells [ 22 ]. The junction between the two materials is called the heterojunction.
Ever since that discovery, the heterojunction has become the basis of OSC design. To
achieve exciton dissociation, two organic materials with band alignment shown in
Fig. 1.3 are placed adjacent to each other. The difference between the HOMO of
material A and the LUMO of material B has to be lower than the potential difference
between the bound electron-hole pair, i.e., the band gap of either material A or B
minus the exciton binding energy. When an exciton is generated in, for instance
material A, it migrates toward the heterojunction. As the potential difference between
LUMO B and HOMO A is lower than that of the energy of the exciton, the transfer of an