Bulk Heterojunction Solar Cells

Main idea behind the bulk heterojunction solar cell (BHJ) configuration is to increase the probability that a photo-generated exciton dissociate into free charge carriers instead of decaying into the groundstate via recombination. When a photon is absorbed by the organic semiconductor, a bound electron-hole pair (exciton) is generated. Typically the exciton is strongly bound (with a binding energy of 0.5-1.0 eV >> kT) due to the low dielectric characteristics of organic materials. This means that the photo-generated polarization on the acceptor or donor is weakly screened and dissociation of the generated exciton is unlikely. The BHJ is created by blending an electron-poor and -rich material generating a molecular morphology minimizing the distance that the exciton needs to travel to reach the donor-acceptor interface. At the interface, the exciton binding energy can be overcome by the potential difference in the electron affinity of the donor and acceptor. There is a compromise between getting a higher interfacial area and better conductivity resulting from the amount of meshing and connectivity of the material domains generated via blending. The donor is usually a conjugated polymer and fullerene derivatives are the common acceptor.

Main working principles

The basic device operation can be considered in four main parts. (1) The exciton (bound electron-hole) generation takes place after the photoabsorption by either the donor or acceptor. The optical absorption gap (Eopt) of the materials determines the range of the solar spectrum to be absorbed. The exciton binding is much larger than that of inorganic semiconductors, thus disassociation is unlikely happen via thermal interactions. (2) The generated exciton diffuses until it reaches the D-A interface or recombines to the ground state. The lifetime of the exciton (typically in 0.1 - 1 ns) has an important role on successfully quenching the exciton. (3) Charge-transfer exciton generated when the electron and hole are still bound but each localized on different material. The charge-transfer exciton still has a significant amount of Coulomb attraction resulting a binding energy of 0.1-0.5 eV. (4) Dissociation of the exciton into free carriers initiate transport to the device electrodes. The transport is a hopping process characteristic of disordered organic semiconductors. The mobility and implicitly the power conversion efficiency depend on the amount of disorder and other macroscopic properties such as temperature and the density of charge carriers. The geminate recombination may also occur if the carriers are not separated or not collected by the electrodes sufficiently, leading to an additional lost in the efficiency.