Polymer Solar Cell - Mechanism

Mechanism

Polymer solar cells usually consist of an electron- or hole-blocking layer on top of an indium tin oxide (ITO) conductive glass followed by electron donor and an electron acceptor (in the case of bulk heterojunction solar cells), a hole or electron blocking layer, and metal electrode on top. The nature and order of the blocking layers – as well as the nature of the metal electrode – depends on whether the cell follows a regular or an inverted device architecture.

In bulk heterojunction polymer solar cells, light generates excitons with subsequent separation of charges in the interface between an electron donor and acceptor blend within the device’s active layer. These charges then transport to the device’s electrodes where these charges flow outside the cell, perform work and then re-enter the device on the opposite side. The cell's efficiency is limited by several factors especially non-geminate recombination. Hole mobility leads to faster conduction across the active layer.

Organic photovoltaics are made of electron donor and electron acceptor materials rather than semiconductor p-n junctions. The molecules forming the electron donor region of organic PV cells, where exciton electron-hole pairs are generated, are generally conjugated polymers possessing delocalized π electrons that result from carbon p orbital hybridization. These π electrons can be excited by light in or near the visible part of the spectrum from the molecule's highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO), denoted by a π -π* transition. The energy bandgap between these orbitals determines which wavelength of light can be absorbed.

Unlike in an inorganic crystalline PV cell material, with its band structure and delocalized electrons, excitons in organic photovoltaics are strongly bound with an energy between 0.1 and 1.4 eV. This strong binding occurs because electronic wavefunctions in organic molecules are more localized, and electrostatic attraction can thus keep the electron and hole together as an exciton. The electron and hole can be dissociated by providing an interface across which the chemical potential of electrons decreases. The material that absorbs the photon is the donor, and the material acquiring the electron is called the acceptor. In Fig. 2, the polymer chain is the donor and the fullerene is the acceptor. After dissociation, the electron and hole may still be joined as a "geminate pair", and an electric field is then required to separate them.

After exciton dissociation, the electron and hole must be collected at contacts. If charge carrier mobility is insufficient, the carriers will not reach the contacts, and will instead recombine at trap sites or remain in the device as undesirable space charges that oppose the drift of new carriers. The latter problem can occur if electron and hole mobilities are not matched. In that case, space-charge limited photocurrent (SCLP) hampers device performance.

Organic photovoltaics can be fabricated with an active polymer and a fullerene-based electron acceptor. Illumination of this system by visible light leads to electron transfer from the polymer to a fullerene molecule. As a result, the formation of a photoinduced quasiparticle, or polaron (P+), occurs on the polymer chain and the fullerene becomes an radical anion (C₆₀-). Polarons are highly mobile and can diffuse away.