The idea ofa polymer solar cell was introduced in Figure 6.10, while a simple example of such a cell was shown in Figure 6.19. Such cells are usually considered among the least expensive to produce, and thereby are important in making solar energy more available. An improvement in this latter type of cell, to an efficiency of 6%, is our next topic .
The improved cell is ofthe tandem type, in which two cells are connected in series. Each of the constituent cells resembles that shown in Figure 6.10, but differs in that a network of internal heterojunctions pervades the charge-separating layers. The overall structure is similarly built on ITO-coated conductive glass and connected on the top with a thin metal electrode. The title of the article, “Efficient Tandem Polymer Solar Cells Fabricated by All-Solution Processing,” makes a claim for efficiency and for a low-cost process.
The materials used in these cells are characterized as semiconducting polymers and fullerene derivatives. Typically, semiconducting polymers have a framework of alternating single and double C—C (sometimes C—N) bonds. Delocalization of the electrons in double bonds over the entire polymer molecule produces a molecular bandgap in the range of 1.5-3 eV. Light absorption in such a material produces an “exciton” or coupled electron-hole pair, which may diffuse but has to be dissociated in order to get a photocurrent. It is found that efficient exciton dissociation occurs if, instead of a single polymer, two separate polymers, called donor and acceptor polymers, are in contact. (This may be a well-defined interface, as in Figure 6.10 right panel, or a mixture of the polymers can create a network of such interfaces.) The exciton is generated in the donor material and charge separation occurs at the interface if the acceptor material has an empty energy level that is lower than the LUMO (lowest unoccupied level) of the donor (this is the level that is transiently occupied by a photoelectron). The exciton dissociation gives an electron in the acceptor material that is thus separated from the hole that remains in the donor. This charge separation gives an effective potential difference, induced by light, leading to a photovoltaic effect. In the work described , the acceptor polymers are loaded with fullerene molecules to improve the electrical conduction. The donor polymers, absorbing the light at different wavelengths by virtue of the chemical structure, act like dyes. The role of a dye was discussed in Figures 6.16-6.18. Exciton propagation is important in these materials, so that a dissociating interface can be reached, or else no external current will result.
highlyconductiveholetransportlayerandITOis conductive indium tin oxide. The P3HT-fullerene and PCPDTBT-fullerene layers are the active charge separation layers, each described as a “bulkheterojunction composite” of a dye polymer donor with the fullerene acceptor. (Left, upper and lower) TEM crosssectional images of portions of the structure. Note the sharp interfaces, and scale bars 20nm (upper) and 100nm (lower).
Light enters through the glass-ITO substrate into the “front cell” at the bottom. Absorption and charge separation in the front cell (PCPDTBT-fullerene) is strong in the UVand in the IR, but most ofthevisible spectrum passes through. The “back cell” (P3HT-fullerene) absorbs and charge-separates the visible portion of the spectrum. Thus, the two junctions harvest complementary portions of the solar spectrum. Because the cells are in series, and the electrical current is constant through the cell, the short-circuit current ofthetandem cell is limited to the smaller ofthe short-circuit currents of the constituent cells.
In each cell, referring to right panel of Figure 7.9, photoelectrons flow vertically upward through the fullerene network of each composite and into the electron – conductive TiOx layers. For the “back cell” (top of figure), these electrons go through the aluminum film into the external load. For the “front cell” the photoelectrons annihilate with holes at the interface of the TiO% and the upper, hole-conducting PEDOT:PSS layer, which is fed photoholes from the (upper) “back cell.” The annihilation of electrons and holes at the TiO%-PEDOT:PSS interface makes the current continuous through the device. (The TiO% layer will not transmit holes from the “back cell” into the “front cell” because its valence band energy, —8 eV relative to vacuum, is too low for holes to enter.) Photoholes generated in the “front cell” (lower) flow down through the lower PEDOT:PSS hole-conductinglayer and annihilate at the
PEDOT:PSS-ITO interface with electrons from the external circuit that flow in through the ITO layer.