*Mechanism for Flexible Solar Cells DOI: http://dx.doi.org/10.5772/intechopen.93818*

*Solar Cells - Theory, Materials and Recent Advances*

the interface being an ohmic contact [13].

and diffusion [12].

In organics the small radius excitons generated as a result of photon absorption is the source of photocarriers. Excitons in organics have a binding energy in the range 200–400 meV, which is significantly higher than the binding exciton energy for semiconductor materials ~2–40 meV. The exciton dissociation should occur at the interface between the donor and acceptor materials with suitable (HOMO) and (LUMO) energy levels because that thermal energy at room temperature is not sufficient (~25 meV) for exciton dissociation to hole and electron in the bulk medium. The mechanism of exciton dissociation is not completely known, however, the charge transfer process between the donor and acceptor components is the major factor controlling the charge separation at the interface. After charge separation, holes and electrons move to the opposite electrodes because of drift

The working principle of a polymer bulk heterojunction (BHJ) solar cell as shown in **Figure 3** is summarized by the creation of an exciton in the active layer, due to light absorption, and then this exciton will separate into two charge carriers at the interfaces between the species that constitute the active layer (typically, a binary blend of a polymer and a fullerene or two polymers, which act as the donor phase and the acceptor phase), with subsequent collection by the electrodes. To get efficient steps, all of these steps must follow very strict limitations. At first, the generated excitons must hop between the molecules reaching an interface between the two phases before recombining (radiatively or non-radiatively). This means that the two phases should be mixed in an optimal structure, with phase domains usually in the order of (10–30) nm (the average exciton diffusion length in polymers). Then, the position of the energy levels at the interface must be favorable for a fast exciton dissociation followed by charge separation (*i.e.*, the electron in the acceptor phase and the hole in the donor phase without successive recombination). After that, the charges must travel inside the respective phases, reaching the collecting electrodes again without a charge recombination: at this point, the energetic level structure at the electrode interfaces plays an essential role, ideally

The conjugated polymer-PCBM bulk heterojunction is currently the best conjugated polymer-based PV cell. One significant improvement to this device structure was made recently by many researchers, who found that the morphology of the blend could be optimized by casting the polymer and PCBM from a solvent that prevents long-range phase separation and enhances the polymer chain packing. This

*A schematic diagram for the working principle of a polymeric bulk heterojunction solar cell [13].*

**414**

**Figure 3.**

avoids the formation of isolated regions of polymer and PCBM in the film and gives the polymer increased hole mobility. This fabricates a device with more than double the EQE of the previous best device and with 2.5% power efficiency under AM1.5G conditions [14].

For organic solar cells, the magnitude of JSC, VOC, and FF depends on parameters such as: light intensity, temperature, composition of the components, thickness of the active layer, the choice of electrodes used, and the solid state morphology of the film. A clear understanding of the device operation and photocurrent Jph generation and its limitations in these devices are required for their optimization and maximization. In order to allow for further design of new materials that can improve the efficiency of this type of solar cells, the relation between the experimental Jph and material parameters (charge-carrier mobility, band gap, molecular energy levels, or relative dielectric constant) needs to be understood and controlled [9].

Two intrinsic issues can be resolved by the bulk heterojunction (BHJ) structure, charge separation and charge transport, in organic layers. These are representative structures; though, they have a fundamental limitation in terms of open circuit voltage (VOC) that is basically determined by the offset energy between the highest occupied molecular orbital (HOMO) of p-type organic semiconductors and the lowest unoccupied molecular orbital (LUMO) of n-type organic semiconductors, even though the work functions of electrodes often affect VOC. A tandem structure can be applied to maximize the power conversion efficiency in organic solar cells by increasing VOC, because the overall voltage becomes the sum of individual VOC values in each sub cell (front and back cells). In addition, the selecting complementary BHJ layers enhances the overall short circuit current density (JSC) of tandem cells, because these layers have different absorption ranges for maximizing solar light harvesting. An adverse effect is also present owing to marginally increased electrical resistances by the presence of additional interfaces and active layers in series connection. As illustrated previously, both normal- type and inverted-type structures are possible by placing suitable electrodes with appropriate work functions on each side. To date, the most popular bulk heterojunction structure is made with the composites of conjugated polymers (p-type) and fullerene derivatives (n-type), leading to polymer:fullerene solar cells. Hence, P3HT polymers have been introduced because they can absorb visible light up to 650 nm and their glass transition temperature approaches ~110C°. The external quantum efficiency (EQE) of

