# Organic Nanostructured Materials for Sustainable Application in Next Generation Solar Cells
### The Operatzon Principle of OPVs
#### 1.2.1 Light Absorption and Exciton Generation  
Increased efficiency is a near-linear driver for lowering the cost of PV power per kilowatt-hour since efficiency is a fundamental parameter in the development of photovoltaic (PV) systems. [PERSON] (S-Q) proposed a theoretical framework for calculating the limiting efficiency of a single junction solar cell in 1961, based on the notion of detailed balancing, which equates the incoming and outgoing photon fluxes for a device under open-circuit circumstances [31]. This Shockley-Queisser limit defines the maximum practicable solar energy conversion efficiency for applied active materials. The photoactive layer must absorb the maximum amount of incoming sunlight as this is the standard by which new photovoltaic technologies are compared (see Figure 4a). The limit is that the maximum solar conversion efficiency is around 33.7% for a single p-n junction photovoltaic cell, assuming typical sunlight conditions (unconcentrated, AM 1.5 solar spectrum), at a  
Figure 3: Stepwise operation of a BHJ OPV device (Reproduced from reference [25] with copyrights permission from Elsevier Science and Technology Journals).  
band-gap of 1.34 eV. The multi-junction concept is the most relevant approach to overcome the Shockley-Queisser limit for single-junction photovoltaic cells.  
Questions have been raised about whether nanostructured organic photovoltaic devices can exceed the Shockley-Queisser limit. As previously stated, for high efficiency, light is absorbed by both donor and acceptor component of the BHJ photoactive layer [25], and light with high sufficient energy levels is absorbed by the OPV. Unlike in the inorganic material where the exciton binding energy (E\({}_{\text{b}}\)) is small enough to reduce the attraction between the electron and hole pairs, organic excitons have large binding energy (typically 0.2-1.0 eV) that drives the electron and hole to recombine after photoexcitation. A solution to this problem was proposed by [PERSON] et al. in 1986 [32] which has been working ever since. It involves combining two organic semiconductors with slightly offset HOMO and LUMO energies, such that the photoexcited electron in the first material (normally the donor) is transferred to the second material (acceptor), allowing for the charges to be separated at the junction between these two materials.  
The maximum possible efficiency is achieved when non-radiative recombination is absent and all generated carriers are either collected as current in the leads or recombine, emitting a single photon per electron-hole pair. Conjugated polymers, in particular, with extremely thin photoactive layers (up to 100 nm), may absorb light at the maximum of their absorption spectrum with a high absorption coefficient, unlike their inorganic silicon-based equivalents, which need hundreds of micrometres. It is important to produce donor  
Figure 4: Illustration of the working principles of an organic photovoltaic solar cell displaying the processes of (**a**) light absorption and exciton generation, (**b**) exciton diffusion and charge dissociation, (**c**) free charge transportation and lastly, (**d**) collection of charge carriers at the electrodes.  
polymers that absorb light in the longer wavelength area for next-generation organic photovoltaics, that is, the absorption border should fall at wavelengths larger than 700 nm. The band-gap of such polymers should be smaller than 2 eV [33].