Nanostructure of organic solar cells

Organic materials with various electron affinities and ionization energies are combined to fabricate a solution- processed photoactive film, sandwiched between cathode and anode layers to form a working organic solar cell (OSC). Physical processes driving the photovoltaic conversion efficiency are strictly bound to that photoactive film; therefore, it is only prudent to focus on its morphology. Molecular interaction between organic donor and acceptor material, along with a chosen solvent, determines the extent of phase separation and corresponding nanostructures of the photoactive film. Phase separation size and the relative purity of domains are directly responsible for efficient charge separation and their transport, respectively. Soft X-ray scattering experiments can quantify the domain size and relative domain purity at microscale. Smaller domain sizes provide large interfacial areas, which facilitates the charge separation toward larger short circuit current, JSC. At the same time, it is also important for the domains to exhibit reasonable purity for efficient charge transport, which would lead to higher fill factor (FF) values. However, these two morphology parameters must be optimized in a photoactive layer to yield a high photovoltaic conversion rate. In addition, the microscale morphology parameters, we had to account for the influential role of complex nanostructures in the organic blend film toward driving the photovoltaic process of OSCs. Hard X-ray scattering experiments are being adopted to probe the molecular-packing information and extent of crystallization of donor and acceptor materials. Preferential face-on orientation provides a favorable scenario for p-p interacting stacks to facilitate better intermolecular charge transfer channels at interfaces, which can result in greater photocurrent generation. At the same time, larger crystalline stacks of donor and acceptor materials expedite free charge transport in their respective domains to ensure excellent collection efficiency. This chapter will present information about the influential correlation between morphology and photovoltaic parameters, as well as a number of film-processing techniques to manipulate or control the photoactive layer morphology toward realizing efficient and stable OSCs.