Materials and Processing Methods to Control Architecture of the Active Materials
As noted above, the morphology of the active layer in organic and hybrid PV cells plays a key
role in determining the overall cell efficiency. The active materials need to be tailored and
structured in such a way as to optimize the key steps of light absorption, exciton diffusion to
interfaces and charge carrier diffusion to electrodes. While significant advances have been made
in the development of novel materials and structures for PV applications, considerable effort is
needed to learn how to properly self–assemble them, to organize the various structures, and
control the morphology of each interface in order to achieve real breakthroughs and realize
disruptive technologies that will bring solar cells technology to the point where it is competitive
with other power sources.
New approaches and various techniques must be developed for the controlled deposition of
photoactive materials with minimum density of defects so that carrier generation, transport, and
collection are optimized. Layered structures are needed to allow confinement of excitons. At the
same time, a bicontinuous morphology is needed to decrease the distance needed for excitons to
diffuse to interfaces (see Figure 60b), and to allow charges to efficiently diffuse to electrode
interfaces. For small molecule-based cells, novel vapor deposition methods are needed that will
allow control of the nanostructure of the materials being deposited (see Figure 60a). For polymer
and hybrid devices, it is anticipated that a combination of deposition methods and control of
molecular architecture can be used to tailor the structure of the active materials.
Figure 60 Bulk heterojunction structures. (a) Left: Controlled growth by vapor deposition of a
small molecule material into pillars which can give rise bulk heterojunction solar cell material.
(b) Right: Bulk heterojunction formed by nanophase segregation of organic PV materials.Self-assembly
Self-assembly is anticipated to play a substantial role in allowing control of the nano- and
mesostructure of the active materials in solar cells constructed from organic, hybrid or inorganic
building blocks. For example, by using polymer blends or block polymers, self-assembly can
give rise to spontaneous formation of nanostructures that separate donor and acceptor regions
allowing for charge carrier diffusion, while maintaining the very high interfacial area needed for
effective charge separation. Alternatively, it is anticipated that advances in the ability to control
self-assembly of quantum confined structures such as quantum dots and rods may allow for