B. Reeja Jayan: Development of Efficient, Stable Organic-Inorganic Hybrid Solar Cells

B. Reeja Jayan is a Ph.D. student in her fifth year working under the supervision of Dr. Manthiram. Her research focuses on the development of efficient, stable organic-inorganic hybrid solar cells.

The major hurdle to widespread adoption of solar cells is the cost. Increasing solar cell efficiency and decreasing manufacturing expenses are the two primary ways to lower the cost. Organic-inorganic hybrid solar cells comprising of nanocrystalline inorganic semiconductor oxides in direct contact with an organic light harvesting polymer can dramatically impact both these areas as they use inexpensive, environmentally compatible raw materials and can be produced by high throughput manufacturing processes like roll-to-roll printing. Despite recent improvements in the solar energy conversion efficiencies of hybrid solar cells, generating solar cells based on semiconductor oxides that exhibit high efficiencies over long lifetimes remains a challenge. Published reports on the long-term stability of these cells are few and often conflicting. The objective of Reeja’s Phd research is to develop efficient and stable hybrid solar cells by systematically investigating the complex relationships among the different materials used in these solar cells, in particular the interfaces among them.

Her dissertation focuses on the hybrid ordered heterojunction solar cell systems consisting of poly(3-hexylthiophene) (P3HT) as the electron donating light absorbing polymer and nanocrystalline titanium dioxide (nc-TiO2) as the electron acceptor. Interaction of incident photons with P3HT generates coulombically bound electron-hole pairs, which dissociate due to a difference in the energy levels of the two components. Once separated, electrons are carried through the network of TiO2 nanocrystals while holes are extracted through P3HT. Such cells normally employ a transparent conducting oxide like fluorine doped tin oxide (FTO) or indium doped tin oxide (ITO) as the negative electrode and a thermally evaporated metal film as the positive electrode. Reeja has been investigating how this metal polymer interface influences the efficiency and stability of nc-TiO2-P3HT hybrid solar cells. A known drawback of these hybrid solar cells is that both cell performance and stability are influenced by the complex interactions between atmospheric oxygen and the electronic structure of TiO2. In this regard, I have also been working on understanding how modifying the TiO2-P3HT interface both chemically and morphologically can affect these photovoltaic parameters.

The hybrid solar cells are characterized by device level I-V measurements using simulated AM 1.5G solar spectrum and quantum efficiency measurements under different atmospheres. The materials and thin films used in these cells are characterized by various techniques such as X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, transmission electron microscopy, and atomic force microscopy. The device test data are then correlated to materials characteristics to establish material-property-performance relationships.

Schematic of Hybrid Solar Cell.

Low cost hybrid solar cells

Cells with annealed Cu electrode exhibit
remarkable stability in absence of oxygen