Novel device architecture for high performance organic solar cells

Organic solar cells offer a promising alternative to conventional solar cells, owing to their low material and fabrication cost, ease of processability and mechanical flexibility. Efforts are being stepping up to push the efficiency of organic solar cells to values competitive with their inorganic counterparts. In this dissertation we focus on novel device architectures to achieve enhanced solar cell efficiencies. We demonstrate that the open circuit voltage of the small molecule solar cells can be enhanced by sandwiching a thin layer of high ionization potential materials at the interface between the donor and acceptor layer. Effect of thickness of the sandwich layer on the short-circuit current and open circuit voltage is investigated. In this device architecture we observe enhancement in the spectral response of the solar cell and as well as increase in VOC by two times.;The second part of the dissertation is focused on solution processible tandem cells where in two polymer bulk heterojunctions with complementary absorption range are connected through a transparent interlayer. A two terminal tandem device architecture in which the two component cells are connected in series was explored. We study the role of the inter-connection layer, constituting a bilayer of n-type and p-type layers, on the efficient functioning of the 2-terminal tandem cell. The effectiveness of the various interfaces within the inter-connection layer is crucial for efficient charge collection and recombination and the factors that influence the interface properties are discussed. Various issues concerning the tandem cell i.e. photocurrent matching, good electrical contact and interface engineering are explored to achieve highly efficient tandem cells. High efficiency tandem cell with power conversion efficiency close to 5.9% have been demonstrated from recently reported low band gap polymer and poly(3-hexylthiophene) (P3HT) systems. An important revelation of this work is the nature of contact between the n-type and p-type layers, which is metal-semiconductor contact as opposed to a tunnel junction in case of inorganic tandem cells. We also show that the efficiencies are overestimated by as much as 10% when high conductivity interlayer is used.;A novel 3-terminal tandem cell in which the two component cells are connected in parallel through a transparent conducting interlayer has been proposed and successfully demonstrated. The advantage of this structure is that electrical power can be extracted from the two component cells independently and as well as when connected in parallel. 3-terminal tandem cell is an ideal design in order to extract maximum efficiency. The most important component of a 3-terminal device is the transparent conducting interlayer acting as a common electrode (anode or cathode) to the two component cells. The interlayer properties have been optimized for the desirable electrical and optical properties and as well as for appropriate workfunction. We observed an added up short circuit current from the component cells when connected in parallel and efficiencies close to 5% have been demonstrated.