New Device Structures for Organic Photovoltaics Using Transfer Printing
Organic photovoltaics provide an inexpensive and lightweight alternative to conventional silicon solar panels. However, low power conversion efficiencies, mainly due to low open-circuit voltage (VOC) prevent them from competing with silicon cells commercially. Poly (3-hexylthiophene-2, 5-diyl) (P3HT) is a polythiophene derivative that is a popular electron-donor material used in organic solar cells. However, the highest VOC reported for well-performing P3HT/fullerene solar cells is around 0.80 V. This is much lower than would be expected for a donor with a 1.8eV bandgap such as P3HT, and indicates over a 44% loss compared with the theoretical limit. The most popular morphology for the active layer in OPVs currently involves a blend deposition method in which acceptor and donor materials are processed together in solution be- fore deposition. While this device architecture allows for increased interfacial surface area between the acceptor and donor, it also increases leakage and recombination at the interface leading to reduced VOC and shunt resistance. We propose a planar device structure that could allow us to suppress interfacial recombination. This device structure could be scaled-up for quick, mass production through roll-to-roll processing. Using print deposition, we have fabricated an unconventional structure consisting of pristine layers of P3HT as an electron donor and [6,6]-penyl-C61-butyric- acid-methyl-ester (PCBM) as the electron acceptor to form a planar cell. Our deposition method also employs print deposition to place thin Lead(II)Sulphide quantum dot (PbS QD) films and PCBM electron acceptor on top of our spin-cast P3HT layer. We investigate electron-hole pair dissociation at the donor-acceptor interface by inserting an additional film between the donor and acceptor layers and observing the impact on VOC. We employ the small molecule, Tris(8-hydroxyquinolinato)aluminium (Alq3), and see an improvement in VOC. We propose that the proper material, when placed at the interface, suppresses non-geminate recombination and can increase VBI. We have extend our investigation to vapor deposited materials, 2,2’,7,7’- Tetrakis-(N,N-di-4-methoxyphenylamino)-9,9’-spirobifluorene (Spiro-OMeTAD) and buckminsterfullerene (C60), and document the observed changes in VBI.