Materials, photophysics and device designs for high efficiency photovoltaics
The development of inexpensive and sustainable renewable energy technologies is of critical importance in fulfilling rapidly growing global energy demand. The seriousness of this endeavour is also amplified by the urgent need to eliminate the usage of fossil-fuels. In the recent past, the cost of power generated using solar photovoltaics (PVs) has dropped below that of power generated by oil and gas, making them an attractive candidate in the renewable energy market.
The Shockley-Queisser limit (S-Q limit), also known as the detailed balance limit, refers to the maximum theoretical efficiency that can be achieved using a single p-n junction solar cell. Silicon PVs, which account for about 90% of the solar-PVs market share, have achieved an overall power conversion efficiency of ~ 27% which is very close to its threshold S-Q limit of ~ 30%. Therefore, in order to further reduce the cost of solar power, it is essential to find technologies capable of surpassing the S-Q limit. In this thesis we discuss three main strategies for surpassing the S-Q limit: (i) tandem solar cells, (ii) singlet fission sensitized solar cells and (iii) triplet-triplet annihilation assisted upconversion.
Following a review of the relevant background theory for organic semiconductors, colloidal quantum dot (CQD) semiconductors and perovskite semiconductors, we discuss their applications in inexpensive, solution-processed, optoelectronic devices. We commence by demonstrating the first prototype of a solution-processed, monolithic tandem solar cell with perovskite and CQDs as the top-cell and bottom-cell active materials respectively. Using a detailed balance model we show that the radiative coupling between the two sub-cells can result in an absolute gain of ~ 11% in the tandem cell’s efficiency. Next, we study a novel solution-processed pentacene precursor for applications in singlet fission sensitized photovoltaics and photon downconversion systems. We observe significant contribution from singlet fission to the photocurrent generated in these devices. Finally, we study organic light-emitting diodes (OLEDs) that are prepared using an efficient singlet fission (SF) molecule to investigate the physics of triplet-triplet annihilation (TTA) in such molecules. We develop a kinetic model based on the Merrifield theory to explain the magnetic-field effect on SF and TTA in these OLEDs.