Mechanistic Understanding of Organic Solar Cells: A Detailed Investigation on Role of Förster Resonance Energy Transfer, Dielectric Constant and Exciton Dynamics

Abstract

Organic solar cells (OSC) have been emerging as promising energy harvesting technology because of their low-cost fabrication, semitransparency, solution processability and ability to be produced in large scale using roll-to-roll processing methods. It was envisaged that, resonance energy transfer (RET) is pivotal for improving device efficiencies of ternary blend solar cells (TBSCs). Taking analogy from natural photosynthetic systems, we have shown that mechanistic deviations of observed RET rates from an expected Förster type mechanism are anticipated in OSCs. But unlikely the highly efficient photosynthetic systems, blend morphology plays an important role in dictating the device performance of OSCs. We extracted new suggested strategies to systematically correlate the R0 with increments in current density (ΔJSC) and to optimize the effect of FRET in enhancing the photocurrent for realizing high efficiency OSCs. Chapter 2 and 3 discuss incorporation of a small molecule and polymer in fullerene and NFA based TBSCs, respectively. FRET between the donor components played an important role in enhancement of PCE and this was established by probing the excited state photophysics using both steady state and transient absorption (TA) spectroscopy. The fullerene based TBSCs had >300 nm thick active layer which implicates its potential application in roll-to-roll processable OSCs. In the NFA based TBSCs, we observed ≥ 10% increment in JSC, which enhanced PCE up to 10.34% for PTB7-Th:PBDB-T:IT4F blend. An important conclusion drawn from this work is although FRET plays an important role in enhancing device photocurrent, its extent is limited by blend morphology, synchronous with our finding in the first chapter. The ultrafast charge generation mechanism of a reported high-performing NFA based OSC, PM6:Y6 is discussed in Chapter 4. The PCE of ~15 % is achieved with external quantum efficiency > 70 % in the 400-900 nm region. TA spectroscopy was employed to understand the charge generation by probing both electron and hole transfer processes at ultrafast timescales. It was concluded that the slow hole transfer rate is limited by singlet exciton diffusion in Y6 molecule and is key to the observed dynamics of charge generation. These results also suggest that ultrafast charge transfer is not a necessary condition for efficient generation of free charges. In Chapter 5, we reported synthesis of three new conjugated copolymers with high dielectric constant (εr). To improve the εr of semiconducting polymers, we followed a two-pronged approach: (i) introduction of TEG sidechains in D-A type semiconducting polymers to enhance the dipolar polarization and (ii) variation in electron-donating strength of donor chromophores in the polymer backbone. This led to the achievement of εr ~ 4-5 at room temperature in MHz frequency range. However, free charge generation could not be achieved for any of the polymers, which is possibly because the low electronic polarization of TEG groups rendering the screening of hole and electrons ineffective. We conclude that concerted efforts should be made to not only improve the εr in MHz frequency regime but also in high frequency regime so as to achieve free charge carrier generation.