The Influence of Polymer Sequence on the Formation of Bulk-Heterojunctions in Organic Solar Cells.

摘要

This thesis summarizes my work on organic solar cells during my graduate studies.;Chapter 2 describes the size-dependent behavior of polymer solar cells measured under partial illumination. We found that ITO resistance is a significant source of power loss because sheet resistance (Rs) increases with area. The non-illuminated part of a partially illuminated device introduces some interesting effects related to the physics of device operation. Specifically, this contributes additional "dark diodes" that connect in parallel with an illuminated cell, giving rise to an apparent decrease in VOC and increase in FF as the illuminated portion of the cell is decreased.;Chapter 3 is a study of a P3HS-b-P3HT block copolymer as a donor material in organic solar cells. Fiber-like nanostructures are formed spontaneously in P3HS-b-P3HT:PCBM devices, and their thermal stability exceeds homopolymer:PCBM devices or ternary mixtures. Although P3HS-b-P3HT contains two distinct electron donor materials, the EQE spectra, hole mobility, Jsc, and PCE exceed that of a physical mixture of the two homopolymers and PCBM.;Chapter 4 compares the photovoltaic properties of two conjugated copolymers with the same composition, P3HS-b-P3HT and P3HS- s-P3HT. The block polymer spontaneously undergoes intrinsic phase separation and the statistical polymer does not. P3HS-b-P3HT devices perform best when the native self-assembled structure is most perturbed, which is accomplished using PC71BM. P3HS-s-P3HT is a polymer that does not form a native phase separated structure. Here vapor annealing can be used to more predictably optimize the polymer:fullerene morphology.;Chapter 1 serves as an introduction to organic solar cells. I will briefly discuss the working mechanism, and describe the device fabrication processes and testing set up that I designed at the beginning of my graduate studies.;Chapter 5 studies the evolution of the electron mobility of two different acceptors with different crystallinity, PC71BM and ICBA, in a crystallized P3HT matrix during a prolonged thermal aging process. ICBA has an electron mobility that is over an order of magnitude lower than PC71BM. Given that both devices use the same polymer donor, it appears that high electron mobility does not correlate with the best device performance. The evolution of the acceptor appears to be the dominant factor that leads to long term changes in devices.;Chapter 6 describes the synthesis of donor-acceptor copolymers with a "blocky" structure. Selenophene is introduced into PBDTTT-C-T in order to improve molecular ordering between polymer chains. Though more ordered morphologies are observed with blocky polymers, the physical mixture of parent polymers perform better in solar cell devices, which is not well correlated with morphology.;Chapter 7 summarizes the above work and discusses the further directions in organic solar cell research.