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Open Access Publications from the University of California

Device Physics and Recombination in Polymer:Fullerene Bulk-Heterojunction Solar Cells

  • Author(s): Hawks, Steven
  • Advisor(s): Schwartz, Benjamin J
  • Tolbert, Sarah H
  • et al.

My thesis focuses on improving and understanding a relatively new type of solar cell materials system: polymer:fullerene bulk-heterojunction (BHJ) blends. These mixtures have drawn significant interest because they are made from low-cost organic molecules that can be cast from solution, which makes them a potential cheap alternative to traditional solar cell materials like silicon. The drawback, though, is that they are not as efficient at converting sunlight into electricity. My thesis focuses on this issue, and examines the loss processes holding back the efficiency in polymer:fullerene blends as well as investigates new processing methods for overcoming the efficiency limitations. The first chapter introduces the subject of solar cells, and polymer:fullerene solar cells in particular. The second chapter presents a case study on recombination in the high-performance PBDTTT polymer family, wherein we discovered that nongeminate recombination of an anti-Langevin origin was the dominant loss process that ultimately limited the cell efficiency. Electroluminescence measurements revealed that an electron back-transfer process was prevalent in active layers with insufficient PC$_{71}$BM content. This work ultimately made strong headway in understanding what factors limited the relatively unexplored but highly efficient PBDTTT family of polymers. In the next chapter, I further explore the recombination mechanisms in polymer:fullerene BHJs by examining the dark diode ideality factor as a function of temperature in several polymer:fullerene materials systems. By re-deriving the diode law for a polymer:fullerene device with Shockley-Read-Hall recombination, we were able to confirm that trap-assisted recombination through an exponential band-tail of localized states is the dominant recombination process in many polymer:fullerene active layers. In the third chapter, I present a generalized theoretical framework for understanding current transients in planar semiconductor devices, like those discussed above. My analysis reveals that the apparent free-carrier concentration obtained via the usual integral approach is altered by a non-trivial factor of two, sometimes leading to misinterpretations of the charge densities and overall device physics. This new perspective could have far-reaching effects on semiconductor research and technology. Finally, in the last two chapters, I discuss the device physics associated with a relatively novel method for fabricating nanoscale polymer:fullerene BHJs: solution sequential processing (SqP). In particular, I compare recombination in SqP vs. traditionally processed blend-cast devices, and demonstrate that SqP is a more scalable method for making BHJ solar cells. In the final chapter, I examine an unexpected discovery that occurred while working on the content in Chapter 5. Specifically, Chapter 6 examines electrode metal penetration in the SqP quasi-bilayer active layer architecture. Therein, we unexpectedly found that evaporated metal can readily penetrate into fullerene-rich layers, up to $\sim$70 nm or more. The details and consequences of this surprising occurrence are discussed in detail.

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