Solar Cells: Nanoscale Organization for Higher Efficiency

Samples of nanowire composites prepared for electron microscopy imaging. These composites are designed to maximize the speed of electron conductivity. Courtesy of Carol Hsin.

While solar cells offer a cleaner alternative to fossil fuels, the latter’s energy efficiency still often surpasses that of the former. To solve this problem, Chinedum Osuji, Yale Professor of Chemical and Environmental Engineering, in collaboration with Dr. Lisa Pfefferle, Dr. Sohrab Ismail-Beigi, and Dr. Andre Taylor of the Yale Nano-Solar group, used nanotechnology to manipulate the organization of solar cells in the “active layer,” where the energy is produced, with the ultimate goal of optimizing solar cells.

The active layer of a solar cell, also known as a photovoltaic (PV) cell, is the region where charge is collected and transported to the electrodes. This charge transport, which produces electric current, occurs at the intersection of electron-abundant materials and electron-deficient materials. These two materials are randomly mixed and sandwiched between two electrodes in current versions of PV cells. The mix usually consists of a polymer and carbon nanoparticles. Sunlight hits the polymer, causing electrons to jump into the nanoparticles, which are better electrons conductors. The freed electron moves to the positive electrode, conducted by the nanoparticles. Its absence leaves a positive “hole” in the polymer, which moves to the negative electrode. But because the polymer and nanoparticle mixture composition is random, the charges wander through their respective material before reaching the electrode.

To improve solar cell efficiency, Osuji targeted this random structure in the active layer. “First, we want to address the convoluted nature of the electron conduction pathway,” Osuji said. “Second, we want regular spacing between the nanomaterials.” Straightening the electron pathway improves the speed of electron conduction, while regular spacing of the nanomaterials improves light-use efficiency by maximizing charge separation.

The lab used zinc oxide as the electron-conducting nanomaterial and a polythiophene as the hole-conducting polymer. In constructing the composite donor-acceptor material, the lab used a “bottom-up” approach, in which zinc oxide “self-assembled” into crystals similar to how sugar crystals form. After synthesizing zinc oxide wires, Candice Pelligra, a Ph.D. student in Osuji’s lab, coated each wire with polymer by soaking wires in a polymer-water solution. By introducing cobalt ions into the ZnO crystal during the synthesis process, she could make the wires more sensitive to the magnetic field driving wire alignment. The coating process ensured periodic repetition of electron accepting and donating materials in a bulk film. In-plane magnetic alignment allowed for straighter electron charge paths, but Osuji and the group have not tested the efficiency of any devices yet, because they were primarily interested in whether they could manipulate the structure.

Candice Pelligra examines tubes of polymer in water solution (black) and cobalt-doped ZnO wires (blue). These substances are used to construct the electron donating/accepting hybrid nanowire composites. Courtesy of Carol Hsin

While the new structure is an improvement, Osuji said that it is still sub-optimal. “When you make solar panels, you want square meters of material,” Osuji said. “But the nanowires cannot be one meter long.” To harness electricity, electrodes have to be connected to the top and bottom of each wire. Since the aligned wires lay end-to-end on a flat surface and electron transport is limited to only a few hundred nanometers, many rows of electrodes and wires are needed to make a PV cell.

The solution is to make the wires stand up on a surface, like a micrometer-tall “block,” Pelligra said. Two sheets of electrodes can sandwich this block and solve the problem. Pelligra is working on this next step, using magnetic field to drive the nanowire alignment.