Overview

MIT researchers have made major strides toward developing solar cells that are inexpensive, efficient, flexible, and transparent using a design that combines two special components. Microscopic fibers called nanowires rapidly carry electrons liberated by solar energy through the solar cell to a flexible, transparent electrode made of graphene, a form of carbon that occurs in one-atom-thick sheets. Using a novel approach involving solutions and relatively low temperatures, the researchers were able to attach those two components together, “growing” nanowires directly on graphene in their lab. Tests with assembled solar cells showed that incorporating the nanowires pushed up device efficiency and that there was no performance penalty for replacing a conventional high-cost, brittle electrode with their version made of abundant, inexpensive carbon.

In the past decade or so, scientists and engineers have begun making photovoltaic solar cells out of organic—that is, carbon-containing—materials rather than inorganic silicon. The potential benefits are many. Conventional silicon solar cells require expensive, meticulous, step-by-step processing and careful handling to be sure they’re not damaged during transport and installation. In contrast, organic solar cells can be manufactured using inexpensive, continuous “roll-to-roll” processing in which individual layers are deposited sequentially on large areas of material. The finished organic cells are lightweight, flexible, and transparent, so they’re easy to handle and can be installed on roofs, windows, electronic device displays, and other surfaces, both flat and curved.

However, two problems have slowed development of this promising technology. First, organic solar cells can’t yet convert visible light into electricity as efficiently as their silicon counterparts can. And second, researchers haven’t been able to find a flexible, transparent, low-cost material for the electrode that carries current out of the cell. Silvija Gradečak, the Thomas Lord Associate Professor in Materials Science and Engineering, and a team of collaborators from four MIT departments have come up with methods of dealing with both of those problems.

The efficiency challenge

The challenge with conversion efficiency, says Gradečak, is getting the right geometry inside the organic cell. Any solar cell requires two materials—a “donor” material that absorbs incoming solar energy and gives off energized electrons, and an “acceptor” material that picks up those electrons and carries them to the electrode, where they exit the device as electrical current. In the usual organic solar cell, two polymers act as the donor and acceptor materials, and they need to be intertwined to provide lots of interfaces for the jumping electrons. The difficulty is controlling the nanometer-scale structure inside the organic cell to achieve those interfaces while providing pathways for the rapid movement of electrons to maximize the current coming out of the device.

To solve that problem, Gradečak has been working to make a “hybrid” solar cell by replacing one of the organic polymers with an inorganic material that will move the electrons more efficiently. But she adds a special twist: She makes the inorganic material into nanowires, microscopic fibers that are a few billionths of a meter in diameter and millions of times longer. Each nanowire is a single crystal, with an extensive surface area and no defects to interfere with the flow of electrons. And Gradečak’s group—the Laboratory for Nanophotonics and Electronics— has an unmatched ability to grow nanowires at any length, diameter, and density desired.

With this approach, there’s no need to worry about interconnecting regions of polymers. The electron-donor material can surround a “forest” of tall, solid nanowires—an overall structure that’s predictable and stable and maximizes contact between the two materials (see the figure below). Assembling the solar cell involves growing the nanowires up from a transparent electrode, infiltrating that forest with the polymer or other electron-donor material, and topping it off with a second electrode. When the solar cell is in use, light enters through the transparent electrode, and electrons knocked loose from the donor material move into nearby nanowires. The electrons travel rapidly through the nanowires to the transparent electrode, out along an external circuit, and back to the second electrode.

Incorporating nanowires into solar cells

In this solar cell design, tall, thin nanowires grow up from a transparent electrode and are surrounded by a light-absorbing polymer or other electron-donor material. A second electrode tops off the system. Light enters through the transparent electrode and energizes electrons in the polymer. The electrons move into the nanowires and flow to the transparent electrode and then out of the device into an external circuit. After powering, say, a light bulb, the electrons return to the second electrode and rejoin the “holes” they left behind. Using nanowires in place of the usual second polymer increases the stability and predictability of the structure and allows the electrons to move more quickly to the surface of the device.

To test their design, Gradečak and her collaborators grew nanowires on a transparent electrode and then deposited a solution containing the donor material on top. Images with a scanning electron microscope (SEM) showed that the solution infiltrated deep into the nanowire array, making good contact with the nanowire surfaces and leaving few voids to reduce performance. And in experiments with fully assembled devices, the presence of the nanowires pushed up efficiency by as much as 35%, depending on the donor material used.

Those results confirm the viability of their hybrid approach. “By combining the organic and inorganic materials, we bridge the advantages of both worlds,” says Gradečak. “We get solar cells that can be processed at a large scale using roll-to-roll methods, but they can still have reasonable power conversion efficiencies.”

Electrode options

Gradečak envisioned one more improvement, namely, a new