The world is excited about solar cells – and with good reason. Imagine the City of the Future, where every exposed surface has solar cells on it, converting the sun’s energy into electricity. This vision could include solar cells on windows, on top of our cars, on the surface of our cell phones, or on our clothes. Instead of using energy from coal or oil, which pollutes the environment, we would be using the ever-present, pollution-free energy available from the sun.

Scientists and engineers must work on improving solar cells if we want to make this futuristic city a reality. The main questions scientists have to answer are:

1. How can we develop solar cells that are cheaper to make?

2. How can we make solar cells that are light enough and flexible enough to coat any surface?

3. How can we make solar cells more efficient, so more of the sun’s light is converted into useable energy?

Silicon: the Master of Today

Today’s commercial solar cells are made out of silicon. The atoms inside silicon are in an ordered structure, forming a crystal. This silicon structure shown in Figure 1 is similar to that of other more familiar crystals like quartz. Crystals are hard, and thin crystals break instead of bending.

For a silicon solar cell to work, it must be made very thin, only a few atoms thick. This super-thin film needs to be properly protected from the elements such as wind and rain, so silicon solar cells are protected by glass. This glass layer and silicon’s brittle quality make conventional solar cells relatively heavy and very inflexible.

Figure 1. Atomic structure of crystalline silicon vs. amorphous organic materials. The atomic structure affects the flexibility of the material.

The Benefits of Organic Materials

Many scientists are excited by using organic materials, instead of silicon, to overcome these challenges. A material is “organic” if it is made up of elements such as carbon, oxygen, nitrogen: the same elements found in living things. The molecules in these materials are disordered, as shown in Figure 1.

When used in a solar cell, organic materials convert light into energy. This reaction is similar to photosynthesis, the process by which plants convert sunlight into food. Through evolution, plants have separately optimized both the absorption of light by the material and the conversion of that absorbed energy to electric charge [1], and using organic materials allows us to exploit this efficiency in solar technology.

There are many benefits to using organic materials in solar cells. First of all, they are much cheaper than silicon solar cells, since they take less energy to make. Because organic materials are disordered, scientists can put them on a surface without caring about their atomic order. To make a silicon solar cell, more energy needs to be invested to create a thin film of precisely crystalline silicon. We can quantify this expenditure by calculating the energy payback time, which is the amount of time it takes for a solar cell to produce the same amount of energy that went into manufacturing it. The payback time for silicon cells is at least 1-2 years, while for organics it is about 1-2 days [2].

Additionally, organic materials are light and flexible. Figure 2a shows a flexible display produced by Samsung that is made of organic materials [3]. Since the molecules in the organic material are disordered, bending the display does not break it. Organic materials for solar cells can be easily deposited on whatever surface we like, as in Figure 2 (b) and (c), and don’t need the protective shield of glass like silicon cells do. This flexibility also means the solar cells can be easily mass-produced using roll-to-roll printing [5], the same process used for printing patterns on fabrics.

Figure 2. (a) Flexible organic light-emitting diode display, produced by Samsung in 2010. (b-c) Clever places to put flexible organic solar cells.

The Main Challenge: Why Organic Solar Cells are Less Efficient

Some companies, such as Konarka [3], are starting to market organic solar cells, but there are challenges that scientists face in the lab to improve organic solar cells before they will be widely accepted. While organic solar cells have all the benefits discussed above, their efficiency is still much lower than that of silicon. Efficiency tells us how much energy from the sun is converted into electricity. Silicon solar cell efficiency is about 15% [6]: only 15% of the incoming energy from the sun becomes useful energy. Organic solar cells have an even lower efficiency, around 10%.

Why is the efficiency of organic solar cells lower? To understand this, we need to understand better how solar cells convert sunlight into electricity. The energy is converted between three different states: photons from the sun get converted into excitons in the solar cell material, which then get converted into electrons to use in electricity.

Figure 3 illustrates this process. The cartoon a) shows a simple model of an atom, with electrons (blue dots) orbiting a nucleus. Because of quantum mechanics, the electrons can only orbit the nucleus at discrete energies, shown by the two black circles, called orbitals. It takes an energy E for an electron to hop from a smaller orbital to a larger one.

Figure 3. The process of converting sunlight into energy.

Sunlight is composed of energy-packets called photons, and each photon has a certain amount of energy, which is associated with its color. When you see a red colored object, it is reflecting photons of light that have low energy, while a blue color reflects photons that have a high energy. When a photon comes from the sun and interacts with the atom, if that photon has the same energy E, then it can give up that energy to the electron and allow it to hop to a higher orbital, shown in (a).

In (b), the electron is now sitting in the higher orbital, with energy it got from the sunlight. The grey dotted circle shows where the electron used to be, which scientists call a hole. Even though a hole is just an absence of an electron, it acts as a particle itself. This electron-hole pair is called an exciton.

The last step is for the electron-hole pair to separate and move through the material, shown in (c), and then the electron can be used as electricity. In silicon, the electron and hole are very weakly bound, so they can easily be separated. Then, it is easy for the electron to move through the crystalline material.

In organics, the exciton is much more strongly bound. Thus the electron-hole pair moves together through the organic. This is a slower process, giving more time for the exciton to lose its energy before it is converted into electricity, thereby reducing the efficiency of organic solar cells.

Progress in the Lab to Improve Efficiency

One way scientists are trying to make solar cells better is by converting more wavelengths of the sun’s spectrum into electricity. A simple solar cell only has one energy that it can convert into electricity, such as the energy E described in Figure 3, and all the other photons in the solar spectrum are simply converted to heat.

One example of current research to improve absorption is designing “tandem cells,” which are stacks of different organic materials, each of which capture a different part of the solar spectrum [7]. Other scientists work on exploiting a physical phenomenon called singlet fission, in which we can split the incoming photon from the sun into two photons in the material, potentially doubling the amount of output electricity [8]. Another promising research area is the development of solar concentrators, which take in the randomly-oriented sun rays and guide the light onto a solar cell, concentrating the amount of light that hits it [9]. With the hard work of scientists and engineers, that City of the Future may be closer than we think.

Jean Anne Currivan is a graduate student in physics at Harvard University.











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