Bench Talk

(Source: Miquel - stock.adobe.com)

Solar cells, otherwise known as photovoltaics (PVs), are a class of renewable energy devices that convert the photons from solar rays into electricity. Solar cells come in many forms, with the most common types composed of inorganic materials. But, new developments in technology have yielded a wide range of thin film solar cells. Notable developments range from the ability for solar cells to be printed using inks to flexible solar cells made of organic materials, solar cells that use quantum dots, and dye-sensitized solar cells (DSSCs). Here, we look at the different types of thin film solar cells that can now be created and the chemistry behind these developments.

Organic solar cells are the second biggest class of solar cells. These organic solar cells typically use polymeric materials—but can use other organic materials—to convert the photons into electricity. Organic solar cells are much cheaper to produce and are much more flexible than inorganic solar cells, but their conversion efficiencies are much lower. Unlike inorganic solar cells, organic molecules can be solution-processed, and design engineers can use these formulations to create much thinner devices than their inorganic counterparts.

The chemical makeup of the polymer is crucial for generating an electric current. Chemical methods can be applied to change the bandgap of the polymer, which allows for electronic tunability. Even though the conversion efficiency of organic solar cells is not as high as inorganic materials, organic materials have a very high optical absorption coefficient, which is why designer engineers can fabricate thinner devices without losing the ability to generate electricity. The chemistry also allows polymers to be processed into formulations that can be printed (printable solar cells) and enables organic solar cells to be transparent, after which they can be used in windows and other areas of buildings.

Many might expect the working mechanisms to be roughly the same as inorganic solar cells. However, the chemical and internal structures are fundamentally different. In inorganic solar cells, dopants are used to change the chemical structure of the inorganic material so that electrons and holes are generated, which are then separated by a depletion region where some of the holes and electrons have already recombined, leading to the separation of the remaining charges. The migration of these separated charge carriers to the opposite side of the depletion region under photon absorption causes a current to flow.

However, organic solar cells are different. Organic solar cells use specific donor and acceptor materials to generate the electrons and holes, rather than doped materials. The organic molecules absorb the photons of light, which generate excitons—an electron and its corresponding hole. Light absorption also causes the electrons within the exciton to become excited, whereupon they move from valence band to the conduction band. The exciton then moves to the interface between the donor and acceptor materials, where it separates into an electron and a hole. This charge separation causes a current to flow because the electrons and holes flow to the electrodes.

Dye-Sensitized Solar Cells (DSSCs) are another emerging class of thin film solar cells, which again have a completely different mechanism for generating an electric current under solar irradiation. It is a class of thin-film solar cells that are semi-transparent and semi-flexible.

In DSSCs, it’s all about the anode. The anode in a DSSC is coated with a semiconducting film, followed by a titanium dioxide layer. This is further followed up by being soaked with a photosensitive dye—commonly a ruthenium complex—that bond to the titanium dioxide layer. The cathode is simply a glass plate coated with a platinum film to act as a catalyst. Between the two electrodes is an electrolyte solution.

As the name suggests, the dye—located in the anode—is key to the current generating mechanism. When the light shines on a DSSC, the dye becomes excited, causing its electrons to shift from a ground state to an excited state. This higher energy enables the dye to overcome the semiconductor’s bandgap, whereupon it becomes oxidized, and an electron is released into the conduction band of the semiconductor. This causes the semiconductor to become conductive, and a current is generated. The electronic balance of the cell is resorted by the electrolyte molecules donating an electron to the dye, where the dye transforms back into a non-excited electronic ground state. The electrolyte is regenerated to its normal electronic state through a reduction reaction at the cathode.

Quantum dot solar cells are not as widely developed as other thin film solar cells, but their interest is growing. Quantum dots are 0D materials (electrons are quantumly confined in all three directions), which are only a few nanometers in size. The size and quantum properties of quantum dots mean they have unique optical absorption and emitting properties. One of the key reasons for using quantum dots is their tunable bandgap. Because they are semiconducting in nature, they work like traditional inorganic semiconductors, but due to the small size of each quantum dot, they essentially act as a multi-junction solar cell.

The tunable bandgap means that they can also be tuned to absorb radiation at many different wavelengths of the electromagnetic spectrum. As it stands, their efficiencies are much lower than other solar cells; however, there is great potential for these devices. Quantum dots are the only type of material used in solar cells that can release more than one electron for each photon absorbed. All other materials have a 1:1 ratio, so quantum dots could potentially increase the conversion efficiency significantly by releasing more electrons for each absorbed photon of light.

Even though inorganic solar cells remain the most common, there are many different types of solar cells out there. Many of the other solar cells are not as efficient, but what they lack in efficiency, they make up for in other properties. One key driver of using non-inorganic materials is that they can be made much thinner, are more flexible, can be optically transparent, and in some cases, printable. The ability to use other materials enables solar cells to be implemented on parts of buildings—such as in windows or on curved architectures—that cannot be covered using traditional inorganic solar cells. This greatly expands the capabilities of solar cells as a class of renewable energy devices and makes them much more versatile. And, like many things, the chemistry of the materials used makes this possible.