Bulk Heterojunction (BHJ) Organic solar cell technology may pave the way to a cheaper, greener, more accessible energy source worldwide. This unique organic solar cell is made from a liquid dye composed of self-assembling anode and cathode layers which can be rolled into a flexible plastic polymer. Once dried, these lightweight plastic sheets are easily fastened to any surface. With one of the smallest manufacturing carbon footprints in green energy technologies to date, BHJ organic solar cells offer a truly sustainable energy source with nearly zero degenerative impact to the environment.
BHJ Organic Solar Cell Mechanics
The BHJ organic photovoltaic device is very similar in principle to common p-n junction photovoltaic devices, such as crystalline silicon. In a p-n junction device, at least two semi-conductive materials are formed together, one material being rich in electrons (p-type) and the other being electron deficient (n-type). When a photon of light strikes the device, an electron is freed from the material via photovoltaic effect and will leave behind a positively charged region called a “hole.” The electron is immediately attracted back to the electron deficient region of the material, but can be redirected towards those regions by using a circuit, which can then be hooked up to some load that the flowing electrons can power. In a bulk heterojunction device, two organic polymers are mixed together which create heterozygous junction regions in the photoactive portion of the device and are spread between two conductors. One organic polymer acts as an electron donor and the other as an electron acceptor. Often times these two materials are artificially “doped,” which means another material is added that makes the donating or accepting materials even more likely to donate or accept electrons. When a photon of light is absorbed by these materials, a species known as an exciton is formed and simultaneously ejects its electron, leaving behind an electron deficient region called a “hole.” Electrons in the excitons of organic compounds are excited into the highest occupied molecular orbital (HOMO), leaving its positively charged hole counterpart in the lowest unoccupied molecular orbital (LUMO). Electrons in the HOMO level freely move to adjacent exciton HOMO levels throughout the material, while holes in the LUMO levels move to adjacent exciton LUMO levels counter to electron flow. Since electrons must have the ability to jump from π-orbitals to π*-orbitals, it is no surprise that the best organic photovoltaic materials are highly conjugated polymers, such as fullerene, having the ability to delocalize a negative charge over a larger area, giving more stability to the negatively charged compound. Finally, Like that of a p-n junction device, the flowing electrons in a bulk heterojunction device can be diverted from their path toward holes via circuit which can then be used to power some load.
Despite the exciting advantages of organic solar cells, there are still many hurdles to consider. One problem is that the excitons produced from polymers have a poor traveling distance, only reaching ~15nm before recombining with a hole site. This issue requires use of extremely thin films to reduce the distance necessary for an electron to travel , however, thin films are unable to absorb as much light as other inorganic solar cells. Another major problem researchers of organic photovoltaic devices currently face is the conductive potential of the materials being used. Organic polymers have thus far proven themselves highly inefficient in energy conversion when compared to inorganic devices because they cannot produce as many free electrons as inorganic compounds can. A new improvement to the organic solar cell has been the addition of a quantum dot layer.
Quantum Dot Enhancers Enter the Scene
Quantum dots are small nanoparticles between 1nm and 10nm in diameter which have unique properties that help boost the electron density in the photoactive region and the overall energy conversion efficiency of the device. The quantum dots’ ability to do so is due to a phenomenon known as “quantum confinement,” which describes an exciton’s behavior when a particle’s energy bands are spread across a limited amount of space due to the particle’s small shape and size. Quantum confinement occurs in particles whose diameters range between 1 and 10nm where its corresponding electron wavelength is the same size or larger than the particle’s diameter (a distance known as the exciton Bohr radius). When a particle is this small, the bandgap distance between its valence bands and conductance bands increase, requiring more energy to excite its electrons. The result is a blue shift in observable emissions spectra of smaller particles, and a red shift corresponding to the lower energy requirements and smaller bandgaps of larger particles. This unique quantum mechanical property allows more exciton reactions per photon than macro-sized particles, theoretically achieving a 7:1 exciton to photon ratio. Quantum dots can be added to bulk heterojunction materials in order to increase the number of exciton per photon reactions, producing more free moving electrons and a higher energy conversion efficiency in the module. In order to incorporate quantum dots into an organic heterojunction, the bandgap energies between organic conductors and quantum dots must be compatible with one another so that they may undergo electronic charge transfer when excited. If the two materials can undergo electronic charge transfer, an electron from the quantum dot layer will be able to move toward the polymer’s outer orbitals and contribute to its HOMO-LUMO interactions. The heterojunction organic material would be spread throughout the quantum dot layer so electrons could transfer to the polymer and flow into the conductive material (titanium oxide). Many quantum dot/polymer pairs are currently being studied to increase electron efficiency in organic solar cells by varying the shape, size, concentration, and constitution of the quantum dot electron donors.
There is much debate over the potential of photovoltaic solar cell technologies as a practical answer to the energy crisis. While many research teams around the world remain hopeful, energy conversion efficiencies still remain low, the consumer cost too great, and much more research is needed if any viable product is to act as an agreeable alternative energy resource. Organic photovoltaic solar cells are the cheapest to manufacture, but to date maintain some of the lowest energy conversion efficiencies which range from 3%-8.1%.
|Photovoltaic Device Type
||Device Efficiency (%)
|Single Crystal GaAs
|Single Crystal Si Cells
|Multicrystal Si Cells
|Cu(In, Ga)Se2 Thin Film
|CdTe Thin Film
|Amorphous Si:H Thin Film
While organic photovoltaics are at the bottom of the list in terms of efficiency, it should be noted that these technologies are among the newest and have been researched the least by comparison. Because organic photovoltaic solar cells are the first to offer a real solution to manufacturing costs as well as the added potential to cover any surface imaginable due to their pliability, scientists have found no difficulty in justifying continued research efforts despite low energy conversion efficiencies. The addition of quantum dot enhancers offers new hope and direction in the future of organic photovoltaics, and possibly a solution to their current shortcomings.