The device we described today is the organic solar cell or organic photovoltaic cell or OPV. The photovoltaic effect is the exact reverse of electroluminescence. That is, it is when light energy Is converted into electrical energy. The idea of using organic solids for conversion dates back to the 50s. But for decades the efficiency of organic cells has remained desperately low. It was only in 1986 that Ching Tang, the very person that invented the two layer OLED one year later, reported on a double-layer organic solar cell that could convert light to electricity with an efficiency of 1%. By that time, the power conversion of silicon solar cells was already around 20%. Then, came two major breakthroughs from the University of California at Santa Barbara. First, the elucidation of the mechanism at the origin of the improved efficiency of the two layer structure, namely, a fast electron transfer, and then, the synthesis of a fullerene-based polymer by Fred Wudl, that led to the bulk heterojunction solar cell. Here we show the current-voltage characteristic of the the cell by Chin Tang. The cell itself is a two layered diode that associates an n type and a p type semiconductors. As will be explained later, the two layers now refer to as an electron acceptor and donor. Here, the donor is copper phthalocyanine and the acceptor a perylene derivative. Light come to the cell through a transparent tin oxide electrode. In conventional solar cells, a photon of energy higher than the energy gap promotes an electron from the top of the Valence bands to the conduction band. This electron is next injected into an external circuit, and then returns to the Valence band, meanwhile generating electrical energy. All in all, light directly creates electrical charges. However, in an organic solar cell, there is an additional step. Light do not directly creates electrical charges. Instead it creates an exciton, which in turn must be converted into an electron and a hole. The issue with organics semiconductors that the excitons are tightly bound, that is, the binding energy is around 1 electric volt, so excitons are difficult to split into electrical charges. The way towards high efficiency opened in the 1990s by two major breakthroughs. First was the discovery of light induced electron transfer between an electron donor D and an acceptor A. The process can be decomposed into four steps. First, the donor is promoted to its excited state D star by the incident photon. Then, the excitation is delocalized between the donor and the acceptor. In the third step, electrical charge separation is initiated and is finalized in the last step, leading to the formation of an electron-hole pair. What is important now is that the lifetime of the delocalized exciton, also called charge-transfer exciton, is much shorter than that of the exciton. In the process described earlier, the acceptor was Fullerene, a bulky molecule made of 60 carbon atoms arranged in a sphere. However, fullerene is insoluble in most organic solvents, and hence difficult to implement. So the second important discovery was the synthesis of a soluble fullerene derivative called PCBM. The operation of the organic solar cell can now be visualized as follows. A photon of energy higher than the optical gap hits the donor and creates an exciton. This exciton can migrate to the donor-acceptor interface through thermal diffusion. When the exciton reaches the interface, the electron drops in the LUMO level of the acceptor, while the hole remains in the HOMO level of the donor. This leads to the formation of the charge transfer exciton, with a reduced binding energy. The next step is charge separation and extraction, which occur under the effect of the built-in electric field in the diode, due to the different work functions of the anode and the cathode. The direction of the field makes both electrons and holes to be extracted and collected at the electrodes. The performance of the first two layer OPVs was limited by the fact that the excitons that can reach the interface are those created at a distance corresponding to their diffusion length. The exciton diffusion length is typically around 10 nanometers; that is, only one tenth of the typical thickness of the donor layer. So, only less than 10% of the excitons could be retrieved. The efficiency of the cells substantially increased with the so-called bulk hetero-junction geometry, when the donor and the acceptor are blended through the thickness of the diode. Here, the donor is in green and the acceptor in orange. Because excitons are electrically neutral, they are insensitive to the electric field, and can defuse in any direction. Here we see that through a horizontal movement, the exciton can easily reach the donor acceptor interface. Besides, electrons and holes are electrically charged and preferentially move along the electric field. Moreover, the holes only move into the donor and electrons in the acceptor. So in the bulk hetero-junction the movement of excitons and electric charges are disconnected. The electrical power provided by solar cell is estimated from its current-voltage curve. A solar cell is basically a diode, and its current-voltage curve in the dark follows the characteristics of a diode, that is, no current under reverse bias and a current that rapidly increases under forward bias. When the diode is illuminated, a photo current adds to the dark current, and the curve shifts along the y-axis. At zero current, one has the open-circuit voltage, Voc, and at zero voltage, the short-circuit current, I sc. The electrical power supplied by the cell is given by the product of the current times the voltage; that is, the area of the orange rectangle in the figure. More precisely, the output power is calculated at the point where the area is at its maximum; that is, at the potential V max and current I max. Mathematically speaking, the maximum delivered power is given by P max equals V max times I max, or alternatively, the open circuit voltage times the short circuit current multiplied by the fill factor, FF. The power conversion efficiency is given by the ratio between the maximum delivered electrical power to the incident light power. To date, the best organic solar cells have a power conversion efficiency of slightly above 10%. This is twice lower than the efficiency of commercial silicon cells. So currently, applications and envisioned in domains where the compatibility of organic device is plastics is at work, for instance, for the fabrication of light, flexible and easily portable cells. Thank you for your attention.
