1An overview on P3HT:PCBM, the most efficient organic solar cell material so far. Name: Ge, Weihao Email: [email protected] Solid State Physics II, Spring 2009 Instructor: Elbio Dagotto Abstract It is not news that organic polymers have been used as solar cell materials. They have a couple of advantages over conventional semiconductors. However, their efficiency remains relatively low due to limited absorption spectra and poor charge mobility. Among these materials, fullerene derivates shows great potential being effective electron acceptor. A combination of narrow-band donor and fullerene derivate is a possible approach to efficient organic cells, including the most efficient organic cell P3HT:PCBM. Here I’ll discuss the ways to increase solar efficiency and give a brief description of efforts made for P3HT:PCBM. key words: organic solar cell, P3HT:PCBM Introduction Solar cell or Photovoltaic cell (or PV cell for short), is the device that converts the radiation of the sun to electricity. Every leaf of a green plant does something similar --- they convert sunlight to chemical energy. Actually, a group of solar cells, the so-called “organic cells”, started by borrowing the idea from leaves. Pigments (including chlorophyll[1]) were used to sensitize titanium-based materials[1,2]. The importance of developing efficient solar cells is obvious. The sun supplies us a clean and unlimited resource of energy, helping us relieve the energy crises and world pollution. The source is out of question while the approach to efficiently utilize it remains a challenge. Ever since 1954, when the first modern Si p-n junction solar cell is invented at bell lab[3], many attempts have been done looking for a high-efficiency low-cost solar cells, leaving several significant milestones. In 1970, Zhores Alferov’s team at USSR developed first highly effective GaAs heterostructure cell. In 1980, the first thin-film cell using Cu2S/CdS was developed at the University of Delaware with efficiency of 10%. In1991, the first dye-sensitized cell was invented. In 1994, there came the first cell that exceeds 30% conversion efficiency using GaInP/GaAs. And in 2006, the “40% efficient barrier” was broken. Besides material research, much has been done for increasing sunlight concentration, carrier collection, and cell stability[4]. Moreover, researches have gone beyond the inorganic world. Despite their low efficiency, the organic polymers have attracted much interest. The poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methylester (PCBM) blends is one of the promising organic solar cell materials. It is the most efficient fullerene derivate based donor-acceptor copolymer so far[5,6]. P3HT:PCBM has reported an efficiency as high as 5%, which is unusual in the organic cell material. Their structures are as shown (Fig 1). PCBM is a fullerene derivative. Because of high hole mobility, it plays the role of electron acceptor in many organic cells. P3HT is among the Polythiophene family,2which is a kind of conducting polymer. It is the excitation of the π-orbit electron in P3HT that gives the photovoltaic effect in the blend. [7] General Principle Here I’ll briefly introduce the microscopic working steps of a solar cell as well as its macroscopic parameters such as short circuit ISC, open circuit voltage VOC, Fill Factor FF, and characteristic resistance. Generally a solar cell operates through four major steps. The first step is to absorb incident photons, which is affected by the macroscopic surface property. Then, the electron-hole pair, the so-called exciton, is produced. This is directly determined by the material’s band structure. The third step is separation of the pairs, determined by the charge distribution inside the cell. And in the end, the generated charge would be collected at electrodes. The major factors of the efficiency of a solar cell is the number of independent charge carriers produced through the procedure. Thus, the second and third steps are mostly focused on. (1) Exciton generation The basic idea how the incident light produces charge carriers seems easy to comprehend. When a photon incidents on a certain material, it would be either scattered or absorbed. For the latter case, the photon should have a minimum energy. Then an electron would be exited to a higher energy state. For example, in alkali metals, the metal got ionized when incident by (UV) beams, and “photoelectric” current is produced. “Photovoltaic” materials operates in a similar way. For example, in a semiconductor, when the incident photon has an energy , an electron in the valence band would be excited to the conducting band, leaving a hole in the valence band. This electron-hole pair is called exciton. In organic materials, equals to the difference between energy of lowest unoccupied molecular orbit (LUMO) and highest occupied molecular orbit (HOMO). Extra energy would be wasted in the form of heat. Thus, an efficient solar cell should have a wide absorb spectrum, so as to create as many pairs as possible. A multiband material would be a good choice. With smaller gaps, the cell can operate in the dark using infrared spectrum. On the other hand, larger gaps leaves little extra energy for UV photons, so that the energy wasted in the form of heat is minimized. Additionally, low optical reflectance is desired. Why not choose an alkali metal as a solar cell material, then, as they also produce charge carrier after illuminated? The reason is quite obvious: energy gap. In a band-structured material, electrons excited to conducting band or LUMO have to cross energy barrier before recombination with holes. In a metal where no Fig 1: chemical structure of PH3T and PCBM3such gap exists, electrons and holes would instantly recombine unless the metal is ionized. Besides higher energy requirement to ionize the metal, vacuum and external field supply is desired to collect all the photoelectrons. In band-structured materials, on the other hand, potential