An Overview on P3HT:PCBM, the Most Efficient Organic Solar Cell Material So Far
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An 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 Cu2S/CdS was developed at the University of Delaware with efficiency of 10%. In1991, the Solar cell or Photovoltaic cell (or PV cell first dye-sensitized cell was invented. In 1994, for short), is the device that converts the there came the first cell that exceeds 30% radiation of the sun to electricity. Every leaf of conversion efficiency using GaInP/GaAs. And a green plant does something similar --- they in 2006, the “40% efficient barrier” was convert sunlight to chemical energy. Actually, broken. Besides material research, much has a group of solar cells, the so-called “organic been done for increasing sunlight cells”, started by borrowing the idea from concentration, carrier collection, and cell leaves. Pigments (including chlorophyll[1]) stability[4]. Moreover, researches have gone were used to sensitize titanium-based beyond the inorganic world. Despite their low materials[1,2]. The importance of developing efficiency, the organic polymers have attracted efficient solar cells is obvious. The sun much interest. The poly(3-hexylthiophene) supplies us a clean and unlimited resource of (P3HT) and [6,6]-phenyl C -butyric acid energy, helping us relieve the energy crises 61 methylester (PCBM) blends is one of the and world pollution. The source is out of promising organic solar cell materials. It is the question while the approach to efficiently most efficient fullerene derivate based utilize it remains a challenge. donor-acceptor copolymer so far[5,6]. Ever since 1954, when the first modern P3HT:PCBM has reported an efficiency Si p-n junction solar cell is invented at bell as high as 5%, which is unusual in the organic lab[3], many attempts have been done looking cell material. Their structures are as shown for a high-efficiency low-cost solar cells, (Fig 1). PCBM is a fullerene derivative. leaving several significant milestones. In 1970, Because of high hole mobility, it plays the role Zhores Alferov’s team at USSR developed of electron acceptor in many organic cells. first highly effective GaAs heterostructure cell. P3HT is among the Polythiophene family, In 1980, the first thin-film cell using 1 Fig 1: chemical structure of PH3T and PCBM which is a kind of conducting polymer. It is should have a minimum energy. Then an the excitation of the π-orbit electron in P3HT electron would be exited to a higher energy that gives the photovoltaic effect in the blend. state. For example, in alkali metals, the metal [7] got ionized when incident by (UV) beams, and “photoelectric” current is produced. General Principle “Photovoltaic” materials operates in a similar Here I’ll briefly introduce the way. For example, in a semiconductor, when microscopic working steps of a solar cell as the incident photon has an energy , an well as its macroscopic parameters such as electron in the valence band would be excited short circuit I SC , open circuit voltage V OC , Fill to the conducting band, leaving a hole in the Factor FF, and characteristic resistance. valence band. This electron-hole pair is called exciton . In organic materials, equals to the Generally a solar cell operates through difference between energy of lowest four major steps. The first step is to absorb unoccupied molecular orbit (LUMO) and incident photons, which is affected by the highest occupied molecular orbit (HOMO). macroscopic surface property. Then, the Extra energy would be wasted in the electron-hole pair, the so-called exciton, is form of heat. produced. This is directly determined by the material’s band structure. The third step is Thus, an efficient solar cell should have a separation of the pairs, determined by the wide absorb spectrum, so as to create as many charge distribution inside the cell. And in the pairs as possible. A multiband material would end, the generated charge would be collected be a good choice. With smaller gaps, the cell at electrodes. The major factors of the can operate in the dark using infrared efficiency of a solar cell is the number of spectrum. On the other hand, larger gaps independent charge carriers produced through leaves little extra energy for UV photons, so the procedure. Thus, the second and third that the energy wasted in the form of heat is steps are mostly focused on. minimized. Additionally, low optical reflectance is desired. Why not choose an alkali metal as a solar (1) Exciton generation cell material, then, as they also produce charge The basic idea how the incident light carrier after illuminated? The reason is quite produces charge carriers seems easy to obvious: energy gap. In a band-structured comprehend. When a photon incidents on a material, electrons excited to conducting band certain material, it would be either scattered or or LUMO have to cross energy barrier before absorbed. For the latter case, the photon recombination with holes. In a metal where no 2 such gap exists, electrons and holes would discussed later. It’s easy to calculate the instantly recombine unless the metal is ionized. binding energy using hydrogen model, Besides higher energy requirement to ionize replacing the reduced mass m by reduced the metal, vacuum and external field supply is effective mass m* and electronic charge by desired to collect all the photoelectrons. In , where is the dielectric constant, we band-structured materials, on the other hand, got [9] potential difference naturally builds up when illuminated. So that’s a better material to convert solar energy to electricity, despite the (2) Exciton separation fact that surface resistivity would somewhat Separation of exciton is the major impair the production of current. It is the difference of various kinds of solar cell energy gap that mainly leads to the difference materials: semiconductors, dye-sensitized of “photovoltaic” materials and materials, and organic polymers. First “photoelectric” materials, the former of which difference lays in the binding energy. In we choose to make solar cell. semiconductors where is large, ~ 0.1 “Photovoltaic effect” is more than just eV, the pair is easy to separate. Meanwhile, photoelectric effect, although the physics for the change in the band structure is trivial. In photon-charge transmission is the same. dye-sensitized materials, the electron is Experiments show that photovoltaic potential excited to the conducting band, leaving the is only proportional to light intensity at low dye molecule in an ionized state. The ionized intensity. With the increase of the intensity, it dye is regenerated by electron transfer from would then go up in a logarithm behavior then the surrounding environment[10]. In other reaches a steady maximum. On the other hand, words, the dye-sensitized material separates simple photoelectric effect goes directly the pair by extracting the hole with electron proportional to light intensity[8]. This might supplied from the environment. In organic be the result of a limited charge carrier supply. polymers, however, things have been different. Moreover, photoelectric cells goes to higher Dielectric constants are small [11]. This potential when illuminated while photovoltaic shifts the absorption spectrum to longer cells can have reversed polarity[8], which wavelength but brings difficulties to separate means not only electrons, but also holes, can excitons. be the charge carriers. Furthermore, to Excitons can be separated if they meet produce a photoelectric current needs external with electric field within their diffusion range potential, which is not necessary for ( ≤ 20nm[12]). The internal field exists in the photovoltaic cells[8], indicating the existence vicinity of junctions, the boarder where two of internal field in photovoltaic cells, which regions meet. It is the result of thermal usually occurs in the vicinity of junctions and equilibrium of the contacted materials. When are essential to exciton separation. two materials with different work functions The actual energy to create an exciton is were brought into contact, electrons would actually less then , considering the fact that flow from the one with lower work function to electron and hole are paired through screened the other until the Fermi surface matches. This Coulomb interaction instead of none. On the builds up an internal field near the contact other hand, we have to supply this energy to surfaces. Here’s some typical junctions. separate the excitons, which would be Schottkey junctions occurs at the 3 interface of metal and semiconductors, or However, due to lattice distortion at the between metal and p-type organic polymers. interface, in real cases it becomes more Due to high resistivity of organic polymers, complicated. Heterostructures have many only the excitons created within the diffusion interesting properties, like superinjection and range from the interface can be separated. window effect. The former increases carrier number and the latter prevents photon Most common solar cells were based on absorption at surface defects, both largely the single p-n junctions, which is the interface improving the photoelectron productivity [13]. of similar semiconductors. This can be In organic materials, the most basic achieved by doping one single crystal with heterostructure is a bilayer of donor and different dopants diffused from each end.