Organic Semiconductors (small molecules)

Team Members: (Dept. of Physics, Indian Institute of Technology Kanpur, India)
Prof. Satyendra Kumar
Dr. Sanjay K. Ram
Dr. Vivek Shukla
Mr. Girish Gupta

Organic semiconductors have shown immense potential in terms of their numerous technological applications which were earlier dominated by inorganic semiconductors. These applications started off with electroluminescent devices, but have since diversified to include electronic devices such as transistors as well. The electronic conductivity of these materials lies between that of metals and insulators, spanning a broad range of 10-9to 103 (Ωcm)-1.
Known organic semiconductors can be broadly classified into two groups on the basis of their molecular weight:
  • Conjugated polycyclic compounds of molecular weight less than 1000, and
  • Heterocyclic polymers with molecular weight greater than 1000.
Polymers are easily deposited as thin films on large areas making them valuable semiconducting materials. Nevertheless, they suffer from a major drawback in that they are not highly soluble in organic solvents, and they lose their mobility upon functionalization to enhance solubility. This has been a major driving force behind the research on small molecules as semiconductors. With small molecules semiconductors, it is possible to control charge transport in a simpler way by modification of various molecular parameters. For example, the ability of these molecules to pack into well-organized polycrystalline films leads to higher mobility compared to polymeric semiconductors.
The study of organic materials (small molecules) in our research group is focused on:
1. Material synthesis and purification
2. Thin film deposition of organic materials (synthesized & commercially available materials) by thermal evaporation technique on the glass/Si/quartz substrates.
3. Structural characterization: AFM, XRD, Spectroscopic Ellipsometry, FTIR
4. Optical Characterization: normal and transient photoluminescence (PL), photoluminescence excitation (PLE), transmissitance and refelectance.
5. Electrical characterization : Temperature dependent dark and photoconductivity on planar and sandwich configuration
6. Device fabrication:
  • Organic light emitting diodes (OLED)
  • Organic thin film transistors (OTFT)
  • Organic solar cells devices
7. Electroluminescence (EL) studies of OLED devices and transistor characteristic studies of OTFT
Organic Light Emitting Diode (OLED)
The device structure of OLED consists of several layers of organic materials sequentially deposited on glass substrate, each layer having a specific purpose that serves to enhance device quality and performance. The schematic representation of an ideal/standard OLED device is shown below.

We have explored several organic materials (small molecules) in our electroluminescent (EL) devices as emitting as well as electron transporting layer like Alq3, Znq2, Cr-doped Alq3, Inq3. The organic materials are usually susceptible to environmental aging and photo-oxidation, which influence their viability for commercial utility. Our studies demonstrate the effects of oxygen, light and environment on these organic materials to enhance the efficiency and lifetime of OLEDs. A simple device structure for these studies, along with the molecular structures of the materials used are shown below.

Organic Thin Film Transistor (OTFT)
Organic thin film transistors (OTFT) have made impressive progress over the past decade. Organic TFTs provide two principal advantages over TFTs based on inorganic semiconductors; they can be fabricated at lower temperatures, and potentially, at significantly lower cost. Low process temperatures, in particular, may allow organic TFTs to be integrated on inexpensive plastic substrates rather than glass. With field effect mobility and current on/off ratio values comparable to amorphous silicon, it becomes increasing likely that organic electronic devices will find use in broad area electronics applications. OTFTs are of interest for such a number of applications as pixel-access devices in active matrix displays, liquid crystal light valves of organic light emitting diodes, switching devices for logic gate memory arrays in smart cards, and low cost integrated circuits on flexible large area substrates.
We have fabricated OTFTs using Pentacene (C22H14), as the active material. Pentacene (C22H14) is a planar molecule composed of five benzene rings. Pentacene has a strong tendency to form molecular crystals. It forms well-ordered films that can be poly or single crystalline depending upon deposition and substrate conditions using vacuum evaporation even at low substrate temperatures. With Pentacene, ordered films are obtained when deposited by thermal evaporation at substrate temperatures as low as 0 οC. While the bulk electrical conductivity of acenes such as Pentacene is very low (~ 10-15 S/cm), Pentacene has been found to have the highest mobilities for hole transport (p-channel). These devices have field-effect mobility as large as 2.2 m2/V-s, comparable to hydrogenated amorphous silicon TFTs. The upper limits in microscopic mobilities of organic molecular crystals, determined at 300 K by time-of-flight experiments, are falling between 1 and 10 cm2/V-s. The high mobility of pentacene is a result of significant orbital overlap from edge-to-face interactions among the molecules in their crystal lattice. The performance of OTFT depends on orientation of molecules, crystal structure, morphology, grain size and defects. In our study we tried to tune these parameters by changing the deposition parameters like nature of substrate (or surface treatment), substrate temperature, deposition rate, film thickness etc during deposition.
OTFT Fabrication
Once the gate oxide is made over Si there are just two possible structures for the source-drain contacts. One is called bottom gate-bottom contact (BG-BC) or bottom electrode TFT design, where drain and source contact metal is patterned on the gate dielectric prior to the active layer deposition. The other is named as bottom gate-top contact (BG-TC) or top electrode TFT design, where both source and drain pads are deposited on the top of an active layer through a shadow mask. The schematic cross sectional view of these structures is as shown below.