Recent developments in the synthesis of organic semiconductors (π-conjugated molecules and polymers) with improved luminescence and charge transport properties have made these materials attractive for developing various optoelectronic devices. Devices of interest are photovoltaic plastic solar cells organic light emitting diodes (OLEDs), polymer lasers, chemical/biosensors etc. Other materials e.g. nanoparticles of inorganic semiconductors and nanotubes/fullerenes also provide opportunity to develop novel photonic devices. The combination of both these materials i.e. polymers and nanoparticles as organic-inorganic composites further expand the application area. At RRCAT we are engaged in basic R & D of these polymer based prototype devices and develop optical probes to characterize optoelectronic properties of these materials. Followings are highlights of the activity at RRCAT:

1. Material development

   (a)   Nanomaterials:

   Metallic and semiconducting nanoparticles have been synthesized using wet chemical technique. Materials capped with suitable organic ligands showed better stability as compared to their uncapped counterparts.  Mechanism of phase transfer catalyst has been demonstrated to transfer some of these nanoparticles from aqueous to non aqueous medium.
    
   Following nanomaterials in colloidal form have been synthesized :
   colloidal gold (Au), Silver (Ag), cadmium sulphide (CdS), Lead
   Sulphide (PbS), Cadmium selenide (CdSe) and Lead iodide (PbI2).

   (b)  Organic/Polymer Materials:
   
   Conjugated organic molecules with donor and acceptor end groups   such as Chalcones, P-Bis-benzylidine cycloalkanones (BBC) and their derivatives,  have been synthesized. Second harmonic generation efficiency of these materials has been measured by Kurtz powder technique.   Improved synthesis and fabrication techniques of   Organic/Inorganic hybrid composites are being developed for organic solar cells applications, higher harmonic generation etc.

2. Device Fabrication
   
   

   Organic thin films, for making devices in sandwich/planar geometries on patterned ITO/FTO coated glass plates, are made using spin casting and thermal evaporation (and Knudsen cell). Film thickness and roughness of organic thin films is measured using KLA-Tencor surface profiler. Optical and electrical characterizations of devices/organic thin films are routinely carried out by source meter unit, LCR meter, transient photoconductivity, linear optical absorption, steady state and time resolved photoluminescence etc.

3. Charge transport in organic films

   A better insight to charge transport mechanism in disordered organic thins films is highly demanded for the design and development of more efficient organic optoelectronic devices. A unified model of charge transport explaining the various charge transport characteristics observed in this class of materials is yet to be developed. A detailed and systematic study on electron and hole mobility as a function of field, temperature, morphology of thin films and other parameters can certainly provide valuable information on the mechanism of charge transport. Current-voltage measurements, transient photoconductivity/time-of-flight (TOF) technique, nonlinear optical (NLO) techniques etc are commonly employed to measure mobility as function of various parameters.

   (a)  Transient photoconductivity or time-of-flight measurement

   Transient photoconductivity or time-of-flight (TOF) technique has been widely used to measure carrier mobility in organic thin films. We have measured hole mobility in N,N’-diphenyl-N,N’-bis(3-methylphenyl)-(1,1’-biphenyl)-4,4’diamine (TPD) doped  in polymers (Polycarbonate (PC) or Polystyrene (PS)) as a function of  field and temperature. Structure of the device used is FTO/TPD:PC/a-Se/Al. Figure 1 show the typical TOF set-up and device structure used.

   
   
   Figure 1

   Measured mobility data are analyzed on the basis of Gaussian disorder model (GDM) of hopping transport. Model predicts that charge transport in disordered organic thin films occurs by means of hopping among localized sites that are subjected to disorder, i.e. both energetic and positional disorder. Our studies assert that these disorder parameters depend largely on the film morphology. We have observed both positive and negative field dependence of mobility upon varying dopant concentration and temperature. Observed results are explained on the basis of low energetic disorder present in the sample due to different morphology of the thin films used in our study. Investigation on morphology revealed large aggregation / crystallization of TPD dye in polymer film. Aggregates of TPD produces more ordered regions which result in low energetic disorder in investigated thin films.

   (b)    Monte Carlo Simulations:

   Monte Carlo simulation of charge transport based on hopping transport is carried out to get better insight on transport mechanism and the effect of film morphology on charge transport in disordered organic systems. Hopping of charge carrier from one site to another is controlled by Miller-Abhrahams equation,

   

   where E is the applied electric field, a is the intersite distance, k is the Boltzmann constant, T is the temperature in Kelvin,is the distance between sites i and j and  is the wave function overlap parameter which controls the electronic exchange interaction between sites. Under the action of applied electric field carriers are allowed to move randomly across the film thickness. Carrier transit time (t) is calculated by averaging over several hundred carriers and is used to calculate mobility using drift mobility equation. Simulation is carried out for different lattice morphologies and also for various field and temperature. Our simulation study also emphasizes the strong influence of morphology on thin films; especially more insight is obtained on the role of energetic disorder on charge transport. Observed experimental results and explanation provided on the basis of different morphology of investigated thin films (aggregation of TPD which results in low value of energetic disorder) is well supported by our simulation studies.

   (c)    Effects of UV light exposure on properties of TPD dye

   For developing organic light emitting diodes the films of  N,N-diphenyl-N,N-bis(3-methylphenyl)-1,1-biphenyl-4.4 diamine (TPD) dye are used very extensively both as hole transport and emissive layers. We have observed improvement in hole injection across metal-organic interface when, prior to film casting, the dye solution (TPD in halomethane solvents like chloroform and dichloromethane) is first exposed to low power UV light. With detailed spectroscopic investigations we have shown that UV light essentially oxidizes TPD in halomethane solvents to generate TPD cations (TPD⁺). Generation of TPD⁺ is referred as p-type(hole) doping of TPD. With increasing UV exposure time an increase in doping concentration and subsequent increase in hole injection efficiency across anode-TPD film interface is observed.  We have shown photo-oxidation of TPD as novel technique of doping TPD. 

   Large effect of UV exposure is seen as substantial changes in luminescence property of TPD. It is known that TPD is a blue emitting dye.  When exposed to UV for longer duration ( ~ few hours) emission changes from blue to green (as shown in figure 2 below). Detailed investigations on photoluminescence properties suggested that the origin of green emission is result of excited state intermolecular interaction amongst the photodegraded products of TPD.  Photodegraded product is observed to be very stable and show large changes in emission color with solvents of different polarity. Further investigations provided evidence of transformation of TPD to acridine based dye as photoproduct. Excimer emission from acridine based photoproduct is the source of green emission.

   

   (2)   Polymer based solar cell

   Photovoltaic device rely upon the dissociation of photogenerated exciton to free carriers with long lifetime. In organic semiconductors (molecular films and polymers) the exciton binding energies are high compared to inorganic counterparts. Therefore to dissociate excitons and separate electrons and holes one makes use of mixtures of electron donating and electron accepting materials. Suitable combination of both donor and acceptors is chosen to maximize photogeneration efficiency and higher carrier mobility.  Improved efficiency requires both the development of materials with superior properties (light absorption, mobility etc.) and an understanding the device physics of polymeric solar cells. 

   To develop a prototype polymer solar cell device we chose combination of organic semiconducting polymer and inorganic QDs as active material.  Polymer MEH-PPV was chosen as donor material and PbS QDs as acceptor. The signature of dissociation of exciton at interface of polymer and QDs is seen as reduced  or quenched PL emission from polymer. We made comparative study of photoluminescence quenching behavior of polymer (MEH-PPV) using in-situ generated and ligand capped PbS nanoparticles. The current technique of fabricating ligand capped nanoparticles:conjugated polymer composite rely upon first synthesizing ligand capped nanoparticles separately, and then mixing them with conjugated polymer. We have observed that this technique is less efficient in quenching polymer PL as compared to the in-situ generated nanoparticles in the polymer itself.  Nanoparticles grown in-situ in the polymer directly provide close contact of two entities that not only eliminates the need for an initial ligand to terminate the nanoparticles growth but it facilitates the efficient charge transfer as well.  Here polymer itself acts as a ligand to control the nanoparticle size. Efficient charge transfer from polymer to PbS is reflected in reduced radiative recombination of electrons and holes i.e. strong photoluminescence quenching in in-situ approach.  Two techniques of fabricating polymer composite are being processed further in order to compare their current-voltage characteristics and charge generation efficiency. Figure below shows the emission quenching from polymer composites.

   

    

Recent Publications

 Journals

1. S. Raj Mohan and M. P. Joshi, “On the field and temperature dependence of hole mobility in TPD doped polycarbonate”,    Synth. Metals, 155, 372,(2005).
2. S. Raj Mohan and M. P. Joshi, "Field dependence of hole mobility in TPD doped Polystyrene" Solid State Commn., 139, 181(2006).   
3. S. Raj Mohan, M. P. Joshi, S. K. Tiwari, V. K. Dixit and T. S. Dhami, “Electrical and Optical characterization of Photooxidized TPD”,J. Mater. Chem, 17, 343 (2007).
4. M. P. Joshi et al, ”Broad-band visible emission from UV-exposed TPD solution”, ASID Digest, p-328, New Delhi,2006.
5. M. P. Joshi et al, “Enhanced Optoelectronic Properties of UV-light induced Photodegraded TPD”  Appl. Phys. A 90, 351 (2008).
6. S. Raj Mohan, M. P. Joshi and M. P. Singh, “Charge transport in disordered organic solids: A Monte Carlo simulation study on the effects of film morphology”, Org. Electr. 9,355 (2008).
7. S. Raj Mohan, M. P. Joshi, M. P. Singh,” Negative field dependence of hole mobility in TPD doped polymers”, Chem. Phys. Lett. 470, 279 (2009).
8. R. A. Ganeev,_H. Singhal, P. A. Naik, J. A. Chakera, A. K. Srivastava, T. S. Dhami, M. P. Joshi, and P. D. Gupta, “Influence of C60 morphology on high-order harmonic generation enhancement in fullerene-containing plasma”, J. Appl. Phys. 106, 103103 (2009).

Conferences

1. M. P. Joshi, A. Pandey and S. Rajmohan, “ Charge injection and transport in Tin Phthalocyanine thin films” Proc. DAE-BRNS National Laser Symp. p-232 (2003).
2. S. Raj Mohan and M. P. Joshi, “On the field and temperature dependence of hole mobility in TPD doped polycarbonate”, DAE-SSPS 2004, p-778, Amritsar, India.
3. Sanjiv K. Tiwari, M. P. Joshi and Sunil Kumar, "Second Harmonic Generation from Corona-Poled DR1-Doped PMMA Thin Films", Proc. NLS, p. 448 (2004).
4. S. Raj Mohan and Mukesh P. Joshi, “Charge Transport Studies on TPD Doped Polystryrene”, Symp on Cond. Matter & Mater. Phys, Vadodara, India.
5. S. K. Tiwari, S. Raj Mohan, M. P. Joshi and T. S. Dhami, ”Photo-induced Changes in Optical Properties of  TPD Solution, National laser symposium, Mumbai, India (2005).
6.  M. P. Joshi , S. Raj Mohan, S. K. Tiwari, V. K. Dixit  and T. S. Dhami, “Improved transport and injection of holes using photo-oxidized TPD”, Photonics-2006, Hyderabad.
7.  M. P. Joshi, S Raj Mohan, S. K. Tiwari, T. S. Dhami, T. Shripathi, U.P. Deshpande, M. K. Singh, H. Ghosh, “Broad-band visible emission from UV-exposed TPD solution” ASID Digest, New Delhi -2006.
8.  M. P. Joshi, S. Rajmohan, Beena Jain, T. S. Dhami and S. K. Tiwari, “On the origin of dual band emission from UV exposed TPD solution”, NLS, Indore, 2006.
9. S. Raj Mohan, M. P. Joshi and M. P. Singh, "Morphology and charge transport in disordered molecular solids: A Monte Carlo simulation study",  Presented at OP-2007, Turku, Finland.
10.  Haranth Ghosh, M. K. Singh, M. P. Joshi and S. Raj Mohan, "Optical absorption in neutral and cationic TPD: Theory and Experiment", Presented at OP-2007, Turku, Finland.
11. T. S. Dhami, M.P. Joshi, S.Raj Mohan and Shweta Mishra, “ Synthesis and characterization of polymer-Nanoparticle composites”, Proc. National Seminar on Photonic Polymers; Materials, Devices and Applications, BITS Pilani, Rajasthan,p-18,2008.
12. T. S. Dhami, M.P. Joshi, S.Raj Mohan and Shweta Mishra, “Fabrication and characterization of conjugated polymer based Nano-composite for solar cell applications” DAE-BRNS International Symposium on Materials Chemistry, BARC Mumbai, during Dec.2-6, p 424 (2008).
13. S. Raj Mohan, M. P. Joshi, T. S. Dhami and Manoranjan P. Singh, “Studies on the electrical and optical properties of conjugated organic systems for optoelectronic device application” Winter School on Chemistry and Physics of Materials, ICAS, Bangalore, December, 2009.