A STUDY OF ORGANIC SEMICONDUCTOR POLYMER MATERIAL AND DEVICE STRUCTURES FOR APPLICATION IN OPTICAL DETECTORS. By SHEETAL LILADHAR BARAI

Transcription

1 A STUDY OF ORGANIC SEMICONDUCTOR POLYMER MATERIAL AND DEVICE STRUCTURES FOR APPLICATION IN OPTICAL DETECTORS By SHEETAL LILADHAR BARAI DEPARTMENT OF ELECTRICAL ENGINEERING INDIAN INSTITUTE OF TECHNOLOGY KANPUR MAY, 2005

2 A STUDY OF ORGANIC SEMICONDUCTOR POLYMER MATERIAL AND DEVICE STRUCTURES FOR APPLICATION IN OPTICAL DETECTORS A Thesis submitted In partial fulfillment of the requirements for the degree of Master of Technology By SHEETAL LILADHAR BARAI To The DEPARTMENT OF ELECTRICAL ENGINEERING INDIAN INSTITUTE OF TECHNOLOGY KANPUR MAY, 2005

3 CERTIFICATE This is to certify the work contained in the thesis entitled A Study of Organic Semiconductor Polymer Material and Device Structures for Application in Optical Detectors by Sheetal Liladhar Barai has been carried out under our supervision and this work had not been submitted elsewhere for a degree. (Dr. Baquer Mazhari) (Dr. R. S. Anand) Department of Electrical Engineering Indian Institute of Technology Kanpur MAY, 2005

4 Abstract In this work, polymer photo-detectors having good electrical and optical characteristics have been demonstrated. It is shown that pure MEHPPV, an electroluminescent material can be used as active material in organic photodetector. Further, devices fabricated using blends of MEHPPV with PCBM as photoabsorbing layer has an order of improved photoresponse with respect to the device having only MEHPPV as active layer. The optimization of the processing conditions and change of device structure has been done in order to get good quality devices. It is shown that the use of aromatic solvent leads to best results. The photoresponse in the device with polymer dissolved in 1-2 Dichlorobenzene is found to be better with maximum ratio of photo current to dark current as 29.9 at -2.2 V, where as the leakage current in the device with MEHPPV dissolved in Chlorobenzene is less. The thickness variation of the active layer is incorporated and it is observed that photo-response is better in the device with thinner active layer. The maximum ratio of the photocurrent to the dark current is in the thinner device that is at very low bias voltage of -0.8V.The leakage current is reduced to -9 x 10-8 A/ cm 2 as the active polymer layer thickness is increased. The device using blend of MEHPPV: PCBM in 1:1 proportion shows a very high ratio of photocurrent density to dark current density that is at a very low applied bias of - 0.6V. The physical demonstration of the photo-detector using MEHPPV: PCBM (1:4) as photoabsorbing layer using an OP-AMP photodetector circuit has been made. The response time of the detector at 680 Ω load was measured to be 450 ns and calculated capacitance value is nf.

5 Acknowledgements I am indebted to my Supervisor, Dr. R. S. Anand, for his help and advice during last one year. His generous support helped me throughout my work. His Advice was most valuable to understand the obtained results and to determine next steps for the work presented in this thesis. A special thanks goes to my Co-Supervisor, Dr. Baquer Mazhari, who was the first person to introduce me to the field of organic electronics. His enthusiasm motivated me to learn more about this interesting and emerging field of electronics. I am grateful to Dr. J. Narain and Dr. Asha Awasthi of Semiconductor Device Laboratory for all their help and support. I would also like to thank my colleagues Mr. Talari Manojaya, Mr. Ramesh and Dr. Anjali Giri, for their encouragements and support during the work. Further, I would like to thank all my fellow students for their co-operation and suggestions for the accomplishment of the work. And finally, I feel a deep sense of gratitude for my parents who taught me the good things that really matter in life.

6 Table of Contents PREFACE ACKNOWLEDGEMENTS LIST OF FIGURES LIST OF TABLES CHAPTER 1 INTRODUCTION 1 CHAPTER 2 BACKGROUND ON POLYMER PHOTODETECTOR 5 CHAPTER 3 FABRICATION OF PHOTODETECTOR Patterning Preparation of Mask Cleaning of ITO coated substrate Resist coating, Pre-bake and UV exposure Developing the resist and post baking ITO Etching Stripping of resist Cleaning Ozone treatment Coating of PEDOT-PSS layer & Vacuum Annealing Spin Coating of Active layer & Solvent removal Cathode Deposition Encapsulation 16 CHAPTER 4 POLYMER PHOTODETECTOR: SINGLE LAYER Introduction Principle of operation Experiments, Results and Discussion Optimizing processing conditions of the device Variation in Device structure Summary 34 iv vii xi

7 CHAPTER 5 POLYMER PHOTODETECTOR: DISPERSED (BULK) 35 HETEROJUNCTION 5.1 Introduction Fabrication details Principle of Operation Experiments, Results and Discussion An OP-AMP Photodetector Circuit Rise Time Measurement Summary 48 CHAPTER 6 CONCLUSION AND FUTURE WORK 50 BIBLIOGRAPHY 52 APPENDIX [A] 53 APPENDIX [B] 57

8 vii LIST OF FIGURES [1] Fig.2.1(a): shows how π bond is formed between two carbon atoms in a molecule 6 [2] Fig 2.2(b): shows when chain of carbon atoms comes together, π electron cloud is formed 6 [3] Fig.2.2: The organic polymer diode can be operated in various modes keeping the planer layered structure the same but by varying the biasing conditions. (a) LED mode, the forward bias is provided between the electrodes with electroluminescence as output. (b) Photodetector mode, the reverse bias is provided between the electrodes and the device is illuminated simultaneously to provide output voltage/current. (c) Photovoltaic mode, no bias is provided but the device is given irradiation to provide output voltage/current. 7 [4] Fig. 3.1: Basic Steps in Fabrication of Polymer Photo Detector. 9 [5] Fig. 3.2: Process of transferring mask features onto the ITO coated substrate 10 [6] Fig.3.3: Design of mask transferred on photographic plate 11 [7] Fig.3.4: ITO coated Glass substrate before patterning 12 [8] Fig.3.5: Photo Resist coated ITO substrate. 12 [9] Fig.3.6: Hardened Photo resist after developing. 13 [10] Fig.3.7: Etched ITO substrate 13 [11] Fig.3.8: Resist stripped and patterned ITO substrate. 13 [12] Fig.3.9: Profile of ITO obtained from Alpha Step 500 Surface profiler 14 [13] Fig.3.10: PEDOT-PSS coated patterned ITO substrate. 15 [14] Fig.3.11: Vacuum annealed Polymer layer on patterned ITO substrates 15 [15] Fig.3.12: Cathode deposited on the substrates 16 [16] Fig.3.13: Glass Encapsulated ITO/PEDOT-PSS/Polymer Active Layer/Ca/Al Device 17 [17] Fig.3.14: Structure of Photodetector 17 [18] Fig.4.1: Chemical Structure (the picture shows one repeat unit of polymer) and energy level diagram of MEHPPV 18 [19] Fig. A: Structure of single layer polymer photodetector 19

9 viii [20] Fig. 4.2: Energy level diagram. Upon irradiation an electron is promoted to LUMO leaving a hole behind in HOMO. Electrons are collected at Al electrode and holes at the ITO electrode. Φ: workfunction, χ: electron affinity, IP: ionization potential, E g : optical band gap.χ MEHPPV = 2.8 ev, IP MEHPPV = 5.2 V, Φ ITO = 4.9eV, Φ Ca = 2.9eV, Φ Al = 4.3eV,Φ PEDOT-PSS = 5.1 ev and Eg MEHPPV = 2.4 ev. [21] Fig. 4.3: Absorption and Emission spectra of conjugated polymer MEHPPV,Courtesy: Absorption maximum= 490nm [22] Fig. 4.4: Reverse J-V characteristics of ITO/PEDOT/MEHPPV/Ca/Al device. Chloroform as organic solvent. 22 [23] Fig 4.5(a): Reverse J-V characteristics of ITO/PEDOT/MEHPPV/Ca/Al device. 1-2-Dichlorobenzene as organic solvent. 23 [24] Fig 4.5 (b): The ratio of photo current density to dark current density versus applied voltage 24 [25] Fig 4.6(a): Reverse J-V characteristics of ITO/PEDOT/MEHPPV/Ca/Al device. Xylene as organic solvent. 24 [26] Fig 4.6 (b): The ratio of photo current density to dark current density versus applied voltage 25 [27] Fig 4.7(a): Reverse J-V characteristics of ITO/PEDOT/MEHPPV/Ca/Al device. Chlorobenzene as organic solvent. 25 [28] Fig 4.7 (b): The ratio of photo current density to dark current density versus applied voltage 26 [29] Fig.4.8(a): Aromatic Conformation 28 [30] Fig.4.8(b): Non- aromatic conformation 28 [31] Fig.4.9(a):Reverse J-V characteristics of ITO/PEDOT/MEHPPV/Ca/Al device with active layer thickness of 70-80nm 30 [32] Fig.4.9(b):Forward J-L-V characteristics of ITO/PEDOT/MEHPPV/Ca/Al device with active layer thickness of 70-80nm under forward bias. 30 [33] Fig. 4.9(c): The ratio of Photo current density to dark current density plotted versus the applied voltage 31

10 ix [34] Fig.4.10(a):Reverse J-V characteristics of ITO/PEDOT/MEHPPV/Ca/Al device with active layer thickness of nm [35] Fig.4.10(b):Forward J-L-V characteristics of ITO/PEDOT/MEHPPV/Ca/Al device with active layer thickness of nm under forward bias [36] Fig. 4.10(c): The ratio of Photo current density to dark current density plotted versus the applied voltage 32 [37] Fig 5.1: Chemical Structure and energy level diagram of PCBM 36 (HOMO LUMO = 2.4eV) [38] Fig. 5.2: In blended device interface is distributed all over the device. Figure shows one such interface. 37 [39] Fig.5.3: Absorption spectra of PCBM and C 60. Absorption maximum for PCBM = 284 nm & 341nm. Courtesy: 38 [40] Fig.5.4(a): Reverse J-V characteristics of the device whose active layer has MEHPPV and PCBM are mixed in 1:1 proportion by weight in Chlorobenzene. 39 [41] Fig.5.4(b) : The ratio of photo current density to dark current density versus applied voltage 40 [42] Fig.5.5(a): Reverse J-V characteristics of the device whose active layer has MEHPPV and PCBM are mixed in 1:2 proportion by weight in Chlorobenzene. 40 [43] Fig.5.5(b) : The ratio of photo current density to dark current density versus applied voltage 41 [44] Fig.5.6(a): Reverse J-V characteristics of the device whose active layer has MEHPPV and PCBM are mixed in 1:4 proportions by weight in Chlorobenzene. 41 [45] Fig.5.6(b) : The ratio of photo current density to dark current density versus applied voltage 42 [46] Fig. 5.7: An OP-AMP Photodetector Circuit. PD: Photodetector, R 1 = 5kΩ, R 2 = 10MΩ, Bias = 6V. 44 [47] Fig.5.8 (a): Light is incident on the polymer photodetector (on left) and the organic polymer LED s are being driven through that using op-amp circuit. [48] Fig. 5.8(b): Light is incident on the polymer photodetector (above right) and inorganic LED s are being driven through that using op-amp circuit 45 45

11 x [49] Fig. 5.9: Rise time measurement setup 46 [50] Fig.5.10 (a): R = 680 ohms, Fig.5.10 (b): R = 2.7k ohms, Fig.5.10 (c): R = 6.2k ohms, Fig.5.10 (d): R = 12k ohms. 46 [51] Fig. 5.11: Rise time Versus Resistance characteristics [52] Fig : A.1 Typical band structure (a) Before ozone treatment ITO( ~4.7 ev) & HOMO of MEHPPV( ~5.2 ev) (b) After ozone treatment ITO (~4.9eV) & HOMO of MEHPPV (~5.2eV) [53] Fig: A.2 (a) when PEDOT-PSS layer is not present, can lead to short (b) PEDOT-PSS layer is present, thus avoiding short 55 [54] Fig: A.3 Typical band structure showing alignment of workfuction with the addition of PEDOT PSS layer. 56 [55] Fig. B.1:Typical system configuration to characterize PLED (PD is photodetector). 57 [56] Fig. B.2: Typical system configuration to characterize photodetector. 58

12 xi LIST OF TABLES [1] Table 4.1: Comparison of the ratio of photo current density to dark current density of the various devices made using various solvent 27 [2] Table 4.2: Comparison of the devices with different active layer (MEHPPV) thickness 33 [3] Table 5.1: Comparison of the ratio of photo current density to dark current density of the various devices made using different proportions of the master solution of MEHPPV and PCBM. [4] Table 5.2: Rise time values of the photodetector for different values of resistance xi

13 1 Chapter 1 INTRODUCTION In recent times there has been intense activity in the field of organic electronics. The main driving force behind such an activity is the apparent capability of the organic materials to affect the various devices of commercial interests. The activity that started with the advent of polymer light emitting diodes (PLED s) [1], now spans over all the disciplines of semiconductor technology covering various devices for different applications [2]. The organic approach finds advantage over inorganic due to its mechanical flexibility, deposition over large substrate, ease of production and low cost. Not only organic small molecules but also the conjugated organic polymers have been studied extensively for the past several years due their potential applications in optoelectronics devices such as PLED s, photovoltaics, photodetectors, FET s & displays. Although Silicon technology has been the dominant technology in the above areas for the past several decades, they suffer from several limitations such as the need for a crystalline substrate which leads to inflexibility, the requirement for ultra-pure silicon wafer as a starting material which leads to high processing costs, limited color sensitivity in photovoltaic/photodetector applications etc. the organic molecules overcome all these limitations and provide a simple, efficient and yet inexpensive alternative for all the above applications.

14 2 The photodetectors/photovoltaic applications are especially the ones that are severely hampered by the prevailing inorganic semiconductor technological limitations. The present day photovoltaic devices are large and bulky and require a huge area for setup. Also, their absorption range is also limited due to their band-gap dependencies. The organic molecules, theoretically, can provide an option for co-evaporation/mixing of molecules which can provide an absorption range much superior than the inorganic materials. The capability to make an organic photo-device on the flexible substrates, make them an attractive option from both mechanical and economical aspects. Photodetectors are widely used in a variety of applications in fields like military, bio-medical, space, traffic, consumer electronics etc. Few of the most common applications of the photodetectors can be listed as: Optical scanners, Wireless LAN, Remote control devices, Automatic lighting controls, Color sensor element for Digital Camera, Flexible photodetectors for detection of optical field with non-planar wave-front, extended large area photodiode arrays for industrial automation, security sensing, night vision instruments etc. The first few applications like Optical scanners, Wireless LAN, Remote control devices, Automatic lighting controls, Color sensor element for Digital Camera, can be achieved using silicon as well as polymer detectors but for the Flexible photodetectors, extended large area photodiode arrays for industrial automation the applications, silicon is found unsuitable. This is because the maximum area possible till date with the silicon as substrate is 12 in diameter where as polymer thin films can very well be coated on infinitely long flexible plastic sheets and glass substrates.

15 3 Also, organic polymer material provide us a number of other advantages as well owing to their unique inherent properties in terms of their simple and inexpensive processability, availability of different band gap materials which provide a broad absorption range and modulation of absorption edge using different dyes. This means that polymer photodetectors having response in entire visible and near IR region are possible [3]. However, the area of organic electronics is still under intense study and requires a greater maturity in terms of understanding of various physical mechanisms as well as optimization of fabrication processes. In terms of photo detector applications, it is required that the photo detector device has a very high photo-sensitivity (high quantum efficiency), low leakage (dark) current, large dynamic range, fast response time and low noise. Out of all the above requirements, the dark current requires special attention because the dark current is the photodetector leakage current, when reverse bias is applied and no light is incident on photodetector. It also limits the photodetector dynamic range which is another critical parameter of photodetector application. Dynamic range of the detector is defined as ratio of the maximum signal level to the minimum signal level that can be detected by the photodetector. If the minimum signal level, equivalent to the leakage/dark current is high then the operating range of the device is reduced, which is an undesirable situation. Hence the dark current needs to be minimized with a high priority in order to achieve an efficient photodetector device with good dynamic range.

16 4 In this thesis, emphasis is given on the photodetector application of the organic conjugated polymer materials. Not much data is available on the processing as well as the mechanism of the process of photo-detection in the organic molecules. These organic photodetector devices normally suffer from a high dark/leakage current and degradation after illumination. The focus of this thesis work is to optimize the dark current of the organic photodetector. Initial work led to the dark current in the range of milli-amperes which leads to almost no photo-response and these devices were prone to degradation after illumination. After the optimization of the various steps of fabrication, dark current was brought down to the levels of nano-amperes with a high photo-response and reasonably good life times. In the present thesis, ITO/PEDOT-PSS/MEHPPV/Ca/Al and ITO/PEDOT-PSS/ MEHPPV-PCBM/Ca/Al devices were fabricated and their current-voltage characteristics are studied. Chapter 2 provides a basic background of photodetectors and the related physical processes. In chapter 3, the fabrication steps of the above mentioned organic devices are discussed in details. In chapter 4 & 5 two different structures of photodetector are studied. The first structure (Chapter 4) uses only one type of polymer and other using a blend of this polymer with Fullerene material (Chapter 5). The above chapters also discuss the results and cause for the analysis drawn in case of the respective devices. Chapter 6 gives the conclusion of the thesis work and the future aspects to the work done in this thesis.

17 5 Chapter 2 Background on Polymer Photodetectors Polymers were initially studied as they have attractive mechanical and structural properties. In mid 1970 s when conducting polymers were discovered, it led to a whole new branch of materials having electrical properties that can range from insulating to semiconducting to conducting [4]. These new semiconducting materials have electronic and optical properties of inorganic semiconductor along with the mechanical flexibility of a polymer. Though the electrical and optical properties are quite similar to those of inorganic, the charge carrier generation and charge transport mechanisms in semiconducting polymers are essentially different from their inorganic counterparts. A brief background necessary for the same is discussed in this chapter. Semiconducting polymers are conjugated polymers which refer to the alternating single and double bonds between the carbon atoms on the polymer backbone. The carbon atoms along with the polymer backbone are sp 2 hybridized, which leaves the hybridized p z orbital sticking up out of the plane of polymer. The electrons in the π-orbital form delocalized electron cloud which is free to conduct as shown in Fig.1.1. Thus the electrical conductance of the Semiconducting polymer comes in picture.

18 6 π e - Cloud (a) C π e - Cloud (b) C Fig.2.1 (a) shows how π bond is formed between two carbon atoms in a molecule (b) shows when chain of carbon atoms comes together, π electron cloud is formed. The molecular levels are grouped in bands. The band structure generally associated with the inorganic semiconductor. In case of organic semiconductor there are HOMO and LUMO levels. The band edge of valance band is referred to as Highest Occupied Molecular Orbital (HOMO) and edge of conduction band is called the Lowest Unoccupied Molecular Orbital (LUMO). The energy gap between HOMO and LUMO levels in conjugated polymer is generally within range of visible photon. When the incoming photon is absorbed electron is promoted to LUMO level, leaving behind hole in HOMO layer. This electron and hole remains on the same polymer chain and are bound to each other by their electrostatic force, commonly known as excitons. Dissociation of these photo generated excitons requires an input of energy of nearly hundreds of mev compared to only a few mev for crystalline semiconductor as they are strongly bound and do not spontaneously dissociate into charge pairs. Therefore the carrier generations do not necessarily result from the absorption of light. A strong driving force such as an electric field should be present to break up the photogenerated excitons. This electric field can be provided externally by applying a reverse bias voltage or can be provided by doping different material with polymer such as carbon nano-

19 7 particles or other polymers. The different material used with the polymers have large difference in the electron affinities and which in turn helps to extract the electron from the polymer chains and make it available for the conduction. As discussed above, the photodetector principle can be summarized as follows: A photodetector/photosensor is an electronic component that detects/senses light and converts the optical signal to the electrical signal. It acts like a transducer. The operation principle of the polymer photodetector can be considered as the combination of the three processes [5] as: 1. Carrier generation by incident light 2. Carrier transport to respective electrode 3. Interaction of current with external circuit to provide output signal. Light Light Light Input Glass ITO Organic Material Al, Ca Glass ITO Organic Material Al, Ca Output current Output Glass ITO Organic Material Al, Ca (a) LED mode (b) Photodetector (c) Photovoltaic mode (Photoconductive) mode mode Bias Fig.2.2: The organic polymer diode can be operated in various modes keeping the planer layered structure the same but by varying the biasing conditions. (a) LED mode, the forward bias is provided between the electrodes with electroluminescence as output. (b) Photodetector mode, the reverse bias is provided between the electrodes and the device is illuminated simultaneously to provide Photoconductive current. (c) Photovoltaic mode, no bias is provided but the device is given irradiation to provide output to drive current through the external circuit.

20 8 The organic polymer diode structure can offer different functions simply by varying the biasing conditions applied to the device as shown in Fig.2.2. Under the forward bias the device emits light and thus acts as a simple Light emitting diode (LED). When the diode is operated under the reverse bias and subjected to the illumination, then it is said to be operating in the photoconductive mode of the photodetector (Fig. 2.2(b)). In the third option an output voltage is obtained when the device is irradiated without any external applied bias. This is termed as the photovoltaic mode of the detector shown in Fig.2.2 (c). This operating mode is similar to that of a solar cell. But the need of large area for the solar cell applications restricts the detector to be called solar cell.

21 9 Chapter 3 Fabrication of Photodetector Polymer Photodetector in this work has been fabricated on commercially procured Indium-Tin-Oxide (ITO) coated glass substrate. ITO, a transparent conducting oxide is used as anode. The ITO film has been characterized for its sheet resistance, thickness and roughness. The measured values are given below: Sheet resistance: 20.4 to 20.9 Ω/ Thickness: ~ nm Mean roughness: ~10.2 nm The basic steps involved in Fabrication of the polymer photodetector are as shown in figure 3.1. Cleaning of ITO coated substrate Patterning of transparent Conducting Anode Coating of active layer Deposition of Cathode Metal Encapsulation Fig. 3.1 Basic Steps in Fabrication of Polymer Photo Detector

22 Patterning ITO coated glass substrate (0.1cm x 5 cm x 5 cm) are subjected to UV Photolithography. The purpose of the lithography process is to transfer the mask feature to the surface of the substrate. Fig. 2.2 shows overview of this typical transfer process. Baking of substrate at 95 to 100 C for 20 min (to remove moisture) Coating of Positive Photo Resist (PPR) 2000 rpm (1 min) Pre-bake in Oven ( C) (30 min) UV Exposure through Mask (2 min) Develop, Rinse & Dry Post Bake in Oven ( C) (30 min) Etching Removal of Resist and cleaning Fig. 3.2 Process of transferring mask features onto the ITO coated substrate.

23 Preparation of Mask The mask is prepared on rubylith with the help of a co-ordinatograph. The mask contains around 19 lines of dimension 0.1 cm x 5 cm. Then the pattern is transferred on photographic plate. The typical picture of mask (particularly for Positive Photo resist) is as shown in fig 3.3. Fig. 3.3 Design of mask transferred on photographic plate Cleaning of ITO coated substrate The ITO substrates are cleaned with RCA 5:1:1 solution (200ml DI + 40ml H 2 O ml NH 4 OH). The substrates are immersed in solution and heated for 30 min, the temperature being C. DI De-ionized Water doesn t have any metallic ion inside. It has ρ = 10MΩ/cm to 18MΩ/cm. For our purpose 10 MΩ/ cm is sufficient. H 2 O 2 Functions to oxidize all organic contaminant (oxidizing agent) NH 4 OH helps in removing heavy metals such as cadmium, cobalt, copper, iron, nickel etc. The substrates are subjected to ultrasonic cleaning in DI for 5 min, followed by drying. The advantages of ultrasonic cleaning can be listed as follows: (i) Ultrasonic cleans the surface and cavities without scratching, brushing or scraping.

24 12 (ii) It takes very short time to clean. Even most complex geometries can be thoroughly cleaned. (iii) Ultrasonic cleaning is very simple and easy to handle. Concentration of chemicals required is very less than in conventional cleaning. The substrates are then baked at 95 to 100 C for 20 min. to remove the moisture content Resist coating, Pre-bake and UV exposure ITO Glass Fig.3.4 ITO coated Glass substrate before patterning A positive Photo Resist (PPR) is flooded with the syringe onto the substrate, followed by spinning at a speed of 2000 rpm for 1 min. PPR ITO Glass Fig.3.5 Photo Resist coated ITO substrate A pre exposure bake of these photo resist coated ITO substrate at a temperature of C is done for 30 min. The UV Exposure of substrates through the pattern mask is carried out in angstroms spectral region with the exposure time of 2 min.

25 Developing the resist and post baking The exposed substrates are developed in developer (DI + KOH) followed by rinse in DI water. The substrates are constantly agitated in these baths during this period. The developed substrates are post baked at C for 30 min. Hardened Resist ITO Glass ITO Etching Fig. 3.6 Hardened Photo resist after developing An ITO etchant consisting of 200 ml DI water, 60ml HCl and 15 ml HNO 3 is used for etching the ITO from the post baked substrate at a temperature between C for 6-7 minutes. Hardened Resist ITO Glass Fig.3.7 Etched ITO substrate Stripping of resist After the etching, the resist is stripped using the acetone. The stripping of positive resist becomes easier as compared to the stripping of negative photo-resist. ITO Glass Fig. 3.8 Resist stripped and patterned ITO substrate

26 Cleaning The resist stripped substrates are rinsed in DI water and then cleaned with RCA 5:1:1 solution (200ml DI + 40 ml H 2 O ml NH 4 OH typically) by heating it for 30 min. Rinsing the substrates in DI water, using ultrasonic shaking for 5 min and then drying completes the cleaning process. The profile of patterned ITO, obtained from profilometer, is shown in fig.3.9, Angstrom Length in Micro Meters Fig. 3.9: Profile of ITO obtained from Alpha Step 500 Surface profiler 3.2 Ozone treatment The substrates after the cleaning are subjected to the ozone treatment for 15 min. Studies suggest that treating the ITO surface with oxygen plasma / ozone increases the work function of ITO, which will also lowers the hole barrier at anode [6-7]. The role of ozone treatment is discussed in Appendix [A]. 3.3 Coating of PEDOT-PSS layer & Vacuum Annealing The PEDOT- PSS (Poly (3, 4-EthyleneDioxyThiophene, Poly (Styrene Sulfonate)) is spun onto the ozone treated substrates for a thickness of the layer ~ 40nm. Subsequently the substrates are vacuum annealed, such that water content is removed. The role of PEDOT is discussed in Appendix [A].

27 15 PEDOT-PSS ITO Glass Fig PEDOT-PSS coated patterned ITO substrate. 3.4 Spin Coating of Active layer & Solvent removal The active polymer layer is then spun onto vacuum annealed substrates at 1000 rpm for 1 min. The substrates are then subjected to anneal in vacuum in order to ensure the entire solvent removal. Annealed polymer layer Annealed PEDOT ITO Glass Fig Vacuum annealed Polymer layer on patterned ITO substrates. 3.5 Cathode Deposition Thermal evaporation is used for the deposition of the cathode layer. This evaporation system consists of a diffusion pump backed by a rotary pump. The base pressure is of the order of 4 x 10-6 to 3 x 10-6 mbar. The deposition is achieved by the application of current through a filament of crucible. This then cause the filament or crucible to heat up and allows the material to simply evaporate and is then deposited upon the polymer coated ITO substrate.

28 16 The width of cathode deposited is restricted to 2mm, using metal mask. Therefore the active area has been 1mm x 2mm. The cathode used is Calcium/ Aluminum. Firstly, calcium is deposited and then aluminum is evaporated for deposition. The 2 lines of cathode deposited by using metal mask is shown in Fig.2.11 ITO lines Cathode deposited (Ca/Al) (a) Top View Cathode (Ca/Al) Annealed polymer layer Annealed PEDOT ITO Glass (b) Side View Fig Cathode deposited on the substrates. 3.6 Encapsulation Since these metals have tendency to be unstable in air hence they have to be encapsulated. The cathode deposited substrate is sealed with a glass plate using U-V epoxy resin, which is then treated with ultra-violet light. The device is now ready for test. Typical system configuration for the characterization is discussed in Appendix [B].

29 17 Glass Encapsulation ITO lines Cathode deposited (Ca/Al) Fig Glass Encapsulated ITO/PEDOT-PSS/Polymer Active Layer/Ca/Al Device. Typical structure of Photo detector is shown in Fig Each process in fabrication has prominent effect on the device performance. Some of the modifications in fabrication and their role on the device performance are studied in detailed in further chapters. + - ALUMINUM CALCIUM PEDOT: PSS GLASS POLYMER ACTIVE LAYER ITO ILLUMINATION Fig Structure of Photodetector

30 18 Chapter 4 Polymer Photodetector: Single Layer 4.1 Introduction Organic thin film diodes made of polymer MEHPPV (Poly (2-Methoxy-5-(2 - Ethyl-Hexyloxy)-1, 4-PhenylVinylene) are generally used as polymer light emitting diodes (PLED s). Very little photoresponse has been observed when the device structure is optimized as LED s. To increase the photoresponse, certain materials like C 60 and its derivatives are usually added to pure electroluminescent material. This chapter gives an idea that even pristine MEHPPV can be used to fabricate a photodetector by optimizing the device structure and processing conditions. The Single layer polymer photodetector is fabricated using only polymer MEHPPV as active layer. The chemical structure of MEHPPV is shown in Fig eV χ LUMO HOMO IP 5.2eV Vacuum level Fig. 4.1: Chemical Structure (the picture shows one repeat unit of polymer) and energy level diagram of MEHPPV

31 Principle of operation: As discussed in chapter 2, the photodetector has a planar-layered structure, where organic light absorbing layer is sandwiched between two different electrodes. One of the electrode is semi-transparent, often ITO, but a thin metal layer can also be used. The other electrode is Ca/ Al. The device structure of single layer photodetector is shown in Fig. A given below. + - ALUMINUM CALCIUM PEDOT: PSS GLASS POLYMER ACTIVE LAYER ITO ILLUMINATION Fig. A: Structure of single layer polymer photodetector The working principle of photodetector is just the reverse of the operation of LED. In LED s an electron is injected from the cathode with the balanced introduction of hole at anode. At some point in organic layer the electron and the hole meets, and gives light upon recombination. The photodetectors working phenomenon is just reverse. When light (photons) is absorbed by an active layer, an exciton is formed. The exciton is basically an electronhole pair bound with an electrostatic force of attraction. These excitons must dissociate to give free charge carriers i.e. a free electron and hole so that they reach respective electrode and provide output current/voltage. In order to achieve the charge separation an electric field is needed. The built-in electric field that arises due to difference in electrode

32 20 work functions is found to be insufficient to split the photogenerated excitons. Thus with external application of electric field this exciton dissociation is achieved. Fig 4.2 explains the charge transfer in single polymer photodetector when the light is incident on it. The Fig. 4.2 shows the energy level diagram of the single layer polymer photodetector. The device is illuminated from the ITO side. When the photon strikes the active layer of the device, an electron is promoted from the HOMO layer to the LUMO energy level. This leaves a hole in HOMO layer, thus forming a neutral exciton on the polymer chain. With the external bias provided, the dissociation of the exciton is achieved. The electron travels to the higher electron affinity electrodes (Calcium and then Aluminum), and then towards the positive terminal of the battery. Similarly the hole travels towards the negative terminal of the battery through the PEDOT PSS layer and ITO. Thus generation, separation and transport of the charge takes place in the detector. hν Energy Φ ITO ITO Φ PEDOT h + P E D O T χ LUMO IP HOMO E g Ca Φ ca e - Al Φ Al Vacuum Level Fig 4.2: Energy level diagram. Upon irradiation an electron is promoted to LUMO leaving a hole behind in HOMO. Electrons are collected at Al electrode and holes at the ITO electrode. Φ: workfunction, χ: electron affinity, IP: ionization potential, E g : optical band gap. χ MEHPPV = 2.8 ev, IP MEHPPV = 5.2 V, Φ ITO = 4.9eV, Φ Ca = 2.9eV, Φ Al = 4.3eV, Φ PEDOT-PSS = 5.1 ev and Eg MEHPPV = 2.4 ev.

33 21 The absorption and emission spectra of the conjugated polymer MEHPPV is given in Fig. 4.3 below. The polymer mostly absorbs in the visible region of the spectrum. The absorption peak is at 490 nm, where as its photo-luminescent maximum is at 585nm. The difference between the two is because of Frank-Condon Shift. Absorption Emission Fig. 4.3: Absorption and Emission spectra of conjugated polymer MEHPPV, Courtesy: Absorption maximum= 490nm. 4.3 Experiments, Results and Discussion The ultimate aim of this work is to fabricate a good quality polymer photodetector with an improved dynamic range and lifetime. Hence the fabrication steps were under continuous modification and the corresponding effects were thoroughly studied. The analysis of the modifications led to an improvement in the overall performance of the device. The ITO/PEDOT/MEHPPV/Ca/Al devices were fabricated as discussed in Chapter 3. Chloroform was used as an initial organic solvent for MEHPPV. The reverse J-V characteristics obtained are shown in Fig There is a set of three measurements taken on a device at a stretch. Firstly the device is kept under dark. The reverse voltage bias is provided and is varied from 0 to -3.5V. The corresponding current values, which

34 22 represent the reverse leakage current of the device (also known as the Dark current), are shown in Fig.4.4 with square symbols. In the second measurement, the device is subjected to illumination with a broad light source. The device current under illumination, which is the Photocurrent of the device, is represented by the circular symbols. Finally, the device is again subjected to the reverse voltage under dark atmosphere. This is done in order to determine any deterioration in the device because of exposure to light. This current named as Dark current (2) is shown by the triangular symbols in Fig 4.4. The graph is plotted for current density versus the applied reverse voltage, device area being 2 mm 2. Current Density(A/cm 2 ) 1E-3 1E-4 Dark current density Photo current density Dark current(2) density Chloroform 1E Reverse Voltage(V) Fig 4.4: Reverse J-V characteristics of ITO/PEDOT/MEHPPV/Ca/Al device. Chloroform as organic solvent. The graph clearly shows that, the dark/leakage current density of the device is too large ~ 1mA/cm 2 and the photocurrent density is of the same order. The dark current (2) density characteristic also follows the photo current density characteristic.

35 23 The optimization is undoubtedly required for the device especially to reduce the leakage current. In this work optimization is achieved by two approaches: Firstly, by varying the processing conditions for the device and secondly, by varying the device structure Optimizing the Processing Conditions of the Device The processing conditions of the device are varied by varying the organic solvent in which the polymer MEHPPV is dissolved. The different organic solvents under test are Chloroform, 1-2-Dichlorobenzene, Xylene and Chlorobenzene. The four different solutions of MEHPPV using above four solvents were prepared keeping the material concentration similar (8mg/cc) and devices were fabricated. The reverse J-V measurements were taken on individual device as shown in Figures 4.4, 4.5, 4.6 and 4.7. Current Density(A/cm 2 ) 1E-4 1E-5 1E-6 1E-7 1E-8 1,2 Dichlorobenzene Dark current density Photo current density Dark current(2) density Reverse Voltage(V) Fig 4.5 (a) : Reverse J -V characteristics of ITO/PEDOT/MEHPPV/Ca/Al device 1-2-Dichlorobenzene as organic solvent.

36 Ratio (P/D) Dichlorobenzene Maximum (P/D) =29.9 at -2.2V Voltage(V) Fig 4.5 (b): The ratio of photo current density to dark current density versus applied voltage 1E-6 Current Density(A/cm 2 ) 1E-7 1E-8 Dark current density Photo current density Dark current(2) density Xylene Reverse Voltage(V) Fig 4.6(a) : Reverse J -V characteristics of ITO/PEDOT/MEHPPV/Ca/Al device Xylene as organic solvent.

37 xylene Ratio (P/D) Maximum P/D = 2.79 at -7.9V Voltage(V) Fig 4.6 (b): The ratio of photo current density to dark current density versus applied voltage 1E-5 Current Density(A/cm 2 ) 1E-6 1E-7 Chlorobenzene 1E-8 Dark current density Photo current density Dark current(2) density Reverse Voltage(V) Fig 4.7(a) : Reverse J-V characteristics of ITO/PEDOT/MEHPPV/Ca/Al device. Chlorobenzene as organic solvent.

38 Chlorobenzene Ratio (P/D) Maximum P/D = at -3.1V Voltage(V) Fig 4.7 (b): The ratio of photo current density to dark current density versus applied voltage The results shown in the above four graphs are tabulated under Table 4.1. It compares the ratio of photo current density to dark current density of different devices made using various solvents mentioned before. It can be seen from the tabulated results that variation in the solvent in which MEHPPV being dissolved does affect the photoresponse of the device.

39 27 Current density (A/cm 2 ) 1,2Dichlorobenzene Chloroform Xylene Chlorobenzene Bias = -3.5V Dark Current E E E E-8 density Photo Current E E E E-6 density Ratio = P / D 8.8 Nearly same Table 4.1 Comparison of the ratio of photo current density to dark current density of the various devices made using various solvents In the device where chloroform was used as the organic solvent for MEHPPV, no response with the illumination is seen. The device also has higher leakage current density as shown in Fig.4.4. With the chloroform as organic solvent, it is also seen that polymer film after spin cast is not good. This is because the evaporation temperature of chloroform is very low. By the time polymer is dropped on the substrate prior to the spinning, chloroform found to be partially evaporated. Hence during the spinning of the polymer, uniform film formation is not achieved. In Xylene as organic solvent, small amount of the photoresponse is observed in Fig In 1-2-Dichlorobenzene the photoresponse is indeed better, but device showed degradation after subjected to the illumination as in Fig There is a remarkable decrease in leakage current when Chlorobenzene was used as solvent. The photocurrent density to the dark current density ratio in the device, with Chlorobenzene as organic solvent, is nearly times that of the device when1-2-dichlorobenzene was used as

40 28 organic solvent. Thus, the overall performance of the device made using C 6 H 5 Cl (Chlorobenzene) is found to be best [8, 9]. The Fig. 4.5(b), Fig. 4.6 (b) and Fig.4.7 (b) shows the ratio of photocurrent density to dark current density plotted versus the applied voltage. It represents the optimal value of the applied bias for the device with respect to get maximum ratio of P/D. The choice of 1-2-Dichlorobenzene as an organic solvent thus can be further explored. Discussion: Basically conjugated polymer films are composed of many polymer chains and it is becoming increasingly clear that the way in which a conjugated polymer films are cast affects the interaction between polymer chains and thus the electrical and optical properties [10]. The degree of interchain interaction in polymer strongly depends on the polymer film morphology. This morphology of conjugated polymer in turn is controlled by the way in which films are processed. The organic solvents are basically divided into two categories Aromatic solvents and Non-aromatic solvents. Chloroform (CHCl 3 ) comes under non- aromatic group where as rest of the solvents falls under aromatic group. The way in which polymer orients itself in the solvent does depend upon the type of solvent as explained with the Fig. 4.8 (a), 4.8(b) Fig. 4.8(a): Aromatic Conformation Fig. 4.8(b): Non- aromatic conformation

41 29 The polymer chains, dissolved in aromatic solvents (like Xylene Chlorobenzene, 1-2-Dichlorobenzene), have relatively open and straight conformation [11] with maximum solute solvent interaction. The benzene rings of the conjugated polymer when dissolved in the aromatic solvent aligns parallel to the surface, thus has planer conformation as described in Fig. 4.8(a). It is expected that aromatic solvents can solvate the π-conjugated segments better than the alkyl side chains [12]. This results in a conformation which has better π-π stacking and subsequently better electrical conduction. On the other hand, non-aromatic solvent (like chloroform) solvate the nonconjugated segments i.e. alkyl side groups of the polymer. These alkyl side groups when interacts among themselves, forms a tight coiled structure. This hinders the conjugation length of the polymer backbone. Also the benzene ring structure of the polymers aligns itself as perpendicular to the surface, with the non-conjugated group laying on the surface as shown in Fig.4.8 (b). Thus the non aromatic solvents results in a polymer conformation with a lower electrical conductivity Variation in Device structure The variation in the device structure is incorporated by varying the active layer thickness. Now taking the best solvent i.e. Chlorobenzene (as discussed in section 4.3.1), the active layer thickness can be varied by varying the spin speed or the by varying the concentration of the polymer in the solvent. In this work, the concentration of the polymers is varied. The concentration is reduced to 4mg of MEHPPV per cc of Chlorobenzene. The reverse J-V and forward J-L-V characteristics of the devices are shown in fig. 4.9(a), 4.9(b), 4.10(a) & 4.10(b) respectively for the thickness d = nm, d = nm.

42 30 Current Density (A/cm 2 ) 1E-5 1E-6 1E-7 1E-8 Dark current density Photo current density Dark current(2) density Chlorobenzene d = nm Reverse Voltage(V) Fig. 4.9(a): Reverse J -V characteristics of ITO/PEDOT/MEHPPV/Ca/Al device with active layer thickness of 70-80nm Current Density (A/cm 2 ) d = nm Current Light Intensity Light Intensity (a.u.) Voltage(V) Fig. 4.9(b): Forward J -L -V characteristics of ITO/PEDOT/MEHPPV/Ca/Al device with active layer thickness of 70-80nm under forward bias.

43 Maximum P/D = at -0.8V Ratio (P/D) Chlrobenzene d = nm Voltage (V) Fig. 4.9(c): The ratio of Photo current density to dark current density plotted versus the applied voltage Current Density(A/cm 2 ) 1E-5 1E-6 1E-7 Chlorobenzene d = nm 1E-8 Dark current density Photo current density Dark current(2) density Reverse Voltage(V) Fig (a): Reverse J-V characteristics of ITO/PEDOT/MEHPPV/Ca/Al device with active layer thickness of nm

44 Current Density (A/cm 2 ) d = nm Current Light Intensity Light Intensity(a.u.) Voltage(V) 0.2 Fig. 4.10(b): Forward J-L -V characteristics of ITO/PEDOT/MEHPPV/Ca/Al device with active layer thickness of nm under forward bias Chlorobenzene d = nm Ratio (P/D) Maximum P/D = at -3.1V Voltage(V) Fig. 4.10(c): The ratio of Photo current density to dark current density plotted versus the applied voltage The above results are presented in Table 4.2. The characteristics in Fig 4.9(b) and 4.10(b) show that under forward bias the thinner device shows better performance than the thicker one. The turn on voltage in thinner device is nearly 4V where as it is much

45 33 higher ~ 8-10V in thicker device. Also the light output in the thinner device is much better. In the device with thicker active layer, the light intensity was found to be poor even at 16V. Active Layer Thickness nm nm Reverse Bias (at -3.5V) Forward Bias Reverse Bias (at -3.5V) Forward Bias Dark Current density = E-8 A/ cm 2 Turn on Voltage= ~ 8-10V Dark Current Density= E -6 A/ cm 2 Turn on Voltage= ~ 4V Photo Current density = E-6 A/ cm 2 Light intensity = poor Photo Current Density= E -5 A/ cm 2 Light intensity = Very Good Ratio P/D = 16.8 Ratio P/D = 7.6 Table 4.2 Comparison of the devices with different active layer (MEHPPV) thickness Comparing the characteristics in Fig.4.9 (a) and 4.10(a) it can be stated that as the device active layer thickness increases there is an improvement in the characteristics of the device. The dark current was found to be less in case of thicker device than in thinner device. Discussion: The dark current is found to be higher in devices having thinner active layer. This may be because of the higher surface roughness in the commercially obtained substrates. The roughness of the substrate has been found to be 10.2 nm. It is preferred to have low value of roughness in order to have low leakage currents. The photo-response in the thinner device was found to be higher as compared to the thicker device. This may be due to the better charge collection of electrodes. In larger

46 34 active layer devices, the charges may be getting recombined before reaching the electrodes. The Fig. 4.9(c) and Fig. 4.10(c) represent the plot of ratio of photocurrent density to the dark current density versus the applied voltage for the devices having active layer thickness nm and nm respectively. It is seen that if we bias the device at -0.8V for the thinner device then the maximum ratio of P/D is obtained as where as in thicker device the bias voltage should be -3.1 V so as to get maximum ratio of It can be said that the device performance of the thinner device is good with respect to irradiation than the thicker device except the leakage current values. 4.5 Summary: The polymer MEHPPV is mostly used in the display like PLED applications. When the device is optimized for the LED operation, very little photoresponse is reported. The chapter shows that even pure MEHPPV can be used for the application in photodetectors. This was achieved by changing the processing conditions and the active layer thickness for the device. The processing conditions were varied by varying the solvent in which the conjugated polymer MEHPPV is dissolved and the change in polymer layer thickness changes the device structure. The photoresponse of the detector using undoped MEHPPV polymer is still less for the application in practical photodetectors. The response can be further improved with the addition of the fullerene/polymer to the pure MEHPPV as discussed in next chapter.

47 35 Chapter 5 Polymer Photodetector: Dispersed (Bulk) Heterojunction 5.1 Introduction The analysis of the pure conjugated polymer single layer device points out that charge separation process in these monocompound layers is rather weak. To improve the charge generation and separation the idea is to use two materials with different electron affinities and ionization potentials. This will favour the exciton dissociation: the electron will be accepted by the material with the larger electron affinity and the hole by the material with the lower ionization potential. Among the known electron acceptor materials Fullerene/ C 60 and its derivatives are very popular. One reason for the popularity of above class of n-type nanoparticles is the lack of wide range of n-type (electron transporting) conjugated polymers. In this work, one derivative of C 60 i.e. PCBM, [6, 6]-Phenyl-C 61 Butyric acid Methyl ester is used. The chemical structure and energy level diagram of PCBM are shown in Fig. 5.1.

48 36 3.7eV χ LUMO IP Vacuum level 6.1eV HOMO Fig 5.1: Chemical Structure and energy level diagram of PCBM (HOMO LUMO = 2.4eV) In analogy to the classical p-n junction concept of electron donor material and electron acceptor material forming bilayer structure [13] was quite of interest initially. However for most of organic semiconductor the film thickness should be more than 100nm in order to absorb more light. It follows that thicker film layers increase light absorption but only small fraction of the excitons will reach the interface and dissociate. But the development of bulk heterojunction concept has opened up new research directions. This bulk heterojunction concept is based on blends of the two organic compounds, one with the donor properties and other with acceptor properties. Device produced in a bilayer structure with a single interface between the electron donor and acceptor had a low efficiency [14] because volume of the active layer where efficient charge separation occurred was limited to a small fraction at the interface. An early study shows that importance of close proximity of the polymer and fullerene for the efficiency of the device. To overcome this problem, a device with a structure fabricated from blend of MEHPPV and PCBM was incorporated [15].

49 Fabrication details: The two master solutions of MEHPPV & PCBM (8mg/cc) were prepared. The blend solutions with defined concentrations were then obtained by mixing the two master solutions in proper molecular ratio. The MEHPPV: PCBM blend films were spin cast from blend solution at room temperature. The device configuration is ITO/PEDOT- PSS/MEHPPV-PCBM/Ca/Al. 5.3 Principle of Operation: By blending the material, the interface is distributed throughout the device as shown in Fig 5.2. The difference in electron affinities creates driving force at the interface between two materials that is strong enough to spilt photogenerated exciton. Hence, generally all photogenerated excitons are likely to find an interface and split before recombining [16]. Fig. 5.2: In blended device interface is distributed all over the device. Figure shows one such interface.

50 38 PCBM C 60 Fig.5.3: Absorption spectra of PCBM and C 60. Absorption maximum for PCBM = 284 nm & 341nm. Courtesy: In blended device, both electron accepting PCBM and a hole transporting conjugated polymer are present throughout the bulk of the device, unlike the normal bilayer device where the hole transport layer and electron transport layer are separately defined. The absorption spectrum of PCBM is given in Fig. 5.3 and that of MEHPPV is already given in Fig Most of the absorption takes place in the conjugated polymer layer. The excitons are generated with the absorption of the light. The dissociation of the excitons into the electron and hole happens quickly in presence of the PCBM. Due to higher electron affinity of PCBM than MEHPPV, electrons are attracted towards the LUMO of PCBM, thus leaving a hole behind on MEHPPV polymer chains. In the photodetector mode these charge carriers are drifted to the respective electrode with the help of external electric field /voltage. Thus the charge transportation becomes faster in the photodetector mode as compared to the photovoltaic mode where no external bias is provided.

51 Experiments, Results and Discussion: While using the blend of MEHPPV and PCBM as photo absorbing layer, the basic aim remains same, to get good device with low dark current and to achieve the maximum photoresponse out of the device. To implement this, concentration of the master materials in blend is varied. The three compositions used in this work are MEHPPV: PCBM ratio being 1:1, 1:2 & 1:4 by weight in organic solvent. Chlorobenzene as common organic solvent is used. Active layer thickness was nm and device area used as 2 mm 2. The corresponding devices were fabricated and the J-V characteristics were taken in dark and illuminated environment shown in fig. 5.4, 5.5 and 5.6. MEHPPV :PCBM (1:1) 1E-4 Current Density (A /cm 2 ) 1E-5 1E-6 1E-7 1E-8 Dark Current density Photo Current density Dark Current density(2) 1E Reverse Voltage(V) Fig.5.4(a): Reverse J-V characteristics of the device whose active layer has MEHPPV and PCBM are mixed in 1:1 proportion by weight in Chlorobenzene.

52 MEH:PCBM (1:1) Ratio (P/D) Maximum P/D = at -0.6V Voltage(V) Fig.5.4(b) : The ratio of photo current density to dark current density versus applied voltage MEHPPV :PCBM (1:2) 1E-5 Current Density(A/cm 2 ) 1E-6 1E-7 1E-8 1E-9 Dark Current density Photo Current density Dark Current density(2) Reverse Voltage(V) Fig.5.5(a) : Reverse J-V characteristics of the device whose active layer has MEHPPV and PCBM are mixed in 1:2 proportion by weight in Chlorobenzene.

53 MEH:PCBM (1:2) Ratio(P/D) Maximum (P/D) = at -3.5V Voltage(V) Fig.5.5(b) : The ratio of photo current density to dark current density versus applied voltage MEHPPV:PCBM(1:4) Current Density(A/cm 2 ) 1E-5 1E-6 1E-7 1E-8 Dark Current density Photo Current density Dark Current density(2) Revese Voltage(V) Fig.5.6 (a) : Reverse J-V characteristics of the device whose active layer has MEHPPV and PCBM are mixed in 1:4 proportion by weight in Chlorobenzene.

54 Ratio(P/D) MEH:PCBM(1:4) Maximum (P/D) = 156 at -3.2 V Voltage(V) Fig.5.6(b) : The ratio of photo current density to dark current density versus applied voltage For the ease of understanding the different results are being summarized in following Table 5.1. Current Density MEHPPV: PCBM MEHPPV: PCBM MEHPPV: PCBM (A / cm 2 ) at -3.5V (1:1) (1:2) (1:4) Dark Current Density E E E-7 Photo Current Density E E E-5 Ratio = P / D Table 5.1: Comparison of the ratio of photo current density to dark current density of the various devices made using different proportions of the master solution of MEHPPV and PCBM.

55 43 The tabulated result shows that as the concentration of the PCBM with respect MEHPPV is increased from 1:1 to 1:2, the ratio of photocurrent density to dark current density increases by almost 20 times at -3.5V. Further, improvement in the ratio is achieved by increasing the PCBM proportion to 4 times. As seen from the table, the absolute value of the photocurrent is decreased as the MEHPPV proportion is decreased. This has been shown that the function of the MEHPPV is light absorption and formation of exciton (bound electron-hole pairs), and that of PCBM is the charge separation. It seems that at blend ratio of 1:4, the MEHPPV acts as the limiting factor in charge generation, and hence we are observing decrease in photocurrent. The above improvement in photocurrent to dark current ratio is mainly coming from the decrease in dark current. As the PCBM concentration is increased (in 1:2 and 1:4 proportions) in the blend the dark current is decreased as compared to the 1:1 proportion. It suggests that PCBM is better capable of covering the surface roughness in comparison to MEHPPV. There may be other reasons, which are yet to be explored. The Fig. 5.4(b), Fig. 5.5(b) and Fig 5.6(b) represent the plot of ratio of photo current density to dark current density versus applied voltage. The graph gives the optimal value of the bias voltage for the device to get maximum of ratio (P/D). The ratio in the device with MEHPPV: PCBM (1:1) is very high as at bias voltage of - 0.6V which is very less. If the leakage current in the device is decreased further at higher voltages the device with blend proportion of 1:1 can be further explored.

56 An OP-AMP Photodetector Circuit: The device configuration where MEHPPV: PCBM blend ratio was kept 1:4 is used in an op-amp photodetector circuit. The idea is to realize similar working of polymer photodetector in place of inorganic photodetector for practical applications. A photodetector produces current that is a linear function of the light intensity incident on it. This current is converted to a voltage by an inverting op-amp in a currentvoltage converter mode. The output voltage depends on the input current. A simple light sensing circuit consisting of a photodiode and an inverting op-amp is shown in Fig.5.7. The anode terminal of the detector is connected to the negative terminal of a 6V supply. To see the output, an organic LED or inorganic LED can be connected to the output pin of the op-amp. Whenever light is incident on the detector, current is generated and then converted into desired voltage through the op-amp in a current-voltage converter configuration. This in turn helps to light up the LED connected to the output. R 2 2 Light PD V V LED Bias R 1 Fig. 5.7: An OP-AMP Photodetector Circuit. PD: Photodetector, R 1 = 5kΩ, R 2 = 10MΩ, Bias = 6V.

57 45 The photographs of the practical demonstration are as shown in Fig.5.8 (a) & 5.8(b) Fig.5.8(a): Light is incident on the polymer photodetector (on left) and the organic polymer LED s are being driven through that using op-amp circuit. Fig. 5.8(b): Light is incident on the polymer photodetector (above right) and inorganic LED s are being driven through that using op-amp circuit Rise Time Measurement: The rise time of the polymer photodetector is measured using the arrangement shown in the Fig The photodetector was irradiated with the blue wavelength (457nm). The blue light source is connected to the function generator generating a square waveform of frequency 15 khz having 50 % duty cycle. The photodetector is biased at - 6V. The output is taken across the resistance R. The value of R is varied from some ohms to kilo ohms and corresponding rise time graphs are captured from the CRO. They are shown in Fig. 5.10(a), 5.10(b), 5.10(c) and 5.10(d) respectively. The corresponding values of R are also mentioned in these figures

58 46 To Y channel of CRO Function Generator Light Sourc PD R To X channel of CRO Bias To Y channel of CRO Fig. 5.9: Rise time measurement setup Reference Pulse Reference Pulse Output Pulse Output Pulse Fig.5.10 (a): R = 680 ohms Fig.5.10(b): R = 2.7k ohms Reference Pulse Reference Pulse Output Pulse Output Pulse Fig.5.10 (c): R = 6.2k ohms Fig.5.10 (d): R = 12k ohms