Effects Of Bending On The Optoelectronic Properties And Failure Mechanism In Organic Photovoltaics

Effects Of Bending On The Optoelectronic Properties And Failure Mechanism In Organic Photovoltaics


This research investigates the effects of bending on the electrical, optical, structural and mechanical properties of flexible organic photovoltaic (OPV) cells. Bulk heterojunction organic solar cells were fabricated on Polyethylene terephthalate (PET) substrates using Poly-3-hexylthiophene: [6, 6]-phenyl-C61-butyric acid methyl ester (P3HT: PCBM) as the active layer and Poly (3, 4-ethylenedioxythiophene) Polystyrenesulfonate (PEDOT: PSS) as the hole injection layer. All the organic layers were deposited by spin coating while the Al cathode was vacuum thermally evaporated. The Indium Tin Oxide (ITO) anode has an average optical transmittance of 85% in the visible spectrum, a sheet resistivity of 60 ohms per square and an average surface roughness of 3nm. The relationship between the optoelectronic performance of the various device layers and the applied mechanical strains has been analyzed. The effects of stress and strain on the current-voltage characteristics of the device and its failure were modeled using the Abaqus software.


1.1 Introduction-1
1.2 The Solar Cell-1
1.2.1 Inorganic Solar Cell Limitations-3
1.3 Organic Electronics-4
1.4 Photovoltaic Energy Conversion-5
1.4 The Make Up of a Photovoltaic Cell-6
1.5 Problem Statement/ Hypothesis-7
1.6 Scope of Work-8
1.7 Arrangement of Work-8
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2.1 Introduction-9
2.2 Background-9
2.2.1 The Bulk Heterojunction-11
2.3 Plasticity-13
2.3.1 Plasticity in Solar Cell Manufacturing-14
2.4 The Electrical Interactions at the Organic Solar Cell Interface-14
2.5 Theory-17
2.6 The Bending Theory-20
2.6.1 Technique One (3 Point Bend Test)-22
2.6.2 Technique Two (4 Point Bend Test)-24
2.7 The efficiency of a conventional photovoltaic cell-24
3.1 Introduction-29
3.2 Experimental Work-29
3.2.1 Dimensions of the Solar Cell-29
3.2.2 Sample Preparation and Cleaning-30
3.2.3 Solar Cell Manufacturing Procedure-30
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1.1 Introduction

The Global consumption of energy on the earth surface is estimated to be 15TW. 32TW is the total geothermal energy available, 870TW is that of wind and direct solar offers up to 86,000TW [1]. With these statistics, solar energy would be the ultimate source of fuel due to its abundance in availability. Ultimately, solar energy would replace the dwindling fossil fuels’ reserves in this new era of cleaner and more efficient energy as the world surges on to new technologies and horizons.

1.2 The Solar Cell

A solar cell is a semiconductor PN junction diode, normally without an external bias, that provides electrical power to a load when illuminated as shown in figure below. Solar cells or photovoltaic devices are devices that can convert efficiently the energy in sunlight into usable electrical energy.

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ASARE, Joseph |Dept. of Theoretical Physics (AUST) 2

Figure 1.1: Current Voltage (IV) characteristics of a PN junction diode

A solar cell under illumination would exhibit the IV characteristics of a forward bias PN junction diode as indicated in the Figure 1.2:

Figure 1.2: Characteristics of a solar cell under light

In the dark, a solar cell would exhibit the Ohmic characteristics curve of a conventional resistor. The efficiency of a solar cell is the ratio of the electrical power it delivers to the December 9, 2011 DECEMBER,2011 ASARE, Joseph |Dept. of Theoretical Physics (AUST) load, to the optical power incident on the cell. Maximum efficiency is when power deliv-ered to the load is Pmax [2].

A solar cell could be either organically or inorganically manufactured. A solar cell made by depositing one or more layers (thin films) of photovoltaic materials on a substrate is called a thin-film photovoltaic cell (TFPV) or thin-film solar cell (TFSC). These have thickness ranges varying from a few nanometers to tens of micrometers. Various deposition methods on a variety of substrates are used to deposit many different many different photovoltaic materials. Therefore the photovoltaic material used categorizes the thin-film solar cell into inorganic and organic forms given as:

1. Amorphous Silicon and other thin-film Silicon

2. Cadmium Telluride Inorganic

3. Copper Indium Gallium Selenide

4. Dye synthesized solar cell and other organic solar cells [3]

1.2.1 Inorganic Solar Cell Limitations

There has been increasing interest in the development of low cost organic electronic devices stimulated by the potential for significant processing cost reductions compared to the cost of their amorphous or crystalline silicon counterparts in solar cell and light-emit-ting devices [4]. Organic electronics has become a vastly developed field in the past two decades due to their promise of low cost, lightweight, versatility of chemical design and synthesis, ease of processing, and mechanical flexibility as compared to inorganic solar cells [5]. The main advantages of the organic semiconductors over inorganic semiconductors in the electronic industry are: easier deposition of thin films, higher degree of sensi December 9, 2011 DECEMBER, 2011 ASARE, Joseph |Dept. of Theoretical Physics (AUST) 4 activity to external agents, a potential lower-cost on large scale production, higher absorption coefficient and greater flexibility [6]. However, the major merit is that very simple and low cost deposition techniques such as; vacuum evaporation, spin coating and the like are used to deposit these organic thin films on some suitable substrates be it glassy or flexible.

1.3 Organic Electronics

The make-up of an electronic device could also be either inorganic or organic. Examples of organic electronics are light-emitting diodes (OLEDs), field-effect transistors (OFETs), solar cells and photo-detectors (Organic Photovoltaics-OPVs). Organic electronics are now applicable to PV technology to solve the growing energy challenges that will take an integral part in future energy production. However, organic electronic devices’ performance and lifetime such as that of OPV depend critically on the properties of the active materials used and their interfaces. An example is how surface energy and work-function greatly affect the charge injection or extraction and transport in organic semiconductors which has been explained further in chapter 2 [5]. Recent improvements in efficiency from ~1% to 8.3%[7] has strengthened the case for organic photovoltaic (OPV) cells thus bringing them closer to commercialization and suggesting that organic electronic devices may evolve to be widely used in both rural and urban applications. Figure 1.3 below shows the schematics of an organic solar cell manufactured in the electronic industries.

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Figure 1.3: The Schematics of an organic photovoltaic cell [Adapted from Korhan Demirkan]

1.4 Photovoltaic Energy Conversion

The ability to do the work of forcing electrons to move especially from the valence band to the conduction band is what is termed as the potential difference or Voltage. The potential difference across a conductor such as copper, silver and gold (they have one valence electron-good conductors) causes current to flow thereby providing electricity (see Figure 1.4)[8]. The conversion of electromagnetic energy such as light (which includes infra-red, visible and ultraviolet) to electric energy in the form of current or voltage is termed photovoltaic energy conversion [9].

Figure 1.4: Electricity generation [8]

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1.4 The Make Up of a Photovoltaic Cell

When the light is incident on the absorber/ material, it experiences a transition from a ground state to an excited state after which this excited state is converted to free negative and positive charge carrier pairs. The free negative charge carrier moves to the cathode whiles the free positive charge carrier move to the anode. During this discriminating transport mechanism, the energetic photo-generated negative charge carriers arriving at the cathode result in electrons which move through an electric circuit losing their energy to electrical loads as they make their way back to the anode to recombine with the arriving positive charge carriers. This recombination process eventually returns the absorber back to its ground state [9].

Figure 1.5: Solar energy spectra [Adapted from Fonash S. J.]

In solar cells’ energy generation, Photon spectra as depicted in Figure 1.5 are much preferred because in the most desirable way one photon translates to an electron-hole pair for the energy conversion (see Figure 1.6 below).
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Figure 1.6: Cross-section of a typical solar cell [Adapted from Fonash S. J.]

1.5 Problem Statement/Hypothesis

Device efficiency and reliability are affected in a drastic manner by the interfaces of the semi-conductor and the electrical contacts. Due to this, the electrical contacts has to be designed in a way as to enable the interfaces exhibit low resistance, low operating voltage and stability to minimize device degradation[10]. Building electronic devices on deform-able and flexible substrates is a requirement for novel large-area electronics, such as electronic textiles, electronic paper, sensor skin for robotics or medical prosthesis, and droppable solar cell or flexible displays. Amorphous silicon and silicon nitride, are brittle inorganic semiconductor device materials and as such crack easily when subjected to a significant amount of mechanical strain. The relationship between the optoelectronic performance of the devices and that of the applied mechanical strain values that are below the fracture strains and the strains associated with the loss of device functionality would be analysed to gain understanding in that light.[11] With this knowledge that bending strains affect the optoelectronic and failure mechanisms in bendable substrates, the brittle glass December 9, 2011 DECEMBER, 2011

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