Characterization of Organically Fabricated Photovoltaic Cells

Advances in Electrical and Electronics lab

Abstract.

The increased need for global decarbonization and development of renewable energy source to safeguard the environment and ensure sustainable energy generation has gained great momentum in the recent past. Of the commonly targeted system is the solar generation alongside wind, tidal energy among other bio generation mechanisms. Solar energy, being the chief source of renewable energy has shifted much of mankind effort to it side in bid to study, develop and operate solar generation. Development of semiconductor physics in 19^th century significantly lead to realization of effective means for solar power harnessing. However, the continued trend and search for cost effective system, High effective system and with higher conversion ratios has led to rapid research and development leading to venturing into organic fabrication mechanism for various reason; to promote proper solar waste management and improve efficiency of performance thus increasing output power. The use of organic based photogeneration may also compromise the cost to pave way to development of Cost effective and efficient systems.

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This documentation serves to present the research and finding on characterization of organically fabricated Photovoltaic cells. Through systematic selection of chemical properties for proper generation the photovoltaic system has been developed. Sophisticated design mechanisms such as ion and chemical implantation have been employed among other related process. The fabricated device is then tested and the characteristics of the product tested for its functionality and characterization under different condition just las in normal mono/polycrystalline photovoltaic (PV) fabricated gadgets.

Aims and objectives.

• To understand basic principles of polymer /fullerene bulk heterojunction solar panel.
• To evaluate power conversion of fabricated organic based photovoltaic cells.
• To analyses key performance parameters of organic photovoltaic cells.
• To come up with more effective techniques or selection material synthesis to enhance organic photovoltaic efficacy.

1.Introduction

In today’s Solar photovoltaic system, the main technology used is the semiconductor physics which usually depends on electron -hole pair (Formation and recombination) for potential difference creation. Though this is the most fundamental concept behind photogeneration, semiconductor generation has portrayed low performance with most efficient system being less than 40 % which calls for increased cost of power generation must high power be need. This has shifted attention to organic PV cell fabrication, a still growing and delicate technology. After the discoveries of new organic photovoltaic materials, a third generation of photovoltaic system made from organic structure has been gaining momentum.

 The organic Photovoltaic cells technology has become a very promising solution to the upcoming energy problem because of their flexible structures, low fabrication costs, ease of processing and adjustable electrical properties. Being in its early growth phase, third generation cells have moved beyond silicon-based cells by using the evaluation of materials innovation technology aimed at achieving 40-60% conversion efficiency. Owing to young market and delicacy of the industry, various research is being carried out to select best material structures to enhance commercialization of organic photovoltaic cells. It is recently reported that efficiencies of these devices have increased to 4-5%. Increasing the performance of these plastic cells is dependent on understanding and suppressing the effects of the performance limitations considering the performance limitations and solar radiation.

 Solar radiation covers the total electromagnetic spectrum, almost half of the energy exists in the short-wave (visible) part and one third of the energy lies in the infrared portions. So, during the last decade, research on the light harvesting started from the visible range and expanded to the entire spectrum. Generally, organic solar cells do not need the p – n junction necessary in semiconductor. In the last 10 years, different organic materials, blends and structures are being investigated to achieve highest efficiencies. Outcomes of these researches are organic Solar cells, tandem solar cells, hybrid solar cells and dye-sensitized solar cells. (Bedeloglu, 2011)

2: Basics of Organic Solar Cell

 Organic Semiconductor Materials and working.

The organic solar cell field started with the small organic molecules (pigments), but real breakthrough was achieved after the development of semiconducting polymers. Incorporating these conjugated polymers into organic solar cells resulted in remarkable improvements within the past years. In 1977, Shirakawa, MacDiarmid, and Heegner demonstrated that the conductivity of conjugated polymers can be controlled by doping. Since then, these conjugated polymers have been used successfully in LEDs and solar cells. What makes these conjugated polymers attractive for solar cells is the bond structure between the carbon atoms (Dawn, 2017).Different from most of the industrial plastics where the insulating properties of them come from the formation of σ -bonds between the neighboring carbon atoms in conjugated polymers. These σ -bonds forming the backbone are alternatingly single or double (Deibel, 2008).

 In other words, in the backbone of the polymer, each carbon atom can only bind to only three of its four neighboring atoms which means that one electron per each carbon atom is left in the pp- orbital. Since this is the case for all the carbon atoms in the backbone, these unbounded electrons mutually overlap between these pp-orbitals, so that they form the π- bonds along the backbone So that the electrons on the π bonds can be delocalizing along this conjugated path to make the conjugated polymer an intrinsic semiconductor. If the π band is filled with electrons, the band is called the highest occupied molecular orbital (HOMO) otherwise called as the lowest unoccupied molecular orbital (LUMO). Under excitation, the electrons in this π band, the polymer chain stays together without falling apart because of the σ- bonds formed between the neighboring carbon atoms. In conjugated systems, π- bonds are covalent chemical bonds, where two lobes of one involved electron orbital overlap two lobes of the other involved electron orbital. Only one of the orbital’s nodal planes passes through both of the involved nuclei. Also, as the band gap of a conjugated system depends on its size, the local HOMO and LUMO positive ions can be changed by changing the conjugation along the polymer’s backbone (Bushen, 2012). Therefore, real conjugated polymers are subject to energetic disorder. The structure can be represented as shown below;

Figure 1: Structural composition of fullerene compounds

Hence, one way to move an electron from the HOMO level to the LUMO level is by light absorption, if the energy of the absorbed photon is equal or greater than the energy of the orbital gap (band gap). Following the absorption of a photon with sufficient energy by the organic semiconductor, an electron moves into the LUMO, leaving a hole behind in the HOMO. However, after the separation, this electron-hole pair cannot be isolated due to electrostatic interactions but forms a tightly bound state called as exciton(Aqil, 2017)(Deibel, 2008). The exciton binding energy for organic semiconductors (in a range of 200–500 mV) is one order of magnitude larger than inorganic semiconductors like silicon, where photo excitant ions lead direct free carriers at room temperature. The working and step toward organic photovoltaic generation can be summarized by the figure below.

(a)    (b)

 (c) (d)

Figure 2: Organic photogeneration process;(a) Light absorption by the organic material, (b)Excitation due to energy band charges resulting to donor -Acceptor pair, (c) Charge diffusion and transport from donor and acceptor excitation points, (d)Charge collection resulting to current flow and I-V characteristics

After the neutral-charge excitons generated via light absorption, they start to transport through diffusion. The diffusion length of an exciton is an important characteristic property of the conjugated polymers for optoelectronic applications that varies from 5nm to 20nm. However, the increase in the diffusion length decreases the exciton lifetime meaning that they either decay or dissociate through the internal mechanism (Anon., 2012). Therefore, the thickness of the conjugated polymer systems is restricted by the exciton diffusion length. This diffusion mechanism is followed by dissociation for the excitons that are close enough to the layer interface. Then, electrons transfer into the acceptor at the donor-acceptor interface remaining the hole behind. The rest of the exciton’s decays through recombination. Therefore, the energy efficiencies of single-layer polymer devices remain typically below 0.1% (Japtag, 2006).

Then, dissociation of the electron-hole pair is transferred into the related electrodes. Efficiency of this process is related to different parameters. Some of which are the exciton diffusion length, carrier drift length and exciton lifetime. In addition, the absorption bands of the materials used have a high impact on the overall efficiency. The absorption band of most conjugated polymer lies in a relatively narrow range of solar spectrum, as they are commonly known as intrinsic wide band gap semiconductors (band gap above 1.4eV).The thermal energy at room temperature (∼25 mV) is not sufficient to efficiently generate free charge carriers in organic materials by exciton dissociation, even at typical internal electric fields (∼10^6–10^7 V/m) it is not an easy process (Deibel, 2008). In many conjugated polymers most of the excitons cannot dissociate into free carriers in a pure layer

Heterojunction structures

While high recombination rates and low efficiencies in one-layer organic solar cells made them inappropriate candidates for the future applications, the discovery of heterojunction organic solar cells opened a new era for this type of cells. According to this approach, polymer layer is made by hole and electron accepting organic materials and photo generated excitons in this layer dissociated into free carriers at the interface (Dawn, 2017). Comparing with the single component solar cells, recombination rate of heterojunction ion solar cells is low. The charges are separated at the donor and acceptor molecule interface, caused by a large potential drop (Tanyi, 2014). When potential difference ΔΦ, the difference between the ionization potential of the donor and the electron affinity of the acceptor exceeds the exciton binding energy, is achieved, excited electron charge is transferred from the higher LUMO level to the lower LUMO level. In other words, electron moves from the donor to the acceptor. The structures can be represented as in figure below;

Figure 3: Charge transfers in various band levels

The efficiency of this exciton dissociation process, called as photo induced charge transfer depends on some conditions. First of all, during the process, free charges can be generated only if the hole remains on the donor side of the interface which is due to the higher HOMO level of the donor. If the HOMO of the acceptor is higher, both electron and hole can be transferred to the acceptor side which leads to an energy loss. (left of the figure above) (Aqil, 2017) (Anon., 2012).The materials in the heterojunction layer should have an appropriate band gap to avoid this kind of an energy loss. A new structure based on the mixture of the electron donor and acceptor material is prepared to increase the interfacial area length and thus to improve exciton dissociation efficiency. This mixture of materials spin coated on the surface to form a heterojunction layer, also called as bulk heterojunction.

Among different polymers, the Buckminster fullerene C60 draws a lot of attention because of its’ strong electron acceptor properties. It’s based on experiments in bulk heterojunction cells that when C60 is mixed with hole conducting materials, photoconductivity increases under illumination. The figure bellows shows a typical configuration of polymeric organic cells structure of PEDOT; PSS electron -hole pair (Wenjai mai, 2016).

Figure 4: Configuration of polymeric cell structure of PEDOT; PSS electrons hole pairing

3.Atomic Force Microscopy (AFM) – thin film surface morphology characterization.

Atomic force microscopy is used in surface science laboratories to obtain images with atomic resolutions of 10-10 m or one tenth of nanometer (Bushen, 2012). This type of microscopy can be effectively applied in the field of polymers to study the surface characteristics of a polymer sample. The instrument is based on the principle that when a tip, integrated to the end of a spring cantilever, is brought within the interatomic separation between the tip and sample, interatomic potentials are developed between the atoms of the tip and the atoms of the surface (systems, 2014). As the tip moves across the surface, the interatomic potentials, force the cantilever to bounce up and down with the changes in contours of the surface. Therefore, by measuring the deflection of the cantilever, the topographic features of the surface can be mapped out.

 (a)

 (b)

Figure 5: Basic structure of AFM ;(a) Principle of AFM, (b)Schematics of essentials of AFM

The atomic force between a sample and tip are measured using a laser and a detector to monitor the cantilever motion. The sample holder moves the sample up and down via a piezoelectric scanning tube so as to maintain the interaction force to a preselected level. A three-dimensional image can finally be constructed by recording the cantilever motion in the Z-direction as a function of sample’s X and Y position (Aqil, 2017) (Bushen, 2012). Theoretically for any material having certain rigidity, such an instrument is always capable of producing surface images with atomic resolutions. The developments in laser technology have made it possible to detect very minute (atomic level) deflections of the cantilever to be detected easily (Bushen, 2012).

AFM can be subjected to minor modifications to suit particular applications. For example, the set-up for AFM noise analysis of polymer surfaces consists of AFM head, which is fixed to a metal base. The metal base is screwed to three-channel piezoelectric flexure stage for fine approach. The piezo stage has active feedback control with sensors for distance control (systems, 2014) (Bushen, 2012).A magnetic sample holder is attached to the XYZ inertia drive for the coarse approach. This assembly is fixed to the three-channel piezo stage. The set-up is placed in a closed chamber to reduce the thermal drift (Japtag, 2006).

To acquire the image resolution, AFMs can generally measure the vertical and lateral deflections of the cantilever by using the optical lever. The optical lever operates by reflecting a laser beam off the cantilever. The reflected laser beam strikes a position-sensitive photo-detector consisting of four-segment photo-detector. The differences between the segments of photo-detector of signals indicate the position of the laser spot on the detector and thus the angular deflections of the cantilever (Snaiclair, 2000).

Figure 5:AFM when working with optical fiber

A sharp probe is brought into proximity with the surface. The probe is oscillated vertically near its mechanical resonance frequency (Aqil, 2017). As the probe lightly taps the surface, the amplitude of oscillation is reduced and the AFM uses this change in amplitude in order to track the surface topography. Into its amplitude, the probe motion can be characterized by its phase relative to a driving oscillator. The phase signal changes when the probe encounters regions of different composition. Phase shifts are registered as bright and dark regions in phase images, comparable to the way height changes are indicated in height images. Fig5. elucidates the principle of phase imaging.

Phase images often show extraordinary contrast for many composite surfaces of technological and scientific interest. These include contamination deposits, discontinuous (i.e. defective) thin films, devices built of composite materials (e.g. magnetic recording heads), and cross-sectional specimens of composite materials (johnstones, 2013; Dawn, 2017). Both inorganic and organic materials can be examined. It is found that phase imaging is more convenient and gentler than other methods which are based on contact mode operation. It routinely achieves lateral resolution of 10 nm. As compared to force modulation microscopy which can be used to study the viscoelastic prometaphase detection mode of AFM provides enhanced resolution (better than 10 nm versus 100 nm in force modulation microscopy (Japtag, 2006)).

 The strong contrast between domains in the images is conspicuous. While the topographic image shows some corresponding features, surface roughness hinders the identification of domains. The phase image allows unambiguous resolution of the different material phases (Japtag, 2006). Typical images obtained through the process are as the one shown in the figure below. They were obtained from in a tapping mode using Agilent 5500AFM.

Figure 6: AFM images; (a) Topography, (c) 3D topography, and (c) phase images. Scan size of 5µm×5µm (R.M.S surface roughness, S_q., of 16.7 nm, highest peak – deepest valley height, S_z, of 112 nm, and highest peak – mean plane height, S_p, of 61.3 nm, mean surface roughness, S_a, 13.1 nm)

4.Optical (UV-Vis) absorption spectra measurement 

With recent technology development and great interest in Nano technology growth, Optical absorption technology has been greatly used to characterize thin film material that has been widely applied in spectroscopy analysis of organic solar films. They have been found useful in extracting material properties of Nano technologically manufactured material such as electromagnetic and optical properties; a great milestone to greater development in Nano structures development and study basics (Anon., 2012) (Dawn, 2017) (Tanyi, 2014). The specific light properties posed by certain classes of materials such as absorptions of light at specific wavelengths can be used to define their transition bands and hence can be used to characterize the material behavior in relation to light absorption which can define its spectroscopy.

All material absorption spectroscopy is based on Beers law which usually expresses light absorption spectroscopy(A) as function of incidence light intensity ( $I_0$

) and transmitted light intensity (I) and can be related by equation below (Japtag, 2006);

$A=\log \left(\frac{I_0}{I}\right)=–{\mathit{log}}_{10}T$

Where $T$

is the transmittance and A gives the absorption spectrum of a given wavelength This equation is mainly applied to most common spectroscopy study machine such as one used in lab; Varian Cary 50 UV-Vis spectrophotometer. The basic parts of a spectrophotometer are a light source, a holder for the sample, a diffraction grating in a monochromator or a prism to separate the different wavelengths of light, and a detector. The radiation source is often a Tungsten filament (300–2500 nm), a deuterium arc lamp, which is continuous over the ultraviolet region (190–400 nm), Xenon arc lamp, which is continuous from 160 to 2,000 nm; or more recently, light emitting diodes (LED) for the visible wavelengths (Wenjai mai, 2016). The detector is typically a photomultiplier tube, a photodiode, a photodiode array or a charge-coupled device (CCD). Single photodiode detectors and photomultiplier tubes are used with scanning monochromators, which filter the light so that only light of a single wavelength reaches the detector at one time (systems, 2014).

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The scanning monochromator moves the diffraction grating to “step-through” each wavelength so that its intensity may be measured as a function of wavelength. Fixed monochromators are used with CCDs and photodiode arrays. As both of these devices consist of many detectors grouped into one- or two-dimensional arrays, they are able to collect light of different wavelengths on different pixels or groups of pixels simultaneously (Tanyi, 2014) (Tress, 2011). The sequential diagram of the process can be illustrated by the figure below;

Figure 7: Simplified schematic of double beam UV -Visible spectrophotometer

During device fabrication it was found that the rate of ion implantation and absorption varies with increasing wavelength. And since increase in wavelength results to decrease in frequency, it’s easy to suggest that the highest rate of ion implantation and absorption takes place at higher frequencies more than at lower frequencies as depicted by linear curve (Linear Abs) and Abs curves shown by figure below. However, surpassing above a given wavelength the rate starts decreasing at same rate suggesting that an implantation has to take place at specific frequency for optimal performance

Figure 8: Absorption spectrum behavior of and a blend of P3HT: PCBM thin films.

Solar organic cell was realized to depict a totally different curves as their counterparts; the inorganic cells although they seem to follow nonlinear current-voltage characteristics like the diode and other semiconducting material. A close extermination of both organic and inorganic cell theoretical curves is shown Fig9;

While the organic cells have and current being placed in the first quadrant, the open source voltage and short circuit current are mainly found in the fourth quadrant. This case also applies to the maximum point of power occurrence (known as maximum power point tracking) which is located at the midway of parabolic curves joining the point of open circuit voltage and short circuit currents.

(a)                                                                                                                                                   (b)

Figure 9: Comparison theoretical I-V characteristics for inorganic and organic solar cells ;(a) I-V characterization of maximum power point, (b)I-V characteristics of organic cells under dark and illuminated conditions respectively.

 5:Organic solar cells parameter analysis.

Form the experimental data, obtained from the resulting specimen, the resulting data was plotted on the excel to reveal the current to voltage characteristic curve resulting to curves shown in figure 7 and 8 for dark and illuminated conditions respectively.

The current to voltage characteristic just likes in semiconductor photovoltaic cells do not follow ohms law and instead shows an exponential rise when the barrier or bond is broken. When the bond is completely broken (as in case of barrier breaking), these devices depicted sharp current rise with low voltage changes for both illuminated and dark condition. At this point the device become completely spoilt and can no longer assume dipodic characteristic (Deibel, 2008).

Using the labelling done on organic cell shown in figure 9 (right) and from illuminated condition, the short circuit current was obtained at coordinate (0.00, -0.0023). This mean that the voltage at this point is zero and current is 0. 0023A.This is the short circuit current of the device.

The open circuit voltage was obtained at a point of curve intersection with X-axis of figure 10, giving rise to coordinate (0.760,0.000). This show that the current at this point is 0 A while the voltage is 0.76 This will give the open circuit voltage of the device.

 (a)

 (b)

Figure 10: I – V characteristic of organic cell obtains from lab analysis. ;(a) Obtained under dark conditions, (b) Curves from data obtained under illumination.

The maximum power can also be deduced from the maximum value of current and voltage obtained by the specimen. The point of maximum power point tracking was found at turning point of curve in figure 10 and was obtained as coordinate (0.58, -0.0017). Thus, the maximum power point and current for the device can be estimated to be 0.58V and 0.0017A.

Having obtained the maximum power yielded by the model, we can also calculate the device fill factor which is usually a function of (Maximum power) MP power, open source voltage and short circuit current shown by equation below.

$\mathit{Fill\; Factor }\left(\mathit{FF}\right)=\frac{\mathit{MP\; voltage}\times \mathit{MP\; current }}{\mathit{Open\; circuit\; voltage}\times \mathit{Short\; circuit\; current}}=\frac{\mathit{Maximum\; power}}{\mathit{Theoritical\; power}}$

$=\frac{0.58\times 0.0017}{0.76\times 0.0023}=0.56407322$

From all obtained parameter then its easy to calculate the efficiency of the solar cell given the rate of irradiation.

6: Conclusions

It was found that the absorption rate during ion implantation stage take place optimally at specific wavelength thus not all wavelength can yield optimal implantation. Increasing or reducing wavelength beyond this point will reduce the rate of ion implantation drastically.

The short circuit current and voltage takes place at 4^th quadrant, with maximum power being depicted at midway between the quadrants. The device parameter was found to have same characteristic interim of range with both theoretical vale and show much similarities as one in semiconductor photovoltaic cells excepts for curve characteristics. The resulting parameter can be summarized as below;

The efficiency of the system is usually a product of solar irradiation and area thus not obtained by calculation as irradiation wasn’t provided, however for this model the efficiency ( $\eta )$

can be obtained as;

$\eta =\frac{ 0.58\times 0.0017\times 0.5640722}{\mathit{Solar\; irradiation }\times \mathit{Cell\; Area}.}=\frac{5.56104\times {10}^{–4}}{\mathit{Solar\; irradiation}\times \mathit{Cell\; Area}}$

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