Evaluating Taiwan’s Solar Energy Potential

Taiwan is an island country off the coast of mainland China that uses fossil fuels to supply 90% of its energy needs.  Taiwan has no domestic fuel production and demand for energy is expected to increase by 37.4% between 2005 and 2025.  As such, it is important for the country to find alternative sources of energy.  Solar power is one option that has become increasingly attractive with the rapid improvement in that field of technology.  Two types of solar technology are mainly discussed in Yue and Huang’s (2011) paper: photovoltaics and solar water heating.  The researchers study the potential of these technologies in Taiwan, taking into consideration factors such as the area available for developing solar technologies, local laws and regulations, and the cost of implementing these technologies.  The study concludes that only 0.02% of Taiwan’s solar energy potential was realized in 2009.  Adopting PV and solar thermal technologies have the potential to reduce carbon emissions by 20.9 and 2.1 million tons each year, respectively.  However, due to high population density and tall buildings, the amount of area that can be covered with solar energy harnessing technology is small compared to the number of households it must support.  As such, solar technologies will have an important, but limited effect on Taiwan’s energy sources.—Alan Hu

CD Yue, GR, Huang. 2011. An evaluation of domestic solar energy potential in Taiwan incorporating land use analysis. Energy Policy 39, 7988–8002.

            Yue and Huang at the University of Kang Ning use a mathematical expression to calculate the annual heat output of solar water heaters by multiplying the total collector area, the annual solar radiation, and average heat efficiency of the solar collector system.  The average electricity output from PV systems can be modeled with another equation that multiplies the total photovoltaic module areas, the annual solar radiation, the module efficiency, and the aggregative coefficient.
            The researchers use the city of Tainan as a case study to apply the models described above.  Yue and Huang consider Tainan’s city laws concerning protruding structures on rooftops and geologic information on the region to estimate the energy potential.  The study determines that solar water heaters can provide 369.1 GWh per year and 628.5 GWh per year through PV systems.  Analysis of buildings depending on height allowed researchers to determine that PV systems and solar water heaters can respectively provide 22% and 44% of energy demand for electricity and hot water for a 12 story building.  In contrast, PV systems and solar water heaters can provide 109% and 217%, respecitvely, of energy demand for electricity and hot water for a 4 story building.
            Yue and Huang apply the same analysis to Taiwan as a whole to determine the potential of solar energy.  PV systems are determined to be able to provide 16.3% of electricity demand whereas solar water heating systems can account for 127.5% of energy demand from heating water.  In 2009, PV systems only provided 0.02% of electricity and solar water heating systems only provided 11.6% of hot water.  As such, both technologies have plenty of room to grow.
            An economic analysis is provided to determine the economic feasibility of the technologies.  Solar water heaters have a lifetime of 20 years and PV systems have an operating lifetime of 25 years.  Using market information and a discounted cash flow, Huang and Yue determined that solar water heaters would recoup their cost in 4 years while the costs of a PV system would not ever be recouped under the current tax regime.
            Widespread adoption of solar energy technologies could have a large impact on Taiwan’s energy policy.  For example, the country would be less prone to natural disasters that disrupt energy distribution infrastructure such as the 1999 Chi-chi earthquake which halted the supply of solar energy from south Taiwan to north Taiwan.  Also, peak load energy consumption occurs over summer in Taiwan due to intense use of air conditioning, which is increasing.  This summer load peak corresponds nicely with the increase in solar energy output over summer due to increased solar irradiance. 
            The study concludes that current exploitation of solar energy potential in Taiwan is far below the maximum potential.  Energy policy needs to be modified to make PV systems economically feasible, as currently, the benefits of the systems do not recoup their costs, but energy autonomy based purely on solar energy is improbable due to high population density and the prevalence of high-rise buildings.  Nevertheless, adoption of solar energies could reduce up to 9% of the country’s carbon emissions.  Yue and Huang recommend further study using land use analysis and believe that despite the many limitations, solar energy can have a sizeable impact on the energy landscape of Taiwan.

Generating Hydrogen Fuel for Electric Vehicles

Fuel cell electric vehicles (FCEVs) are automobiles that use hydrogen fuel instead of carbon-based fuels.  Because the byproduct of burning hydrogen fuel is water, it is a much cleaner form of energy at the vehicle level.  Large companies such as General Motors plan to begin selling FCEVs by 2015, and over 100 FCEVs Chevrolets have collectively driven over one million miles.  But the widespread adoption of FCEVs is hampered by the current cheap prices of carbon fuels and the general lack of hydrogen fuel infrastructure.  Researchers believe, however, that solar energy hydrogen generation systems based in single homes are a viable system for fueling FCEVs.  Kelly et al. (2011) at General Motors have already built a photovoltaic (PV) powered electrolyzing/storage/dispensing (ESD) system for use as a single FCEV home fueling system.  However, the system has only been tested for 14 days, and the widespread effect of day to day operation of the system on its efficiency is unknown.  The next step is to measure the efficiency and other characteristics of such systems.—Alan Hu
N.A., Kelly, T.L., Gibson, D.B., Ouwerkerk. 2011. Generation of high-pressure hydrogen for fuel cell electric vehicles using photovoltaic-powered water electrolysis. International Journal of Hydrogen Energy 36, 15803–15825.

            Kelly et al. at General Motors previously built PV-ESD system consisting of a set of solar arrays and an ESD system.  Four solar arrays were used, each having 10 Sanyo HIP-190BA3 modules.  The modules were wired parallel in each array, and each array was wired parallel to the electrolyzer.  As such, the PV system voltage output was equal to that of one module whereas the system current output was 40 times that.  The solar array tilt angle could be significantly altered to maximize solar irradiance depending on the season.  The second part of the PV-ESD system is the electrolyzer/storage/dispenser system.  The Avalence electrolyzer used was cylindrical and could contain hydrogen and oxygen produced at high pressures up to 6500 psi.  Due to problems in past experiments, however, the system was not run at 6500 psi but rather at 2000 psi.  As such, the system could store only 2kg of hydrogen as opposed to 6kg.
            In order to evaluate the performance of the PV-ESD system, Kelly et al. used the Sandia Photovoltaic Array Performance Model (SPAPM) to measure the voltage, current, and power values of the PV-ESD system.  The SPAPM uses a set of outdoor performance measurements and output current at two other voltage values to determine the I-V curve of the PV module.  Researchers analyzed the I-V curves of the PV and electrolyzer separately.  A resistive load bank was used to measure the I-V curve of the PV-ESD on a sunny day while a Sorensen DC power supply was used to measure the I-V curve of the electrolyzer.  These figures were used to calculate important PV figures through the SPAPM model including module efficiency and maximum power.
            After constructing a theoretical model, researchers began actual testing of the PV-ESD system.  The study was run from November 2008 to October 2009 with a period of inactivity between December 16, 2008 and March 24, 2009 due to the inability of the ESD to run in sub-freezing temperatures.  The study was run for a total of 109 days between sunrise and sunset, though for about 10% of the days, less than one hour of data was collected due to problems in the electrolyzer system. Researchers found that on sunny days, system efficiency started low, peaked, and then dipped at around noon.  In the afternoons, after the noon dip, efficiency rose until the sunset drop.  On cloudy days, the PV efficiency showed sensitivity to short increases in solar irradiance caused by passing clouds and the associated increase in temperature.  In general, researchers found that PV efficiency was tied to solar irradiance, the temperature of the PV system, and the impedance of the load that it is connected to.  The ESD efficiency was dependent on the operating voltage and the electrolysis cell temperature.  Electrolyzer operative voltage depended on its impedance and electrolysis cell temperature affected the electrolyte conductivity.
            Based on the experience of the researchers various possibilities for improvement were considered.  The experimenters recommended that the anode and cathode be reversed, that compression energy stored in high pressure hydrogen be captured and used to do work, that the membrane of the electrolyzer be made alkaline, that excess heat from the PV, which decreases its efficiency, be transported to the ESD, which increases its efficiency.  In general, the experiment was a proof-of-concept for a single FCEV fueling system.  The first phase of the experiment concerned the design and construction of the PV-ESD system while the second tested the built PV-ESD system.  It was found that the electrolyzer responded well to the constantly changing solar irradiance caused by passing clouds and that day to day temperature variations did not decrease its efficiency.  The coupling factor of the combined PV-ESD system was calculated by comparing maximum power and efficiency of the PV and the actual power and efficiency of the PV.  The researchers concluded that 1) the system operated without any major failures of the high-pressure electrolysis system, which had previously been a problem, 2) solar energy to hydrogen efficiency averaged 8.2%, 3) coupling factor averaged 0.91, 4) the system produced 0.67 kg of hydrogen over a full day of operation, and 5) solar to hydrogen efficiency is less than a third as efficient on an energy utilization per mile basis as solar battery charging.

Hydrogen Fueled Homes

A hydrogen economy is often seen as a greener alternative to the current carbon economy.  Hydrogen is the most common element found on earth and can be easily transported and burned as a fuel.  However, the earth’s hydrogen is trapped in its vast seas and in other compounds.  As such, before hydrogen gas can be utilized, it must be extracted and separated from its source.  One method of generating hydrogen gas is by splitting water molecules into their oxygen and hydrogen components.  The process involves running an electrical current through the water, which commonly requires the burning of fossil fuels, but the current needed to split water can be generated in another fashion—through photovoltaics.  The combination of solar cell and hydrogen fuel technology solves a fundamental problem for solar energy: energy can now be stored for use even when the sun is absent.  While widespread adoption of hydrogen fuel technology is currently economically infeasible, homes powered by photovoltaic generated hydrogen fuel are a possible future.  Shah et al. (2011) describe and analyze a hypothetical hydrogen home built in Wallingford, Connecticut.
Alan Hu
Shah, A., Mohan, V., Sheffield, J., Martin, K. 2011. Solar powered residential hydrogen fueling station. International Journal of Hydrogen Energy 36, 13132–13137.

            Wallingford was chosen as the site of the hydrogen home due to its existing hydrogen infrastructure developed by Proton Energy Systems and Hydrogen Highway.  It was also believed that due to the presence of existing infrastructure, there would be higher public acceptance of hydrogen homes.  The home, in two levels, includes bedrooms, bathrooms, a closet, a kitchen, a dining room, a living room, and a garage.  Built with the architectural heritage of the region in mind, the home has steeply sloped roofs.
            The home is to be powered by a total of 60 PV panels arranged in two arrays.  The first, containing 18 panels, is directed 19º off the East-west axis and is designed to capture maximum solar energy in the mornings.  The second array, containing 42 cells, is pointed due south to maximize energy output in the afternoon and evenings.  The specific orientation of the solar arrays can be adjusted to keep output high across different seasons.  Researchers assumed 4.74 hours of daylight per day and an energy utilization of 32.8 MWh/yr.
            The hydrogen system of the home comprises a high pressure hydrogen electrolyzer and three storage tanks.  The electrolyzer turns on when the pressure inside the storage tanks drops below 138 bar and off when the pressure reaches 165 bar.  Hydrogen fuel stored in the tanks is then piped to the hydrogen vehicle.  The vehicle is assumed to commute 56 km per day at a fuel mileage of 71 km per kg hydrogen, thus requiring 0.8 kg of hydrogen per day.
            Safety precautions of the hydrogen power system include the use of hydrogen detectors, an emergency shutoff button, fire extinguishers, remote emergency stops, and a pressure relief system.  The power is disconnected to prevent ignition from electrical sources.  Regular inspection is advised and warning signs are placed around the hydrogen fueling station.  Significant failure scenarios such as the vehicle colliding with the storage tank, fueling nozzle, leakage of hydrogen gas, or hydrogen overfill are all taken into account.
            The researchers developed a wheel-to-wheel analysis of the hydrogen home.  The Wallingford hydrogen home is estimated to require 95 kWh per day assuming a house consumption rate of 21 kWh per day and a vehicle consumption rate of 74 kWh per day.  The average daily output by the PV cells is 90 kWh per day which means that 5 kWh per day will need to be drawn from the power grid.  A comparison of power consumption between the hydrogen car and a normal car shows that the hydrogen car produces 34.8 grams of carbon per mile whereas a normal vehicle produces 272.2 grams of carbon per mile.
            The researchers calculate that 16.5 metric tons of carbon dioxide are saved each year with the use of the hydrogen home.  The energy efficiency improvement for the Wallingford home is calculated to be 23% and the hydrogen vehicle uses 13% of the carbon of a normal vehicle.

Hydrogen Fuel Through Bacteria

Various different strategies have been proposed to harness solar energy efficiently.  Photovoltaics directly translate solar energy into an electric current.  Solar thermal collectors use the heat from solar energy to create thermal energy.  Another branch of technology uses the electricity generated from photovoltaics to perform hydrolysis—the process of splitting a water molecule into hydrogen gas and oxygen.  The resulting hydrogen gas can be burned as a clean alternative to the more common carbon based fuels.  Hydrolysis, however, is an energy intensive endeavor and currently the cheapest way to perform it is to burn fossil fuels.  Recent research into the various processes of performing hydrolysis has analyzed the potential of using organisms’ natural pathways of creating hydrogen gas.  In particular, Anabaena sp. PCC 7120 possesses biological pathways that create hydrogen gas when starved of nitrogen in their culture medium.  Under nitrogen starvation, the bacteria begin producing specialized cells called heterocysts that house special nitrogen fixing enzymes called nitrogenases.  Nitrogenases react with atmospheric nitrogen to create ammonia and hydrogen gas.  The process requires energy input in the form of the universal biological medium of energy, adenosine triphosphate (ATP), and can only function in the absence of oxygen.  Two types of hydrogenases exist in Anabaena: uptake hydrogenase (Hup), which absorbs excess hydrogen and decreases hydrogen production, and bidirection hydrogenase (Hox), about which less is known, but is theorized to play a part in the oxidation of hydrogen and the creation of nutrition in conjunction with photosynthesis.  During photosynthesis, carbon dioxide, absorbed from the ambient atmosphere, is broken down into carbohydrates and frees oxygen gas.  The carbohydrates produced are essential to the creation of ATP whereas the production of oxygen gas inhibits the hydrogenases.  This means that Hup, which absorbs hydrogen produced, will function at a lower level, increasing the hydrogen gas output of Anabaena.  More ATP also translates to more nitrogenase action.  Anabaena can thus create hydrogen gas in a nitrogen starved state through nitrogenases and hydrogenases.  In an attempt to maximize the production of hydrogen gas, Marques et al. (2011) examine the effect of two hydrogenases on hydrogen production by comparing the hydrogen output of wild type Anabaena, mutants with no Hup (hupL), mutants with no Hox (hoxH), and mutants with neither Hup nor Hox (hupL/ hoxH).—Alan Hu
Marques, AE., Barbosa, AT., Jotta, J., Coelho, MC., Tamagnini, P., Gouveia, L. 2011. Biohydrogen production by Anabaena sp. PCC 7120 wild-type and mutants under different conditions: Light, nickel, propane, carbon dioxide and nitrogen. Biomass and Bioenergy 35, 4426–4434.

            Marques et al. from various research institutes in Portugal measured the hydrogen output of various strains of Anabaena under different environmental conditions.  The Anabaena sp. PCC 7120 and its mutants were provided by Professor Sakurai at Waseda University.  The specimen were grown in 500 ml flasks with medium under a constant air temperature of 25ºC and a set level of irradiance.  Growth was evaluated over 42 days in terms of optical density and chlorophyll α content.   Hydrogen production trials were conducted with all four cultures of Anabaena, were transferred to 120 ml glass bottles called photobioreactors (PBRs).  Each PBR was filled with 30 ml of culture and various environmental variables such as gas atmosphere, light intensity, and nickel concentration were manipulated in various trials.  Heterocysts were counted under a light microscope and the number of heterocysts per 100 vegetative cells was noted.  Hydrogen production was measured with a gas chromatograph.
            It was found that the HupLstrain under high light intensity produced the high levels of hydrogen.  The researchers observed that light is the primary driver of ATP levels, the higher of which allows more nitrogenase to generate hydrogen gas.  Researchers also observed that higher light intensity led to higher oxygen levels, which in turn increased hydrogen production levels by inhibiting the hydrogenases.  Discontinuous light was found to have a positive effect on hydrogen production in all specimens except for the wild type.  Increasing nickel concentration in the culture medium was also found to have increased hydrogen production across most of the specimens.  The HupL strain was again found to have the highest hydrogen production under increased carbon dioxide scenarios.  Under a propane atmosphere, the HupL/HoxH produced the highest levels of hydrogen and heterocysts.
            Marques et al. conclude that the HupL mutant in general provided the best hydrogen production figures.  However, it is admitted that current biohydrogen production rates are not high enough to be used on an industrial scale, and that more research needs to be done toward the subject. 

Modeling and Experimental Validation of a New Hybrid Photovoltaic Thermal Collector.

Photovoltaics and solar thermal collectors are often competing technologies used to harness the energy of the sun.  Photovoltaic (PV) systems convert solar energy directly into an electric current whereas solar thermal collectors harness the heat of solar radiation to generate electricity in a process similar to the one used in conventional power plants.  Both systems have advantages and drawbacks.  Photovoltaics generate lower conversion rates under direct sunlight and experience a decrease in efficiency when the temperature of the solar cell increases.  Current technology also does not allow electricity to be stored for later use.  Solar thermal collectors reach relatively high temperatures under a clear sky which allows them to reach higher conversion rates.  Thermal energy can be efficiently stored in the form of fluid storage tanks or molten salts.  However, solar thermal collectors generate little power under diffused light conditions while photovoltaics can continue operating under such weather.  The weaknesses and advantages of both systems complement each other and there have been attempts to integrate the two systems into a hybrid cell.  Touafek et al. (2011) have attempted to create precisely such a hybrid photovoltaic thermal collector (PVT collector).  The PVT collector consists of a photovoltaic layer placed adjacent to a thermal collector unit.  As such, excess heat that decreases the efficiency of the photovoltaic cell will be transferred to the solar thermal unit, which can generate electricity with the heat.  Touafek et al. present the results of their modeling and experimentation on a prototype hybrid PVT collector.—Alan Hu
Touafek, K., Haddadi M., Malek A. 2011. IEEE Transactions on Energy Conversion 26, 176-183.

            Touafek at the Unit of Applied Research in Renewable Energy, Haddadi at the Algerian National Polytechnic University, and Malek at Renewable Energy Development Center at Algeria constructed a prototype PVT collector and created a theoretical framework used to calculate various indicators.  The PVT collector prototype consists of two main components: the first is the photovoltaic section of the system and the second is the thermal collector.  The photovoltaic section of the system consists of three layers in the following order: tempered glass, a PV cell, and Tedlar (used as a backsheet to protect the PV system).  Immediately adjacent to the Tedlar layer is thermal collector which essentially is a system of pipes that pumps cooling fluid.  Any excess heat generated by the PV system is absorbed and transferred by the pipes and coolant. 
Toaufek et al. develop a numerical model that attempts to take into account all sources of heat that may affect the PVT collector, including heat from ambient air, from the ground, from the sky, from solar radiation absorbed by the glass, and from other sources.  Also included in the model is the transfer of heat between components of the PVT collector.  For example, heat transferred from the glass to the PV cell through conduction is considered in the calculations.  Toaufek et al. are able to isolate the temperature of the solar cell and the fluid through the model.  The two figures are important as the goal of the PVT collector is to minimize the temperature of the PV cell and to maximize the temperature of the fluid.
The researchers use the temperature of the solar cell and the temperature of the fluid to judge the optimal thickness of the pipe material.  Thicknesses of 0.01, 0.02, 0.03, 0.04, and 0.05 m were tested and the thickness that maximized fluid temperature and minimized solar cell temperature.  Toaufek et al. continue to present calculations from the model and from experimentation to validate the accuracy of their model.  The paper claims a total efficiency of 80%, though it is unclear if this refers to EQE or thermal efficiency.  Further research and experimentation is recommended in the conclusion.

Effect of Electron Donor Concentration on Power Conversion

Organic photovoltaics (OPVs) operate by separating two electrodes with a layer of organic polymeric material that generates a current between the two ends of the diode. Initial designs of OPVs used a single layer of organic material that, when struck by sunlight, generates excitons due to electrons vacating the Highest Occupied Molecular Orbital (HOMO) and filling the Lowest Unoccupied Molecular Orbital (LUMO). Due to the existing difference of charge between the two ends of the circuit, the excitons flow to the positive electrode, establishing a current. This design, however, yielded extremely low power conversion rates due to the weakness of the electric field established by the electrodes. Bilayer OPVs, which had electron donor and acceptor layers of organic material, were subsequently developed to address this issue. The two layers of the bilayer OPV generated themselves electrostatic forces that helped generate a stronger electric field. Bilayer OPVs still yielded low power conversion rates, however, due to the discrepancy between the diffusion length of excitons and the length of the polymer layer required to absorb enough sunlight. Bulk heterojunction (BHJ) cells combine the donor and acceptor layer of the bilayer OPV into a single bulk layer. This design solves the issue of matching exciton diffusion length with the length of the polymer layer and has so far yielded the highest power conversion rates of any OPV. An central question in growing BHJ cells is the question of what acceptor/donor ratio is optimal for achieving higher conversion rates. Zhang et al. (2011) have demonstrated through their experimentation that low donor concentration (under 10%) and the presence of molybdenum oxide (MoOx) can yield relatively high conversion ratios across a diversity of donor materials.—Alan Hu

Zhang, M., Wang H., Tian H., Geng Y., Tang C. 2011. Bulk Heterojunction Photovoltaic Cells with Low Donor Concentration. Advanced Materials 23, 4960–4964.

Zhang et al. at the University of Rochester and the Changchun Institute of Applied Chemistry grew cells composed of a sequence of indium tin oxide (ITO), molybdenum oxide (MoOx), TAPC:C60/C70, bathophenanthroline (Bphen), and aluminum (Al). As the researchers were primarily interested in measuring the effect of the BHJ layer TAPC:C, the dimensions and composition of the other layers were kept constant: ITO (90nm), MoOx (2nm), Bphen (8nm), Al (100nm). Researchers then modeled and tested the cells for indicators including the short circuit density, voltage, fill factor, power conversion efficiency, calculated hole mobility, and calculated electron mobility. Hole and electron mobility, the speed at which an electron can move through a semiconductor, were calculated with Bässler’s model.

The experiments yielded results that indicated a low concentration of donors was optimal. Voc increased as donor concentration decreased until 5%, below which Voc began to drop quickly. The same was found to be true for Jsc, which increased until donor concentration dropped below 5%. Similarly, the fill factor peaked at 0.53, which occured when donor concentration was between 5% and 10%. High donor concentrations, on the other hand, yielded poorer results: when TAPC concentration was 50%, fill factor was 0.29. External quantum efficiency (EQE) also peaked with a low TAPC concentration. At TAPC 5%, EQE was about 60% while at TAPC 50%, EQE was only 10%.

Researchers also found that the commonly used HOMO/LUMO gap rule did not directly affect Voc as long as donor concentrations were sufficiently low. Instead, the Voc was largely determined by the Schottky barrier, a difference in charge formed between a metal and semiconductor, created between the MoOx and BHJ layers.

Zhang et al. (2011) achieved an impressive conversion ratio of 5.23% in their tests. They concluded that a low donor concentration maximized efficiency in BHJ OPVs and recognized the importance of the MoOx for generating the electric field

Operating Lifetimes of Organic Photovoltaics

Organic photovoltaics (OPVs) are a type of solar cell that has recently been receiving much research.  Though such polymer solar cells have low conversion efficiencies when compared with expensive nonorganic multijunction solar cells, OPVs are also much cheaper to produce and can be inexpensively manufactured in large quantities.  Low conversion efficiencies is not the only drawback of OPVs; organic materials are much more vulnerable to degradation from environmental factors, and the lifetimes of OPVs must be seriously considered as part of the cost benefit analysis.  Peters et al. (2011) present conclusions from their study of the efficiency decay of two types of OPVs: the well-studied P3HT and relatively new PCDTBT devices.  The study concludes that the less researched PCDTBT devices decayed at a slower rate than P3HT though the P3HT had, in general, a higher absolute conversion efficiency figure.  Researchers expect that further development of PCDTBT solar cells will raise their conversion efficiency while maintaining their long lifetime.—Alan Hu
Peters, C., Sachs-Quintana, I., Kastrop, J., Beaupre, S., Leclerc, M., McGehee, M., 2011. High Efficiency Polymer Solar Cells with Long Operating Lifetimes. Advanced Energy Materials 1, 491–494.

Peters et al. at Stanford University and University of Laval test the lifetimes of P3HT and PCDTBT devices by exposing an experimental group of the two types of solar cells to a standardized environment.  Initial device efficiencies of P3HT and PCDTBT respectively were 4 ± 0.05%. and 5.5 ± 0.15%.  Since UV radiation is known to cause defects in polymer solar cells and that commercial OPVs are likely to carry UV blockers, an LG sulfur plasma lamp, which emits very little UV radiation, was used as a source of light.  The solar cells were kept in a dark room for a week after fabrication before being placed under the lamp.  The cells were then aged at maximum power point for 4400 hours under one-sun intensity at 37ºC.  Light intensity was calibrated with a National Renewable Energy Lab-certified silicon photodiode.  Both light intensity and temperature were measured every 5 seconds; current-voltage curve data was collected every hour.  The researchers defined burn-in of solar devices as the short period of exponential loss in efficiency after initial use.  Lifetime ends by convention when the efficiency of the device has dropped below 80% of its initial value.
Results from the experiment show that PCDTBT solar cells had lower efficiency ratios immediately after burn-in but suffer from less degradation than P3HT.  PCDTBT had a burn-in period of about 400 hours after which its efficiency figures remained relatively stable.  The VOC and fill factor of PCDTBT in particularly remained practically flat for 4000 hours after the initial burn-in.  P3HT devices on the other hand experienced a roughly 10% drop in efficiency per 1000 hours. This was caused by a simultaneous decrease in both VOC  and JSC­.
            A burn-in demarcation was set at 1300 hours and the lifetime of each type device tested was found with a linear regression.  Assuming 5.5 hours of one-sun intensity per day and 365 days per year, the average lifetimes of PCDTBT and P3HT solar cells were found to be 6.2 and 3.2 years respectively. As such, PCDTBT solar cells demonstrated a clear advantage over P3HT solar cells in terms of durability; one particular PCDTBT device was so remarkably stable that it was projected to have a lifetime of 11 years.
            A laser beam-induced current map, a test used to determine if a portion of the solar cell has lost effectiveness, shows that both P3HT and PCDTBT showed no loss of device area after 200 hours of aging.
            PCDTBT solar cells experience a more dramatic loss of efficiency over the burn-in period but are far more stable than P3HT solar cells after this initial period.  P3HT cells currently hold a conversion efficiency advantage over PCDTBT cells though considering the nascent stage of research into the PCDTBT polymer, PCDTBT cells have the potential for large advances in efficiency.  The researchers predict that as PCDTBT cells become more optimized, both longevity and greater efficiency can be achieved in the inexpensive OPVs.

Feasibility and Costs of II-VI Materials in Multijunction Solar Cells

Solar energy is often hailed as the successor to fossil fuels as the planet’s main source of energy. However, solar cells face various issues affecting widespread adoption including prohibitive costs and low energy conversion rates. Currently, multijunction solar cells are the most efficient. These cells are able to boost conversion rates by employing different junctions of semiconductors that utilize different wavelengths of light. The most commonly used semiconductors belong to the III-V group due to perceived advantages over II-VI group semiconductors. Garland et al (2011) argue that II-VI semiconductors are both more efficient and less expensive than III-V semiconductors. Results from models and initial experimentation indicate that II-VI solar cells are 3–4% more efficient than III-V solar cells.—Alan Hu

Garland, J. W., T. Biegala, M. Carmody, C. Gilmore, and S. Sivananthan. Next-generation Multijunction Solar Cells: The Promise of II-VI Materials. Applied Physics Letters 109, 102423(2011).

Garland and colleagues of EPIR Technologies project III-V and II-VI solar cell output with the commonly accepted “standard” model put forth by Xu et al. (2010) in an earlier paper. The model uses a beta coefficient that takes such factors into account as the dimensions and doping (the molecular makeup) of the semiconductor. The beta is calculated through a best fit line describing modeled data and figures from real world performance of the latest III-V solar cell. Projected efficiency is then computed by dividing the sum of the junction outputs by the power input. The researchers supplement their projected figures with real world figures generated through actual experimentation. Garland and colleagues also grew II-VI semiconductors and collected empirical data on the solar cells.

The results from both projections and empirical observation supported the argument in favor of II-VI solar cells. Calculated efficiency for a III-V solar cell under one sun, a measure of sun intensity, was 43.7% whereas the figure for a II-VI solar cell was 49.7%. Empirical observations agree with these results: III-V solar cells were observed to have achieved 38.6% under one sun whereas II-VI solar cells achieved 44.5% under the same conditions.

The study also argues that II-VI solar cells could bring about significant reductions in manufacturing cost. The authors claim that due to the sturdy nature of silicon wafers used in the production of II-VI solar cells and lower costs of growing II-VI crystals, nearly all associated costs of creating II-VI semiconductors are lower than those of creating III-V semiconductors. Specifically, molecular beam epitaxy (MBE), which is a process for growing crystals, can be replaced by a cheaper production line method of production due to the nature of II-VI materials.

The lower cost of II-VI solar cells means that medium concentration photovoltaics can be used instead of high concentration photovoltaics. III-V solar cells were relatively costly; as such, it was cheaper to have fewer solar cells and instead have a system of mirrors that concentrated sunlight onto a small area of solar cells. This meant the costs of a solar energy field were increased by the installation of such tracking systems. The cheaper II-VI solar cells allow for a relatively larger area of solar cells and less complicated tracking systems.

The search for a commercially viable alternative to fossil fuels continues. As long as solar cells are more expensive and less effective than existing energy sources, the widespread adoption of solar energy is unlikely. However, incremental advances in photovoltaic technology are gradually cheapening the cost of solar energy. The improvement of 3–4% in energy conversion rates brought about by the use of II-VI semiconductors is one small step toward a greener future.

D. Xu, T. Biegala, M. Carmody, J. W. Garland, C. Grein, and S. Sivananthan. Proposed monolithic triple-junction solar cell structures with the potential for ultrahigh efficiencies using II–VI alloys and silicon substrate. Applied Physics Letters. 96, 073508(2010).