Flexible polymer photovoltaic modules with incorporated organic bypass diodes to address module shading effects

We present experimental results on large-area low-cost processed flexible organic photovoltaic (OPV) modules incorporating organic bypass diodes to eliminate the negative effects of shading on the module power output. A fully organic-based structure (organic solar module combined with an organic bypass diode) is essential to allow monolithic interconnection of the bypass diode during the solar module production within the same printing steps. The origin of shading losses in organic photovoltaic modules is analyzed in detail, and guidelines for the design and architecture of flexible OPV modules are derived. Inorganic and organic diodes were tested on their functionality as bypass diodes, and a set of diode specifications to minimize shading losses is summarized. Organic bypass diodes were found to efficiently reduce the adverse shading effects in OPV modules.     

Large-area organic photovoltaic module—Fabrication and performance

Here we describe the fabrication of the largest (233 cm² total area) organic photovoltaic (OPV) module (polymer:fullerene) to be certified by the National Renewable Energy Laboratory (NREL). OPV solar cells were fabricated at Plextronics by spin coating a blend of poly 3-hexylthiophene-2,5 diyl (P3HT) and [6,6] phenyl C₆₁ butyric acid methyl ester (PCBM) on top of our hole transport layer (HTL), Plexcore^® OC. In laboratory-scale devices (0.09 cm²), this system routinely exhibits power conversion efficiencies exceeding 3.7%. This P3HT:PCBM active layer and HTL ink system was used to scale up to the larger area module (15.2 cm×15.2 cm module size, i.e. 233 cm² total area; 108 cm² active area), which was certified by NREL as having 1.1% total area efficiency (3.4% active area efficiency).     

Organic photovoltaic devices with a crosslinkable polymer interlayer

Organic photovoltaic devices with a photo-crosslinkable interlayer were fabricated. This photo-crosslinkable interlayer acted as a leakage current reducing buffer layer. The performance of the small area OPV cell (0.04 cm²) was enhanced by the increase in the short circuit current and the fill factor. When a larger area cell (1 cm²) was used, the performance of OPV cell increased when the appropriate interlayer thickness was used. In the case of a 10 cm×10 cm module, the power conversion efficiency was about double than that without the interlayer. The insertion of the interlayer increased the current extraction by lowering the barrier height and attenuated the fill factor reduction by enhancing the rectification with a better leakage current sealing. From this study, it is clearly proved that the insertion of the appropriate photo-crosslinkable layer improves the performance of OPV devices, the effect was especially evident for large area cells.     

Morphological and electrical effect of an ultrathin iridium oxide hole extraction layer on P3HT:PCBM bulk-heterojunction solar cells

An ultrathin iridium layer was treated with O₂-plasma to form an iridium oxide (IrO_x), employed as a hole extraction layer in order to replace poly(3,4-ethylenedioxythiophene):poly(styrene-sulfonate) (PEDOT:PSS) in organic photovoltaic (OPV) cells with poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester (P3HT:PCBM). The IrO_x layer affects the self-organization of the P3HT:PCBM photo-active layer due to its hydrophobic nature, inducing a well-organized intraplane structure with lamellae oriented normal to the substrate. Synchrotron radiation photoelectron spectroscopy results showed that the work function increased by 0.57 eV as the Ir layer on ITO changed to IrO_x by the O₂-plasma treatment. The OPV cell with IrO_x (2.0 nm) exhibits increased power conversion efficiency as high as 3.5% under 100 mW cm⁻² illumination with an air mass (AM 1.5G) condition, higher than that of 3.3% with PEDOT:PSS.     

Outdoor testing of PV module temperature and performance under different mounting and operational conditions

Outdoor performance of photovoltaic (PV) modules primarily depends on the instantaneous plane-of-array irradiance (G_poa) and PV module temperature (T_pv). T_pv can be estimated from the ambient temperature (T_amb) and the G_poa as T_pv=T_amb+k_TG_poa. The coefficient k_T depends strongly on the way the PV module is mounted (open rack, ventilated or unventilated roof mounting, etc.), wind speed and also on the module type. In the presented paper, open rack mounted and unventilated roof integrated cases of PV module installation are experimentally and theoretically examined. Linear relationship with k_T is upgraded with a nonlinear one based on the energy balance model and measured data. Nevertheless, T_pv is also affected by the module’s regime of operation. The T_pv dependency on different regime of operation (open-circuit and maximum power point tracking) for two types of PV modules in two regimes is reported. Differences are discussed in light of energy balance equation within thermal management, where impact of the PV module conversion efficiency on T_pv is also shown.     

A better solar module performance obtained by employing an infrared water filter

In this work the authors report the results obtained from the characterization of a S_i photovoltaic module, which was protected with an infrared filter (water filter). The photovoltaic parameters such as short circuit current (I_sc), open circuit voltage (V_oc), maximum current (I_m), maximum voltage (V_m), fill factor, efficiency (η) and maximum power of the module were determined and compared with and without an infrared filter. A noticeable improvement in the photovoltaic parameters of the module was observed when a water filter was employed.     

Enhanced heat dissipation of V-trough PV modules for better performance

A concentrator photovoltaic (PV) module, in which solar cells are integrated in V-troughs, is designed for better heat dissipation. All channels in the V-trough channels are made using thin single Al metal sheet to achieve better heat dissipation from the cells under concentration. Six PV module strips each containing single row of 6 mono-crystalline Si cells are fabricated and mounted in 6 V-trough channels to get concentrator V-trough PV module of 36 cells with maximum power point under standard test condition (STC) of 44.5 W. The V-trough walls are used for light concentration as well as heat dissipation from the cells which provides 4 times higher heat dissipation area than the case when V-trough walls are not used for cooling. The cell temperature in the V-trough module remains nearly same as that in a flat plate PV module, despite light concentration. The controlled temperature and increased current density in concentrator V-trough cells results in higher V_oc of the module.     

Bacterial Foraging Algorithm based solar PV parameter estimation

The abundance and non-polluting nature of solar energy has aroused the interest of many researchers. This worldwide attention of photovoltaic panels has led to the need of generating accurate model for solar photovoltaic (PV) module before proceeding to the installation part. However, accurate modeling of solar PV characteristics is difficult; since the manufacturer’s datasheet provides only four values such as V_mp, I_mp, V_oc, and I_sc. Further, for accurate modeling precise estimation of model parameters at different environmental conditions are very essential. On the other hand, optimization technique is a very powerful tool to obtain solutions to complex non-linear problems. Hence, in this paper, Bacterial Foraging Algorithm is proposed to model the solar PV characteristics accurately. A new equation has been evolved to determine the values of V_oc, V_mp accurately; since these values decides the closeness of the simulated characteristics. Model parameters are extracted for three different types of solar PV panels. A systematic evaluation and performance comparison of Bacterial Foraging Algorithm with other optimization techniques such as Genetic Algorithm and Artificial Immune System has been done and the best computational technique is derived based on performance criteria such as accuracy, consistency, speed of convergence and absolute error. Extensive computations are carried out for the proposed method, as well as for Genetic Algorithm and Artificial Immune System to substantiate the findings.     

Sensitivity analysis and more accurate solution of photovoltaic solar cell parameters

A systematic solution method to obtain the five-parameter model of the photovoltaic (PV) solar cells (modules or arrays) in case of only the few data (V_oc, I_sc, V_mp, and I_mp) available from the manufacturers. To preset the proper initial values (such as R_sh) within their ranges and select the proper iterative expressions when solving the nonlinear equation system through a concise computer procedure, the basic ranges of parameters are defined and the sensitivities of parameters are graphically analyzed on condition that as few simplifications as possible are made. The stability and effectiveness of the solved five parameters are kept and the convergence is very fast owing to the automatic error corrections during iterative computation. The resultant I-V and P-V characteristics and the experimental results are almost unanimous and show small errors, especially near the maximum power point.     

Exergoenvironmental analysis and optimization of a cogeneration plant system using Multimodal Genetic Algorithm (MGA)

In the present work, a combined heat and power plant for cogeneration purposes that produces 50 MW of electricity and 33.3 kg/s of saturated steam at 13 bar is optimized using genetic algorithm. The design parameters of the plant considered are compressor pressure ratio (r_AC), compressor isentropic efficiency (η_comp), gas turbine isentropic efficiency (η_GT), combustion chamber inlet temperature (T₃), and turbine inlet temperature (TIT). In addition, to optimally find the optimum design parameters, an exergoeconomic approach is employed. A new objective function, representing total cost rate of the system product including cost rate of each equipment (sum of the operating cost, related to the fuel consumption) and cost rate of environmental impact (NO_x and CO) is considered. Finally, the optimal values of decision variables are obtained by minimizing the objective function using evolutionary genetic algorithm. Moreover, the influence of changes in the demanded power on various design parameters are parametrically studied for 50, 60, 70 MW of net power output. The results show that for a specific unit cost of fuel, the values of design parameters increase, as the required, with net power output increases. Also, the variations of the optimal decision variables versus unit cost of fuel reveal that by increasing the fuel cost, the pressure ratio, r_AC, compressor isentropic efficiency, η_AC, turbine isentropic efficiency, η_GT, and turbine inlet temperature (TIT) increase.     

Outdoor performance of amorphous silicon and polycrystalline silicon PV modules

The daily watt-hour efficiency (η_Wh) and daily integrated output power (P_Wh) of the a-Si and poly-Si module have been used to examine the performances of both modules on the basis of two years' data accumulated at outdoor conditions. Results from the analysis of experimental data taken under incident solar energy higher than 3.0 kWh/m² per day show that the annual average of η_Wh of the a-Si module is about 95% and 92.5% of its efficiency at STC condition at the first and second year, respectively, while the values are nearly unchanged at about 89% for the poly-Si module. During a one year period, the average P_Wh of the a-Si and poly-Si module was about 60% and 56%, respectively, of their calculated output power at STC condition, so that the P_Wh for each watt-peak (W_p) of the maximum power of a-Si module is about 11% higher than that of the poly-Si module.     

Maximum power output of a solar PV module at various latitudes as influenced by the swing angle of the sun

The effect of the maximum swing angle of the sun (0°, 15°, 30°, 45° and 60°) away from the normal is observed on the maximum power output of an amorphous silicon solar PV module using a solar simulator. Studies reveal that as the panel inclination is increased, the maximum power produced (P_mp) by the module decreases. Solar equations are used to compute the maximum swing angle of the sun away from the normal position of the panel at noon (λ) during March, June, September and December months and are computed for various selected locations such as Mumbai, Ludhiana, Fargo, London and Moscow. An analogy between the simulated study and λ (for real operation conditions) for polar-mounted inclinations of PV panels is established and the effect of λ on the percent reduction of maximum power produced (P_mp) by the PV solar panel is studied.     

Co-sintering anode and Y2O3 stabilized ZrO2 thin electrolyte film for solid oxide fuel cell fabricated by co-tape casting

Fabricating a large-area unit cell is very important for the development of solid oxide fuel cell (SOFC) stack. In this study, details of sintering process of half cell with NiO-yttria stabilized zirconia (YSZ) anode-supported YSZ thin electrolyte film fabricated by co-tape casting have been discussed. The results demonstrates that the shrinkages and shrinking rates mismatches between the electrolyte and the anode can be controlled by the organic additive content in the anode slurry composition and heating rate. Low heating rate suppresses the cracks formation in the electrolyte films. A warp-free unit cell with size of 100 × 100 mm² and dense electrolyte has been successfully fabricated. A power of 22.2 W, with a power density of 0.27 W cm⁻² has been achieved at 0.7 V and 750 °C in O₂/humidified H₂ atmosphere. The area specific resistance of the cell is 1.20 Ω cm² at 0.7 V and 750 °C.     

Use of porous baffles to enhance heat transfer in a rectangular channel

An experimental investigation was carried out to measure module average heat transfer coefficients in uniformly heated rectangular channel with wall mounted porous baffles. Baffles were mounted alternatively on top and bottom of the walls. Heat transfer coefficients and pressure loss for periodically fully developed flow and heat transfer were obtained for different types of porous medium (10, 20, and 40 pores per inch (PPI)) with two window cut ratios (B_h/D_h=1/3 and 2/3) and two baffle thickness to channel hydraulic diameter ratios (B_t/D_h=1/3 and 1/12). Reynolds number (Re) was varied from 20,000 to 50,000. To compare the effect of foam metal baffle, the data for conventional solid-type baffle were obtained for (B_t/D_h=1/3). The maximum uncertainties associated with module Nusselt number and friction factor were 5.8% and 4.3% respectively. The experimental procedure was validated by comparing the data for the straight channel with no baffles (B_h/D_h=0) with those in the literature [Publications in Engineering, vol. 2, University of California, Berkeley, 1930, p. 443; Int. Chem. Eng. 16 (1976) 359]. The use of porous baffles resulted in heat transfer enhancement as high as 300% compared to heat transfer in straight channel with no baffles. However, the heat transfer enhancement per unit increase in pumping power was less than one for the range of parameters studied in this work. Correlation equations were developed for heat transfer enhancement ratio and heat transfer enhancement per unit increase in pumping power in terms of Reynolds number.     

Improved size distribution of AgBiS2 colloidal nanocrystals by optimized synthetic route enhances photovoltaic performance

Ternary silver bismuth sulfide (AgBiS₂) colloidal nanocrystals (NCs) have been recognized as a photovoltaic absorber for environmentally-friendly and low-temperature-processed thin film solar cells. However, previous synthetic methods involving hot injection of sulfur precursors into metal oleate precursor solutions do not provide a balance between nucleation and growth, leading to AgBiS₂ NCs with broad size distributions. Here, we demonstrate the modified synthetic route that size distribution of AgBiS₂ NCs can be improved by pre-adding the non-coordinating 1-octadecene (ODE) solvent into metal precursor solutions, leading to controlled concentration of coordinating oleic acid with improved hot-injection synthetic conditions. The addition of ODE as a non-coordinating solvent to metal precursor/oleic acid solution significantly suppresses variations in the concentration of coordinating oleic acid after injection of the sulfur precursor solution, leading to a homogenous reaction between the metal and sulfur precursors. For photovoltaic devices fabricated using the resultant AgBiS₂ NCs, the champion device shows power conversion efficiency (PCE) of 5.94% with an open-circuit voltage (V_OC) of 0.52 V. This performance is better than that a control device (PCE of 5.50% and V_OC of 0.49 V) because of the reduced energetic disorder and band tail broadening originating from the uniformly-sized AgBiS₂ NCs.     

Fabrication of organic photovoltaic cells with double-layer ZnO structure

The fabrication process of a photovoltaic cell with a structure of indium-tin-oxide (ITO)/double ZnO/C₆₀/poly(3-hexylthiophene) (PAT6)/Ag has been investigated. The C₆₀/PAT6 heterojunction of this cell was fabricated by spin-coating a chloroform solution of PAT6 onto the C₆₀ thin film formed on double-layer ZnO-coated ITO. The fabrication of this double-layer ZnO was a new method, which was a composite of a sputtered ZnO layer and oriented zinc oxide nanograins layer fabricated at low temperature (343 K). Insertion of the double-layer ZnO in the photovoltaic cells produced enhanced performance with the power conversion efficiency of 1.31% under AM1.5 illumination.     

Organic photovoltaics for low light applications

Here we report on organic photovoltaic's (OPV) suitable for low light applications. In this paper, we illustrate the impact of R_s and R_p for indoor and outdoor applications. In addition, we propose a simple physics approach to predict the behavior of organic solar cells under various illumination intensities through electrical modeling. The combination of simulation and modeling allows to define a set of design rules for OPVs under low light illumination. The performance of various organic solar cells under low light intensity is compared with our predictions and excellent correlation is found. OPV shows high performance under low light conditions.     

Spray deposition of electrohydrodynamically atomized polymer mixture for active layer fabrication in organic photovoltaics

Printing and spray technologies are the most recent and novel approaches to form organic photovoltaics (OPV) with inexpensive, high speed, and environmentally friendly process. With an electrohydrodynamic atomization (EHDA) approach, the active layer composed of polymer mixture (P3HT:PCBM) was successively fabricated. Operating conditions for obtaining the stable cone jet mode were determined with various applied voltages and liquid feed flow rates. The size distribution of EHDA droplets was characterized by aerodynamic particle sizer (APS) measurement. The mode diameters of the droplets were 580 and 670 nm, respectively, when the liquid flow rates were 1 and 20 μl/min. The maximum power conversion efficiency of 0.48% was obtained under AM 1.5 solar simulation for an OPV device fabricated in air.     

Improved the efficiency of small molecule organic solar cell by double anode buffer layers

Small molecule organic solar cell with an optimized hybrid planar-mixed molecular heterojunction (PM-HJ) structure of indium tin oxide (ITO)/ poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT: PSS) doped with 4 wt% sorbitol/ pentacene (2 nm)/ copper phthalocyanine (CuPc) (10 nm)/ CuPc: C₆₀ mixed (20 nm)/ fullerene (C₆₀) (20 nm)/ bathocuproine (BCP) (10 nm)/Al was fabricated. PEDOT: PSS layer doped with 4 wt% sorbitol and pentacene layer were used as interlayers between the ITO anode and CuPc layer to help the hole transport. And then the short-circuit current (J_sc) of solar cell was enhanced by inserting both the PEDOT: PSS (4 wt% sorbitol) and the pentacene, resulting in a 400% enhancement in power conversion efficiency (PCE). The maximum PCE of 3.9% was obtained under 1sun standard AM1.5G solar illumination of 100 mW/cm².     

Fabrication and transport mechanisms of 2-(2,3-dihydro-1,5-dimethyl-3-oxo-2-phenyl-1H-pyrazol-4-ylimino)-2-(4-nitrophenyl)acetonitrile/p-silicon hybrid solar cell

2-(2,3-Dihydro-1,5-dimethyl-3-oxo-2-phenyl-1H-pyrazol-4-ylimino)-2-(4-nitrophenyl) acetonitrile (DOPNA) is an organic compound; it has been synthesized and examined as a photovoltaic material in thin-film form grown on single crystal of p-type silicon wafer substrate (p-Si). A polycrystalline structure has been formed in as-synthesized powder. Nano-crystallite grains are formed in the as-deposited film. The dark current-voltage (I-V) characteristics of Au/p-DOPNA/p-Si/Al heterojunction diode measured at different temperatures ranging from 298 to 423 K have been investigated. The operating conduction mechanisms, series and shunt resistances, rectification ratio, ideality factor and potential barrier height were determined. The capacitance-voltage (C-V) measurements of the device were performed in dark condition and analyzed to determine carrier concentration and built-in potential. Solar cell parameters were also evaluated and the power conversion efficiency was estimated as 5.74%.