The influence of electrode buffer layers on the performance of polymer photovoltaic devices

The influence of electrode buffer layers (EBLs) on the performance of polymer photovoltaic (PV) devices based on blends of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C₆₁-buytyric acid methyl ester (PCBM) has been investigated. Copper phthalocyanine (CuPc), bathocuproine (BCP) and pentacene are used as EBLs of solar cells to improve the photovoltaic performances, including short circuit current density (Jsc), open circuit voltage (Voc), power conversion efficiency (PCE) and fill factor (FF). The obtained results suggest that the insertion of EBLs can greatly improve the Jsc and PCE of the PV devices. And the effect of thickness of the four EBLs has also been explored and compared. The PV device with 1 nm BCP cathode buffer layer gets the highest PCE, which is 2.6 times that of the PV device without EBL.     

Study of the effect of annealing process on the performance of P3HT/PCBM photovoltaic devices using scanning-probe microscopy

We have studied the effect of annealing process on the performance of photovoltaic devices based on the bulk heterojunction of poly(3-hexylthiophene) and [6,6]-phenyl-C₆₁ butyric acid methyl ester (P3HT/PCBM). By means of atomic force microscopy (AFM) and scanning of near-field microscopy (SNOM), we can observe the morphology evolution of the annealed P3HT/PCBM composite films. We also studied the changes of optical properties by absorption spectroscopy and the changes of composition distribution of annealed composite films. The results indicate the P3HT in the composite film gradually becomes an ordered structure with annealing. The ordered P3HT facilitates the charge transport. However, the film exhibits a large-scale (1 μm) PCBM aggregation after annealing for an extended period of time. The disrupted bi-continous phase retards the charge transport. Thus, the device efficiency reaches the highest (2.308%) after annealing at 140 °C for 30 min but decreases to 0.810% after 60 min annealing.     

Nanostructured polymer blends (P3HT/PMMA): Inorganic titania hybrid photovoltaic devices

We have fabricated a photovoltaic (PV) device based on the polymer blends of (poly(3-hexylthiophene) (P3HT)/polymethylmethacrylate (PMMA)) and inorganic TiO₂ nanorod bulk heterojunction. The optimized photovoltaic device with 1.6 wt% PMMA concentration has a power conversion efficiency of 0.65% under simulated AM 1.5 illumination (100 mW/cm²), which is 38% more efficient than the device without the incorporation of PMMA. Furthermore, the PMMA-included device gives a short-circuit current density of 2.57 mA/cm², an open-circuit voltage of 0.53 V, and a fill factor of 0.48. Our studies have shown that having optimal PMMA concentration in the photovoltaic devices helps to smoothen the surface of the hybrid thin film, broaden the absorption spectrum, and improve the electrical conductivity. The results implying improvement in cell performance can be illustrated using atomic force microscopy (AFM), a UV/vis spectrophotometer and electrical measurements.     

Numerical analysis and optimization of a spectrum splitting concentration photovoltaic–thermoelectric hybrid system

This paper presents the numerical modeling and optimization of a spectrum splitting photovoltaic–thermoelectric (PV–TE) hybrid system. In this work, a simulation model is established in consideration of solar concentration levels and several heat dissipation rates. Exemplarily, the performance of a hybrid system composed of a GaAs solar cell and a skutterudites CoSb₃ solar thermoelectric generator (TEG) is simulated. Analysis under different conditions has been carried out to evaluate the electrical and thermal performance of the hybrid system. Results show that the cutoff-wavelength of the GaAs–CoSb₃ hybrid system is mainly determined by the band gap of solar cell, when the solar concentration ratio is ranged between 550 to 770 and heat transfer coefficient h = 3000–4500 W/m² K, the hybrid system has good electrical performance and low operating temperatures. Based on the analysis of the GaAs–CoSb₃ hybrid system, guidelines for the PV–TE system design are proposed. It is also compared with a PV-only system working under the same cooling condition; results show that the PV–TE hybrid system is more suitable for working under high concentrations.     

Influence of the microstructure on the supercapacitive behavior of polyaniline/single-wall carbon nanotube composites

Polyaniline/single-wall carbon nanotube (PANI/SWCNT) composites were prepared by in situ potentiostatic deposition of PANI onto SWCNTs at the potential of 0.75 V versus SCE, with the aim to investigate the influence of microstructure on the specific capacitance of PANI/SWCNT composites. It was found that the specific capacitance of the PANI/SWCNT composites is strongly influenced by their microstructure, which is correlated to the wt.% of the PANI deposited onto the SWCNTs. The optimum condition, corresponding to the highest specific capacitance, 463 F g⁻¹ (at 10 mA cm⁻²), was obtained for 73 wt.% PANI deposited onto SWCNTs. The specific capacitance of the PANI/SWCNT composite electrode was highly stable, with a capacitive decrease of 5% during the first 500 cycles and just 1% during the next 1000 cycles, indicative of the excellent cyclic stability of the composite for supercapacitor applications.     

Maximising the energy output of a PVT air system

Simultaneously generating both electricity and low grade heat, photovoltaic thermal (PVT) systems maximise the solar energy extracted per unit of collector area and have the added benefit of increasing the photovoltaic (PV) electrical output by reducing the PV operating temperature. A graphical representation of the temperature rise and rate of heat output as a function of the number of transfer units NTUs illustrates the influence of fundamental parameter values on the thermal performance of the PVT collector. With the aim of maximising the electrical and thermal energy outputs, a whole of system approach was used to design an experimental, unglazed, single pass, open loop PVT air system in Sydney. The PVT collector is oriented towards the north with a tilt angle of 34°, and used six 110 Wp frameless PV modules. A unique result was achieved whereby the additional electrical PV output was in excess of the fan energy requirement for air mass flow rates in the range of 0.03–0.05 kg/s m². This was made possible through energy efficient hydraulic design using large ducts to minimise the pressure loss and selection of a fan that produces high air mass flow rates (0.02–0.1 kg/s m²) at a low input power (4–85 W). The experimental PVT air system demonstrated increasing thermal and electrical PV efficiencies with increasing air mass flow rate, with thermal efficiencies in the range of 28–55% and electrical PV efficiencies between 10.6% and 12.2% at midday.     

Investigation of multi-donor bulk-heterojunction photovoltaic cells based on P3HT:PCBM system

(2,7-bis[5′-(9,9-dioctylfluorene-2-yl)-2,2′-dithienyl-5-yl]-9,9-dioctylfluorene) (F3Th4) was used as a secondary electron donor material in the poly(3-hexylthiophene) (P3HT):[6,6]-phenyl C61-butyric acid methyl ester (PCBM) bulk-heterojunction photovoltaic cell. It is shown that the combination of F3Th4 with P3HT allows strong light absorption. The mechanism of charge transfer in the multi-donor PV cell was investigated; it shows that efficient energy transfer takes place from F3Th4 to P3HT. However, the short-circuit current (J_SC) of the multi-donor bulk-heterojunction photovoltaic (PV) cell still decreased. The possible reason for the smaller photocurrent is worsening of transport property after addition of F3Th4.     

Light harvesting in organic solar cells

We present a methodology which allows designing photonic crystals slabs (PCs) able to couple incident light into “slow Bloch modes” (SBMs) and dealing with their incorporation in an organic solar cell (OSC). We theoretically study different structures based on the same couple of organic materials (poly-3-hexylthiophène (P3HT) as donor and [6,6]-phenyl-C61-butiryc acid methyl ester (PCBM) as acceptor): a 2D photonic crystal based on a perfectly ordered P3HT/PCBM blend (placed in the air), a 1D photonic crystal based on a nanostructured PEDOT:PSS (Poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate)) layer embedded in a P3HT:PCBM host matrix (first placed in the air and then inserted in an organic solar cell) and finally a 1D photonic crystal based on a nanostructured P3HT:PCBM layer covered by a metallic electrode and inserted in an OSC ( with and without nanostructuration of the PEDOT:PSS layer). We show that the light coupling into SBMs in an OSC depends on vertical interferences and that optical spacers are needed. We then demonstrate that the P3HT:PCBM active layer nanostructuring covered by a thick metallic electrode exhibits the highest gain (4% in the 400–700 nm spectral range) thanks to a simultaneous optimisation of the optical properties of the photonic crystal (coupling of SBM) and of the stack of the organic solar cell (vertical interferences).     

Design,preparation, and durability of TiO2/SiO2 and ZrO2/SiO2 double-layer antireflective coatings in crystalline silicon solar modules

This paper reports the use of a combination of numerical calculations and experimental work to establish the optimum photovoltaic transmittance (T_pv) and durability of the quarter wave, the quarter-half wave, and the non-quarter wave double-layer TiO₂–SiO₂ and ZrO₂–SiO₂ antireflective coatings (ARCs) on solar glass towards practical photovoltaic applications. Numerical calculations based on 4 × 4 propagation matrix method indicated that the non-quarter wave double-layer ARCs exhibited higher T_pv values than those of the quarter wave and the quarter-half wave ARCs. Such calculated values are in good agreement with the experimental T_pv values. For examples, the T_pv values for the non-quarter wave double-layer TiO₂–SiO₂ and ZrO₂–SiO₂ ARCs prepared by sol–gel reached 94.4 ± 0.1% and 94.3 ± 0.1%, respectively. In terms of the coating durability, the non-quarter wave double-layer coatings with a dense and thicker TiO₂ or ZrO₂ barrier layer on solar glass exhibited less than 1% reduction in T_pv after 96 h highly-accelerated temperature and humidity stress test (HAST), as compared with the standard single-layer porous SiO₂ used in industry which tested in the same HAST conditions to be greater than (15.4%) after 48 h. Single crystalline Si modules encapsulated by the non-quarter wave TiO₂–SiO₂ or ZrO₂–SiO₂ AR-coated glass are more durable, with only less than 10% degradation in efficiency after 48 h HAST, as compared with Si modules encapsulated by single-layer porous SiO₂ AR-coated glass which have signification loss in efficiency (circa. 21.8%).     

Technical and economic assessment of grid-independent hybrid photovoltaic–diesel–battery power systems for commercial loads in desert environments

Solar photovoltaic (PV) hybrid system technology is a hot topic for R&D since it promises lot of challenges and opportunities for developed and developing countries. The Kingdom of Saudi Arabia (KSA) being endowed with fairly high degree of solar radiation is a potential candidate for deployment of PV systems for power generation. Literature indicates that commercial/residential buildings in KSA consume an estimated 10–45% of the total electric energy generated. In the present study, solar radiation data of Dhahran (East-Coast, KSA) have been analyzed to assess the techno-economic viability of utilizing hybrid PV–diesel–battery power systems to meet the load requirements of a typical commercial building (with annual electrical energy demand of 620,000 kW h). The monthly average daily solar global radiation ranges from 3.61 to 7.96 kW h/m². NREL's HOMER software has been used to carry out the techno-economic viability. The simulation results indicate that for a hybrid system comprising of 80 kWp PV system together with 175 kW diesel system and a battery storage of 3 h of autonomy (equivalent to 3 h of average load), the PV penetration is 26%. The cost of generating energy (COE, US$/kW h) from the above hybrid system has been found to be 0.149 $/kW h (assuming diesel fuel price of 0.1 $/L). The study exhibits that for a given hybrid configuration, the operational hours of diesel generators decrease with increase in PV capacity. The investigation also examines the effect of PV/battery penetration on COE, operational hours of diesel gensets for a given hybrid system. Emphasis has also been placed on unmet load, excess electricity generation, percentage fuel savings and reduction in carbon emissions (for different scenarios such as PV–diesel without storage, PV–diesel with storage, as compared to diesel-only situation), cost of PV–diesel–battery systems, COE of different hybrid systems, etc.     

Experimental study of natural convection in PV roof solar collector

The aim of this paper is to propose the PV roof solar collector (PV-RSC) to investigate the natural convection heat transfer and estimated the convective heat transfer coefficient in the channel. The experimental set-up was composed of a PV panel on the upper layer and the lower layer is aluminum plate of the channel. The inclination angle and air gap of channel were fixed at 30° and 15 cm, respectively. The channel width is 0.7 m, and length is 1.2 m. The data analysis were confirmed the effect of radiative exchange influent to natural convection within the channel. On the basis of the experimental results, an empirical formula is found; the Nu as a function of Ra_s sin30, that is Nu_s = 0.3282 (Ra_s sin30)^0.2249. The correlation obtained to range 3 × 10⁸ < Ra_s sin30 < 7 × 10⁸. A comparison between PV-RSC and normal PV panel, it was confirmed that the PV-RSC could be generated electric power than that normal PV panel by about 30 W; and also the percentage of power generation increase was rising about 25% throughout the day.     

Optical efficiency of a PV–thermal hybrid CPC module for high latitudes

The advantage of PV–thermal hybrid systems is their high total efficiency. By using concentrating hybrid systems, the cost per energy produced is reduced due to simultaneous heat and electricity production and a reduced PV cell area. In this article, the optical efficiency of a water-cooled PV–thermal hybrid system with low concentrating aluminium compound parabolic concentrators is discussed. The system was built in 1999 in Älvkarleby, Sweden (60.5° N, 17.4° E) with a geometric concentration ratio of C=4 and 0.5 kW_p electric power. The yearly output is 250 kWh of electricity per square metre solar cell area and 800 kWh of heat at low temperatures per square metre solar cell area. By using numerical data from optical measurements of the components (glazing, reflectors, and PV cells) the optical efficiency, η_opt, of the PV–CPC system has been determined to be 0.71, which is in agreement with the optical efficiency as determined from thermal and electrical measurements. Calculations show that optimised antireflection-treated glazing and reflectors could further increase the electric power yield.     

Improvement of polymer/fullerene solar cells by controlling geometry of the ITO substrate surface

We have found that the short-circuit current, J_sc, of polymer/fullerene [RR-P3HT/C₆₀] solar cells has a clear dependence on the surface roughness of the ITO/glass substrate. We prepared an ITO surface with an average roughness, R_a, of 0.7–11 nm by chemical etching. At first J_sc increases with the increase in ITO surface roughness and then gradually decreases. The maximum performance was obtained at R_a≈4 nm. J_sc is also high with a very flat surface of R_a=0.7 nm. This feature can be attributed to the trade-off between the increase in absorption light path length and film-quality deterioration.     

Solar hybrid systems with thermoelectric generators

The possibility of using of thermoelectric generators in solar hybrid systems has been investigated. Four systems were examined, one working without radiation concentration, of the traditional PV/Thermal geometry, but with TEGs between the solar cells and heat extractor, and three other using concentrators, namely: concentrator – TEG ? heat extractor, concentrator ? PV cell ? TEG ? heat extractor, and PV cell – concentrator – TEG – heat extractor. The TEGs based on traditional semiconductor material Bi₂Te₃ and designed for temperature interval of 50–200 °C were studied experimentally. It was found that the TEG’s efficiency has almost linear dependence on the temperature difference ΔT between its plates, reaching 4% at ΔT = 155 °C (hot plate at 200 °C) with 3 W of power generated over the matched load. The temperature dependencies of current and voltage are also linear; accordingly, the power generated has quadratic temperature dependence. The experimental parameters, as well as parameters of two advanced TEGs taken from the literature, were used for estimation of performance of the hybrid systems. The conclusions are drawn in relation to the efficiency at different modes of operation and the cost of hybrid systems, as well as some recommendations in relation to optimal solar cells for applications in these systems.     

Embodied energy analysis of photovoltaic (PV) system based on macro- and micro-level

In this paper the energy payback time and CO₂ emissions of photovoltaic (PV) system have been analyzed. The embodied energy for production of PV module based on single crystal silicon, as well as for the manufacturing of other system components have been computed at macro- and micro-level assuming irradiation of 800–1200 W/m² in different climatic zones in India for inclined surface. The energy payback time with and without balance-of-system for open field and rooftop has been evaluated. It is found that the embodied energy at micro-level is significantly higher than embodied energy at macro-level. The effect of insolation, overall efficiency, lifetime of PV system on energy pay back time and CO₂ emissions have been studied with and without balance of system. A 1.2 kW_p PV system of SIEMENS for mudhouse at IIT, Delhi based on macro- and micro-level has been evaluated. The CO₂ mitigation potential, the importance and role of PV system for sustainable development are also highlighted.     

High-conductivity large-area semi-transparent electrodes for polymer photovoltaics by silk screen printing and vapour-phase deposition

Transparent electrodes based on PEDOT were prepared using a variety of techniques suitable for large area applications from 3,4-ethylenedioxythiophene (EDT) and Fe(III)-tosylate. High conductivities were obtained (∼20 Ω⁻¹) with moderate transmission in the UV-visible range 350–600 nm. We subsequently demonstrate the application of PEDOT electrodes to flexible polyethyleneterphthalate plastic substrates (PET) prepared by this procedure for polymer photovoltaic devices with active areas of 4.2 cm² using a 1:4 w/w mixture of MEHPPV and PCBM. We obtain typical efficiencies of 0.2% under simulated sunlight (AM1.5 at 1000 W m⁻²).     

Optimal inverter sizing considering cloud enhancement

A study of a photovoltaic (PV) array and inverter system installed in San Diego, California, was conducted in order to determine the energy losses due to inverter saturation (capping of inverter power output due to the PV array power output exceeding the inverter maximum power rating). Two mechanisms of saturation were considered: cloud enhancement (refers to an increased diffuse component of irradiance caused by clouds surrounding the unobstructed solar disk) and clear sky exceedance. For inverter sizing ratios (defined as R = inverter maximum AC output rating/PV DC rating) of R = 0.81 and R = 0.87 the annual energy losses as a percent of annual energy production were 2.65% and 2.20% using 1-s measurement resolution. Annual energy losses were calculated by aggregating the difference between modeled power (assuming no saturation occurred) and measured power. Losses due to cloud enhancement dominated the total losses, especially for R = 0.87. Increasing inverter size reduces saturation losses during high irradiance conditions, but decreases inverter conversion efficiency under low irradiance conditions. Increasing the sizing ratio to R = 1.22 would result in a maximum amount of energy production at our site. Averaging on timescales from 1-s to 1-h was performed to demonstrate that cloud enhancement losses can only be quantified using 10-s or finer measurements.     

Microthermophotovoltaic power generator with high power density

A promising energy source for portable MEMS devices, microthermophotovoltaic (micro-TPV) power generator, is described in this paper. The system mainly consists of a micro SiC combustor, and a GaSb photovoltaic (PV) cell array and a simple nine-layer dielectric filter, with a volume of about 3.1 cm³. When the flow rate of H₂ is 4.2 g/h, and H₂/O₂ ratio is 0.7, the system is able to deliver 3.06 W electrical power, the open-circuit voltage and short-circuit current are 2.31 V and 1.74 A, respectively, the result is a power density of about 1.0 W/cm³ (1 MW/m³). The effect of all kinds of factors on the performance of the micro-TPV system is also discussed in this paper.     

Analysis of off-grid hybrid wind turbine/solar PV water pumping systems

While many remote water pumping systems exist (e.g. mechanical windmills, solar photovoltaic, wind-electric, diesel powered), few combine both the wind and solar energy resources to possibly improve the reliability and the performance of the system. In this paper, off-grid wind turbine (WT) and solar photovoltaic (PV) array water pumping systems were analyzed individually and combined as a hybrid system. The objectives were to determine: (1) advantages or disadvantages of using a hybrid system over using a WT or a solar PV array alone; (2) if the WT or solar PV array interfered with the output of the other; and (3) which hybrid system was the most efficient for the location. The WT used in the analysis was rated at 900 W alternating current (AC). There were three different solar PV arrays analyzed, and they were rated at 320, 480, and 640 W direct current (DC). A rectifier converted the 3-phase variable voltage AC output from the WT to DC before combining it with the solar PV array DC output. The combined renewable energies powered a single helical pump. The independent variable used in the hybrid WT/PV array analysis was in units of W/m². The peak pump efficiency of the hybrid systems at Bushland, TX occurred for the 900 W WT combined with the 640 W PV array. The peak pump efficiencies at a 75 m pumping depth of the hybrid systems were: 47% (WT/320 W PV array), 51% (WT/480 W PV array), and 55% (WT/640 W PV array). Interference occurred between the WT and the different PV arrays (likely due to voltage mismatch between WT and PV array), but the least interference occurred for the WT/320 W PV array. This hybrid system pumped 28% more water during the greatest water demand month than the WT and PV systems would have pumped individually. An additional controller with a buck/boost converter is discussed at end of paper for improvement of the hybrid WT/PV array water pumping system.     

Hybrid poly (3-hexylthiophene)/titanium dioxide nanorods material for solar cell applications

We conducted an extensive study on poly(3-hexylthiophene) (P3HT) in combination with titanium dioxide (TiO₂) nanorods hybrid material for polymer solar cell applications. The device performance critically depends on the morphology of the hybrid film that will be determined by the molecular weight of P3HT, the solvent type, the hybrid compositions, the surface ligand on the TiO₂ nanorods, film thickness, process conditions, and so on. The current–voltage characteristic of the device fabricated in air has shown a power conversion efficiency of 0.83% under air mass (AM) 1.5 illumination using high molecular weight (65,000 D) P3HT, high boiling point solvent trichlorobenzene, and pyridine-modified TiO₂ nanorods with a film thickness of about 100 nm. The Kelvin probe force microscopy (KPFM) study of hybrid films shows large-scale phase separation with domain size greater than 10 nm, which may be the main factor limiting device performance.