Comparison of potential solar electricity output from fixed‐inclined and two‐axis tracking photovoltaic modules in Europe

We present an approach to determine the potential energy gains of flat plate non‐concentrating photovoltaic systems for the case of two‐axis tracking and two inclination angles with fixed orientation (assuming biannual adjustment) compared to the configuration of single fixed optimum angle. The calculation is based on the Photovoltaic Geographic Information System (PVGIS), which integrates modelling tools with the pan‐European solar radiation database. The results indicate that in the case of a PV system with two seasonal inclination angles, the maximum yearly gains, compared to the single fixed optimum angle, do not exceed 60–70 kWh per kW_p in the Mediterranean region, while in the Baltic and North Sea regions this configuration gives less than 20 kWh extra. For the case of two‐axis tracking, the relative energy gain compared to single fixed optimum angle is highest in the Northern latitudes but the absolute gain is much higher in the South. Typical yearly gains in Portugal and the Mediterranean region are in the range of 400–600 kWh per kW_p. The smallest absolute increase is found in the Northwest and Central Europe including the British Isles, where it is lower than 250 kWh per kW_p. For crystalline silicon we also investigate the effects of temperature and shallow‐angle reflectivity on the comparison between fixed and tracking systems. While both effects reduce the overall energy output, the temperature degradation is stronger for tracking systems while the reflectivity reduces output more for fixed systems. The combined effect is almost equal for fixed and two‐axis tracking systems. Copyright © 2007 John Wiley & Sons, Ltd.     

Towards realization of a large‐area light‐emitting diode‐based solar simulator

Solar simulators based on light‐emitting diodes (LEDs) have shown great promise as alternative light sources for indoor testing of photovoltaic cells with certain characteristics that make them superior to the traditional solar simulators. However, large‐area uniform illumination more suitable for larger cells and module measurements still remain a challenge today. In this paper, we discuss the development and fabrication of a scalable large‐area LED‐based solar simulator that consists of multiple tapered light guides. We demonstrate fine intermixing of many LED light rays and power delivery in the form of a synthesized AM 1.5 spectrum over an area of 25 cm × 50 cm with better than 10% spatial nonuniformity. We present the spectral output, the spatial uniformity, and the temporal stability of the simulator in both the constant current mode and the pulsed‐mode LED operation, and compare our data with the International Electrotechnical Commission standards on solar simulators for class rating. Although the light intensity with our current design and settings falls short of the standard solar AM 1.5 intensity, this design and further improvements open up the possibility of achieving large‐area, high‐power indoor solar simulation with various desired spectra. Copyright © 2011 John Wiley & Sons, Ltd.     

Broadband Finite‐Difference Time‐Domain Modeling of Plasmonic Organic Photovoltaics

We develop accurate finite‐difference time‐domain (FDTD) modeling of polymer bulk heterojunction solar cells containing Ag nanoparticles between the hole‐transporting layer and the transparent conducting oxide‐coated glass substrate in the wavelength range of 300 nm to 800 nm. The Drude dispersion modeling technique is used to model the frequency dispersion behavior of Ag nanoparticles, the hole‐transporting layer, and indium tin oxide. The perfectly matched layer boundary condition is used for the top and bottom regions of the computational domain, and the periodic boundary condition is used for the lateral regions of the same domain. The developed FDTD modeling is employed to investigate the effect of geometrical parameters of Ag nanospheres on electromagnetic fields in devices. Although negative plasmonic effects are observed in the considered device, absorption enhancement can be achieved when favorable geometrical parameters are obtained.     

Automated dual‐axis planar solar tracker with controllable vertical displacement for concentrating solar microcell arrays

Concentrator photovoltaics are able to maintain power output for extended hours while reducing the quantity of expensive high performance compound semiconductor solar materials. One of the limitations in concentrator photovoltaics is the bulky and heavy form factors of solar tracking systems which usually require dedicated installation locations and resistance to wind loading. This paper describes a planar solar tracker that requires only micro or millimeter scale lateral and vertical displacements of a microlens array to maintain both the optimum lateral locations and focal lengths at various incident angles of light, using dual‐axis actuations. Experimental results for lateral solar tracker were obtained under a solar simulator considering the effects of the rotation and revolution of the Earth, and under the Sun on a rooftop with the actual system, to demonstrate the concept and practical performance. Copyright © 2016 John Wiley & Sons, Ltd.     

New method for lateral mapping of bimolecular recombination in thin‐film organic solar cells

The best organic solar cells are limited by bimolecular recombination. Tools to study these losses are available; however, they are only developed for small area (laboratory‐scale) devices and are not yet available for large area (production‐scale) devices. Here we introduce the Intermodulation Light Beam‐Induced Current (IMLBIC) technique, which allows simultaneous spatial mapping of both the amount of extracted photocurrent and the bimolecular recombination over the active area of a solar cell. We utilize the second‐order non‐linear dependence on the illumination intensity as a signature for bimolecular recombination. Using two lasers modulated with different frequencies, we record the photocurrent response at each modulation frequency and the bimolecular recombination in the second‐order intermodulation response at the sum and difference of the two frequencies. Drift‐diffusion simulations predict a unique response for different recombination mechanisms. We successfully verify our approach by studying solar cells known to have mainly bimolecular recombination and thus propose this method as a viable tool for lateral detection and characterization of the dominant recombination mechanisms in organic solar cells. We expect that IMLBIC will be an important future tool for characterization and detection of recombination losses in large area organic solar cells. Copyright © 2016 John Wiley & Sons, Ltd.     

Quinacridone‐Based Electron Transport Layers for Enhanced Performance in Bulk‐Heterojunction Solar Cells

A water/alcohol‐soluble small molecule based on the commercially available pigment quinacridone is employed as an electron transport layer in organic photovoltaics. The quinacridone derivative is utilized in solution‐processed bulk‐heterojunction solar cells to improve primarily the fill factor of the devices, contributing to an upwards of 19% enhancement in the power conversion efficiency relative to the control devices with no electron transport layer. The facile synthesis of the quinacridone derivative coupled with the ease of device fabrication via solution processing provide a simple, yet effective means of improving the performance of existing organic photovoltaic cells.     

In‐Situ Switching from Barrier‐Limited to Ohmic Anodes for Efficient Organic Optoelectronics

Injection and extraction of charges through ohmic contacts are required for efficient operation of semiconductor devices. Treatment using polar non‐solvents switches polar anode surfaces, including PEDOT:PSS and ITO, from barrier‐limited hole injection and extraction to ohmic behaviour. This is caused by an in‐situ modification of the anode surface that is buried under a layer of organic semiconductor. The exposure to methanol removes polar hydroxyl groups from the buried anode interface, and permanently increases the work function by 0.2–0.3 eV. In the case of ITO/PEDOT:PSS/PBDTTT‐CT:PC71BM/Al photovoltaic devices, the higher work function promotes charge transfer, leading to p‐doping of the organic semiconductor at the interface. This results in a two‐fold increase in hole extraction rates which raises both the fill factor and the open‐circuit voltage, leading to high power conversion efficiency of 7.4%. In ITO/PEDOT:PSS/F8BT/Al polymer light‐emitting diodes, where the organic semiconductor's HOMO level lies deeper than the anode Fermi level, the increased work function enhances hole injection efficiency and luminance intensity by 3 orders of magnitude. In particular, hole injection rates from PEDOT:PSS anodes are equivalent to those achievable using MoO₃. These findings exemplify the importance of work function control as a tool for improved electrode design, and open new routes to device interfacial optimization using facile solvent processing techniques. Such simple, persistent, treatments pave the way towards low cost manufacturing of efficient organic optoelectronic devices.     

Techno‐economic analysis of three different substrate removal and reuse strategies for III‐V solar cells

The high cost of wafers suitable for epitaxial deposition of III‐V solar cells has been a primary barrier to widespread use of these cells in low‐concentration and one‐sun terrestrial solar applications. A possible solution is to reuse the substrate many times, thus spreading its cost across many cells. We performed a bottom‐up techno‐economic analysis of three different strategies for substrate reuse in high‐volume manufacturing: epitaxial lift‐off, spalling, and the use of a porous germanium release layer. The analysis shows that the potential cost reduction resulting from substrate reuse is limited in all three strategies––not by the number of reuse cycles achievable, but by the costs that are incurred in each cycle to prepare the substrate for another epitaxial deposition. The dominant substrate‐preparation cost component is different for each of the three strategies, and the cost‐ranking of these strategies is subject to change if future developments substantially reduce the cost of epitaxial deposition. Copyright © 2016 John Wiley & Sons, Ltd.     

Exciton‐Charge Annihilation in Organic Semiconductor Films

Time‐resolved optical spectroscopy is used to investigate exciton‐charge annihilation reactions in blended films of organic semiconductors. In donor–acceptor blends where charges are photogenerated via excitons, pulsed optical excitation can deliver a sufficient density of temporally overlapping excitons and charges for them to interact. Transient absorption spectroscopy measurements demonstrate clear signatures of exciton‐charge annihilation reactions at excitation densities of ≈10¹⁸ cm^?3. The strength of exciton‐charge annihilation is consistent with a resonant energy transfer mechanism between fluorescent excitons and resonantly absorbing charges, which is shown to generally be strong in organic semiconductors. The extent of exciton‐charge annihilation is very sensitive not only to fluence but also to blend morphology, becoming notably strong in donor–acceptor blends with nanomorphologies optimized for photovoltaic operation. The results highlight both the value of transient optical spectroscopy to interrogate exciton‐charge annihilation reactions and the need to recognize and account for annihilation reactions in other transient optical investigations of organic semiconductors.     

Solvent‐Induced Morphology in Polymer‐Based Systems for Organic Photovoltaics

Studies on the influence of four different solvents on the morphology and photovoltaic performance of bulk‐heterojunction films made of poly(3‐hexylthiophene) (P3HT) and [6,6]‐phenyl‐C₆₁ butyric acid methyl ester (PCBM) via spin‐coating for photovoltaic applications are reported. Solvent‐dependent PCBM cluster formation and P3HT crystallization during thermal annealing are investigated with optical microscopy and grazing‐incidence wide‐angle X‐ray scattering (GIWAXS) and are found to be insufficient to explain the differences in device performance. A combination of atomic force microscopy (AFM), X‐ray reflectivity (XRR), and grazing‐incidence small‐angle X‐ray scattering (GISAXS) investigations results in detailed knowledge of the inner film morphology of P3HT:PCBM films. Vertical and lateral phase separation occurs during spin‐coating and annealing, depending on the solvent used. The findings are summarized in schematics and compared with the IV characteristics. The main influence on the photovoltaic performance arises from the vertical material composition and the existence of lateral phase separation fitting to the exciton diffusion length. Absorption and photoluminescence measurements complement the structural analysis.     

Lending Triarylphosphine Oxide to Phenanthroline: a Facile Approach to High‐Performance Organic Small‐Molecule Cathode Interfacial Material for Organic Photovoltaics utilizing Air‐Stable Cathodes

Cathode interfacial material (CIM) is critical to improving the power conversion efficiency (PCE) and long‐term stability of an organic photovoltaic cell that utilizes a high work function cathode. In this contribution, a novel CIM is reported through an effective and yet simple combination of triarylphosphine oxide with a 1,10‐phenanthrolinyl unit. The resulting CIM possesses easy synthesis and purification, a high T _g of 116 °C and attractive electron‐transport properties. The characterization of photovoltaic devices involving Ag or Al cathodes shows that this thermally deposited interlayer can considerably improve the PCE, due largely to a simultaneous increase in V _oc and FF relative to the reference devices without a CIM. Notably, a PCE of 7.51% is obtained for the CIM/Ag device utilizing the active layer PTB7:PC₇₁BM, which far exceeds that of the reference Ag device and compares well to that of the Ca/Al device. The PCE is further increased to 8.56% for the CIM/Al device (with J _sc = 16.81 mA cm^?2, V _oc = 0.75 V, FF = 0.68). Ultraviolet photoemission spectroscopy studies reveal that this promising CIM can significantly lower the work function of the Ag metal as well as ITO and HOPG, and facilitate electron extraction in OPV devices.     

Boron Subphthalocyanine Chloride as an Electron Acceptor for High‐Voltage Fullerene‐Free Organic Photovoltaics

High‐efficiency fullerene‐free single‐heterojunction (SHJ) organic photovoltaic (OPV) cells consisting of tetracene (Tc) as a typical donor material and boron subphthalocyanine chloride (SubPc) as an acceptor material are reported. Cells containing SubPc as a direct replacement for C₆₀ exhibit an ～60% improvement in open circuit voltage (V_oc) achieving a maximum V_oc of 1.24 V, which is amongst the highest values acheived to date for SHJ devices. This resulted in an overall improvement of ～60% in power conversion efficiency from 1.8%, for Tc/C₆₀ cells, to 2.9% for Tc/SubPc. The OPV device results are complemented by soft X–ray photoelectron spectroscopy (PES) measurements of the interfacial energetics of both systems. The results demonstrate that SubPc shows considerable promise as an electron acceptor material for future cell designs.     

Functionalized Graphene as an Electron‐Cascade Acceptor for Air‐Processed Organic Ternary Solar Cells

Functionalized graphene nanoflakes (GNFs) are used as an electron‐cascade acceptor material in air‐processed organic ternary bulk heterojunction solar cells. The functionalization is realized via the attachment of the ethylenedinitrobenzoyl (EDNB) molecule to the GNFs. Simulation and experimental results show that such nanoscale modification greatly influences the density of states near the Fermi level. Consequently, the GNF‐EDNB blend presents favorable highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels to function as a bridge structure between the poly[N‐9′‐heptadecanyl‐2,7‐carbazole‐alt‐5,5‐(4′,7′‐di‐2‐thienyl‐2′,1′,3′‐benzothiadiazole)] (PCDTBT) and the [6,6]‐phenyl‐C71‐butyric‐acid‐methyl‐ester (PC₇₁BM). The improved exciton dissociation and charge transport are associated with the better energy level alignment of the ternary blend and the high electrical conductivity of the GNFs, which act as additional electron transport channels within the photoactive layer. The resulting PCDTBT/GNF‐EDNB/PC₇₁BM ternary organic solar cells, fabricated entirely under ambient conditions, exhibit an average power conversion efficiency enhancement of ≈18% as compared with the binary blend PCDTBT/PC₇₁BM.     

All‐Day Operating Quaternary Blend Organic Photovoltaics

The unique properties of organic photovoltaics (OPVs) offer great promise in emerging applications such as wearable electronics or the Internet of Things. For their successful utilization, OPV operation should be designed for versatile irradiation circumstances in addition to solar light since they should be capable of providing electric power when there is no sunlight or when they operate indoors. Here, a quaternary OPV (Q‐OPV) as a semitransparent, colorful energy platform that operates efficiently under both solar and artificial light irradiation is demonstrated. The experimentally optimized Q‐OPV shows a broadened spectral response and improved charge transport process with suppressed recombination, thereby providing high output powers that are sufficient to autonomously operate low‐power electronic devices. In addition, the Q‐OPV benefits from improved morphological stability with a reduced driving force for grain growth by the increased entropy in the quaternary blend system. The important features of the Q‐OPV platform such as semitransparency, high tolerance to film thickness, and color codability, while pursuing the improved performance and thermal durability, further open new opportunities as an all‐day (24/7/365) power generator in broad practical applications.     

Particle conservation in the hot‐carrier solar cell

In conventional p–n junction solar cells, carrier multiplication by impact ionisation, is negligible, owing to the low temperature of the electron–hole pairs. This leads to particle conservation between the net number of absorbed photons and the number of electron–hole pairs withdrawn from the cell. In hot‐carrier solar cells, in which electrons are at a high temperature by assuming suppression of electron–phonon scattering, such particle conservation leads to peculiar results. Numerical calculations show that entire current–voltage characteristics with meaningful values of temperature and chemical potential do not exist. If the energy at which electron–hole pairs are extracted is smaller than the average energy of absorbed photons, the temperature of the electrons and holes becomes much larger than the tem perature of the sun. When the extraction energy is larger than the average energy of the absorbed photons, an entire current–voltage curve cannot always be obtained. It follows that impact ionisation and Auger recombination cannot be neglected when the thermal energy of the electron–hole pairs is comparable to the bandgap of the absorber. Accounting for these processes results in current–voltage characteristics that are well behaved. Copyright © 2005 John Wiley & Sons, Ltd.     

Analyzing historical cost trends in California's market for customer‐sited photovoltaics

This paper reports highlights from a detailed statistical analysis of the cost of customer‐sited, grid‐connected photovoltaic installations in the largest solar market in the United States: California. It is based on an analysis of 18 942 PV systems, totaling 254 MW_AC, either funded or approved for funding under two separate rebate programs overseen by the California Energy Commission and the California Public Utilities Commission. We find that: (1) solar costs have declined substantially over time; (2) policy incentives have impacted pre‐rebate installed costs, and some cost inflation is apparent; (3) economies of scale have driven down costs for larger systems; (4) systems installed in new home developments and in affordable housing projects have experienced much lower costs than the general retrofit market; and (5) installer experience and type have affected costs, but the effects differ by program. Results hold important implications for solar suppliers and customers, and for policymakers designing incentive programs. Copyright © 2006 John Wiley & Sons, Ltd.     

Spin‐ and Spray‐Deposited Single‐Walled Carbon‐Nanotube Electrodes for Organic Solar Cells

Organic bulk‐heterojunction solar cells using thin‐film single‐walled carbon‐nanotube (SWCNT) anodes deposited on glass are reported. Two types of SWCNT films are investigated: spin‐coated films from dichloroethane (DCE), and spray‐coated films from deionized water using sodium dodecyl sulphate (SDS) or sodium dodecyl benzene sulphonate (SDBS) as the surfactant. All of the films are found to be mechanically robust, with no tendency to delaminate from the underlying substrate during handling. Acid treatment with HNO₃ yields high conductivities >1000 S cm^?1 for all of the films, with values of up to 7694 ± 800 S cm^?1 being obtained when using SDS as the surfactant. Sheet resistances of around 100 Ω sq^?1 are obtained at reasonable transmission, for example, 128 ± 2 Ω sq^?1 at 90% for DCE, 57 ± 3 Ω sq^?1 at 65% for H₂O:SDS, and 68 ± 5 Ω sq^?1 at 70% for H₂O:SDBS. Solar cells are fabricated by successively coating the SWCNT films with poly(3,4‐ethylenedioxythiophene):poly(styrene sulphonate) (PEDOT:PSS), a blend of regioregular poly(3‐hexylthiophene) (P3HT) and 1‐(3‐methoxy‐carbonyl)‐propyl‐1‐phenyl‐(6,6)C₆₁ (PCBM), and LiF/Al. The resultant devices have respective power conversions of 2.3, 2.2 and 1.2% for DCE, H₂O:SDS and H₂O:SDBS, with the first two being at a virtual parity with reference devices using ITO‐coated glass as the anode (2.3%).     

Additive‐Morphology Interplay and Loss Channels in “All‐Small‐Molecule” Bulk‐heterojunction (BHJ) Solar Cells with the Nonfullerene Acceptor IDTTBM

Achieving efficient bulk‐heterojunction (BHJ) solar cells from blends of solution‐processable small‐molecule (SM) donors and acceptors is proved particularly challenging due to the complexity in obtaining a favorable donor–acceptor morphology. In this report, the BHJ device performance pattern of a set of analogous, well‐defined SM donors— DR3TBDTT ( DR3 ), SMPV1 , and BTR —used in conjunction with the SM acceptor IDTTBM is examined. Examinations show that the nonfullerene “All‐SM” BHJ solar cells made with DR3 and IDTTBM can achieve power conversion efficiencies (PCEs) of up to ≈4.5% (avg. 4.0%) when the solution‐processing additive 1,8‐diiodooctane (DIO, 0.8% v/v) is used in the blend solutions. The figures of merit of optimized DR3:IDTTBM solar cells contrast with those of “as‐cast” BHJ devices from which only modest PCEs <1% can be achieved. Combining electron energy loss spectrum analyses in scanning transmission electron microscopy mode, carrier transport measurements via “metal‐insulator‐semiconductor carrier extraction” methods, and systematic recombination examinations by light‐dependence and transient photocurrent analyses, it is shown that DIO plays a determining role—establishing a favorable lengthscale for the phase‐separated SM donor–acceptor network and, in turn, improving the balance in hole/electron mobilities and the carrier collection efficiencies overall.     

Materials‐Scale Implications of Solvent and Temperature on [6,6]‐Phenyl‐C61‐butyric Acid Methyl Ester (PCBM): A Theoretical Perspective

The ability to detail how molecules pack in the bulk and at the various materials interfaces in the active layer of an organic solar cell is important to further understanding overall device performance. Here, [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PCBM), a preferred electron‐acceptor material in organic solar cells, is studied through molecular dynamics (MD) simulations; the goal is to examine the effects of temperature and trace solvents on the packing and morphological features of bulk PCBM. Solubility (miscibility) parameters, melting and order‐disorder transitions, surface energies, and orientational distributions as a function of different starting configurations are discussed. On the basis of the derived morphologies, electronic structure calculations and a kinetic Monte Carlo approach are combined to evaluate the parameters impacting electron mobility in crystalline and amorphous PCBM structures.     

Semitransparent Energy‐Storing Functional Photovoltaics Monolithically Integrated with Electrochromic Supercapacitors

Energy‐storing functional photovoltaics, which can simultaneously harvest and store solar energy, are proposed as promising next‐generation multifunction energy systems. For the extension of conventional organic photovoltaics (OPVs), electrochromic supercapacitors (ECSs) are monolithically integrated with semitransparent (ST) quaternary blend‐based OPVs (ST Q‐OPVs) to achieve compact, energy‐efficient storage with great aesthetic appeal. In particular, ST Q‐OPVs with low‐power‐consumption ECSs allow full operation, even under low‐intensity irradiance, including artificial indoor light circumstances, and thereby exhibit potential for all‐day operating energy suppliers. The prepared ST energy‐storing functional photovoltaics also serve as a backup power source for external electronic equipment (e.g., light‐emitting diodes, and sensor nodes for Internet of Things) by consuming charged power. In addition to features that include unrestricted operation under any circumstances, color tunability, feasibility of designs with various shapes, rapid charging/discharging, and real‐time indication of stored energy levels, ST energy‐storing functional photovoltaics could potentially be applied in electronic devices such as advanced smart windows or portable smart electronics.