Breakthroughs in Photonics and Energy

The role of photonics in energy

[+] Author Affiliations
Zakya H. Kafafi, Nelson Tansu

Lehigh University, Center for Photonics and Nanoelectronics, Department of Electrical and Computer Engineering, Bethlehem, Pennsylvania 18015, United States

Raúl J. Martín-Palma

Universidad Autónoma de Madrid, Departamento de Física Aplicada, Cantoblanco, Madrid 28049, Spain

Ana F. Nogueira

University of Campinas, Laboratório de Nanotecnologia e Energia Solar, Campinas-SP 13083970, Brazil

Deirdre M. O’Carroll

Rutgers University, Department of Materials Science and Engineering, Department of Chemistry and Chemical Biology and IAMDN, Piscataway, New Jersey 08854, United States

Jeremy J. Pietron

Surface Chemistry Branch, Code 6170, Naval Research Laboratory, Washington, DC 20375, United States

Ifor D. W. Samuel

University of St Andrews, Organic Semiconductor Centre, SUPA, School of Physics and Astronomy, St. Andrews, Fife KY16 9SS, United Kingdom

Franky So

North Carolina State University, Department of Materials Science and Engineering, Raleigh, North Carolina 27695, United States

Loucas Tsakalakos

General Electric–Global Research Center, Electrical Technologies and Systems, Micro and Nano Structures Technologies, Photonics Laboratory, Niskayuna, New York 12309, United States

J. Photon. Energy. 5(1), 050997 (Oct 12, 2015). doi:10.1117/1.JPE.5.050997
History: Received May 9, 2015; Accepted August 3, 2015
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Open Access Open Access

Abstract.  In celebration of the 2015 International Year of Light, we highlight major breakthroughs in photonics for energy conversion and conservation. The section on energy conversion discusses the role of light in solar light harvesting for electrical and thermal power generation; chemical energy conversion and fuel generation; as well as photonic sensors for energy applications. The section on energy conservation focuses on solid-state lighting, flat-panel displays, and optical communications and interconnects.

The most comprehensive ancient account of the science of light is a millennium old, dating from the Islamic golden age when the Arab scholar Ibn al-Haytham (Alhazen) composed and published his seminal seven-volume Book on Optics. It was nearly 700 years later that Christiaan Huygens developed a wave theory of light, while Newton proposed a corpuscular (particle) theory of light. In his book, A Dynamical Theory of the Electromagnetic Field, Maxwell mathematically unified light, electricity, and magnetism.1 This modern theory of light is only 150 years old!

These great milestones were followed by the work of Einstein,2 who introduced the concept of photons to explain photoelectric effect data in terms of light energy being carried in discrete, quantized packets. He demonstrated that only photons with a certain threshold frequency (energy) impinged upon a metal surface can cause the ejection of electrons. The discovery of the law of this photoelectric effect was a foundation of the quantum revolution and was the primary reason for the Nobel Prize awarded to Einstein in 1922.3

A recent and elegant demonstration of light’s dual nature was reported by the group of Carbone at École Polytechnique Fédérale de Lausanne (EPFL) in Switzerland, who captured a snapshot of the concurrent behavior of light as both a wave and a stream of particles4 (Fig. 1).

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Fig. 1
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Energy-space photography of light confined on a nanowire simultaneously shows both spatial interference and energy quantization.4

Understanding the quantum nature of light and electrons was crucial to the discovery of other phenomena that involve the interaction of light with electric charges, such as the photoconductive effect, i.e., photoconductivity, the photovoltaic (PV) effect, and the photoelectrochemical (PEC) effect.

In this paper, we will review some of the enabling photonic technologies that followed based on last century’s fundamental discoveries related to the interaction of light and matter. Section 1 gives an overview of the role that photonics plays in power generation through electrical, thermal, and chemical energy conversion, as well as in sensor technology. Section 2 describes technological advances where photonics is important for energy conservation. It covers solid-state lighting, flat-panel displays, and optical communications and interconnects. Advances in photonics such as the development of the worldwide optical communications network that carried the news of Charles Kao’s receiving the Nobel Prize in 2009,5 thanks to his proposal 50 years ago to use optical fibers to transmit phone calls across great distances, will be highlighted. Other examples will include the revolution in solid-state lighting that earned Isamu Akasaki, Hiroshi Amano, and Shuji Nakamura the Nobel Prize in Physics in 2014.6 This award was based on their development of efficient blue light-emitting diodes (LEDs), enabling bright and energy-saving white light sources. This work was preceded by the seminal work of R. N. Hall et al.7 on diode lasers, as well as that of Holonyak and Bevacqua8 and Allen and Grimmeiss,9 who developed red LEDs in the early 1960s, and that of Round10 and Losev,11 who reported electroluminescence from carborundum early in the 20th century. The 21st century witnessed another revolution in display technology with the development and commercialization of flat-panel displays. The technology on which these flat-panel displays is based will soon undergo another transformation with the introduction of bright, efficient, and stable organic light-emitting diodes (OLEDs).

The sun supplies the energy that allows all life on earth. The enormous power of the sun can be used in three main ways. The first is the conversion of sunlight to electrical power, which is discussed in Sec. 2.1. The second is the conversion of sunlight to thermal power, which is briefly described in Sec. 2.2. The third is the conversion of sunlight directly to fuels, which is the topic of Sec. 2.3. In addition, the role of photonic sensors in energy harvesting and the production of power are covered in Sec. 2.4.

Solar Light Harvesting and Electrical Energy Conversion

Most solar cells on rooftops now convert sunlight into electricity using the PV effect first demonstrated by Becquerel.12 With the advent of the silicon PV cell in 1954,13 humankind increasingly found ways of using semiconductors for many energy conversion applications. Among the first applications of solar PVs was their use in space to power control systems and other critical functions on space vehicles.14 This subsequently grew into the application of PVs in residential solar systems for off-grid applications and has matured into grid-connected residential systems and solar power plants in the last 15 years.15 The progress in solar PVs in terms of cell power conversion efficiencies (PCEs) can be seen in the chart from the (US) National Renewable Energy Laboratory (NREL) shown in Fig. 2. NREL makes certified measurements of PV cells, and the chart shows how different solar PV technologies have developed as a function of time (for small-area, champion solar cells). As will be explained below, the highest efficiency technologies are accompanied by very high cost and complexity, so the dominant solar cells are in the middle of the efficiency range. The newest technologies are at the bottom right of the chart, but their rapid rate of development and efficiency improvement could make them the solar PV technologies of the future. In a single-junction solar cell, the amount of light absorbed and the maximum voltage obtainable both depend on the bandgap of the semiconductor. This leads to a trade-off between the amount of light absorbed and the open-circuit voltage. The theoretical limit for the PCE of a single-junction PV cell under AM1.5 irradiation is calculated to be 33.7% using the detailed balance theory developed by Shockley and Queisser in 196116 and taking into account thermalization losses and low-bandgap losses in the solar cell.

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Fig. 2
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Latest chart on the progress of the power conversion efficiency of solar cells reported by the National Center for Photovoltaics at National Renewable Energy Laboratory (NREL).17

Inorganic solar cells

The most important commercially available PV materials systems today are based on inorganic semiconductors. The vast majority of solar modules are based on silicon, which is the second most abundant material in the earth’s crust, hence, these modules are of relatively low cost. The record silicon solar cell efficiency is 25.6%. Modules with a PCE of 21%, fabricated by applying a thin film of amorphous silicon-based heterojunction to a silicon surface, are commercially available.18 The majority of commercial Si modules have PCEs in the range of 14% to 18%. The modules are typically fabricated by manufacturing Si wafers with the correct doping level to form a p–n junction, along with suitable front and back contact (typically screen-printed). Both single-crystalline and multi/polycrystalline wafers are used in the industry. The cells are then laid out on a backing sheet and strung together with metal tabs. An adhesive layer is applied to bond the front glass cover, ensuring that good edge moisture barriers are also in place. Photonics plays an important role in Si-based solar cells and modules since it is an indirect bandgap semiconductor and hence has a relatively long absorption depth across the solar spectrum. Most importantly, light-trapping structures must be fashioned on the front surface of the wafers to ensure that long wavelength light is adequately trapped within the solar cell and is not lost from reflection at the back contact and subsequent impingement on the front surface (within the escape cone). Light trapping is typically achieved by etching the front surface of the wafers to form a random texture or by an anisotropic etch that forms pyramidal structures on the Si surface.19 Another important photonic aspect of Si modules is related to the optical losses that can occur at the glass/adhesive interface, as well as at the glass to air interface due to reflection. Significant advances to minimize such optical losses have been made over the last two decades.20 Further discussion of these topics is found in Sec. 2.1.3.

Another commercially important class of materials is based on inorganic thin films, primarily related to the CdTe21 and Cu(In,Ga)Se2 (CIGS) materials systems.22 CdTe solar cell efficiencies have been reported at 19.6% and CIGS at 20.5%. Until recently, thin-film solar cells have been lower in cost than Si solar cells due to the fact that they are direct bandgap semiconductors, and hence, a very thin layer (typically 1 to 5μm) can be deposited using low-cost deposition processes directly onto a large-area glass substrate. This significantly lowers the manufacturing cost. One can use the glass growth substrate as the window layer, or can use it as a backing layer and add another glass window layer. In both cases, it is possible to manage the light using suitable antireflective coatings (ARC, see below).23 Today Si and CdTe-based modules are competitive in cost (typically measured in $/Watt).

III–V semiconductor materials are also important for high-efficiency PV conversion. GaAs cells have reached 1 Sun efficiencies of 28.8%, and multijunction III–V cells have achieved an efficiency of 37.9% at 1 Sun.18Figure 2 gives a summary of the record efficiencies achieved by these different PV cells over the last few decades. III–V solar cells have been reserved for specialty applications, primarily for space due to their exceptionally high cost (ca. three orders of magnitude higher cost per square meter than Si). This high cost is due to several factors, including the low abundance of group III and V elements, the high expense of growing III–V crystals, the lack of a good diffusion process for forming the p–n junction (an epitaxial layer is needed), the need for another surface passivation layer to minimize surface recombination, and the need for expensive materials/processes to form good Ohmic contacts. An ARC is also always needed on the III–V surface. On the other hand, the high efficiency of III–V solar cells/modules is attributed to the direct bandgap nature of these semiconductors, which implies a short absorption depth as well as a high charge carrier mobility and high minority carrier lifetimes due to the low nonradiative losses that occur in the crystal. Surface recombination is readily minimized in III–V cell technologies by applying a suitable passivation layer. Another advantage is the fact that the bandgap of 1.5 eV gives the optimal balance of absorption of the solar spectrum and open-circuit voltage. Photon recycling in III–V solar cells has been critical in improving their performance and led to a record PCE of 28.8% for a single-junction cell.24 Recent efforts to produce lower-cost III–V modules have allowed penetration into previously unattainable markets.25,26

The state-of-the-art high-performance inorganic solar cells technology is primarily pursued using multijunction stacked tandem solar cells.27 Using materials with different bandgaps that overlap with the visible and near-IR solar spectral region leads to the absorption of photons over a wide range of energies with a small loss of photon-to-electrical energy conversion, thereby overcoming the Shockley–Queisser limit. Tandem structures require the development of appropriate absorber materials and the matching of the current density throughout all the stacked layers. Progress has been made using primarily III–V multijunction tandem cells with record efficiencies of 44.4% under concentration (302 Suns) for InGaP/GaAs/InGaAs solar multijunction cells.27 Recent work has reported the superior lasing characteristics of GaInNAs quantum well lasers with very low threshold.2830 By taking advantage of this progress, the integration of III–V multijunction cells with GaInNAs-containing alloys as one of the cell materials has enabled a record PCE of 43.5%.31 Other materials including InGaN32 and InN33 are currently being pursued, while others such as GaAsBi34 are being considered for multijunction solar cells. One way of improving the cost-effectiveness of these very expensive but highly efficient solar cells is to use a concentrator, i.e., to harvest light over a large area but deliver it to small but very efficient solar cells35 (e.g., planar Fresnel lenses36).

Another recent development deals with the application of various nanostructures to produce novel architectures for solar cells and, in some cases, to take advantage of new energy conversion physics. If classes of nanostructures are considered by their dimensionality, zero-dimensional quantum dots (QDs) have been applied to building new solar cell architectures that utilize quantum confinement in unique ways. For example, Green et al.37 have proposed an all-silicon QD tandem device and are making progress toward the fabrication of such structures.38 QDs have also been applied as novel photonic coatings that take advantage of either up- or downconversion of portions of the solar energy spectrum in order to more effectively use bands that are lost due to thermalization or due to being sub-bandgap (and hence not absorbed).39 One-dimensional (1-D) nanostructures, such as inorganic nanowires and carbon nanotubes, have also been explored in various novel device architectures. For example, silicon nanowires and III–V nanowire solar cells have been demonstrated4042 and show promise for producing low-cost, high-efficiency, flexible solar cells.43 Two-dimensional (2-D) quantum well nanostructures have been employed to convert near-IR photons and, hence, boost the efficiency of III–V solar cells.44 Finally, three-dimensional inorganic nanoarchitectures have been fabricated and show promising cell performance.45

Organic and hybrid solar cells

As depicted in Fig. 2 and discussed above, PV cells based on crystalline inorganic semiconductors have reached impressive PCEs of >28% in single-junction and 38% in multijunction device architectures under full-sun illumination.46 However, this class of PV cells suffers from high production and energy costs, which result in long financial and energy payback times.47 This drawback has fostered the development of a new generation of solution-processable solar cells,4850 which benefit from low-cost materials (e.g., organic molecular and polymeric materials), high-throughput manufacturing methods, such as reel-to-reel coating, and low-energy expenditure.51 This new class of PVs is referred to as emerging PV cells and can be categorized according to the materials used and mechanisms invoked: PEC or dye-sensitized solar cells (DSSCs), organic (molecular and polymer) solar cells, hybrid organic–inorganic solar cells, and QD solar cells. An overview of these emerging and promising technologies is given with the best cell PCEs achieved so far appearing at the bottom right of the NREL chart shown in Fig. 2. Since they are relatively new compared to their inorganic counterparts, it is not surprising that their efficiencies are much lower than those of the most established inorganic solar cells. However, the slope of their PCEs has been quite steep over the last few years, especially for the promising PV technology based on organic–inorganic hybrid materials (e.g., perovskites).

DSSCs, also known as PEC cells, were the first emerging PV technology to reach acceptable efficiency values. Consequently, DSSC research has initially attracted the largest number of researchers and the largest amount of industry cooperation, as well as the largest number of solar panel prototypes and related products. Since the first demonstration in 1991 by O’Regan and Graetzel,52 and after two decades of research and development, DSSCs with an iodine/tri-iodide (I/I3) liquid electrolyte have achieved light to electric PCEs of >11% using ruthenium dyes,53>12% based on zinc porphyrin dyes,54 and >10% based on metal-free organic dyes.55 Recently, through the molecular engineering of zinc porphyrin sensitizers, Grätzel and coworkers have reported solar cells with 13% PCE. The new sensitizers feature the prototypical structure of a donor–π-bridge–acceptor, maximize electrolyte compatibility, and improve light-harvesting properties.56 Using SM315 dye with the cobalt (II/III) redox shuttle resulted in DSSCs that exhibit a high open-circuit voltage (VOC) of 0.91 V, short-circuit current density (JSC) of 18.1mAcm2, and a fill factor (FF) of 0.78. However, there are several problems that limit the mass production and long-term stability of such devices, such as leakage and evaporation of the liquid electrolyte, and corrosion of the metal-based current collectors (silver, copper, etc.) by iodine. As a result, substantial effort has been made to find alternative electrolytes and/or introduce new concepts to develop electrolyte-free DSSCs. In solid-state DSSCs (ss-DSSCs), the liquid electrolyte is entirely replaced by a solid organic hole transport material (HTM). For the solid-state version (ss-DSSC), PCEs >20% have been predicted,57 using low-cost materials,58 low temperature processing (<150°C), and reel-to-reel fabrication methods.59,60

Organic (molecular and polymeric) solar cells represent another and different category in the class of emerging PVs. These solid-state devices have attracted a lot of attention in the last 20 years as one of the most promising technologies for low-cost solar energy conversion. A key difference from their inorganic semiconductor counterparts results from their low dielectric constant and gives rise to strongly bound upon solar light absorption at room temperature. A chemical potential is required between the hole (p-type) transporter/electron donor and the electron (n-type) transporter/acceptor in order to separate the photogenerated carrier charges and often involves the use of a strong electron acceptor. One subclass of these devices is based on a light-harvesting material that consists of a mixture of a conjugated polymer as the p-type material or hole transporter/electron donor and a fullerene derivative as the n-type material or electron transporter/acceptor. The operating mechanism in these organic solar cells, discovered in the mid-1990s by Sariciftci et al.,61 is based on photoinduced electron transfer from a conducting polymer such as a poly(p-phenylene vinylene) (PPV) derivative to a fullerene derivative. This finding opened up a very rich and intensive research field involving the synthesis of new conjugated polymers and copolymers with low-bandgap and high charge carrier mobilities, novel fullerene derivatives with better solubility, morphology control of the active light-harvesting layer, and different solar cell configurations. In particular, the so-called bulk heterojunction (BHJ) in which the donor and acceptor are mixed together is most widely used because it enables efficient charge generation even in materials with very limited exciton diffusion length. Further refinements included novel contact materials aiming to give better interfaces for electron and hole collection, and the investigation of ground and excited states to understand the complex mechanisms involving exciton formation and diffusion, charge-transfer state formation and separation followed by charge transport and collection.

Organic solar cells with BHJ architectures using conducting polymers and fullerene molecules are by far the most studied,62,63 achieving PCEs close to 10% using one polymer as an absorber (i.e., single-junction devices), in both regular and inverted device structures.6467 The chemical nature of the electron donor polymers has evolved from simple structures, such as PPV and poly(3-hexylthiophene) (P3HT), to more complex copolymers specially designed to have both electron donor and acceptor groups in their structures in order to reduce their bandgap. One example is the newly synthesized narrow-bandgap semiconducting polymer poly [[2,6′-4,8-di(5-ethylhexylthienyl)benzo[1,2-b;3,3-b] dithiophene] [3-fluoro-2[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]], which when used in the light-harvesting layer led to a very efficient cell that delivered a certified PCE of 9.94%.67 Recently, Yang and coworkers have fabricated a multijunction solar cell having a PCE exceeding 11%, suggesting great potential for tandem architectures in organic photovoltaic (OPV) research68 (see Fig. 3).

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Fig. 3
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(a) JV characteristics and (b) action spectra, external quantum efficiency as a function of wavelength, for single-junction photovoltaic (PV) cells based on poly(3-hexylthiophene):ICBA, PTB:[6,6]-phenyl-C71 butyric acid methyl ester (PC71BM), and LBG:PC71BM as the active light-harvesting layers. (c) JV characteristics and (d) device configurations (front subcell/back subcell) of inverted double-junction tandem OPVs. (Reproduced with permission from Ref. 68 © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Small molecules offer some advantages over conducting polymers, such as synthetic flexibility, ease of purification, less batch-to-batch variation in properties, higher hole and electron mobilities in some cases, and intrinsic monodispersity. Efficient molecular BHJ solar cells have recently been reported in which merocyanine dyes, squaraine dyes, fused acenes, triphenylamine, benzofuran benzothiadiazole, diketopyrrolopyrrole, benzodithiophene, and other chromophores, such as oligothiophenes and push-pull organic dyes, have been employed as the light-harvesting donor components.6971 State-of-the-art OPV devices achieved PCEs up to 10%.7275 Recently, molecular OPVs based on oligothiophenes with five thiophene units in the backbone and 2-(1,1-dicyanomethylene)rhodanine as end capping units exhibited a PCE of 10%.76

State-of-the-art solution-processed devices generally rely on the BHJ of polymer electron donors and fullerene electron acceptors.77 Fullerene derivatives, such as [6,6]-phenyl-C61 butyric acid methyl ester (PC61BM) and [6,6]-phenyl-C71 butyric acid methyl ester (PC71BM), have played a leading role as electron acceptor materials; however, they suffer from poor absorption in the visible and near-IR regions. Recently, nonfullerene electron-accepting chromophores, such as perylene diimide, naphthalene diimide, vinazene, fluoranthene-fused imide, benzothiadiazole, rhodanine, and diketopyrrolopyrrole, have been incorporated with success in BHJ molecular and polymeric solar cells.7884 So far, state-of-the-art BHJ devices based on nonfullerene acceptors have shown PCE up to 6%.84

Hybrid organic–inorganic solar cells have structures similar to the cells described above where the electron-accepting fullerenes are substituted with inorganic semiconductors (e.g., TiO2, ZnO, CuInS2, PbS, CdSe, and CdTe) nanoparticles.8588 Chalcogenide nanoparticles with quantum confinement properties have also been considered and used as sensitizers in DSSCs.89,90 These solar cells offer a series of advantages over the more traditional BHJ OPVs based on the polymer/fullerene systems. Some of the expected advantages are (1) a contribution to light absorption by an inorganic acceptor can lead to the generation of more photocarriers, due to their larger linear absorption coefficients compared to those of fullerene derivatives; (2) the absorption of nanoparticles can be tuned to cover a broad solar spectral range, as a result of modification of their size and shape, complementary to that of the organic electron donor/hole transporter; (3) the physical dimensions of some inorganic semiconductors can be tailored to produce 1-D nanostructures, to allow efficient exciton dissociation, i.e., charge separation and electron transporting pathways simultaneously; (4) ultrafast and efficient photoinduced charge carrier transfer between the electron acceptor (inorganic nanoparticles) and the electron donor (the organic semiconductor); (5) the acceptors have relatively high electron mobility; and (6) good photo- and chemical stability.91 However, to date, PCEs achieved for hybrid organic–inorganic solar cells are significantly lower than those of OPV devices, which is primarily due to the challenges in controlling the interface between the nanoparticles and the polymer, and achieving a well-defined matrix with a continuous percolation network. Furthermore, the presence of surface traps on the nanoparticles can be problematic for achieving good charge generation and carrier transport. An efficiency of 3.5% has been achieved in a BHJ architecture when combining CdSe nanorods and PbS QDs with different polymers.92,93 More recently, silicon and an organic polymer (e.g., P3HT) and carbon nanotubes have been combined to give hybrid solar cells with efficiencies reaching 11%.94,95

A recent breakthrough occurred in the last couple of years with the demonstration of highly efficient solar cells based on organic/inorganic lead halide perovskite absorbers. These light-harvesting organic–inorganic materials have transformed the field of organic/inorganic hybrid PV devices.96,97 Solution processed PVs incorporating perovskite absorbers, such as CH3NH3PbI3 and CH3NH3PbI3xClx, have achieved efficiencies of 20.1% (certified, but not stabilized) in solid-state device configurations, surpassing liquid DSSCs, evaporated and tandem organic solar cells, as well as various thin-film inorganic PVs, thus establishing perovskite-based solar cells as a robust candidate for expanding the worldwide PV market. In 2009, hybrid perovskites were first used in the work of Kojima et al.98 as the sensitizer (absorber) in a DSSC with liquid electrolyte, generating only a 3.8% efficiency. The breakthrough, however, came in 2012 when the group of Snaith at the University of Oxford and the group of Grätzel at EPFL combined the perovskite with the well-known DSSC solid-state HTM, 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenyl-amine)-9,9′-spirobifluorene (spiro-OMeTAD).99,100 The Oxford group demonstrated that a mesoporous TiO2 scaffold is not necessary and that the hybrid perovskites are able to transport both electrons and holes. In subsequent reports, efficiency values of 15% were attained in different systems with various morphologies, either in nanoheterojunction or planar thin-film configurations with different chemical compositions and preparation routes.101106 To date, efficiency values exceeding 16% have been reported with two quite different configurations, using CH3NH3PbI3 perovskite in a classical solid-state DSSC and in a thin-film planar configuration with CH3NH3PbI3xClx, as shown in Fig. 4.107,108 The use of a mixed solvent of γ-butyrolactone and dimethylsulfoxide followed by toluene drop-casting leads to solar cells with a certified PCE of 16.2% and no reported hysteresis.109 The lack of hysteresis, which had been an obstacle for the stable operation of perovskite devices, was observed recently using thin films of organometallic perovskites with millimeter-scale crystalline grains with efficiencies close to 18%.110 Stabilization of the perovskite phase based on formamidinium lead iodide (FAPbI3) with methylammonium lead bromide (MAPbBr3) as the light-harvesting unit in a bilayer PV architecture improved the PCE of the solar cell to >18%.111 Interestingly, electron transport layer-free and hole transport layer-free perovskite solar cells have also been reported, simplifying the device structure, thus opening up an interesting route to low-cost production of solar cells.112114

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Fig. 4
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(a) Mesoscopic perovskite solar cell with mesoporous TiO2 layer and (b) planar structure without a mesoporous TiO2 layer. Thin film on fluorine-doped tin oxide (FTO) is an n-type semiconductor. HTM stands for hole transporting material. In the mesoscopic structure, electrons can be collected directly and/or via TiO2 layer. (Reproduced with permission from Ref. 115 © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The excellent PCEs achieved for solar cells based on perovskites are attributed to their long extended electron–hole diffusion length (>1μm),116,117 broad solar light absorption (visible to near-IR), good solubility in organic media, and high carrier mobility. However, identifying the basic working mechanisms, which are still being debated, improving film quality, and improving device stability are important challenges that need to be addressed before serious consideration for their future development and commercialization.

Colloidal QD thin-film solar cells (CQD cells), depicted in Fig. 5, are also part of the newly emerging PV technology. These devices have the potential to reduce manufacturing costs due to their solution processability, light weight, and being amenable to large area deposition. CQDs can be utilized in Schottky,118,119 p–n heterojunction,120,121 hybrid BHJ,91 and as the sensitizers in PEC solar cells.122 CQDs are nanometer-sized particles that are below the Bohr exciton radius of the specified material. CQDs are synthesized by rapid injection of the reactant into the reaction medium with stabilizing agents under an inert gas atmosphere. The role of the stabilizing agent, such as oleic acid and oleylamine, is to control growth rate, particle size, and dispersion of the QDs. However, it may hinder the route for high-efficiency CQD-based PVs. The reduction of the interparticle spacing and passivation of surface traps in CQDs remain as primary challenges.

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Fig. 5
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(a) Schematic diagram and (b) cross-sectional scanning electron micrograph (SEM) of a colloidal quantum dot (CQD)-based organic photovoltaic device with a PbS CQD film and TiO2/ZnO junction. (Reprinted with permission from Ref. 118 © 2012 Macmillan Publishers, Ltd: Nature Nanotechnology.)

Among several candidates, PbS CQDs are of interest due to their ease of bandgap tunability in the visible region, and their simple, high-yield synthesis. Sargent122 and McDaniel et al.123 showed that performing a hybrid ligand exchange (long organic ligands are replaced simultaneously by short ligands and by halide ions, e.g., CdCl2) results in efficient CQD solar cells in a p:n configuration with metal oxides (TiO2, ZnO). The PCE was improved to 8.5% by reducing the TiO2 thickness layer to 10 nm in order to increase the depletion width.124 As CQD and perovskite solar cells develop further, their environmental impact will need to be considered, leading to a search for lead and cadmium free materials.

Light management in solar cells

An essential function of photonics is to optimize light coupling into PV device and to improve light trapping within the absorbing light-harvesting layer. Since solar power converters primarily rely on inorganic semiconductors, there is a significant discontinuity in the refractive index between air or the encapsulant material, and the silicon or other inorganic semiconductors, such as III–V (e.g., GaAs, InP, etc.) and II–VI (e.g., CdTe) compounds—the discontinuity in the refractive index can be as high as 4.125 This refractive index discontinuity leads to significant Fresnel reflection losses at the interface across the solar spectrum that can reduce the performance (i.e., PCE and energy yield) of the solar energy conversion system. It should also be noted that, in reality, a PV module consists not only of the semiconductor device, but also of various protecting layers, such as glass and protective/adhesive transparent polymers, and hence, the full module must be considered as an optical system when designing proper light-management approaches.20,126

The most widely used photonic method for minimizing reflection losses from the semiconductor surface is the single-layer or double-layer ARC.127 These coatings are typically designed to minimize the reflection via interference of optical plane waves and consist of thin, transparent films of silicon nitride, silicon oxide, titanium oxide, and other similar compounds. Of course, it is possible to design more advanced multilayered ARCs to further minimize reflection losses; however, for solar PV applications, this is typically too costly to implement. It is noted that such ARCs are deployed directly on the active solar cell and are then embedded within the polymer adhesive used to bind the protective glass cover for the solar module. Note that the glass itself, being a dielectric in the UV to near-IR portion of the solar spectrum, has a much lower refractive index than silicon, and hence, the outer module reflection loss is lower, i.e., ca. 4% reflection loss. Nevertheless, in recent years, glass manufacturers have also started to implement ARCs on the outer surface of the module, which, of course, provides additional requirements on the durability of the coating from mechanical handling during manufacturing and installation to environmental effects in the field, such as erosion.128

As the solar industry expanded rapidly in the last decade, it became apparent that thin-film ARCs have limitations with regard to their performance. This has to do with the fact that the reflection is often minimized for only a select region of the solar spectrum and also has a strong dependence on the angle of incidence. The surface reflectance increases rapidly as sunlight goes from normal incidence to high angles that arise during the morning and evening hours, which can lead to significant (up to ca. 30%) energy production losses.129 One solution is to have an active system that tracks the sun (see Sec. 2.4.1), but such systems are expensive and complicated, so passive systems that reduce these losses are of considerable interest. In recent years, the application of nanostructured surfaces (e.g., micro- or nanopillars or nanowires) that can be used to minimize reflection across the full solar spectrum and also up to high angles has been explored to address this problem.130,131 These structures typically produce a graded effective refractive index that leads to an ultralow, broadband reflection that is also omnidirectional; hence, these structures have been categorized as being omnidirectional antireflective (ODAR) layers.132 A related concept has been to develop ODAR layers that can also act as downconverting layers, hence better optimizing the spectrum that the semiconductor absorbs to avoid thermalization losses when high-energy photons are absorbed.133

Indeed, downconversion is part of a broader light-management strategy being investigated by researchers worldwide to more effectively utilize the solar spectrum. High-energy (short wavelength) photons lead to thermalization losses when charge carriers are excited above the conduction band edge. Alternatively, low-energy photons (long wavelengths) are not absorbed by the semiconductor and, hence, also lead to losses with respect to what the PCE of the solar cell could be if these photons were properly utilized. Calculations have shown that the limiting efficiency of a single-junction solar cell could go from 30.9% to 39.6% using a downconverting (or so-called quantum cutting) layer on the surface of the solar cell, whereas using an upconverting layer on the backside of the solar cell can increase the limiting efficiency (under 1 Sun illumination) to 47.6%.134,135 Research has focused on developing new optically active materials that can achieve up- or downconversion (e.g., QDs).38,136

In addition to minimizing surface and interface reflections, light-trapping methods have become important as the light-harvesting active layers have become thinner over the past few decades (thicknesses are approaching a few tens of microns for silicon-based solar cells; are on the order of a micron or less for thin-film inorganic solar cells; and are in the region of 10 to 100 nm for organic solar cells). When the semiconductor active layers become thinner, material costs are reduced and, in some cases, the electrical properties are improved (e.g., due to reduced bulk electron–hole recombination). However, the optical density of the active layer is reduced and in-coupled solar radiation may not be fully absorbed during the first (or second) pass of light through the material. Therefore, methods to trap or increase the path length of light within the semiconductor layer have been developed to increase the effective optical density of the material.137,138 The use of highly reflective back electrodes, textured back reflectors or electrodes, and textured semiconductor surfaces has been found to improve light trapping and, hence, the PCE in a host of PV devices. However, there is a limit (known as the ergodic or light-trapping limit) to the number of passes that light can take within a thin-film active layer using textured surfaces or back reflectors, which is proportional to 4n2.139,140 Approaches have been demonstrated that break this limit over relatively narrow wavelength ranges using photonic crystals (PCs), dielectric micro and nanostructures, and other photonic and plasmonic nanostructures.141 For light trapping over the broad solar spectrum in thin-film PVs, texturing of back reflectors and semiconductor surfaces is still the most practical approach. Besides light trapping, for semiconductor materials with fast radiative recombination rates [high internal quantum efficiencies (QEs)], such as GaAs, photon recycling by means of highly reflective back electrodes and controlling interference effects within the active layer have enabled record single-junction solar cell PCEs.26,142,143

With the development of thin-film optoelectronic devices that have active layer thicknesses below the diffraction limit (i.e., <300nm); such as small molecule and polymeric OPVs in which the active light harvesting layer thickness ranges from 10 to 40 nm and up to 250nm, respectively, light-management techniques using nanophotonic structures, particularly metallic plasmonic structures, have emerged.144156 Metal nanoparticles and structured metallic thin films can localize incident light to subwavelength dimensions due to the excitation of surface plasmons—collective electron oscillations of the free electrons at the surface of metals. However, for such effects to be strong, the metals must exhibit weak intra and interband electronic absorption losses. Therefore, noble metals are typically used for subwavelength thin-film light management using plasmonics. However, even noble metals behave as nonideal metals at optical frequencies; therefore, parasitic absorption loss is always present to some degree. Despite these losses, well-designed plasmonic nanostructures can greatly improve light harvesting for a range of energy applications that employ very thin layers or small absorber material volumes.

For example, arrays of discrete plasmonic nanoparticles and nanoantennas have been shown to effectively harvest and localize incident light within semiconductor thin films and, therefore, improve light absorption and electron–hole pair generation rate at the nanoscale. Far-field scattering by such nanostructures placed on top of or within thin-film PV devices have been shown to improve short-circuit current densities by 20% to 70% (compared to analogous devices without the plasmonic nanostructures) through the increased effective path length of incident solar radiation with the semiconductor absorber layer.145151 Optimization of the albedo (ratio of scattering crosssection to extinction cross-section), location, degree of shadowing, and shape of the nanostructures is typically required to achieve significant improvements in short-circuit current density. In addition to far-field scattering, near-field localization or trapping of incident light into guided surface plasmon polariton modes at the metal/semiconductor interface using nanostructured metal electrodes, such as nanotextured or nanopatterned metal thin films and metasurfaces, is also of interest for increasing the effective absorption depth of semiconductor thin films.

Additionally, (semi)transparent metal nanowire arrays and nanohole arrays can both facilitate light trapping and in-coupling to guided surface plasmon polariton modes and also act as transparent electrodes.148,157161 Near-field localization using the aforementioned structures and using plasmonic nanoantenna structures has also been found to increase the generation rate of electron–hole pairs, which can lead to increased PCE in thin-film PV devices. More recently, photosensitization of wide-bandgap semiconductors and generation and collection of hot electrons on plasmonic nanostructures were shown to be an alternative means to harvest solar energy for applications in PVs, photocatalysis, and solar-to-fuel energy conversion.152,162164 Plasmonic absorber materials have also been considered for light harvesting without the need for semiconductor materials.165,166