Toward Highly Efficient Nanostructured Solar Cells Using Concurrent Electrical and Optical Design

Toward a theoretical limit of solar cell efficiency with light trapping and sub-structure

Efficiency limit of solar cells with index-near-zero photon management layers

The application of metallic nanoparticles leads to an increase in the efficiency of solar cells due to the plasmonic effect. We explore various scenarios of the related mechanism in the case of metallized perovskite solar cells, which operate as hybrid chemical cells without p-n junctions, in contrast to conventional cells such as Si, CIGS or thin-layer semiconductor cells. The role of metallic nano-components in perovskite cells is different than in the case of p-n junction solar cells and, in addition, the large forbidden gap and a large effective masses of carriers in the perovskite require different parameters for the metallic nanoparticles than those used in p-n junction cells in order to obtain the increase in efficiency. We discuss the possibility of activating the very poor optical plasmonic photovoltaic effect in perovskite cells via a change in the chemical composition of the perovskite and through special tailoring of metallic admixtures. Here we show that it is possible to increase the absorption of photons (optical plasmonic effect) and simultaneously to decrease the binding energy of excitons (related to the inner electrical plasmonic effect, which is dominant in perovskite cells) in appropriately designed perovskite structures with multishell elongated metallic nanoparticles to achieve an increase in efficiency by means of metallization, which is not accessible in conventional p-n junction cells. We discuss different methods for the metallization of perovskite cells against the background of a review of various attempts to surpass the Shockley–Queisser limit for solar cell efficiency, especially in the case of the perovskite cell family.

With the global demand for renewable energy sources, there is a race in the scientific community to build a highly efficient solar cell. Nanostructured solar cells are a class of photovoltaic devices which is a strong candidate for high efficiency energy generation. Hot carrier solar cells are a kind of nanostructured solar cells, for offering high power conversion efficiency, compared with the conventional single absorber solar cell, through the process of extracting power from photo-generated high energetic carriers prior to thermalization to lattice via phonons. In this paper, we have analyzed the band structure, thermodynamic properties, and hot carrier transport in solar cell.

Nanostructured solar cells have the potential to provide a low-cost alternative to more traditional thin film solar cell technologies. Of particular interest are nanostructured solar cells with inorganic semiconductor absorbers, due to their favorable absorption properties. Such devices include quantum-dot-sensitized solar cells (QDSSCs), extremely thin absorber solar cells (ETASCs), and colloidal quantum dot solar cells (CQDSCs). However, these device architectures suffer from high rates of internal recombination and other problems associated with their extensive internal surface areas. Interfacial surface treatments have proven to be a highly effective means to improve the electronic properties of these devices, leading to overall gains in efficiencies. In this Perspective, we focus on three types of interfacial modification: band alignment by molecular dipole layers, improved CQD film mobilities by ligand exchange, and reduced recombination by interfacial inorganic layers. Select examples in each of these categories are highlighted to provide a detailed look at the underlying mechanisms. We believe that surface modification studies in these devices-QDSSCs, ETASCs, and CQDSCs-are of interest not only to these fields, but also to the broader photovoltaics community.

The application of inverse titania opals in nanostructured solar cells

Organic-inorganic lead halide perovskite, initially applied as active layer in light-emitting diode (LED) and field-effect transistor ( FET) devices[1], has been gradually introduced into the development of solar cells and plays such critical role as either light absorber or hole conducting material (HCM). This kind of hybrid layer structure makes perovskite as a promising candidate in photovoltaic applications, mainly due to its attractive chemical versatility that can readily tune the electronic coupling between inorganic sheets, and eventually tailor its optical/electronic properties[2]. In order to understand the performance of a photovoltaic device especially involving such novel material for its further optimization consideration, exploring the photogenerated charge accumulation and transport is imperative to be highlighted. Here we report the work on perovskite (CH3NH3PbI2Cl[3])-based nanostructure solar cell, including the cell fabrication and characterization, and particularly provide the analysis on charge carrier properties on the basis of dynamic electrochemical measurements. In this research stage, we targets more than achieving relatively fair energy conversion efficiency, instead we hope to uncover internal mechanisms of charge carrier movement and subsequently the substantial function of perovskite in photovoltaic device.The stack structure of the mesoscopic solar cell we developed for this investigation is shown in Fig. 1(a). The nanoporous semiconductor oxide membrane was fabricated by screen-printing technique on fluorine doped tin oxide (FTO) glass substrate with terpineol-based paste made by commercial-available TiO2 powder (Aldrich, d=25nm). The work electrode coated with the TiO2 paste was sintered under airflow at 550oC for 3 h. Iodide-chloride mixed-halide perovskite was synthesized from a N,N-Dimethylformamide solution of CH3NH3I and PbCl2 (3:1 molar ratio) [3]. The precursor solution was deposited on porous TiO2 surface via spin-coating or dipping process, and then was dried at 100oC. After drying, obvious color variation indicated the formation of perovskite layer. The function of the perovskite layer in nanostructure solar cells strongly depends on the deposition method. In particular, the shape of nanocrystalline CH3NH3PbI2Cl infiltrating into TiO2 porous membrane, probably with quantum-dot structure, is confirmed by electron microscopy micrographs. One thing should be noted is that, combining the band diagram shown in Fig.1(b), the extremely thin CsSnI2.95F0.05 layer (referring our previous work for its synthesis[4]) deposited on counter electrode performs more like energy ‘staircase’ for hole transport rather than pure hole conducting material. Meanwhile, a heterojunction-like cell (called ‘blank’ cell) containing only CH3NH3PbI2Cl layer on the TiO2film is produced for comparison.In order to thoroughly understand the decisive mechanisms of charge accumulation and transport behaviors on photovoltaic conversion, a systematic characterizations including photocurrent transient, open-circuit voltage decay (OCVD) and electrochemical impedance spectroscopy (EIS) were carried out under different operation conditions. Correspondingly the effective lifetime and diffusivity of charge carriers are extracted to investigate the transport properties of photon-generated electrons. Particularly, chemical capacitance (Cμ) is examined since it assists to monitor the Fermi level of electrons and density of states (DoS) in perovskite absorber, which significantly differentiates from conventional dye (e.g. N719) sensitized solar cells on the aspect of charge generation and accumulation in light absorber material.Consequently, our research demonstrates the promising HCM-free nanostructure solar cells involving solution-processed perovskite material. Fair light absorption and charge accumulation capability make self-assembled CH3NH3PbI2Cl behave as both sensitizer and charge transport material, which helps to constitute new kind of solid-state sensitized solar cells with a much simpler configuration. Reference 1. D. B. Mitzi, K. Chondroudis, and C.R. Kagan, Organic–inorganic electronics. IBM J. Res. & Dev. , 2001. 45(1): p. 29-45.2. Etgar, L., Semiconductor Nanocrystals as Light Harvesters in Solar Cells. Materials 2013. 6: p. 445--59.3. Lee, M.M., et al., Efficient hybrid solar cells based on meso-superstructured organometal halide perovskites. Science, 2012. 338 ( 6107): p. 643-647.4. Sun, X., et al., Effects of calcination treatment on the morphology, crystallinity, and photoelectric properties of all-solid-state dye-sensitized solar cells assembled by TiO2 nanorod arrays. Phys. Chem. Chem. Phys., 2013. 15: p. 18716-18720.

Despite their maturity and market dominance, crystalline silicon (c-Si) solar cells and modules continue to develop rapidly, with increasing efficiencies and reducing cost ($/W). This contribution describes recent progress at SERIS with industrially relevant crystalline silicon solar cells and modules, covering both p-type and n-type cells. Since the costs of the solar cells and modules as a proportion of the costs of complete PV systems continue to fall, a premium is placed on cell efficiency as this reduces the balance of system (BOS) cost (in $/W) of PV systems. Thus, cost-effective monocrystalline high-efficiency silicon solar cells such as SERIS’ 23.5% efficient passivated-contact technology (“monoPolyTM”) have enormous market potential. Good progress is also being made in the SERIS laboratory with small-area perovskite/silicon tandem solar cells, giving efficiencies of over 25% in 4-terminal configuration. Keywords: Solar energy, photovoltaic devices, crystalline silicon solar cells, PERC solar cell, monoPolyTM solar cell, passivated contacts, perovskite solar cells, perovskite/silicon tandem solar cells.

Nowadays, the latest technological innovations demand energy conversion/storage devices and they have accelerated the consumption of energy. So, this requires to develop the sustainable and environment-friendly devices that can harvest energy most prominently and reduce the usage of fossil fuels. In this regard, the most inventive renewable energy devices are supercapacitors, fuel cells, photovoltaic materials, thermoelectric materials, lithium-ion batteries, and so on. Solar cells are a good candidate for clean energy. Researches are working on solar cells to develop the highest efficiency, environment-friendly, and non-toxic solar cell devices. Photovoltaic solar cells are promising candidates that can harvest energy directly from sunlight and are known as photovoltaic devices. Recently, chalcogenides-based 2D nanomaterials have attracted the attention of scientists as they have emerged as very promising materials for solar cell applications. The reason behind this is that they can solve both toxicity and stability problems simultaneously faced by conventional solar cells. There are different classes of chalcogenides-based 2D nanomaterials. Over the past years, metal chalcogenides (metal monochalcogenides, metal dichalcogenides) and transition metal chalcogenides have experienced important progress that is leading to the discovery of very promising compounds and derivatives for solar cell applications. This is because of the advantages that chalcogenide-based 2D nanomaterials technology offers like applicability onto flexible substrates, high stability and aesthetical appearance, reliability, monolithic deposition, superior temperature coefficient, and potential in tandem applications. This book chapter will present a detailed review of II-VI chalcogenides-based 2D nanomaterials for solar cell application. Moreover, types of various II-VI chalcogenides-based nanomaterials, their recent synthesis progress, optical properties, and their potentials in solar cells like hybrid inorganic nanostructure-conductive polymer composite solar cells, all-inorganic thin-film nanostructure solar cells, and quantum-dots-sensitized solar cells will be discussed.

We predict that high temperature Bose-Einstein condensation of charge transfer excitons can be achieved in organic-two-dimensional (2D) material heterostructures, at ∼50-100 K. Unlike 2D bilayers that can be angle-misaligned, organic-2D systems generally have momentum-direct low-energy excitons, thus favoring condensation. Our predictions are obtained for ZnPc-MoS2 using state-of-the-art first-principles calculations with the GW-BSE approach. The exciton energies we predict are in excellent agreement with recent experiments. The lowest energy charge transfer excitons in ZnPc-MoS2 are strongly bound with a spatial extent of ∼1-2 nm and long lifetimes (τ0 ∼ 1 ns), making them ideal for exciton condensation. We also predict the emergence of inter-ZnPc excitons that are stabilized by the interaction of the molecules with the 2D substrate. This novel way of stabilizing intermolecular excitons by indirect substrate mediation suggests design strategies for singlet fission and exciton multiplication, which are important to overcome the Shockley-Queisser efficiency limit in solar cells.

Nanostructures in III–V solar cells have been widely investigated. Due to the enhancement in photon absorption, higher short-circuit current has been achieved in a number of previous reports. However, many of the nanostructured solar cells suffer from low open circuit voltage and low fill factor, which result from poor junction quality and high non-radiative recombination. Such problems can be resolved by utilizing both a nanostructured III–V window layer and planar p-n junction. In this work, we demonstrate the design and optimization of nanostructured window solar cells through a set of carefully designed experiments. A systematic study based on device fabrication, characterization and simulation is presented. This work shows that reducing absorption and minority carrier loss inside the nanostructured window are key to further improve the performance of nanostructured window solar cells.

Successfully designing an ideal solar cell requires an understanding of the fundamental physics of photoexcited hot carriers (HCs) and the underlying mechanism of unique photovoltaic performance. Harnessing photoexcited HCs offers the potential to exceed the thermodynamic limit of power conversion efficiency, although major loss channels employing ultrafast thermalization of HCs severely restrict their utilization in conventional bulk-absorber-based solar cells. Spatially confined semiconductors, especially 2D van der Waals (vdW) materials, offer several advantages, such as strong Coulomb interaction, high exciton binding energy, strong carrier–carrier scattering and weak carrier–phonon coupling, resulting in slow HC cooling and restricted loss channels. This Review provides a detailed mechanistic understanding of the HC cooling dynamics in confined vdW layered materials for efficiently utilizing HCs and discusses the role of carrier multiplication in designing a solar cell with the power conversion efficiency exceeding the Shockley–Queisser limit. Additionally, we analyse the major energy loss channels that limit the efficiency of a conventional solar cell, as well as the promises held by the 2D vdW heterostructures for an efficient HC solar cell. Furthermore, we highlight the challenges and opportunities involved in successfully utilizing HCs in practical solar cells with efficiencies beyond the thermodynamic limit. Conventional solar cells suffer from the thermalization energy loss of hot carriers, which sets the Shockley–Queisser limit. This Review highlights the insightful mechanisms of slow hot carrier cooling dynamics and efficient carrier transport/extraction in 2D van der Waals heterojunctions, following opportunities in practical hot carrier solar cells.

The introduction of semiconductor and metallic nanostructures in photovoltaic materials is an active and promising area of development. This is due to the existence of several characteristics of nanostructures that improve the functionality of photovoltaic materials, for example by an enhancement of efficiency in solar cells, an increase of collected sun light and by the use of up- and down-converters to change the energy of incident photons. Nanostructures for solar cells offer, in comparison with single junction solar cells, new photovoltaic mechanisms for transforming sun light energy into electrical energy. A critical issue to understand the behavior of nanostructures based solar cells is to know their structural characteristics (size, shape, strain, distribution and composition) at nanoscale. In addition to this, the knowledge of the processes of nucleation, segregation and intermixing in nanostructures at nanoscale, and in some cases to atomic scale, is another important point to improve their design. This communication reviews our contribution toward the understanding of these issues and present some prospective studies and experiments in this context, by the use and development of methods based in aberration-corrected scanning transmission electron microscopy, high resolution electron microscopy and other complementary techniques. These methods are applied to several semiconductor nanostructures for intermediate band solar cells and some prospective results are presented in relation to their potential use for nanostructured thin film solar cells. We also present a method to prepare special samples to correlate, in individual or some few nanostructures, high resolution structural properties with other functional properties for the better design of photovoltaic materials. The nanostructures studied by aberration-corrected scanning transmission electron microscopy that are reviewed in this paper consist of III-V quantum dots and wires, silicon wires and strain-compensated stacked nanostructures grown by epitaxial techniques.

Light-absorption enhancement is one of the key research areas related to the development of high-efficiency crystalline silicon (c-Si) solar cells. Surface structuring which can reduce not only the surface reflection but also increase the optical path length is an efficient way to increase light absorption in a broadband wavelength range. Among the surface structures, vertically aligned silicon microwires (MWs) have been extensively investigated as a means for developing highly efficient c-Si solar cells because of the outstanding broadband antireflection and radial junction effect which enables efficient charge collection. The incident light is absorbed along the long MW axis, while photo-induced carriers can be collected along the short radial direction. To realize the highly efficient radial junction c-Si solar cells, we have developed novel technologies such as fabrication process of MWs, high conductive and transmittance top electrode, shape-controlled MWs, and high purity doping process. The high-aspect-ratio MWs (> 10:1) were successfully fabricated through both optimized metal-assisted chemical etching (MACE) and deep reactive ion etching (DRIE) processes. To achieve the high efficiency of MW radial junction solar cells, we developed the high conductive and transparent top electrode to replace the conventional bus-finger electrode which has a significant shading loss. We devised a novel micro-grid top electrode which shows superior transmittance (over 97%) and low sheet resistance (less than 30 Ω/□). By applying the micro-grid electrode on the top surface, the MWs solar cells showed outstanding fill factor (81.2%) and improved efficiency (16.5%). Although our MWs radial junction solar cell showed improved efficiency with the micro-grid electrode, it needs to increase the light absorption capability to maximize the efficiency. As an efficient way to decrease the flat-top-surface reflection of the MWs and increase the light absorption property of the radial junction solar cells, a tapered-MW structure was employed using a simple wet-etching process. When a c-Si wafer with MWs is dipped in a silicon etchant (RSE-100, transene), the top part of the MWs that has a shorter diffusion path compared to the bottom part is etched more quickly because of the different chemical diffusion path lengths leading to the formation of tapered MWs. Since the diameter of the tapered MWs gradually increased from the top to the bottom, the tapered MWs can act as a buffer layer to compensate for the mismatch between the refractive indexes of air (1) and the silicon substrate (4). Thus, the surface reflection of the tapered MWs was observed to be less than 2% at a wavelength of 550 nm. The tapered MW based radial junction solar cells exhibit improved efficiency up to 18.9% thanks to the enhanced light absorption property. As the last step for optimizing the device structure of the MWs solar cells, we developed high purity doping process using acid dopant sources that showed improved minority carrier lifetime (from 79.29 µs to 272.24 µs). Accordingly, we achieved high efficiency (20.2%) MWs radial junction solar cell by applying all of the developed our technologies such as the micro-grid electrode, tapered MWs, and high purity doping process. At present, we are aiming at developing an ideal passivation layer to achieve the more than 25% efficiency of the radial junction solar cells. Therefore, we believe the MW structures with the suggested technologies become a foundational technology for the highly efficient radial junction solar cells.

This chapter will focus on the recent advances on the traditional and modern solar cell technologies, notably, (a) silicon solar cells, (b) multi-junction solar cells, (c) perovskite solar cells, and (d) dye-sensitized solar cells. Research efforts focused on improvement of the stability and the efficiency of each type of cells will be mentioned. While the current industrial market is predominantly dominated by silicon solar cells, other photovoltaic cells (b–d) show immense promise to overtake the silicon PV market in near future. The most efficient silicon solar cell reported reaches an efficiency of over 26%. This efficiency was achieved by fabricating a cell with an interdigitated back contact, combining n-type and p-type amorphous silicon to collect both holes and electrons. The back contact is separated from the front contact by crystalline silicon, with the front contact covered by an amorphous silicon passivation layer and an antireflective coating. A close competitor of silicon solar cells, known as multi-junction solar cells, displays power conversion efficiency as high as 46% using a solar concentrator. However, due to difficulty in cell fabrication with elevated cost, application of this type of cell is mostly limited to extraterrestrial purposes. A low-cost alternative of multi-junction cell is a perovskite solar cell. The best efficient perovskite solar cell with a power conversion efficiency of 23.9% was achieved by using a complex semitransparent organic–inorganic perovskite material with a high bandgap absorber, Cs0.1(H2NCHNH2)0.9PbI2.865Br0.135 combined with a low bandgap absorber, c-Si, for the back contact. While the perovskite solar cells are promising candidates as low-cost substitute to silicon solar cells, stability remains an issue for the former. On the other hand, dye-sensitized solar cells are cost-effective and chemically stable, with a best reported efficiency of 13% using a panchromatic donor–π–acceptor-based designed SM315 as an organic sensitizer; power conversion efficiency of this type of cell is still to be improved to overtake the silicon PV market.