New generation solar cell
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太阳能光伏发电是解决目前日益严重的能源与环境问题的一种有效手段。经过数十年的发展，电池的种类从传统的晶硅电池发展到各类新型太阳能电池，包括半导体薄膜电池、有机薄膜电池、敏化电池以及钙钛矿电池等。特别是钙钛矿太阳能电池，在短短几年内实现了光电转换效率的飙升，被Science评为“2013年十大科学突破”之一，成为目前新型太阳能电池的研究热点之一。新型太阳能电池的快速发展不仅有效促进了材料和器件设计，加深了对复杂体系的电荷产生、输运过程的研究，增强了不同光电领域的交叉发展，也成为一门新兴的材料和光电交叉学科。
本刊特组织“新型太阳能电池”专题，这些论文对各类新型电池的工作机理、关键材料和器件设计等方面进行了创新研究和发展现状介绍。我们希望，通过对传统和各类新型太阳能电池的深入研究，能够加速实现低成本、高效率地利用太阳能。
2015, 64 (3): 033301.
doi: 10.7498/aps.64.033301
Abstract +
Perovskite solar cells with a solid-state thin film structure have attracted great attention in recent years due to their simple structure, low production cost and superb photovoltaic performance. Because of the boost in power conversion efficiency (PCE) in short intervals from 3.8% to 19.3% at present, this hybrid cells have been considered as the next generation photovoltaic devices. It is expected that the efficiencies of individual devices could ultimately achieve 25%, which is comparable to the single-crystal silicon solar cell.In this article, the perovskite absorber, its basic device structure, and operating principles are briefly introduced. Since most of the high efficiency perovskite solar cells employ hole transporting materials (HTM), they could benefit the hole transport and improve the metal-semiconductor interface in the cells. This perspective gives analyses of some effective hole transporting materials for perovskite solar cell application. The hole transporting materials used in perovskite solar cell are classified into six categories according to their structures, including triphenylamine-based small molecule HTM, small molecule HTM containing N atom, sulfur-based small molecule HTM, sulfur-based polymer HTM, polymer HTM containing N atom and inorganic HTM. Emphasis is placed on the interplay of molecular structures, energy levels, and charge carrier mobility as well as device parameters. A critial look at various approaches applied to achieve desired materials and device performance is provided to assist in the identification of new directions and further advances.
2015, 64 (3): 038103.
doi: 10.7498/aps.64.038103
Abstract +
This review surveys the application of graphene and its derivatives in organic solar cells, used as interfacial layers: including anode interfacial layers, cathode interfacial layers, and intermediate layers in a tandem device. Research work has be done for increasing the electroconductivity by reducing the oxide to partially oxidized graphene, as well as chemically modifying or making composite interfacial layer. Additionally, the researches on graphene derivatives and combined interfacial layers used as a cathode interfacial layer or an intermediate layer in the tandem device are discussed. Finally, this review suggests that graphene and its derivatives are potential to be used in perovskite solar cells.
2015, 64 (3): 038104.
doi: 10.7498/aps.64.038104
Abstract +
Perovskite solar cell, which is prepared by using the organic-inorganic hybrid halide CH3NH3PbX3 (X = I, Cl and Br), receives widespread attention because of its solution processability and high photon-to-electron conversion efficiency. The highest reported photon-to-electron conversion efficiency is that using CH3NH3PbI(3-x)Clx as an absorber. It is reported that the diffusion length is greater than 1 micrometer in this mixed halide perovskite. The method most commonly used in preparing CH3NH3PbI(3-x)Clx film is the one-step pyrolysis method, which has a complex reaction mechanism. In this paper, we review the work about CH3NH3PbI(3-x)Clx perovskite, in which emphasis is put on the importance of the preparation process, and analyze the role of CH3NH3I in the one-step pyrolysis method for fabricating the CH3NH3PbI(3-x)Clxperovskite layer. Scanning electron microscope images show that CH3NH3I can improve the coverage and crystallinity of the perovskite layer for precursors in low CH3NH3I concentrations (CH3NH3I/PbCl2=2.0 and 2.5). For precursors in high CH3NH3I concentrations (CH3NH3I/PbCl2=2.75 and 3), this change is not obvious. X-ray photoelectron spectroscopy confirms the change of coverage, and indicates that the content of Cl in CH3NH3PbI(3-x)Clx will be less than 5% for precursors with high CH3NH3I concentrations (CH3NH3I/PbCl2>2.5). Perovskite solar cells based on CH3NH3PbI(3-x)Clx with different Cl dopant concentrations are studied by photoelectric measurements. Photocurrent density-photovoltage curves show that the performance of the devices increases with the increase of CH3NH3I concentration in precursors. And the incident-photon-to-current conversion efficiency (IPCE) measurements indicate that the devices fabricated by using precursors with high CH3NH3I concentration have a relatively high external quantum efficiency. These results imply that only CH3NH3PbI(3-x)Clx with very low Cl dopant concentration will be effective material for photovoltaic application. To further understand the difference between these devices during working condition, transient photovoltage/photocurrent measurements are carried out to investigate the carrier dynamics in the device. Transient photovoltage decay curves indicate that high Cl dopant concentration may decrease the photoelectron lifetime in CH3NH3PbI(3-x)Clx, and results in a relative low open-circuit photovoltage in the corresponding photovoltaic devices. The charge transport time in the devices of various Cl concentrations are studied by transient photocurrent decay method. CH3NH3PbI(3-x)Clx with low Cl dopant concentration has relative short transport time, which can avoid the recombination process and increase the charge collection efficiency.
2015, 64 (3): 038401.
doi: 10.7498/aps.64.038401
Abstract +
The development of highly efficient and low-cost solar cells is the key to large-scale application of solar photovoltaic technology. In recent years, the solution-processed organic-inorganic perovskite solar cells attracted considerable attention because of their advantages of high energy conversion efficiency, low cost, and ease of processing. The ambipolar semiconducting characteristic of perovskite enables the construction of planar heterojunction architecture to be possible in perovskite-based solar cells. This kind of architecture avoids the use of mesoporous metal oxide film, which simplifies the processing route and makes it easier to fabricate flexible and tandem perovskite-based solar cells. Planar heterojunction perovskite solar cells can be divided into n-i-p type and p-i-n type according to the charge flow direction. Two interfaces are formed between perovskite film and hole/electron transport layer, where efficient charge separation can be realized. Hole and electron transport layers can form separated continuous paths for the transport of holes and electrons, thus beneficial to improving exciton separation, charge transportation, and collection efficiency. In addition, this planar architecture avoids the use of high temperature sintered mesoporous metal oxide framework; this is beneficial to expanding the choice of the charge transport materials. In this paper, we review the recent progress on the planar heterojunction perovskite solar cells. First, we introduce the material properties of perovskite, the evolution of device architecture, and the working principle of p-i-n type and n-i-p type planar heterojunction perovskite solar cells. Then, we review the recent progress and optimization of planar heterojunction perovskite solar cells from every aspect of perovskite preparation and the selection of electron/hole transport materials. Finally, we would like to give a perspective view on and address the concerns about perovskite solar cells.
2015, 64 (3): 038402.
doi: 10.7498/aps.64.038402
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Analysis of the DC current-voltage (I-V) characteristics is an effective approach to investigate the charge transport properties in a solar cell. The perovskite solar cell attracted wide research interest in the past two years due to their outstanding photovoltaic capacity. However, the charge transport characteristics and working principles of this kind of cells have not been clearly clarified. In this work, the I-V characteristics of the perovskite solar cell have been investigated from the experimental and theoretical perspective views. Moreover, the S-shaped I-V feature coming from the limitation of interfacial charge transport was focused on. With a series connected diode model, the I-V characteristics of the solar cell are investigated and simulated. It is found that the charge accumulation appears gradually when the interfacial charge transport velocity is decreased, lowering the output of the cell. When the interfacial charge transport decreases gradually, the short-circuit current density and the fill factor of the cell also decrease obviously. In experiments, limitations of charge transport at the front and back contacts of the cell have been designed, successfully producing varied S-shaped I-V features. It is found that both in the hole transport material-free and in the p-i-n perovskite solar cells, the S-shaped I-V characteristics can appear. Moreover, the origins of these features in various experimental conditions have also been discussed, which can be the energy barriers or large charge transport resistances in the cell. These energy barriers and resistances will lower the charge transport velocity and may cause charge accumulation, thus leading to the appearence of the S-shaped features. Meanhiwle, the emerging S-shaped I-V curves all have their own features, which may be due to the specific interfacial energy band structures. Thus, to promote the cell performance, the charge transport and interface energy barrier should be attached importance to and carefully designed. This work directly shows the interface factors that can significantly affect the cell performance, and gives a theoretical guide in cell design. By considering these limiting factors, the cell fabrication has been carefully designed with the control on the thickness of the mesoporous layer and the perovskite absorber film deposition, and a forward-swept efficiency of 15.5% can be achieved without any modification of the cell.
2015, 64 (3): 038403.
doi: 10.7498/aps.64.038403
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Due to their high efficiency and low cost, organic-inorganic hybrid perovskite solar cells are attracting growing interest recently. For the most commonly studied perovskite CH3NH3PbI3, optimization of the morphology and crystallinity of CH3NH3PbI3 thin films can greatly improve the efficiency of perovskite solar cells. A homogenous and uniform perovskite film can prevent direct contact between the hole transport layer and the electron transport layer, and thus can significantly reduce charge recombination. And the high crystallinity perovskite film facilitates fast charge transportation and injection. Various studies have proved that solvent has a critical influence on both the morphology and the crystallinity of perovskite thin films. In this work, we thoroughly studied the influence of the normally used N, N-Dimethylformamide (DMF) and r-butyrolactone (GBL) solvents on perovskite morphology, crystallinity, as well as the solar cells efficiency. When using DMF as the solvent, the efficiency is only 2.8%, while the efficiency of the cell obtained based on GBL can reach 10.1%. SEM and HRTEM are employed to study the morphology and crystallinity of these two kinds of perovskite films. The perovskite film prepared using solvent DMF shows a rough capping layer consisting of strip-like perovskite crystals, and the filling of meso-TiO2 is poor. Compared with DMF, the GBL perovskite film shows a better capping layer structure consisting of large perovskite domains, and the filling of meso-TiO2 is improved as well. This great difference in capping layer morphology and meso-TiO2 filling is one reason for the different performance. Besides morphology, different defect concentrations in these two kinds of perovskite films are another crucial issue. By Combined XRD and UV techniques, the mechanisms how perovskite precipitats from DMF and GBL solutions can be disclosed. In DMF, because of its low spoiling point of 153 ℃, most of DMF solvent volatilize by spin-coating, and an intermediate MOF structure of PbI2: MAI: xDMF is formed. During thermal annealing, the unstable MOF structure breaks down and a large amount of dislocations form in perovskite films, which highly restrict the charge transport. However, the spoil point of GBL (206 ℃) is higher than that of DMF, which makes it hard to be fully volatilized by spin-coating. During the following thermal treatment, the solubility of perovskite is lowered with increasing temperature. So perovskite crystallites precipitate from the GBL first and then gradually grow up with the volatilization of the excess solvent. We finally find that coordination between the solvent and the PbI2 plays a big role on the morphology and the crystallinity of the solution-processed perovskite film, and this is responsible for the difference of the device performance.
2015, 64 (3): 038404.
doi: 10.7498/aps.64.038404
Abstract +
Preparation of Perovskite solar cell, an emerging low-cost photovoltaic technology in rapid development, has provided a ray of hope to solve the energy problem. However, its low reproducibility and stability limit the wide application of this potential technology. In this review, we summarize the recent progress with a focused discussion on some key issues in the development of perovskite solar cells. Starting from the analysis of basic structure and working principles, we first discuss the perovskite-based light harvesting layer and the general strategy to control its spectrum response. We also demonstrate the effect of film morphology on the device performance and the reproducibility which requires very uniform thin films. Then we discuss the major function of electron transporting layer and hole blocking layer, and point out the importance of compact hole blocking layer with less nano-scaled pinholes. For the hole transporting layer, we focus the discussion on the stability problem induced by widely used dopants that can improve the hole conductivity in the hole transporting layer while the dopants' deliquescent behavior also can induce the decomposition of perovskite-based light harvesting layer with a rapid degradation of the whole device. The potential approaches to solve this stability problem, such as using a dopant-free hole transporting material or making device without any hole transporting materials, are also discussed. Finally, we are in prospect of overcoming the main challenges in the future research for high performance perovskite solar cells.
2015, 64 (3): 038405.
doi: 10.7498/aps.64.038405
Abstract +
Recently solid-state organic-inorganic hybrid solar cells based on perovskite structured materials have evidenced a great breakthrough due to their perfect light absorption and charge transfer optoelectronic properties. The power conversion efficiencies have exceeded 20.1% during the last 5 years, since the first report on perovskite solar cells with an efficiency of 3.8% in 2009. Remarkably, perovskite solar cells with a planar-heterojunction structure have achieved an efficiency of 19.3%, and the perovskite solar cells with conventional mesoporous structure have achieved a certified efficiency above 16.7%. This review article first introduces the development of the third generation of solar cells from dye-sensitized solar cells to the perovskite solar cells, and then focuses on the optical and physical properties of the perovskite materials and their application in solid-state solar cells. We discuss the performance characteristics and advantages of the perovskite solar cells having mesoporous, planar heterojunction, flexibility, and hole-conductor-free structure respectively, and the charge collection layer which is applied in perovskite solar cells, such as semiconductor oxide (TiO2, Al2O3, ZnO and NiO) and PEDOT:PSS, etc. More over this review article introduces the charge transport materials, including P3HT, spiro-OMeTAD, PTAA, and PCBM, as well as different photoabsorption material, such as CH3NH3PbI3, CH3NH3PbBr3 and CH3NH3PbI3-xClx, etc. aiming to analyze their performance characteristic in the perovskite solar cells with different configurations; and the main factor related to the performance. Finally, this review elaborates the perspective and understanding of the perovskite solar cells and points out the critical point and expectation for improving the performance of perovskite solar cells further.
2015, 64 (3): 038406.
doi: 10.7498/aps.64.038406
Abstract +
Recently, antimony selenide (Sb2Se3) has been proposed as an alternative earth-abundant absorber material for thin film solar cells. Sb2Se3 is a simple V2-VI3 binary compound with an orthorhombic crystal structure and a space group of Pnma 62. It is a staggered layered compound consisting of parallel 1D (Sb4Se6)n ribbons held together by weak van der Waals forces. Sb2Se3 has a direct band gap of approximately 1.15 eV with a large absorption coefficient (>105 cm-1, at short wavelength) and a low grain growth temperature (～300^{o}C), facilitating the fabrication of low-cost thin film solar cells. Moreover, it is a simple binary compound in single phase with a fixed composition, which provides a much simpler growth chemistry than the multicomponent Cu2ZnSn(S,Se)4. In addition, it is stable upon exposure to the ambient air, thus having a better prospect for long-term stability than the organic-inorganic halide perovskite solar cells. Theoretical analysis indicates that the efficiency limit is >30% for single junction Sb2Se3 solar cells. Various approaches, including vacuum evaporation, electrodeposition, spray pyrolysis, and chemical bath deposition (CBD), have been explored to produce Sb2Se3 thin films; however, it is only in these years that Sb2Se3 solar cells have been reported by our group as well as by others. Seok's group presented the deposition of Sb2Se3 on mesoporous TiO2 films by thermal decomposition of Sb2Se3 single-source precursors, and fabricated Sb2Se3-sensitized inorganic-organic heterojunction solar cells with a remarkable efficiency of 3.21%. Tena-Zaera's group fabricated the FTO/TiO2/Sb2Se3/CuSCN/Au heterojunction device and achieved 2.1% device efficiency; their Sb2Se3 was obtained by an electrodeposition route and CuSCN served as a hole conducting layer. Different from the above Sb2Se3-sensitized solar cells reported by other groups, our group is the first in the world working on Sb2Se3 thin film solar cells so far as wu know. We have fabricated a hydrazine solution-processed TiO2/Sb2Se3 heterojunction solar cell, achieving 2.26% device efficiency (Voc = 0.52 V, Jsc = 10.3 mA/cm2 and m FF = 42.3%). In addition to the solution processing method, thermal-evaporated substrate and superstrate CdS/Sb2Se3 thin film solar cells with 2.1% and 1.9% efficiencies respectively were also demonstrated by our group. Recently, we have further improved the superstrate device performance to 3.7% (Voc=0.335 V, Jsc=24.4 mA/cm2, and m FF=46.8%$) by using a post selenization step. Selenization can compensate the Se loss during thermal evaporation, attenuate selenium vacancy-related recombination loss and hence improve the device performance. In summary, this paper summarizes the recent research progress in Sb2Se3-related researches, including material properties of Sb2Se3, synthesis of Sb2Se3 nanomaterials and thin films, theoretical studies on electrical properties, device configuration and efficiency improvement of Sb2Se3 sensitized and thin film solar cells. This review also presents a perspective on future development of Sb2Se3 solar cells.
2015, 64 (3): 038802.
doi: 10.7498/aps.64.038802
Abstract +
Ever since the first organic-inorganic hybrid halogen perovskite solar cell was first used as a photo-voltaic material in 2009, reports on this type of solar cell have grown exponentially over the years. Up till May 2014, the photo-energy conversion efficiency of the perovskite solar cell have already achieved an efficiency approaching 20%. Surpassing the efficiency achieved by organic and dye synthesized solar cell, the perovskite solar cell is in good hope of reaching the efficiency compatible with that of mono-crystalline silicon solar cell, thus it is going to be the star in photo-voltaic industry. In a perovskite solar cell, the film-formation and electron-mobility in the electron transfer layer can dramatically affect its efficiency and life-span. Especially in the up-right structured device, the mesoscopic structures of the electron-transfer layer will directly influence the growth of the perovskite layer. The present researches of electron transport materials mainly focus on three aspects: (1) How to improve the instability in mesoporous TiO2-mesosuperstructured solar cells, that arises from light-induced desorption of surface-adsorbed oxygen. (2) How to obtain TiO2 or other electron transport materials at low temperature (sub 150 ℃) in order to be applicatable in flexible devices. (3) How to substitute the mesoporous TiO2 or compact TiO2 transport layer by organic or composite materials. This article devides the materials that are used to make the electron-transfer layer into three distinct groups according to their chemical composition: i.e. metal oxides, organic small molecules, and composite materials, and introduces about the role they play and the recent development of them in constructing the perovskite solar cell.
2015, 64 (3): 038803.
doi: 10.7498/aps.64.038803
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In 2009, organic-inorganic hybrid perovskite was first used as the light-absorbing material for solar cells. The rapidly increased efficiency, simple preparation process, and low cost have aroused widespread concern. The last five years have witnessed the increase of the power conversion efficiency in the organic-inorganic hybrid perovskite solar cells from 3.8% to 19.3%. At present, most researches focus on how to improve the photoelectric conversion efficiency rather than the stability. With the improvement of the power conversion efficiency, people have realized that the long-term stability is also one of the key issues in practical applications.The present preliminary researches indicate that there are two main factors connected with the stability. One is the stability of the perovskite materials, including thermal stability and humidity stability; the other is the stability of solar devices, mainly related to the design and optimization of devices' structure. To solve the problems of stability of perovskite materials, the main point is its crystal structure. Based on the tolerance factor related to the stability of the perovskite lattice structure, choosing a more suitable size of the moiety can reduce its sensitivity to humidity and improve its stability. To design the device structure, we should try to select a hydrophobic material to protect the perovskite materials from being affected by the surrounding environment. Researches have so far showed that by optimizing the design of the solar cell structure via combining the elements utilized and the bonding interface work, the stability of the hybrid perovskites solar cell is supposed to be entirely solved, and this will determine the practical process of hybrid perovskite photovoltaic materials. However, by the moment, the study on stability of perovskite solar cells is far from being sufficient.
2015, 64 (3): 038804.
doi: 10.7498/aps.64.038804
Abstract +
Hybrid polymer-based solar cells (HPSCs) that use conjugate polymers as electron donor (D) and inorganic semiconductor nanocrystals as electron acceptor (A) are novel photovoltaic devices. HPSCs integrate the properties of organic polymer (flexibility, ease of film formation, high absorption coefficient) and inorganic nanostructures (high electron mobility, high electron affinity, and good stability), and have the extra advantages, such as the rich sources of synthesized nanostructures by wet chemistry, tunable and complementary properties of assembled components, solution-processibility on a large scale at low cost and light-weight, etc. Amongst various inorganic semiconductor materials, the nanostructured metal oxides are the promising electron acceptors for HPSCs, because they are environment-friendly, transparent in visible spectrum and easy to be synthesized. After a brief introduction to the current research status, working principles, device architecture, steady-state and dynamic characterizations of HPSCs, this paper mainly reviews our recent research advances in the HPSCs using ZnO and TiO2 nanostructures as main electron acceptor and transporter, with emphasis on the theoretical models for charge carrier transport dynamics, design and preparation of efficient materials and devices, and the device performance related with nanostructural characteristics. Finally, the main challenges in the development of efficient HPSCs in basic researches and practical applications are also discussed. The main conclusions from our studies are summarized as follows: (i) IMPS and IMVS are powerful dynamic photoelectrochemical methods for studying the charge transport dynamics in HPSCs, and our theoretical models enable the IMPS to serve as an effective tool for the mechanistic characterization and optimization of HPSC devices. (ii) Using a multicomponent photoactive layer with complementary properties is an effective strategy to achieve efficient HPSCs. (iii) Using the complementary property of components, enhancing the dissociation efficiency of excitons, and improving the transport properties of the acceptor channels with reduced energy loss to increase collection efficiency all are the effective measures to access a high photocurrent generation in HPSCs. (iv) The band levels of components in the photoactive layer of HPSCs are aligned into type II heterojunctions, in which the nanostructured component with the lowest conduction band edge acts as the main acceptor/transporter; the maximum open-circuit voltage (Voc) in HPSCs is determined by the energy difference between the highest occupied molecular orbital (HOMO) level of conjugated polymer and the conduction band edge of the main acceptor, but the Voc in practical devices correlates strongly with the quasi-Fermi levels of the electrons in the main acceptor and the holes in the polymer. While passivating the surface defects on the main acceptor, increasing spatial e-h separation, and enhancing the electron density in conduction band of the main acceptor will facilitate the increase in Voc. (v) There is no direct correlation among Voc, photogenerated voltage (Vph) and electron lifetime (τe), and they may change in the same or the opposite trend when the same or different factors affect them, therefore one should get insight into the intrinsic factors that influence them when discussing the changes in Voc, V_{ph} and τe that are subject to nanostructural characteristics.
2015, 64 (3): 038805.
doi: 10.7498/aps.64.038805
Abstract +
The efficiency of solar cells based on organic-inorganic hybrid perovskite materials has a rapid growth from 3.8% in 2009 to 19.3%. The perovskite material (CH3NH3PbX3) exhibits advantages of high absorbing coefficient, low cost, and easily synthesised, which achieved extremely rapid development in recent years and gains great concern from the academic circle. As we know, perovskite materials not only serve as light absorption layer, but also can be used as either electron or hole transport layer. Consequently, various structures are designed based on the function of the perovskite, such as the solid-state mesoscopic heterojunction, meso-superstructured planar-heterojunction, HTM-free and the organic structured layers. Besides, it is also attractive for its versatility in fabrication techniques: one-step precursor solution deposition, two-step sequential deposition, dual-source vapor deposition, and vapor-assisted solution processing etc. This review mainly introduces the development and mechanism of the perovskite solar cells performance and the fabrication methods of peroskite films, briefly describes the specific function and improvement of each layer, and finally discusses the challenges we are facing and the development prospects, in order to have a further understanding of perovskite solar cells and lay a solid foundation for the preparation of new structures of the perovskite solar cells.
2015, 64 (3): 038806.
doi: 10.7498/aps.64.038806
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Quantum dot sensitized solar cells (QDSCs) appear to be one of the promising photovoltaic candidates, due to the lower cost of obtaining materials and assembling processes, as well as the advantages of their QD sensitizers which exhibit properties of tailoring the absorbance spectrum to near-infrared (NIR) regions, the multiple exciton generation (MEG), hot electron extraction, etc. However, the difficulty of QDs penetrating into TiO2 mesoporous film remains to be an obstacle for the development of QDSCs, which comes from (1) their larger size (1-10 nm) compared with dye molecules, (2) steric hindrance from the long chain organic ligands on the surface, and (3) the lack of terminal functional group of the ligand with affinity to TiO2. These issues imply the importance of implementing an efficient QD deposition method in the fabrication process. Based on summarizing the advantages and shortcomings, this review demonstrates the development of the QD deposition approaches in direct growth deposition methods: the chemical bath deposition (CBD) method, the successive ionic layer adsorption and reaction (SILAR) method, and the pre-synthesized QD deposition methods: linker-assisted deposition (LA), direct absorption (DA) and electrophoretic deposition (EPD). As an overall comparison to be taken for all these deposition approaches, the pre-synthesized QD deposition method has outperformed the direct growth deposition method due to the use of pre-synthesized high quality QD sensitizers for better performance in surface chemistry. Especially, the LA approach in this method exhibits its excellence of fast and uniform QD deposition with high coverage, as well as in building high efficiency QDSC devices. Specifically, the improved structure of the sensitizers such as the inverted type-I, type-II core/shell structures and alloyed configuration through surface ion-exchange, has been employed to boost the charge injection and depress the charge recombination, benefited from LA pre-synthesized QDs deposition method. The advantages of the LA method are fully illustrated by the examples of the most recent work in the achievement of reaching the record efficiency of QDSCs. Finally, outlooks have been given on possible approaches to realize further improvement of fabricating the QDSCs with excellent performance at higher levels.
2015, 64 (3): 038807.
doi: 10.7498/aps.64.038807
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This paper introduces the history and current research status of the novel ferroelectric-semiconductor coupled photovoltaic devices, in which a ferroelectric field of polarized dipoles from nanoparticles separates the photogenerated carriers. Fabrication of such devices by combining a CdS nanodipole and a CdTe absorber via a feasible method is described, which involves a phase segregation process of CdS from a CdS-CdTe pseudobinary system. An irregular behavior is observed on this type of devices, i.e. the hysteresis of open circuit voltage due to external bias of direct-current (DC) electric field. Other macroscopic and microscopic evidences of the dipole field photovoltaic effect are also described. Meanwhile, similar photovoltaic mechanism observed in other types of solar cells are also discussed, such as organic photovoltaic devices and quantum dot devices with photo-induced dipole polarization field, piezo-phototronic devices, ferroelectric photovoltaic devices, as well as perovskite solar cells. It is apparent that the polarization field of dipoles not only exists in the various types of photovoltaic devices, but also may dominate the behavior of devices. Therefore, we propose that a new concept of dipole field semiconductor devices could be properly used to explain the photovoltaic behavior of both junctional and un-junctional devices. The junctional devices could function with either pn junction or Schottky junction, while the un-junctional devices include all the devices mentioned above. We expect that various innovation should be inspired by this concept in photovoltaic community.
