Fabrication and characteristics of n-Si/c-Si/p-Si heterojunction solar cells using hot-wire CVD

A double-side (bifacial) heterojunction (HJ) Si solar cell was fabricated using hot-wire chemical vapor deposition. The properties of n-type, intrinsic and p-type Si films were investigated. In these devices, the doped microcrystalline Si layers (n-type Si for emitter and p-type Si for back contact) are combined with and without a thin intrinsic amorphous Si buffer layer. The maximum temperature during the whole fabrication process was kept below 150 °C. The influence of hydrogen pre-treatment and n-Si emitter thickness on performance of solar cells have been studied. The best bifacial Si HJ solar cell (1 cm² sample) with an intrinsic layer yielded an active area conversion efficiency of 16.4% with an open circuit voltage of 0.645 V, short circuit current of 34.8 mA/cm² and fill factor of 0.73.     

Solution-liquid-solid synthesis of CuInSe₂ nanowires and their implementation in photovoltaic devices

CuInSe? (CIS) nanowires were synthesized by solution-liquid-solid (SLS) growth in a high boiling solvent using bismuth nanocrystals as seeds. The nanowires tended to be slightly deficient in In and exhibited either cubic or hexagonal crystal structure, depending on the synthesis conditions. The hexagonal structure, which is not observed in bulk crystals, appears to evolve from large concentrations of twin defects. The nanowires could be compressed into a free-standing fabric or paper-like material. Photovoltaic devices (PVs) were fabricated using the nanowires as the light-absorbing layer to test their viability as a solar cell material and were found to exhibit measurable PV response.     

Effects of annealing on the polymer solar cells based on CdSe-PVK electron acceptor

CdSe-poly(N-vinylcarbazole) (CdSe-PVK) nanocomposite was synthesized and utilized as the electron acceptor in the active layer of polymer solar cells. The photovoltaic properties of the polymer solar cells based on poly(3-hexylthiophene) (P3HT):CdSe-PVK as the active layer were investigated in detail. The effects of annealing temperature (100-200 °C) and time (5-60 min) on the device performance were studied. At annealing temperature of 150 °C for 30 min, the device demonstrated an optimal efficiency of 0.235% under AM 1.5 (100 mW cm⁻²) solar simulated light irradiation. The improved efficiency under the optimal conditions was confirmed by the highest light harvest in UV-vis spectra due to the increased crystallinity of P3HT after thermal annealing. Photoluminescence of these devices also exhibited that the quench effect increases with the increasing of annealing temperature, indicating that the charge separation between electron-donating (P3HT) and electron-accepting (CdSe-PVK) molecules was increased after heat treatment. Atomic force microscopy (AFM) images showed that the phase segregation and 3D interpenetrating networks of P3HT:CdSe-PVK were responsible for the enhancement of the device efficiency.     

Fabrication and characterizations of PCDTBT: PC71BM bulk heterojunction solar cell using air brush coating method

Organic solar cells have been significantly attracted due to the need to develop an inexpensive clean and sustainable renewable energy source. We report on the fabrication of poly [N-9″-hepta-decanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)]/[6, 6]-phenyl-C₇₁-butyric acid methyl ester blend active layer using airbrush spray-coating method in mixed solvents. Optical absorption of the active layers was analyzed using UV–visible spectral studies in the wavelength range from 300 to 800 nm. The surface morphology of the active layers deposited with different parameters (spraying time and substrate-nozzle distance) was examined using atomic force microscopy. The current density–voltage (J–V) characteristics of photovoltaic cells were measured under the illumination of simulated solar light with 100 mW cm^?2 (AM 1.5G) by an Oriel 1000 W solar simulator. The power conversion efficiency of the solar cell is more than 5 %.     

Effect of solution processed salt layers on the device performances of polymer solar cells

We report the solution processed Li salt layers (i.e. LiBF₄, and LiTFSI) in poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methylester (PCBM) bulk heterojunction solar cells, which facilitate electron injection at the interface between active layer and Al electrode. The Li salt layers are deposited on top of P3HT:PCBM active layer by simple drop-casting combined with controlled evaporation process. The solar cells employing Li salt layers exhibit the increase of short-circuit current (J_SC) and fill factor (FF) by 10% when compared with devices without such an injection layer, resulting in about 28% increase of power conversion efficiency. The effect of Li salt layers on the device performances is investigated with current–voltage (J–V) characteristics and impedance spectroscopy measurements.     

Improved charge separation properties of organic hetero-junction solar cells by self-assembled monolayers anchored Ag nanoparticles

We investigate the effect of self-assembled monolayers and localized surface plasmons of silver nano-particles on an organic solar cell consisting of zinc phthalocyanine as an active layer. The device was fabricated by covalent attachment of silver nanoparticles on n-type silicon substrates using self-assembled monolayer of 4-mercaptophenol. Power conversion efficiency is increased up to 8 times as compared to a reference device with merely 0.13% photo-conversion efficiency containing no self-assembled monolayers and silver nano-particles. We believe that improved conductivity at the interface due to the aromatic self-assembled monolayer and the increased local electric field experienced by the active layer in presence of silver nano-particles act in synergy towards the higher population of excitons and dissipation of charge.     

Conductivity enhancement of PEDOT:PSS thin film using roll to plate technique and its characterization as a Schottky diode

The conductivity of poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) was improved by pressing the PEDOT:PSS thin film using roll to plate system. PEDOT:PSS thin film was deposited on polyethylene terephthalate using electrohydrodynamics atomization technique. The physico-chemical properties of the pressed thin film at different loads were compared with an un-pressed sample. The electrical properties show that the film conductivity has been increased by four times. An optimized pressing load was found to have good conductivity and transmittance of the thin film. A hybrid device (PEDOT:PSS/F8BT/ZnO/Ag) was fabricated using layer by layer method with PEDOT:PSS as anode. The I–V characterization showed that the device with pressed PEDOT:PSS showed higher current densities. The results give a promising future of PEDOT:PSS in electronics device applications using printed electronics techniques.     

Transfer-printing of active layers to achieve high quality interfaces in sequentially deposited multilayer inverted polymer solar cells fabricated in air

Polymer solar cells (PSCs) are greatly influenced by both the vertical concentration gradient in the active layer and the quality of the various interfaces. To achieve vertical concentration gradients in inverted PSCs, a sequential deposition approach is necessary. However, a direct approach to sequential deposition by spin-coating results in partial dissolution of the underlying layers which decreases the control over the process and results in not well-defined interfaces. Here, we demonstrate that by using a transfer-printing process based on polydimethylsiloxane (PDMS) stamps we can obtain increased control over the thickness of the various layers while at the same time increasing the quality of the interfaces and the overall concentration gradient within the active layer of PSCs prepared in air. To optimize the process and understand the influence of various interlayers, our approach is based on surface free energy, spreading parameters and work of adhesion calculations. The key parameter presented here is the insertion of high quality hole transporting and electron transporting layers, respectively above and underneath the active layer of the inverted structure PSC which not only facilitates the transfer process but also induces the adequate vertical concentration gradient in the device to facilitate charge extraction. The resulting non-encapsulated devices (active layer prepared in air) demonstrate over 40% increase in power conversion efficiency with respect to the reference spin-coated inverted PSCs.     

Enhancements in device efficiency of poly(3-hexylthiophene): [6,6]-phenyl C61-butyric acid methyl ester based solar cells with incorporation of bathocuproine

The overall enhancement of poly(3-hexylthiophene): [6,6]-phenyl C61-butyric acid methyl ester based organic solar cell with a thin layer of bathocuproine (BCP) inserted between active layers and aluminum (Al) cathodes was investigated. X-ray and ultraviolet photoemission spectroscopy show that no reaction occurs at the active layer/BCP interface and 2 nm of BCP could effectively suppress the chemical reactions between Al and active layers. Atomic force microscope images also indicate that BCP layers can provide smoother contact surfaces with Al cathodes and suppress the generation of shunt leakage, resulting in larger V_oc and better power conversion efficiency of devices.     

Characterization of nanoporous silicon layer to reduce the optical losses of crystalline silicon solar cells

Reduction of optical losses in crystalline silicon solar cells by surface modification is one of the most important issues of silicon photovoltaics. Porous Si layers on the front surface of textured Si substrates have been investigated with the aim of improving the optical losses of the solar cells, because an anti-reflection coating and a surface passivation can be obtained simultaneously in one process. We have demonstrated the feasibility of a very efficient porous Si AR layer, prepared by a simple, cost effective, electrochemical etching method. Silicon p-type CZ (100) oriented wafers were textured by anisotropic etching in sodium carbonate solution. Then, the porous Si layers were formed by electrochemical etching in HF solutions. After that, the properties of porous Si in terms of morphology, structure and reflectance are summarized. The structure of porous Si layers was investigated using SEM. The formation of a nanoporous Si layer on the textured silicon wafer result in a reflectance lower than 5% in the wavelength region from 500 to 900 nm. Such a surface modification allows improving the Si solar cell characteristics. An efficiency of 13.4% is achieved on a monocrystalline silicon solar cell using the electrochemical technique.     

Thermal effects and plasma damage upon encapsulation of polymer solar cells

A barrier structure consisting of silicon oxide and silicon nitride films was deposited via plasma-enhanced chemical vapor deposition (PECVD) for the encapsulation of polymer solar cells (PSCs). The total concentration of the solution and the ratio of P3HT and PCBM on the performance of polymer solar cells were studied by UV-Vis absorption spectroscopy, atomic force microscopy and photocurrent measurement. Base on these measurements, there is a compromise between light absorption and phase separation with increasing blend concentration. The PSCs were annealed at 80, 100, 120 and 140 °C for 10-60 min to investigate the thermal effects and to estimate the best deposition temperature of the barrier layers. Nevertheless, the devices with the encapsulation of barrier layers had relatively low power conversion efficiencies (PCE) of 0.98% comparing to the devices heated in the PECVD system (1.57%) at the same condition of 80 °C for 45 min due to the plasma damage during the film deposition process. After inserting a 5-nm TiO_x layer between Al/barrier structure and active layer against the plasma damage, the annealed devices presented an average PCE of 2.26% and demonstrated over 50% of their initial value after constant exposure to ambient atmosphere and sunlight for 1500 h.     

Hierarchical placement and associated optoelectronic impact of carbon nanotubes in polymer-fullerene solar cells

Since their discovery, carbon nanotubes (CNTs) have been considered to be promising candidates for polymer-based solar cells, but their functional incorporation and utilization in such devices have been limited due to processing bottlenecks. Here, we demonstrate the realization of controlled placement of a single-walled CNT (SWNT) monolayer network at four different positions in polymer-fullerene bulk-heterojunction (BHJ) solar cells. SWNTs were deposited by dip-coating from a hydrophilic suspension, and a very brief, largely nondestructive argon plasma treatment of the active layer was utilized for incorporation of a SWNT layer within or above it. We demonstrate that SWNTs on the hole-collection side of the active layer lead to an increase in power conversion efficiency (PCE) of the photovoltaic devices from 4 to 4.9% (under AM 1.5 G, 1.3 suns illumination). This is the highest reported PCE for polymer-based solar cells incorporating CNTs, upon consideration of expected scaling of device parameters for 1 sun illumination. We also observe that SWNTs deposited on the top of the active layer lead to major electro-optical changes in the device functionality, including an increased fluorescence lifetime of poly-3-hexylthiophene (P3HT).     

低温离子渗硫层的结构和减摩性能

采用低温离子渗硫技术在４５和ＧＣｒ１５钢表面形成一定厚度的渗硫层。采用金相,ＳＥＭ＋ＥＤＸ,ＸＲＤ等方法分析了渗层结构。采用ＳＲＶ往复式磨损试验机。在液体石蜡润滑条件下对渗层的摩擦学性能进行了研究。     

Influence of anode roughness and buffer layer nature on organic solar cells performance

Fluorine doped transparent conductive tin oxide thin films (FTO) of different surface roughness have been deposited by chemical vapor deposition (FTO_SOL), classical chemical spray pyrolysis (FTO_CSP), and spray pyrolysis onto heated substrates using infra red irradiation (FTO_IRSP); the three deposition methods inducing different surface roughness. It was found that the different FTOs presented similar electrical properties while their structural, morphological and optical properties were related to surface properties. These FTO films have been used as anode in multilayer organic solar cells, based on coupled donor/acceptor-copper phthalocyanine/fullerene. To improve solar cell performance, buffer layers of different natures have been tried at the anode/organic material interface. Deposition of a thin molybdenum oxide film onto FTO smooth films afforded reproducible devices with performance similar to those obtained with indium tin oxide anodes. However, cell efficiency decreased as FTO surface roughness increased. The degree of degradation depended on the nature of the buffer layer. We show that it is necessary to use buffer layer material that allows consistency and completeness of the electrode coverage.     

Effect of cold rolling and subsequent annealing on hot pressed Ni/Al laminates

Nickel/Aluminum laminates have been prepared by hot pressing of stacked alternate layers of very thin foils of nickel and aluminium. It is found that ductile nickel layers exist in hot pressed Ni/Al laminates which are annealed up to 600 °C. However, at these temperatures Al-rich intermetallic compounds are also formed with a very thin irregular layer of pure aluminium and few voids. As the annealing temperature is increased, nickel layer is consumed and Ni-rich intermetallic compounds are formed. Cold rolling up to 8% reduction in area resulted in the cracking of the regions having intermetallic compounds with cracks perpendicular to the rolling direction. The tensile strength values are high for the laminates which contain ductile Ni layers and reach a value of 320 MPa for the specimens which are hot pressed at 420 °C. However, the laminates which are annealed at 800 °C show brittle fracture and lower tensile strength of 174 MPa. The fractographic examination indicated that Ni/Al laminates which are hot pressed at 420 °C undergo plastic deformation before breaking. On the other hand, Ni/Al laminates which are annealed at 800 °C with Ni present in the form of intermetallic compounds only, exhibit brittle intergranular fracture.     

Microstructure and Transport Properties of Compaction-Modified In Situ Fe/MgB<Subscript>2</Subscript> Wires

This study has shown the effect of excessive mechanical deformation by pressing and consecutive heat treatment, respectively, on the microstructure and transport properties of mono-core sintered in situ Fe/MgB₂ wires. These wires were pressed into short tapes under 1 GPa uniaxial pressure. Transport properties like critical temperature, transition width and engineering critical current density of the same samples were determined before and after pressing and after annealing. Phase formation and grain size of the samples were studied with XRD. The SEM analysis indicated that the mechanical deformation reduced initial voids to a large extent but caused cracks, increased the number of grain boundaries and introduced weak links. The subsequent heat treatment on the pressed samples improved the microstructure and the grain connectivity. Possibility of a final densification of the reacted in situ wires was discussed in terms of transport properties.     

Modified processing conditions for optimized organic solar cells with inkjet printed P3HT:PC61BM active layers

Inkjet printing can be used to deposit the functional layers of organic solar cells and it offers advantages over spin coating such as the possibility to print films with user-defined patterns. In this study, inkjet printing was utilized to deposit polymer:fullerene solar cell active layers and different drying and annealing conditions were examined in order to optimize device performance. Low fill factors of approximately 30% were found for devices with printed active layers that were dried at 100 °C and a considerable shift in the fill factor of up to 60% was seen after post-annealing at 150 °C. Changes in the fill factor corresponded to an increase in device efficiency from ～1.3% to ～2.4% after post-annealing. An alternative active layer drying procedure was used based on solvent annealing which resulted in high fill factors of 60% and efficiencies of ～2.4% without post-annealing. Blend films were examined with atomic force microscopy, ultra-violet visible spectroscopy and X-ray photoelectron spectroscopy. It was determined that solvent annealed, inkjet printed active layers are considerably rougher and show enhanced organization with respect to films that were dried at 100 °C. Two preparation routes are provided for devices with printed active layers with acceptable efficiencies based on quick drying and post-annealing or slow drying (solvent annealing).     

Overcoming Moisture-Induced Degradation in Organic Solar Cells

Unencapsulated organic solar cells are prone to severe performance losses in the presence of moisture. Accelerated damp heat (85 °C/85% RH) studies are presented and it is shown that the hygroscopic hole-transporting PEDOT:PSS layer is the origin of device failure in the case of prototypical inverted solar cells. Complementary measurements unveil that under these conditions a decreased PEDOT:PSS work function along with areas of reduced electrical contact between active layer and hole-transport layer are the main factors for device degradation rather than a chemical reaction of water with the active layer. Replacements for PEDOT:PSS are explored and it is found that tungsten oxide (WO₃) or phosphomolybdic acid (PMA)—materials that can be processed from benign solvents at room temperature—yields comparable performance as PEDOT:PSS and enhances the resilience of solar cells under damp heat. The stability trend follows the order PEDOT:PSS << WO₃ < PMA, with PEDOT:PSS-based devices failing after few minutes, while PMA-based devices remain nearly pristine over several hours. PMA is thus proposed as a robust, solution-processable hole extraction layer that can act as a one to one replacement of PEDOT:PSS to achieve organic solar cells with significantly improved longevity.     

Pseudohalide‐Exchanged Quantum Dot Solids Achieve Record Quantum Efficiency in Infrared Photovoltaics

Application of pseudohalogens in colloidal quantum dot (CQD) solar‐cell active layers increases the solar‐cell performance by reducing the trap densities and implementing thick CQD films. Pseudohalogens are polyatomic analogs of halogens, whose chemistry allows them to substitute halogen atoms by strong chemical interactions with the CQD surfaces. The pseudohalide thiocyanate anion is used to achieve a hybrid surface passivation. A fourfold reduced trap state density than in a control is observed by using a suite of field‐effect transistor studies. This translates directly into the thickest CQD active layer ever reported, enabled by enhanced transport lengths in this new class of materials, and leads to the highest external quantum efficiency, 80% at the excitonic peak, compared with previous reports of CQD solar cells.     

A Chemically Orthogonal Hole Transport Layer for Efficient Colloidal Quantum Dot Solar Cells

Colloidal quantum dots (CQDs) are of interest in light of their solution-processing and bandgap tuning. Advances in the performance of CQD optoelectronic devices require fine control over the properties of each layer in the device materials stack. This is particularly challenging in the present best CQD solar cells, since these employ a p-type hole-transport layer (HTL) implemented using 1,2-ethanedithiol (EDT) ligand exchange on top of the CQD active layer. It is established that the high reactivity of EDT causes a severe chemical modification to the active layer that deteriorates charge extraction. By combining elemental mapping with the spatial charge collection efficiency in CQD solar cells, the key materials interface dominating the subpar performance of prior CQD PV devices is demonstrated. This motivates to develop a chemically orthogonal HTL that consists of malonic-acid-crosslinked CQDs. The new crosslinking strategy preserves the surface chemistry of the active layer beneath, and at the same time provides the needed efficient charge extraction. The new HTL enables a 1.4× increase in charge carrier diffusion length in the active layer; and as a result leads to an improvement in power conversion efficiency to 13.0% compared to EDT standard cells (12.2%).