Nanoantennas for enhanced light trapping in transparent organic solar cells

We propose a light-trapping structure offering a significant enhancement of photovoltaic absorption in transparent organic solar cells operating at infrared, while the visible light transmission keeps sufficiently high. The main mechanism of light trapping is related with the excitation of collective oscillations of the metal nanoantenna arrays, characterized by advantageous field distribution in the volume of the solar cell. It allows more than triple increase in infrared photovoltaic absorption.     

A transparent,solvent-free laminated top electrode for perovskite solar cells

A simple lamination process of the top electrode for perovskite solar cells is demonstrated. The laminate electrode consists of a transparent and conductive plastic/metal mesh substrate, coated with an adhesive mixture of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate), PEDOT:PSS, and sorbitol. The laminate electrode showed a high degree of transparency of 85%. Best cell performance was achieved for laminate electrodes prepared with a sorbitol concentration of ~30 wt% per milliliter PEDOT:PSS dispersion, and using a pre-annealing temperature of 120°C for 10 min before lamination. Thereby, perovskite solar cells with stabilized power conversion efficiencies of (7.6 ± 1.0)% were obtained which corresponds to 80% of the reference devices with reflective opaque gold electrodes.     

Light-trapping design of graphene transparent electrodes for efficient thin-film silicon solar cells

In this paper, the performance of solar cells with graphene transparent electrodes is compared with cells using conventional indium tin oxide (ITO) electrodes, and it is demonstrated the optical absorption of solar cells with bare graphene structure is worse than that of bare ITO structure because of the higher refractive index of graphene. To enhance the light trapping of graphene-based thin-film solar cells, a simple two-layer SiO₂/SiC structure is proposed as antireflection coatings deposited on top of graphene transparent electrodes, and the thickness of each layer is optimized by differential evolution in order to enhance the optical absorption of a-Si:H thin-film solar cells to the greatest degree. The optimization results demonstrate the optimal SiO₂/SiC/graphene structure can obtain 37.30% enhancement with respect to bare ITO structure, which has obviously exceeded the light-trapping enhancement of 34.15% for the optimal SiO₂/SiC/ITO structure. Therefore, with the aid of the light-trapping structure, the graphene films are a very promising indium-free transparent electrode substitute for the conventional ITO electrode for use in cost-efficient thin-film silicon solar cells.     

Long-term stable silver nanowire transparent composite as bottom electrode for perovskite solar cells

Nano Research - As the most promising alternative to traditional indium tin oxide (ITO), silver nanowire (AgNW) composite transparent electrodes with improved stabilities compared with that of the...     

Dithiapyrannylidenes as efficient hole collection interfacial layers in organic solar cells

One inherent limitation to the efficiency of photovoltaic solar cells based on polymer/fullerene bulk heterojunctions (BHJs) is the accumulation of positive charges at the anodic interface. The unsymmetrical charge collection of holes and electrons dramatically decreases the short-circuit current. Interfacial layers (IFLs) such as poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) have no effect on the unbalanced electron/hole transport across the BHJ. We report here on the use of dithiapyrannylidenes (DITPY), a new class of planar quinoid compounds, as efficient hole-transporting/electron-blocking layers in organic solar cells based on poly(3-hexylthiophene)/[6,6]-phenyl-C(61)-butyric acid methyl ester (P3HT:PCBM) BHJs. Inserting a 15-nm-thick IFL of 4,4'-bis(diphenyl-2,6-thiapyrannylidene) (DITPY-Ph(4)) between the indium-tin oxide electrode and the P3HT:PCBM BHJ prevents detrimental space-charge effects and favors recombination-limited currents. Current-sensing atomic force microscopy reveals a drastic increase of the hole-carrying pathways in DITPY-Ph(4) compared to PEDOT:PSS. In ambient conditions, photovoltaic cells using DITPY-Ph(4) exhibit an 8% increase in the current density, although the conversion efficiency remains slightly lower compared to PEDOT:PSS-based devices. Finally, we present a detailed analysis of the photocurrent generation, showing that DITPY-Ph(4) IFLs induce a transition from unproductive space-charge-limited currents to recombination-limited currents.     

Facile construction of high-electrocatalytic bilayer counter electrode for efficient dye-sensitized solar cells

To improve the mechanical rigidity of the electrocatalyst and assure a higher number density of catalytic sites of the counter electrode in dye-sensitized solar cells (DSCs), we have extended widely applied titanium tetrachloride treatment to construct a rough scaffolding underlayer for the platinized counter electrode. Field-emission scanning electron microscopy and atomic force microscopy images clearly depicted the platinum nanoparticles with a diameter of ca. 10 nm homogeneously distributed on the scaffolding underlayer of the bilayer counter electrode and thus led to a characteristically high surface roughness. The electocatalytic activity of this novel bilayer counter electrode was measured and compared with the corresponding properties of conventional sputtered Pt electrode. Interestingly, electrochemical impedance spectroscopy and cyclic voltammetry measurements further demonstrated the notably larger electrochemical active surface area and thereby higher electrocatalytic activity of the bilayer counter electrode. Consequently, under standard 1 sun illumination (100 mW cm(-2), AM 1.5), device with this bilayer counter electrode achieved a considerably improved fill factor of 0.67 and overall energy conversion efficiency of 7.09%, which was apparently higher than that of 0.60 and 6.37% for sputterd Pt electrode. Therefore, this present method paves a facile and inexpensive way to prepare high-electrocatalytic bilayer counter electrode in DSCs.     

Solution-processable graphene mesh transparent electrodes for organic solar cells

Graphene mesh electrodes (GMEs) with good conductivity and transparency have been fabricated by the standard industrial photolithography and O₂ plasma etching process using graphene solutions. Organic photovoltaic (OPV) cells using GMEs as the transparent electrodes with a blend of poly-(3-hexylthiophene)/phenyl-C₆₁-butyric acid methyl ester (P3HT/PC₆₁BM) as the active layer have been fabricated and exhibit a power conversion efficiency (PCE) of 2.04%, the highest PCE for solution-processed graphene transparent electrode-based solar cells reported to date.      

ZnO nanostructures for efficient perovskite solar cells

Controlled ZnO nanostructures were synthesized via spin-coating and in situ thermal decomposition processing using ZnO paster with/without zinc acetate as precursor. The perovskite CH₃NH₃PbI₃ solar cells (PSCs) based on these ZnO nanostructures were fabricated and their photovoltaic performances have also been investigated. Effects of zinc acetate concentration on morphologies of ZnO nanostructures and the photovoltaic properties of corresponding PSCs have been discussed. Interestingly, the morphologies of ZnO nanostructures were varied from separate nanoparticles to interconnect net-like nanostructures and the space of ZnO nanoparticles became large when the concentration of zinc acetate was increased from 0 to 0.13 M. The space and the connection degree of ZnO nanostructure obtained from 0.05 M zinc acetate are the best choice for perovskite infiltration and charge transport, which leads to corresponding cells have highest power conversion efficiencies (PCE) of 9.30 %. Post-treatment of ZnO nanostructures improved further Voc and FF, leading to PCE to 13.1 %.     

Radial electron collection in dye-sensitized solar cells

We introduce a new photoelectrode architecture consisting of concentric conducting and semiconducting nanotubes for use in dye-sensitized solar cells (DSSCs). Atomic layer deposition is employed to grow indium tin oxide (ITO) within a porous template and subsequently coat the high area photoelectrode with amorphous TiO 2. Compared with control devices lacking a current collector within the pores, the new photoelectrode geometry exhibits dramatically higher current densities, an effect attributed to the radial collection of electrons.     

High mobility transparent conducting oxides for thin film solar cells

A special class of transparent conducting oxides (TCO) with high mobility of > 65 cm² V^− 1 s^− 1 allows film resistivity in the low 10^− 4 Ω cm range and a high transparency of > 80% over a wide spectrum, from 300 nm to beyond 1500 nm. This exceptional coincidence of desirable optical and electrical properties provides opportunities to improve the performance of opto-electronic devices and opens possibilities for new applications. Strategies to attain high mobility (HM) TCO materials as well as the current status of such materials based on indium and cadmium containing oxides are presented. Various concepts used to understand the underlying mechanisms for high mobility in HMTCO films are discussed. Examples of HMTCO layers used as transparent electrodes in thin film solar cells are used to illustrate possible improvements in solar cell performance. Finally, challenges and prospects for further development of HMTCO materials are discussed.     

Fabrication of platinum nanoparticle counter electrode for highly efficient dye-sensitized solar cells by controlled thermal reduction time

We report the effect of the thermal reduction time during sintering on the electrocatalytic activity and the morphology of platinum nanoparticles (Pt-NPs) fabricated using thermal decomposition method. A uniform and dense distribution of Pt-NPs on fluorine-doped tin oxide glass substrate was achieved by controlling the thermal reduction time higher than 15 min and this morphology of Pt-NPs was responded for high electrocatalytic performance of counter electrode (CE). As expected, the excellent electrocatalytic activity with low charge-transfer resistance of 1.04 Ω cm² and highly conductivity of Pt-NPs CE prepared at the thermal reduction time of 15 min during sintering was obtained, which was desirable for dye-sensitized solar cells. The energy conversion efficiency of 9.43 % was obtained for the thermal reduction time of 15 min with fill factor of 63.05 %, J _sc of 18.82 mA cm^?2 and V _oc of 795 mV.     

Compound parabolic concentrator probe for efficient light collection in spectroscopy of biological tissue

We describe a compound parabolic concentrator (CPC)-based probe for enhanced signal collection in the spectroscopy of biological tissues. Theoretical considerations governing signal enhancement compared with conventional collection methods are given. A ray-tracing program was used to analyze the throughput of CPC's with shape deviations and surface imperfections. A modified CPC shape with 99% throughput was discovered. A 4.4-mm-long CPC was manufactured and incorporated into an optical fiber-based near-infrared Raman spectrometer system. For human tissue samples, light collection was enhanced by a factor of 7 compared with collection with 0.29-NA optical fibers.     

An efficient metal-free sensitizer for dye-sensitized solar cells

A novel metal-free organic dye consisting of a phenothiazine donor, a 3,4-ethylenedioxythiophene bridge, and a cyanoacrylate acceptor is synthesized and its optical, electrochemical and photovoltaic properties are characterized. A solar cell employing the metal-free dye exhibits a maximum solar energy to an electricity conversion efficiency of 6.72% under AM 1.5 solar simulator (100 mW cm^− 2). The results suggest that dye based on a phenothiazine donor and 3,4-ethylenedioxythiophene π-spacer is a promising candidate for high performance dye-sensitized solar cells.     

Charge collection and pore filling in solid-state dye-sensitized solar cells

The solar to electrical power conversion efficiency for dye-sensitized solar cells (DSCs) incorporating a solid-state organic hole-transporter can be over 5%. However, this is for devices significantly thinner than the optical depth of the active composites and by comparison to the liquid electrolyte based DSCs, which exhibit efficiencies in excess of 10%, more than doubling of this efficiency is clearly attainable if all the steps in the photovoltaic process can be optimized. Two issues are currently being addressed by the field. The first aims at enhancing the electron diffusion length by either reducing the charge recombination or enhancing the charge transport rates. This should enable a larger fraction of photogenerated charges to be collected. The second, though less actively investigated, aims to improve the physical composite formation, which in this instance is the infiltration of mesoporous TiO(2) with the organic hole-transporter 2,2',7,7'-tetrakis(N,N-di-p-methoxypheny-amine)-9,9'-spirobifluorene (spiro-MeOTAD). Here, we perform a broad experimental study to elucidate the limiting factors to the solar cell performance. We first investigate the charge transport and recombination in the solid-state dye-sensitized solar cell under realistic working conditions via small perturbation photovoltage and photocurrent decay measurements. From these measurements we deduce that the electron diffusion length near short-circuit is as long as 20?μm. However, at applied biases approaching open-circuit potential under realistic solar conditions, the diffusion length becomes comparable with the film thickness, ～2?μm, illustrating that real losses to open-circuit voltage, fill factor and hence efficiency are occurring due to ineffective charge collection. The long diffusion length near short-circuit, on the other hand, illustrates that another process, separate from ineffective charge collection, is rendering the solar cell less than ideal. We investigate the process of TiO(2) mesopore infiltration with spiro-MeOTAD by examining the cross-sectional images of and performing photo-induced absorption spectroscopy on devices with a range of thickness, infiltrated with spiro-MeOTAD with a range of concentrations. We present our interpretation of the mechanism for material infiltration, and by improving the casting conditions demonstrate efficient charge collection through devices of over 7?μm in thickness. This investigation represents an improvement in our understanding of the limiting factors to the dye-sensitized solar cell. However, much work, focused on composite formation and improved kinetic competition, is required to realize the true potential of this concept.     

晶体硅太阳能电池少子寿命测试方法

少数载流子寿命(简称少子寿命)是半导体晶体硅材料的一项重要参数,它对半导体器件的性能、晶体硅太阳能电池的光电转换效率都有重要的影响。分别介绍了常用的测量晶体硅和晶体硅太阳电池少子寿命的各种方法,包括微波光电导衰减法(MW-PCD),准稳态光电导方法(QSSPC),表面光电压(SPV),IR浓度载流子浓度成像(CDI),调制自由载流子吸收(MFCA)和光束(电子束)诱导电流(LBIC,EBLC),并指出了各种方法的优点和不足。