Efficiency enhancement in dye-sensitized solar cells by in situ passivation of the sensitized nanoporous electrode with Li2CO3

This work entails a method to improve the performance of dye-sensitized nanocrystalline TiO₂ solar cells by adding surface passivating elements to the electrolyte. The presence of either CO₂, Li₂CO₃ or K₂CO₃ in electrolyte increases both the photocurrent and the photovoltage, resulting in higher overall conversion efficiency of these solar cells. The additives are used to form a passivation layer of lithium carbonate on the dye free surface of the TiO₂ nanoparticles and the conductive substrate. This layer suppresses the rate of the main recombination reaction between the photoinjected electrons and the oxidized ions in the electrolyte solution. While blocking part of the recombination, the lithium carbonate layer allows motion of the Li⁺ ions towards the TiO₂ surface for charge screening. Consequently using this simple treatment, the conversion efficiency of dye-sensitized solar cell most improved by 17.2% (from 6.4% to 7.5%).     

Efficiency enhancement from [60]fulleropyrrolidine-based polymer solar cells through N-substitution manipulation

A series of fulleropyrrolidine derivatives (FPx, x = 1–8) with alternating N-phenyl or N-methyl group were prepared as acceptors for polymer solar cells (PSCs) with the purpose of investigating the effect of N-substitutions on the photovoltaic properties of fullerene materials. More importantly, the morphology studies by means of atomic force microscopy (AFM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and space charge limited current (SCLC) measurements revealed that FP1 with N-phenyl group possessed not only appropriate miscibility with P3HT but also high electron mobility, which may account for its optimal photovoltaic properties.     

Efficiency improvement of InGaP/GaAs/Ge solar cells by hydrothermal-deposited ZnO nanotube structure

In this paper, a zinc oxide (ZnO) nanotube, fabricated by the hydrothermal growth method on triple-junction (T-J) solar cell devices to enhance efficiency, is investigated. Compared to those of bare T-J solar cells (without antireflection (AR) coating) and solar cells with Si₃N₄ AR coatings, the experimental results show that the T-J solar cells, which use a ZnO nanotube as an AR coating, have the lowest reflectance in the short wavelength spectrum. The ZnO nanotube has the lowest light reflection among all experimental samples, especially in the range of 350 to 500 nm from ultraviolet (UV) to visible light. It was found that a ZnO nanotube can enhance the conversion efficiency by 4.9%, compared with a conventional T-J solar cell. The Si₃N₄ AR coatings also enhance the conversion efficiency by 3.2%.The results show that a cell with ZnO nanotube coating could greatly improve solar cell performances.     

Water-soluble polyaniline/graphene prepared by in situ polymerization in graphene dispersions and use as counter-electrode materials for dye-sensitized solar cells

Water-soluble polyaniline/graphene nanocomposites have been prepared via a simple in situ polymerization of aniline in graphene dispersion. TEM measurement confirmed that polyaniline was homogeneously coated on the graphene sheets. The nanocomposites solution can be used for film fabrication by common technology, such as drop coating. When these different polyaniline/graphene nanocomposites were applied as the counter electrode materials for dye-sensitized solar cells, the short-circuit current density and power-conversion efficiency of the devices were measured to be 12.19 mA cm⁻² and 4.46%, respectively, which was comparable to 5.71% for the cell with a Pt counter electrode under the same experimental conditions.     

Efficiency of bulk-heterojunction organic solar cells

During the last years the performance of bulk heterojunction solar cells has been improved significantly. For a large-scale application of this technology further improvements are required. This article reviews the basic working principles and the state of the art device design of bulk heterojunction solar cells. The importance of high power conversion efficiencies for the commercial exploitation is outlined and different efficiency models for bulk heterojunction solar cells are discussed. Assuming state of the art materials and device architectures several models predict power conversion efficiencies in the range of 10–15%. A more general approach assuming device operation close to the Shockley–Queisser-limit leads to even higher efficiencies. Bulk heterojunction devices exhibiting only radiative recombination of charge carriers could be as efficient as ideal inorganic photovoltaic devices.     

石墨烯材料在本体异质结太阳电池中的研究进展

总结了近年来功能化石墨烯在本体异质结太阳电池(BHJ-SCs)中的研究进展。针对石墨烯在不同结构层中的应用分别进行了阐述,主要介绍石墨烯作为透明阳极导电层材料和活性层材料在BHJ-SCs中的应用。     

A simple and scalable graphene patterning method and its application in CdSe nanobelt/graphene Schottky junction solar cells

We have developed a simple and scalable graphene patterning method using electron-beam or ultraviolet lithography followed by a lift-off process. This method, with the merits of: high pattern resolution and high alignment accuracy, being free from additional etching or harsh processes, being universal to arbitrary substrates, and being compatible to Si microelectronic technology, can easily be applied to diverse graphene-based devices, especially in array-based applications, where large-scale graphene patterns are desired. We have applied this method to fabricate CdSe nanobelt (NB)/graphene Schottky junction solar cells, which have potential applications in integrated nano-optoelectronic systems. A typical as-fabricated solar cell shows excellent photovoltaic behavior, with an open-circuit voltage of ～0.51 V, a short-circuit current density of ～5.75 mA cm(-2), and an energy conversion efficiency of ～1.25%. We attribute the high performance of the cell to the as-patterned high-performance graphene, which can form an ideal Schottky contact with CdSe NB. Our results suggest that both the developed graphene patterning method and the as-fabricated CdSe NB/graphene Schottky junction solar cells have reachable application prospects.     

Performance enhancement of ITO/oxide/semiconductor MOS-structure silicon solar cells with voltage biasing

In this study, we demonstrate the photovoltaic performance enhancement of a p-n junction silicon solar cell using a transparent-antireflective ITO/oxide film deposited on the spacing of the front-side finger electrodes and with a DC voltage applied on the ITO-electrode. The depletion width of the p-n junction under the ITO-electrode was induced and extended while the absorbed volume and built-in electric field were also increased when the biasing voltage was increased. The photocurrent and conversion efficiency were increased because more photo-carriers are generated in a larger absorbed volume and because the carriers transported and collected more effectively due to higher biasing voltage effects. Compared to a reference solar cell (which was biased at 0 V), a conversion efficiency enhancement of 26.57% (from 12.42% to 15.72%) and short-circuit current density enhancement of 42.43% (from 29.51 to 42.03 mA/cm²) were obtained as the proposed MOS-structure solar cell biased at 2.5 V. In addition, the capacitance-volt (C-V) measurement was also used to examine the mechanism of photovoltaic performance enhancement due to the depletion width being enlarged by applying a DC voltage on an ITO-electrode.     

Efficiency enhancement of dye-sensitized solar cells with PAN:CsI:LiI quasi-solid state (gel) electrolytes

While many attempts have been made in the recent past to improve the power conversion efficiencies of dye-sensitized solar cells (DSSCs), only a few reports can be found on the study of these cells using binary iodides in the gel polymer electrolyte. This paper reports the effect of using a binary mixture of (large and small cation) alkaline salts, in particular CsI and LiI, on the efficiency enhancement in DSSCs with gel polymer electrolytes. The electrolyte with the binary mixture of CsI:LiI = 1:1 (by weight) shows the highest ionic conductivity 2.9 × 10^?3 S cm^?1 at 25 °C. DC polarization measurements showed predominantly ionic behavior of the electrolyte. The density of charge carriers and mobility of mobile ions were calculated using a newly developed method. The temperature dependent behavior of the conductivity can be understood as due to an increase of both the density and mobility of charge carriers. The solar cell with only CsI as the iodide salt gave an energy conversion efficiency of ~3.9 % while it was ~3.6 % for the cell with only LiI. However, the electrolyte containing LiI:CsI with mass ratio 1:1 showed the highest solar cell performance with an energy conversion efficiency of ~4.8 % under the irradiation of one Sun highlighting the influence of the mixed cation on the performance of the cell. This is an efficiency enhancement of 23 %.     

Improving the performance of dye-sensitized solar cells with TiO2/graphene/TiO2 sandwich structure

This study investigates the extent to which the TiO₂/graphene/TiO₂ sandwich structure improves the performance of dye-sensitized solar cells (DSSCs) over that of DSSCs with the traditional structure. Studies have demonstrated that the TiO₂/graphene/TiO₂ sandwich structure effectively enhances the open circuit voltage (V_oc), short-circuit current density (J_sc), and photoelectrical conversion efficiency (η) of DSSCs. The enhanced performance of DSSCs with the sandwich structure can be attributed to an increase in electron transport efficiency and in the absorption of light in the visible range. The DSSC with the sandwich structure in this study exhibited a V_oc of 0.6 V, a high J_sc of 11.22 mA cm^-2, a fill factor (FF) of 0.58, and a calculated η of 3.93%, which is 60% higher than that of a DSSC with the traditional structure.     

Efficiency improvement of silicon solar cells enabled by ZnO nanowhisker array coating

An efficient antireflection coating is critical for the improvement of silicon solar cell performance via increased light coupling. Here, we have grown well-aligned ZnO nanowhisker (NW) arrays on Czochralski silicon solar cells by a seeding-growth two-step process. It is found that the ZnO NWs have a great effect on the macroscopic antireflection effect and, therefore, improves the solar cell performance. The ZnO NW array-coated solar cells display a broadband reflection suppression from 500 to 1,100 nm, and the minimum reflectance smaller than 3% can easily be achieved. By optimizing the time of ZnO NW growth, it has been confirmed that an increase of 3% relatively in the solar cell efficiency can be obtained. These results are quite interesting for the application of ZnO nanostructure in the fabrication of high-efficiency silicon solar cells.     

Preparation of graphene/multi-walled carbon nanotube hybrid and its use as photoanodes of dye-sensitized solar cells

This study employed a solution-based method to prepare a 3-D hybrid material comprising graphene and acid-treated multi-walled carbon nanotubes (MWCNTs). The adsorption of MWCNTs on graphene reduces the π–π interaction between graphene sheets resulting from steric hindrance, providing a subsequent reduction in aggregation. Optimal proportions of MWCNTs to graphene (2:1) enabled the even distribution of individual MWCNTs deposited on the surface of the graphene. The hybrid 3-D material was incorporated within a TiO₂ matrix and used as a working electrode in dye-sensitized solar cells (DSSCs). The hybrid material provides a number of advantages over electrodes formed of either MWCNTs or graphene alone, including a greater degree of dye adsorption and lower levels of charge recombination. In this study, DSSCs incorporating 3-D structured hybrid materials demonstrated a conversion efficiency of 6.11%, which is 31% higher than that of conventional TiO₂-based devices.     

Efficiency enhancement of non-selenized Cu(In,Ga)Se2 solar cells employing scalable low-cost antireflective coating

In this study, a non-selenized CuInGaSe₂ (CIGS) solar device with textured zinc oxide (ZnO) antireflection coatings was studied. The ZnO nanostructure was fabricated by a low-temperature aqueous solution deposition method. With controlling the morphology of the solution-grown tapered ZnO nanorod coatings, the average reflectance of the CIGS solar device decreased from 8.6% to 2.1%, and the energy conversion efficiency increased from 9.1% to 11.1%. The performance improvement in the CuInGaSe₂ thin-film solar cell was well explained due to the gradual increase of the refractive index between air and the top electrode of solar cell device by the insertion of the ZnO nanostructure. The results demonstrate a potential application of the ZnO nanostructure array for efficient solar device technology.     

Low-temperature synthesis of multilayer graphene/amorphous carbon hybrid films and their potential application in solar cells

ABSTRACT: The effect of reaction temperature on the synthesis of graphitic thin film on nickel substrate was investigated in the range of 400 [DEGREE SIGN]C to 1,000 [DEGREE SIGN]C. Amorphous carbon (a-C) film was obtained at 400 [DEGREE SIGN]C on nickel foils by chemical vapor deposition; hybrid films of multilayer graphene (MLG) and a-C were synthesized at a temperature of 600 [DEGREE SIGN]C, while MLG was obtained at temperatures in excess of 800 [DEGREE SIGN]C. Schottky-junction solar cell devices prepared using films produced at 400 [DEGREE SIGN]C, 600 [DEGREE SIGN]C, 800 [DEGREE SIGN]C, and 1,000 [DEGREE SIGN]C coupled with n-type Si demonstrate power conversion efficiencies of 0.003%, 0.256%, 0.391%, and 0.586%, respectively. A HNO3 treatment has further improved the efficiencies of the corresponding devices to 0.004%, 1.080%, 0.800%, and 0.820%, respectively. These films are promising materials for application in low-cost and simple carbon-based solar cells.     

Nanoarchitecture of variable sized graphene nanosheets incorporated into three-dimensional graphene network for dye sensitized solar cells

A novel three-dimensional (3D) nanoarchitecture consisting of hybrid graphene nanosheets (GNs)/graphene foam (GF) was fabricated on the FTO conducting substrate as a high efficient counter electrode (CE) for dye sensitized solar cells (DSSCs). The GNs with various sized such as large-sized heat-reduced graphene nanosheets (H-GNs) and small-sized laser-reduced graphene quantum dots (L-GQDs) were synthesized and used as catalytic materials incorporated into a 3D GF network, respectively. In this design, the aggregations and restacking of GNs were efficiently reduced, which is beneficial for increasing the amount of the active defective sites at the edges of graphene to the electrolyte solution. Especially, L-GQDs with smaller dimension less than 100 nm have more active defective sites at edges, providing superiority over the large-sized H-GNs in terms of electrocatalytic activity. Meanwhile, the GF network with high conductivity provides fast electron transport channels for charge injection between the GNs and FTO. The DSSC with this hybrid CE exhibited energy conversion efficiency (η) of 7.70% with an open circuit voltage (V_OC), short circuit photocurrent density (J_SC) and fill factor (FF) of 760 mV, 15.21 mA cm⁻², and 72.0%, respectively, which is comparable to that of the conventional Pt CE (7.68%).     

Performance enhancement of polylactide by nanoblending with POSS and graphene oxide

We report the effect of filler modification on the properties of polylactide (PLA)‐based nanocomposites, where graphene oxide (GO) nanosheets and polyhedral oligomeric silsesquioxane (POSS) nanocages are employed as nanofillers. The organically treated nanofillers are termed as GO‐functionalized and POSS‐functionalized. The synthesis of the nanocomposites was carried out via in situ ring‐opening polymerization of lactic acid (LA). The following four naocomposite systems were prepared, characterized, and compared to achieve a better understanding of structure‐property relationship (1) PLA/GO‐functionalized, (2) PLA/POSS‐functionalized, (3) PLA/physical mixture of GO‐functionalized and POSS‐functionalized, and (4) PLA/GO‐graft‐POSS (with eight hydroxyl groups). As revealed by the thermal and mechanical (nanoindendation) characterization, that the nanocomposites having a combination of GO and POSS as nanofiller, either as physical mixture of GO‐functionalized and POSS‐functionalized or as GO‐graft‐POSS, is far more superior as compared with the nanocomposites having individually dispersed nanofillers in the PLA matrix. Observed enhancement is attributing to the synergistic effect of the nanofillers as well as better dispersion of the modified‐fillers in the matrix. POLYM. COMPOS., 35:118–126, 2014. © 2013 Society of Plastics Engineers     

Junction characteristics of chemically-derived graphene/p-Si heterojunction solar cell

Experimental and theoretical investigations on the heterojunction of silicon (Si) with chemically derived graphene have been presented. The stability study of graphene oxide and reduced graphene oxide (rGO) in aqueous medium were performed by visual observation and surface charge measurement. The detailed characterization by FT-IR, UV–Vis, and Raman spectroscopy exhibited the formation of rGO with a high optical band gap of 3.6 eV. The atomic force microscopy analysis for rGO sample revealed the formation of flakes with thickness ≈ 10 nm. The rGO was spin-coated on the p-Si substrate for fabrication of a heterojunction device, with the structure of rGO/p-Si. In the fabricated device, incident light was transmitted through the thin rGO film to reach the junction interface, generating photoexciton, and thereby a photo-conversion efficiency of 0.02% was achieved. The theoretical simulation of rGO/p-Si heterojunction device using solar cell capacitance simulation 1D software showed the efficiency of 1.32%. Such large deviations in efficiency between experiment and theory have been discussed in details.     

Enhanced charge extraction for all-inorganic perovskite solar cells by graphene oxide quantum dots modified TiO2 layer

All-inorganic cesium lead bromide (CsPbBr₃) perovskite solar cells have been attracting growing interest due to superior performance stability and low cost. However, low light absorbance and large charge recombination at TiO₂/CsPbBr₃ interface or within CsPbBr₃ film still prevent further performance improvement. Herein, we report devices with high power conversion efficiency (9.16%) by introducing graphene oxide quantum dots (GOQDs) between TiO₂ and perovskite layers. The recombination of interfacial radiation can be effectively restrained due to enhanced charge transfer capability. GOQDs with C-rich active sites can involve in crystallization and fill within the CsPbBr₃ perovskite film as functional semiconductor additives. This work provides a promising strategy to optimize the crystallization process and boost charge extraction at the surface/interface optoelectronic properties of perovskites for high efficient and low-cost solar cells.