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Hybrid solar or photo-voltaics cells have organic/inorganic materials that accept light and transform holes. The materials in hybrid cells are used as the acceptor and electron carrier in the structure. These photovoltaic devices have a significant property. They are very cheap in cost and also reliable for the transmission of power. The organic material is mixed with immense electron transport material to form the photoactive layer. These different type of materials are made in photoactive layer by hetro-junction . While one material get interaction with the other one, the power conversion efficiency becomes greater than as compare to a single material.
In reality these act as a photon like any other source of light and the absorber. As a result the movement of charge happens through the whole process of activity.
Hybrid solar cells with silicon nanocrystals and P3HT polymer
Polymer based solar cells represent a low cost type of thin film solar cells, with, however, up to now rather low efficiencies. In the most common cells based on P3HT (poly (3-hexylthiophene) the maximum efficiency of 5% to 5.2% (5% with P3HT or 8.13%) is e.g. limited by the low carrier mobility in the material. Due to this low mobility the cells cannot be made thicker than about 100 to 300 nm. Consequently, only part of the sun light is absorbed.
As a way out of this limitation an organic/inorganic hybrid concept has been suggested in which
P3HT is combined with silicon nanostructures. In these hybrid cells the highly doped silicon can act as a
finely dispersed conductor and at the same time increases light scattering. In this way both, electrical conductance and light absorption, are increased as the polymer/silicon interface also charge carrier
separation may occur. As a nanoscale conductor silicon nanowires have been suggested, which can be prepared by VLS (vapor liquid solid) growth or by etching of silicon wafers. The doped silicon nanowires prepared by etching where mechanically pressed into the polymer, fullerene blend. As compared to reference cells without nanowires these cells delivered a higher short circuit current
(11.6 mA/cm² versus 7.17 mA/cm²). Due to a low open circuit voltage of 400 mV the efficiency increased from 1.2% to 1.9% only. In P3HT cells without an ITO/PEDOT-PSS layer system were presented into which silicon nanowires were introduced. These cells, however, showed a very low
efficiency, probably due to a too thick polymer layer. Silicon nanoclusters 3 to 20 nm in diameter instead of nanowires were used previously. The clusters were prepared by plasma induced CVD from silane and introduced into P3HT. The short circuit current reached 3.5 mA/cm², two orders of magnitude higher than without the nanoclusters. Here a hybrid solar cell is presented consisting of a P3HT mixture into which silicon nanowires introduced, on a ITO/PEDOT-PSS layer system on glass.
The silicon nanowires were prepared by etching. It started from a highly p-doped silicon (100) wafer (8â‹…1018 to 8â‹…1019 cm-3 phosphorus, resistivity 1 to 5 mâ„¦cm). The wafers were cleaned by rinsing in acetone (2 min) and ethanol (2 min). The natural oxide layer on the wafer was removed by dipping first into 40% HF followed by dipping into 2% HF. Then the wafers were rinsed by de-ionized water and
dried in a nitrogen flow. Etching of nanowires was performed in a two step process. First silver nanoparticles were generated at the silicon surface by placing the wafer into a solution of silver nitrate and HF in water (0.02m AgNO3, 5m HF 1:1) for 30 s. Then the wafers were dipped into a mixture of HF and H2O2 (5m HF, 30%H2O2 10:1) for 4 min to etch the nanowires. The wafers were rinsed in DI water and the silver nanoparticles were removed by nitric acid (65% HNO3, 15 min). The oxide layer on the nanowire system on the wafer was removed by HF (2%, 2 min.). This procedure passivates the silicon surface by hydrogen so that reoxidation is prevented for several hours. The wafers were then placed in a glovebox. To remove the nanowires from the wafer the system was sonicated in
chlorobenzene for 10 min (ultrasonic cleaner Bandelin electronic).
To prepare the absorber layer the nanowire containing chlorobenzene was mixed with defined amounts of a P3HT solution (P3HT Rieke 4002, 99.5% purity from Solenne in chlorobenzene) to get a defined
concentration of P3HT in chlorobenzene(1:0.8 w/w). Solar cells were prepared in the different ways. The 70-100 nm of the photoactive layer (P3HT:fullerene: nanowires in chlorobenzene) was applied, again by spincoating. An annealing step at 150°C followed. Both annealing steps were done under argon, spin-coating was performed in ambient air). A 50 nm thick aluminum contact layer was deposited by evaporation through a shadow mask. In this way 8 cells each 25 mm² in area were prepared on one substrate.
I-V-curves were recorded under ambient conditions at AM1.5 white light, 100 mW/cm². The layers were investigated by optical microscopy as well as SEM.
The structure of the etched nanowires is shown in the SEM image of Fig. 2. The wires are about 4 to 5 µm long with a density of 60 to 100 wires/µm².
For sonication a 3x4 mm² part of the wafer was immersed into 0.5 ml of chlorobenzene. After 13 min of
sonication the wafer looks as shown in Fig. 3. Locally the nanowires were removed and dispersed into the chlorobenzene liquid. The dispersion was drop cast onto a 1"x1" glass substrate and the chlorobenzene was evaporated. The result is shown in the SEM image. Obviously during sonication the nanowires brake into parts of various lengths ranging from 100 nm to several µm. In addition, some nanowire agglomeration occurs. A rather high density of nanowires on the substrate is reached. For solar cells preparation the procedure was used based on spin-coating. The layer system without Al contact was investigated by optical microscopy and by SEM. The optical micrograph shows a layer with some inclusions several µm apart. The SEM demonstrates that the inclusions consist of nanowire agglomerates and of single nanowires which are completely covered by a thin polymer layer.
Parameters of hybrid solar cells with different nanowire concentrations and, as a reference, without
nanowires, measured under AM1.5, 100 mW/cm². The cell efficiency is increased by the nanowires
from 3.77% to 4.13% i.e. by 10% relative. There is no systematic dependency of the efficiency on the amount of nanowires to be observed. This may be due to the fact that the nanowires are not distributed randomly in the polymer but form aggregates. The high nanowire density reached by drop casting was not achieved by spin coating up to now. Nevertheless, even the lower density of nanowires is enough to increase the cell efficiency remarkably. We do not attribute this to higher light absorption since the low nanowire density is not expected to lead to higher light scattering. Instead we attribute the
higher efficiency to a better charge separation, a better charge transport, or a better percolation path for the charge transport along the fullerene clusters due to the presence of nanowires.
P3HT stands for poly(3-hexylthiophene). It is a conjugated polymer based on thiophene
rings, i.e., conjugated ring with four carbon atoms and one sulfur. Oligothiophenes
and polythiophenes have been widely studied for electronic (thin-film transistors)
and opto-electronic (solar cells) applications. The most efficient solar cells are made by these
materials with power changing efficiencies about 4.5% and short-circuit current densities higher
than 10mAcm. In addition, both accelerated and real, outdoor ageing measurements
have shown that these devices are among the most stable organic ones.
Advantages of Nanowire Solar Cells
There are distinct advantages regarding the use of radial junctions. First, the junction interface extends along the length of the NWmaximizing the junction area. Second, since the radial configuration yields orthogonalized pathways for light absorption and carrier collection, a carrier collection distance comparable to or even smaller than the minority carrier diffusion length (Ln ) is possible. This allows photogenerated carriers to reach the p/n regions or electrodes (if in the surrounding geometry) with
high efficiency and substantially low bulk recombination, a key limitation of conventional planar solar cells. Recent theoretical studies have supported that core/shell NW structures improve light absorption and carrier collection efficiencies when compared to planar PV devices. Although, generally, generation and separation efficiencies are improved, design and optimization issues remain in core/shell NW solar
cells. The study addresses the effects of different design parameters on the performance of solar cell devices, namely the minority carrier lifetime (Ï„ ), open-circuit voltage (VOC), short-circuit current density (JSC), fill factor (FF), and eventually, conversion efficiency (Î·). Like planar structures, optimization considerations must be made with respect to some of the geometric parameters of NW structures due to the inverse behavior of the JSC and This study offered guidance to later experimental studies. The axial junction can be formed by introducing different doping along the axial direction of NWs, by growing vertical NW arrays on substrates with opposite doping, or by creating rectifying metal contacts to unintentionally doped NWs. In axial junction NWs, the carrier transport direction is parallel to the NW growth direction and axial junctions do not have the same advantages in charge separation that radial junctions do.
Hetero structures, such as tandem stacking of multiple p/n junctions and multiple quantum well (MQW) structures, can also be integrated in series in axial or radial junctions. The fabrication of these tandem cell structures leads to a maximization of the absorption of solar spectrum by using multiple materials with different band gaps and a minimization the hot electron effect. Also, quantum wells can act
as absorbers of additional photons, thus resulting in increased JSC and Î·.
Moreover, vertical NW solar cells offer enhanced energy conversion efficiency due to enhanced light absorption, improved charge separation and improved charge collection. The vertical NW array geometry with varied size and/or composition along the NW axis enhances light absorption due to the wave guiding effect, reduces surface reflectance and minimizes angular dependence. Lastly, the formation of hetero structure solar cells enables potential for further enhancements in overall efficiency.
One of the approaches to enhance the power conversion efficiency of Si NW-array-based solar cells is to decorate them with nanoparticles (NPs) or quantum dots (QDs). Materials such as insulators, metals, and semiconductors are widely used for this purpose, each contributing a different improvement due to a different mechanism. Insulators provide efficient light scattering. These NPs disperse the incoming light towards the Si NWs in a way that light absorption is maximized and AOI effect is suppressed. Alternatively, metal NPs, such as Pt and Au, can introduce plasmonic effects besides efficient light scattering. These NPs improve light trapping due to the interaction between the surface plasmon and metal NPs. For this reason, plasmon-enhanced solar cells are recognized as the next generation of solar cells. Also, Si NWs decorated with Pt NPs show exceptional catalytic activity at interfaces with liquids. Semiconductor QDs normally have high refractive index, which is believed to offer better light trapping performances. On the other hand, semiconductor NPs provide additional absorption over a broad spectrum due to their high absorption coefficient.
It is finally observed that by adding silicon nanowires to the P3HT absorber layer of polymer solar cells the cell efficiency can be increased by 10% relative, even if the density of nanowires are low. The improvement is due to an increase of the short circuit current and of the fill factor. The reason for
the improvement is not exactly clear at this stage. We just can exclude an optical effect of light scattering for which the nanowire density is too low. More work will be carried out to reach a higher nanowire density without agglomeration. Only then a systematic optimization of the cells can be performed.
The drop-casting may be more effective than spin-coating to get higher densities.
In addition the nanowire etching process has to be revived to get more homogeneous nanowire ensembles. At last to test VLS grown nanowires in a similar cell concept. However, from a technological point of view the etched nanowires may be more interesting concerning low-cost production.
Hybrid solar cells composed of organic semiconductors and inorganic nanostructures are an area of deep study as they are alternatives to organic bilayer3 and bulk hetero-junction device structures. The organic/inorganic hybrid system has opened new opportunities for the development of future generation solar cells, new device technologies, and a platform to study three dimensional morphology. A multitude of concepts have been demonstrated by combining p-type donor polymers with n-type acceptor inorganic nanostructures such as CdSe, TiO2, and . One-dimensional (1-D) inorganic semiconductor nanostructures are among some of the most attractive nanomaterials for solar cell devices because they provide a direct path for charge transport.
Other advantages include high carrier mobilities, solution processability, thermal and ambient stability, and a high electron affinity necessary for charge injection from the complementary organic donor material. Silicon nanowires are an example of this class of materials that have been used for hybrid solar cells. Poly(3-hexylthiophene) (P3HT)/Silicon nanowire composite solar cells are benchmark systems that have attained power conversion efficiencies ranging from 0.02% to 2%. Despite the vast efforts in this area of research, solar cells based on hybrid composites have yielded efficiencies only close to those of organic bilayer devices and significantly less than organic bulk heterojunction solar cells. Knowledge regarding interfacial charge separation and/or transport in hybrid nanowire devices is only partly understood. If this class of materials is to play a part in the future of next generation solar cells, then there must be an improved fundamental understanding of the organic/inorganic interface in order to increase power conversion efficiencies. While nanowire array and bulk inorganic/organic blend devices are technologically relevant, their electrical properties depend on nanostructure size, uniformity, crystallinity, phase segregation, interfacial interactions, mobility, trap density, and many other factors. For macroscopic devices, these parameters can vary significantly over the active area, making it difficult to attribute any change in performance to a particular phenomenon. Single nanowire devices allow for more precise control over and characterization of the properties listed above, greatly reducing the uncertainty in data interpretation. In this study, we utilize end-functionalized P3HT to chemically graft the organic component to an p-type Silicon nanowire, producing a p-n core-shell nanowire from which we subsequently fabricated a single nanowire solar cell. We end-functionalized P3HT and quaterthiophene with a phosphonic ester and acid, respectively, and self-assembled the semiconductors onto the Silicon surface in the solution phase to yield organic shells with thicknesses of about 5-20 nm. We present results on the synthesis and characterization of the organic/Silicon composites, high-resolution transmission electron microscopy (TEM) of the organic/Silicon interface, and results on the photovoltaic characteristics of individual nanowire devices. The nanowire devices yield low efficiencies of about 0.03% but provide an effective platform for isolating and studying the many phenomena that affect bulk hybrid solar cell performance. Silicon nanowires were prepared via solution and vaporphase synthesis as previously described. Both methods can produce high-quality, single crystalline nanowires with lengths of several micrometers and diameters ranging from 30 to 100 nm.