Solar cells are power converters which transform energy carried by sun-light into electrical energy. This conversion is based on energy transfer from photons to electrons.
To achieve power production, electrons must remain in an excited state long enough
to reach the outer circuit. In other words, the desexcitation rate (either radiative or
non-radiative) must be much lower than the excitation rate under illumination. In
addition, an energetic asymmetry is required to drive these excited electrons in a particular direction, thus creating a macroscopic current.
HYBRID SOLAR CELL
Hybrid solar cells or photovoltaics have organic materials that absorb light as the donor and transport holes. Inorganic materials in hybrid cells are used as the acceptor and electron transporter in the structure. The hybrid photovoltaic devices have a significant potential for not only low-cost by daily researches but also scalable solar power conversion. The organic material is mixed with a high electron transport material to form the photoactive layer. The two materials are assembled together in a hetrojunction type photoactive layer. By placing one material into contact with each other, the power conversion efficiency can be greater than a single material.
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One of the materials acts as the photon absorber and excition donor, and the other facilitates exciton dissociation at the junction by charge transfer.
Hybrid solar cells based on blends of silicon nanocrystals (Si NCs) and poly-3(hexylthiophene) (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. Huang presented hybrid
solar cells based on P3HT:PCBM absorbers on a ITO/PEDOT-PSS layer system. 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 we present a hybrid solar cell consisting of a P3HT mixture into which we introduce silicon nanowires, on a ITO/PEDOT-PSS layer system on glass.
The silicon nanowires were prepared by etching. We started from a highly n-doped silicon (100) wafer (8⋅1018 to 8⋅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
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chlorobenzene for 10 min (ultrasonic cleaner Bandelin electronic RK 255 H).
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 following way: Onto a ITO covered glass substrate (Merck Displays 5x5 cm², 125 nm ITO, surface resistance 13 â„¦) a 80 nm thick PEDOT:PSS (poly(3,4)-ethylenedioxythiophene)-poly (styrenesulfonate) from H. C. Starck) layer was prepared by spin-coating followed by a drying step at 120°C. Then
~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 (Steuernagel solar simulator). The layers were investigated by optical microscopy as well as SEM (Joel 6300F).
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. Some of the most efficient allorganic
solar cells to date are based on P3HT blended with fullerene derivatives, with
power conversion efficiencies about 4.4% 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.
Hybrid solar cells composed of organic semiconductors and inorganic nanostructures are an area of immense study as they are alternatives to organic bilayer3 and bulk heterojunction 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 ZnO. 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. Regioregular P3HT was prepared from 2-bromo-3-hexyl-5-iodothiophene through the Grignard metathesis (GRIM) reaction to afford a bromine-terminated polymer with a molecular weight of 7000 Da as determined by MALDI-TOF spectroscopy. End-functionalization was carried out by reacting P3HT-Br with butyllithium and
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