Perovskite quantum dots (PQD) can be created using a ligand assisted reprecipitation method at room temperature with affordable equipment. These PQDs can exhibit much higher photoluminescence (PL) than bulk perovskite films of the same material. In this study, methylammonium lead bromide (MAPbBr3) quantum dots were created using energetically aligned capping ligands of trans-cinnamic acid (TCA) and 3,3-Diphenylpropylamine (DPPA). The bandgap of the PQDs was adjusted by varying the quantity of ligands added to the solution during the ligand assisted reprecipitation process. Prototype light emitting diodes (LEDs) were created using the PQD thin films.
KEYWORDS: Solar cells, Photons, Waveguides, Monte Carlo methods, Photovoltaics, Solar concentrators, Absorption, Quantum efficiency, Scattering, Ray tracing
Luminescent solar concentrators (LSCs) have the ability to receive light from a wide range of angles, concentrating the captured light onto small photo active areas. This enables greater incorporation of LSCs into building designs as windows, skylights and wall claddings in addition to rooftop installations of current solar panels. Using relatively cheap luminescent dyes and acrylic waveguides to effect light concentration onto lesser photovoltaic (PV) cells, there is potential for this technology to approach grid price parity. We employ a panel design in which the front facing PV cells collect both direct and concentrated light ensuring a gain factor greater than one. This also allows for flexibility in determining the placement and percentage coverage of PV cells during the design process to balance reabsorption losses against the power output and level of light concentration desired. To aid in design optimization, a Monte-Carlo ray tracing program was developed to study the transport of photons and loss mechanisms in LSC panels. The program imports measured absorption/emission spectra and transmission coefficients as simulation parameters with interactions of photons in the panel determined by comparing calculated probabilities with random number generators. LSC panels with multiple dyes or layers can also be simulated. Analysis of the results reveals optimal panel dimensions and PV cell layouts for maximum power output for a given dye concentration, absorbtion/emission spectrum and quantum efficiency.
Size- and structure-dependent efficiency enhancement methods are studied for luminescent solar concentrators (LSCs) fabricated by casting organic laser dyes into PMMA matrixes. The enhancement are achieved mainly by attaching a white diffuser with an airgap at the bottom of the LSC and adding refractive index matched optical gel between the LSC's edges and the attached photovoltaic cells. The size-dependent efficiency enhancement is studied for a single layer by changing the size up to 120 cm. The results show that the enhancement from the white diffuser drops and then tends to plateau at a certain size of LSC. This also applies to multilayer LSCs. Together with optical gel, the efficiency enhancement is higher for multilayer structures than that for single layers. We also demonstrate the optimal length for the design of LSCs due to reabsorption of dyes. These results could be applied to optimize the design of other LSCs.
A luminescent solar concentrator (LSC) generally is a sheet of highly transparent materials embedded with luminescent
materials. Incident sunlight is absorbed by the luminescent materials, and then emitted through down conversion process
at longer wavelengths. A large portion of the emitted light is trapped in the sheet and travels to the edges where
photovoltaic solar cells are attached. In this study, we investigate the optical enhancement methods for LSCs with
different sizes mainly by using optical gel and white diffuser. The largest tested LSC is up to 1.2m in length and with
geometrical gain 64. This is, as we know, the largest reported size. It yields electrical gain 3.9 by optical enhancements.
And the optical efficiency is still as large as 10%. The study shows that the enhancement by white diffuser is more
sensitive to the size of the LSCs than that of the optical gel. Such enhancement drops with the increase of the sizes of
LSC, but tends to plateau at certain size.
Quantum dot (QD) luminescent solar concentrator (LSC) uses a sheet of highly transparent materials doped with
luminescent QDs materials. Sunlight is absorbed by these quantum dots and emitted through down conversion process.
The emitted light is trapped in the sheet and travels to the edges where it can be collected by photovoltaic solar cells. In
this study, we investigate the performance of LSCs fabricated with near infrared QDs (lead sulfide) and compared with
the performance of LSCs containing normal visible QDs (CdSe/ZnS), and LSCs containing organic dye (Rhodamine B).
Effects of materials concentrations (related to re-absorption) on the power conversion efficiency are also analyzed. The
results show that near infrared QDs LSCs can generate nearly twice as much as the output current from normal QDs and
organic dye LSCs. This is due to their broad absorption spectra. If stability of QDs is further improved, the near infrared
QDs will dramatically improve the efficiency of LSCs for solar energy conversion with lower cost per Wp.
Photoluminescence (PL) of poly[2-methoxy-5-(2'-ethylhexyloxy)-p-phenylene vinylene] (MEH-PPV) in the presence
of Silver nanoparticles (NP) is studied. The purpose of this research is to understand the PL distance
dependence of plasmon-polymer separation and a correlation between the surface enhanced fluorescence (SEF)
and polymer molecular weight. Distinct peaks in PL are found for plasmon-polymer separations ranging from
near the far field to the near field, under 100 nm. Extinction of the devices shows that changes in absorption
cannot explain all enhancement in PL and suggests that a modification of the radiative lifetime is modified. The
dependence of the photoluminescence of MEH-PPV on molecular weight shows variation but overall suggests
chain length does not affect film quenching. This is largely attributed to the large polydispersity of the polymer
materials used.
Conjugated polymers are unique materials for use in the development of chemical and biological sensors because of their widely tunable optical and electrical properties that allow them dual functionality as both the sensing element and the signal transducer. Furthermore, as optical photoluminescence based sensors, electroactive polymers are found to exhibit high sensitivity due to the ability of the analyte of interest to quench the photoluminescence of the entire polymer chain. In order to produce a more chemically robust thin film for use as a "solid-state" optical sensor, we succeeded in grafting various poly (3-alkyl-thiophene)s to optically transparent substrates such as glass, quartz, and ITO coated glass. This was accomplished by first grafting a thiophene monomer to the surface then chemically growing the films via oxidative polymerization. XPS studies indicated that each chemical step was accurately understood. The polythiophene growth, unaltered by sonication and tape peeling tests, was uniform across the substrate and could be directed by selective silanization of the substrate. Film thicknesses range from 20 to 200 nm and exhibit varying degrees of surface roughness, depending on the polymerization process. The reaction times and solvents were varied in order to optimize the desired film properties. The absorption and photoluminescence properties of the thin films compared well with literature on spun-cast polythiophene films, as did the electrical conductivities of the doped and undoped material. The photoluminescence intensities of the films are found to be unaffected by paraquat in water but are sensitive to trace amounts of ferric chloride in acetonitrile with measurable Stern Volmer constants.
We present time-resolved photoluminescence studies in conjunction with device characterization of a variety of heterojunctions with poly-(3-hexylthiophene), or P3HT, as a means to understand how exciton dynamics affect device performance. We find that blends of P3HT with the electron-transporting polymer CN-ether-PPV and with the fullerene derivative PCBM result in ~4-fold and ~15-fold improvements in short-circuit currents, respectively, over neat-film P3HT on TiO2 solgel. Despite efficient charge-transfer in P3HT:PCBM films, as evidenced by enhanced device performance and quenched steady-state luminescence, we observe only moderate reduction of the excited state lifetime, due to the already efficient non-radiative pathways in P3HT. We observe evidence for a new state that we assign to an exciplex in blends of P3HT with the electron-transporting polymer CN-ether-PPV. The exciplex state, which confirms the existence of charge-transfer between the two polymers, may account for the enhanced device performance of these blends by acting as a scavenger for excitons that would otherwise decay rapidly via non-radiative pathways. The long-range order of P3HT is disrupted when spin-cast on rough TiO2 nanoparticles, and this results in a blueshift of the PL spectrum and a new long-lived decay component that we attribute to long-lived intrachain polarons. P3HT on smooth TiO2 solgel films shows little or no quenching of the excited state, despite known charge transfer from P3HT to TiO2.
The main factors inhibiting higher conversion efficiencies in plain polymer layer sandwich photovoltaic devices are the low exciton dissociation efficiency and the low carrier mobilities in the polymer. We consider two different blend approaches for increasing these qualities. NiO (or LiNiO) hole transporting nanoparticles are blended into the photoactive polymer MEH-DOO-PPV in an attempt to increase hole mobility across the device. Improvements to device performance were not significant at these blend concentrations. Devices made using blends of hole and electron transporting polymers M3EH-PPV and CN-ether-PPV showed increased dissociation efficiency and gave power conversion efficiencies of up to 0.6% with stable electrodes.
Efficient conversion of electrical to optical energy in organic light-emitting diodes (OLEDs) depends on balancing the flux of holes injected at the anode with that of electrons at the cathode. In this paper, we discuss several concepts related to optimizing the power efficiency of OLEDs, and put them in the context of analytic and numerical models for OLED operation. A simple argument is used to relate the charge injection rate from each electrode to measurable properties of the organic layer, deriving he equivalent of the Richardson-Dushman equation for the metal- organic interface. We discuss the role of charge density in dictating the importance of both space charge effects and recombination. These ideas are illustrated with experimental data form device structures which exemplify the various types of behavior predicted.
Polymer light-emitting diodes, based for example on MEH-PPV, are known to be susceptible to oxidative degradation. This leads to loss of conjugation, i.e. lower carrier mobility and higher operating voltage, and to the formation of carbonyl species, i.e. to luminescence quenching. In-situ FTIR has revealed that ITO can act as the source of oxygen. In order to explore further the mechanism of oxidation and to provide guidance for its elimination, we have studied the behavior of MEH-PPV LEDs prepared with a variety of conducting polymer anodes including polyaniline and polythiophene derivatives cast from various solvents and with various molecular and polymeric dopants. In all cases examined, it is found that polymer anodes lead to significant improvement in lifetime over devices with ITO as the anode contact. Moreover, in contrast to the variability observed for ITO anodes, conducting polymers with polymers with polymeric dopants yield consistently good devices with power efficiencies of about 0.5 percent at 5 volts and brightness in excess of 1000 cd/m2. Anodes prepared with small molecule dopants are more variable and exhibit short term behavior which suggests interfacial electrochemistry. We describe the device characteristics in the context of a model of hole-dominated bipolar charge injection with Langevin recombination.
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