KEYWORDS: Solar cells, Organic photovoltaics, Solar energy, Polymers, Photovoltaics, Photovoltaic materials, Organic semiconductors, Thin film solar cells
Fullerene-free organic solar cells (OSCs) have attracted significant interest in the research community over the past few years. Their efficiency has risen rapidly, with multiple reports of record power conversion efficiencies (PCEs) breaching 14%. While encouraging, these performance metrics are often achieved with the utilization of toxic halogenated solvents for the fabrication process, which is less attractive for large-scale manufacturing. Dimeric perylene diimide (PDI) electron transport materials are currently considered amongst the key candidates for the realization of low-cost, highefficiency “green-processed” OSCs. The low-cost synthetic versatility of the PDI skeleton allows for a range of chemical “fine-tuning” and the chromophore has excellent photochemical stability and strong light absorption in the visible region.
This report will detail our research into OSCs using PDI dimers as the electron acceptors and active layer fabrication using non-halogenated solvents. PTB7-Th was chosen as the donor material, owing to its good solubility in nonhalogenated solvents, complementary light absorption and suitable energy level alignment for pairing with our PDI acceptors. Two different PDI dimers having linear and branched alkyl chains are studied.
We have previously shown that PTB7-Th:PDI based solar cells with active layers processed from 2Me-THF, o-xylene, or 1,2,4-trimethylbenzene could reach PCEs from 5-6%. The processing solvent can be extended to toluene with solar cells exhibiting PCEs of 5%. Thus, this work highlights the many processing options for the PTB7-Th / N-annulated PDI dimer active layer combination.
Quantum dots (QDs) have numerous applications in optoelectronics due to their unique optical properties. Novel hybrid
organic light-emitting diodes (OLEDs) containing QDs as an active emissive layer are being extensively developed. The
performance of QD–OLED depends on the charge transport properties of the active layer and the degree of localization
of electrons and holes in QDs. Therefore, the type and the density of the ligands on the QD surface are very important.
We have fabricated OLEDs with a CdSe/ZnS QD active layer. These OLEDs contain hole and electron injection layers
consisting of poly(9-vinyl carbazole) and ZnO nanoparticles, respectively. The energy levels of these materials ensure
efficient injection of charge carriers into the QD emissive layer.
In order to enhance the charge transfer to the active QD layer and thereby increase the OLED efficiency, the QD surface
ligands (tri-n-octyl phosphine oxide, TOPO) were replaced with a series of aromatic amines and thiols. The substituents
were expected to enhance the charge carrier mobility in the QD layer. Surprisingly, the devices based on the original
TOPO-coated QDs were found to have the best performance, with a maximum brightness of 2400 Cd/m2 at 10 V. We
assume that this was due to a decrease in the charge localization within QDs when aromatic ligands are used. We
conclude that the surface ligands considerably affect the performance of QD–OLEDs, efficient charge localization in QD
cores being more important for good performance than a high charge transfer rate.
Solar energy converters based on organic semiconductors are inexpensive, can be layered onto flexible surfaces, and
show great promise for photovoltaics. In bulk heterojunction polymer solar cells, charges are separated at the interface of
two materials, an electron donor and an electron acceptor. Typically, only the donor effectively absorbs light. Therefore,
the use of an acceptor with a wide absorption spectrum and high extinction coefficient and charge mobility should
increase the efficiency of bulk heterojunction polymer solar cells. Semiconductor nanocrystals (quantum dots and rods)
are good candidate acceptors for these solar cells. Recently, most progress in the development of bulk heterojunction
polymer solar cells was achieved using PCBM, a traditional fullerene acceptor, and two low band gap polymers, poly[N-
9'-heptadecanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (PCDTBT) and poly
(PTB7). Therefore, the possibility of combining these polymers with semiconductor nanocrystals
deserves consideration.
Here, we present the first comparison of solar cells based on PCDTBT and PTB7 where CdSe quantum dots serve as
acceptors. We have found that PTB7-based cells are more efficient than PCDTBT-based ones. The efficiency also
strongly depends on the nanocrystal size. An increase in the QD diameter from 5 to 10 nm causes a more than fourfold
increase in the cell efficiency. This is determined by the relationship between the nanoparticle size and energy spectrum,
its pattern clearly demonstrating how the mutual positions of the donor and acceptor levels affect the solar cell
efficiency. These results will help to develop novel, improved nanohybrid components of solar cells based on organic
semiconductors and semiconductor nanocrystals.
Semiconductor quantum dots (QDs) are characterized by high extinction coefficients adjustable by varying the nanoparticle size and a high quantum yield of charge generation. They have the advantage of efficient charge transfer from QDs to organic semiconductors. An advanced photovoltaic cell where a SnO2/ITO electrode is covered with layers of CdSe QDs integrated in a polyimide (PI) organic semiconductor (about 100 nm thick) and Cu–phthalocyanine (20–40 nm thick) has been developed.Laser-induced photoluminescence analysis has permitted the optimization of the QD concentration in the PI matrix. Special attention has been paid to the electrode surface quality, including the effect of oxygen-plasma treatment of the transparent SnO2/ITO electrode surface on the heterostructure photoconductivity. The mechanisms of excitation and charge transfers from QDs to the organic semiconductor and their effects on the efficiency of solar radiation conversion to electricity are discussed. Photovoltaic study of the structures developed has been performed, and the effect of the Cu–phthalocyanine layer on their photoconductivity has been estimated. The photovoltaic efficiency of optimized PI–CdSe hybrid structures approaches that of the best performing systems based on the MEH–PPV organic semiconductor. Incorporation of CdSe QDs in MEH–PPV has been demonstrated to increase the photovoltaic efficiency of the system by 50%, thus allowing the development of novel QD-based inorganic/organic hybrid materials with considerably improved photovoltaic properties.
Multilayer structures based on matrices of CdSe and CdSe/ZnS nanoparticles in polyimide (PI) and poly[2-methoxy-
5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) were prepared and investigated. A comparison of
photoluminescence of the various matrices excited by visible and ultraviolet laser radiation was carried out. The
main contribution to the photoluminescence was made by the organic semiconductors. Quantum yield of the
luminescence of CdSe nanoparticles embedded in organic semiconductor matrix was found to be lower then that of
the individual CdSe nanoparticles dispersed onto a glass substrate. This difference was shown to be a result of
charge transfer from the nanoparticles to organic molecules. The nanoparticles were responsible for photovoltage in
thin layers of the PI/nanoparticles composites. Processing of a surface of electrodes and organic semiconductors by
oxygen plasma increased the photovoltaic efficiency.
Solid thin films of CdSe and CdSe/ZnS nanoparticles on different templates have been investigated under
influence of power visible laser radiation. Composite structures including organic semiconductors and CdSe and
CdSe/ZnS nanoparticles films have been fabricated. Luminescent and electron properties of the structures have been
investigated. Luminescence quantum yield of these nanocomposite structures is shown to exceed that of the films of
organic dyes by two orders of magnitude for CdSe nanoparticles with ZnS shell. As for quantum dots without the shell
their luminescence quantum yield appears to fall drastically in the films and even in the matrices of organic
semiconductors compared to the solution. The presence of CdSe films in multi-layer structures of polyimides leads to
abrupt increase in their conductivity by several orders of magnitude. The prospects of development of photovoltaic
elements and light-emitting devices including the films with high concentration of CdSe and CdSe/ZnS quantum dots
are discussed.
Optical properties of the films with high concentration of semiconductor core-shell CdSe/ZnS nanocrystals under
action of visible laser radiation in a wide range of power densities have been investigated. It's shown that in the films
with ultimate concentration ofthe nanocrystals a quantum-size effect is observed. High concentration of the nanocrystals
in the films and the presence of dipoles caused by nanoparticles asymmetry lead to strong shift of quantum-size peaks in
absorption and luminescence spectra compared to the solution and the films with low concentration of the nanoparticles.
The altitude of the shift depends on the thickness of the films and varies from 35 nm to 50 nm. The luminescence spectra
of the films don't change until the power density of exciting laser radiation exceeds 1x106 W/cm2. The regimes of laser
action on the films of the nanoparticles with power densities beyond the threshold of films destruction (from 5x106 W/cm2
to 1x109 W/cm2) have been investigated.
CdSe/ZnS quantum dots in solution ad in condensed phase have been investigated by methods of laser induced
luminescence. Anti-stokes photoluminescence (APL) of CdSe/ZnS nanoparticles in the solutions and in the films has
been studied under action of laser radiation of various wavelengths. The dependencies of APL of CdSe/ZnS
nanoparticles ensembles on exciting radiation intensity, temperature and quantum dots concentration have been studied.
It is shown that the mechanism of APL formation in CdSe/ZnS nanoparticles is thermal.
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