Perovskite-based multijunction solar cells are cost-effective strategies to deliver power conversion efficiencies beyond the theoretical limit for single-junction solar cells. In this presentation, we address important factors limiting the performance and longevity of wide-bandgap perovskite solar cells and strategies to achieving record all-perovskite triple-junction solar cells. In addition, by connecting an all-perovskite tandem solar cell with a water electrolysis cell, we demonstrate a solar-to-hydrogen efficiency of 17.8%.
The world of organic solar cells (OSC) have been taken by storm by the recent developments in non-fullerene acceptors (NFAs) with record efficiencies being published in close succession. However, not all polymers that previously performed well in fullerene based devices are suitable for use in a blend with NFAs. This seems to be especially the case for diketopyrrolopyrrole (DPP) based polymers and currently there is a lack of understanding as to why. In our research we aim to improve on solar cell performance of DPP-NFA blends and elucidate the reason behind their suboptimal performance in order to extend the range of useful polymers in high efficiency (near infra-red absorbing) NFA OSCs. Preliminary results look promising with over 50% EQE improvement using a ternary solvent system which we currently attribute to better packing in the IEICO-4F phase.
KEYWORDS: External quantum efficiency, Polymers, Fullerenes, Solar cells, Solar energy, Absorption, Data modeling, Heterojunctions, Binary data, Transmission electron microscopy
In ternary bulk heterojunction solar cells based on a semiconducting biphenyl-dithienyldiketopyrrolopyrrole copolymer donor and two different fullerene acceptors that distinctly differ in electron affinity, the open-circuit voltage is found to depend in a slightly sublinear fashion on the relative ratio of the two fullerenes in the blend. Similar effects have previously been observed and have been attributed to the formation of an alloyed fullerene phase possessing electronic levels that are the weighted average of the two components. By analyzing the contribution of the charge transfer (CT)-state absorption to the external quantum efficiency of the ternary blend solar cells as a function of composition, we find no evidence for a CT state formed between the polymer and an alloyed fullerene phase. Rather, the results are consistent with the presence of two distinct CT states, one for each polymer–fullerene combination. The two-state CT model does not, however, explain the sublinear behavior of the open-circuit voltage as a function of the blend composition.
Liquid crystal (LC, Merk 5 CB) is infiltrated into active, InAs quantum dots embedded, InGaAsP membrane type
nanocavities to investigate the possible effect of the LC orientation on active cavity tuning. The tuning is demonstrated
thermally and thermo-optically. The thermal tuning showed that the cavity modes can be tuned in opposite directions and
exhibits a sudden change at the clearing temperature. The mechanism relies on the existence of both ordinary and
extraordinary refractive indices of the liquid crystal due to its molecular alignment inside the voids. It shows that the
electric field distribution of cavity modes can have a substantial component parallel to the LC director. The average
electric field orientation with respect to the LC orientation can be mode dependent, so that different modes can be
dominated by either branch of the LCs refractive index. Thermo-optic tuning of the modes is obtained when the power of
the excitation laser is increased from 40 μW to 460 μW. A large and a reversible blueshift of more than 10 nm of the
cavity modes is observed which is attributed to temperature induced liquid transport. InGaAsP type of nanocavities,
without InAs quantum dots were infiltrated with PbSe colloidal quantum dots to obtain a comparison of internal light
sources either in the semiconductor or in the holes.
One of the limitations of present organic solar cells is the relatively poor spectral overlap
of their absorption bands with the solar spectrum. Semiconducting polymers as poly(3-hexyl
thiophene) have a bandgap higher than 2.0 eV (600 nm), thereby limiting the maximum
possible absorption of the solar spectrum to about 30%. A way to overcome this limitation is a
tandem solar cell where two bulk heterojunction single cells are stacked in series, each with a
different bandgap. The combined absorption then covers a broader region of the solar
spectrum. So far, solution-processed tandem solar cells have not been realized due to
incompatibility of the solvents. We demonstrate a solution-processed polymer tandem cells by
stacking two single cells in series. The tandem cell consist of two bulk heterojunction subcells separated by a thin semitransparent electrode of gold. This middle electrode serves in
three different ways; as a charge recombination centre, as a protecting layer for first cell
during spin coating of the second cell, and as a semitransparent layer that creates optical
cavities, which allows tuning of the optical transmission through the first (bottom) cell to
optimize the optical absorption of the second (top) cell. To cover a broader region of the solar
spectrum we combined a small bandgap polymer (λmax ~ 850 nm) with a large bandgap
polymer (λmax ~ 550 nm). These sub cells are electronically coupled in series, which leads to
an open-circuit voltage that equals the sum of each sub cell. A high open-circuit voltage of 1.4
Volt is achieved. The current density of the tandem cell follows the current of the top cell,
which has a lower, limiting current. The tandem architecture and proper materials give us the
possibility to cover a very broad spectral range of the solar spectrum to make highly efficient
organic solar cells in the near future.
We describe the optimization of bulk heterojunction type photovoltaic devices from blends of ZnO nanoparticles and conjugated polymers. The photovoltaic effect of these devices depends on the choice of solvent, the amount of ZnO, and the thickness of the active blend layer. Optimized solar cells have an estimated AM1.5 performance of 1.6%. Incident photon to current conversion efficiencies (IPCE) show that up to 40% of the incident photons can be collected as charges. At high light intensities the performance of the cell drops, due to a decreasing fill factor (FF).
A new low band gap polymer (pBEHTB) with an absorption onset at 800 nm is reported. When combined with a soluble fullerene derivative (PCBM), efficient electron transfer occurs after excitation of the polymer. Bulk heterojunction solar cells have been prepared with a response up to 800 nm, and an estimated power conversion efficiency of 0.9 %.
In hybrid polymer photovoltaics, conjugated polymers are combined with wide bandgap metal oxide semiconductors like TiO2 or ZnO. Reported maximum power conversion efficiencies (PCE) at AM1.5G conditions for a hybrid polymer bulkheterojunction device are up to 1.6 %. In this paper we report on the current-voltage characteristics of bi-layer devices consisting of TiO2 and a conjugated polymer. Several polymers with different optical bandgap were studied. The maximum External Quantum Efficiency (EQE) of the devices is comparable, but the PCE differs considerably (0.2-0.5%). The differences can for a large part be explained by the differences in optical bandgap of the polymers. It is shown that a low band gap is beneficial for the short circuit current, but does not automatically result in a high PCE as relative shifts of the highest occupied molecular orbital (HOMO) energy levels of the polymers reduce the open circuit voltage (Voc). The calculations show that a PCE up to ~ 19 % can be achieved using the maximum possible Voc and a fill factor of 80%. Judicious engineering of material combinations is required to achieve such a power output, and it expresses the need for a continuing search on potentially low cost, efficient metal oxide/polymer BHJ structures.
We describe a simple and new method to create hybrid bulk heterojunction solar cells consisting of ZnO and conjugated polymers. A gel-forming ZnO precursor, blended with conjugated polymers, is converted into crystalline ZnO at temperatures as low as 110 °C. In-situ formation of ZnO in MDMO-PPV leads to a quenching of the polymer photoluminescence. Positive charges on the MDMO-PPV are formed after photoexcitation, indicating electron transfer from the polymer to ZnO. Results without full optimization already give photovoltaic cells with an estimated performance over 1% under AM1.5 illumination. The large effect of the processing conditions on the photovoltaic effect of the solar cells, indicate that there are several parameters that require control. The choice of solvent, type of atmosphere, and the relative humidity during spin coating, are important for optimization of the photovoltaic effect. These solar cells are made from cheap materials, and via simple processing and can be regarded as promising for further research.
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