The Luminescent Solar Concentrator (LSC) consists of a transparent polymer plate containing luminescent particles.
Solar cells are connected to one or more sides of the polymer plate. Part of the light emitted by the luminescent particles
is guided towards the solar cells by total internal reflection.
About 25% of the dye emission is typically emitted within the optical escape cone of the matrix material and is lost due
to emission from the top. We study the application of selectively-reflective cholesteric layers to reduce these losses. We
have implemented these mirrors in the ray-tracing model for the LSC. The simulations show that an optimum in
performance can be obtained by selecting an appropriate centre wavelength of the cholesteric mirror.
External Quantum Efficiency measurements were performed on LSC devices with a mc-Si, GaAs or InGaP cell and a
dichroic mirror. This mirror shows a similar behavior as the cholesteric mirror. The results show that for a 5x5 cm<sup>2</sup> LSC
the mirror does improve the EQE in the absorption range of the dye.
Tandem solar cells, in which two individual cells are stacked on top of each other, offer the potential to increase the
efficiency significantly compared to a single cell on the same area. To reach maximum efficiency, each cell in the stack
must have a distinctive spectral response and the current in each cell must be similar. This requires smart selection of
materials, proper cell design and appropriate layer thickness. Tandem polymer solar cells can be made by processing two
individual cells from solvent based liquids, separated by a recombination layer. Potential candidates for the
recombination layer are 1) a combination of a ZnO layer and a pH-neutral PEDOT:PSS layer, 2) a TiOx layer combined
with a normal PEDOT:PSS layer. We will discuss the properties of the suggested recombination layers. To determine the
performance of tandem cells, accurate spectral response measurements are crucial. Spectral response measurements of a
polymer tandem cell show that the response of each subcell can be measured only when a bias light with sufficient
intensity and suitable spectrum is applied. We will discuss the special requirements for the spectral response set-up that
are needed in order to successfully discriminate between the responses of each subcell.
The Luminescent Solar Concentrator (LSC) consists of a transparent plate with solar cells connected to one or more
sides. The plate contains luminescent species, like e.g. organic dyes or quantum dots. Part of the light emitted by the
luminescent species is guided towards the solar cells by total internal reflection, the plate functioning as a waveguide.
We have developed a ray-tracing model that describes the experimental results and can be used to determine the different
loss mechanisms in the LSC. We will present a parameter study using this model which indicates some of the important aspects of the LSC.
Luminescent concentrator (LC) plates with different dyes were combined with standard multicrystalline silicon solar cells. External quantum efficiency measurements were performed, showing an increase in electrical current of the silicon cell (under AM1.5, 1 sun conditions, at normal incidence) compared to a bare cell. The influence of dye concentration and plate dimensions are addressed. The best results show a 1.7 times increase in the current from the LC/silicon cell compared to the silicon cell alone. To broaden the absorption spectrum of the LC, a second dye was incorporated in the LC plates. This results in a relative increase in current of 5-8% with respect to the one dye LC, giving. Using a ray-tracing model, transmission, reflection and external quantum efficiency spectra were simulated and compared with the measured spectra. The simulations deliver the luminescent quantum efficiencies of the two dyes as well as the background absorption by the polymer host. It is found that the luminescent quantum efficiency of the red emitting dye is 87%, which is one of the major loss factors in the measured LC. Using ray-tracing simulations it is predicted that increasing the luminescent quantum efficiency to 98% would substantially reduce this loss, resulting in an increase in overall power conversion efficiency of the LC from 1.8 to 2.6%.
In hybrid polymer photovoltaics, conjugated polymers are combined with wide bandgap metal oxide semiconductors like TiO<sub>2</sub> 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 TiO<sub>2</sub> 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 (V<sub>oc</sub>). The calculations show that a PCE up to ~ 19 % can be achieved using the maximum possible V<sub>oc</sub> 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 present a highly fluorescent polymer poly[2,7-(9,9'-dioctylfluorene)-alt-1,4-bis(1-cyanovinyl-2-thienyl)-2-methoxy-5-(3,7-dimethyloctyloxy)phenylene] (PF1CVTP), that was found to perform exceptionally well as electron acceptor in polymer photovoltaic devices when mixed with poly(2-methoxy-5-(3,7-dimethyloctyloxy)-1,4-phenylenevinylene) (MDMO-PPV) as electron donor. The optical and electrochemical properties of the blend were investigated. Both the quenching and the position of the oxidation and reduction waves indicate that charge transfer could take place if the blend is illuminated. Solar cell devices were made of blends containing different ratios of donor and acceptor. Maximum external quantum efficiency of more than 50 % was obtained and a power conversion efficiency of up to 1.5 % was measured under AM1.5 G (100 mW/cm<sup>2</sup>) conditions.
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.