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.
KEYWORDS: Polymers, External quantum efficiency, Absorption, Solar cells, Quantum efficiency, Luminescence, Electrodes, Transmission electron microscopy, Photovoltaics, Solar energy
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/cm2) conditions.
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.
Polymer bulk hetero junction solar cells were made from poly(2-methoxy-5-(3,7-dimethyloctyloxy)-1,4-phenylene-vinylene) (MDMO-PPV) as donor and poly(cyanoetherphenylenevinylene) (PCNEPV) derivatives as acceptor material. In this paper we start out with discussing the synthesis of the materials. Subsequently, the main issues concerning the devices are treated. Annealing the devices yielded devices with encouraging efficiencies of 0.5% (1 sun, 100mW/cm2), as calculated from the maximum power points (MPP). AFM studies revealed that this anneal step improves especially the interface of the active layer with the under laying PEDOT:PSS, although mobility and morphology changes can not be ruled out. Lowering the molecular weight (Mw) of the MDMO-PPV gave a slight improvement of the device performance. Decreasing the Mw of the acceptor material, MDMO-PCNEPV (PCNEPV derivative with the same side chains as MDMO-PPV) and optimizing the layer thickness led to a device with an efficiency of 0.65%. Finally we looked into the influence of the nature of the side chains on the acceptor polymer. The results suggest that the closer the resemblance between donor and acceptor is the better the device performance.
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