Influence of Polymer Phase, Polymer/Nanoparticle Ratio and Organic Additives on the Performance of Hybrid Solar Cells

Polymer/copper indium sulfide (CIS) nanoparticle hybrid solar cells represent an interesting solar cell system combining advantages of inorganic semiconducting materials with these of solution processable, lightweight and flexible and polymers. In this study, we prepare the CIS nanoparticles directly in the polymer matrix. This in situ formation of the CIS nanoparticles from copper and indium xanthates as precursors makes them, besides other benefits, suitable for cheap production routes at temperatures compatible with flexible substrates. 1

So far, the highest power conversion efficiencies (PCEs) of solar cells prepared via this route have been obtained with the conjugated polymers PSiF-DBT and PCDTBT. An important factor currently limiting the overall PCE of these solar cells is the strong thickness dependence of the PCE. Best PCEs were achieved with thicknesses of 60-70 nm and in layers with this thickness only a part of the incoming light can be absorbed and used for conversion. In this study, we investigated which influences the polymer phase, the polymer/nanoparticle ratio, morphology and the hybrid interface have on the characteristic solar cell parameters and the thickness dependence of the PCE. We compared the currently commonly used PCDTBT to state of the art conjugated polymers used in polymer/fullerene cells (e.g. PBTTT-14, DT-PDPP2T-TT or PffBT4T) which show higher charge carrier mobility or lower dependency of PCE on absorber film thickness. As the polymer/CIS nanoparticle ratio is also strongly influencing morphology, and is thereby critical for the efficiency, these ratios have been also investigated for all investigated polymers. Additionally, the influence of different molecular weights of the polymers was tested. In particular, for testing PffBT4T with a high molecular weight, which led to polymer/fullerene cells with very high PCEs (up to 10.8%),2 the coating and processing conditions of the absorber layer fabrication had to be adjusted as this polymer requires elevated temperatures to from stable solutions and to be processed. Regarding the hybrid interfaces and the inorganic phase, we incorporated organic additives into the fabrication process aiming to modify the polymer/nanocrystal interface and we also added hexylamine, which is known for improving the crystallinity of the nanoparticle phase.3 Morphological aspects in the absorber layers were analyzed by AFM or TEM and the optical properties of the films were studied by UV-VIS absorption spectroscopy. The electrical properties of the prepared solar cells were characterized by current-voltage (IV) curves and external quantum efficiency (EQE) measurements.