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Investigation of the thermal stability of nanoimprinted comb structures in a conjugated polymer and their application in hybrid solar cells

Comb-shaped nanostructures were prepared in a low band gap polymer using nanoimprint lithography (NIL) and their thermal stability was investigated using time resolved GISAXS.

S. Dunst et al. , ACS Appl. Mater. Interfaces 6, 7633 (2014).

Figure 1. (a) SEM images of NIL-imprinted comb structures (periodicity: 500 nm) directly after the NIL process and after a temperature treatment at 195 oC; (b) GISAXS images of a NIL-structured PSiF-DBT film (periodicity: 180 nm) at 65, 105, 143 and 200 oC. The red boxes indicate the vertical and horizontal areas for integration. (c) Evolution of the out-of-plane scattering signal of a NIL-structured PSiF-DBT film during a heating run to 200 oC with a heating rate of 10 oC/min, approx. every 10th measurement is shown, the curves are shifted vertically for better visibility; (d) Temperature-dependent changes in out-of-plane intensity of the “line structure peak” at approx. qz = 0.9 nm-1; (e) Temperature-dependent changes in out-of-plane intensity of the wings of the Yoneda peak at approx. qz = 0.55 nm-1. Adapted with permission from S. Dunst et al, ACS Appl. Mater. Interfaces 6, 7633 (2014). Copyright (2014) American Chemical Society

 

These measurements showed that the comb structures in the conjugated polymer are stable up to a temperature of 145 oC, which enabled us to apply them in nanostructured organic/inorganic hybrid solar cells. The nanostructured solar cells revealed improved power conversion efficiencies compared to flat bilayer devices.
The absorber layer of organic/ inorganic hybrid solar cells consists of a mixture of a conjugated polymer and a nanostructured inorganic semiconductor. The nanostructured inorganic semiconductor can be present either in the polymer matrix as a random network of nanoparticles or, in a more defined way, as an ordered nanostructure. For obtaining high power conversion efficiencies (PCEs) of these devices, a large interface between polymer and nanoparticle phase, where charge separation of electrons and holes occurs, is beneficial. However, also continuous domains in both phases towards the respective electrode are needed for fast charge transport of electrons and holes to the electrodes in order to limit recombination.
In this study, defined comb structures were prepared using NIL in the conjugated polymer PSiF-DBT, see Fig. 1a and 2a, and it was envisaged to use them in absorber layers of hybrid solar cells by filling the comb-shaped structure with copper indium sulphide. This can be realised by coating the nanostructured layer with a solution containing copper and indium xanthates, as schematically illustrated in Fig. 2a. The metal xanthates can be subsequently thermally converted at moderate temperatures to a copper indium sulphide layer covering the nanostructure.
For a successful preparation of nanostructured hybrid solar cells via this approach, the nanostructures have to be sufficiently thermally stable. Therefore, we performed a time resolved GISAXS study at the Austrian SAXS beamline at Elettra, which revealed a good thermal stability of the polymeric nanostructures. In the GISAXS images (see Fig. 1b), which were acquired at low temperatures, a semicircle-like chain of intensity maxima is visible, which is characteristic for periodic line structures oriented parallel to the x-ray beam. The evolution of the out-of-plane scattering signal shows that the most intense parts of this semicircle-like feature (peak at qz ~ 0.9 nm-1) are present up to a temperature of approx. 145 oC (Fig. 1c and d), which proves a good stability of the nanostructure up to this temperature. Moreover, at around 145 oC the intensity of the wings of the Yoneda peak at qz ~ 0.55 nm-1 (see Fig. 1e) increases, which suggests that the roughness of the polymer layer becomes higher at this temperature and the well-defined structure is lost. Time resolved grazing incidence wide angle x-ray scattering (GIWAXS) measurements pointed out that the formation of the copper indium sulphide nanocrystals from the metal xanthates starts shortly before the comb structures start to be unstable and that the formation of the nanocrystalline metal sulphide is completed at a temperature of about 150 - 160 oC.

 

In a next step, nanostructured solar cells were prepared by coating the nanostructured polymer layers with the metal xanthate solution and subsequent annealing at 160 oC to form the copper indium sulphide. Indium tin oxide (ITO) as well as aluminium were used as electrodes. In Fig. 2b, a scanning electron microscopy (SEM) image of a cross section of a device with a nanostructured interface is presented, which proves that the comb-shaped polymer structure (appearing darker) is filled with copper indium sulphide (lighter areas). The current-voltage characteristics of the solar cells (Fig. 2c) exhibit a significantly improved PCE of the nanostructured device compared to a similarly prepared flat bilayer device. The increase in PCE (approx. 0.3% for a nanostructured device compared to 0.1-0.15% for a flat bilayer device) is based on a distinct improvement of the short circuit current in the nanostructured solar cells, which can be ascribed to the increased interface area.
In this study, we demonstrated that the preparation of nanostructured hybrid solar cells via this approach is generally feasible. To optimize the devices, it is envisaged to further reduce the periodicity of the nanoimprinted comb structures, which is 180 nm in the solar cells prepared in this study. Moreover, the temperature of the thermal conversion of the metal xanthates should be kept as low as possible in order to retain a well-defined nanostructure.
 
 
Figure 2.  (a) Schematic illustration of the NIL process for the preparation of comb structures and the infiltration process towards defined hybrid layers and chemical structures of the used polymer PSiF-DBT and the Cu and In xanthates; (b) SEM image of a cross section of a hybrid solar cell comprising a polymer/CIS absorber layer with a nanostructured interface (periodicity: 180 nm) prepared at 160 oC; (c) IV curves measured in the dark and under 100 mW/cm2 illumination of hybrid solar cells with flat and nanostructured interfaces prepared at 160 oC. Adapted with permission from S. Dunst et al, ACS Appl. Mater. Interfaces 6, 7633 (2014). Copyright (2014) American Chemical Society.
 

Retrieve article
Nanoimprinted Comb Structures in a Low Bandgap Polymer: Thermal Processing and Their Application in Hybrid Solar Cells;
S. Dunst, T. Rath, A. Radivo, E. Sovernigo, M. Tormen, H. Amenitsch, B. Marmiroli, B. Sartori, A. Reichmann, A.-C. Knall and G. Trimmel;
ACS Appl. Mater. Interfaces 6, 7633 (2014).
10.1021/am5009425
Last Updated on Tuesday, 06 February 2018 10:26