Polymer/Nanocrystal Hybrid Solar Cells: Influence of Molecular Precursor Design on Film Nanomorphology, Charge Generation and Device Performance

Figure 1. Chemical structures of the used cadmium xanthate and the different alkyl moieties, TEM images of poly(3-hexylthiophene)/CdS hybrid layers prepared using a xanthate bearing a propyl (upper image) and a dimethylpentyl (bottom image) alkyl chain and the current/voltage characteristics of the corresponding hybrid solar cells (green: propyl, purple: dimethylpentyl).

In this study, polymer/nanocrystal films are prepared via thermal conversion of metal xanthates to metal sulfide nanocrystals directly in the polymer matrix without capping ligands for the nanocrystals. The effects of different alkyl chains of the metal xanthates on film nanomorphology and optoelectronic properties are investigated (Figure 1). As only little was known about the nanocrystal growth and the formation of the nanomorphologies during the heating step, this is thoroughly characterised on the nanometer scale by time resolved simultaneous GISAXS and GIWAXS (grazing incidence small and wide angle X-ray scattering) measurements performed at the SAXS beamline of Elettra Trieste (Figure 2).
It is found that longer alkyl chains of the metal xanthates lead to a better mixing of the polymer and nanoparticle phase and to smaller domain sizes in the absorber layers, while using short ethyl moieties results in the biggest domain sizes. As revealed by time resolved GISAXS and GIWAXS measurements, the formation of these different morphologies is determined by agglomeration of the nanocrystals, while the nanocrystal size is similar in all the investigated samples. The agglomeration of the nanocrystals is influenced by growth temperature and growth kinetics of the nanocrystals. In addition, phase separation of polymer and metal xanthate phase in the precursor layer before the heating step plays an important role.
 

Moreover, the influences of the different nanomorphologies on the optoelectronic properties of the films are studied by microsecond and femtosecond transient absorption spectroscopy and a remarkable effect on device performance is revealed. While in finer mixed morphologies down to a certain domain size more efficient photoinduced charge generation was observed, the highest power conversion efficiencies were obtained in solar cells prepared using metal xanthates bearing propyl moieties, which have an absorber layer nanomorphology with medium sized domains. This originates from the fact that besides efficient charge generation also charge transport and recombination play a crucial role in the devices, which become more and more an issue when going to finer mixed morphologies.

Figure 2.  a) GIWAXS patterns of a polymer/Cd-butyl xanthate sample measured between 22.9 and 35° 2 theta during a heating run from room temperature to 200°C showing the evolution of the most intense peak of CdS at around 27° 2 theta. The increasing temperature (T) is indicated with a black arrow; b) Normalised integrated intensity of the GIWAXS patterns of the different samples plotted against time and temperature; c) GISAXS images of the ethyl and butyl xanthate samples at different temperatures during the heating run.