SPATIALLY MAPPING THE NANO-SCALE MORPHOLOGY OF HIGH EFFICIENCY ORGANIC SOLAR CELLS

High efficiency organic solar cells require judicious control of bulk heterojunction (BHJ) morphology. However, characterizing the obtained morphologies so that it can be correlated with processing conditions, is technically challenging. Here we will show how selective diffusion and retention of organometallic precursors in different phases can be utilized to spatially map BHJ internal morphology and phase separation.

Generally, Atomic Layer Deposition (ALD) is used to grow metal oxide coatings on functionally terminated surfaces. When organic non-reactive films are exposed to ALD organometallic precursors, the precursors can diffuse through the free volume, i.e. amorphous and disordered regions, of the film and deposit inside it. Under such conditions, the spatial distribution of the inorganic phase, effectively maps the disordered and amorphous domains in the film.

Fig. 1. Cross section BSE HRSEM of PCE11:[60]PCBM BHJ (30:70 wt%) on Si substrate after low-temperature ALD

Here we demonstrate the feasibility of this approach to map the bulk heterojunction (BHJ) morphology of high efficiency organic solar cell films composed of PCE11 and [60]PCBM, a common polymer donor and small molecule acceptor, respectively. Fig. 1 shows a back-scattered (BSE) HRSEM cross section image of a PCE11:[60]PCBM BHJ after exposure to ALD sequences of diethyl zinc and water. The bright regions are associated with a polymer-rich phase that retained large amounts of ZnO. In contrast, the film also shows dark regions which are associated with a small molecule-rich phase that does not contain ZnO. We will show that the distribution and size of the different domains depends on BHJ composition and processing conditions allowing us to correlate the morphology and phase separation with the preparation conditions and OPV power conversion efficiency.