Investigation of the heterojunction morphology in donor/acceptor stacks by energy filtered transmission electron microscopy
Authors: Chouliaras Georgios , Wollgarten Markus , Schindler Wolfram , Fostiropoulos Konstantinos 
Published: December 2016

One challenge in the development of efficient organic photovoltaic devices (OPVs) is the optimization of the donor/acceptor(D/A) interface morphology, as this has tremendous impact on the electrical transport properties. In an optimized absorber layer morphology, excitons should be generated always in close proximity to the D/A interface to reach it within their lifetime, where theydissociate into free charge carriers [1‐3]. In OPVs however the free charge carriers exhibit a small mobility which requires short pathways towards the electrodes to prevent recombination. Substrate temperature during deposition as well as surface chemistry are important process parameters [4‐7]. In this respect alternating D/A layers are promising candidates for an optimized active layer morphology [8‐10].

In this contribution we present results obtained from a stack assembly of three Zn‐Phtalocyanine/C
60
layer (3 nm/3 nm nominal thickness) pairs (for further details of the preparation see [11]). The complete OPV device is shown in Fig. 1. The morphology of the individual D/A layers was investigated after each deposition step of ZnPc or C
60
, respectively. For this, energy filtered electron transmission microscopy in a Zeiss LIBRA 200 FE operated at 80 kV was used. In order to be able to investigate the same area after each deposition step, a special finder TEM grid, coated with a graphene monolayer was used. Zero‐loss filtered images were acquired with an energy filter slit width of 10 eV. In Fig. 2 we show images which where obtained after the first, second and third deposition, respectively (deposition at room temperature (RT)). The first image (Fig. 2a) shows that the ZnPc film is not closed, but islands have formed. The same can be observed for the successive C
60
layer (Fig. 2b). However, the latter agglomeration is stronger, thus larger clusters are formed, indicating a higher surface mobility of C
60
. In the further course of the deposition steps (not shown) the contrast between image features of the first two layers decreases. This indicates that the successive deposition steps lead to closed films. Such a morphology is unfavorable as there are limited percolation paths from the different interfaces towards the respective electrodes. The morphology in the layer stack changes substantially when the substrate temperature is increased to 80 °C. Figure 3 shows the corresponding morphologies after the 1
st
, 2
nd
and 3
rd
deposition step, respectively. In contrast to the experiment at RT agglomeration is observed in all following layers. In order to investigate the inter‐layer links the morphology in layer
n
was visualized by calculating the difference image d
n
= Image
n
– Image
n‐1
. The result for the first six layers is also depicted in Fig. 3. We observe a high porosity and a well established crosslinking between the ZnPc on the one hand and the C
60
layers on the other. These morphology results in an improved efficiency of η=2.1 % compared to η=0.5 % for the device deposited at RT.