Mapping Orientational Order in a Bulk Heterojunction Solar Cell with Polarization-Dependent Photoconductive Atomic Force Microscopy

Alan J. Heeger, University of California-Santa Barbara, DMR 0856060

(a) pd-pcAFM photocurrent map of a p-DTS(FBTTh2)2 BHJ blend with in-plane polarization vector in the direction shown by the red arrow. Micron-scale changes in the photocurrent are visible across the sample. (b) Sum of two photocurrent maps taken from the same region with orthogonal polarizations. The uniformity shows that the “polarization-averaged” photoresponse is nearly constant. (c) Molecular orientation as reconstructed from four polarized photocurrent maps. (d) +1/2 and (e) -1/2 liquid crystalline disclinations are frequently observed.

We introduce polarization-dependent, photoconductive atomic force microscopy (pd-pcAFM) as a probe of orientational order and nanoscale optoelectronic properties with ~20 nm resolution. Using the donor 7,7’-(4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b’]dithiophene-2,6- diyl)bis(6-fluoro-4-(5’-hexyl[2,2’-bithiophen]-5-yl)benzo[c][1,2,5]thiadiazole), p-DTS(FBTTh2)2, we show significant spatial dependence of the nanoscale photocurrent when illuminated with polarized light for both pristine and bulk heterojunction (BHJ) blends up to 7.0% PCE due to the local alignment of the molecular transition dipoles.

By mapping the polarization dependence of the nanoscale photocurrent, we estimate the molecular orientation and orientational order parameter at resolution beyond an optical microscope. Micron-scale, liquid crystalline order was observed in all films in agreement with complementary electron microscopy experiments. The pd-pcAFM data also allows estimation of the nano-scale orientational order parameter at each position on the film; the average was found to exceed 0.3 in all films, an unexpected and significant value. HR-TEM and dark-field imaging carried out by C.J. Takacs, graduate student in Physics supported by NSF-DMR-0856060. TEM facilities were supported by the NSF- MRSEC (DMR05-20415). Manuscript published in ACS Nano.

Si

SS

NS N

NSN

SS

SS

R1R1

R2R2

FF

(transition dipole)

(transmitted) (absorbed)

polarized light

local photocurrent ∝ optical absorption

c

e

d

500 nm

a b

3 um 3 um

Mapping Orientational Order in a Bulk Heterojunction Solar Cell with Polarization-Dependent Photoconductive Atomic Force Microscopy

Alan J. Heeger, University of California-Santa Barbara, DMR 0856060

Education The development of polarization-dependent photoconductive atomic force microscopy is a classic example of interdisciplinary science involving a combination of photonics, chemistry, materials science and device physics. The pd-pcAFM technique is a novel approach to investigating the nanomorphology of organic solar cell materials that was proposed and co-developed by C. J. Takacs, a physics graduate student. He used a combination of dark-field and HRTEM to map the orientational order of small crystallites, identified the link between the morphology and polarization-dependent charge generation, and developed the pd-pcAFM data analysis methods. The results demonstrate that these BHJ materials are not random formations of “crystalline” and “amorphous” structures but a multifaceted manifestation of self-assembly over a broad range of length scales much larger than typically considered.

OutreachOrganic solar cells fabricated by low-cost printing

technology continue to be of interest.

• They offer unique semi-transparent PV modules for building integrated products (for example. windows, Greenhouses, etc);

• They enable thin, flexible, light-weight and rugged products;

• They can be manufactured by high throughput roll to roll processes.

• They have a low carbon footprint compared to inorganic solar cells.

• The manuscript describing the liquid-crystalline structure observed with pd-pcAFM and TEM has been published in ACS Nano.

• Professor Heeger regularly gives invited (Keynote and Plenary) lectures at conferences in the United States, Europe and Asia.