The goal of the project is to understand interactions between donor (D) and acceptor (A) small molecules in bulk heterojunctions, which determine their optoelectronic properties.

This year we focused on (i) separating effects of D/A LUMO energy offsets, of molecular packing in the solid state, and of D/A separation on the optoelectronic properties of D/A materials and (ii) quantifying effects of acceptor addition and of external parameters on charge photogeneration, transport, trapping and recombination via numerical simulations of photocurrent.

Highlights include: Strong effects of molecular packing and of the D/A separation on the PL properties of D/A films were observed in D/A composites with close to zero D/A LUMO offset. In D/A composites with a 0.55 eV D/A LUMO offset, exciplex formation was observed. PL properties and electric field-induced dissociation of the exciplex depended on the molecular packing and D/A separation. In all composites, fast (ns scale) charge photogeneration was observed. At lower A concentrations, photocurrents were higher in composites with a nonzero D/A LUMO offset. However, at higher A concentrations, all D/A composites showed reduced photocurrents as compared to pristine D films, due to an increased disorder that reduced charge carrier mobility.We developed a computational model for simulating transient photocurrents in our devices. The model revealed competing charge generation pathways in pristine D and D/A films and enabled us to extract efficiency of each pathway, as well as charge carrier mobilities, trap densities, trap depths, and other parameters.

B. Johnson, M. J. Kendrick, and O. Ostroverkhova, J. Appl. Phys. 114, 094508 (2013), DOI:10.1063/1.4820259. K. Paudel et al., submitted to J. Phys. Chem. C (2013). K. Paudel et al., Proc. of SPIE 8827, 8827-25 (2013), B. Johnson, K. Paudel, M. J. Kendrick, and O. Ostroverkhova, " Proc. of SPIE 8830, 8830-67 (2013).

Figures: (1) Chemical structures of the molecules and (2) side group choices. Energy levels and PL properties for D/A composites with zero D/A LUMO energy offset (3-5) and with 0.55 eV D/A LUMO energy offset (6-7). (8) Simulated transient photocurrent data in pristine D films (smooth lines) and experimental data (light gray) . Inset of (8) shows the effect of A addition to the transient photocurrent amplitude, depending on the A concentration and the type of the A.

600 675 750 825

0.2

0.4

0.6

0.8

1.0

PL inte

nsi

ty (ar

b.u

.)

Wavelength(nm)

0% 2% 5% 7% 10%

Pn-TIPS-F8:exciplex

R= TES, TIPS, TSBS R= TIPS, TCHS

600 700 800 900 10000.0

0.2

0.4

0.6

0.8

1.0

0% 2% 5% 7% 10%

ADT-TIPS - Fconcentration:

PL inte

nsity (arb

.u.)

Wavelength(nm)

0 1 2 3 4 5 6 7 8

0.01

0.1

1

0%

PL in

tensi

ty (ar

b.u

.)

Time(ns)

IRF

CTransient PL

-

+

Δ LUMO = ~0.55 eVDonor

Acceptor

Si

SiTIPS TCHS

TSBS

Si

R’s

Δ LUMO = 0Donor Acceptor

0 2 4 6 8 10-0.2

0.0

0.2

0.4

0.6

Inte

grat

ed P

L in

tent

sity

(a.

u.)

Acceptor concentration(%)

PristineDonor

Donor Acceptor

Donor Acceptor

0 1 2 3 4 5 6 7

0.1

1

P

eak

I ph/P

eak

I ph,p

rist

ine

Acceptor Concentration(%)

Pn-TCHS-F8 Pn-TIPS-F8 ADT-TIPS-F ADT-TSBS-F

Pristine ADT-TES- F

(1)

(6)(3)

(4)

(2)

(5) (8)

a. b. a. b.

c. d.

(7)

Designing intermolecular interactions for high performance small-molecule bulk heterojunctionsOksana Ostroverkhova, Oregon State University, DMR 1207309

Mattson is performing test runs of optical and electrical measurements at liquid nitrogen temperatures.

Alex is processing single-molecule fluorescence data.

Undergraduate students have been actively involved in our research project. Over the past year, four undergraduate students were regularly involved in our experimental and computational components of the project. Mattson Thieme (right), a senior undergraduate student majoring in physics, joined our group last year. He is working on measurements of optical absorption and PL. He will next focus on conducting temperature dependent measurements at both macroscopic and microscopic levels. Mattson works under the supervision of a postdoctoral scholar Dr. Keshab Paudel.

Alex Robertson (left), a senior majoring in nuclear engineering, has been conducting data processing and computational parts of the project. Alex, in collaboration with previous graduate students, has developed software for analyzing single-molecule fluorescence data and currently is involved both in the experimental and in the computational aspects of the project. He works together with graduate student Rebecca Grollman and postdoctoral researcher Dr. Keshab Paudel.Two more undergraduate students, Afina Neunzert and Kyle Peters, who contributed to the project last year, graduated in the summer of 2013 and are currently pursuing graduate studies in physics at the University of Michigan and Case Western Reserve University, respectively.

Over the past year, the PI completed editing a book titled “Handbook of organic materials for optical and optoelectronic devices: properties and applications” (Woodhead Publishing, Cambridge, UK), which came out in August 2013.

Designing intermolecular interactions for high performance small-molecule bulk heterojunctionsOksana Ostroverkhova, Oregon State University, DMR 1207309