Fakultät Mathematik und Naturwissenschaften Institut für Angewandte Photophysik http://www.iapp.de Efficient Organic Solar Cells based on Small Molecules C. Körner, R. Fitzner, C. Elschner, F. Holzmüller, P. Bäuerle, K. Leo and M. Riede DPG AKE Meeting 2013 AKE 7.2 05.03.2013
Organic Solar Cells flat heterojunction (FHJ) charge transport easy to process limited to thin layers (little absorption, low currents) metal electron transport layer (ETL) acceptor donor:acceptor donor hole transport layer (HTL) bulk heterojunction (BHJ) high currents (small D/A distances) thicker layers possible charge transport disturbed higher recombination multi-parameter optimization glass + ITO Slide 2
Reasons/Ways for Improvement towards Higher Efficiency material purity? processing issues?!!molecules!! module integration? contacts materials? stack design? Slide 3
current density (ma/cm 2 ) Processing: Substrate Heating 20 10 FHJ BHJ BHJ heated 0-10 T sub = RT T sub = 88 C substrate heating -1.5-1.0-0.5 0.0 0.5 1.0 bias (V) Influence morphology increase FF in BHJ solar cells charge transport (nano-)crystallinity phase separation Slide 4
Molecules: Variability! The 2006 K. Schulze C. Uhrich R. Schüppel D. Wynands M. Levichkova H. Ziehlke C. Körner C. Elschner 2013 Material System Slide 5
The DCVnT-Zoo 3T 4T 5T 6T Me Et Bu ** http://www.gimp-werkstatt.de/colorieren.php Slide 6
The DCVnT-Zoo 15 24 33 Similar properties different performance Slide 7
The Starting Point outstanding performance for compound 33 high efficiency of 6.1% reached already for nonoptimized standard device 15 15 24 33 24 33 T sub = 90 C Fitzner, Elschner et al., J. Am. Chem. Soc. 2012, 134, 11064 Slide 8
Thin Film Morphology via GIXRD 15 24 33 15:C 60 24:C 60 33:C 60 C 60 pristine films mixed films pristine films: highest crystallinity for comp. 15 mixed films: highest crystallinity for comp. 33 ongoing investigations! Fitzner, Elschner et al., J. Am. Chem. Soc. 2012, 134, 11064 15 24 33 Slide 9
Roadmap for High Efficiency optimize mixing ratio DCV5T/C60: 2:1 optimal substrate temperature: T sub =80 C optimize thickness of the active blend layer: 35-40nm d optimize thickness of window layer: 35-40nm d Slide 10
current density j (ma/cm 2 ) 7.2% reached! 2 0.7 0-2 -4-6 -8-10 -12 0.0 0.2 0.4 0.6 0.8 1.0 h EQE (norm.) h EQE 0.6 0.5 0.4 0.3 0.2 0.1 0.0 300 400 500 600 700 800 voltage V (V) wavelength (nm) certified efficiency at Fraunhofer ISE: h = 7.2% Slide 11
extinction coefficient k Summary molecules and thin film morphology are crucial for high efficiency record efficiency of 7.2% achieved for small molecule organic solar cells Further optimization by increased absorption red-shifted absorption (optical gap at 1.4eV) tandem structures 1.5 1.0 0.5 0.0 300 400 500 600 700 800 900 wavelength (nm) Slide 12
Acknowlegdments OSOL-group at IAPP Thank You For Your Attention Slide 13
IAPP Seminar Dissertation Talk - 2012/06/28 Slide 14
Thanks to the OSOL group Appendix
Introduction leight-weight flexible cheap colorful transparent superior temperature, angle and low-light performance http://www.fotoskizzen.de/deutschland/ fotografie/frankfurt-glasfassade.jpg building integration facades mobile applications pictures taken from Slide 16
Solar Cell Efficiency Development 12.0% Sources: NREL (www.nrel.gov/ncpv/images/efficiency_chart.jpg); heliatek press releases Slide 17
Charge Carrier Generation Scheme C. Tang, APL 48, 183 (1986) electrode donor acceptor electrode LUMO 1 2 3 4 LUMO 5 HOMO photon absorption 5 4 HOMO (exciton generation) charge transfer charge collection exciton diffusion charge transport Slide 18
Organic Solar Cells metal acceptor donor glass + ITO Slide 19
Organic Solar Cells metal Why transport layers? electron transport layer (ETL) acceptor set active layer to maximum of optical field (window layers) donor independent of contact material hole transport layer (HTL) avoid charge transport to wrong electrode glass + ITO Slide 20
Solar Cell Characterization jv-characteristics P el j V PCE P P out in j MPP V P in MPP V oc FF 1 j sc MPP PCE j sc V P oc in FF Slide 21
Substrate Heating Review David Wynands: /Franz Selzer: DCV6T-Bu(1,2,5,6) DCV4T Substrate heating Substrate heating Slide 22
Comparison 33 vs 15 Variation of Substrate Temperature j [ma/cm²] 30 20 10 0-10 33 RT 80 C 110 C -1.0-0.5 0.0 0.5 1.0 U [V] critical temperature higher for compound 15 improvement up to T sub =80 C above a critical temperature, j sc and V oc are decreased j [ma/cm²] 30 20 10 0-10 15 RT 80 C 110 C 140 C -1.0-0.5 0.0 0.5 1.0 U [V] Slide 23
EQE Morphological Changes upon Substrate Heating C 60 0.8 0.6 15 absorption T sub = 80 C/110 C DCV5T absorption 0.4 T sub = RT 0.2 T sub = 140 C 0.0 300 400 500 600 700 800 wavelength [nm] Koerner et al., DCV4T, Organic Electronics 13, (2012) Slide 24
Substrate Heating Detailed Investigations surface topography (AFM): increased roughness phase demixing 90 C PL quenching decrease: 30 C DCV4T:C 60 2:1, 20nm C 60 (15nm) excitons are lost structure investigations (GIXRD): glass together with Chris Elschner, in coop. with Univ. of Stanford absorption decrease: p-stacking / molecular orientation changed decrease in j sc Slide 25
IAPP Seminar Dissertation Talk - 2012/06/28 Slide 26
Organic Solar Cells electrode absorber electrode electrode acceptor absorber donor electrode Slide 27
Organic Solar Cells LUMO E electrode absorber electrode HOMO very strong exciton binding energy negligible photocurrent! Slide 28
absorption Organic Solar Cells x molecules: broad-band narrow-band absorbers solar radiation intensity electrode acceptor donor electrode electrode acceptor photoactive material donor wavelength electrode Slide 29
Substrate Heating Review David Wynands: DCV6T-Bu(1,2,5,6) Similar results for DCV6T-Et(2,2,5,5) Substrate heating et. al. DPG AKE Dresden 2013 Slide 30
Substrate Heating Solar Cell Characteristics DCV4T x x x x x x x x Au (50nm) ETL (55nm) DCV4T:C 60 2:1, 20nm x x x x x C 60 (15nm) glass + ITO x x x et. al. DPG AKE Dresden 2013 Slide 31
Substrate Heating Blend Layer Absorption decrease in absorption j sc directed transmission integrated transmission A 1 R T DCV4T C 60 DCV4T:C 60 2:1, 20nm C 60 (15nm) glass et. al. DPG AKE Dresden 2013 Slide 32
Substrate Heating Blend Layer Topography (AFM) 30 C 50 C 70 C 90 C AFM-pictures taken by Franz Selzer 90 C 30 C DCV4T:C 60 2:1, 20nm C 60 (15nm) glass et. al. DPG AKE Dresden 2013 Slide 33
Substrate Heating Photoluminescence Quenching reference: blends: DCV4T:C 60 2:1, 20nm C 60 (15nm) glass S 1 x CT D/A S 0 donor et. al. DPG AKE Dresden 2013 Slide 34
@ 90 C Substrate Heating Photoluminescence Quenching reference: blends: DCV4T:C 60 2:1, 20nm C 60 (15nm) glass DCV4T (13.3nm) C 60 (6.7nm) C 60 (15nm) glass et. al. DPG AKE Dresden 2013 Slide 35
Substrate Heating Structure Investigations (GIXRD) increased crystallinity of DCV4T and C 60 phase : 30 C (blue) 90 C (orange) Change in p-stacking direction (out-of-plane in-plane reflections) = explanation for decrease in absorption (unfavorable molecule orientation to incoming light) et. al. DPG AKE Dresden 2013 Slide 36