Low Cost Plastic Solar Cells

Transcription

1 Low Cost Plastic Solar Cells Solar cells --- Power from the Sun --- must be and will be --- a significant contribution to our energy needs. Two problems that must be solved with solar: 1. Cost 2. Area

2 Semiconducting and Metallic Polymers inks ---- with electronic functionality! Plastic Substrate The Dream Solar Cells Functional Ink

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5 Plastic Solar Cells Ultrafast charge separation with quantum efficiency approaching Unity! ħω RO OR 1992 RO OR RO OR e- RO OR 50 femtoseconds!!

6 Ultrafast electron transfer is important --- Charge transfer is 1000 times faster than any competing process, and Back charge transfer is inhibited Therefore, Quantum efficiency for charge separation approaches unity! Every photon absorbed yields one pair of separated charges! (Route to photovoltaics and photodetectors)

7 Ultrafast photo-induced charge separation ---- But how do we create a material with charge-separating junctions everywhere?? ħω RO OR RO RO OR OR 50 fs e- RO OR All must be accomplished at nanometer length scale!

8 O OMe S P3HT n PCBM Bulk Heterojunction Material Bicontinuous interpenetrating network Self-assembled nanoscale material with charge-separating junctions everywhere!

9 Bulk D-A Heterojunction Material A self-assembled nanomaterial Electrical Contact Electrical Contact Must break the symmetry --- use two different metals with different work functions. Electrons will automatically go toward lower work function contact and holes toward higher work function contact

10 Origin of V oc --- when irradiated with high light intensity, Fermi levels must be equal (holes in the π-band and electrons in PCBM LUMO): V oc E PCBM LUMO -E polymer homo π*-band π*-band PCBM LUMO PCBM LUMO V oc --- small V oc --- larger π-band PCBM HOMO π-band PCBM HOMO PCBM PCBM semiconducting polymer semiconducting Polymer (deeper HOMO)

11 Solar Cell Performance Device structure: ITO/PEDOT/P3HT:PCBM/Al. w/o anneal Eff = 5.0% V OC = V FF = 68% 70 o C, 30 min 150 o C, 30 min Series resistance decreased from 113 Ω to 7.9 Ω

12 Bulk Heterojunction Material --- Optimize Nanomorphology Before 150 o C anneal 150 o C anneal 30 min 150 o C anneal 2 hours O OMe S n P3HT PCBM TEM images of the P3HT/PCBM interpenetrating network

13 After annealing for 10 minutes at 150 o C Spatial Fourier Transform 10min annealing 30min annealing 2hour annealing P.S.D.(a.u.) High temperature annealing o C 1. Stable for long times at High-T minutes sufficient to lock in the nano-scale morphology Spatial Frequency(1/nm) Quasi-periodic with periodicity of nm

14 80 nm Period Å Distance from any point in the material to a charge separating interface Å Less than the mean exciton diffusion length --- High efficiency for charge separation.

15 New Device Architecture: Optical Spacer Conventional Device Glass ITO PEDOT Active Layer Al Light intensity in Dead zone

16 New Device Architecture: Optical Spacer Conventional Device Glass ITO PEDOT Active Layer Al Device with Optical Spacer Glass ITO PEDOT Active Layer Al Light intensity in Dead zone Optical spacer

17 Device architecture: Optical spacer OR OR Ti OR + H 2 O OH OH Ti OH OH Glas s P3HT PED :PCB OT:P M SS ITO TiOX Al um inu m Ti O O Ti O O O Ti Ti O TiO X Charge separation layer

18 Optical Spacer can improve the Power Conversion Efficiency Green light (532 nm) Efficiency η e = 8.1% 12.6% 25 % - 50% increase in efficiency AM1.5 (solar spectrum) Efficiency Optical spacer important for low mobility materials where simply making the active layer thicker is not an option η e = 2.3% 5.0%

19 What can we expect to achieve?? Single layer with efficiency > 5% demonstrated with P3HT Band gap too large ---Missing more than half the solar spectrum Opportunity: Potential for factor of 2 improvement using polymer with smaller band gap.

20 Semiconducting polymers with smaller band gap? Zhengguo Zhu (a) (ZZ50) N S ( ) n N (c) S S (b) O OCH 3 S n Absorption and photoresponse out to 900 nm in the IR. Should be capable of 7% in single cell and >10% in a Tandem Cell with P3HT Photocurrent (arb.u.) Photocurrent (arb.u.) 1E-4 1E-5 1E-6 1E-7 1E-8 1E-4 1E-5 1E-6 1E-7 1E-8 ZZ50/ZZ50:PCBM P3HT/P3HT:PCBM Energy (ev)

21 With ZZ Progress --- but still missing all incident power in the IR beyond 900 nm. Need smaller gap materials

22 Morphology Control withalkane-dithiols as Processing Additives (Nature Materials, Published online May ) Add 2.5% alkanedithiol into solvent (chlorobenzene) 1,3-propanedithiol (blue), 1,4-butanedithiol (green), 1,6-hexanedithiol (orange) and 1,8-octanedithiol (red) Spectra sharpen and red-shift Higher j sc --- higher efficiency

23 Processing Additives for Morphology control S S Zhengguo (a) Zhu S N (ZZ50) N (c) ( ) n b O OCH 3 η =2.8% Factor of 2 Improvement by using octane-dithiol as processing additive η =5.5% Nature Materials Published online May Best performance for a single cell architecture

24 Processing Additives for Morphology Control of Bulk Heterojunction Materials Mechanism: (i) Selective (differential) solubility of the fullerene component (ii) Higher boiling point than the host solvent.

25 Absorption Spectra PCPDTBT film (green) PCPDTBT:C71-PCBM films processed with 1,8-octanedithiol --- Before removal of C71-PCBM with alkanedithiol (black), After removal of C71-PCBM with alkanedithiol (red) The PCBM is completely removed by the alkane-dithiol!

26 Multi-layer Tandem Cell (equivalent to two solar cells in series) Al TiO x P3HT:PC 70 BM PEDOT:PSS TiO x O R OR Ti O R Back cell Front cell Al TiO x P3HT:PC 70 BM PEDOT:PSS TiO x PCPDTBT:PCBM PEDOT:PSS ITO Glass T i O H O O + H OH 2 O Ti OH O T i O OH T i O Ti O Ti O X

27 Tandem Cell (Multilayer architecture equivalent to two solar cells in series) Current Density (ma/cm 2 ) PCPDTBT single cell P3HT single cell Tandem cell Bias (V) Open circuit voltage doubled --- Efficiency 6.5%

28 We can do even better ---- Increased performance of each of the two subcells --- Already demonstrated (using the Processing Additive). Goal for coming months: 8-10 %

29 In the Tandem Cell, the TiOx layers serve six separate functions: 1. Optical spacer that redistributes the light intensity to optimize the efficiency of the back cell. 2. With TiOx layer between the charge separating layer and the aluminum cathode --- much improved air stability 3. The TiOx functions as a low resistance electron transport layer 4. TiOx layer breaks the symmetry and thereby creates the open circuit voltage. 5. TiOx functions as a hole blocking layer (top of the valence band at ev) 6. TiOx layer enables the fabrication of tandem cells. The transparent TiOx layer is used to separate and connect the front cell and the back cell.

30 Polymer Solar cells: Present status % with P3HT (and with several other materials with similar spectra) What can we expect (hope) to achieve?? New Architecture --- optical spacer % improvement New Polymers with energy gap well matched to the solar spectrum Opportunity for x2 improvement Better charge collection efficiency (optimize morphology) Opportunity for 25% improvement Increased open circuit voltage (deeper HOMO for semiconducting polymer) Opportunity: Potential for > 50% improvement Tandem Cell --- Opportunity: Potential for > 50% improvement

31 Reality of each of these has been demonstrated. Goal: Achieve all these advances in the same system (1.25 x 2 x 1.25 x 1.5 x 1.5 = 7) Clear vision of technology pathway to BHJ solar cells with efficiencies exceeding 20%

32 New materials are the key to progress Examples: LeClerc and colleagues (JACS published on the web 12/21/07) N S N * N C 8 H 17 C 8 H 17 S X S n * With X = acceptor such as 3.6% with PCBM N N Yong Cao and colleagues (in press) 5.4% with PCBM Note: Both have high V oc V Deeper HOMO

33 Air-stable polymer electronic devices Thin layer of amorphous TiO x (x<2) improves performance and Enhances Lifetime Polymer Based Solar Cells Polymer LEDs Polymer FETs Adv Mater. 2007

34 Bulk Heterojunction Solar Cells Single TiO x passivation layer signficantly enhances the lifetime Factor of 100! Efficiency (%) Conventional with TiO x Time (Hours) Goal: Simple inexpensive barrier materials will be sufficient for achieving long lifetime.

35 Background scientific information a b PL Intensity (Normalized) Glass/PF Glass/TiO x /PF Glass/PF/TiO x Glass/TiO x /PF/TiO x Fresh Film 15 hours at 150 o C Wavelength (nm) without TiO x layer with TiO x layer Binding Energy (ev) O 1s Intensity (C 1s - Normalized) TiOx prevents oxidation of polyfluorene (PF) XPS of the O1s/C1s ratio in the PF after 48 hrs at 150 o C in air

36 Similar enhancement in lifetime for LEDs and FETs LEDs FETs Luminous Efficiency (cd/a) with TiO x Conventional Time (Hours)

37 Rooftop testing of BHJ solar cells at KONARKA

38 Lifetime: Rooftop and Accelerated Degradation Flexible devices packaged with commercial, low cost barrier films (Konarka) Active layers Barrier Film Metal Transparent Electrode PET Barrier Film UV-Filter Metal Illumination Flexible Organic P3HT:PCBM Bulk-Heterojunction Modules with more than 1 Year Outdoor Lifetime Jens A. Hauch, Pavel Schilinsky, Stelios A. Choulis, Richard Childers, Markus Biele and Christoph J. Brabec Long-Lived Flexible Organic Solar Cells Jens A. Hauch, Pavel Schilinsky, Stelios C. Choulis, Sambatra Rajoelson and Christoph J. Brabec

39 Outdoor rooftop lifetime (Konarka) 1.6 Sep. 20th 2006 Nov. 7th Normalized Power output [a.u.] Efficiency -10 Temperature S O N D J F M A M J J A S O N D Month Temperature [ C] 15.00% 10.00% 10.86% Efficiency actually increased --- Drop in power output after 1 year due to small shift in the maximum power point (from 0.87V to 0.8V). 5.00% 0.00% -5.00% % % 3.31% 0.14% Efficiency Voc Jsc FF -6.80% Rel. Change

40 Accelerated Degradation --- Flexible devices packaged with commercial, low cost barrier films (Konarka) Normalized FF [a.u.] Normalized V oc [a.u.] 105% 100% 95% 90% 85% 105% 100% 95% 90% 85% 80% 75% 65 C/85%rh 65 C 65 C/1sun Time [hrs] 65 C/85%rh 65 C 65 C/1sun Time [hrs] Normalized Efficiency [a.u.] Normalized J sc [a.u.] 110% 100% 90% 80% 70% 60% 50% 110% 100% 90% 80% 70% 60% 50% Time [hrs] 65 C/85%rh 65 C 65 C/1sun 65 C/85%rh 65 C 65 C/1sun Time [hrs]

41 Goal: Efficiency > 10-15% with polymer bulk heterojunction nano-materials Low Cost Roll-to-Roll manufacturing by printing and coating technologies

42 Acknowledgement Professor Kwanghee Lee ---UCSB and GIST (Korea) Dr. Jin Young Kim Prof G. Bazan, D. Moses, J.Peet Christoph Brabec and colleagues at Konarka

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