Roll to Roll Processing, Demonstration and Advanced Materials for Polymer Solar Cells The Solar Energy Programme Martin Helgesen, PhD. Frederik C. Krebs, Professor
Quick Status and Potential of Polymer Solar cells Outdoor lifetime > 1 year reported, Hauch et al., Sol. Energy Mater. Sol. Cells (2008), 92, 727 Lifetime >10000 hours (vacuum) for thermocleavable polymer based on PT, Krebs et al. Chem. Mater. 17 (2005) 5235 Why OPV Existing solar cells expensive Complex to make generally need highly specialized systems No solar cell technology can be made on the 1 GW/day scale Only silicon PV has no abundance problem (bad for CIGS, CdTe, OPV ) Materials and process are the key for low cost, high volume OPV The Unification Challenge Stability Process R2R process at Risø - OPV has a steep learning curve (lifetime is not accounted for) Efficiency Efficiency >8% for lab-device (Solarmer, Konarka) Efficiency >2-3% for R2R products (Konarka Technologies) No special physics limits OPV performance. 10-15% is imagined. 2
R2R Process at Risø DTU Low cost and large scale process 1. It must be an all solution process in air 2. For some processes we still use vacuum coating (i.e. ITO, metals) 3. Processing of subsequent layers must not affect underlying layers (i.e. complementary solvents, soluble precursor polymers) 4. Flexibility might not be needed in the product but is in the process 5. Materials + Knowhow + equipment A process is a combination of: Device architecture Materials Process equipment/techniques Knowhow 3
Risø DTU multi task process equipment Inline coating and printing machine Two ovens and three different coating and printing methods. Fast manufacturing speeds 45 seconds for an A4 sheet Low capital investment Simple printing equipment from small to large scale Ink feed Slot-die coating head 4
Fast continous R2R study 5
Is R2R coating scientific? accuracy and parsimony Automatic R2R characterisation Little materials usage and very fast (~35s pr sample) (i.e. 60 mg P3HT, 60 mg PCBM for 200 devices) Alstrup et al. ACS Appl. Mater. Interfaces, 2010, 2, 2819 2827 6
Process One PET-ITO-ZnO-Active-PEDOT:PSS-silver: 5 layers where 2 are screen printed and 3 are slot-die coated. Full R2R, all solution, all air, semitransparent, flexible 2.3% PCE, modules comprising 8 serially connected cells where active area of each stripe is 15 cm 2 Performance close to lab scale device (2.7%, Risø) 1.5 m/minute/layer, ~ 30s per module, ~ 6 m 2 hour -1 Outdoor lifetime (T 80 ) 0.5-1 year with simple encapsulation (PET lamination) 7
Same R2R Machinery - ITO Free ProcessTwo ITO free - PET/Ag(np)/ZnO/AL/PEDOT:PSS/Ag All solution - No vacuum - Nontransparent - Flexible 0.3% PCE (poor transmission) Process Four 4-layer structure, full R2R, ITO free, only patterning of the last layer Does use vacuum and a solid metal back electrode Area equivalent of Si cells Cu/Ti/AL/PEDOT:PSS/Ag 8
What are plastic solar cells good for? Application and Demonstration Serves the purpose of honing our understanding of the capacity of the technology It helps us in transferring the technology to industry (from lab to pilot scale) Bringing technology into application Bringing OPVs to public awareness Getting feedback Demonstration Provides know-how Provides insight into processing costs Suntiles Grid connected PSC 9
Making it useful - Lighting Africa 100 Million people with no light after 18:00 PM (subsaharan Africa) A budget of 28-56 year -1 for lighting (kerosene) The possibility for a low cost light weight OPV solution (< 3 ) 10
Demonstration projects Lighting Africa Field tests in Zambia (2009) Intuitive in handling <50g, <1mm Flexible, portable Potential for Low Cost Nonhazardous Shortcomings: User instructions The flexibility Unique Switching Charging instructions 11
Demonstration project Ligting Africa Cost analysis (per lamp) ~5 min ~35 /W p 12
We have to make many to get the cost down PCE = 2.8% Voc = 8.61 V Isc = 22.08 ma FF = 51.4%. New lamp Smaller outline (Even smaller outlines possible) Cost analysis (~8 /W p ) 1) Man Power and machine time ~16-20% 2) Encapsulation ~15-20% 3) ITO ~25-30% 4) Other Layers ~30-40% Modules based on 16 serially connected solar cells with a total active area of 35.5 cm 2 Best performance reached for a single module was 2.8% (average ~2%) Lower cost (~8 /W p ) through optimization of ink usage and larger quantities at lower cost Required the production of around 10 kw p to reach a cost of 8.1 /W p 13
Advanced Materials for improved photochemical stability Stability Process Combination in one material Efficiency 14
Stability: Known Degradation Processes Diffusion of O 2 and H 2 O into device Diffusion of electrodes into device (Al and ITO) Metal oxide formation (insulating layer) Photochemical reactions with the Active layer Norrman et al., Sol. Energy Mater. Sol. Cells 90 (2006) 213 Jørgensen et al., Sol. Energy Mat. & Sol. Cells 92 (2008) 686 Norrman et al., ACS Appl. Mater. Interfaces, 1 (2009) 102 15
Must use photochemically stable materials Rule of thumb 34 different polymers relevant to PSCs studied. broad range of polymer types (purely donor, donor/acceptor, thermocleavable, etc.) and chemical structures The total amount of absorbed photons was monitored versus ageing time over the range λ 1 - λ 2 under 1 sun Screening allowed for the description of general rules for the p-conjugated polymer photochemical stability (i.e. donor-acceptor nature, sidechain type) Manceau et al. J. Mater. Chem. 2011, 21, 4132-4141 16
Some examples in air Influence of the donor group Stability: fluorene<cpdt<si-cpdt<thiophene. Quaternary carbon site can be readily oxidized Si less easily oxidized Unsubstituted thiophene most stable (>600 h) Influence of the side-chains Stability (cleaved): fluorene<cpdt<si-cpdt<thiophene, but different timescale Cleavage of side-chains systematically led to improved stability P4 only showed 20% decrease during 1000 h irradiation Thermocleaved 17
Photochemical stability study General rules: 1. The use of exocyclic double bonds in the main backbone (MEH PPV, MDMO PPV) leads to a poor stability and should be avoided 2. Moieties containing a quaternary C-site are very unstable (e.g. fluorene, cyclopentadithiophene) because of the oxidazability of this site 3. The presence of readily cleavable bonds (such as C N or C O) also limits stability 4. Side-chains play a key role in conjugated polymer degradation and their cleavage largely improves stability by a factor 2-20 Manceau et al. J. Mater. Chem. 2011, 21, 4132-4141 18
Thermocleavable Materials What are they? Side chains provide solubility and allow solution processing Thermal treatment: - Leads to a more rigid and stable morphology and higher chromophore density - The diffusion phenomena is slowed down due to harder material - Photochemical reactions associated with the side chains are avoided 19
Thermocleavable materials P3MHOCT Thermocleavable labile ester bond PT available by solution processing Stability: P3MHOCT<P3CT<PT Elimination of sidechains improve stability with a factor 4 Photochemical Stability Thermocleaved P3MHOCT P3CT PT P3HT Thermocleavable group P3MHOCT:PCBM films 20
Thermocleavable materials: PSDT-DTZ Thermocleavable Bulkiness of chains on SDT unit is very significant (i.e. PCE, Thermocleavage) Thermocleavage leads to improved PCE with hxsdt. IPCE (350-700nm) enhanced 1 6% GIWAXS: Inhibiting effect of ethylhexyl chains vs. hexyl chains on the polymer packing higher degree of crystallinity of PhxSDT-DTZ compared to PehSDT-DTZ V oc = 0.66 V J sc = 5.93 ma/cm 2 FF = 0.37 η = 1.5% V oc = 0.66 V J sc = 3.52 ma/cm 2 FF = 0.37 η = 0.9% Helgesen et al. Polym. Chem. 2011, 2 (11), 2536-2542 21
Evaluation of stability: PhxSDT-DTZ Thermocleavable 100 suns (10 W/cm 2 ) T50: Stability of PhxSDT-DTZ more than twice the one of P3HT Thermocleavage increase T50 with 50% Including PCBM in film doubles T50 Sunlight concentration setup For quick screening of the photo-degradation 1-100 suns MEH-PPV Helgesen et al. Polym. Chem. 2011, 2 (11), 2536-2542 22
Water Processing No solar cell technology has low environmental impact during manufacture today not even OPV Table: Cumulative energy requirements for raw materials production. 23
Existing water soluble polymers 24
Water processing Water soluble Thermocleavable non-ionic Søndergaard et al, Adv. Energy Mater., 1, (2011), 68-71 25
Water processing - Device architecture Ag Aluminium Paste (water) PEDOT:PSS (water/ipa) Soluble in water:iso-propanol:thf (47.5 : 47.5 : 5) Active layer (water/ipa/thf) ZnO (water) Transparent substrate/ito Modified PCBM 26
Water processing Aqueous solutions ZnO Polymer: PCBM PEDOT: PSS Ag paste 27
Perspective and Outlook Polymer solar cells can be made in air by R2R coating Large volume with relative stability is possible The technology is ready for demonstration and niche products (Risø) Thermocleavage in a Roll-to-Roll Process? PET and PEN substrate show significant deformation Polyimide (Kapton) deformations Thermocleavable group Krebs et al, Appl. Mater. interfaces, 2, (2010), 877 28
Thank you for your attention J. Goldenberg, Science 315, 808 (2007) 29