Global warming in the last millennium

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1 Polymer solar cells why, status and challenges René Janssen DPI Annual Meeting, November 25-26, 28 Global warming in the last millennium Global energy power demands: Billion people use 12 TW Billion people use 2 TW ource: Marty Hoffert, New York University 1

2 nly solar energy can provide 2 TW in 25 Tide/cean Currents.7 TW Wind 14 TW Geothermal 1.9 TW olar 1 TW at Earth surface 1 TW (technical value) Biomass 5-7 TW Hydroelectric 1.2 TW technically feasible.6 TW installed capacity Apart from nuclear power, all sources of energy used nowadays (coal, oil, natural gas, biomass, water, wind) ultimately originate from the sun. ource: Arthur Nozik, NREL Courtesy: Christoph Brabec, Konarka What are the costs for producing solar energy? 2

3 Photovoltaic solar cell efficiencies & costs crystalline materials thin film materials organics increasing tatus of 25 3J sc-i mc-i CI a-i dye polymer How can we reach 2 TW PV-energy in 25? Required power: 2 TW unlight: 1 W/m 2 12 h a day Efficiency: 1% Required area: 4, km 2 = /(1 1% 12/24) Compare: 5, km 2 = pain How can we reach this area in ~3 years? Required 36.5 km 2 solar cells a day this is a strip of 365 km by 1 m every day for 3 years The (only?) solution : plastic solar cells Roll-to-roll production Lightweight Low cost ource: Konarka 3

4 Photosynthesis : or how coal, oil, gas and biomass are made today N N Mg N N Bulk-heterojunction solar cells light electron transfer absorption glass transparent electrode nm metal electrode donor acceptor nanoscopic mixing of donor and acceptor to overcome ~1 nm exciton diffusion length R. H. Friend et al., Nature 1995, 376, 498 A. J. Heeger et al., cience 1995, 27,

5 ub-picosecond hole and electron transfer Pump 51 nm MDM-PPV ( ) n h+ e- Me PCBM Pump 67 nm nm 67 nm -ΔT/ /T nm..1 ΔT/T Probe 97 nm nm 63 nm Energy (ev) Time delay (ps) Paul van Hal, Appl. Phys. A. 24, 79, 41. Polymer solar cells contain two components: electron donor and electron acceptor ( ) n MDM-PPV metal interface layer semiconducting polymer transparent conductive polymer transparent conductive oxide PCBM Me glass ean haheen et al., Appl. Phys. Lett. 21, 78, 841 Jeroen van Duren, Adv. Funct. Mater. 22, 12,

6 2.5% efficient organic plastic solar cells V C =.87 V ( ) n MDM-PPV PCBM EQE Me J (ma/cm 2 ) FF =.6 J C = 4.9 ma/cm Voltage (V) ( V I) FF = max 1 V oc I sc Wavelength [nm] ean haheen et al., Appl. Phys. Lett. 21, 78, 841 Martijn Wienk (ECN) and Jeroen van Duren (TU/e) Importance of processing and morphology Me ( ) n Toluene η < 1% Chlorobenzene η > 2.5 % 5μm 5μm 2 nm 2 nm see also: T. Martens et al., ynth. Met. 23, 138, 243 Joachim Loos and Xiaoniu Yang (TU/e) 6

7 Phase separation at higher temperatures 13 C Δh = 1 nm 1 13 C Δh = 6 nm C Δh = 17 nm 2 13 C Δh = 32 nm More efficient P3HT:PCBM solar cells C 6 H 13 n Me Pavel chilinsky, Appl. Phys. Lett. 22, 81, 3885 Franz Padinger, Adv. Funct. Mater 23, 13, 85 I sc = 8.7 ma/cm 2 V oc = 58 mv FF =.55 η e = 2.8%. 7

8 Annealing of P3HT:PCBM solar cells annealing at 12 C for 6 min on complete devices Xiaoniu Yang, TU/e regioregular P3HT M w = 1 g mol -1 M w /M n = 2.14 V oc =.615 V FF =.61 J sc = 7.2 ma cm -2 η e = 2.7 % AM1.5 1 W/m 2 TEM after spin coating from chlorobenzene P3HT.39 nm* PCBM.46 nm after annealing at 11 C for 6 min P3HT.39 nm* PCBM.46 nm * P3HT whiskers: K.J. Ihn, J. Moulton, P. mith, J. Polym. ci. Part B: Polym. Phys. 1993, 31,

9 What makes an solar cell efficient? Absorption efficiency r how many photons are absorbed? Quantum efficiency r how many photons are converted into electrons? Energy efficiency r what is the final (chemical) potential of the electrons generated? hockley-queisser limit: 31% efficiency for a single junction cell Losses in P3HT:PCBM cells Me photon energy > 1.9 ev!!!!! n FF =.65 V oc =.62 V PCBM P3HT Photons In (x 1-18 ) Electrons ut Power In % 4.% Power ut Wavenlength (nm) Wavelength (nm) 9

10 Voltage loss: ev P3HT PCBM 1.9 ev driving force for charge separation =.9 ev 4.2 HM D -LUM A = 1. ev loss at electrodes 2 x ~.2 =.4 V V oc.62 V J. Halls et al., Phys Rev B. 1999, 6, 5721 V. Mihailetchi et al., J. Appl. Phys. 23, 94, 6849 M. C. charber et al., Adv. Mater. 26, 18, 789 Theoretical efficiencies (%) Theroretical Efficiency ( α =.9 ev α =.2 ev Band gap energy α + β (ev) assuming: FF =.7, EQE =.9 and.2 V loss at each electrode E g = α+β V oc β-.4 V LUM HM Donor α β LUM HM Acceptor Goal: α+β 1.4 ev α small Martijn Wienk (TU/e) 1

11 PFTBT:PCBM (8 wt. %) solar cells: 4.2% N N C 1 C1 Me PCBM PFTBT n LiF / Al Active layer PEDT:P IT Glass smu 75 % of the absorbed photons generate current V oc =1V 1. FF =.54 J sc = 7.7 ma/cm 2 η e = 4.2 % AM1.5 1 W/m 2 Lenneke looff, Appl. Phys. Lett. 27, 9, Inganäs & M. R. Andersson, Adv. Mater. 23, 15, 988 PBBTDPP2:PCBM solar cells: 4% Improved solar cell performance PBBTDPP2 :PCBM (1:2).5.4 EQ E.3.2 H 25 C 12 N N C 12 H 25 n wavelength (nm) PBBTDPP2 Ratio 1:2 J 2 sc (ma/cm ) FF V oc (V) η (%) PBBTDPP2:[6]PCBM PBBTDPP2:[7]PCBM M. Wienk, J. Gilot, M. Turbiez, Adv. Mater. 28, 2,

12 PCDTPBT:PCBM solar cells: ~4%.5.4 EQE Wavelength N N E g = 1.4 ev n Ratio 1:2 J sc (ma/cm 2 ) FF V oc (V) η (%) PCPDTBT:[6]PCBM PCPDTBT:[7]PCBM Munazza hahid, Jan Gilot (TU/e) Muhlbacher, Adv. Mater. 26, 18, Peet, Nature. Mater. 27, 6, 497. Where is the limit? CT 1 X 1 1 ΔG CT PET CT ΔG CRT CRT PET Energy [ev] T 1 film E g,pt E T film T 1 T 1 CT Δ X film E CT PT PT E HM (D) - E LUM (A) I IIa IIb 1 and T 1 represent the lowest singlet and triplet states in the D A combination 12

13 Results from 18 D-A blends in PET PT PT ΔGCT = E CT Eg = E HM(D) E LUM(A) Eg nly -.1 ev is enough to make PET from optical photoinduced absorption, fluorescence, and cell performance Effective levels scale with E CT and V oc Implication: There is a loss of.5 ev going from E CT to V oc 2 2 E CT,max [ev] PL 1 1 V C [V] 1 2 PT PT E HM (D) - E LUM (A) [ev] PT PT ECT = EHM(D) ELUM(A) +.29 PT PT evoc = E HM (D) E LUM (A).18 ev ev 13

14 Minimal energy losses? -ΔG CRT <.1 ev prevents CRT -ΔG CT >.1 ev enables PET Energy [ev] film E g,pt film E T >.6 >.4 Δ =.3.2 film E CT PT PT E HM (D)-E LUM (A) ev C Dirk Veldman (TU/e) Conclusion: There will be at least.6 ev loss from E g to V C How is this in good cells? Polymer bulk-heterojunctions E g V oc E g -V oc η PCPDTBT : [7]PCBM PiF-DBT : [6]PCBM P3HT : [6]PCBM PF1TBT : [6]PCBM PBBTDPP2 : [7]PCBM MDM-PPV : [7]PCBM mall molecule heterojunctions CuPc : C DCV5T / C Dirk Veldman (TU/e) 14

15 Refinement Wavelength [nm] PF1TBT:[6]PCBM (.7 ev,, ), (Theore etical) Efficiency [%] 1 5 EQE = 65% FF =.65.6 V ptical band gap energy [ev] PCPDTBT:[7]PCBM (.76 ev,, ), PBBTDPP2:[7]PCBM (.8 ev,, ) P3HT:[6]PCBM (1.9 ev,, ). o we may hope for ~11% Charge generation in polymer solar cells Efficiencies of polymer solar cells are increasing to ~5% recently. Do we really understand the fundamental processes?

16 Charge separation only partly understood? 1 % 8 % cientific question: What are the requirements for full charge separation art very low electrical fields? Multi-junction polymer solar cells wide band gap BHJ metal low band gap BHJ e - e - h + h + n p recombination junction AM1.5 1 W/m 2 3% for a single band gap cell 42% for a tandem cell 49% for a triple junction device 68% for an infinite stack 16

17 Aim: create transparent electron and hole transporting layers to recombine charges Back cell e - Recombination layer Hole transporting layer Electron transporting layer I,V h + e - Front cell h + Light Zn in acetone 2 nm. olution-processed double junction solar cell V.82 V LiF/Al P3HT:PCBM PEDT Zn MDM-PPV:PCBM PEDT:P IT Glass + Current Density (ma/c cm 2 ) V front cell back cell tandem V oc = 1.53 V I sc = 3. ma/cm 2 FF =.4 MPP = 1.8 mw/cm Voltage (V) Jan Gilot, Appl. Phys. Lett. 27, 9,

18 olution-processed triple junction solar cell LiF/Al P3HT:PCBM PEDT Zn MDM-PPV:PCBM PEDT Zn MDM-PPV:PCBM PCBM PEDT:P IT Glass - + Current Density (ma/c cm 2 ) V 8 solution processed layers on top of each other processing time ~1 min. Jan Gilot, Appl. Phys. Lett. 27, 9, x.82 V 2.19 V front cell back cell triple junction Voltage (V) V oc = 2.19 V I sc = 2.6 ma/cm 2 FF =.37 MPP = 2.1 mw/cm 2 Large stacks: six-fold junction - 17 solution processed layers on top of each other /cm 2 ) Current Density (ma/ V oc =.75 V I sc = 3.5 ma/cm 2 FF =.48 MPP = 1.3 mw/cm 2 ingle junction Double junction Triple junction ix fold junction Voltage (V) V oc = 1.53 V I sc = 3. ma/cm 2 FF =.4 MPP = 1.8 mw/cm 2 V oc = 2.19 V I sc = 2.6 ma/cm 2 FF =.37 MPP = 2.1 mw/cm 2 ingle Junction Double Junction Triple Junction LiF/Al P3HT:PCBM PEDT Zn MDM-PPV:PCBM PEDT Zn MDM-PPV:PCBM PEDT Zn MDM-PPV:PCBM PEDT Zn MDM-PPV:PCBM PEDT Zn MDM-PPV:PCBM PEDT:P IT Glass + V oc = 3.46 V I sc = 1.6 ma/cm 2 FF =.32 MPP = 1.7 mw/cm 2 ix-fold Junction 18

19 Tandem solar cell using different band gaps - LiF/Al PBBTDPP2:PCBM n PEDT Zn PF1TBT:PCBM PEDT:P IT Glass Absorption (%) PFTBT:PC 6 BM PBBTDPP2:PC 6 BM Wavelength (nm) C 12 H 25 J. Gilot (TU/e) N N Me N N C 1 H 21 C1 H 21 C 12 H 25 n PBBTDPP2 [6]PCBM PF1TBT n Preliminary Preliminary performance results from solar simulator - 1 LiF/Al PBBTDPP2:PCBM n PEDT Zn PF1TBT:PCBM PEDT:P IT Glass + 78 nm 155 nm Current Density (ma A/cm 2 ) Voltage (V) TU/e J C (R) = 5.3 ma/cm 2 V C = 1.57 V FF =.47 η est = 3.9 % olar simulator (ECN) J C () = 5. ma/cm 2 V C = 1.54 V FF =.46 η = 3.6 % one month later J. Gilot (TU/e) after UV illumination 19

20 To remember and think about Renewable energy is a must for our future society Polymer solar cells might be an option to contribute to that goal because of speed of production at low cost Efficiencies of about 1% seem within reach but are a challenging goal New cell designs and concepts may increase efficiencies to ~15% Acknowledgement TU/e Dirk Veldman Jan Gilot tefan Meskers Martijn Wienk Arjan Zoombelt Munazza hahid hahid Ashraf Johan Bijleveld Joachim Loos vetlana van Bavel ECN joerd Veenstra Jan Kroon RUG Paul Blom Bert de Boer TN Marc Koetse Jorgen weelssen 2