| 1 Plastic solar cells M. A. Loi Zernike Institute for Advanced Materials University of Groningen, The Netherlands

| 2 Overview 1 st hour Solar cells in general Solar Radiation p-n junction The organic version 2 nd hour Improving plastic solar cells Low band-gap polymers Charge transfer states is detrimental?

| 3 Solar Cells I ›Long duration power supply Satellites Space vehicles Remote locations on earth ›Valid alternative to fossil fuels ›Pollution free

| 4 ›Photovoltaic effect Becquerel (1839) Fritts {Selenium} (1883) Ohl {semiconductor junction solar cell}(1946) Chapin, Fuller, Person {Silicon p-n junction solar cells} (1954) Solar Cells II

| 5 Motivations ›ENERGY Increasing energy need Exhaustion of fossil fuels Diversification of energy sources Energy for all (2 billion people without electricity) ›ECOLOGY Pollution of environment CO2 Responsible Climate change ›ECONOMY Energetically independent

| 6 Solar Radiation ›Every second in the sun 6 x kg H 2 → He + 4 x J ☼ ›At the average distance of the earth the solar radiation is 1353 W/m 2 ›The atmosphere attenuates the solar radiation Absorption water - IR Absorption Ozone – UV Scattering Air Mass

| 7 Air Mass ›Air mass = the path length of the light from a celestial source relative to that at the zenith at sea level. ›increases as the angle between the source and the zenith increases (AM38 at the horizon). ›Out of the atmosphere AM0 ›On earth surface with sun at the zenith AM1 ›Average for terrestrial applications - 45˚ from the zenith AM1.5 AM= sec  zenith angle

| 8 Solar spectrum

| 9

| 10 Solar cells – inorganic case ›Single bandgap material Photons with h <E g lost energy Photons with h =E g used energy Photons with h >E g (h -E g ) lost energy Illuminated p-n junction

| 11 P-n junction solar cells

| 12 Ideal solar cell I L current produced by solar radiation I s diode saturation current R L load resistance Shockley diode equation A device area

| 13 IV characteristics Short circuit current Open circuit voltage

| 14 Ideal solar cell

| 15 IV characteristics-realistic Shunt resistance – leakage current Series resistance Junction, impurity concentration

| 16 IV characteristics-realistic The effect R SH is negligible Rs in Si solar cells 

| 17 Conversion efficiency ›FF; I L ; V oc should be maximized for efficient solar cells! Fill factor Conversion efficiency EQE or IQE, quantum efficiency-percentage of photon converted in carriers (I SC )

| 18 Ideal efficiencies

| 19 Real efficiency

| 20 Plastic Solar cells

| 21 Pro & con Advantages ›tailoring of opto-electronic properties ›large areas ›low temperatures (RT) ›processing from solution ›roll to roll manufacturing ›light weight ›transparent ›low cost…….maybe… Power paint?

| 22 Problems ›low ambient stability ›strongly bound excitons (Frenkel like) ›Exciton diffusion length rather short 5-20 nm. ›low mobility of charge carriers μ n (c-Si) > 1000 cm 2 /Vs μ h (polymer) ≈ 0.1 cm 2 /Vs ›difficult to obtain low band-gap materials Pro & con

| 23 Nevertheless Disposable low-end applications!

| Frenkel exciton Stronghly bound (0.4 eV in PPV); radius  5 Å Molecola - + Charge Transfer exciton Polarons Molecular semiconductors coulomb interaction elettron-phonon coupling To start - photoexcitations

| 25 Triplet excitons Frenkel excitons Ground state Fluorescence Intermolecular excitons non radiative states Non radiative-emission Phosphorescence

| 26 The first examples ›Early works inspired by nature (photosynthesis) ›Porphyrins, phthalocyanines, perylenes (xerography), merocyanines ›Organic heterojunction devices: p-type / n-type organic semiconductors ›– 1970’s until 1995: organic heterojunction bilayers ›– 1985 Tang cell: PTCBI (45 nm) and CuPc (25 nm) ›1% efficiency

| 27 The Kodak approach Tang et al., APL 2005

| 28 The polymer approach! ›Active layer: bulk heterojunction - hole conducting material - electron conducting material ›Operation principle: Exciton photoexcitation Diffusion of the excitons towards the organic-organic interface Charge separation/electron transfer Transport of charge carriers towards the electrodes

| 29 Photoinduced Charge Generation MDMO PPV 3,7 - dimethyloctyloxy methyloxy PPV PCBM 1-(3-methoxycarbonyl) propyl-1- phenyl [6,6]C 61 DONOR ACCEPTOR N. S. Sariciftci et al., Science 258, 1474 (1992) An ultra-fast e - transfer occurs between Conjugated Polymer / Fullerene composites upon illumination. The transition time is less than 40 fs. exciton Back transfer very slow!  s - ms

| 30 The driving force! ›Electron affinity fullerene derivatives! Polymer PCBM -6 eV -5.2 eV -4.2 eV -3.5 eV

| 31 Bulk Heterojunctions h MDMO-PPV PCBM e-e- ITO on Glass / Plastic e-e- P+P+ e-e- e-e- e-e- e-e- e-e- P+P+ Al Electrode e-e-

| 32 P-Solar Cells - FILM PREPARATION Spin Casting is a easy coating technique for small areas. Material loss is very high. Doctor Blade Technique was developed for large area coating Doctor Blade has no material loss

| 33 Production - Large Area a) b) Large Area Thin Film Production using Doctor/Wire Blading

| 34 Plastic Solar Cells - CONTACTING The cathode electrode is applied by evaporation. Different electrodes are used for different applications. Sealing is absolutely necessary for an increased life time of plastic solar cells.

| 35 Characterization under A.M. 1.5

| 36 Bulk Heterojunctions h MDMO-PPV PCBM e-e- ITO on Glass / Plastic e-e- P+P+ e-e- e-e- e-e- e-e- e-e- P+P+ Al Electrode e-e-

| 37 The morphology issue… S. E. Shaheen, Appl. Phys. Lett., 78, 841–843 (2001) 2,5% < 1%

| 38 Now.. Organic solar cells performances depend on the material properties and microscopic structure of the bulk heterojunction! P3HT 4,5-5.0 % > 60 polymers checked last 5 years!

| 39 Optimization  eff = I sc * V oc * FF / I inc I sc Tuning of the Transport Properties - Mobility V oc Tuning of the Electronic Levels of the Donor Acceptor Systems FFTuning of the Contacts and Morphology I inc Tuning of the Spectral Absorbance/Absorbing more light (low bandgap)

| 40 The future?

| 41 Intermezzo!

| 42 Organic Solar cells Polymer (donor) PCBM (acceptor) Power conversion efficiency ~ 5 - 6% bulk heterojunction 3D heterostructure hole conducting material + electron conducting material

| 43 Remember-Organic Solar Cells ›Working mechanism-steps Excitons photoexcitation Diffusion of the excitons towards the interface Charge separation/electron transfer Transport of charge carriers towards the electrodes ›Organic solar cells performances depend on the material properties the microscopic structure of the bulk hetero-junction

| 44 The driving force! ›Donor and acceptor LUMO energy offset! Polymer PCBM -6 eV -5.2 eV -4.2 eV -3.5 eV Ultrafast phenomena!

| 45 Enhancing devices efficiency ›Optimize the materials properties Matching solar spectrum! NIR materials Relative position of the energy levels of the donor and acceptor  optimal offset between LUMO (D) – LUMO(A) for electron transfer at least 0.3 – 0.5 eV  P3HT:PCBM: LUMO (D) – LUMO(A) ~1.1 eV ›Optimize the morpholog microscopic phase separation ( exciton diffusion length ~ 5 – 7 nm ) presence of a percolation pathway

| 46 Remember-Solar cells parameters ›J SC – short-circuit current ›J ph – photocurrent ›FF – fill factor: ›V OC – open circuit voltage ›  power conversion efficiency LUMO (A) Donor Acceptor Energy (eV) V oc HOMO (D) LUMO (D)

| 47 Energy(eV) P3HT:PCBM The reduction of the LUMO offset power conversion efficiency ~ 3.8 % LUMO offset ~ 1.1 eV V oc ~ 0.59 V V oc Donor Acceptor bisPCBM LUMO offset ~ 1.0 eV V oc ~ 0.73 V Power conversion efficiency ~ 4.5 % !!! M. Lenes et al, Adv. Mater. 2008, 20, 2116

| 48 PL of thermally annealed films The devices performance: P3HT:PCBM – 3% P3HT:bisPCBM – 3.6%  electron transfer is more efficient for P3HT:PCBM P3HT:bisPCBM –  PL ≈ 60 ps P3HT:PCBM –  PL ≈ 41 ps

| 49 PL of solvent annealed films The devices performance: P3HT:PCBM – 3.8% P3HT:bisPCBM – 4.6%  electron transfer is more efficient for P3HT:PCBM P3HT:bisPCBM –  PL ≈ 38 ps P3HT:PCBM –  PL ≈ 31 ps

| 50 AFM measurements P3HT:PCBMP3HT:bisPCBM spin coated slow dried 3.9 nm 12.4 nm 10.7 nm 4.6 nm surfaces is smoother for samples prepared by thermal annealing difference in RMS roughness between P3HT:PCBM and P3HT:bisPCBM 10x10  m

| 51 The blend in solution P3HT:PCBM & P3HT:bisPCBM  PL ≈ 156 ps → the efficiency of the electron transfer is the same in both blends

| 52 Conclusion 1 ›Increasing power conversion efficiency by tailoring the energy levels P3HT:bisPCBM – higher V oc P3HT:bisPCBM – higher power conversion efficiency P3HT:PCBM – faster PL decay in the thin film In solution – the same PL decay ›A small reduction of the LUMO offset does not have a significant influence on the electron transfer ›The P3HT:bisPCBM blend is limited by diffusion - the morphology can be still optimize

| Frenkel exciton Stronghly bound (0.4 eV in PPV); radius  5 Å Molecule - + Charge Transfer exciton Polarons Molecular semiconductors coulombic interaction elettron-phonon coupling Remember-Photoexcitations

| 54 Intermediate state? M. A. Loi et al., Adv. Funct. Mat. 17, 2111 (2007) CT like intermediate states are considered for modelling IV of solar cells V.D. Mihailetchi et al., PRL (2004) Recent reports consider the energy transfer from the polymer to the PCBM as the first step of the charge separation

| 55 PCBM F8BT Very poor PV performances!! Energy transfer?

| 56 M. A. Loi et al., Adv. Funct. Mat. 17, 2111 (2007) PCBM F8BT Energy transfer?

| 57 Energy Transfer? The polymer PL decay becomes very fast upon PCBM blending Energy transfer?

| 58 No clear evidence! Long rise-time also in pristine PCBM M. A. Loi et al., Adv. Funct. Mat. 17, 2111 (2007) Energy transfer to the PCBM singlet state then transferred to the triplet state. S. Cook et al., APL (2006)

| 59 Electron transfer F8DTBT PCBM  ~4% Polyfluorene copolymers promising low-band gap materials for PV applications M. Svensson et al., Adv. Mat. 15, 988 (2003); Q. Zhou et al., Appl. Phys. Lett. 84, 1653 (2004); F. Zhang et al., Adv. Funct. Mater. 16, 667 (2006)

| 60 PCBM F8DTBT

| 61 Charge transfer excitons Red-shift with the increasing of the average dielectric constant ε(PCBM) ~ 3.9; ε(Polymer) ~ Rydberg-like transitions

| 62  PCBM  1 = 320 ps;  2 = 3.1 ns PCBM  1 = 350ps;  2 = 1.0 ns M. A. Loi et al., Adv. Funct. Mat. 17, 2111 (2007)

| 63 PCBM F8DTBT Ground state interaction?

| 64 PCBM F8BT

| 65 Charge separation There is an intermediate state between the Frenkel exciton and the free charge!

| 66 Is it general? Are more systems showing this phenomena? Typical of narrow band gap polymers? D. Muehlbacher et al. Adv. Mater Solar cells efficiency:  =3.2 % FIRST narrow band-gap O-semiconductor PCPDTBT

| 67 Narrow band-gap polymer PCPDTBT blend PCBM 1:1

| 68 Concentration dependence 0 PCBM 1/11 PCBM 1/3 PCBM 1/2 PCBM Charge Transfer Exciton (1,0) (10,1) (2,1) (1,1)

| 69 New excited state in the blend with long decay time! 0 PCBM  = 100ps 1/11 PCBM  1 = 50ps  2 = 1190ps 1/3 PCBM  = 510ps 1/2 PCBM  = 478ps 1/2 PCBM  = 5ps (ns) 0 PCBM  = 150ps ExcitonCT Exciton

| 70 CTE detrimental for PV? PCPDTBT/PCBM 1:1

| 71 Conclusions II Evidences of an excited state intermediate between the exciton and the free carriers in heterojunctions containing narrow band gap polymers – present also in working devices! Additives can reduce the CTE component acting on the microstructure of the blend. The suppression of the intermediate states in bulk hetero- junctions is extremely important for the optimization of organic solar cells.  PCPDTBT/PCBM 3.8% → 5.5%