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Solar Cells Lecture 5: Organic Photovoltaics

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Muhammad A. Alam (2011), "Solar Cells Lecture 5: Organic Photovoltaics," http://nanohub.org/resources/11950.

Muhammad A. Alam (2011), "Solar Cells Lecture 5: Organic Photovoltaics," http://nanohub.org/resources/11950.

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  • 1. NCN Summer School: July 2011 Notes on the Fundamental of Solar Cell Lecture 5Physics of Organic Solar Cells M. A. Alam and B. Ray alam@purdue.edu Electrical and Computer Engineering Purdue University West Lafayette, IN USA 1
  • 2. copyright 2011 This material is copyrighted by M. Alam under the following Creative Commons license: Conditions for using these materials is described at http://creativecommons.org/licenses/by-nc-sa/2.5/2 Alam 2011
  • 3. Outline1) Introduction: Rationale of organic solar cells2) Planar Heterojunction OPV3) Checkerbox Heterjunction OPV4) Bulk Heterojunction devices5) Percolation, fluctuation, and efficiency limits6) Conclusions Alam 2011 3
  • 4. Different types of solar cells p-n p-i-n m-i-m Crystalline Silicon Amorphous silicon Flexible organic Alam 2011 4*Google Images
  • 5. Economics of solar cells C-Si CdTe a-Si CIGS OPV Material/m2 207 50-60 64 100-125 37 Process/m2 123 86 73 130 23-37 Total/m2 350 130 138 230 50-80 Cost/W 1.75 0.94 -1.2 0.9-1.4 1.63 1-1.36If efficiency exceeds 15% and lifetime 15 years, $/W ~0.36 • All costs are approximate • J. Kalowekamo/E. Baker, Solar Energy, 2009. • Goodrich, PVSC Tutorial, 2011. Alam 2011 5
  • 6. Outline1) Introduction: Rationale of organic solar cells2) Planar Heterojunction OPV3) Checkerbox Heterjunction OPV4) Bulk Heterojunction devices5) Percolation, fluctuation, and efficiency limits6) Conclusions Alam 2011 6
  • 7. Basics: Excitons, electrons, and holes H atom rB m = 0.1m0 , κ =10 mq4 1 1  m E1 Si ~13.6 meV rBSi ~ 53 A E1 = = 13.6 × 2   eV 32π  ε 0 κ 2 2 2 2 κ  m0  4πε 0  2 m  rB = κ = 0.53 × κ  0  A mq 2  m m = 0.1m0 κ = 3 Charge neutral excitons happilyE1 poly ~151 meV rBpoly ~ 15 A diffusing around … Alam 2011 7
  • 8. Basics: Excitons, electrons, and holes E1 poly ~151 meV rBpoly ~ 15 A χB χMχM χA χM χM Heterojunctions takes the exciton apart, Build-in field sweeps electrons/holes away 8
  • 9. Bilayer Plastic Solar Cells Side view Band-diagram Acceptor DonorExciton recombination before dissociation at the junction makes it a poor cell … Alam 2011 9
  • 10. Photocurrent in bilayer cells Jex µn S µn E + S Jex~ Dexτ ex 0 J ph  γ L ,n γ L ,p  =  −  Jex  γ L ,n + γ R ,n + γ rec γ L ,p + γ R ,p + γ rec    Jex JL ,ex = =  ex 1 − e −W /4  ex    qG qG = µn E(0) = δ τ n ≡ S γ L ,n γ rec  ex ≡ Dexτ ex J ph µn E(0) Vbi − V = E(0) = Jex = qG  ex W 2 ≫  ex Wn Jex µn E(0) + S Jex = qG W 4 W 2 ≪  ex 10
  • 11. Photocurrent in bilayer cellsDefective interface J ph  γ L ,n γ L ,p =  −  Jex  γ L ,n + γ R ,n + γ rec γ L ,p + γ R ,p + γ rec    Jex γ n(0)2 + S + µn E  n(0) ≡ γ n(0)2 + υT n(0) =    −υ + υ 2 + 4γ J  J ph =  T T ex  υT  2γ    Interface recombination a key challenge Alam 2011 11
  • 12. Dark current in bilayer organic PV Defective interface 0 Approx. 10 J (mAcm -2) Exact -10Jd ≈ γ n(0)p(0) 10 0 0.5 1≈ γ nL e − ni2,0  − qEWn / KB T − qEWp / KB T × pR e Voltage (V)   γ nL pR e − q(Vbi −V )/ KBT − ni2,0    Alam 2011 12
  • 13. Summary: Total current in bilayer organic PVJex μ =10-4 cm2/V.s=  ex 1 − e −W /2  ex   qG 0  −υ + υ 2 + 4γ J  -2 JdJ ph =  T  υT JT,num J (mAcm -2) T ex  2γ  -4   -6Jd γ nL pR e − q(V  bi −V )/ KB T − ni2,0   -8 JT,approx JT,anall -10 -0.2 0 0.2 0.4 0.6 = J ph + Jd JT Voltage (V) Anomalously low fill factor! 13
  • 14. Current collection and charge pileup V=0.6 Energy V=0.0J ph µn E(0) Vbi − V = E(0) <Jex µn E(0) + S Wn x 10 9 8 0 7 V=0.2 -2 Jd JT,num 6 J (mAcm )-2 E-field 5 -4 4 V=0.4 -6 3 -8 JT,anall 2 -10 V=0.6 -0.2 0 0.2 0.4 0.6 0 Voltage (V) Position The electrons stay too long close to a dangerous region … 14
  • 15. Better mobility improves Fill factor μ =1e-4 μ =1e-3 0 0 -2 JT,num -2 JT,numJ (mAcm -2) J (mAcm ) -2 -4 -4 -6 -6 JT,anall -8 -8 JT,anall -10 -10 -0.2 0 0.2 0.4 0.6 -0.2 0 0.2 0.4 0.6 Voltage (V) Voltage (V) Higher mobility improves Fill factor Nonlinear series resistance …. Alam 2011 15
  • 16. The problem with planar heterojunction … Lex : Dexτ ex Lex : Dexτ ex Making such thin film is essentially difficult, the layers will short out … 16
  • 17. Outline1) Introduction: Rationale of organic solar cells2) Planar Heterojunction OPV3) Checkerbox Heterjunction OPV4) Bulk Heterojunction devices5) Percolation, fluctuation, and efficiency limits6) Conclusions Alam 2011 17
  • 18. Checkerboard organic solar cells Decoupling exciton and electron-hole paths Lex : Dexτ ex McGehee, MRS Bulletin, Feb. 2009Jex JL ,ex = = N ×  ex 1 − e −Ws /2  ex  2   A. Javey, Nature Materials, July 2009qG qG 18
  • 19. the balancing act … Finger density …S NF ~1 2S 2 VF = WS 2 Fraction of the charge collected/finger … F(S) ~ 4S × Dexτ ex 2S 2 = 2 Dexτ ex S Two blocksW Total charge collected … Jex = qG ×VF × F(S)× NF ~ qG ×W Dexτ ex S Alam 2011 19
  • 20. S Photo and dark currents (like a p-i-n diode) Photocurrent with distributed recombination J ph W  γ L ,n γ L ,p  = Jex − R(V ) ∫ 0 dx  −   γ L ,n + γ R ,n γ L , p + γ R , p    W 2 LD W  2 LD W  = W× log cosh ≅W − coth  γ R = υ0 e − E(W − x )/θ W W 2 LD  W 2 LD γ L = υ0 Dark current with distributed recombination  µn (V − Vbi ) / d  qV = A  + q(V −V )/ k T  e Jd nkB T − 1 e bi B + 1   Sokel and Hughes, JAP, 53(11), 1982. Alam 2011 20
  • 21. meso-structured organic solar cellsbilayer checkerboard Mixed Layers Jex JL ,ex = = 2N ex 1 − e −Ws /2  ex    qG qG AW = NWs 2W 2N = A Ws 21
  • 22. Outline1) Introduction: Rationale of organic solar cells2) Planar Heterojunction OPV3) Checkerbox Heterjunction OPV4) Bulk Heterojunction OPV5) Percolation, fluctuation, and efficiency limits6) Conclusions Alam 2011 22
  • 23. Processing of a plastic bulk heterojunction cell Solvent Nature Materials, 2009Polymer-A Polymer-B(Donor) (Acceptor) 100 Y (nm) 50 0 0 50 100 X (nm) Anneal for a certain duration Cahn-Hilliard Eq: at moderate temperature ∂ϕ  ∂f  Phase Separation occurs = M0  ∇ 2 − 2κ∇ 4ϕ  ∂t  ∂ϕ  through Spinodal Decomposition 23
  • 24. Process model for phase segregation Free energy: f mix= U − TS Ø0 Free energy, f(a.u) 0.5Enthalpy Entropy Cahn-Hilliard Eq: 0 ∂ϕ  2 ∂f 4  = M0  ∇ − 2κ∇ ϕ  ∂t  ∂ϕ  ØA ØB -0.5 0 0.5 1 Surface Energy of mixing tension Volume fraction, Ø (x,y) free energy contains polymer details 24 Ray et al., Solar Energy Mat and Solar Cells, 2011
  • 25. Demixing and self-organization in thin films Anneal time Grain-size W(t) ~ ( ta ) 1/ 3 1/ 3 W(t) ~ t a Anneal time (sec) Alam 2011 25
  • 26. Response of BHJ cells to light pulses Electron Movie: How excitons are taken Holes apart by the heterojunction Alam 2011 26
  • 27. Photocurrent in BHJ OPV Anneal time 1/ 3 L(ta)W(t) ~ t a W(ta) AreaL(t)× W(t) = 2 AreaL(t) ~ 2 × t1/ 3 a 27 Alam 2011
  • 28. Photocurrent and exciton flux Q Dex 1 Exciton flux (a.u) Jex = : τ ex τ ex t1/ 3 a Anneal time (a.u.) L(ta) Area L(t) ~ 2 × t1/ 3 aW(ta) Q = q Dexτ ex × L(ta ) Form defines function …. Should we anneal at all ? 28
  • 29. Photocurrent and annealing time Ray et al., Solar Energy Mat and Solar Cells, 2011 10 Exciton Flux JSC (mA/cm2) 8 1/R 6 ta(opt) 4 2 1 2 Anneal time (a.u.) 10 Anneal Time (min) 10 29
  • 30. How do they compare ? Bilayer Typical BHJ Ordered BHJ Ray et al., PVSC, 2011 15 Bi-layer Typical BHJ Ordered BHJ η = 3.56 % η = 5.5 % η = 6.1 % 10 Jsc = 6.6 mAcm -2 Jsc = 14.6 mAcm -2 Jsc = 15.2 mAcm -2 Voc = 0.66V Voc = 0.63VCurrent (mA/cm2) Voc = 0.66V 5 FF = 81.5 % FF = 59.5 % FF = 63.3 % Low current 0 Bi-layer Typical BHJ Poor fill factor -5 -10 -15 Ordered BHJ 0 0.2 0.4 0.6 Voltage (V) Alam 2011 30
  • 31. Outline1) Introduction: Rationale of organic solar cells2) Planar Heterojunction OPV3) Checkerbox Heterjunction OPV4) Bulk Heterojunction devices5) Percolation, fluctuation, and efficiency limits6) Conclusions Alam 2011 31
  • 32. The challenge of a 15% cell …New polymers with Change solventsmaller bandgap and mixing ratioSolubility issues Percolation dictates ~1:1 volume ratioOptimize anneal time Regularize morphology Very limited play Random close to optimal Conflicting system requires constraints the design, Need to consider all improvement within this context 32
  • 33. Mixing ratio and percolation Fluctuation and 1 Mixing RatioConnected Volume 100 nm 0.8 1 200 nm 1:1 Connected Volume 0.6 0.8 1:2 0.6 0.4 0.4 0.2 0.2 0 1:3 0 0 0.2 0.4 0.6 0.8 10 10 2 10 4 Fraction of P3HT Anneal Time (s) Moving away from 1:1 ratio is challenging ….
  • 34. Ordered vs. Disordered Morphology Ray et al., PVSC, 2011 6.5 180 6 160 Regular BHJ 5.5 Efficiency (%) 140 (mA/cm 2) 5 Regular BHJ 120 4.5 Random BHJ Random BHJ SC 100 J 4 3.5 80 3 60 5 10 15 20 25 5 10 15 20 25 <W D> <W D> 0.66 0.7 0.65 Regular BHJ 0.68 Regular BHJ 0.66 0.64 (V) 0.64 Random BHJ FF 0.63 OC 0.62 Random BHJ V 0.62 0.6 0.61 0.58 0.6 5 10 15 20 25 5 10 15 20 25 <W D> <W D>Reduced variability, optimal for a range of anneal conditions 34
  • 35. Fundamental constraint of reliability (i) t a = 0 s (ii) t a = 10 2 s (iii) t a = 10 3 s Continued coarsening with anneal time(a) Ta =120 0 C n(ProcessingTemperature) WC (ta ,Ta ) ∝  Deff (Ta )ta    (i) t s = 10 4 s (ii) t s = 10 5 s (iii) t s = 10 6 s Deff (Ta ) = D0 e -EA / kTa (b) TS = 80 0 C TS= 800, 1000, 1200 C Cluster Size, WC (nm)Stress Temperature ,TS (i) t s = 10 4 s (ii) t s = 10 5 s (iii) t s = 10 6 s 50 (c) TS = 100 0 C W C~ tSn 20 (i) t s = 10 4 s (ii) t s = 10 5 s (iii) t s = 10 6 s (d) TS = 120 0 C 10 2 4 6 10 10 10 Stress Time (s)The difference between Arizona sun and an oven is negligible 35
  • 36. Derivation of the reliability formula Lex JSC (t0 + t s ) WC (t0 ) JSC ∝ = ts = 10 hrs 100 hrs WC (t) JSC (t0 ) WC (t0 + t s ) n  Deff (T0 )t0    = n  Deff (T0 )t0 + Deff (Ts )t s    -n  -E A / kTS  ts  = 1+ e     teq     500 Ts= 800, 1000, 1200 C 1 JSC (deg) = 10, 20, 30% Cluster Size, WC (nm) Lifetime (days) 400 50 JSC (norm) 0.8 300 WC~ tan Ea = 1.2 eV 0.6 200 n = .25 20 0.4 100 10 0.2 Ts= 800, 1000, 1200 C 0 2 4 6 3 4 5 6 20 30 40 10 10 10 10 10 10 10 Anneal Time (s) Anneal Time (s) Operating T (0C)Ray et al., APL, 2011 Lifetime less than a year! 36
  • 37. ordered vs. spinodal filmsBulk Heterojunction Checkerboard Double Gyroid Good … better … Best … Alam 2011 37
  • 38. ConclusionsOrganic solar cells promises a low cost PVtechnology, lightweight, easy to install. Also, abeautiful physics problem with biomimetic transport.Theory explains optimum of anneal time, therationale of 1:1 mixing ratio, the fundamentalconstraints of reliability, limits of Voc and FF.Reliability, variability, and efficiency are importantconcerns. Self assembled regularized structure, newclass of optics, lower bandgap materials, may help usreach 15% efficiency targets. Alam 2011 38

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