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.

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Solar Cells Lecture 5: Organic Photovoltaics

  1. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 26. Response of BHJ cells to light pulses Electron Movie: How excitons are taken Holes apart by the heterojunction Alam 2011 26
  27. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 37. ordered vs. spinodal filmsBulk Heterojunction Checkerboard Double Gyroid Good … better … Best … Alam 2011 37
  38. 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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