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Solar Cells Lecture 4: What is Different about Thin-Film Solar Cells?

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Muhammad A. Alam (2011), "Solar Cells Lecture 4: What is Different about Thin-Film Solar Cells?," http://nanohub.org/resources/11949.

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Solar Cells Lecture 4: What is Different about Thin-Film Solar Cells?

  1. 1. 2011 NCN@Purdue-Intel Summer School Notes Summer School: July 2011 NCN on Photovoltaic Devices Notes on the Fundamental of Solar Cell Lecture 14What is Different about Thin-Film PV M. A. Alam and S. Dongaonkar alam@purdue.edu Electrical and Computer Engineering Purdue University West Lafayette, IN USA
  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. The lecture series on solar cells Introduction to Solar cells Physics of Crystalline Solar cells Simulating Solar Cells What is different about thin film solar cell Organic photovoltaics superposition and recombination Alam 2011 3
  4. 4. Outline of the lecture1) Background information about thin film solar cells2) Photo current from the transmission perspective3) Dark current, shunt conduction, and weak diodes4) Variability, reliability, and lifetime of solar cells5) Conclusions Alam 2011 4
  5. 5. Different types of solar cells p-n p-i-n m-i-m Crystalline Silicon Amorphous silicon Flexible organic Si too thick and expensive … Alam 2011 5*Google Images
  6. 6. 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 1-1.36 c-Si installation, labor, etc. $3.75/W Others … $1.00-1.50/W …. but thin film solar cell has their own problems !• All costs are approximate(J. Kalowekamo/E. Baker, Solar Energy, 2009. Goodrich, PVSC Tutorial, 2011. 6
  7. 7. Features of thin film solar cells (2) Thin doped region (1) Thin absorption layer(3) Contact diffusion (4) Grain boundaries Laser Scribe Al a-Si:H TCO Glass (5) Series connection Alam 2011 7
  8. 8. Equivalent circuit of thin film solar cells IT Idark IT RL Iph Voc RL Pmax = Iph × Voc × FF IphIR Id ISH = Iph − I0 (e qV / kBT − 1) I ⇒ VOC = ( kT q ) ln(1 + I ph I0 ) Superposition does not hold … 8
  9. 9. Outline of the lecture1) Background information about thin film solar cells2) Photo current from the transmission perspective3) Dark current, shunt conduction, and weak diodes4) Variability, reliability, and lifetime of solar cells5) Conclusions Alam 2011 9
  10. 10. Basics of current flow Wrong contact loss +Recombination loss 4 qυ Jn ≠ Jn = 0 L Jn = Jn − J p = 3qυ0 − 2qυ0 L L J= J −J = J −J L n L p L n R n= 4 qυ0 − 2qυ0 3 2  = ×  υ0 − υ0  6q 6 6  4 2 = 6q × υ0 − 6q × υ0  γ L ,n γ L ,p  6 6 = qG ×  −  γ L ,n γ L ,p  γ L ,n + γ R ,n + γ rec γ L ,p + γ R ,p + γ rec    = qG × − qG × γ L ,n + γ R ,n γ L ,p + γ R ,p Alam 2011 10
  11. 11. Basics of transmission over a barrier −EB ,L kBT γ L←R = Ae −EB ,R kBT γ R←L = Ae Right RightLeft Contact Left ContactContact Contact −EB ,L kBT γ L = Ae γ R = Ae −EB ,R kBT Left Contact Device Right Contact Alam 2011 11
  12. 12. Photocurrent without recombination J ph  γ L ,nW γ L ,p  = qG ∫0 dx  γ L ,n + γ R ,n − γ L ,p + γ R ,p      γ R = υ0 e= W dx  γ L ,n − E ( W − x )/ kT γ R ,n  ∫0  γ L ,n + γ R ,n − γ L ,n + γ R ,n   γ L = υ0   W  υ0 υ0 e − E(W − x )/ kT  = ∫0 dx   υ0 + υ0 e − E ( W − x )/ kT −  υ0 + υ0 e − E(W − x )/ kT    2L W  2L W  RL = D log cosh W× ≅ W  D − coth  kT W 2 LD  W 2 LD  LD ≡ E ‘Price length’ and point of no return …. Alam 2011 12
  13. 13. Properties of ‘Sokel’ photo-currentSokel and Hughes, JAP, 53(11), 1982. J ph 2L W γL  γR = W × D log cosh qG W 2 LD VA=0  2 LD W  ≅W − coth   W 2 LD  γL = γR Jn qG Vbi=VA>0 Vbi VA γL = γR Vbi γL < γR VA>Vbi=0Voltage dependent photocurrent, different from Si p-n junction …
  14. 14. Blocking layer and photocurrentJ ph W  γ L ,n γ R ,n  γR ~ 0qG ∫ 0 dx   −  γ L ,n + γ R ,n γ L ,n + γ R ,n  VA=0  J ph = qGW J ph qG Vbi γR ~ 0 VA Vbi=VA>0 With blocking layer … Blocking is essential for many types of thin film PV …. 14
  15. 15. Photocurrent with recombination Crandall, JAP, 54(12), 19823  n ≡ υ0 × τ n = µn × E × τ nJ ph W  γ L ,n γ L ,p  W ∫ dx  − = ∫ dx e − x /=  c 1 − e −W /  c    c    qG  γ L ,n + γ R ,n + γ rec γ L ,p + γ R ,p + γ rec  0 0   J ph qG {  } = W − W −  n 1 − e −W /  n   15
  16. 16. Photo-current in crystalline cells with electron mirror without electron mirror J ph J L ,n JL ,p = − =  n 1 − e −W /  n  ~  n    n ≡ Dnτ n  W qG qG qG Voltage independent photocurrent is unaffected by electron mirrors Electron blocking layer does suppress dark current, increases Voc. 16
  17. 17. Numerical validation: Effect of blocking layer With blocking Without blocking Dτ  W Dτ  W For low quality Si PV, blocking is not essential 17
  18. 18. Photocurrent with field/recombination J ph W υ0 e − x /  n − υ0 e −(W − x )/  k e −(W − x )/  n  = ∫ dx  qG 0   υ0 + υ0 e −(W − x )/  k   W  e − x /  n − e −( W − x )/  k e −( W − x )/  n = ∫ dx  − ( W − x )/  k  0  1+ e   2 LD J ph W  ≅W − coth  qG  W 2 LD  nph kT LD ≡ E Matches with numerical simulation very well … Alam 2011 18
  19. 19. Outline of the lecture1) Background information about thin film solar2) Photo current from transmission perspective3) Dark current, shunt conduction, and weak diodes4) Variability, reliability, and lifetime of solar cells5) Conclusions Alam 2011 19
  20. 20. Dark current without recombination γL γR Jn qnLυ0 − qnRυ0 γL + γR γL + γR nL ,0 nR ,0 = γ R ,0 γ L ,0 ND NA −E = Ae − qVbi k T γ L ,0 Ae B ,0 B kB T nR ,0 = ni2 N A γ R ,0 = A ×1 ⇒ γ L ,0 γ R ,0 = e −qV bi kB T nL ,0 = ND nL ,0 = nR ,0 e + qVbi kB T Vbi ni2 − qVbi kB TnR ,0 = ni2 N A nL ,0 = e NA ER ←L = qVbi Alam 2011 EL ←R = 0 20
  21. 21. Calculating dark current without recombination γ L = Ae −q(V bi − V ) kB T γL γR Jn qnL ,0υ0 − qnR ,0υ0 γ R= A ×1 γL + γR γL + γR ni2 + qVbi kB T nL ,0 = e qυ0 NA Jn γL + γR ( nL ,0γ L − nR ,0γ R ) qυ0 ni2 qVbi e kB T − q ( Vbi −V )/ kB T e − 1 e − q ( Vbi −V )/ kB T + 1 NA   Vbi ni2  µn (V − Vbi ) / d  qVb  + q(V −Vbi ) / kBT  e − 1 kB T q NA e + 1    ni2 ni2   µn (V − Vbi ) / d  qV   + q(V −Vbi )/ kBT  e − 1 ≡ I0 e qV − 1 kB T kB TJd = Jn + J p = q  +  N A ND   e   + 1     21
  22. 22. Theory and practice of thin film dark IV Theory ExperimentId ≡ I0 e qV  kB T − 1  Butterfly wings! A real solar cell IV seldom looks like textbook IV! These wings helped create many complicated models. 22 Alam 2011
  23. 23. Contact diffusion and shunt conduction (2) Thin doped region (1) Thin absorption layer (4) Grain boundaries(3) Contact diffusion A real solar cell IV seldom looks like textbook IV 23
  24. 24. Parasitic shunt leakage@ low voltage shunt@ high voltage intrinsic Dongaonkar and Alam JAP, 2010 24
  25. 25. Interpretation of ‘shunt’ leakage J n = qnµn E Va = 2 2J L 3 2 3 εµn dE qn = 9εµn dx κε 0 J ( Va ) = Va2 8L3 V δ +1 * EA Ishunt = Aµ 2δ +1 γ= kT L * G. Paasch et al., JAP 106, 084502 (2009) 25
  26. 26. Features of shunt leakage V δ +1 Symmetry ExponentsIshunt = Aµ 2δ +1 L L-3 Dongaonkar and Alam, EDL, 2010 26
  27. 27. Outline of the lecture1) Background information about thin film solar2) Photo current from transmission perspective3) Dark current, shunt conduction, and weak diodes4) Variability, reliability, and lifetime of solar cells5) Conclusions Alam 2011 27
  28. 28. Contact diffusion and shunt conduction (2) Thin doped region (1) Thin absorption layer (3) Grain boundaries(4) Contact diffusion 28 Alam 2011
  29. 29. L I Variability and weak diodes I0 ≈ Iph (Voc ) e − qVoc / kT = Iph (Voc )(e ( oc ) − 1) q V −V / kT I l 2 × n × Iph (Voc1 )(e ( oc1 ) − 1) q V −V / kT = Iph (Voc 2 )(e ( oc 2 ) − 1) q V −V / kT = L2 l 2 ≈ e ( oc1 oc 2 ) ! q V −V / kT nI I0 (e qV / kT − 1) − Iph (V )I0 (e qVoc / kT − 1) = (Voc ) Iph Like an impact crater, a single um-sized weak diode can drain away 1-10 mm region !!Voc = kBT × ln(1 + Iph I0 ) (karpov, PRB 69, 045325, 2004.) 29
  30. 30. (5) Series connection, shadow degradation, and a very weak diode 30 Alam 2011
  31. 31. Being in shadow stresses the device Shaded Load line panelShaded cell operatingoperating pointpoint Initial cell Initial panel operating point operating point Shaded cells can get reverse biased! 31
  32. 32. Shadow degradation Dongaonkar et al. IRPS 2011 Dark currentNormal timeoperation Shadow Operating degradation voltage 32
  33. 33. Light induced degradation Y. Nakata, et al., JJAP, 1992 Slope – t1/3 T. Shimizu, JJAP, 2004 D. L. Staebler, et al., APL, 1977.Ongoing discussion aboutexponent n~1/3 33 M. Stutzmann, et al., PRB, 1985
  34. 34. Reaction-Diffusion Model for LID G H2 Reaction: 1011 Density of DBs [cm-3]dNDB = kF N0 G - kR NDB NH ~0 1010 dtFree H Generation: Slope n ~0.3 109 dNH dNDB = - kH NH 2 dt dt 108 2 /3  kf N0G  10-4 10-2 100 102 NDB ∝ ( 3kH ) 1/ 3 1/ 3  t  k   LS Time [sec] 34  r 
  35. 35. Light induced degradation in crystalline PV metal finger n+ emitter ~ 100 nm p absorber ~ 100 µm p+ electron mirror ~ 100 nm back reflectorBoron-doped Czochralski (Cz) crystalline PV equallysusceptible to LID. Float-zone and/or Ga doping better. Alam 2011 35
  36. 36. Extrinsic and Intrinsic Solar Cell Reliability • ElectrochemicalIntrinsic • Light Induced Extrinsic corrosion Degradation • Moisture Ingress • Hot spot • Glass fracture breakdown • Inverters reliability • Shunt leakage • Delamination • Shadow degradation • Improper insulation • Weak diodes • Bypass diode failure Source - www.nrel.gov/pv/performance_reliability/pdfs/failure_references.pdf 36
  37. 37. conclusionsEconomic incentive to develop thin film solar cell.The unique features of thin film PV make photo-current voltage dependent, increases probability offormation of weak diodes and shunts.In addition to the extrinsic reliability issues, we needto worry about shadow degradation, light induceddegradation, etc.The reliability/variability are key concerns – makingmodules less efficient than individual cells. Alam 2011 37
  38. 38. Reference for imageshttp://www.viraload.com/2008/08/28/avasolar-raises-104m-for-thin-film-solar/http://www.solarserver.com/solar-magazine/solar-news/current/kw42/nanosolar-to-expand-thin-film-cigs-solar-cell-manufacturing-to-115mw.htmlhttp://www.solarthinfilms.com/active/en/home/photovoltaics/photovoltaics_and_thinfilms/thinfilm_photovoltaics.htmlhttp://gotpowered.com/2011/ge-develops-thin-film-photovoltaic-panels/ http://kypros.physics.uoc.gr/images/web2.gif 38

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