Basic Limitations to Third generation PV performance

Transcription

1 Basic Limitations to Third generation PV performance Pabitra K. Nayak Weizmann Institute of Science, Rehovot, Israel THANKS to my COLLEAGUES Lee Barnea and David Cahen. Weizmann Institute of Science Juan Bisquert, Un. Jaume I, Spain Antoine Kahn, Princeton University In Solar Cells Most Solar Energy is Lost as Heat e- Energy e- Single p-n junction solar cell hν hν p-type n-type useable photovoltage (qv) h+ space 1

2 Lowest Loss Laboratory PV cells (1-4 cm 2 ): ~ [7%] 2% single crystalline Si ~ [8%] % single jctn. PX thin films (CIGS, Si) ~ [9%].4% dye sensitized solar cells (DSSC) ~ [92%] 8.3 % organic polymer / molecule η = Data from Solar Cell Eff #38, Progr in PV, 11 Definition of efficiency: Electrical PowerOUT % Solar Radiative Power IN Parameters of efficiency η = Electrical PowerOUT % Solar Radiative Power IN Electrical power OUT Electrical Power OUT = V MP J Mp = V oc J sc Fill Factor 2

3 Maximal possible vs. experimental photocurrents Current density(ma/cm 2 ) 4 c-si CIGS DSC-Black InP GaAs CdTe DSC- N719 OPV Solarmer a-si OPV Konarka (J sc max ) J MP J sc Absorption Edge (ev) P. K. Nayak, J. Bisquert and D. Cahen; Adv. Mater. 11 V MP / E G : Limit or Opportunity? Cell type (absorber) RT bandgap abs. edge[ev] qv MP * [ev] Energy loss qv MP /E G [%] sc-si GaAs InP CdTe Cu(In ~.7 Ga ~.3 )Se ~1.1 a.6 ~. 2 a-si:h ~ ~1.3 ~4 DSSC (black dye) (red N719) ~1.3 ~ ~.7 ~1.1 ~42 ~41 (OPV) Solarmer KONARKA ~1.6 ~ ~ ~41 ~41 Photosynthesis fuel ~1.8 (.7) 1. (42) * for best performing cell of given type 3

4 Shockley-Queisser or detailed balance limit COST as function of minimal excitation energy qv hv - qv operational (ev) SQ- Limit Loss a-si PS OPV Konarka OPV Solarmer CuGaSe 2 DSC-N719 DSC-Black CIGS c-si InP CdTe GaAs (GaIn)P Absorption Edge (ev) P. K. Nayak, J. Bisquert and D. Cahen; Adv. Mater. 11 S-Q based on R.Milo,WIS Electron-hole pair: Organic vs. Inorganic PV cells Exciton binding energy < kt dissociation by space charge region E-field MOLECULAR PICTURE Exciton binding energy >> kt requires donor/ acceptor, (D/A) type structure Organic semiconductor Inorganic semiconductor from A. Kahn, Princeton U 4

5 Electron transfer & Vibronic relaxation D*A D + A - λ rel (1) λ Energy (ev) A - λ = λ rel (1) + λ rel (2) Energy (ev) G* G DA E g Vibronic Relaxation after electron transfer A λ rel (2) Nuclear co-ordinate Loss = λ rel (hole) + λ rel (electron) λ rel (hole) = ~1meV (UPS) G rec λ rel (electron) = ~1meV (DFT) K et = K o exp -[( G +λ) 2 /4λk B T] Nuclear co-ordinate Entropy factor on dissociation..4 Coulomb potential Coulomb potential - Entropy Factor.3 Energy (ev) e-h separation length (nm) Entropy contribution = k B Tln(Ω) where Ω corresponds to number of available molecular pairs Energy (ev) D*A Hot CT state D + A - DD + DAA - A (LRS) Nuclear co-ordinate

6 Static and Dynamic disorder Tail states Kera, Yamane and N. Ueno, Progress in Surface Science, 9 Do Tail States Matter??? In a- Si, tail states cause a loss of ~.3 V. Organic materials also show presence of tailstates, due to impurities and, esp. structural dis-order. From J. Bisquert, UJ, Spain 6

7 % EQE

8 3 + 8% EQE + Fill factor loss (n=2) % EQE + Fill factor loss (n=2) + Tail state loss =.2eV

9 3 + 8% EQE + Fill factor loss (n=2) + Tail state loss =.2eV + Vibronic loss =.2eV % EQE + Fill factor loss (n=2) + Tail state loss =.2eV + Vibronic loss =.2eV + Dielectric loss =.2eV

10 % EQE + Fill factor loss (n=2) + Tail state loss =.2eV + Vibronic loss =.2eV + Dielectric loss =.2eV + (Dielectric + vibronic) =.3eV Summary There are limits, beyond Shockley-Queisser for photo-conversion with organics (OPV, DSSC, PS, AP) disorder vibronic coupling ~.2 ev ~.3 ev low dielectric constant~.2 ev Σ =. -.7 ev 1.3eV optimum band gap is not required for Organic systems. High(er) optical absorption edge systems should be considered. Ways to beat those limits Filling the tail states with permanent dopants? Smart design of materials with low vibronic loss

11 Thank you Maximum current from solar cell Max Current density(ma/cm 2 ) 4 q λ = λ edge λ = φ ( λ ) d λ Absorption Edge (ev) q is the elementary charge, λ is the wavelength, and Φ is photon flux ( AM 1.) 11

12 Cell type (absorber) V oc / E G : Limit or Opportunity? RT bandgap abs. edge[ev] V oc [V] Voltage loss [V] qv oc /E G [%] sc-si GaAs InP CdTe Cu(In ~.7 Ga ~.3 )Se ~ ~ a-si:h ~ ~.8 ~1 DSSC (black dye) (red N719) ~1.3 ~ ~ ~ Org.polymer Konraka solarmar ~1.6 ~ ~49 ~49 P. K. Nayak, J. Bisquert and D. Cahen; Adv. Mater

13 (non) concentrator; single-& multijunction homo- & hetero-junction; MIS Current Types of PV Devices Primarily based on solid-state electronic material systems Elemental Semiconductors Single or multi-crystal Polycrystalline Amorphous thin film Inorganic Compound Semiconductors Single crystal Polycrystalline thin film Organic, Excitonic (molecules, polymer) Polycrystalline Interpenetrating network Nanocrystalline; dye-sensitized Si,Ge (Ga,In)(As,P) Cu(In,Ga)Se 2 CdTe P3HT-PCBM Ru-dye+TiO 2 Solar Cell (r)evolutions nex generations 1 st generation Si 2 nd generation CdTe, CIGS 3 d generation TiO nm Quantum dot Cells 4 cm Single- & multicrystalline poly-crystalline µm micro-crystalline & amorphous nano-crystalline ~ nm cheaper? simpler? organic (polymer/ small molecules 13