3 rd Japan-German Bilateral Workshop on Molecular Electronics 21-23, Jan., 2009, 烏丸 Kyoto Hotel, Japan Supra-hierarchical nano-structured organic thin film solar cell I nstitute of Advanced Energy Kyoto University Susum u Yoshikaw a
Contents □ Insertion of TiO 2 ETL in polymer solar cell of P3HT:PCBM □ Preparation of PEDOT:PSS polymer brush as HTL in polymer solar cell □ Concept of Supra-hierarchical nano-structured cell (Hybrid solar cell ）
Biography of Organic Solar Cell & Novel Architecture of OSC with Supra-Hierarchical Nano-Structure Al OSC for Next-Generation Al Al V organic n-type semi- Al semi- V conductor n-type semiconductor conductor V p-type semi- V conductor p-type semiconductor Metal ITO Glass ITO Glass Schottky junction ITO Glass Heterojunction (Calvin Cell) Glass Bulk heterojunction (Tang Cell) Supra-Hierarchical Nano-Structure (Sariciftci Cell) η < 0.01% (SHNS) with C 60 η > 7% η = 1% η = 4% n-type (Acceptor) 2006 supra-hierarchical nano-structured cell ( Yoshikawa ) 2004 tandem heterojunction photovoltaic cell (Forrest, Uchida ) 2000 bulk MDMOPPV/PCBM heterojunction PV cell (Brabec) 1996 C 60 -linked molecular type PV cell ( Imahori ) 1995 bulk MEHPPV/PCBM heterojunction PV cell (Heeger) p-type (Donor) 1995 bulk polymer/polymer heterojunction PV cell (Friend) 1994 bulk polymer / C 60 heterojunction PV cell （ Heeger ） TiO 2 1993 polymer/ C 60 heterojunction PV cell (Sariciftci) 1991 dye-sensitized TiO 2 PV cell (Graetzel) 1991 bulk dye/dye heterojunction PV cell ( Hiramoto ) 1990 tandem PV cell ( Hiramoto ) Donor/ Acceptor 1986 heterojunction PV cell (Tang) S O H 3 C C C O PEDOT S CH 2 Device architecture of OSC with SHNS 1958 photo-induced current with MgPor (Calvin) S S O 3 S CH CH 2 Topics are focused to improvements in device structures.
Introduction Introduction Current status of polymer solar cell Organic Solar ・ Recently high efficiencies over 5% are reported. Cells ・ Poly(3-hexylthiophene)/PCBM is commoly used as a high carrier mobility system. ・ Bu Bulk h lk hetero rojunction on is is im important f for h r high ghly ly Dye Organic efficie cient ce cell- ll-stru ructure. Sensitized TF Cells Cells Small Polymer Molecule PV Problems of Organic PV Advantages of polymer PV ・ Low efficiency ・ Fabrication under ambient atmosphere ・ Low durability ・ Potential low-cost manufacturing
Organic Semi-conductors Comparison, organic and inorganic meterials mobility Cohesion anisotropy [cm 2 /Vs] Force Chemical Inorganic ~1000 few times bonding van der Organic ~10 ~100 times Waals organic materials: origin of electric conduction ⇒ planer π -conjugated molecule
Fundamental Limitations to Organic Solar Cells Fundamental Limitations to Organic Solar Cells The exciton and carrier diffusion bottleneck Since L D is short & μ is little, there exists a trade off in thickness. Problems Maximum: exiciton collection & Minimum series resistance thin film Maximum: absorption thick film How to solve these problems? Using a bulk heterojunction Using material with long range order Using thin HTL and ETL with EBN and HBN Using tandem cells (capture more light in thin layer Using 1D nanostructured array for carrier path （ Supra-hierarchical nano-structured cell)
□ Insertion of electron transport layer (ETL) in polymer solar cell ・ P3HT: PCBM with TiO 2 layer Organic Thin Film solar Cell Enhanced efficiency and stability in P3HT:PCBM bulk heterojunction solar cell by using TiO 2 as electron transport layer
Necessity of hole blocking layer in polymer cell ITO glass TCO HTL PEDOT:PSS LAL P3HT:PCBM ← TiOx 層 HTL Nothing (LiF) Al anode To achieve highly efficient charge transfer and charge collection Poly(3-hexyl [6,6]-phenyl C60- thiophene) butyric acid methyl ester Poly(3,4-ethylenedioxythiophene)- [PCBM] [P3HT] poly(styrensulfonate)[PEDOT:PSS]
Necessity of HBL (hole blocking layer) Necessity of HBL (hole blocking layer) in illumination in dark 0.1 Under Increase in a 0.0 illumination, I-V back electron 2 ) C urrent density (m A /cm curve shows a -0.1 transfer less rectification. -0.2 Au/P3HT/Al Necessary to introduce a new barrier -0.3 -0.4 Conditions for HBL -0.5 -0.6 n-type semiconductor -1.0 -0.5 0.0 0.5 1.0 1.5 Voltage (V) LUMO is between Al and PCBM Fig.3 I-V curve of Au/P3HT/Al in dark and under illumination. 3.7 A wide band gap In dark, P3HT/Al 4.2 4.2 interface forms a Al Schottky barrier to PCBM prevent a back electron TiO ₂ transfer and shows a 6.1 favorable rectification. 7.4 NTCDA as HBL O O C C Ag/C 60 /CuPc/ITO O O 2 μ m C C O O Hiramoto, Appl. Phys. Lett., 86 , 063509 (2005).
Energy diagram of TiO 2 Energy diagram of TiO 2 P.V.A. Hal et. al., J. Adv. Mater. 2003, 15, No.2 Hal et al. reported room TiO ₂ (７7～ Ti-alkoxide • temperature 91%) over night 3.3 3.7 Al TiO 2 prepared by this 4.4 P3HT method has an PCBM 4.3 ITO amorphous structure PEDOT 4.8 TiO ₂ 5.0 UV-Vis and cyclic 5.2 voltammetry suggest that 6.1 band gap is 3.7 eV 、 HOMO is 8.1 eV 、 8.1 LUMO is 4.4 eV Fig.8 Flat band potentials of ITO/PEDOT/P3HT:PCBM/Al. Use of TiOx layer as optical spacer, K. Lee, et. al., Adv. Mater. 2006, 18, 572
Effects of TiO TiO 2 film thickness on photovoltaic properties on photovoltaic properties Effects of 2 film thickness 0.7 4.0 (a) (b) efficiency (%) 3.0 FF 0.5 2.0 1.0 0.0 0.3 0 10 20 30 0 10 20 30 thickness (nm) thickness (nm) (c) (d) オプティカルスペーサではない 10.0 0.6 9.0 Isc (mA/cm 2 ) 0.5 Voc (V) 8.0 0.4 7.0 0.3 6.0 5.0 0.2 0 10 20 30 0 10 20 30 thickness (nm) thickness (nm) Fig.10 Effect of TiO ₂ on Efficiency (a), FF (b), Voc (c), Isc (d).
Evaluation method of organic solar cell Evaluation method of organic solar cell Series resistance （ Rs ） Simulated sun light of A.M. and parallel resistance 1.5G 100mW/cm² was (Rp) illuminated onto the cell. j ∂ R V R = Rp ≈ ≈ p ＋ ＋ R R R R = ∂ V 0 s s p p j R j j R s − rev p ＋ sc s 1 exp( ) J sc V V T T j sc j D j Rp V D V J max Rp ∂ R V = ≈ dv/dj = Rp Rs p ＋ R R = ∂ j 0 s s j R j V rev p ＋ oc Equivalent circuit of solar cells. 1 exp V V T T V oc V max Voltage (V) Reduce in an internal resistance I-V curve of solar cells. → decrease in Rs → increase in Isc ⋅ P V J = = max max max FF ⋅ ⋅ V J V J oc sc oc sc Improve of carrier selectivity ⋅ ⋅ → increase in Rp P V J FF η = × = × max oc sc 100 100 → increase in Voc P P light light
Rp inclease inclease and and Rs Rs decrease by decrease by TiOx TiOx insertion insertion Rp ① Series resistance （ Rs ） and parallel Reduce in an internal resistance resistance （ Rp ） → decrease in Rs 1400 14 Rp x Rs Improve of carrier selectivity 1200 12 → increase in Rp 1000 10 Rp 800 Rs 8 600 6 dV = R 400 p 4 dJ ( 0 ) 200 2 0 0 dV = R 0 10 20 30 s dJ ( V ) oc Thickness Method of calculating Rs and Rp. Rp and Rs of devices containing TiO ₂ layer with several thicknesses. Insertion of TiO 2 layer induced a marked increase in Rp → Improve of carrier selectivity
Rectification ratio Rectification ratio ② Change in rectification ratio I (-2V) (RR) (RR) = - I (2V) 1 Rectification Ratio 0.1 Curren I (-2V) t 0.01 density 0.001 2V 0.0001 -2V Voltage 0 10 20 30 I (2V) Thickness (nm) Fig.13 The ratio of the current density of devices with several Fig.14 Method of calculating thickness TiO ₂ at ＋ 2V an － 2V in dark. RR. Improvement of rectification by insertion of TiO 2 layer → TiO 2 layer blocks hole carriers.
Conclusions Conclusions Improved cell structure with TiO 2 layer on active layer (P3HT:PCBM) is quite promising. TiO 2 layer acts as a hole blocking layer. Optimal 4% conversion efficiency was obtained with very high fill factor under ambient atmospheric condition without sealing. Isc of the device with TiO 2 decreased only 6% after 100 hour illumination, showing high durability under ambient atmospheric condition. 1.0 12 0.9 10 0.8 2 ) 0.7 Current density(mA/cm 8 0.6 IPCE Eff = 4.05 (%) 0.5 6 =9.72 (mA/cm 2 ) I sc 0.4 V oc = 0.60 (V) 0.3 4 = 0.70 FF 0.2 2 0.1 0.0 0 300 400 500 600 700 800 -0.1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Wave length (nm) Voltage (V) Fig.22 I-V curve of the best cell. Fig.23 IPCE spectrum of the best cell.
□ Preparation of hole transport layer (HTL) using PEDOT:PSS polymer brash Organic Thin Film solar Cell
Rolls of Hole Transporting Layer (HTL) Organic Thin-Film Solar Cells Metal anode (Al) Electron Transporting Layer (ELT) Light Absorption Layer (LAL) Hole Transporting Layer （ HTL) PEDOT:PSS Poly(3,4-ethylenedioxythiophene)-poly(styrensulfonate ） ITO cathode PEDOT/PSS ● PEDOT:PSS films help to planarize the ITO surface ● PEDOT:PSS films appear to make the ITO surface more uniformly electroactive ● PEDOT:PSS layer reduces the electrode surface polarity, making it more compatible with nonpolar components of OPVs ● PEDOT:PSS layer appears to increase the effective work function of the resulting substrate highly rectification (electron blocking) ability (large carrier mobility and/or conductivity), transparency, low resistance at the interface, and so on. The HTL plays a key role in the OPVs.
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