Quantum Dot-Sensitized Solar Cells Photovoltaic Properties and - PowerPoint PPT Presentation

Quantum Dot-Sensitized Solar Cells ― Photovoltaic Properties and Photoexcited Carrier Dynamics ― T. . Toy oyod oda 1, 1,2 and nd Q. S Shen hen 1, 1,2, 2,3 1 Depa epartment ent o of E Eng ngineer neering ng S Sci cience ence The Unive ivers rsity ity of f Ele lectro tro-Commu mmunic icatio ations Chof ofu, , Tok okyo o 182-8585, 8585, Ja Japa pan 2 CREST, 3 PRE RESTO, Ja Japa pan S n Sci cience ence and nd Techno echnology A Agency ency (JS JST)

Main Research Plan  In order to achieve higher photovoltaic con- version efficiency of quantum dot-sensitized solar cell (QDSSC) , further basic research on 1) nanostructured TiO 2 electrode and 2) sensitizer is important and essential (with characterizations of morphology, structure, optical absorption, charge transfer, energy transfer, recombination processes , and so on).

Advantages of Semiconductor Quantum Dot as Sensitizer ▶ Quantum confinement allows for energy gap tunable across the solar spectrum. ▶ Higher optical absorption resulting from quantum confinement. ▶ Larger intrinsic dipole moment which may lead to rapid charge separation and band alignment. ▶ Inorganic nature. ▶ Possib ibility ility of f multiple exciton generation .

Motivations  Morphologies of TiO 2 electrodes and choices of sensitizers are important factors for sensitized solar cells. → In this study, we prepare inverse opal TiO 2 (& TiO 2 nanotube) electrodes adsorbed with multilayered semiconductor quantum dots of CdS/CdSe .  Relaxation processes in photoexcited carriers are important factors not only for basic studies but applied research in solar cells. → In this study, we characterize the ultrafast photoexcited carrier dynamics of inverse opal TiO 2 electrodes with multilayred semiconductor quantum dots.

Preparation of CdS/CdSe QDs on NC and IO TiO 2 1. Nanostructured TiO 2 electrodes 2. Adsorption with CdS QDs 1) • TiO 2 nanoparticles ( 15 nm ) at 10 ℃ • PEG ( Molecular weight : 500000 ) • Acetylaceton • Pure water 20 mM CdCl 2  Mix and Paste applied 66 mM NH 4 Cl onto FTO glass  Annealed at 450 ºC 140 mM Thiourea Nanostructured 230 mM Ammonia for 30 min TiO 2 electrodes 1) R. Jayakrishnan et al., Semicond. Sci. Technol. 11 (1996) 116. 3. Adsorption with CdSe QDs 2) 4. Passivation with ZnS 3) at 10 ℃ In order to improve its photo-stability 2 cycles 80 mM CdSO 4 120 mM N(CH 2 COONa) 3 80 mM Na 2 SeSO 3 0.1 M Zn(CH 3 COO) 2 0.1 M Na 2 S 2) S. Gorer and G. Hodes, J. Phys. Chem. 98 (1994) 5338. 3) S.M.Yang et al., J. Mater. Chem. 12 (2002) 1459.

Improvement Electrode Structure FTO FTO Disordered Structure Ordered Structure (Nanocrystalline TiO 2 ) (Inverse Opal TiO 2 ) Advantages of inverse opal structure in solar cell application are, (1) Smooth electron transport owing to topological inter- connected material. (2) Better penetration of sensitizer owing to macroporous structure. (3) Enhancement of optical absorption owing to photon localization.

Preparation of Inverse Opal TiO 2 Electrode Template Preparation FTO glass evaporated slowly at 40 o C polystyrene latex colloids self-assembling (0.1 wt% ) latex template 2% TiCl 4 in CH 3 OH hydrolysis 30min calcination and annealing at 450 3 times o C (60 min) self-assembling template heat dropping inverse opal treatment at TiO 2 electrode 80 o C(10 min) L. J. Diguna et. al., Jpn. J. Appl. Phys. 45 (2006) 5563.

SEM Images Inverse Opal (IO) TiO 2 IO TiO 2 adsorbed with CdSe QDs 8 h 24 h Top View Top View 70nm 190nm 500 nm 500 nm 500 nm Chemical adsorption 8h Cross Section Cross Section For longer adsorption,  increase of CdSe QD size and amount. 1.5 µm 2 µm

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Sample ： TiO 2 /CdSe/ZnS BF-STEM Image HAADF-STEM Image TiO 2 CdSe

Photoacoustic Spectra Wav Wavel elen engt gth (nm) s) arb.units) 800 800700 700 600 600 500 500 400 400 300 300 CdS ( bulk ) : Eg= 2.4 eV Bulk CdSe CdSe ( bulk ) : Eg= 1.7 eV 1 sity (ar E 1 ensi PA inten Norm. PA TiO 2 0.1 0. CdS (0.5h) CdSe (6h) No CdS (0.5h)/CdSe (6h) 1.6 1. 2.0 2. 2. 2.4 2. 2.8 3.2 3. 3. 3.6 4.0 4. 4.4 4. Effective mass apprpximation 1) Ph Photon en ener ergy gy (eV) eV) π 2 2 h ∆ = − = = E E E D a ( 2 ) Estimation of average size of each g 1 µ 2 a 2 quantum dots 1 1 1 m = + : electron effective mass e m µ m m : hole effective mass CdS QDs : 4.4 nm (0.5 h) h e h : first excited energy E 1 CdSe QDs : 7.0 nm (6 h) 1) L. E. Brus, J. Chem. Phys. 80 (1984) 4403.

IPCE Spectra Wav Wavel elen engt gth (nm) 800700 800 700 600 600 500 500 400 400 300 300 TiO 2 80 80 CdS CdS ( (0. 0.5h) 5h) CdS CdSe ( (6h) 6h) CdS CdS ( (0. 0.5h) 5h)/CdS /CdSe ( (6h) 6h) 60 60 PCE (%) IPCE 40 40 20 20 0 1.5 1. 2. 2.0 2. 2.5 3. 3.0 3. 3.5 4.0 4. 4.5 4. 5.0 5. Photon en Ph ener ergy gy (eV) eV) Adsoption of CdS QDs at 0.5 hours and CdSe QDs at 6 hours (multi-layered) shows higher IPCE value than CdSe QDs (single-layered) at 6 hours. 55 ％ → 75 ％

Photovol voltai aic P c Perf rforma rmance ce Short circuit current density(J sc ) Photovoltaic conversion efficiency ( η ) max power of solar cell Open circuit voltage (V oc ) η = X 100% power of input light max power Fill factor (FF) = Jsc x Voc x FF ideal power X 100% = power of input light J sc Sandwich Configuration Ideal Power Electrolyte TiO 2 /CdS/CdSe (polysulfide) Current density Max Power brass Input light (100 V oc mW/cm 2 ) Voltage FTO spacer Cu 2 S

J-V Characteristics cm 2 ) ity (mA/cm 12 12 TiO iO 2 ( (IO)/CdS /CdS ( (0. 0.5h) 5h)/CdS /CdSe ( (6h) 6h) TiO iO 2 ( (IO) /CdS /CdSe ( (6h) 6h) 10 10 t Density 8 6 current 4 FTO tocu 2 Photo 0 0. 0.0 0.2 0. 0.4 0. 0.6 0. 0. 0.8 Vo Voltage age (V) V) η Sample J sc V FF oc (mA/cm 2 ) (V) (%) CdSe (6h) 7.7 0.72 0.53 2.9 CdS (0.5h)/CdSe(6h) 10.3 0.75 0.49 3.8

CdS/CdSe QDs on TiO 2 nanotube electrodes 8 2 ) A/cm C dS( 3h) /C dSe( 3h) 7 C dSe( 3h) Photocur r ent Densi ty ( m 6 C dSe( 3h) /C dS( 3h) 5 -e -e 4 3 2 Ti Ti 1 0 0. 0 0. 1 0. 2 0. 3 0. 4 0. 5 0. 6 Vol tage ( V) CdS/CdSe CdSe(3h) CdSe/CdS

Improved Transient Grating (TG) Technique Detailed understanding of photoexcited carrier dynamics is required to understand and improve photovoltaic properties of solar cells, that is satisfied using ultrafast transient grating technique. Titanium/sapphire laser wavelength: 775 nm; pulse width: 150 fs; intensity: 2 µ J/pulse Optical delay line Probe beam OPG/OPA Advantages  Easy and compact. Chopper  High sensitivity. Grating → Measurements under Sample low pump intensity.  Suitable for rough surfaces or optically scattering samples. K. Katayama et al., Appl. Phys. Lett . 82 (2003) Lock-in Amplifier 2775.

TG response in IO TiO 2 /CdSe electrode 1) Norm.TG signal (arb. units) 1 In air ① relaxation ② （ electron transfer, trap ） ② CdSe QD － CB LUMO ＋ IO TiO 2 HOMO relaxation ① VB TiO 2 (IO)/CdSe (6h) (hole trap) 0.1 0 20 40 60 80 100 Time (ps) − τ − τ = + + t t y t A e / A e / y ( ) 1 2 1 2 0 ① ② ① : Relaxation in fast process → hole trap ② : Relaxation in slow process → electron transfer and/or trap 1) Q. Shen, Y. Ayuzawa, K. Katayama, T. Toyoda, Appl . Phys . Lett. 97 (2010) 263113.

TG response in IO TiO 2 /CdS/CdSe electrode Norm.TG signal (arb. units) 1 ① In air relaxation ② ② （ electron transfer, trap ） CdSe QD － LUMO CdS QD CB ＋ HOMO IO TiO 2 relaxation ① VB TiO 2 (IO)/CdSe (6h) （ hole trap ） 0.1 TiO 2 (IO)/CdS (0.5h)/CdSe (6h) 0 20 40 60 80 100 Time (ps) Faster relaxation process can be observed in relaxation ② on TiO 2 /CdS/CdSe electrode than that of CdSe. τ 1 (ps) τ 2 (ps) Sample − τ − τ = + + t t y t A e A e y / / ( ) 1 2 1 2 0 98 ± 10 8 ± 1 TiO 2 (IO)/CdSe (6 h) ① ② 9 ± 1 67 ± 7 TiO 2 (IO)/CdS (0.5 h)/CdSe(6 h)

Velocity constant in electron relaxation processes IO TiO 2 /CdS/CdSe electrode CdSe QD － ・ decrease in relaxation time τ 2 in CdSe QDs. LUMO CB CdS QD HOMO k 2 (10 10 s -1 ) sample τ 2 (ps) IOTiO 2 TiO 2 /CdSe 98 1.0 50% increase VB TiO 2 /CdS/CdSe 67 1.5 1 k 2 : electron velocity constant = = + k k k k τ r et 2 2 k ｒ : velocity constant (trap) 2 k et : velocity constant (transfer) increase in k et

Summary (Japan)  Quantum confinement effect by multilayerd CdS/CdSe quantum dot on inverse opal TiO 2 electrode can be observed by photoacoustic spectroscopy.  Photosensitization by multilayered CdS/CdSe quantum dot on inverse opal TiO 2 electrode is realized and the suitable adsorption time is existed for the photocurrent.  The maximum photovoltaic conversion efficiency of 3.8% can be achieved on inverse opal TiO 2 electrode adsorbed with multilayered CdS/CdSe quantum dots, having the correlation with ultrafast carrier dynamics (faster electron velocity constant in CdS/CdSe than in CdSe).