Synthesis and structural optimization of functional photosensitizers for dye sensitized solar cells

M.S Thesis Synthesis and Structural Optimization of Functional Photosensitizers for Dye-sensitized Solar Cells Graduate School of Yeungnam University Department of Chemical Engineering Major in Chemical Engineering LE THI THUY Advisor: Professor Jae Hong Kim February 2018 M.S Thesis Synthesis and Structural Optimization of Functional Photosensitizers for Dye-sensitized Solar Cells Advisor: Professor Jae Hong Kim Presented as M.S Thesis February 2018 Graduate School of Yeungnam University Department of Chemical Engineering Major in Chemical Engineering LE THI THUY Le Thi Thuy’s M.S Thesis is approved Committee member Ahn, Kwang-Soon signature Committee member Kim, Jae Hong signature Committee member Han, Yoon Soo signature February 2018 Graduate School of Yeungnam University ACKNOWLEDGEMENT Firstly, I would like to express my deepest gratitude to my advisor, Professor Jae Hong Kim for his invaluable advice, motivation and never-ending encouragement throughout my studies His guidance always challenged me knowledgeably and provided a perfect atmosphere that I needed to grow as a researcher I strongly believe that without the help from Prof Kim I could not have enough will to live in Korea, not even mentioning about completing my thesis today Besides my supervisor, I would like to appreciate other members of M.S thesis defense committee, Prof Kwang-Soon Ahn and Prof Yoon Soo Han for their contribution of time and understanding comments to improve the quality of this M.S thesis Next, I am extremely thankful to Dr Suresh for his backing and support me in all time of research And I also would like to say thank you to members of LOFM for their assistance, support and friendship during the time I stayed in Korea Without them, I would face a lot of troubles in my experiments Last but not least, I would like to thank my family for their unconditional love, reassurance and encouragement through all my life especially during the time I live overseas February 2018 Le Thi Thuy ABSTRACT Since Gratzel and O'Regan reported high solar-cell performances for dyesensitized solar cells (DSSCs) based on polypyridyl ruthenium (II) complex dyes adsorbed on a nanocrystalline n-type semiconductor TiO2 electrode in 1991, DSSCs have expected extensive consideration as a new generation of maintainable photovoltaic devices because of their high incident-solar-light-to-electricity conversion efficiency, colorful and attractive natures, and low cost of production A characteristic DSSCs is created with a dye-absorbed wide band gap oxide semiconductor electrode, such as TiO2, ZnO, or NiO; a liquid electrolyte containing I-/I3- redox couples; and a platinum-coated counter electrode In the working electrode of the device, the dye sensitizer is very important factor; its function is light harvesting and electronic transition Up to now, the sensitizer can be divided into two general classes: the metal complex sensitizers and the metal-free organic sensitizers The DSSCs using the metal-complex sensitizer such as N3, N179 or black dye have been achieved high efficiency over 13% However, the metal-complex sensitizers have some difficulties such as limited resource, low molar extinction coefficient (ε) and high cost, which will limit their applications in DSSCs of large-scale To get rid of these problems, the focus has been shifted to metal-free organic sensitizers owing to compared with metal complexes, metal-free organic dyes have also attracted significant attention caused by the benefits of easier preparation and purification, higher structural flexibility, environmental kindliness and prevention of noble metals This thesis began with the series of organic sensitizers triphenylamine-based metal-free organic dyes (D1-D3) with different electron acceptors, such as 2cyanoacetic acid, rhodanine-3-acetic acid or 5-oxo-1-phenyl-2-pyrazolin-3- carboxylic acid, connected through anthracene and thiophene π-spacers were synthesized and applied for DSSCs The photophysical and electrochemical properties of these dyes were investigated and their performance as sensitizers in DSSCs was measured Electrochemical studies showed that the LUMO energy levels can be tuned by changing the anchoring groups with different electron withdrawing ability The power conversion efficiencies of the DSSCs based on D1-D3 decrease as the electron withdrawing ability of their anchoring groups increase in the order of D1 < D2 < D3 and D1-based device showed the higher power conversion efficiency of 1.27% In the study on the phenothiazine as an electron donor, three novel dyes with single donor-acceptor (T1), double donor-acceptor (T2) and multiple anchoring group (T3) organic dyes have been synthesized to investigate the influence of the hexyloxy benzene unit between the two chromophores and the different number of anchoring groups on the performance of DSSCs The T3 with double branches phenothiazine bases device shows a broader and higher IPCE as well as photocurrent density (Jsc) with an improved photovoltage (Voc) In contrast, T1 with single branch presents a reasonably low IPCE within the whole spectral region, along with Jsc and Voc of 10.51mA/cm2 and 0.67 V, respectively The higher Jsc and Voc gained with the device based on T2 dyes and show a highest conversion efficiency of 5.02% TABLE OF CONTENTS Chapter 1: INTRODUCTION 1.1 Background 1.2 Introduction of dye-sensitized solar cells (DSSCs) 1.2.1 Operating principle of dye-sensitized solar cells 1.2.2 Fabrication of DSSCs 1.3 Scope of work and research objective Chapter 2: CHARACTERIZATION OF DYE-SENSITIZED SOLAR CELLS 2.1 Key component of dye-sensitized solar cells 2.1.1 Transparent conducting glass 10 2.1.2 TiO2 as the photoelectrode 10 2.1.3 Dye Sensitizer 11 2.1.4 Electrolyte 13 2.1.5 Counter electrode 15 2.2 Key efficiency parameters of dye-sensitized solar cells 16 2.2.1 Incident photon to current conversion efficiency (IPCE) 16 2.2.2 Current-voltage characteristics (J/V curves) 17 2.2.3 Electrochemical impedance spectroscopy (EIS) of DSSCs 19 Chapter 3: EFFECT OF ANCHORING GROUP IN ANTHRACENE/THIOPHENE-BRIDGED TRIPHENYLAMINE BASED ORGANIC DYES FOR DYE-SENSITIZED 22 3.1 Introduction 22 3.2 Experiment details 24 3.2.1 Materials and instruments 24 3.2.2 Synthesis 25 3.2.3 Assembly and Characterization of the DSSCs 29 3.3 Results and discussion 30 3.3.1 Design and synthesis 30 3.3.2 Optical properties 31 3.3.3 Electrochemcal properties 34 3.3.4 Photovoltaic properties 35 3.4 Conclusion 39 Chapter 4: SYNTHESIS AND PHOTOVOLTAIC PERFORMANCE OF NOVEL PHENOTHIAZINE SENSITIZERS CONTAINING HEXYLOXY BENZENE UNIT AND MUTI -ACCEPTOR FOR DYE-SENSITIZED SOLAR CELLS 41 4.1 Introduction 41 4.2 Experimental 44 4.2.1 Synthesis 44 4.2.2 Instrumental Analysis 50 4.2.3 Assembly and Characterization of the DSSCs 50 4.3 Results and discussion 52 4.3.1 Absorption properties in solution and on TiO2 films 52 4.3.2 Electrochemcal properties 54 4.3.3 Photovoltaic performances of the DSSCs 55 4.4 Conclusions 63 Chapter 5: CONCLUSION 65 REFERENTS 67 Figure 4.8 Electrochemical impedance spectra measured under illuminated condition As shown in Table 4.3, there was little difference in the R2 values because the same counter electrode (Pt) and electrolyte was used On the other hand, there was a substantial difference in the R3 values, which indicates that the charge transfer behavior between TiO2 and the electrolyte is changed significantly, which is due probable to surface modification with different number of donor and acceptor groups of dyes R3 values of 17.94, 17.35, 17.70, 19.96 (Ω) were obtained for the T1, T3, T4 based devices, respectively 59 Sample R1 (Ω) R2 (Ω) R3 (Ω) Ws(Ω) T1 6.543 5.647 17.94 3.754 T2 7.336 5.87 17.35 3.735 T3 6.969 6.569 17.7 6.924 Table 4.3 EIS analysis of the DSSCs under illumination condition Figure 4.9 shows the Nyquist plots for the DSSCs based on the dyes, and Table 4.4 lists the corresponding parameters under dark condition T3 T2 T1 -18 Z"(ohm) -15 -12 -9 -6 -3 10 20 30 40 50 60 Z' (ohm) Figure 4.9 Electrochemical impedance spectra measured in the dark for DSSCs sensitized by T1-T3 Under dark impedance analysis, recombination resistance (R3) values of 37.05, 43.21, 28.11 and 20.55 Ω were obtained for the T1, T2 and T3 based devices, respectively 60 Sample R1(Ω) R2 (Ω) R3 (Ω) Ws(Ω) T1 7.061 5.855 37.05 3.813 T3 6.892 6.005 43.21 3.851 T4 6.985 6.716 28.14 8.615 Table 4.4 EIS analysis of the DSSCs under dark condition To confirm the electron life-times in the DSSCs, we also measured opencircuit photovoltage decay (OCVD) curves The OCVD technique is a method of monitoring the subsequent decay of photovoltage Voc after turning off the illumination in a steady state Figure 4.10 shows that Voc decay curves of DSSCs with organic phenothiazine based photosensitizers were recorded during relaxation from an illuminated quasi-equilibrium state to the dark state 61 -0.7 T1 T2 T3 Voltage (V) -0.6 -0.5 -0.4 -0.3 10 15 Time (s) Figure 4.10 Voc decay curves of DSSCs with T1- T3 based organic photosensitizers Figure 4.11 shows electron life-times calculated from the OCVD curves, according to the following equation: 𝜏𝑒 = − 𝑘𝐵 𝑇 𝑑𝑉𝑜𝑐 −1 𝑒 { 𝑑𝑡 } (4.1) Where kBT is the thermal energy, e is the positive elementary charge, and dVoc/dt is the first derivative of the open-circuit voltage The photovoltaic decay rate is directly related to electron life time (𝜏𝑒 ) because as the illumination of the DSSC at the open circuit is interrupted, excess electrons are diminished through recombination [54] The T2 dye exhibited much longer electron life times compared to T1 and T3 which is in good agreement with the results obtained from Figure 4.11 62 Lifetime (s) 100 T1 T2 T3 10 0.1 0.3 0.4 0.5 0.6 0.7 Voltage (V) Figure 4.11 The electron life-time derived as a function of Voc These results can suggest that the alkoxyl group connecting double donoracceptor chromospheres in one dye molecule significantly suppressed the recombination rate, resulting in the much longer electron life time Therefore, T2 provided much faster electron transport and significantly extended electron lifetime, giving rise to the significantly improved electron collection efficiency because the value of electron collection efficiency is determined by using two factors such as electron transport and recombination rates 4.4 Conclusions Three novel dyes (T1-T3) based on adjustments of phenothiazine with single and double donor-acceptor, introduced of the alkoxy group present in molecular and different number of anchoring groups were designed and synthesized The dye T3 with double branches phenothiazine and multiple acceptor was found higher to the 63 dye T1 with single branch in deference to effective suppression of the charge recombination, increased Voc and electron lifetime Furthermore, the dye T2 with double branches of phenothiazine with a hexyloxy benzene unit prominently displayed the best results in all parameters with Jsc of 10.81 mA/cm2, Voc of 0.68 V, FF of 68.07% and ɳ of 5.02% This work provides valuable and essential information for developing and designing new organic dyes for highly efficient DSSCs Further structural modifications of the dye T2 to obtain even better power conversion efficiency of DSSCs performance is in development 64 Chapter 5: CONCLUSION As one of the crucial parts in DSSCs, the sensitizer is one of the key components of these cells for high power conversion efficiency It is still challenging to search for optimum sensitizers which are capable of absorbing the whole region of visible light to get the high power conversion efficiency Promising strategies to gain higher molecular absorptivity of sensitizers, metal-free organic dyes-based ones in this case The objective of this work is to improve the photovoltaic performance of DSSCs using different types of sensitizer In the chapter 3, three triphenylamine-based dyes (D1-D3) with different electron anchoring/acceptors, 2-cyanoacetic acid (CA), rhodanine-3-acetic acid (RA) and 5-oxo-1-phenyl-2-pyrazolin-3-carboxylic acid (OPCA), were designed and synthesized as sensitizers for DSSCs The effects of the different electron acceptors on the photophysical, electrochemical and photovoltaic properties were examined The dye with the OPCA acceptor unit showed the longest maximum absorption wavelength and the dye with the CA acceptor unit showed highest molar absorption coefficient The overall conversion efficiency of the DSSC based on dye with the CA unit 1.27% was higher than those of the DSSCs based on the dye with the RA unit 0.91% and the dye with OPCA unit 0.70% Cyanoacetic acid proved to be the best electron acceptor in the D-π-A dye for improving the high cell conversion efficiency The phenothiazine-based dye containing electron-rich nitrogen and sulfur heteroatoms in a heterocyclic structure with high electron-donating ability, and its 65 non-planar butterfly conformation can sufficiently inhibit molecular aggregation and the formation of intermolecular excimers Meanwhile, the N(10)-substituent on can further enhance the charge separation at the oxide solution interface The structural features of phenothiazine-based dye make it a promising type of sensitizers for DSSCs Thus, in the chapter 4, a series of new organic dyes (T1-T3) based on the phenothiazine unit were synthesized, in which a hexyloxy benzene unit as a π-bridge to connect double phenothiazine as a donor, and the multiple anchoring groups was introduced The T2 based device show a bester conversion efficiency compare with T1 and T3 of 5.02% A novel intramolecular sensitization strategy to enhance the optical response and, ultimately, the photovoltaic efficiency of DSSCs was demonstrated In particular, we believe that this new strategy will facilitate the search for new efficient sensitizers 66 REFERENTS [1] BP, Statistical Review of World Energy, (2006) [2] S Solomon, D Qin, M Manning, Z Chen, M Marquis, K.B Averyt, 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이후 저렴한 생산비용에 비해 높은 광전변환효율의 장점으로 많은 관심을 받아왔다 염료감응태양전지는 TiO2 로 구성된 나노입자, 태양광 흡수율을 높이는 염료, 전해질, 투명전극 등으로 구성된 광합성원리를 이용한 유기태양전지의 한 종류이다 염료감응태양전지는 기존의 일반화 되어 있는 실리콘방식 태양전지와는 달리 자원적인 제약이 적고 대기압하에서, 프린팅 방식 등을 이용하여 제조할 수 있는 장점이 있어 고가의 설비투자를 줄이고 대량 제조가 용이한 것이 장점이다 또한 태양광의 입사각도, 날씨변화 등에 대한 전기발전효율이 우수하고 실내에서의 간접광에 의해서도 발전이 가능한 특징을 가지고 있다 그리고 유기염료가 흡착되므로 칼라를 구현할 수 있어 다양한 칼라 디자인을 접목할 수 72 있는 등 기존의 실리콘 방식에서는 해결될 수 없는 많은 장점들을 가지고 있어 광전변환소자와 모바일 기기 Application 응용분야에 대한 연구노력이 계속되어 오고 있고 광전변환소자기술의 원천특허 일부가 ‘08 년 10 월 만료되면서 상용화 개발에 대한 연구가 더욱 활발해질 것으로 기대되고 있다 본 학위기는 염료감응태양전지용 유기염료의 합성에 대한 연구로 기존의 루테늄계 염료를 대신할 수 있는 다양한 유기 소재를 합성하여 광감응제로써의 적용 가능성을 타진한 결과이다 Triphenyl amine 계 전자주게와 다양한 전자받게 구조를 π-spacers 를 통하여 연결하여 분자 내 전자주게-받게 시스템을 적용한 intramolecular chargetransfer chromophore 를 설계하고 합성하였다 전자 주게 및 받게의 분자구조를 최적화하여 기능성 유기염료의 HOMO 및 LUMO 에너지 레벨을 조정하였고, 이를 통하여 다양한 광흡수영역을 가지는 유기염료를 합성할 수 있었다 또한, phenothiazine 계 전자주게 구조를 적용하여 유기염료를 합성하였으며, phenothiazine 구조는 두 벤젠환을 전자주게 특성이 강한 S 와 N 으로 연결한 구조로 전자 주게 특성 및 분자 회합특성을 제어할 수 있어 이미 료감응태양전지의 유기염료로 많은 연구가 진행된 분자이다 본 연구에서는 이러한 phenothiazine 계 염료를 alkyl 기로 연결하여 분자 회합 특성을 선천적으로 제어하여 이에 따른 광전변환 거동을 확인하였다 탄소 수 개의 alkyl 기에서 가장 우수한 광전변환 효율을 확인할 수 있었다 따라서 본 연구의 결과로 다양한 기능성의 유기염료를 합성하여 루테늄계 염료를 대체할 수 있는 염료감응태양전지 광감응제를 합성할 수 있었으며, 광감응제의 분자간 회합특성의 제어는 소자의 광전변환 효율에 직접적인 영향이 있음을 확인할 수 있었다 73 ... Thesis Synthesis and Structural Optimization of Functional Photosensitizers for Dye- sensitized Solar Cells Advisor: Professor Jae Hong Kim Presented as M.S Thesis February 2018 Graduate School of. .. 1.2 Introduction of dye- sensitized solar cells (DSSCs) 1.2.1 Operating principle of dye- sensitized solar cells 1.2.2 Fabrication of DSSCs 1.3 Scope of work and research objective... stability under standard reporting conditions Chapter 2: CHARACTERIZATION OF DYE- SENSITIZED SOLAR CELLS 2.1 Key component of dye- sensitized solar cells The DSSCs is composed of a photoanode and a photoinert