Effects of passivation treatment on performance of CdS/CdSe quantum-dot co-sensitized solar cells

Carrier recombination can greatly reduce the efficiencies of quantum-dot sensitized solar cells (QDSSCs). This work aims to find a general preparation route to reduce carrier recombination in QDSSCs. The effects of a series of passivation treatments on CdS/CdSe quantum-dot (QD) co-sensitized solar cells are investigated. The QDs were synthesized on a nanoporous TiO2 electrode by the successive ionic layer adsorption and reaction processes. The different types of treatment included a blocking layer, a fluoride-ion coating, a ZnS coating, annealing, a TiO2 scattering layer and an Au counterelectrode. The power conversion efficiency was observed to become three times larger after treatment. The effectiveness of each treatment method is as follows in descending order: blocking layer≅TiO2 scattering layer>Au counterelectrode>F− ions and ZnS coatings>annealing. The best cell yields a current density of 14.6 mA/cm2 and a respectable power conversion efficiency of η=3.11% under AM1.5 sun. The passivation proceduremakes a useful general guide for researchers for the preparation ofQDSSCs

Synthesis, optical and photovoltaic properties of silver chalcogenides-Ag2S and Ag2Se quantum dots as sensitizers for solar cells application

We present a new photosensitizer - Ag2S quantum dots (QDs) - for solar cells. The QDs were grown by the successive ionic layer adsorption and reaction deposition method. The assembled Ag2S-QD solar cells yield a best power conversion efficiency of 1.70% and a short-circuit current of 1.54 mA/cm2 under 10.8% sun. The solar cells have a maximal external quantum efficiency (EQE) of 50% at λ=530 nm and an average EQE of ~ 42% over the spectral range of 400-1000 nm. For the family of silver chalcogenide system-Ag2Se quantum dots (QDs), the external quantum efficiency (EQE) spectrum of the assembled cells covers the entire solar power spectrum of 350-2500 nm with an average EQE of ~ 80% in the short-wavelength region (350-800 nm) and 56% over entire solar spectrum. The effective photovoltaic range of Ag2S and Ag2Se were ~ 2-4 and 7-14 times, respectively broader than that of the cadmium calcogenide system—CdS and CdSe. The photocurrent that Ag2Se generates is four times higher than that of N3 dye. The best solar cell yields power conversion efficiencies of 1.76% and 3.12% under 99.4% and 9.7% sun, respectively. We also have demonstrated of Ag2S/Ag2Se co-sensitized solar cells with polysulfide redox couple. Our best efficiency at one sun is 1.27% featuring CuS counterelectrode, which is higher than single QDs under the same kind of electrolyte and an average EQE entire solar spectrum ~ 68%. A higher photocurrent than that of single QDs can be generated from this double-layered QDs is almost five times compared with N3 dye. The results show that silver chalcogenide element can be used as a highly efficient broadband sensitizer for solar cells.我們介紹一種新有機染料光敏化劑，硫化銀量子點太陽能電池。藉由連續的離子層吸收與反應沉積法成長量子點(QDs)。組裝硫化銀量子點太陽能電池產生最佳功率轉換效率1.70%, 短路電流1.54 mA/cm2在10.8%太陽光照下。在光譜400-1000 nm的範圍間，波長530 nm且平均外部量子效率42%太陽能電池具有最大的外部量子效率(EQE) 50%。銀硫族化合物系統族中的硒化銀(Ag2Se)量子點所組裝的太陽能電池其外部量子效率光譜涵蓋所有太陽功率光譜350-2500 nm,具有平均EQE 80%, 在短波長區間中(350-800 nm)有56%是在所有太陽光譜之上。硫化銀和硒化銀的有效光伏範圍分別是鎘硫族化合物系中硫化鎘和硒化鎘的2-4倍及7-14倍相較下更為寬廣。硒化銀產生的光電流比N3染料高出4倍。最佳太陽能電池產生功率轉換效率達1.76% 和3.12% 分別在太陽照度99.4% 和9.7%. 我們也已証明使用硫化銀和硒化銀做為共敏化劑與聚硫化物做為氧化還原結合所做的太陽能電池，在一日照下我們得到最佳的效率是1.27%並以硫化銅做為輔助電極，在完全太陽光譜相同種類的電解質和平均EQE是比單層量子點高出68%。在這種雙層量子點結構中比單層量子點太陽能電池有更高的光電流，與N3染料相比較幾乎是高出5倍。這一結果顯示銀硫族化合物元素能被使用在太陽能電池使其為更高效率寬頻帶染敏化劑。Acknowledgment..................................................................................................................i Abstract (Chinese)...............................................................................................................iii Abstract (English)................................................................................................................iv Contents...............................................................................................................................v List of Tables.......................................................................................................................ix List of Illustrations..............................................................................................................xi Chapter 1 Introduction 1.1 Background and development of Solar Cells……………………...…………..1 1.2 Motivation………………………………………………………...………….13 Chapter 2 Theoretical Background 2.1 Properties of Nanoscale materials……………………………...…...………..13 2.2 Physical properties of titanium dioxide (TiO2)……………………...........….17 2.3 Physical properties of silver chalcoginide systems………………….....….....20 2.3.1 Silver sulfide (Ag2S).............................................................................20 2.3.2 Silver selenide (Ag2Se)..........................................................................22 2.4 Electrolyte solution………………………………………………….….....…23 2.4.1 Polysulfide electrolyte…………………………………………...……23 2.4.2 Polyiodide electrolyte……………………………………………........24 2.5 Counter electrode………………………………………………….………....25 2.6 Air mass……………………………………………………………….....…..26 2.7 Ultra-violet Photoelectron Spectroscopy (UPS)…………………………..28 Chapter 3 Experimental details 3.1 Chemicals and Substrates for QDSCs………………………………………..30 3.2 Instrument……………………………………………………………………31 3.3 Experimental steps…………………………………………………………...32 3.3.1 Substrate……………………………………………………………….32 3.3.2 Preparation of TiO2 photoelectrodes…………………………………..33 3.3.3 Synthesis of Ag2S QDs………………………………………………..35 3.3.4 Synthesis of Ag2Se QDs………………………………………………36 3.3.5 Observation of structural characteristics of QDs……………………...37 3.3.6 Assembly of Ag2S and Ag2Se QD solar cells…………………………37 3.4 Optical analysis and photovoltaic measurements……………………………40 3.4.1 Absorption spectra measurement……………………………………..40 3.4.2 Measurement on the efficiency of solar cells………………………….42 3.4.3 Calculation of the efficiency of solar cells…………………………….44 3.4.4 External quantum efficiency measurement……………………………46 3.4.5 Ultra-violet Photoelectron Spectroscopy (UPS)…………………47 Chapter 4 Results and discussion 4.1 Ag2S QDSC system………………………………………………………….48 4.1.1 Optical absorption spectra of Ag2S QD-loaded TiO2..........................48 4.1.2 Morphology characteristics of Ag2S QDs…………………………….49 4.1.3 Photovoltaic measurement of Ag2S QD-loaded TiO2 photoelectrodes………………………………………………………..50 4.1.3.1 Optimal dipping time and solvent ratio……………………….50 4.1.3.2 Optimal SILAR cycle…………………………………………53 4.1.3.3 External Quantum efficiency (EQE) of Ag2S QD- loaded TiO2 photoelectrodes………………………….………55 4.1.4 Effect of ZnS coating………………………………………………….56 4.1.5 Measurement on power dependence of the Ag2S QD-coated TiO2 photoelectrodes……………………………..58 4.2 Ag2Se QDSC system…………………………………………………………59 4.2.1 Optical absorption spectra of Ag2Se QD-loaded TiO2………………..59 4.2.2 Structural characterization of the Ag2Se QDs………………………...61 4.2.3 Photovoltaic characteristics of Ag2Se QDs…………………………...63 4.2.3.1 Optimal SILAR cycle…………………………………………63 4.2.3.2 Effect of Ti-iP and ZnS coating……………………………..64 4.2.3.3 Comparison of the iodide/triiodide and the polysulfide electrolytes…………………………………...66 4.2.4 External Quantum Efficiency (EQE) of Ag2Se QD-loaded TiO2 photoelectrodes…………………………………………………….68 4.2.5 Measurement on power dependence of the Ag2Se QD- coated TiO2 photoelectrodes………………………………………….72 4.3 Ag2S/Ag2Se double-layered QDSC system…………………………………73 4.3.1 Optical absorption spectra of Ag2S/Ag2Se QDs co-sensitized TiO2 photoelectrodes………………………………………………….73 4.3.2 Structural characterization of Ag2S/Ag2Se QDs……………………...74 4.3.3 Photovoltaic characteristics of Ag2S/Ag2Se QDs co-sensitized solar cells……………………………………………………………...76 4.3.4 External Quantum Efficiency (EQE) of Ag2S/Ag2Se QD -loaded TiO2 photoelectrode………………………………………..80 4.4 Determination of energy diagram from UPS spectra………………………...81 Chapter 5 Conclusions 5.1 Conclusions…………………………………………………………………..85 5.2 Further works and suggestions……………………………………………….86 References......................................................................................................................89 Appendix A A.1 JCPDS file of Ag2S and Ag2Se……………………………………………..97 A.1.1 JCPDS file of monoclinic α-phase structured-silver sulfide (Ag2S) number 11-0688……………………………………………….97 A.1.2 JCPDS file of orthorhombic structured-silver selenide (Ag2Se) Number 71-2410………………………………………………………98 Appendix B B.1 International publications……………………………………………………9

ZnO-Nanorod Dye-Sensitized Solar Cells: New Structure without a Transparent Conducting Oxide Layer

Conventional nanorod-based dye-sensitized solar cells (DSSCs) are fabricated by growing nanorods on top of a transparent conducting oxide (TCO, typically fluorine-doped tin oxide—FTO). The heterogeneous interface between the nanorod and TCO forms a source for carrier scattering. This work reports on a new DSSC architecture without a TCO layer. The TCO-less structure consists of ZnO nanorods grown on top of a ZnO film. The ZnO film replaced FTO as the TCO layer and the ZnO nanorods served as the photoanode. The ZnO nanorod/film structure was grown by two methods: (1) one-step chemical vapor deposition (CVD) (2) two-step chemical bath deposition (CBD). The thicknesses of the nanorods/film grown by CVD is more uniform than that by CBD. We demonstrate that the TCO-less DSSC structure can operate properly as solar cells. The new DSSCs yield the best short-current density of 3.96 mA/cm2 and a power conversion efficiency of 0.73% under 85 mW/cm2 of simulated solar illumination. The open-circuit voltage of 0.80 V is markedly higher than that from conventional ZnO DSSCs