PbS Quantum Dots Based Thin-film Solar Cells

Currently,85%of the world’s energy requirements are fulfilled by exhaustible fossil fuels that have detrimental consequences on human health and the environment.Moreover,the global energy demand is predicted to be doubled by 2050.Facing the energy crisis caused by the exhaustible fossil fuels and the greenhouse effect caused by excessive greenhouse gas emissions,it is imperative to achieve environmental protection and sustainable development of energy issues around the world.As a technology of converting solar light into electricity directly,photovoltaic devices are one of the most important approaches to solve the serious energy and environmental problems.As the leading one for the third generation of solar cells,quantum dot solar cells（QDSCs）have already made a rapid progress in recent years.Currently,the performance of QDSCs has been improved continuously and its power conversion efficiency（PCE）reached to 13.4%,and there is still much room to improve it,showing its both important theoretical investigation and application values.Currently,few understand the physical mechanism of QDSCs and a few reported its underlain physical mechanism,in this thesis,we started from synthesizing Pb S colloidal QDs（CQDs）and its ligand-exchange,and then fabricated and investigated the underlain physical mechanism for the enhanced efficiency of a series of Pb S based thin film solar cells,and we got some interesting conclusions listed as following:（1）We have studied the physical mechanism of lead sulphide（Pb S）based QDSCs by electrochemical impedance spectroscopy（EIS）.We fabricated two kinds of devices Au/Pb S-EDT/Pb S-TBAI/Zn O/ITO（Device A）and Au/Pb S-TBAI/Zn O/ITO（Device B）in which Pb S-TBAI and Pb S-EDT are Pb S CQDs where the original oleic acid（OA）ligands were exchanged with tetrabutylammonium iodide（TBAI）and ethanedithiol（EDT）,respectively.The average PCE of device A and device B are 7.71%and 5.12%,respectively,and the maximum PCE of device A reached to 8.18%.Then,we analyzed the recombination resistance,carrier lifetime,capacitance and conductivity of these two kinds of typical devices by EIS.By insetting the Pb S-EDT layer,parameters such as the recombination resistance,carrier lifetime,capacitance and conductivity of device A are greatly improved,resulting to its improvement of PCE.（2）We have enhanced the device performance of Pb S based QDSCs by post-synthetic annealing.The influence of post-synthetic annealing treatment on device performance of a typical heterojunction solar cell ITO/Zn O/Pb S/Au was studied at different annealing temperatures,and we found its PCE increase from 3.26%to 4.52%after its annealing at140℃,showing 38.6%enhancement.The short circuit-current density(J_SC)increased from14.4 m A/cm² to 20.6 m A/cm² but the open-circuit voltage(V_OC)slightly decreased from 0.47V to 0.43 V.Then,we characterized the device with X-ray diffraction（XRD）,absorption spectrum,EIS and ultraviolet photoelectron spectroscopy（UPS）.The annealing treatment caused the necking phenomenon and the sintering process of the Pb S QDs film,and then,leading to improvement of electronic coupling between each QDs.Also,the Fermi level of Pb S film shows a downward band bending at the interface of Pb S-TBAI/Zn O after the annealing treatment,resulting to the decrement of V_OC.（3）We have improved the efficiency of Pb S QDSCs by using graphene oxide（GO）as the interface modification layer.In order to improve the interface contact between electrode and Pb S QDs film,we fabricated device Au/GO/Pb S-TBAI/Zn O/ITO.After the annealing treatment,the PCE of the solar cells was improved from 4.74%to 5.35%as compared with the reference device Au/Pb S-TBAI/Zn O/ITO.The reason is that the GO played as hole-transporting layer after its annealing treatment,and it can effectively reduce both the defect traps at the interfaces and charge carriers recombination in the bulk,thereby enhancing the device performance.（4）We have improved the efficiency of polymer solar cells by inserting Pb S QDs film as the infrared absorption layer.Based on the band alignment engineering and appropriate ligands exchange strategy,we fabricated organic–inorganic hybrid solar cells ITO/PEDOT:PSS/P3HT/Pb S/PC₆₀BM/Ca/Al.By optimizing the thickness of the Pb S QDs layer to be of 51nm,a PCE of 3.08%was achieved,showing 91%enhancement over that（1.61%）of the heterojunction organic solar cell ITO/P3HT/PCBM/Ca/Al.This is mainly due to the enhancement of J_SC（13.4 m A/cm²）,which is two times higher than that（6.17 m A/cm²）of the device without Pb S QDs film,resulting to the PCE enhancement of organic-inorganic hybrid solar cells.