In Situ Tem Investigation On Quantum Dots Solar Cell

Quantum dot?QD?photovoltaic devices are promising for application in solar energy conversion owing to their facile low cost synthesis method,tunable band gap,and high theoretical power conversion efficiency?PCE?.However,the experimentally observed PCE values thus remain far lower than the theoretical value,which is detrimental to the development and application of QD solar cell.The initial impetus for higher efficiency compels us to explore the relationship between interface and crystal structure and photocurrent which has,however,been questioned limited by present technology and experiment.Furthermore,the structure change of photoanode and reaction process at its interface under light irradiation remains elusive,especially at the atomic level.In this thesis,we developed a novel type of photoelectric TEM holder that can realize imaging at atomic level,and investigated the interface,crystal orientation and deformation,and crystal structure evolution on the performance of QD solar cell through in situ TEM method.Our work provides insights on the mechanism of PCE from atomic level,and offers guidelines and technical supports to design QD solar cell devices.The main achievements are summarized as follows:1.Development of novel photoelectric TEM holder that enables picoampere-precision current measurement and TEM imaging at the atomic scale simultaneously.?1?We introduced optical field with intensity and wavelength adjustment into TEM and carefully designed a current noise shielding system that enables picoampere-precision current measurement.?2?By designing ultra-high output voltage,we obtained stable TEM imaging at the atomic scale.Using the high-precision photoelectric holder,we measured the order of magnitude of picoampere photocurrent from a prototype device of quantum dots solar cell assembled inside TEM.Our results confirm that the novel holder can realize simultaneous atomic-level visualization and high precision photocurrent measurements on photoelectric devices.2.The effect of interface with electrode materials on the performance of QD solar cells has been investigated.?1?A single TiO₂-nanowire/CdSe-QD heterojunction solar cell device?QDHSC?was constructed using the custom-designed photoelectric TEM holder.We utilized a mobile metallic counter-electrode,which can move back and forth to attach or detach from the QD coated NW,to precisely tune the interface area and then measured the photocurrent of different interface area.The results show that the photocurrent decreases with the increase of interface defects.The measured PCE of the QDHSC reaches an impressive value of 19.8%,which is obtained by optimizing interfaces based on a statistical analysis of results from multiple experiments in the“nanolab”built in a TEM.?2?Based on optical antenna effect,we calculated wavelength dependence of optical absorption efficiency Q_absbs and the results show that Q_abs is beyond 100%at the certain wavelength.Additionally,we further analyzed the effect of interface defect density on the power conversion efficiency quantitatively.Theoretical simulation has shown that the ultra-high conversion efficiency achieved in the QDHSC results from nanostructure with resonance absorption and a much smaller number of interfacial defects.3.The relationship between crystal structure electrode materials and photocurrent density has been probed.?1?By combing of high-resolution transmission electron microscopy?HRTEM?at atomic level and current monitoring pico-ampere level,we find that maximum short photocurrent density of3.6*10⁶mA/cm² can be generated when photogenerated electrons transport along the layers of TiO₂?101?plane.?2?From the recorded photocurrent and dynamic HRTEM images,the photocurrent increases with the decrease of the lattice spacing of TiO₂ NW and CdSe QDs.Based on DFT calculation results,band positon is altered with the variation of lattice spacing,which promotes the photogenerated electron injection and transport and thus results in the increase of photocurrent.4.The relationship between the evolution of crystal structure of electrode materials and performance degradation of quantum dots sensitized solar cell has been studied at atomic level.?1?Quantum dots sensitized solar cell of nanostructure was fabricated using in situ TEM technology.Through the photocurrent measurement and HRTEM imaging in real time,we find that the structure of CdSe quantum dots changes from single crystalline to amorphous phase.The defects induced by amorphous phase serves as recombination center for the photogenerated electron,which degrades the photocurrent.?2?Based on electron diffusion model,we simulated the effect of defect density on the photocurrent quantitatively.The results show that photocurrent degrades with the increase of defect density.This thesis provides an insight into the mechanism of the interfacial effect,crystal orientation and evolution in solar cells and paves the way to designing high-efficiency QD-based solar cells as well as other photovoltaic devices.