Reaserch On Colloidal Quantum Dots Solar Cells With Photo-carrier Radiometry

Colloidal quantum dots(CQD) materials which can be applied on photodetector, photodiode, solar cells and so on, have been the world research focus in recent years. The third generation solar cells based on those materials promote the solar cell industry due to its merits of low-costs, broadband solar spectra absorption and roll-to-roll fabrication. However, the optical-electronic properties and many experiments phenomena of CQD are still opaque which restricts the application. Measuring accurate exciton lifetime is the first step for understanding the optical-electronic properties of CQDs. Photoluminescence(PL) is the common method for exciton lifetime measurements, but its low testing accuracy impedes the formation of accurate physical models for dynamic mechanism of exciton transport in CQDs. Photo-carrier radiometry(PCR), as a frequency domain PL modulation technology is all-optical, non-destructive, non-contact and fast which is applicable for soft and vulnerable CQD testing. Moreover, the frequency modulation property of PCR makes the tests of CQDs with high signal to noise ratio and accuracy, especially the PCR phase signal. But there lacks of matched models and method of PCR for CQDs materials. In this work, a CQD-PCR model was developed based on conventional PCR method and exciton transportation properties in CQDs were tested and analyzed.Firstly, a CQD-PCR theoretical model was developed, this promote the application area of PCR for semiconductor electrical parameters measurements from 3D bulk materials to 0D CQD materials.(1) Three CQD-PCR models describing lifetime distribution have been developed to analyze PCR frequency scan signals: the first one based on Gaussion CQD size distribution can yield homogeneous and inhomogeneous Voigt lifetime broadening. The second one is based on direct ad hoc fits to PCR amplitude and phase data, reconstructing a superposition of multiple independent lifetime line spectra. The last one is a rigorous variational method developed to invert simultaneous integral equations in the lifetime distribution function(spectrum) and involves similar fits to the PCR in-phase and quadrature signal channels to reconstruct the lifetime spectrum.(2) A PCR theory with a rate equation model taking into account the lowest 1S-1S exciton state splitting and multi-phonon-assisted carrier trapping to states outside a PbS QD was also developed.Secondly, a PCR experimental system was build up. The correctness and applicability of the CQD-PCR models were verified by the PCR frequency scan experiments of two PbS CQD thin films which were handled with different surface passivation methods(un-coupled and coupled sample), that means a new technological approach was found for exciton transportation properties measurements in CQDs.(1) More accurate exciton mean-lifetime has been found and lifetime distribution spectra at various temperatures in the PbS thin films have been reconstructed. During the research process, the Voigt model is only applicable for un-coupled sample, while the ad hoc model and the variational model are applicable for both samples. The lifetime distribution spectra reconstructed from the variational model with highest reliability could distinguish simultaneous multiple exciton decay mechanisms, this is the first case in related research fields. We found that, samples with different inter-dot spacing show different lifetime distribution spectra and exciton transport properties, which has great significance for understanding the exciton relaxation mechanisms in CQDs. For non-interacting(uncoupled) CQDs LO phonon-exciton interactions limit the radiative lifetime and luminescence-photon-flux throughout the entire 100 – 300 K temperature range. For interacting(coupled) CQDs, variational and ad hoc lifetime line spectra exhibit similar mechanisms to uncoupled CQDs with the important addition of direct exciton-to-exciton transitions in the form of efficient transport(hopping, tunneling or FRET) at all temperatures which result in de-excitation dynamics dominated by radiative emission channels. PCR method is more accurate than PL.(2) The correctness of the rate equation model was verified by analyzing the temperature-dependence of exciton lifetime distribution. The radiative τR and non-radiative lifetimes τNR of the two samples were extracted, and the lowest 1S-1S splitting energy were calculated.(3) Three supplementary PCR experiments were also carried out, which show coincident results with PCR frequency scan results. PCR intensity scan results show that the PCR signals from PbS CQD thin films are linear under laser exposure. Activation energy of the non-radiative recombination center was found from PCR temperature scan data. PCR time scan results show that the two samples have different trap states.Thirdly, preliminary PCR experiments were carried out on PbS CQD solar cell for the first time. The current-voltage curves were tested at various temperatures. PCR frequency scans were done and exciton lifetime distribution spectra of the PbS CQDs which is the light absorption layer in the solar cell were reconstructed. It was found that the exciton lifetime in the solar cell is longer than in the thin films.In summary, CQD-PCR method and experiments were finished for CQD properties research. Three CQD-PCR frequency scan signal analyzing models were deveploded. Based on this, exciton lifetime distribution spectra for the two PbS CQD thin film samples were reconstructed and accurate exciton lifetime values were calculated. In this work, the precision of exaction lifetime measurements were improved and the exciton decay pathways in CQDs were deeply researched. Under the trend of nano-materails and nano-devices wild application in optoelectronics, microelectronics, material science and biomedicine field, CQD-PCR method will be a more maturity and accurate testing technology.