Study On The Charge Carriers Dynamics In Organic Solar Cells

Organic solar cells are attracting more and more interest and focus of researchers with the breakthrough of non-fullerene organic solar cells in recent years.With the efforts of many researchers,more and more efficient donor materials or acceptor materials have been synthesized,so the efficiency of devices has been greatly improved.However,the detailed physical understanding of organic semiconductor devices still lags behind their application,on account of fundamental differences in the optoelectronic properties of these materials compared to conventional semiconductors because of their high energy and spatial disorder.Therefore,the photoelectric conversion process is also much more complicated.Researchers will study the operation mechanism of organic solar cells from various direction and charge carrier dynamics is an important direction.This thesis is mainly concerned with the charge carrier dynamics in organic solar cells.The carrier dynamics includes carrier generation,transport,extraction and recombination,and each process has its corresponding mechanism.This thesis mainly focuses on the carrier recombination process.The recombination processes in organic solar cells consist of geminate recombination and non-geminate recombination.The geminate recombination refers to the combination of photogenerated electrons and holes before they have been separated.On the contrary,the non-geminate recombination refers to the recombination of an electron or a holes that have been successfully separated,and they recombine with other carriers or traps.In the second chapter of this thesis,we set up a transient photoelectric test system to test the transient photovoltage and transient photocurrent of organic solar cell devices.Its technical principle is to use pulsed laser to stimulate organic solar cell devices under different light intensity to produce transient photovoltage or transient photocurrent signals.Analysing these data systematically,we can obtain the carrier dynamics information of organic solar cells,including the concentration,lifetime and recombination rate of the carriers.Through theoretical analysis,we can express the open circuit voltage with carrier dynamics information,and study the influence of recombination on open circuit voltage.We prepare many kinds of organic solar cell devices and tested them by transient photoelectric test to measure their concentration,lifetime and recombination rate,and calculate the open circuit voltage with these data.It is found that the error between the calculated Voc and measured Voc is very small,proving that the open circuit voltage is mainly limited to non-geminate recombination.The credibility of the measured carrier dynamics information is also verified.Finally,through further theoretical analysis,it is found that the open circuit voltage is related to the competitive relationship between the recombination current and the photogeneration current in addition to the energy level between acceptor and donor..In the third chapter of this thesis,we study the influence of solvent vapor annealing on the carrier dynamics of the device by transient photoelectric test system.We report a high fill factor small molecule device.After proper solvent vapor annealing,the fill factor can reach 78%.In order to systematically study the influence of the solvent vapor annealing on the carrier dynamics,we have prepared some devices with different solvent annealing time as the research object,and tested their concentration,lifetime and recombination rate information.Their J-V curves are reconstructed according to theoretical analysis with these information.It is found that the optimal device fits best,which proves that the optimal device is only limited by non-geminate recombination,without geminate recombination.However,there is a large deviation in the reconstruction of the as cast devices.We think that the effect of geminate recombination should be considered for the as cast device,so we simulate the effect of geminate recombination by a simple model,and the JV curve is rebuilt again,and the better fitting results are obtained.It is proved that the as cast device is indeed limited by both geminate recombination and non-geminate recombination.At the other point of view,it is also shown that the solvent vapor annealing treatment does affect the carrier dynamics,which effectively inhibits the geminate recombination and thus improves the fill factor of the device.In the last chapter,we first introduce the trap assisted recombination via tail.The concept of tail is introduced to represent the trap state distribution of the shallow energy level.It is considered that the tail state obeys the exponential distribution in the band gap.Based on the tail recombination model and using the method of numerical simulation,the basic semiconductor equation is solved,and the distribution of the concentration and potential in the space under different applied voltages is obtained,thus the J-V characteristic curve of the device can be simulated.In addition,by adjusting the generation rate,we can also simulate the variable light intensity J-V test,and calculate the relationship between the open circuit voltage and the light intensity,so as to find the ideal factor of the device.At the same time,we can calculate the change of concentration with the intensity of light,so we also realize the simulation of charge extraction experiment.We can know that the carrier dynamics of organic solar cell system can be well studied with the tail recombination model.Finally,on the basis of the tail compound model,the influence of the energy and space on the carrier dynamics is studied by adjusting the slope of the tail and the thickness of the active layer,respectively.It is found that the greater the tail slope is,the more serious the device recombination is.On the other hand,the thickness of the organic solar cells is very thin,so the carrier distribution in the layer is very inhomogeneous,which will also affect the recombination of the carrier.