PSS/Si Hetero-interface:Properties And Applications

The performances of Si-based semiconductor devices are crucially dependent on the surface/interface properties.For example,a high level of surface passivation is a basic prerequisite for high-efficiency crystalline Si?c-Si?solar cells.Three passivation schemes,SiO₂,Al₂O₃ and hydrogenated amorphous silicon?a-Si:H?thin films,have been successfully developed for Si photovoltaic technologies.However,these traditional schemes involve a severe requirement of either vacuum and/or high deposition temperature as well as the possible utilization of explosive gas sources,which hinders the efforts to further reduce the cost of solar cells and modules.Surface passivation by cost-effective organic layers has been established many years ago,but there was not a significant break through in high-quality passivation.In this PhD thesis,a polystyrenesulfonate?PSS?thin film was found to be a novel passivation scheme,providing a superior passivation effect on the Si surface.An effective minority carrier lifetime(?_eff),of up to 28.6 ms was observed for a high resistivity Si wafer with double-side PSS coating.Such an exceptionally high?_effff is close to the record value of 29ms on an n-type wafer passivated using SiO₂ by Kerr and Cuevas.Based on this finding,this thesis has carried out research work including two aspects,PSS/Si interface properties and their applications in the specific devices.In the first part,the thesis focuses on the PSS/Si interface properties and passivation mechanism,as follows:?i?Previously,PSS usually shows a form of organic acid solution,and served as a dispersant for aqueous poly?3,4-ethylenedioxythiophen??PEDOT?to yield PEDOT:PSS thin films.The passivation effect of poly-3,4-ethylenedioxythiophene?PEDOT?:PSS thin films with the different percentage of PSS was investigated.It was found that the?_effff values were higher for the sample with a higher percentage of PSS.However,when PSS species in PEDOT:PSS thin film was decreased,passivation was penalized.This necessitates the introduction of a pure PSS thin film as the passivation layer for better passivation.When PSS is coated as a form of thin film on c-Si surface,the excellent passivation effect was obtained.These results indicate that the major contributor to passivation is from the PSS species in the PEDOT:PSS blend material system rather than from the PEDOT species.?ii?The?_effff evolution over time at the different atmosphere and light soaking was investigated.It was found:1)the passivation by both PEDOT:PSS and PSS thin film was enhanced in O₂ atmosphere;2)for the passivation stability,PEDOT:PSS is opposite to PSS,i.e.the passivation by PEDOT:PSS was enhanced in O₂ atmosphere and light soaking first and then remains stable,but the passivation by PSS was increased to a very high value and then gradually decreased in O₂ atmosphere,and dropped dramatically to the failure in the light soaking.?iii?The above phenomenon is linked to the PSS/Si interface properties and the carrier transport.X-ray photoelectron spectroscopy was employed to explore the interface nature of PSS/Si,which demonstrates that high-quality passivation is dominated by Si surface oxidization related to PSS.It was known by the first-principles calculation that the oxidization was achieved by O in the sulfonic acid group in PSS molecule grafted on Si surface dangling bonds.An external field is applied to execute the charge injection at PSS/Si interface using ITO and c-Si as top and bottom electrode,respectively.The?_effff increases after poled by a positive voltage and decreases by applying a negative voltage,presenting a switchable interface states.Based on the above results,we proposed that the passivation of Si surface by PSS obeys an electrochemical oxidation mechanism at the PSS/Si interface,which can be controlled by electron transfer at the Si surface.When the Si surface acts as an electron donor,an oxidation state was obtained and vice versa.Now,those findings above?ii?can be explained:due to the high work function of PEDOT:PSS,the internal electric field(E_in)of PEDOT:PSS/Si interface favours the holes transfer to the interface,leading to the occurrence of the oxidation state and the stable passivation;on the other hand,however,PSS thin film was found to be a low work function n-type material in this thesis,so an opposite E_inn with PEDOT:PSS/Si was built at PSS/Si interface,which favours the occurrence of the deoxidation state and the unstable passivation.?iv?To improve the stability of the PSS passivation effect,the schemes for the change of the internal electric field direction were introduced.By adding MoO₃ particles into a PSS matrix to form PSS:MoO₃?PSSMO?composite thin film,we incorporated the advantages of both PSS?passivation?and MoO₃?hole transport?to develop versatile PSSMO thin films.?v?Further,we investigated the origin of PSS passivation,i.e.,which species in PSS molecule the high quality passivation stems from?The several materials,with the different functional groups,were tried for Si surface passivation.It was found that one S=O double bond plus one-O is a minimal passivation unit,and two S=O double bonds plus one hydroxyl,i.e.,the sulfonic acid group,provides the higher quality passivation effect.Thus,a very important conclusion is abstracted:materials,with the sulfonic acid group,probablely provide passivation effect,leading to the establishment of functional group passivation system.The technical features of this type of functional group passivation system can be summarized:the origin of passivation is the sulfonic acid group,the mechanism of passivation is the electrochemical passivation,and the passivation level need to be enhanced by some external conditions,such light soaking,oxygen environment and bias voltage.In the second part of the thesis,based on the understanding of the property and mechanism of PSS passivation on the Si surface,further investigation of the devices was performed to examine the real application,including the following:?vi?PSS/Si interface passivation was introduced in photovoltaic technologies.1)The excellent passivation scheme can be utilized for the characteristics of Si wafer bulk lifetime.Without the requirement of solar cell fabrication,Si wafer quality can be evaluated.2)It is found that PSS thin film is highly transparent and possesses suitable refraction index,thus functions well as anti reflection coating for Si wafer based solar cells.An increase in power conversion efficiency is obtained when PSS thin film is coated on the front surface of IBC solar cells without any antireflection/passivation layers,resulted from the increased short circuit current and open circuit voltage.The performance stability of the solar cell can be solved using a front floating junction.The same efficacy is also achived by other material in functional group passivation system.For example,the power conversion efficiency of increases from 15.8%to 18.4%after“Polystyrene-block-poly?ethylene-ran-butylene?-block-polystyrene,sulfonated,cross-linkable”thin film coated on the front surface of the IBC solar cell.3)For the PEDOT:PSS/n-Si organic-inorganic hybrid solar cells,light-induced enhancement?LIE?effect in photovoltaic performance was observed,unlike light-indueced degradation?LID?effect of most solar cells.The reduced density of interface states under light soaking is found to be responsible for the LIE of the hybrid solar cells.?vii?A hysteresis loop of minority carrier lifetime vs voltage is found in PSS/Si organic-inorganic hybrid heterojunctions,implying an interfacial memory effect.Capacitance-voltage and conductance-voltage hysteresis loops are observed and reveal a memory window.The fatigue and retention characteristics for two interface states were measured,demonstrating a well reliability.The first-principle calculations demonstrate that the nature of the interfacial memory effect is the field-induced charge accumulation and depletion of PSS/Si interface.These results offer a concept of organic-inorganic hybrid interface memory being optically or electrically readable,low-cost,and compatible with the flexible organic electronics.