Studies Of Photovoltaic Properties Of Perovskite Solar Cells Based On Amorphous Tungsten Oxide Electron Transport Layer

Perovskite solar cells(PSCs)have emerged as one of the most promising photovoltaic devices due to their simple processing,rich resource,and high efficiency,the power conversion efficiency(PCE)of which has achieved 22.1%presently.Among them,flexible PSCs stick out owing to their intrinsic bendable,lightweight,and easily integrated characteristics.Therefore,it is very significant to design and research flexible PSCs for the further development of PSCs.All the functional layers of flexible PSCs must be prepared at low temperature(150 ? or less)because the maximum withstanding temperature of common flexible conductive substrates(ITO/PEN,e.g.)is about 150 ?.In traditional PSCs,only the electron transport layers(ETLs)requires high temperature(450 ? or more)annealing process.Thus,preparing ETLs at low temperature is the bottleneck problem for flexible PSCs.Currently,low temperature ETLs usually employ inorganic crystalline n-type semiconductors where the long-periodic and complex hydrothermal crystallization process is indispensable for crystalline materials due to the crystallization thermodynamics.As we know,the preparation of amorphous semiconductor materials can completely get rid of the limit of crystalline thermodynamic in theory,which indicates that amorphous materials possess great potential in low temperature ETLs field.However,basic and applied researches for amorphous materials are quite few in the field of PSCs up to now.It is acknowledged that WOx is a stable n-type semiconductor,whose suitable band structure and high electron mobility satisfy the basic requirements of ETLs.At the same time,it is very simple to prepare amorphous WOx at low temperature by solution method.Therefore,we consider that amorphous WOx possesses important research value in this field.Until now,the research about amorphous WOx is very less.Hence,we analyzed the photovoltaic performance of PSCs through realizing the controllable preparation of amorphous WOx-ETLs in this thesis.In addition,we optimized the preparation strategy of amorphous ETLs and revealed the working mechanism in device.The results are as follows:Solution-processed amorphous WOx thin films were prepared facilely with WCl6 as precursor at low temperature and employed as ETLs in PSCs.Results indicated that a large quantity of nanocaves were observed in WOx thin films.In comparison with the conventional TiO2-ETLs,the WOx-ETLs exhibited comparable light transmittance but higher electrical conductivity.Compared with the TiO2-based PSCs,PSCs based on WOx-ETLs exhibited a comparable PCE(8.99%),larger short circuit current density,but lower open circuit voltage and fill factor.Electrochemical characterization indicated that the unsatisfied open circuit voltage and fill factor were caused by the inherent charge recombination.This study demonstrated that this material was an excellent candidate for ETLs.Reducing the trap states and optimizing the energy band structures can improve the performance of WOx-based PSCs.Hence,TiOx was used to modify WOx for fabricating inorganic composite ETLs at low temperatures of 70 ? or even room temperature.This route is significant because the feasibility of fabricating inorganic ETLs without any thermal treatment is confirmed for the first time.The WOx-based amorphous and composite ETLs exhibited favorable optical and electric properties.Electrochemical characterization indicated that modification with TiOx could raise the Fermi level and simultaneously suppress charge recombination.As a result,the photovoltaic performance of planar PSCs was notably improved.The mole ratio of Ti/W in composite ETLs was optimized and a high PCE of 14.47%was obtained in the PSCs based on ETLs fabricated at 150 ? when the mole ratio of Ti/W was 20%.Meanwhile,PSCs with ETLs fabricated at 70 ? and room temperature were also very efficient given that maximum PCEs of 13.45%and 11.56%were achieved,respectively.This work presents a new strategy to develop amorphous,inorganic and composite functional materials for ETLs in efficient PSCs to reduce production cost and energy payback time further.With WCl6 as precursors,the acid substance released from the reaction can corrode the conductive layer of substrate.Hence,W(C2H5O)5 was used as precursors to prepare WOx-ETLs for efficient flexible PSCs.NbOx was used to modify the amorphous WOx-ETLs,which improved the electrical conductivity of WOx-based ETLs by enhancing donor density,reducing interfacial depletion width,and minimizing trap states.Consequently,photovoltaic performance of the planar flexible PSCs were improved,and high PCEs of 15.65%and 13.14%were achieved when ETLs were fabricated at 120 ? and room temperature,respectively.In addition,the effect of ETLs thickness on the hysteresis behavior of PSCs was carefully analyzed.A capacitance CE across the ETLs in ITO/ETLs/perovskite structure was proposed.This capacitance could well elucidate the effect of ETLs thickness on hysteresis behavior.Such proposed modification strategy and mechanism are expected to facilitate the development of novel functional materials.