Research On Modification Of Electron Transport Layer And Active Layer In Perovskite Solar Cells

The past decade has witnessed a rapid evolution of perovskite solar cells(PSCs),an unprecedented photovoltaic technology with both high power conversion efficiency(PCE)and potentially low cost.While the record PCE of PSCs is now reaching 25.2%,rivaling that of silicon-based solar cells,the state-of-the-art PSCs still suffer from large nonradiative recombination losses,making their PCE still below the Shockley-Queisser limit.Together with problems such as hysteresis behavior and unsatisfactory stability,restricting the commercial application of perovskite photovoltaics.It is well established that further improvements of PSCs performance will require suppression of all nonradiative recombination(trap-assisted recombination)losses to unlock the full thermodynamic potential of PSCs.The main content of this thesis is focused on the modification of the electron transport layer(ETL)and perovskite active layer by using strategies such as doping and interface engineering to obtain efficient and stable PSCs.As a widely used ETL in PSCs,TiO2 has some problems(low electron mobility and UV instability,etc.),and doping is an effective way to improve its material properties.By using boron-doped TiO2 as an improved ETL,the PCE and stability of PSCs are improved,and the hysteresis behavior of PSCs is significantly suppressed.Boron doping not only effectively passivates the oxygen vacancy defects in TiO2,increasing the electron mobility and conductivity,but also improves the energy level matching between TiO2 and perovskite.This resulted in enhanced electron transfer and reduced nonradiative recombination at the interface,leading to a PCE improvement from 19.06%to 20.51%.The charge accumulation is reduced due to the balanced carrier transport at the interfaces,resulting in negligible hysteresis of the PSCs.In addition,due to the improved interface contact between TiO2 and perovskite(enhanced interface bonding and reduced interface defects),the stability of PSCs is also enhanced.In addition to ETL doping,modification of the ETL/perovskite interface is also an effective way to suppress nonradiative recombination and improve PSCs performance.(1)The interfacial nonradiative recombination loss of PSCs is significantly reduced with the introduction of a CeOx interlayer between TiO2 and perovskite,which creates a cascade path for electron transport.This optimally positioned band alignment can facilitate the electron transfer,and suppress the electron back transfer from TiO2 to perovskite.Due to the suppression of interfacial recombination,the potential loss is reduced from 520 mV to 300 mV,leading to improvements of open-circuit voltage(Voc)and PCE of the PSCs.(2)The balance between charge transfer and recombination at the SnO2/perovskite interface is regulated by controlling SnO2 film thickness.With the optimized SnO2 film thickness,the interfacial charge balance is enhanced,and a high PCE of 20.79%was obtained.Moreover,due to low temperature processing,trap states still exist at the SnO2 surface and the SnO2/perovskite interface,which limits the further improvement of the PSCs performance.By introducing a diethylenetriaminepentaacetic acid(DTPA)interlayer between SnO2 and perovskite,the interface defects are effectively passivated,resulting in suppressed trap-assisted nonradiative carrier recombination.The Voc of PSCs is increased from 1.06 V to 1.12 V with the champion PCE up to 21.18%.Meanwhile,the environmental stability of the PSCs is also enhanced.There are many defects in the perovskite film.Doping of perovskite with additives can reduce or passivate the defects and inhibit nonradiative carrier recombination.(1)By using biuret containing multiple Lewis base groups as an additive,the crystallization process of the perovskite film can be regulated and the defects at the surface and grain boundaries can be passivated.The interaction between biuret and perovskite components can slows down the crystallization rate,leading to perovskite films with larger grains preferential orientation and low defect density,Meanwhile,the remnant biuret in the final perovskite films can also passivate the ionic defects and act as a molecular lock to prevent thermal degradation of perovskite.(2)High-quality perovskite films are essential for efficient and stable PSCs.However,it is challenging to fabricate formamidinium-based planar PSCs via sequential deposition,mainly due to the small penetration depth of the large size formamidinium cation into a compact PbI2 film during perovskite formation.The author has demonstrated that the addition of CaI2 into the PbI2 precursor can effectively improve the film quality and photovoltaic performance of planar PSCs.The introduction of CaI2 not only regulates the formation process of perovskite film,but also passivates detrimental defects accumulated at the surface and grain boundaries of perovskite films,inhibiting nonradiative carrier recombination in the PSCs.This thesis shows that the control of nonradiative recombination at the ETL/perovskite interface and perovskite film is critical to the performance of PSCs.Doping of ETL and perovskite layer,and modifying the ETL/perovskite interface can effectively suppress nonradiative recombination and improve the PSCs performance.