Study On The Effects Of Interface And Carrier Transporting Layer On The Performance Of P-i-n Planar Perovskite Solar Cells

The use of interfacial modification and efficient hole transporting layers（HTLs）is important for the advancement of high-performance perovskite solar cells（PSCs）.In this thesis,we improved the performance of p-i-n planar PSCs by employing a polyelectrolyte as interlayer at“electron transporting layer（ETL）/metal electrode”interface and small molecules as HTL,respectively.The mechanism of device performance enhancement was studied.1.A self-doping polyelectrolyte,perylene diimide-based polyelectrolyte（PDI-Br）,was utilized as interlayer to improve the efficiency and stability of p-i-n PSCs;on the basis of PDI-Br interlayer and solvent annealing,a power conversion efficiency（PCE）of19.12%was achieved in p-i-n PSCs.Critically,the energy-level mismatch between the ETL and metal electrode in p-i-n PSCs creates energy barriers to charge extraction and transport at the interface.In order to address this issue,PDI-Br was used as interlayer.PDI-Br interlayer can effectively lower the work function of different metal electrodes,such as silver（Ag）,gold（Au）,and copper（Cu）,due to the formation of a large negative interfacial dipole.When inserting PDI-Br interlayer between ETL and Ag,the fill factor（FF）of the device exhibited an obvious rectification,thus leading to the enhancement of PCE.Additionally,the PDI-Br acted as a protective layer at“ETL/metal electrode”interface,which improved the stability of the PSCs.To further improve the PCE of the devices,we used solvent annealing as bulk optimization.Through solvent annealing,polycrystalline perovskite films made by one-step method afforded large grain sizes and high degrees of crystallinity,thus promoting lower trap density and reduced nonradiative recombination.These advances of PDI-Br interlayer and solvent annealing yielded p-i-n PSCs with a maximum PCE of 19.12%,along with a superior long-term stability.Moreover,the devices using Au or Cu as metal electrode also affording PCEs>17%,which showcased the versatility of PDI-Br interlayer and solvent annealing to achieving efficient and stable p-i-n PSCs.2.Novel small molecular materials,4,4′-bis（4-（di-p-toyl）aminostyryl）biphenyl（TPASBP）and 1,4′-bis（4-（di-p-toyl）aminostyryl）benzene（TPASB）were employed as HTLinp-i-nPSCsincomparisonwithpoly（3,4-ethylenedioxythiophene）:poly（styrenesulfonic acid）（PEDOT:PSS）and HTL-free structure,revealing the effects of different HTLs on perovskite film properties and device performance;a high PCE of 18.72%was realized based on TPASBP.Compared to the case of PEDOT:PSS,PSCs based on small molecular HTLs exhibited a～40%higher PCE,indicating the crucial role of HTL in the achievement of high-performance PSCs and providing a new perspective for the selection of efficient and non-doped HTLs in p-i-n PSCs.In p-i-n structure,HTL functionalizes as the P-type layer for constructing the junction and plays hole-transport and hole-selective roles for efficient hole collection and reducing interface carrier recombination.The HTL also affects the morphology of the perovskite layer fabricated on it.We studied the effect of different HTLs on the performance of p-i-n PSCs by using HTL-free,PEDOT:PSS,TPASBP,and TPASB as HTL,respectively.Relative to the HTL-free PSCs and the devices containing PEDOT:PSS HTL,PSCs based on TPASBP or TPASB afforded a better contact at the“indium tin oxide（ITO）/perovskite”interface.Compared to PEDOT:PSS,TPASBP or TPASBP afforded dense perovskite films with larger grain sizes,better crystallinity,and stronger absorption.TPASBP and TPASB also exhibited better abilities of hole extraction and transport rather than PEDOT:PSS.The PSCs containing TPASBP or TPASB also showed lower reverse saturation current density（J₀）and larger built-in potential(V_bi).In addition,we investigated the relation between the properties of substrates and perovskite films,which implied that a hydrophobic surface helps the formation of perovskite with good quality.Lastly,we achieved a best PCE of 18.72%by using TPASBP HTL in p-i-n PSCs.3.As a HTL in p-i-n PSCs,N,N′-bis（naphthalen-1-yl）-N,N′-bis（phenyl）benzidine（NPB）exhibited better energy alignment with CH₃NH₃Pb I₃ as well as excellent electrical and optical properties,which afforded a PCE of 19.96%,along with good stability.To further improve the PCE of p-i-n PSCs,especially the open circuit voltage(V_OC)of the devices,NPB was utilized as an efficient,dopant-free HTL in p-i-n PSCs to relplace the commonly used PEDOT:PSS.Morphological characterization indicated that with NPB as the underlying HTL,larger perovskite grains（average grain size～300 nm）were produced relative to PEDOT:PSS（～200 nm）.NPB-coated sbustrates also afforded perovskite films with better crystallinity,pure CH₃NH₃Pb I₃,and stronger absorption relative to PEDOT:PSS.The insertion of the NPB layer effectively reduced recombination loss,as measured by electrochemical impedance spectroscopy（EIS）.Moreover,the NPB layer afforded more efficient hole extraction and transport from the perovskite layer to the HTL.All of these advantages of the NPB HTL contributed to the outstanding enhancement of short circuit current density(J_SC).Based on the analysis of Mott-Schottky,NPB produced a V_bi of 0.84 V in the devices,which was higher than PEDOT:PSS case（0.77 V）,likely due to the lower highest occupied molecular orbital（HOMO）of NPB relative to the work function of PEDOT:PSS.Furthermore,there was a much lower J₀ in NPB-containing devices compared to the PSCs based on PEDOT:PSS,and a lower J₀ yields a larger V_OC in the devices.Thus,a combination of energy-level alignment and reduced recombination loss explained the observed V_OC enhancement in NPB-containing devices.In addition,the hydrophobic property of NPB helped to improve device stability.In summary,a best PCE of 19.96%was realized when using NPB as the HTL,along with a J_SC of 22.92 m A/cm²,a V_OC of 1.11 V,and an FF of 78.4%.NPB-based devices also exhibited a substantially improved stability.