Stability And High-Efficiency Perovskite Solar Cells

Organic-inorganic metal halide perovskite solar cells（PSCs）have achieved certified power conversion efficiency（PCE）of 25.5%,which is regarded as one of the most promising high efficiency solar cells.However,there are still many problems to be solved and stability of the PSCs is still major concern.In this paper,the stability of PSCs is discussed from the aspects of material design and synthesis,interface optimization,energy level matching,device structure.The perovskite layer,electron transport layer and the interface between the layers are mainly studied.Therefore,the stability and photovoltaic performance of PSCs are improved simultaneously.（1）Stable and high-efficiency methylammonium-free perovskite solar cellsThe thermal instability of methylammonium（MA⁺）cation can cause the degradation of the perovskite film,remaining a risk for the long-term stability of the devices.Herein,a unique method is demonstrated to fabricate highly phase-stable perovskite film without MA by introducing cesium chloride（Cs Cl）in the double cation（Cs⁺,FA⁺）perovskite precursor.Moreover,the amount of Br^-is regulated and leading to high PCE and excellent thermal stability with methylammonium-free composition(Cs_0.17FA_0.83)Pb(I_0.89Br_0.08Cl_0.03)₃.As a result,MA-free PSCs with 0.15M Pb Br₂ achieve a PCE of 20.50%and remarkable long-term stability.Moreover,the unencapsulated device in Ar retains about 80%of the original efficiencies after a 1000h aging study.（2）Hydrophobic conjugated polymer as effective interlayer for air-stable efficient perovskite solar cellsAmong various promising HTMs,doped Spiro-OMe TAD has been the most common HTMs in the high-efficiency PSCs.However,the additives in Spiro-OMe TAD can absorb moisture and cause the degradation of perovskite layers,leading to severe air-instability of devices.Herein,conjugated polymer consists of alkylated di（thienothienyl）ethylene（DTTE）units and diketopyrrolopyrole（DPP）moieties（PD-10-DTTE-7）as a new effective interlayer between perovskite and doped Spiro-OMe TAD to achieve air-stable efficient PSCs.Its superb hole mobility(9.54cm² V^-1 s^-1)and suitable highest occupied molecular orbital level（-5.33 e V）are preferable to the hole injection and transport at the interface thus enhancing the device PCE.As a result,the MAPb I₃ solar cells with the PD-10-DTTE-7 interlayer enhanced PCE（18.83%）and achieve remarkable device air-stability,Moreover,the unencapsulated device retains about 95%of the original efficiencies after being stored in the air for 50 days.（3）Tuning the Fermi-level of Ti O2 mesoporous layer by lanthanum doping towards efficient perovskite solar cellsTuning the band alignment is proved to be an effective way to facilitate carrier transportation and thus enhance the PCE of solar cells.We demonstrated here that La³⁺doping in the mesoporous Ti O₂ layer of a mesostructured PSC.Systematic analysis reveals that doping La³⁺into Ti O₂ raises the Fermi level of Ti O₂ through scavenging oxygen and inducing vacancies,which subsequently increases the open circuit voltage(V_OC)and the fill factor（FF）while reducing the series resistance of the PSC using La³⁺-doped Ti O₂ as a mesoporous layer.As a result,the PCE of PSC based on La³⁺-doped Ti O₂,which is appreciably higher than the PCE of a device with undoped Ti O₂（27.3%）.（4）Engineering long-term stability into perovskite solar cells via application of a multi-functional TFSI-based ionic liquidThe long-term stability and air stability are still major concerns for PSCs.Therefore,we employ a special ionic liquid（[Bcim][TFSI]）as an additive in the triple cation（Cs⁺,FA⁺,MA⁺）perovskite precursor,which contains an imidazolium cation functionalized with nitrile（CN^-）groups and the bis（trifluoromethylsulfonyl）imide（TFSI^-）anion.Due to a combination of the electron donating centers in the ionic liquid and the highly stable hydrophobic TFSI^-anion,highly crystalline and large grain size perovskite films can be fabricated.As a result,the modified triple cation（Cs⁺,FA⁺,MA⁺）perovskite is protected by the[Bcim][TFSI],to afford PSCs that exhibit a PCE of 21.06%.The long-term stability of unencapsulated devices in N₂retain>95%of their original efficiencies after 1000 h aging.