Regulating The Composition And Interface For Efficient And Stable Perovskite Solar Cells

Photoelectric conversion is one of the effective strategies for utilizing clean solar energy.As an emerging thin-film solar cell,perovskite solar cells(PSCs)have many advantages including high power conversion efficiency(PCE),solution process,and low cost,etc.and are expected to become a new photovoltaic power generation technology comparable to the commercial crystalline silicon solar cells.Since the first application of solid-state hole transport materials to PSCs in 2012,their PCE has jumped from less than 10%to 25.7%in just one decade.Although the PCE has been significantly improved,there is still a certain distance to reach its theoretical limit,and"efficiency improvement" will still be the main theme of the development of perovskite photovoltaics in the next few years.The prerequisite for high-performance PSCs requires the preparation of smooth,compact,pinhole free,high-quality perovskite films with large grain sizes,good crystallinity,preferred orientation and without nonphotoactive phases.Regulating the perovskite composition is regarded as one of the most effective strategies,and the interfacial charge transport between the perovskite and the charge transport layer is also one of the main factors affecting the device performance directly.Strategies such as managing the energy level alignment,passivating the defects and enhancing the conductivity and carrier mobility of the charge transport layer can effectively improve the charge transport.In addition,the stability of PSCs is another key factor that determines whether they can achieve commercial applications.Organic spacer cation intercalation in three-dimensional(3D)perovskite can obtain two-dimensional/quasi two-dimensional(2D/quasi-2D)perovskite,which not only improves the ambient stability of the device,but also achieves structural tunability by regulating the number of inorganic octahedral layers between adjacent organic spacer cations.Another strategy for improving the stability of the PSCs is to replace organic-inorganic hybrid perovskite with all-inorganic perovskite.Compared to the commonly used organic-inorganic hybrid perovskite in high-efficiency PSCs,the all-inorganic perovskite obtained by replacing the A-site organic cation in the organic-inorganic hybrid perovskite lattice with cesium ions(Cs+)has better thermal stability and potential application values in tandem solar cells.Therefore,it has also received widespread attention in recent years.In this dissertation,we focused on the composition regulation and interface engineering of quasi-2D,organic-inorganic hybrid and all-inorganic PSCs to improve their efficiency and stability and carried out the following four works:1.The structure and size of organic spacer cations play a key role in improving the performance of 2D/quasi-2D PSCs.In the reported works,organic spacer cations with MA+(CH3NH3+)as the "head" are mostly used,while organic spacer cations with FA+(NH2CH=NH2+)as the "head" have been rarely reported.In this work,we applied the 4-chloro-phenylformamidinium cation(CPFA+)in the quasi-2D Ruddlesden-Popper(RP)perovskite,and optimized the molar ratios of MAI(CH3NH3I)and MACl(CH3NH3Cl)to manipulate the crystal growth of quasi-2D perovskite thin films.A series of characterizations were carried out to demonstrate the formation of quasi-2D CPFA2MAn-1Pbn(I0.857Cl0.143)3n+1(n=9,the n value is determined according to the molar ratio of the precursor),and we systematically studied the effects of MAI/MACl molar ratios on the quasi-2D perovskite crystal growth,defect state density,and energy level alignment.The PCE of the quasi-2D PSCs based on the optimal MAI/MACl molar ratio(MAI:MACl=1:1)reaches as high as 14.78%.Compared with the 3D MAPbI3 PSCs,the CPFA-based quasi-2D PSCs exhibited better ambient storage stability and maintained about 80%of the initial PCE after more than 2000 hours,while the 3D MAPbI3 PSCs stored under the same conditions decay to about 45%of the initial PCE after 1100 hours.2.Until now,almost high-performance single-junction PSCs use FA-dominated(FA:formamidinium)perovskite(abbreviated as FAPbI3)as the light-absorbing layer.However,the δ-FAPbI3 phase is easy to form during the fabrication of FAPbI3 films,and the formation of δ-FAPbI3 phase is usually accompanied by the generation of a large number of defects.In this work,we incorporated pre-synthesized CsPbBr3 crystals into the FAPbI3 perovskite to inhibit the formation of non-photoactive δ-FAPbI3 phase,enhance the crystallinity of α-FAPbI3 perovskite,improve crystal orientation and reduce deep level defects,etc.Optical and electrical characterizations reveal that the elimination of deep-level hole defects not only suppresses defect-assisted nonradiative recombination,but also enhances hole mobility to be more balanced with electron mobility,leading to a significant increase in charge extraction efficiency.As a result,the PCE of the corresponding PSCs is as high as 25.09%(certified at 24.66%),and the fill factor(FF)achieves a ultra-high value of 86.9%reaching 96.3%of its ShockleyQueisser(S-Q)theoretical limit(1.55 eV:90.21%).FF loss analysis shows that the addition of CsPbBr3 can simultaneously reduce the loss caused by series resistance,parallel resistance and non-radiative recombination.3.High-performance single-junction conventional structured PSCs are all adopted 2,2’,7,7’-tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9-spirobifluorene(spiroOMeTAD)as hole transport layer(HTL),however,the spiro-OMeTAD HTL suffers from low conductivity and hole mobility.Enhancing its hole transport properties not only depends on the doping process of dopants,but also relies on the post-treatment oxidation process.In this work,we developed a thermal oxygen treatment(TOT)method to oxidize spiro-OMeTAD films,improving its hole transport performance efficiently.Unlike the conventional dry air treatment(DAT)method commonly used in the literature,the pure oxygen environment and higher temperature can accelerate the movement of oxygen molecules to accelerate the oxidation process of the spiroOMeTAD film.At the same time,this strategy can also avoid the erosion of moisture and help to improve the stability of PSCs.By optimizing the temperature and time of TOT,the spiro-OMeTAD film exhibits very excellent electrical conductivity and hole mobility,leading to a reduced series resistance of charge transport.In addition,compared to the spiro-OMeTAD after DAT method,the downshifted Fermi level(EF)and highest occupied molecular orbital(HOMO)of spiro-OMeTAD after TOT method facilitates the hole carrier injection from the perovskite into spiro-OMeTAD.As a result,we can obtain conventional structured PSCs with certified efficiency of 25.34%and FF exceeding 87%which reaches 96.5%of the S-Q theoretical limit.To the best of our knowledge,this is the first time that PSCs achieve a certified FF of more than 87%.4.All-inorganic perovskite has good thermal stability and has potential application in tandem solar cells,however,the energy level mismatch between all-inorganic perovskite and charge transport layer and a large number of defects on the surface and grain boundary lead to a severe open-circuit voltage(VOC)loss of the PSCs.In this work,we introduced a Ba(OH)2 interlayer between the SnO2/ZnO electron transport layer(ETL)and CsPbI2.25Br0.75 to promote the crystal growth and improve the energy level alignment between the SnO2/ZnO ETL and the perovskite layer.The enlarged grain size(more than 2 μm)helps to reduce the defect-rich grain boundaries,thereby reducing the defect state density,and the more matched energy level structure is conducive to reducing the interface recombination of carriers,so that the PCE of CsPbI2.25Br0.75 PSCs reaches up to 17.32%with a Voc of 1.242 V,and the VOC loss is 0.623 V.As the thickness of the Ba(OH)2 interface layer increases,the VOC increases gradually,but the morphology of the CsPbI2.25Br0.75 perovskite film gradually deteriorates.In order to further improve the VOC,we choose choline chloride(CHCl)as an interlayer to passivate the defects located at surface and grain boundary of CsPbI2.25Br0.75 perovskite,leading to a VOC of 1.336 V and VOC loss of 0.529 V.