Research On All-Inorganic Perovskite/Organic Tandem Solar Cells

In the past decade,perovskite solar cells have been a hot research field worldwide.However,due to the existence of thermal and transmission losses,the power conversion efficiency（PCE）of single-junction perovskite solar cells is limited by the theoretical limits of Shockley-Queisser（S-Q）.Heat loss is mainly caused by the thermal relaxation of excitation electrons towards the edge of the band when the energy of some shortwavelength photons in the sunlight exceeds the bandgap of the perovskite material,while transmission loss is caused by the non-absorption of photons when the energy of some long-wavelength photons in the sunlight is lower than the bandgap of the perovskite material.In order to reduce the losses and improve the PCE of solar cell,constructing allinorganic perovskite/organic tandem solar cells is one of the effective strategies.CsPbI₂Bris the most representative material among many all-inorganic perovskites,and the bandgap of 1.92 eV making it more competitive in the field of tandem solar cells.Therefore,in this paper,a series of tandem solar cells have been designed and fabricated using all-inorganic perovskite CsPbI₂Br as the active layer material of the front subcell,while combining narrow bandgap organic materials as the active layer of rear subcell.The main work is asfollows:（1）In order to solve the problem of interface loss in single-junction CsPbI₂Br solar cells,using the Cl@MZO replace the traditional SnO₂ as ETL,which was successfully introduced into all-inorganic CsPbI₂Br-based perovskite solar cells,and the high-quality CsPbI₂Br thin film was prepared.Cl@MZO shows suitable energy level matching with the CsPbI₂Br layer,improving the speed of charge extraction and transfer.In addition,due to the high activation energy of carrier recombination,charge recombination in the device is effectively suppressed.On the other hand,this work introduces a series of conjugated polymers without dopants as hole transport materials.Among them,PM6 HTL not only has a deep highest occupied molecular orbital energy levels,but also its π-π directional stacking is conducive to vertical hole transport,and forms a quasi ohmic contact between PM6 HTL and the electrode,thereby promoting charge carrier transport.Finally,under the dual effects of ETL and HTL,CsPbI₂Br-based devices achieved a PCE of 17.05%,which lays a good foundation for the subsequent preparation of CsPbI₂Br-based tandem devices.（2）In order to broaden the spectral absorption range and improve the PCE of solar cells,all-inorganic perovskite/organic tandem solar cells were constructed using CsPbI₂Br and PCE10:IEICO-4F as ultraviolet visible and near-infrared light absorbing layer materials,respectively.The front subcell exhibits a high transmittance of nearly 80%in the near infrared region,which ensures that long-wavelength and low energy photons can be irradiated into the organic rear subcell.By carefully optimizing the thickness of the active layers and interconnection layer,the short-circuit current density of the front and rear subcells achieved a good-matching.Finally,the all-inorganic perovskite/organic tandem solar cells based on CsPbI₂Br/PCE10:IEICO-4F achieved a PCE of 20.31%,significantly higher than the PCE of single-junction perovskite and organic solar cells.The optical simulation calculates the energy field distribution in the optimal device and verifies the reliability of the experimental results.（3）In order to further improve the PCE of all-inorganic perovskite/organic tandem solar cells,firstly,the photovoltaic performance of three single-junction organic devices based on PM6 donor hybrid films was compared.The PM6:BTP-eC9 thin film with the best photovoltaic performance was selected as the light absorbing layer material of rear subcell.Next,by adjusting the donor/recipient（D/A）ratio of PM6:BTP-eC9 films,a method for adjusting the light field distribution of all-inorganic perovskite/organic tandem solar cells was successfully developed.It is found that the D/A weight ratio and the thickness of the light absorbing layer of the rear absorber significantly affect the photovoltaic performance of the tandem solar cell.When the D/A weight ratio of the PM6:BTP-eC9 light absorbing layer is 1:1.8 and the thickness reaches 150 nm,the rear subcell has the highest utilization efficiency for photons.Under this condition,all-inorganic perovskite/organic tandem solar cell based on CsPbI₂Br/PM6:BTP-eC9 achieved a PCE of 23.35%.（4）In order to solve the problem of poor water stability of CsPbI₂Br in all-inorganic perovskite/organic tandem devices,HPbI3,HPbBr3 and HCOOCs were used as precursors in this work,replacing the traditional precursor systems（PbI2,PbBr2,and CsI）that use to prepare the CsPbI₂Br thin films.A new complex（HCOOH·Cs+）will be generated in the precursor system,and the complex can not only delay the crystallization of perovskite to form highly-quality CsPbI₂Br films,but also stably exist in a humid air environment with a relative humidity of 40%.Photovoltaic performance tests show that the HPbX3-CSPbI2Brbased single-junction device that fabricated based on this precursor system can achieve a high PCE of 17.22%and a high open circuit voltage（VOC）of 1.361 V,and can still maintain an initial PCE of 90%after being stored in 40%ambient air for 500 hours.At the same time,the corresponding HPbX3-CsPbI₂Br/PM6:BTP-eC9 based tandem solar cellachieved a high PCE of 23.12%,with a VOC loss of only 0.024 V.After storing the device in ambient air with a relative humidity of 40%for 500 hours,it can still maintain 92%of the initial PCE.