Controlling The Band Gap To Improve Open-circuit Voltage In Metal Chalcogenide Based Perovskite Solar Cells

In the past few years,the methylammonium lead halide perovskite（MAPbX₃）based solar cell has led to the development of high efficiency photovoltaic devices,due to its direct band gap,large absorption coefficient and high carrier mobility properties.Up to now,a landmark photoelectric conversion efficiency over 21% has been obtained by using spiro-OMeTAD as hole-transport material（HTM）,but the expensive cost and complicated synthetic procedure of this material may limit its future application.Furthermore,the presence of hygroscopic lithium salt in organic HTMs could easily lead to the degradation of perovskite by oxygen and moisture.Searching for more convenient HTMs to keep high cell efficiency is a great challenge.Inorganic HTMs are promising alternatives because of their suitable energy band alignment with perovskite,high hole mobility,solution processable and comfortable stability.Resistance of HTMs can affect the fill factor of the perovskite solar cell and the band gap of HTMs will affect the open circuit voltage(V_oc)of device.Herein,we employed the band-tunable metal chalcogenides with high hole mobility(>10cm²·V^-1·s^-1)as HTM into perovskite solar cells.By exploring the influence of band gap on cell performances,we provided theoretical evidences for the future selection of HTMs.This thesis contains following contents:（1）The influence of S/Se ratio on perovskite performances:We employed the band-tunable quaternary CZTS QDs with high hole mobility(12.6 cm²·V^-1·s^-1)as HTM into perovskite solar cells and tuned their band gap by replacing sulfur with selenium atom.Under the same method and condition,CZTS QDs and CZTSe QDs were successfully synthesized,open circuit voltage of perovskite solar cells respectively achieved 945 mV,808 mV.By analyzing the energy level alignment and recombination process at perovskite/HTM interface,we found that the V_oc losses of CZTSe device was mainly attributed to its shallower valence band rather than the larger recombination process.（2）Replacing indium with gallium atom to improve open circuit voltage:The V_oc of CZTS device was relatively low,we applied ternary CIS QDs as HTM into perovskite solar cells and tuned their band gap by replacing indium with gallium atom.The CGS device achieved an impressive conversion efficiency of 14.35%.By modulating the conductibility and band-gap structure of hole-transport layer,we showed the physical origin of the V_oc losses in metal chalcogenides HTM based perovskite solar cells.This further approves that a wide band gap of metal chalcogenide HTM is more convenient to achieve a high V_oc for perovskite solar cells.