Study Of Solid-state Tin Sulfide Quantum Dot-sensitized Solar Cell

Quantum dot-sensitized solar cell (QDSSC) is one of the third generation solarcells, which makes use of inorganic semiconductor nanoparticles (generally smallerthan10nm) as light absorption layer. The size of the particles can be tuned throughchanging the synthesis condition (temperature, reaction time, density of precursor, etc.)so that ideal band alignment can be designed. Owing to the quantum confinement effectbrought out by the nanoparticles QDSSC can convert wide range of light its theoreticalconversion efficiency is reported to be75%.Much effort has been paid to the synthesis and application of II-VI and IV–VIseries semiconductors, such as PbS, PbSe, PbTe and SnTe. In these cases the toxicityand radio-active property are significant issues. Plus one more problem should beovercome: liquid electrolyte’s temperature stability problem. In this research tin sulfide(SnS) is chosen for the nanoparticles which is less toxic than the materials introducedpreviously and easy to get. Also organic charge transport materials such asspiro-OMeTAD as solid state electrolyte. The aim of this research is to fabricate SnSQDSSC applying spiro-OMeTAD as solid state electrolyte (ss-QDSSC). Plusinvestigate their energetic alignment carefully by measuring the conduction band’s (CB)level and valence band’s (VB) level shift through XPS measurement.Different size of nanoparticles (3-4,5-6,7-8,11-12,13-14nm) are prepared bycontrolling the synthesis temperatures (60,90,120,150,180℃). Smaller SnSexhibited more prominent quantum confinement effect and thus the band gapwas enlarged leading to blue-shifted sunlight absorption threshold. XPS resultsconfirmed that VB’s downward shift and CB’s upward shift contributedcomparatively to the enlarged band gap for pure SnS QDs. XRD measurementconfirmed that the phase of the as-prepared SnS was orthorhombicherzenbergite.Different sized tin sulfide quantum dot (SnS QDs) has been assembled on TiO2films by using bifunctional surface modifiers (MPA). Porous TiO2/SnS QD/solid stateelectrolyte (Spiro-OMeTAD) structure is designed and applied to solid state quantumdot-sensitized solar cell (ssQDSSC). SnS QDs inject electrons into TiO2nanoparticlesthus enable generation of photocurrent in a photoelectrochemical solar cell. Along with the size down of nanoparticles, in TiO2/SnS structure, the conduction band minimum(CBM) between TiO2and SnS has tendency to decrease, however in every synthesistemperature reached to0.8eV. This can provide enough driving power to injectelectrons. Meanwhile, the sensitizer regeneration driving force, that is thedifference between the valence band maximum (VBM) of SnS and the workfunction of the electrolyte, inversely got to improve which was caused by thedownward shift of SnS’s VB.The performances of QDSSC were compared and SnS synthesized an90℃turnedout to have the best performance among those cells. Photo current density of0.078mA/cm2, open circuit voltage of0.342V, fill factor of38.2%, and overall energyconversion efficiency of0.01%was measured.