Organic-inorganic hybrid perovskites as light absorbing/hole conducting material in solar cells
Solar cells involving two different perovskites were manufactured and analyzed. The
perovskites were (CH3NH3)PbI3 and (CH3NH3)SnI3. Both perovskites have a
shared methyl ammonium group (MA) and are used as both light absorbing material
and hole conducting material (HTM) in this project. The preparation procedures for
the complete device were according to previous attempts to make stable
organic-inorganic hybrid perovskites and involved different layers and procedures.
Both perovskites were manufactured by mixing methyl ammonium iodide with either
lead iodide or tin iodide in different concentrations. This was then deposited on a
600nm thick mesoporous TiO2 layer. Deposition of the hole-transporting material
(HTM) was done by spin-coating 2,2´,7,7´-tetrakis-(N,N-dip-methoxyphenylamine)
9,9´-spirobifluorene, also called spiro-OMeTAD. Lastly thermal evaporation was used
to deposit a silver electrode.
Different measurements were done on the light absorbing materials. The lead
perovskite solar cell device was subjected to illumination with Air Mass 1.5 sunlight
(100mW/cm2) which produced an open circuit voltage Voc of 0.645 V, a short circuit
photocurrent Jsc of about 7 mA/cm2, and a fill factor FF of 0.445. This resulted in a
power conversion efficiency (PCE) of about 2% and an incident photon to current
efficiency (IPCE) of up to 60%.
The tin perovskite has not been used in solar cells before and the initial results
presented here shows low performance using the same device construction as for the
lead perovskite. However, the incident photon to electron conversion affirms that
there is a current in the visible region, and IPCE of 12.5 % was observed at 375nm.
UV-visible NIR measurement was used to analyze the light absorption of the
perovskite structures and a broader light absorption was observed for the lead
perovskite compared to the tin perovskite.
X-ray diffraction (XRD) analyzing was done on both perovskite materials using
different concentrations and both with and without nanoporous TiO2 film. Both
perovskites demonstrate very similar peaks with some exceptions.
Photo-induced absorption (PIA) measurement was used for the purpose of showing
the magnitude of charge separation or hole transfer in the light absorbing material,
both when using the perovskites as a light absorber and a hole conductor. This is
measured by analyzing the hole injection from the excited light absorber into the
HTM. Hole transfer was observed for the lead perovskite (when used as light
absorber) and tin perovskite (when used as hole conductor).
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