Organic-inorganic hybrid perovskites as light absorbing/hole conducting material in solar cells

Abstract:

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).