KEYWORDS: Perovskite solar cells; MAPbI3; grain boundaries; single crystalline; power
conversion efficiency; ambient stability.
Perovskite solar cells (PSCs) have shown impressive efficiency due to their direct bandgap,
low exciton binding energy, low processing temperature, ambipolar nature, and excellent
optoelectronic properties. However, their rapid degradation in ambient conditions, especially
when exposed to moisture, hinders their real-world testing and deployment.
This thesis focuses on understanding the stability aspects of methyl ammonium lead iodide
(MAPbI3) based PSCs and providing suitable solutions to enhance the efficiency. Strategies to
improve the ambient stability of n-i-p architecture MAPbI3-based PSCs are addressed by (i)
hole transport material (HTM) modification, (ii) grain boundary engineering, and (iii) precursor
purity engineering.
Inorganic CuSCN as an alternative to the organic HTM which suppresses easy moisture
penetration is demonstrated. By modifying the hole transport layer in the PSCs, a power
conversion efficiency (PCE) of 10% and stability for 1500 h under ambient conditions (25±3
°C and 50±10% RH) is obtained. Apart from improved stability in the inorganic HTM devices,
the role of the CuSCN-Au interface was shown to prevent moisture ingress, unlike the spiroAu and MAPbI3-Au interfaces, which facilitate easy penetration. CuSCN was found to have a
protective role in maintaining the device efficiency and any decrease in the same with time is
only due to the decomposition of the perovskite, as demonstrated experimentally.
Grain boundaries in the perovskite absorber layer, which act as active sites for moisture
penetration, are engineered to improve the stability further. MAPbI3 films with 100 times
larger grain size than conventional films are fabricated using the inverse temperature
crystallization technique. With these films, stability of 5000 h with a modest PCE of 3.2%
under ambient conditions is achieved. The growth of single crystalline MAPbI3 perovskite
film with areal uniformity of ~2 mm2
and thickness of ~10 μm on PTAA/ITO substrates
are presented via a modified ITC method. The antisolvent promoted faster nucleation at a
lower temperature. Decreased growth time from 50 h to 30 h and temperature from 110 °C
to 70 °C did not hamper the film quality or stability. Novel circular shaped films favourable
for photovoltaic applications were obtained.
A novel technique employing single crystals is designed to draw a consensus between
efficiency and stability. The precursor ink quality derived from single crystals is examined
for its purity and optical properties were found to be better than its powder-derived
counterparts. The absence of impurities and high boiling point solvents in the precursor
resulted in rapid crystallization during spin coating, eliminating unnecessary intermediate
phases. This novel ink required no additional compounds or post treatment for high
functioning stable PSCs. Improving the precursor purity of MAPbI3 ink, used to fabricate
perovskite films, played a crucial role in enhancing both efficiency and stability. A PCE of
16.7% with stability of 3500 h under ambient conditions is achieved by obtaining the
precursor ink from single crystals through a vapor-mediated approach. These
advancements are crucial for making perovskite solar cells viable for outdoor applications
where various environmental conditions over extended periods are unavoidable.
The areas for improvement and future scope of PSCs are discussed.