The
band diagram in heterojunction solar cells is of utmost importance
when visualizing the possibility of charge separation and carrier
transport. The diagram should in principle be drawn from the viewpoint
of the charge carriers in the devices. While considering solar cells
based on conjugated organics and/or inorganic compound semiconductors,
we have shown in this Perspective that scanning tunneling spectroscopy
(STS) can draw appropriate band diagrams, which the carriers would
encounter during the separation and transport processes. Moreover,
differential conductance (dI/dV)
images in scanning tunneling microscopy are capable of energy mapping;
one can therefore map domains of the materials in a bulk heterojunction
(BHJ). Correlation between morphology of the components in BHJs and
device performance can therefore be envisaged through STS.
We introduce antimony-doped
hybrid perovskite compounds in planar
inverted solar cells. Here, we report in-depth and systematic studies
on the formation of the perovskite layer through a modified two-step
spin-coat method. In this method, the “loading time”
of CH3NH3I on a “wet” PbI2 layer was varied in achieving a complete conversion to the
perovskite material. The “loading time” that in turn
also controlled morphology of the perovskite layer along with the
antimony content in perovskite compounds was varied to optimize the
solar cell performances. The effect of dopant content has affected
the band diagram, which was drawn from density of states of the components
as derived from their scanning tunneling spectroscopy. The solar cell
parameters were then correlated with the experimental band diagram
of the heterojunctions. In Cu@NiO|CH3NH3Pb0.92Sb0.08I3|PCBM p–i–n
heterojunctions, we have achieved a high open-circuit voltage of 1.13
V with an energy conversion efficiency of 12.8%. The solar cell parameters
have been correlated with junction properties, which have been studied
through in-depth analysis of diode characteristics.
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