Dual-source vapor-phase deposition enables low-temperature fabrication of high-performance planar structure perovskite (CHNHPbI) solar cells (PSCs), applicable in tandem devices or for industrial production with high homogeneity. Herein, we report low-temperature fabrication of high-efficiency PSCs by dual-source vapor-phase deposition and significance of TiO surface modification with [6,6]-phenyl C butyric acid methyl ester (PCBM) on cell performance. Co-evaporation of PbI and CHNHI, as confirmed by X-ray diffraction and high-resolution transmission electron microscopy analyses, results in CHNHPbI layers with a well-crystallized tetragonal phase formed on both TiO and TiO/PCBM electron-transport layers (ETLs). The devices with PCBM interlayer between TiO and CHNHPbI showed remarkably higher performance than those with TiO only, which was attributed to enhance charge extraction and reduced recombination at the TiO/PCBM/CHNHPbI interface. The devices composed of evaporated CHNHPbI on top of the TiO/PCBM and [2,2',7,7'-tetrakis( N, N-di- p-methoxyphenyl-amine)-9,9'-spirobifluorene] (Spiro-OMeTAD) as hole-transport material demonstrated power conversion efficiencies of 17.1% (reverse scan) and 13.4% (forward scan) with stabilized efficiency of over 16%, which is, to the best of our knowledge, the highest efficiency reported for evaporated perovskite solar cells using low-temperature fabrication method involving compact TiO layer as ETL. Furthermore, we show that this process can be used to deposit a CHNHPbI layer on top of a textured silicon substrate, which is the first step for preparing perovskite-silicon tandem devices with enhanced antireflection and light-trapping properties.
Perovskite
solar cells have become a game changer in the field of photovoltaics
by reaching power conversion efficiencies beyond 23%. To achieve even
higher efficiencies, it is necessary to increase the understanding
of crystallization, grain formation, and layer ripening. In this study,
by a systematic variation of methylammonium iodide (MAI) concentrations,
we changed the stoichiometry and thereupon the crystal growth conditions
in MAPbI3 perovskite solar cells, prepared by a two-step
hybrid evaporation–spin-coating deposition method. Utilizing
X-ray diffraction, scanning electron microscopy, atomic force microscopy,
photoluminescence, and current–voltage (J–V) characterization, we found that a relatively lower concentration
of MAI, or in other words higher content of remnant and unconverted
PbI2, correlates with smaller and stronger interconnected
grains, as well as with an improved optoelectronic performance of
the solar cells and mitigation of hysteresis. The possible explanations
are grain and interface passivation by the excess PbI2 and
a positive contribution of the grain boundaries to current extraction.
Homogeneous layer
formation on textured silicon substrates is essential
for the fabrication of highly efficient monolithic perovskite silicon
tandem solar cells. From all well-known techniques for the fabrication
of perovskite solar cells (PSCs), the evaporation method offers the
highest degree of freedom for layer-by-layer deposition independent
of the substrate’s roughness or texturing. Hole-transporting
polymers with high hole mobility and structural stability have been
used as effective hole-transporting materials (HTMs) of PSCs. However,
the strong intermolecular interactions of the polymers do not allow
for a layer formation via the evaporation method, which is a big challenge
for the perovskite community. Herein, we first applied a hole-transporting
terthiophene polymer (PTTh) as an HTM for evaporated PSCs via an in
situ vapor-phase polymerization using iodine (I2) as a
sublimable oxidative agent. PTTh showed high hole mobility of 1.2
× 10–3 cm2/(V s) and appropriate
energy levels as HTM in PSCs (E
HOMO =
−5.3 eV and E
LUMO = −3.3
eV). The PSCs with the in situ vapor-phase polymerized PTTh hole-transporting
layer and a co-evaporated perovskite layer exhibited a photovoltaic
conversion efficiency of 5.9%, as a proof of concept, and high cell
stability over time. Additionally, the polymer layer could fully cover
the pyramidal structure of textured silicon substrates and was identified
as an effective hole-transporting material for perovskite silicon
tandem solar cells by optical simulation.
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