20 research outputs found
Effective Bandgap Lowering of CdS Deposited by Successive Ionic Layer Adsorption and Reaction
SILAR (successive ionic layer adsorption and reaction)
is a solution
process used to deposit semiconductors that has become very common
in the past few years as a method to deposit light absorbers in nanoporous
solar cells. Films of CdS (possibly the most commonly deposited semiconductor
using this method) often show an anomalously low apparent bandgap
(lowered by as much as 10%) for sufficiently thick films, although
this effect has been ignored in most cases. Here, we study this bandgap
lowering and show that it is due not so much to a lower bandgap but
rather to a particularly long absorption tail that extends far into
the red, and that is amplified by a large optical thickness in the
high surface area nanoporous films. The tail is presumably due to
as yet unidentified, but probably bulk defects in the CdS. Additionally,
the absorption coefficient of the SILAR CdS was nearly twice as high
as normal values
How Important Is the Organic Part of Lead Halide Perovskite Photovoltaic Cells? Efficient CsPbBr<sub>3</sub> Cells
Hybrid organicāinorganic lead
halide perovskite photovoltaic
cells have already surpassed 20% conversion efficiency in the few
years that they have been seriously studied. However, many fundamental
questions still remain unanswered as to why they are so good. One
of these is āIs the organic cation really necessary to obtain
high quality cells?ā In this study, we show that an all-inorganic
version of the lead bromide perovskite material works equally well
as the organic one, in particular generating the high open circuit
voltages that are an important feature of these cells
Band Alignment and Internal Field Mapping in Solar Cells
The internal fields and band offsets developing at individual interfaces, a critical aspect of device performance, are generally inaccessible by standard electrical tools. To address this problem, we propose chemically resolved electrical measurements (CREM) capable of resolving the internal details layer-by-layer. Applied to nanoporous photovoltaic cells, we thus extract a realistic band diagram for the multi-interfacial structure and, in particular, resolve the two p-n-like junction fields built spontaneously in the device. The lack of homogeneity common to many of these nanoporous cells is exploited here to āseeā deep into the cell structure, beyond the typical depth limitations of the surface-sensitive technique. Further information on the cell operation under ārealā working conditions is achieved by studying the charge trapping at each specific layer under optical and electrical stimuli. Our methodology overcomes a missing link in device characterization and in fundamental studies of nanoscale solid-state devices
Band Alignment in Partial and Complete ZnO/ZnS/CdS/CuSCN Extremely Thin Absorber Cells: An Xāray Photoelectron Spectroscopy Study
In
all solar cells, and especially in extremely thin absorber (ETA) solar
cells, proper energy band alignment is crucial for efficient photovoltaic
conversion. However, available tabulated data usually do not agree
with actual results, and in most cases, <i>V</i><sub>oc</sub> values lower than expected are achieved. In fact, ETA cells suffer
from a very low <i>V</i><sub>oc</sub>/<i>E</i><sub>gap</sub> ratio, such as in ZnO/CdS/CuSCN cells. Here, we investigate
limiting factors of ZnO/CdS/CuSCN ETA cells, applying X-ray photoelectron
spectroscopy (XPS), chemically resolved electrical measurement (CREM),
Kelvin probe, and <i>I</i>ā<i>V</i> characterization.
We show that electric fields are gradually developed in the cell upon
increased absorber thickness. Moreover, an accumulation layer, unfavorable
for the solar cell function, has been revealed at the oxideāabsorber
interface An effective chemical treatment to prevent formation of
this accumulation layer is demonstrated
CsSnBr<sub>3</sub>, A Lead-Free Halide Perovskite for Long-Term Solar Cell Application: Insights on SnF<sub>2</sub> Addition
Solar cells based on āhalide
perovskitesā (HaPs) have demonstrated unprecedented high power
conversion efficiencies in recent years. However, the well-known toxicity
of lead (Pb), which is used in the most studied cells, may affect
its widespread use. We explored an all-inorganic lead-free perovskite
option, cesium tin bromide (CsSnBr<sub>3</sub>), for optoelectronic
applications. CsSnBr<sub>3</sub>-based solar cells exhibited photoconversion
efficiencies (PCEs) of 2.1%, with a short-circuit current (<i>J</i><sub>SC</sub>) of ā¼9 mA cm<sup>ā2</sup>,
an open circuit potential (<i>V</i><sub>OC</sub>) of 0.41
V, and a fill factor (FF) of 58% under 1 sun (100 mW cm<sup>ā2</sup>) illumination, which, even though meager compared to the Pb analogue-based
cells, are among the best reported until now. As reported earlier,
addition of tin fluoride (SnF<sub>2</sub>) was found to be beneficial
for obtaining good device performance, possibly due to reduction of
the background carrier density by neutralizing traps, possibly via
filling of cation vacancies. The roles of SnF<sub>2</sub> on the properties
of the CsSnBr<sub>3</sub> were investigated using ultraviolet photoemission
spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS) analysis
Higher Open Circuit Voltage and Reduced UV-Induced Reverse Current in ZnO-Based Solar Cells by a Chemically Modified Blocking Layer
Solid-state semiconductor-sensitized
solar cells require a thin, dense hole-blocking layer at the conducting
glass substrate (F-doped tin oxide (FTO)) to prevent shorting beween
the FTO and hole conductor. We found that by adding a small amount
of Sb ions to a ZnO chemical deposition bath a thin (few tens of nanometers
thick) dense and uniform layer of Sb-incorporated ZnO forms. Here
we investigate the electronic properties of this layer in comparison
to the continuous ZnO layer at the base of the ZnO rods formed in
the standard preparation. Devices incorporating the Sb-incorporated
dense layer followed by a standard ZnO nanorod growth, onto which
CdS or CdSe was grown followed by a CuSCN hole conductor, showed 100ā200
mV higher photovoltage together with occasional improvement in the
short-circuit current. Electrochemical and electrical measurements
indicated complete coverage of the FTO substrate by both preparations;
however, the shunt resistance (resistance to a reverse leakage current)
in the cells (and films) made using the Sb-incorporated ZnO layer
is dramatically increased. Using bias-dependent incident photon-to-electron
conversion efficiency studies, we found that an increased dark or
leakage current develops in the cell on illumination with UV light
together with application of a forward bias. This can be explained
by the presence of a āSchottky junctionā at the FTO\ZnO
interface. This increased leakage current is significantly larger
in cells without the Sb-incorporated ZnO compact layer
Rain on Methylammonium Lead Iodide Based Perovskites: Possible Environmental Effects of Perovskite Solar Cells
The
great promise of hybrid organicāinorganic lead halide
perovskite (HOIP)-based solar cells is being challenged by its Pb
content and its sensitivity to water. Here, the impact of rain on
methylammonium lead iodide perovskite films was investigated by exposing
such films to water of varying pH values, simulating exposure of the
films to rain. The amount of Pb loss was determined using both gravimetric
and inductively coupled plasma mass spectrometry measurements. Using
our results, the extent of Pb loss to the environment, in the case
of catastrophic module failure, was evaluated. Although very dependent
on module siting, even total destruction of a large solar electrical
power generating plant, based on HOIPs, while obviously highly undesirable,
is estimated to be far from catastrophic for the environment
Cesium Enhances Long-Term Stability of Lead Bromide Perovskite-Based Solar Cells
Direct comparison between perovskite-structured
hybrid organicāinorganic
methylammonium lead bromide (MAPbBr<sub>3</sub>) and all-inorganic
cesium lead bromide (CsPbBr<sub>3</sub>), allows identifying possible
fundamental differences in their structural, thermal and electronic
characteristics. Both materials possess a similar direct optical band
gap, but CsPbBr<sub>3</sub> demonstrates a higher thermal stability
than MAPbBr<sub>3</sub>. In order to compare device properties, we
fabricated solar cells, with similarly synthesized MAPbBr<sub>3</sub> or CsPbBr<sub>3</sub>, over mesoporous titania scaffolds. Both cell
types demonstrated comparable photovoltaic performances under AM1.5
illumination, reaching power conversion efficiencies of ā¼6%
with a poly aryl amine-based derivative as hole transport material.
Further analysis shows that Cs-based devices are as efficient as,
and more stable than methylammonium-based ones, after aging (storing
the cells for 2 weeks in a dry (relative humidity 15ā20%) air
atmosphere in the dark) for 2 weeks, under constant illumination (at
maximum power), and under electron beam irradiation
Deleterious Effect of Negative Capacitance on the Performance of Halide Perovskite Solar Cells
Negative capacitance in photovoltaic devices has been observed and reported in several cases, but its origin, at low or intermediate frequencies, is under debate. Here we unambiguously demonstrate a direct correlation between the observation of this capacitance and a corresponding decrease in performance of a halide perovskite (HaP; CsPbBr3)-based device, expressed as reduction of open-circuit voltage and fill factor. We have prepared highly stable CsPbBr3 HaPs that do not exhibit any degradation over the duration of the impedance spectroscopy measurements, ruling out degradation as the origin of the observed phenomena. Reconstruction of current-voltage curves from the impedance spectroscopy provided further evidence of the deleterious role of negative capacitance on photoconversion performance
Impedance Spectroscopic Indication for Solid State Electrochemical Reaction in (CH<sub>3</sub>NH<sub>3</sub>)PbI<sub>3</sub> Films
Halide perovskite-based solar cells
still have limited reproducibility,
stability, and incomplete understanding of how they work. We track
electronic processes in [CH<sub>3</sub>NH<sub>3</sub>]ĀPbI<sub>3</sub>(Cl) (āperovskiteā) films <i>in vacuo</i>, and in N<sub>2</sub>, air, and O<sub>2</sub>, using impedance spectroscopy
(IS), contact potential difference, and surface photovoltage measurements,
providing direct evidence for perovskite sensitivity to the ambient
environment. Two major characteristics of the perovskite IS response
change with ambient environment, viz. -1- appearance of negative capacitance <i>in vacuo</i> or post<i>-vacuo</i> N<sub>2</sub> exposure,
indicating for the first time an electrochemical process in the perovskite,
and -2- orders of magnitude decrease in the film resistance upon transferring
the film from O<sub>2</sub>-rich ambient atmosphere to vacuum. The
same change in ambient conditions also results in a 0.5 V decrease
in the material work function. We suggest that facile adsorption of
oxygen onto the film dedopes it from n-type toward intrinsic. These
effects influence any material characterization, i.e., results may
be ambient-dependent due to changes in the materialās electrical
properties and electrochemical reactivity, which can also affect material
stability