Influence of Selective Nucleation on the One Step Chemical Bath Deposition of CdS/ZnO and CdS/ZnS Composite Films

We show how the composition and the morphology of composite films deposited by one step chemical bath deposition (CBD) can be controlled by varying kinetic factors, in particular by selective nucleation, as well as varying the rate of growth of specific phases. ZnS/CdS composite films are obtained from a solution containing a mixture of Zn and Cd salts and thiourea under conditions where nucleation of zinc oxide is not favorable, while CdS/ZnO films grow if ZnO nucleation is promoted. In the absence of Cd in the deposition solution, only ZnS forms under all the conditions used by us. The role of Cd in directing the deposition to ZnO is believed to be due to the higher affinity of Cd for S compared to Zn, therefore preventing formation of ZnS in the early stages of deposition and allowing ZnO to nucleate. This demonstrates the importance of interaction between the various components in the solution. Addition of ethanol to the deposition solution, believed to serve as a dehydrating-assisting agent for Zn(OH)2, had a crucial role on the deposition outcome by accelerating formation of ZnO in the bulk of the solution, and leading to more rapid depletion of Zn with respect to Cd

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

Charge Overlap Interaction in Quantum Dot Films: Time Dependence and Suppression by Cyanide Adsorption

Chemical bath deposited films of CdSe nanocrystals (<4 nm) are shown to exhibit time-dependent spectral red shifts, caused by increasing overlap of the electron wave functions in adjacent nanocrystals. Treatment of these “aggregated” films with aqueous KCN solution results in repulsion of the wave functions due to the strongly adsorbed negatively charged cyanide and thus electronic decoupling of the physically connected nanocrystals. The previously reported band gap increase due to cyanide adsorption on nominally uncoupled nanocrystals is also described here in more detail

Reproducible Chemical Bath Deposition of ZnO by a One-Step Method: The Importance of “Contaminants” in Nucleation

Reproducible Chemical Bath Deposition of ZnO by a One-Step Method: The Importance of “Contaminants” in Nucleatio

How Important Is the Organic Part of Lead Halide Perovskite Photovoltaic Cells? Efficient CsPbBr₃ Cells

Hybrid organic–inorganic 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 “Is the organic cation really necessary to obtain high quality cells?” 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 in Partial and Complete ZnO/ZnS/CdS/CuSCN Extremely Thin Absorber Cells: An X‑ray 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>_oc values lower than expected are achieved. In fact, ETA cells suffer from a very low <i>V</i>_oc/<i>E</i>_gap 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>–<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 oxide–absorber interface An effective chemical treatment to prevent formation of this accumulation layer is demonstrated

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 “see” deep into the cell structure, beyond the typical depth limitations of the surface-sensitive technique. Further information on the cell operation under “real” 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

Sb₂S₃-Sensitized Nanoporous TiO₂ Solar Cells

Extremely Thin Absorber (ETA) solar cells were made using chemical-bath-deposited Sb2S3 as the absorber and TiO2/CuSCN as the interpenetrating electron/hole conductors. A solar conversion efficiency of 3.37% at 1 sun illumination was obtained. Surface oxidation of the Sb2S3 formed a passivation layer on Sb2S3: without this oxidation, much poorer cells were obtained. Preliminary stability measurements showed good stability over 3 days of illumination (at 60 mW/cm2) under load

CsSnBr₃, A Lead-Free Halide Perovskite for Long-Term Solar Cell Application: Insights on SnF₂ Addition

Solar cells based on “halide perovskites” (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 (CsSnBr₃), for optoelectronic applications. CsSnBr₃-based solar cells exhibited photoconversion efficiencies (PCEs) of 2.1%, with a short-circuit current (<i>J</i>_SC) of ∼9 mA cm^–2, an open circuit potential (<i>V</i>_OC) of 0.41 V, and a fill factor (FF) of 58% under 1 sun (100 mW cm^–2) 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 (SnF₂) 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 SnF₂ on the properties of the CsSnBr₃ 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 100–200 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 “Schottky junction” at the FTO\ZnO interface. This increased leakage current is significantly larger in cells without the Sb-incorporated ZnO compact layer