In this paper we report on systematic studies conducted for the improvement in both the device structure and the materials quality of perovskite based solar cells (PSCs). We have incorporated TiO2 nanorods, of length around 350-400 nm, in the device structure. Such structures were grown by solvothermal technique directly on the glass/FTO substrates. Characterization by femtosecond transient absorption (fs-TA) spectroscopy indicates that the incorporation of TiO2- nanorod array (NA) greatly enhances the collection efficiency of the photo-generated carriers due to substantial reduction in carrier diffusion distance. To improve the crystallinity of the perovskite films we performed systematic studies on cryoassisted growth of the material. The technique eliminates the need for environmentally harmful anti-solvents and enables decoupling of the nucleation and crystallization phases by inhibiting chemical reactions in the precursor films rapidly cooled by immersion in liquid nitrogen. Furthermore, the technique leads to uniform precipitation of precursors due to the supersaturation condition in the residual solvents at cryogenic temperature resulting in highly uniform coverage of the films. Systematic characterization of the films by low-frequency noise and photothermal deflection technique indicate significant in the trap density of the films which is attributed as the main underlying reason for the observed improvement in the power conversion efficiency of the device. A high efficiency of 21.4% is achieved for our champion device.
We present investigations on the growth of high quality CH3NH3PbI3 (MAPI) thin films using both vapor and solution techniques. Recent work on perovskite film growth indicates critical dependencies of the film quality on the nucleation and crystallization steps requiring: i.) uniform distribution of nucleation sites; and ii.) optimal crystallization rate that facilitates the growth of a compact, continuous film with low density of pinholes. Our work shows that the hybrid chemical vapor deposition technique (HCVD) technique is well suited for the deposition of evenly distributed nucleation sites and the optimization of the crystallization rate of the film through detailed monitoring of the thermal profile of the growth process. Signficant reduction in the defect states is recorded by annealing the perovskite films in O2. The results are consistent with theoretical studies by Yin et al. 1 on O and Cl passivation of the shallow states at the grain boundary of MAPI. Their work provides the theoretical basis for our experimental observations on the passivation of shallow states by oxygen annealing. High quality films were achieved through detailed management of the carrier gas composition and the thermal profile of the nucleation and crystallization steps.
Hybrid organic-inorganic perovskite solar cells have attracted lots of attention in recent years. Growth and properties of perovskite layer and its relationship to photovoltaic performance have been extensively studied. Comparably less attention was devoted to the research of the influence of electron transporting layer (ETL). Conventionally, TiO2 is selected as ETL. However, photocatalytic property of this transparent conductive metal oxide reduces the stability of perovskite solar cells under illumination. To realize the commercialization, the stability of perovskite solar cell must be improved. In this study, we replace TiO2 by In2O3, which is not only transparent and conductive, but also has little photocatalytic effect and it has higher electron mobility than TiO2. Investigation on different solution process methods of In2O3 as ETL is demonstrated.
We performed a comprehensive study of the effect of transition metal oxide anode interlayer in bulk heterojunction solar cells based on P3HT:PCBM. We have investigated the influence of different metal oxides including tungsten oxide (WO3), vanadium oxide (V2O5) and molybdenum oxide (MoO3) on the solar cell performance. In addition, the influence of different deposition techniques (solution process and e-beam deposition/ thermal evaporation) has also been investigated. We found that deposition techniques play a significant role on the film quality and morphology and hence affect the photovoltaic performance. Obtained results are discussed in detail.
ZnO as a wide band gap semiconductor is of significant interest for various applications, including dye-sensitized solar cell (DSSC) and photocatalytic degradation of organic pollutants. For DSSC, although the performance of ZnO-based devices is generally inferior to TiO2-based ones, it is still of interest due to its high electron mobility. While the relationship between the material and the device performance are complicated, many studies have been focused on morphologies and surface area of the nanomaterials. The studies of the effect of the material properties such as the types and concentrations of native defects on the DSSC performance have been scarce. For photocatalytic degradation of pollutants, many reports showed ZnO has a higher or similar efficiency compared to the commonly used TiO2. Reports have also pointed out the important role of native defects of ZnO in its photocatalytic activity. Nevertheless, the effect of the type and location of the defects has been contradictory in the literature indicating that there is a complex relationship. Therefore, we will discuss the effect of ZnO native defects on the dye adsorption, charge transport and hence the DSSC performance. We will also discuss their influence on reactive oxygen species (ROS) generation and photocatalytic dye degradation. As photoluminescence (PL) is a common methodology in studying native defects of ZnO, the relationship between PL, DSSC performance and photocatalytic properties will also be investigated. Preliminary results showed a higher overall PL intensity would result in a better device performance and higher photocatalytic activities.
We investigated the influence of ITO properties on the performance of bulk heterojunction solar cells. The morphology, electrical and optical properties of ITO electrodes were characterized. The power conversion efficiency of cells made on different ITO substrates varied significantly from 2.3% to 3.1%. It was found that for a higher sheet resistance ITO substrate, the sheet resistance was the dominant parameter affecting the performance of the based solar cells, while for the lower ( below 20 ohm/square) sheet resistance, transmittance in the region where polymer strongly absorbs and the surface roughness of the substrate had significant effect on the solar cells performance.
In this work, semi-transparent inverted polymer solar cells with poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate; PEDOT:PSS) top electrodes were fabricated by spin-coating process. Poly(3-hexylthiophene; P3HT):[6,6]-phenyl C61-butyric acid methyl ester (PCBM) was used as a model material combination for a bulk heterojunction solar cell, because this material combination has been frequently studied, and its properties and performance have been well established. For enhancing the wetting of P3HT:PCBM blend film, different plasma etching conditions were tried. In addition, different high boiling point organic additives were tried to enhance the conductivity of PEDOT:PSS. The performance of solar cells with different fabrication conditions for the top electrode was compared. The best performance was obtained for Ar plasma etching to improve wetting of PEDOT:PSS and the addition of ethylene glycol to improve conductivity.
We performed a systematic study of the effect of processing conditions on the performance of P3HT:PCBM solar
cells. We have investigated the influence of the source material, solution preparation (stirring vs. sonication),
additives (such as 1,8-octanedithiol), pre-formation of P3HT nanowires, and annealing on the device performance
and/or morphology and phase separation in the active layer. Furthermore, the influence of spin-coating PCBM on
top of P3HT and PCBM on top of the P3HT:PCBM layer has been investigated. We found that all of these factors
affect the performance of the solar cells, although there are several alternative methods which can result in similar
improvements of the performance. We found that the improvement trends for various procedures (additives, PCBM
top layer, P3HT nanowires, etc.) are similar and also strongly dependent on the different source materials. The main
factor determining in the obtainable efficiency is the solution preparation and the source materials. Obtained results
and the implications on further improvement of solar cell performance are discussed in detail.
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