Mark Speirs

Origin of the increased open circuit voltage in PbS-CdS core-shell quantum dot solar cells

Lead sulfide quantum dots (PbS QDs) show great potential for efficient, low cost photovoltaic applications. Currently, device efficiencies are limited by the high density of trap states caused by lattice imperfections on the QD surface. Introducing a thin shell of a wide bandgap semiconductor to the QD surface is a promising method to passivate these trap states. Here we demonstrate solar cells made from PbS–CdS core–shell QDs, yielding a 147 mV increase in VOC compared to core only PbS QDs. We explore the physical reason for this enhancement and demonstrate that it is indeed caused by improved passivation of the PbS surface by the CdS shell, leading to a lower electron trap density.

Temperature dependent behaviour of lead sulfide quantum dot solar cells and films

Despite increasing greatly in power conversion efficiency in recent times, lead sulfide quantum dot (PbS QD) solar cells still suffer from a low open circuit voltage (VOC) and fill factor (FF). In this work, we explore the temperature dependent behavior of ∼9% efficient solar cells. In the temperature range of 290 to 230 K, we find increased VOC and FF values without significant degradation of the short circuit current, leading to up to 10.3% efficiency at 230 K. The change in VOC is driven by the decrease of the reverse saturation current which fits the PN-junction model. Using Schottky and single carrier devices, we measure the carrier mobility, diffusion lengths, and doping concentrations of PbS QD films with tetrabutylammonium iodide and ethane dithiol ligands as a function of temperature. Both mobility and diffusion length are found to decrease with decreasing temperature while device performance increases, indicating that the 260 nm thick active layer is fully depleted. Finally, we propose that further optimization of the doping concentrations could help achieve increased device performance at room temperature.

Increased efficiency in pn-junction PbS QD solar cells via NaHS treatment of the p-type layer

Lead sulfide quantum dot (PbS QD) solar cell efficiencies have improved rapidly over the past years due in large part to intelligent band alignment considerations. A pn-junction can be formed by connecting PbS layers with contrasting ligands. However, the resulting doping concentrations are typically low and cannot be effectively controlled. Here, we present a method of chemically p-doping films of thiol capped PbS QDs. P-n junction solar cells with increased doping in the p-type layer show improved short circuit current and fill factor, leading to an improvement in the power conversion efficiency from 7.1% to 7.6%. By examining Schottky diodes, field effect transistors, and the absorption spectra of treated and untreated PbS QDs, we show that the improved efficiency is due to the increased doping concentration in the thiol capped QD layer and to denser packing of the PbS QD film.

Increasing photon absorption and stability of PbS quantum dot solar cells using a ZnO interlayer

We compared PbS quantum dot (QD) solar cells with different cathode interlayer materials, namely, LiF and ZnO nanoparticles, using the same device structure. Solar cells fabricated with the ZnO interlayer gave a power conversion efficiency of 4.8%, which is higher (above the experimental variation) than the 4.1% efficiency obtained with a LiF interlayer. We found that the ZnO interlayer alters the spatial distribution of the optical field, leading to an increase in external quantum efficiency in the visible range. Furthermore, devices with ZnO as interlayer showed more stable performance than the ones using LiF, with practically no power conversion efficiency degradation after 1 month inside a N2 glovebox.

Organic and Hybrid Solar Cells

Over the last two decades, the need for cheap, clean, and renewable energy has been the driving force behind the field of solution-processable organic photovoltaics (OPV). In contrast to their inorganic counterparts, typical organic semiconductors possess a low relative dielectric constant (~ 3–5), resulting in the formation of a Frenkel exciton upon photoexcitation, as opposed to free charge carriers. Thus, a donor–acceptor heterojunction to separate the exciton is necessary. In 1992, ultrafast electron transfer from conducting polymers to fullerene molecules was reported by Saricifti et al. The subsequent synthesis of a soluble fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PC60BM) by Hummelen et al., opened the pathway to the first solution-processable organic solar cells (OSCs) with appreciable efficiency. Almost two decades later, PCBM is still the most widely used acceptor molecule and has been a crucial factor for the rapid progress of the OSC field.

Colloidal Inorganic–Organic Hybrid Solar Cells: Organic and Hybrid Solar Cells

Over the last two decades, the need for cheap, clean, and renewable energy has been the driving force behind the field of solution-processable organic photovoltaics (OPV). In contrast to their inorganic counterparts, typical organic semiconductors possess a low relative dielectric constant (~ 3–5), resulting in the formation of a Frenkel exciton upon photoexcitation, as opposed to free charge carriers. Thus, a donor–acceptor heterojunction to separate the exciton is necessary. In 1992, ultrafast electron transfer from conducting polymers to fullerene molecules was reported by Saricifti et al. The subsequent synthesis of a soluble fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PC60BM) by Hummelen et al., opened the pathway to the first solution-processable organic solar cells (OSCs) with appreciable efficiency. Almost two decades later, PCBM is still the most widely used acceptor molecule and has been a crucial factor for the rapid progress of the OSC field.

Colloidal Inorganic–Organic Hybrid Solar Cells

Over the last two decades, the need for cheap, clean, and renewable energy has been the driving force behind the field of solution-processable organic photovoltaics (OPV). In contrast to their inorganic counterparts, typical organic semiconductors possess a low relative dielectric constant (~ 3–5), resulting in the formation of a Frenkel exciton upon photoexcitation, as opposed to free charge carriers. Thus, a donor–acceptor heterojunction to separate the exciton is necessary. In 1992, ultrafast electron transfer from conducting polymers to fullerene molecules was reported by Saricifti et al. The subsequent synthesis of a soluble fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PC60BM) by Hummelen et al., opened the pathway to the first solution-processable organic solar cells (OSCs) with appreciable efficiency. Almost two decades later, PCBM is still the most widely used acceptor molecule and has been a crucial factor for the rapid progress of the OSC field.