James A. Dorman

Research Update: Physical and electrical characteristics of lead halide perovskites for solar cell applications

The field of thin-film photovoltaics has been recently enriched by the introduction of lead halide perovskites as absorber materials, which allow low-cost synthesis of solar cells with efficiencies exceeding 16%. The exact impact of the perovskite crystal structure and composition on the optoelectronic properties of the material are not fully understood. Our progress report highlights the knowledge gained about lead halide perovskites with a focus on physical and optoelectronic properties. We discuss the crystal and band structure of perovskite materials currently implemented in solar cells and the impact of the crystal properties on ferroelectricity, ambipolarity, and the properties of excitons.

Toward High-Efficiency Solution-Processed Planar Heterojunction Sb2S3 Solar Cells

Low‐cost hybrid solar cells have made tremendous steps forward during the past decade owing to the implementation of extremely thin inorganic coatings as absorber layers, typically in combination with organic hole transporters. Using only extremely thin films of these absorbers reduces the requirement of single crystalline high‐quality materials and paves the way for low‐cost solution processing compatible with roll‐to‐roll fabrication processes. To date, the most efficient absorber material, except for the recently introduced organic–inorganic lead halide perovskites, has been Sb2S3, which can be implemented in hybrid photovoltaics using a simple chemical bath deposition. Current high‐efficiency Sb2S3 devices utilize absorber coatings on nanostructured TiO2 electrodes in combination with polymeric hole transporters. This geometry has so far been the state of the art, even though flat junction devices would be conceptually simpler with the additional potential of higher open circuit voltages due to reduced charge carrier recombination. Besides, the role of the hole transporter is not completely clarified yet. In particular, additional photocurrent contribution from the polymers has not been directly shown, which points toward detrimental parasitic light absorption in the polymers. This study presents a fine‐tuned chemical bath deposition method that allows fabricating solution‐processed low‐cost flat junction Sb2S3 solar cells with the highest open circuit voltage reported so far for chemical bath devices and efficiencies exceeding 4%. Characterization of back‐illuminated solar cells in combination with transfer matrix‐based simulations further allows to address the issue of absorption losses in the hole transport material and outline a pathway toward more efficient future devices.

Influence of interfacial area on exciton separation and polaron recombination in nanostructured bilayer all-polymer solar cells.

The macroscopic device performance of organic solar cells is governed by interface physics on a nanometer scale. A comb-like bilayer all-polymer morphology featuring a controlled enhancement in donor-acceptor interfacial area is employed as a model system to investigate the fundamental processes of exciton separation and polaron recombination in these devices. The different nanostructures are characterized locally by SEM/AFM, and the buried interdigitating interface of the final device architecture is statistically verified on a large area via advanced grazing incidence X-ray scattering techniques. The results show equally enhanced harvesting of photoexcitons in both donor and acceptor materials directly correlated to the total enhancement of interfacial area. Apart from this beneficial effect, the enhanced interface leads to significantly increased polaron recombination losses both around the open-circuit voltage and maximum power point, which is determined in complement with diode dark current characteristics, impedance spectroscopy, and transient photovoltage measurements. From these findings, it is inferred that a spatially optimized comb-like donor-acceptor nanonetwork alone is not the ideal morphology even though often postulated. Instead, the energetic landscape has to be considered. A perfect morphology for an excitonic solar cell must be spatially and energetically optimized with respect to the donor-acceptor interface.

Effect of dopant concentration on visible light driven photocatalytic activity of Sn1−xAgxS2

Tin(iv) sulfide (SnS2), as a mid-band-gap semiconductor shows good potential as an excellent photocatalyst due to its low cost, wide light spectrum response and environment-friendly nature. However, to meet the demands of large-scale water treatment, a SnS2 photocatalyst with a red-shifted band gap, increased surface area and accelerated molecule and ion diffusion is required. Doping is a facile method to manipulate the optical and chemical properties of semiconductor materials simultaneously. In this work, SnS2 photocatalysts with varied Ag doping content are synthesized through a facile one-step hydrothermal method. The product is characterized by XRD, SEM, TEM and UV-Vis spectrometry. The photocatalytic activity of the as-prepared Sn1-xAgxS2 is studied by the degradation of methylene blue (MB) dye under solar light irradiation. It is found that increasing the Ag dopant concentration can effectively increase the solar light adsorption efficiency of the photocatalyst and accelerate heterogeneous photocatalysis. The optimal concentration of Ag dopant is found to be 5% with the highest rate constant being 1.8251 hour-1. This study demonstrates that an optimal amount of Ag doping can effectively increase the photocatalytic performance of SnS2 and will promote the commercialization of such photocatalysts in the photocatalytic degradation of organic compounds.

Synergistic effects of interfacial modifiers enhance current and voltage in hybrid solar cells

To unleash the full potential of hybrid solar cells, it is imperative to get significant photocurrent contribution from both the sensitizing dye and the polymeric hole transporter. Here we report on the interfacial modifier 4-mercaptopyridine (4-MP), which induces controlled orientation of poly(3-hexylthiophene) (P3HT), the most widely used hole transporting polymer for hybrid solar cells, at the interface. 4-MP optimizes the charge separating interface between P3HT and a squaraine dye-decorated TiO2, inducing enhanced contribution to photocurrent generation by the polymer. In combination with 4-tert-butylpyridine, which enhances the open circuit potential in dye-sensitized and hybrid solar cells but reduces the photocurrent, a synergistic effect is observed and it is possible to enhance both open circuit voltage and photocurrent simultaneously. Similar effects on device performance are also found for two other commonly used dye molecules, a fullerene derivative and a common indoline dye.

Decoupling optical and electronic optimization of organic solar cells using high-performance temperature-stable TiO2/Ag/TiO2 electrodes

An electrode structured with a TiO2/Ag/TiO2 (TAT) multilayer as indium tin oxide (ITO) replacement with a superior thermal stability has been successfully fabricated. This electrode allows to directly tune the optical cavity mode towards maximized photocurrent generation by varying the thickness of the layers in the sandwich structure. This enables tailored optimization of the transparent electrode for different organic thin film photovoltaics without alteration of their electro-optical properties. Organic photovoltaic featuring our TAT multilayer shows an improvement of ∼12% over the ITO reference and allows power conversion efficiencies (PCEs) up to 8.7% in PTB7:PC71BM devices.

Role of the Metal-Oxide Work Function on Photocurrent Generation in Hybrid Solar Cells

ZnO is a widely used metal-oxide semiconductor for photovoltaic application. In solar cell heterostructures they not only serve as a charge selective contact, but also act as electron acceptor. Although ZnO offers a suitable interface for exciton dissociation, charge separation efficiencies have stayed rather poor and conceptual differences to organic acceptors are rarely investigated. In this work, we employ Sn doping to ZnO nanowires in order to understand the role of defect and surface states in the charge separation process. Upon doping we are able to modify the metal-oxide work function and we show its direct correlation with the charge separation efficiency. For this purpose, we use the polymer poly(3-hexylthiophene) as donor and the squaraine dye SQ2 as interlayer. Interestingly, neither mobilities nor defects are prime performance limiting factor, but rather the density of available states around the conduction band is of crucial importance for hybrid interfaces. This work highlights crucial aspects to improve the charge generation process of metal-oxide based solar cells and reveals new strategies to improve the power conversion efficiency of hybrid solar cells.

Chapter 5:The Role of Nanostructured Metal Oxides in Hybrid Solar Cells

Nanostructured metal oxides have been implemental to the development of hybrid, organic, and perovskite solar cells due to their wide bandgaps, chemical stability, and tunable electronic properties. This chapter covers the fabrication of nanostructured metal oxides for all applications in hybrid solar cells, including transparent conducting oxides (TCOs), electron/hole blocking layers, and charge transport layers. While each layer plays a unique role in the device operation, they share fundamental properties that can be engineered during their synthesis. Specifically, the role of doping and energy level manipulation, high interfacial surface area for charge separation, and ordered nanostructure arrays for photon manipulation are highlighted. The materials presented here are divided into two main groups, 1D and 2D nanostructures for TCOs and TiO2 nanocrystals for electron transport layers. The goal of this chapter is to convey a broad range of top-down and bottom-up synthetic methods that are common throughout semiconductor research but have played a vital role in the development of next generation photovoltaics.