Voltage Deficit in Wide Bandgap Perovskite Solar Cells:The Role of Traps, Band Energies, and Effective Density of States

Wide-bandgap ((Formula presented.) 1.7 eV) perovskite solar cells (PSCs) are plagued by relatively low open-circuit voltages. This is problematic as they are key to achieving perovskite silicon tandems, which can boost the potential of silicon solar cells. Performance in PSCs is widely considered to be limited by recombination at the interface between the perovskite and the transport layer (TL). Here, a number of design rules to increase the open-circuit voltage of wide-bandgap PSCs are introduced. A numerical device model that includes a detailed description of the interfacial recombination processes is presented. The combined effects of interface traps, ions, band alignment, and transport properties are introduced to identify the critical parameters for improving the open-circuit voltage. A large number of devices are simulated by picking random combinations of parameters and are looked for trends. It is shown that interface recombination can be suppressed by reducing the minority carrier density close to the interface with the TLs. It is demonstrated that the alignment of energy levels is only part of the story; the effective densities of states are of equal importance. The results pave the way to achieving high open-circuit voltages, despite a significant density of interface defects

Carrier-carrier Coulomb interactions reduce power factor in organic thermoelectrics

Organic semiconductors are excellent candidates for low temperature thermoelectric generators. However, such thermoelectric applications require materials be doped and highly conductive. Here, we show how doping affects the Seebeck coefficient in organic semiconductors using kinetic Monte Carlo simulations. Employing a hopping transport approach, we demonstrate that at high dopant loading, carrier-carrier interactions can reduce the Seebeck coefficient. This results in systems with intrinsic disorder, still following Heike's formula for thermopower at high dopant density. Reducing these carrier-carrier interactions results in an increased Seebeck coefficient and power factor. Specifically, a realistic reduction in carrier-carrier interactions can increase the power factor by more than a factor 15, increasing ZT above 1 for organic thermoelectrics

The Role of Thermalization in the Cooling Dynamics of Hot Carrier Solar Cells

The hot carrier solar cell (HCSC) concept has been proposed to overcome the Shockley Queisser limit of a single p–n junction solar cell by harvesting carriers before they have lost their surplus energy. A promising family of materials for these purposes is metal halide perovskites (MHP). MHPs have experimentally shown very long cooling times, the key requirement of a HCSC. By using ensemble Monte Carlo simulations, light is shed on why cooling times are found to be extended. This article concentrates on the role of thermalization in the cooling process. The role of carrier–phonon and carrier–carrier interactions in thermalization and cooling is specified, while showing how these processes depend on material parameters, such as the dielectric constant and effective mass. It is quantified how thermalization acts as a cooling mechanism via the cold background effect. The importance of a low degree of background doping is to achieve the observed extended cooling times. Herein, it is mapped out how perovskites should be tuned, their material parameters, carrier concentration, and purity, in order to realize a HCSC. It contributes to the debate on the cooling times in MHPs and the suitability of tin perovskites for HCSCs.</p

Can Ferroelectric Polarization Explain the High Performance of Hybrid Halide Perovskite Solar Cells?

The power conversion efficiency of photovoltaic cells based on the use of hybrid halide perovskites, CH3NH3PbX3 (X = Cl, Br, I), now exceeds 20%. Recently, it was suggested that this high performance originates from the presence of ferroelectricity in the perovskite, which is hypothesized to lower charge recombination in the device. Here, we investigate and quantify the influence of mesoscale ferroelectric polarization on the device performance of perovskite solar cells. We implement a 3D drift diffusion model to describe the solar cell operation. To account for the mesoscale ferroelectricity, we incorporate domains defined by polarization strength, P, in 3D space, forming different polarization landscapes or microstructures. Study of microstructures with highly-ordered polarized domains shows that charge transport and recombination in the solar cell depends significantly on the polarization landscape viz. the orientation of domain boundaries and the size of domains. In the case of the microstructure with random correlated polarization, a realistic scenario, we find indication of the existence of channels for efficient charge transport in the device which leads to lowering of charge recombination, as evidenced by the high fill factor (FF). However, the high open-circuit voltage (VOC), which is typical of high performance perovskite solar cells, is unlikely to be explained by the presence of ferroelectric polarization in the perovskite

Identification of the dominant recombination process for perovskite solar cells based on machine learning

Over the past decade, perovskite solar cells have become one of the major research interests of the photovoltaic community, and they are now on the brink of catching up with the classical inorganic solar cells, with efficiency now reaching up to 25%. However, significant improvements are still achievable by reducing recombination losses. The aim of this work is to develop a fast and easy-to-use tool to pinpoint the main losses in perovskite solar cells. We use large-scale drift-diffusion simulations to get a better understanding of the light intensity dependence of the open-circuit voltage and how it correlates to the dominant recombination process. We introduce an automated identification tool using machine learning methods to pinpoint the dominant loss using the light intensity-dependent performances as an input. The machine learning was trained using >2 million simulations and gives an accuracy of the prediction up to 82%. Le Corre et al. demonstrate the application of machine learning methods to identify the dominant recombination process in perovskite solar cells with 82% accuracy. The machine learning algorithms are trained and tested using large-scale drift-diffusion simulations, and their applicability on real solar cells is also demonstrated on devices previously reported

Charge Carrier Extraction in Organic Solar Cells Governed by Steady-State Mobilities

Charge transport in organic photovoltaic (OPV) devices is often characterized by steady-state mobilities. However, the suitability of steady-state mobilities to describe charge transport has recently been called into question, and it has been argued that dispersion plays a significant role. In this paper, the importance of the dispersion of charge carrier motion on the performance of organic photovoltaic devices is investigated. An experiment to measure the charge extraction time under realistic operating conditions is set up. This experiment is applied to different blends and shows that extraction time is directly related to the geometrical average of the steady-state mobilities. This demonstrates that under realistic operating conditions the steady-state mobilities govern the charge extraction of OPV and gives a valuable insight in device performance

Device physics of polymer:fullerene bulk heterojunction solar cells

Plastic solar cells bear the potential for large-scale power generation based on materials that provide the possibility of flexible, lightweight, inexpensive, efficient solar cells. Since the discovery of the photoinduced electron transfer from a conjugated polymer to fullerene molecules, followed by the introduction of the bulk heterojunction (BHJ) concept, this material combination has been extensively studied in organic solar cells, leading to several breakthroughs in efficiency, with a power conversion efficiency approaching 5 %. This article reviews the processes and limitations that govern device operation of polymer.-fullerene BHJ solar cells, with respect to the charge-carrier transport and photogeneration mechanism. The transport of electrons/holes in the blend is a crucial parameter and must be controlled (e.g., by controlling the nanoscale morphology) and enhanced in order to allow fabrication of thicker films to maximize the absorption, without significant recombination losses. Concomitantly, a balanced transport of electrons and holes in the blend is needed to suppress the build-up of the space-charge that will significantly reduce the power conversion efficiency. Dissociation of electron-hole pairs at the donor/acceptor interface is an important process that limits the charge generation efficiency under normal operation condition. Based on these findings, there is a compromise between charge generation (light absorption) and open-circuit voltage (V-oc) when attempting to reduce the bandgap of the polymer (or fullerene). Therefore, an increase in V-oc of polymer.-fullerene cells, for example by raising the lowest unoccupied molecular orbital level of the fullerene, will benefit cell performance as both fill factor and short-circuit current increase simultaneously

Engineering the Thermoelectrical Properties of PEDOT:PSS by Alkali Metal Ion Effect

Engineering the electrical properties of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) holds great potential for various applications such as sensors, thermoelectric (TE) generators, and hole transport layers in solar cells. Various strategies have been applied to achieve optimal electrical properties, including base solution post-treatments. However, the working mechanism and the exact details of the structural transformations induced by base post-treatments are still unclear. In this work, we present a comparative study on the post-treatment effects of using three common and green alkali base solutions: namely LiOH, NaOH, and KOH. The structural modifications induced in the film by the base post-treatments are studied by techniques including atomic force microscopy, grazing-incidence wide-angle X-ray scattering, ultraviolet–visible–near-infrared spectroscopy, and attenuated total reflectance Fourier-transform infrared spectroscopy. Base-induced structural modifications are responsible for an improvement in the TE power factor of the films, which depends on the basic solution used. The results are explained on the basis of the different affinity between the alkali cations and the PSS chains, which determines PEDOT dedoping. The results presented here shed light on the structural reorganization occurring in PEDOT:PSS when exposed to high-pH solutions and may serve as inspiration to create future pH-/ion-responsive devices for various applications