Introduction
Photovoltaic solar energy conversion to electricity is growing as a significant player of the forthcoming renewable revolution. The lowering of the cost of established technologies and the development of new active materials is providing more effective solutions to attract investments in photovoltaics. In this context, the aim of ELDORADO project is to open a new pathway to the integration of an active layer of 2D perovskites with dielectric nanoparticles (NPs) into the top cell (TC) of a tandem solar device with the goal of optimizing the light management and consequently also the power conversion efficiency (PCE) of the device. In fact, despite tremendous effort spent on that topic, the best performing 3D perovskite/Si tandem device still far from the theoretical predicted PCE limit.
2D perovskites present strikingly different optoelectronic properties with respect to the 3D materials that can be exploited for a new concept of tandem PV. In recent years, it has been discovered that layered 2D perovskites can be engineered to funnel energy and/or charge carriers from thinner to thicker quantum wells located on opposite sides of the film. Such layered systems can direct transfer energy in a manner similar to a natural light-harvesting antenna, enhancing the charge separation.
Boosting of the device performance will also be carried out by light management introducing dielectric NPs inside the active layer of 2D perovskite and/or within charge transporting layers. Indeed, the proposed light management strategy aims to tune the perovskite TC light harvesting to maximize the tandem device PCE, efficiently managing the photon capture in the range of 600 - 780 nm, where the extinction coefficient of the perovskites rapidly decays while the Si bottom cell effectively generates charges.
The role of the simulations will be fundamental in driving the production of the optimal composition of the 2D perovskite active layer, the implementation of the most efficient light management strategy and the optimization of the final tandem device.
Objectives
ELDORADO has three main sequential objectives, the first of which is the development of a 2D perovskite active layer where an efficient 2D or quasi-2D perovskite absorber layer will be optimized for a tandem device with a c-Si bottom cell. The optimal composition of the perovskite material will be guided by simulations and advanced characterization optimizing the light harvesting and transmission. The second objective is the development of a suitable layer of Si NPs to be inserted into the perovskite absorber or at the 2D perovskite/charge transporting layer interfaces to optimize the light management and the efficiency of the device. The third and last objective is the realization of a tandem proof-of-concept perovskite/c-Si tandem device employing a two-terminal mechanically stacked architecture with PCE above the current state of the art.
TOR VERGATA (UTV) ROLE: The unit UTV will contribute to ELDORADO with the design, engineering, realization and testing of the perovskite TC and perovskite/Si tandem device: the expertise of Dr. Antonio Agresti involves the development and characterization of new generation PV devices such as dye sensitized solar cells (DSCs), perovskite-based devices, tandem SCs. Dr. Agresti currently leads the research unit for the Graphene Flagship SH5-GRAPHES project successfully designing, developing and characterizing large-area PV devices based on perovskite and two-dimensional materials. UTV will participate to perovskite formulation and optimization, sample preparation (substrate/2D perovskite), perovskite TC structure optimization (evaluated through the PCE and EQE measurements), sample preparation based on semi-cell (substrate/ETL/perovskite, substrate/ETL/perovskite/HTL, substrate/perovskite/HTL etc.). Realization of full perovskite TC and realization of the final perovskite/Si tandem prototype. Evaluation of charge transfer in perovskite TC through transient photovoltage/potocurrent techniques, evaluation of perovskite TC and tandem device performance through I-V curve acquisition.
UNIPG ROLE: Within the ELDORADO project, the UNIPG unit will develop an experimental setup for the elecrical characterization of PSCs in light and dark conditions. EIS will provide relevant information on the internal mechanisms of the cell relating to carrier transport and recombination. Information extracted from EIS analysis, and from the device and materials characterization provided by the other RUs, will be used to develop suitable mathematical models of the PSCs, in order to perform realistic device simulations. Simulations will be used to investigate optimal design parameters and material choice, optimizing light management, absorption spectra and, overall, the efficiency of the tandem PSC.
ISM-CNR ROLE: The research unit will perform time-resolved investigations of the optical properties of 2D materials by transient absorbance/reflectance and photoluminescence studies in a temporal range that goes from 50 fs to tens of ns and in a spectral window of 240-3000 nm. The samples will be investigated by steady-state and time-resolved spectroscopies with the aim to study the charge transfer processes within 2D materials on the time scales of 10-100’s of ps. PL and TR-PL measurements will be performed at RT and low Steady-state temperature to characterize the defectivity and the photon recycling properties of the 2D materials. All these characterizations will be carried out on thin films, half-cells and on the final TC, studying the effects of the interfaces on the active 2D material. Moreover, the research unit will synthesize spherical dielectric Si NPs by femtosecond laser ablation in liquids and characterize forward and backward scattering and absorption of the prepared films.
Figure from 10.1021/acs.jpclett.7b01857
a) Structure of 3D an 2D perovskites: illustration of the structures of low-dimensional perovskites with different numbers of perovskite layers (n). The pure 2D perovskite (n=1) has a R_2 B X_{3n+1} structure, where R is a bulky oranic cation. For n>1, the quasi-2D perovskites arrange into a R_2 A_{n-1} B_n X_{3n+1} structure. b) Distribution of as-grown multiple-phase quasi-2D perovskite films: schematic diagram of the arrangement of the multiphase perovskite film. c) and d) Steady-state absorption and PL spectroscopy of spin-coated quasi-2D Ruddlesden-Popper perovskite thin films. e) Schematic diagram of morphology and charge tranport model of the control and target devices.
Target
2D perovskites have the general formula R_2 A_{n-1} B_n X_{3n+1}, where R is an additional bulky organic cation (aliphatic or aromatic alkylammonium) that acts as a spacer between the inorganic sheets and n defines the number of inorganic layers held together. When n=1 (R_2 B X_4), the organic spacer isolates single layers of the inorganic framework in a pure 2D perovskite form. Increasing the number of layers (n>1) and introducing a small organic cation (A), namely methylammonium (MA) or formamidinium (FA), induce the formation of multi-layered, quasi-2D perovskites, which converge into the 3D structure for n=∞. Their large bandgaps (Eg) and exciton binding energy (Eb) results in a narrow absorption and photoluminescence (PL) profile that broadens as the number of layers increases. The large Eb is due to the mismatch between the dielectric constants of the organic and inorganic layers and the low dielectric screening from the organic sheets. For these reasons, 2D perovskites present strikingly different optoelectronic properties with respect to the 3D materials that can be exploited for a new concept of tandem PV.
Since only theoretical simulations of 2D perovskite/Si tandem SCs have been carried out to date, the effectiveness of this strategy requires the realization of prototypes and their successive optimization. Following this route, ELDORADO aims to optimize the 2D perovskite-based TC in terms of: i) perovskite absorber composed of a mixture of different n-layered perovskites for effective light harvesting, absorption, transmission, and charge separation ii) device structure and adjacent layer interface engineering for PCE improvement. These results will be fundamental for the choice of the best tandem device architecture able to retain low levelized costs of electricity (LCOE), thanks to a suitable realization strategy for the TC compatible with the existing silicon mass-production line.
From the light management point of view, dielectric NPs have been proposed as a means to enhance the performance of PV devices. In ELDORADO an initial test period will be devoted to optimize the integration process of commercial Si NPs in the TC structure and to put together the experimental set-up for the characterization of the absorption and scattering properties of the films. Indeed, the proposed strategy aims to tune the perovskite TC light harvesting for maximizing the tandem device PCE efficiently managing the photon capture in the range of 600 - 780 nm, where the extinction coefficient of the perovskites rapidly decays while the Si bottom cell effectively generates charges. The dielectric NPs and optimization of the optical design may play a key role in leading to a very significant reduction of the perovskite film thickness and hence of the amount of lead present in the device.
The role of the simulations will be fundamental in driving the production of the best composition for the 2D perovskite active layer, the implementation of the most efficient light management strategy and the optimization of the final tandem device. Electro-optical simulation of the device will be performed in order to investigate optimal design parameters and the theoretical limits of the PSC that will be produced. Two key points will be investigated: 1) the current generated in the new perovskite TC that has to match the current generated in the bottom commercial Si cell; 2) the propagation of photons from the top to the bottom cell, and their respective absorption spectra, that have to be distinct in the perovskite and the Si in order to maximize the efficiency. Only in a first stage, for the implementation and validation of the physical models, electrical and optical simulations will be performed separately, but it is evident that the two points must be investigated concurrently, since the generated current is directly related to the absorption and transfer rates of photons.
Optical simulations are needed to investigate the propagation and absorption of photons inside the cell, including photons from an external light source and photons produced by radiative recombination. In general, optical simulations require the solution of Maxwell equations, performed by means of finite difference or finite element methods. A much-simplified approach is possible in one-dimensional simulations, as the photons can propagate along only one direction. This is the so-called transfer matrix method (TMM), whose applicability to SCs has been demonstrated.