Verification of a generalized Planck law for luminescence radiation from silicon solar cells

GaAs/GaInP nanowire solar cell on Si with state-of-the-art Voc and quasi-Fermi level splitting

With their unique structural, optical and electrical properties, III-V nanowires (NWs) are an extremely attractive option for the direct growth of III-Vs on Si for tandem solar cell applications. Here, we introduce a core-shell GaAs/GaInP NW solar cell grown by molecular beam epitaxy on a patterned Si substrate, and we present an in-depth investigation of its optoelectronic properties and limitations. We report a power conversion efficiency of almost 3.7%, and a state-of-the-art open-circuit voltage (V_OC) for a NW array solar cell on Si of 0.65 V. We also present the first quantification of the quasi-Fermi level splitting in NW array solar cells using hyperspectral photoluminescence measurements. A value of 0.84 eV is obtained at 1 sun (1.01 eV at 81 suns), which is significantly higher than qV_OC. It indicates NWs with a better intrinsic optoelectronic quality than what could be expected from TEM images or deduced from electrical measurements. Optical and electronic simulations provide insights into the main absorption and electrical losses, and guidelines to design and fabricate higher-efficiency devices. It suggests that improvements at the n-type contact (GaInP/ITO) are key to unlocking the potential of next generation NW solar cells.

Photon Recycling in Semiconductor Thin Films and Devices

Photon recycling (PR) plays an important role in the study of semiconductor materials and impacts the properties of their optoelectronic applications. However, PR has not been investigated comprehensively and it has not been demonstrated experimentally in many different kinds of semiconductor materials and devices. In this review paper, first, the authors introduce the background of PR and describe how this phenomenon was originally identified in semiconductors. Then, the theory and modelling of PR is reviewed and some of the important parameters that are used to quantify PR are highlighted. Next, a variety of the methods used to achieve and characterize PR in materials and devices are discussed. Examples of how the performance parameters of different types of optoelectronic devices are affected by PR are described. Finally, a summary of the roles of PR in semiconductor materials and devices and an outlook on how PR can be used to solve existing problems and challenges in the field of optoelectronics are provided. From this review, it is apparent that PR can have a positive impact on optoelectronic device performance, and that further in-depth theoretical and experimental studies are needed to rigorously demonstrate the advantages and importance of PR.

Mechanisms and Applications of Steady-State Photoluminescence Spectroscopy in Two-Dimensional Transition-Metal Dichalcogenides

Two-dimensional (2D) transition-metal dichalcogenide (TMD) semiconductors exhibit many important structural and optoelectronic properties, such as strong light-matter interactions, direct bandgaps tunable from visible to near-infrared regions, flexibility and atomic thickness, quantum-confinement effects, valley polarization possibilities, and so on. Therefore, they are regarded as a very promising class of materials for next-generation state-of-the-art nano/micro optoelectronic devices. To explore different applications and device structures based on 2D TMDs, intrinsic material properties, their relationships, and evolutions with fabrication parameters need to be deeply understood, very often through a combination of various characterization techniques. Among them, steady-state photoluminescence (PL) spectroscopy has been extensively employed. This class of techniques is fast, contactless, and nondestructive and can provide very high spatial resolution. Therefore, it can be used to obtain optoelectronic properties from samples of various sizes (from microns to centimeters) during the fabrication process without complex sample preparation. In this article, the mechanism and applications of steady-state PL spectroscopy in 2D TMDs are reviewed. The first part of this review details the physics of PL phenomena in semiconductors and common techniques to acquire and analyze PL spectra. The second part introduces various applications of PL spectroscopy in 2D TMDs. Finally, a broader perspective is discussed to highlight some limitations and untapped opportunities of PL spectroscopy in characterizing 2D TMDs.

Emission Control from Transition Metal Dichalcogenide Monolayers by Aggregation-Induced Molecular Rotors

Organic-inorganic (O-I) heterostructures, consisting of atomically thin inorganic semiconductors and organic molecules, present synergistic and enhanced optoelectronic properties with a high tunability. Here, we develop a class of air-stable vertical O-I heterostructures comprising a monolayer of transition-metal dichalcogenides (TMDs), including WS₂, WSe₂, and MoSe₂, on top of tetraphenylethylene (TPE) core-based aggregation-induced emission (AIE) molecular rotors. The created O-I heterostructures yields a photoluminescence (PL) enhancement of up to ∼950%, ∼500%, and ∼330% in the top monolayer WS₂, MoSe₂, and WSe₂ as compared to PL in their pristine monolayers, respectively. The strong PL enhancement is mainly attributed to the efficient photogenerated carrier process in the AIE luminogens (courtesy of their restricted intermolecular motions in the solid state) and the charge-transfer process in the created type I O-I heterostructures. Moreover, we observe an improvement in photovoltaic properties of the TMDs in the heterostructures including the quasi-Fermi level splitting, minority carrier lifetime, and light absorption. This work presents an inspiring example of combining stable, highly luminescent AIE-based molecules, with rich photochemistry and versatile applications, with atomically thin inorganic semiconductors for multifunctional and efficient optoelectronic devices.

Quantifying Quasi-Fermi Level Splitting and Mapping its Heterogeneity in Atomically Thin Transition Metal Dichalcogenides

One of the most fundamental parameters of any photovoltaic material is its quasi-Fermi level splitting (∆µ) under illumination. This quantity represents the maximum open-circuit voltage (V_oc ) that a solar cell fabricated from that material can achieve. Herein, a contactless, nondestructive method to quantify this parameter for atomically thin 2D transition metal dichalcogenides (TMDs) is reported. The technique is applied to quantify the upper limits of V_oc that can possibly be achieved from monolayer WS₂ , MoS₂ , WSe₂ , and MoSe₂ -based solar cells, and they are compared with state-of-the-art perovskites. These results show that V_oc values of ≈1.4, ≈1.12, ≈1.06, and ≈0.93 V can be potentially achieved from solar cells fabricated from WS₂ , MoS₂ , WSe₂ , and MoSe₂ monolayers at 1 Sun illumination, respectively. It is also observed that ∆µ is inhomogeneous across different regions of these monolayers. Moreover, it is attempted to engineer the observed ∆µ heterogeneity by electrically gating the TMD monolayers in a metal-oxide-semiconductor structure that effectively changes the doping level of the monolayers electrostatically and improves their ∆µ heterogeneity. The values of ∆µ determined from this work reveal the potential of atomically thin TMDs for high-voltage, ultralight, flexible, and eye-transparent future solar cells.

Planck's generalised radiation law and its implications for cathodoluminescence spectra

Cathodoluminescence (CL) is an important analytical technique for probing the optical properties of materials at high spatial resolution. Interpretation of CL spectra is however complicated by the fact that the spectrum depends on the carrier injection density of the incident electron beam. Here a generalised version of Planck's radiation law is used to uncover the evolution of CL spectra with injection under steady-state conditions. The importance of the quasi-Fermi level is highlighted and it is shown that steady-state luminescence is suppressed when the carrier distributions undergo a population inversion. The theory is consistent with some well-known luminescence phenomena, such as the blue shifting of donor-acceptor pair transitions with increased injection, and its predictions are experimentally verified on CdTe and GaN, which are exemplar thin-film solar cell and light emitting diode materials respectively. Furthermore, the discussion is broadened to include pulsed illumination in time resolved CL, where the carrier distribution is dynamically evolving with time.

Ultralow Absorption Coefficient and Temperature Dependence of Radiative Recombination of CH3NH3PbI3 Perovskite from Photoluminescence

Spectrally resolved photoluminescence is used to measure the band-to-band absorption coefficient α(BB)(ℏω) of organic-inorganic hybrid perovskite methylammonium lead iodide (CH₃NH₃PbI₃) films from 675 to 1400 nm. Unlike other methods used to extract the absorption coefficient, photoluminescence is only affected by band-to-band absorption and is capable of detecting absorption events at very low energy levels. Absorption coefficients as low as 10⁻¹⁴ cm⁻¹ are detected at room temperature for long wavelengths, which is 14 orders of magnitude lower than reported values at shorter wavelengths. The temperature dependence of α(BB)(ℏω) is calculated from the photoluminescence spectra of CH₃NH₃PbI₃ in the temperature range 80-360 K. Based on the temperature-dependent α(BB)(ℏω), the product of the radiative recombination coefficient and square of the intrinsic carrier density, B(T) × n(i)², is also obtained.

How much can guided modes enhance absorption in thin solar cells?

Absorption enhancement in thin metal-backed solar cells caused by dipole scatterers embedded in the absorbing layer is studied using a semi-analytical approach. The method accounts for changes in the radiation rate produced by layers above and below the dipole, and treats incoherently the subsequent scattering of light in guided modes from other dipoles. We find large absorption enhancements for strongly coupled dipoles, exceeding the ergodic limit in some configurations involving lossless dipoles. An antireflection-coated 100-nm layer of a- Si:H on Ag absorbs up to 87% of incident above-gap light. Thin layers of both strong and weak absorbers show similar strongly enhanced absorption.

On the origin of the open-circuit voltage of polymer-fullerene solar cells

The increasing amount of research on solution-processable, organic donor-acceptor bulk heterojunction photovoltaic systems, based on blends of conjugated polymers and fullerenes has resulted in devices with an overall power-conversion efficiency of 6%. For the best devices, absorbed photon-to-electron quantum efficiencies approaching 100% have been shown. Besides the produced current, the overall efficiency depends critically on the generated photovoltage. Therefore, understanding and optimization of the open-circuit voltage (Voc) of organic solar cells is of high importance. Here, we demonstrate that charge-transfer absorption and emission are shown to be related to each other and Voc in accordance with the assumptions of the detailed balance and quasi-equilibrium theory. We underline the importance of the weak ground-state interaction between the polymer and the fullerene and we confirm that Voc is determined by the formation of these states. Our work further suggests alternative pathways to improve Voc of donor-acceptor devices.