Highly Reflective Dielectric Back Reflector for Improved Efficiency of Tandem Thin-Film Solar Cells

We report on the prototyping and development of a highly reflective dielectric back reflector for application in thin-film solar cells. The back reflector is fabricated by Snow Globe Coating (SGC), an innovative, simple, and cheap process to deposit a uniform layer of TiO2 particles which shows remarkably high reflectance over a broad spectrum (average reflectance of 99% from 500 nm to 1100 nm). We apply the highly reflective back reflector to tandem thin-film silicon solar cells and compare its performance with conventional ZnO:Al/Ag reflector. By using SGC back reflector, an enhancement of 0.5 mA/cm2 in external quantum efficiency of the bottom solar cell and an absolute value of 0.2% enhancement in overall power conversion efficiency are achieved. We also show that the increase in power conversion efficiency is due to the reduction of parasitic absorption at the back contact; that is, the use of the dielectric reflector avoids plasmonic losses at the reference ZnO:Al/Ag back reflector. The Snow Globe Coating process is compatible with other types of solar cells such as crystalline silicon, III–V, and organic photovoltaics. Due to its cost effectiveness, stability, and excellent reflectivity above a wavelength of 400 nm, it has high potential to be applied in industry

Superior Self-Powered Room-Temperature Chemical Sensing with Light-Activated Inorganic Halides Perovskites

Hybrid halide perovskite is one of the promising light absorber and is intensively investigated for many optoelectronic applications. Here, the first prototype of a self-powered inorganic halides perovskite for chemical gas sensing at room temperature under visible-light irradiation is presented. These devices consist of porous network of CsPbBr3 (CPB) and can generate an open-circuit voltage of 0.87 V under visible-light irradiation, which can be used to detect various concentrations of O2 and parts per million concentrations of medically relevant volatile organic compounds such as acetone and ethanol with very quick response and recovery time. It is observed that O2 gas can passivate the surface trap sites in CPB and the ambipolar charge transport in the perovskite layer results in a distinct sensing mechanism compared with established semiconductors with symmetric electrical response to both oxidizing and reducing gases. The platform of CPB-based gas sensor provides new insights for the emerging area of wearable sensors for personalized and preventive medicine.H.C. and M.Z. contributed equally to this work. A.T. gratefully acknowledges the support of Australian Research Council (ARC) DP150101939, ARC DE160100569, and Westpac 2016 Research Fellowship. M.Z., S.H., and A.W.Y. H.-B. acknowledge the support of the Australian government via financial support from the ARC through the DP160102955 program and the Australian Renewable Energy Agency. K.R.C. acknowledges the support of an ARC Future Fellowship. The financial support from ARC through DP160102955 is also acknowledged

Transient Photovoltage in Perovskite Solar Cells: Interaction of Trap-Mediated Recombination and Migration of Multiple Ionic Species

It is highly probable that perovskite solar cells (PSCs) are mixed electronic-ionic conductors, with ion migration being the driving force for PSC hysteresis. However, there is much that is not understood about the interaction of ion migration with other processes in the cell. The key question is: what factors of a PSC are influenced when ions are free to move? In this contribution, we employ a numerical drift-diffusion model of PSCs to show that the migration of both anions and cations in interaction with trap-mediated recombination in the bulk and/or at the surfaces of the perovskite absorber can manifest both current-voltage hysteresis and unusual nonmonotonic PSC photovoltage transients. We identify that a key mechanism of this interaction is the influence of the net ionic charge throughout the perovskite bulk - which varies as the ions approach new steady-state conditions - on the distribution of electrons and holes and subsequently the spatial distribution of trap-mediated recombination modeled after Shockley Read Hall (SRH) statistics. Relative to intrinsic recombination mechanisms, SRH recombination can be highly sensitive to local asymmetries of the electron-hole population. We show that this sensitivity is key to replicating nonmonotonic transients with multiple time constants, the forms of which may have suggested multiple processes. This work therefore supports the conceptualization of the hysteretic behavior of PSCs as dominated by the interplay between ion migration and trap-mediated recombination throughout the perovskite absorber.This project received funding from the Australian Renewable Energy Agency ARENA and the Australian Centre for Advanced Photovoltaics (ACAP)

Interface passivation using ultrathin polymer–fullerene films for high-efficiency perovskite solar cells with negligible hysteresis

Interfacial carrier recombination is one of the dominant loss mechanisms in high efficiency perovskite solar cells, and has also been linked to hysteresis and slow transient responses in these cells. Here we demonstrate an ultrathin passivation layer consisting of a PMMA:PCBM mixture that can effectively passivate defects at or near to the perovskite/TiO2 interface, significantly suppressing interfacial recombination. The passivation layer increases the open circuit voltage of mixed-cation perovskite cells by as much as 80 mV, with champion cells achieving Voc ∼ 1.18 V. As a result, we obtain efficient and stable perovskite solar cells with a steady-state PCE of 20.4% and negligible hysteresis over a large range of scan rates. In addition, we show that the passivated cells exhibit very fast current and voltage response times of less than 3 s under cyclic illumination. This new passivation approach addresses one of the key limitations of current perovskite cells, and paves the way to further efficiency gains through interface engineering.Australian Renewable Energy Agency; Australian Research Council; MSTC (Grant No. 2016YFA0301300), NNSFC (Grant No. 11674402) and GSTP (Grant No. 201607010044, 201607020023

Diffuse reflectors for improving light management in solar cells: a review and outlook

Pigment based diffuse reflectors (DRs) have several advantages over metal reflectors such as good stability, high reflectivity, and low parasitic absorption. As such, DRs have the potential to be applied on high efficiency silicon solar cells and further increase the power conversion efficiency. In this paper, we perform a thorough review on the notable achievements to date of DRs’ application for photovoltaics. We outline unique attributes of these technologies and discuss the theoretical and laboratory development working towards overcoming the challenges of transferring to high efficiency silicon solar cells. In order to understand the potential of DRs for high efficiency silicon solar cells, we provide a qualitative analysis of the impact of front reflection, rear absorption and the angular distribution on the useful light absorption in silicon wafers. By including this discussion, we provide an outlook for the application of DR in reaching maximum photo-current for high efficiency silicon solar cells

Photoluminescence enhancement towards high efficiency plasmonic solar cells

We demonstrate enhanced absorption in silicon wafers when plasmonic nanoparticles are added to a conventional rear contact structure. A rear side light trapping with plasmonic nanoparticles and various thicknesses of Si 3N4 layer is studied and compared

Evaluating plasmonic light trapping with photoluminescence

We use photoluminescence measurements to quantify the light trapping for a range of plasmonic structures. By combining Ag nanoparticles as a scattering structure and diffuse white paint as a back surface reflector (BSR) on silicon wafers, we can achieve aThis work is a part of an ARC Linkage Project. K. R. Catchpole acknowledges the support of an ARC Australian Research Fellowship. D. Macdonald acknowledges the support of ARC Future Fellowship

Influence of Annealing and Bulk Hydrogenation on Lifetime Limiting Defects in Nitrogen-Doped Floating Zone Silicon

A recombination active defect is found in as-grown high-purity floating zone n-type silicon wafers containing grown-in nitrogen. In order to identify the properties of the defect, injection-dependent minority carrier lifetime measurements, secondary ion mass spectroscopy measurements, and photoluminescence lifetime imaging are performed. The lateral recombination center distribution varies greatly in a radially symmetric way, while the nitrogen concentration remains constant. The defect is shown to be deactivated through high temperature annealing and hydrogenation. We suggest that a nitrogen-intrinsic point defect complex may be responsible for the observed recombination

Highly Reflective Dielectric Back Reflector for Improved Efficiency of Tandem Thin-Film Solar Cells

We report on the prototyping and development of a highly reflective dielectric back reflector for application in thin-film solar cells. The back reflector is fabricated by Snow Globe Coating (SGC), an innovative, simple, and cheap process to deposit a uniform layer of TiO2 particles which shows remarkably high reflectance over a broad spectrum (average reflectance of 99% from 500 nm to 1100 nm). We apply the highly reflective back reflector to tandem thin-film silicon solar cells and compare its performance with conventional ZnO:Al/Ag reflector. By using SGC back reflector, an enhancement of 0.5 mA/cm2 in external quantum efficiency of the bottom solar cell and an absolute value of 0.2% enhancement in overall power conversion efficiency are achieved.We also show that the increase in power conversion efficiency is due to the reduction of parasitic absorption at the back contact; that is, the use of the dielectric reflector avoids plasmonic losses at the reference ZnO:Al/Ag back reflector. The Snow Globe Coating process is compatible with other types of solar cells such as crystalline silicon, III–V, and organic photovoltaics. Due to its cost effectiveness, stability, and excellent reflectivity above a wavelength of 400 nm, it has high potential to be applied in industry

Light trapping efficiency comparison of Si solar cell textures using spectral photoluminescence

The band-to-band absorption enhancement due to various types of light trapping structures is studied experimentally with photoluminescence (PL) on monocrystalline silicon wafers. Four basic light trapping structures are examined: reactive ion etched texture (RIE), metal-assisted etched texture (MET), random pyramid texture (RAN) and plasmonic Ag nanoparticles with a diffusive reflector (Ag/DR). We also compare two novel combined structures of front side RIE/rear side RAN and front side RIE/rear side Ag/DR. The use of photoluminescence allows us to measure the absorption due to band-to-band transitions only, and excludes parasitic absorption from free carriers and other sources. The measured absorptance spectra are used to calculate the maximum generation current for each structure, and the light trapping efficiency is compared to a recently-proposed figure of merit. The results show that by combining RIE with RAN and Ag/DR, we can fabricate two structures with excellent light trapping efficiencies of 55% and 52% respectively, which is well above previously reported values for similar wafer thicknesses. A comparison of the measured band-band absorption and the EQE of back-contact silicon solar cells demonstrates that PL extracted absorption provides a very good indication of long wavelength performance for high efficiency silicon solar cells.This Program has been supported by the Australian Government through the Australian Renewable Energy Agency (ARENA). KRC is grateful for the support of a Future Fellowship from the Australian Research Council