Hieu T. Nguyen

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.

Uncertainty analysis for the coefficient of band-to-band absorption of crystalline silicon

We analyze the uncertainty of the coefficient of band-to-band absorption of crystalline silicon. For this purpose, we determine the absorption coefficient at room temperature (295 K) in the wavelength range from 250 to 1450 nm using four different measurement methods. The data presented in this work derive from spectroscopic ellipsometry, measurements of reflectance and transmittance, spectrally resolved luminescence measurements and spectral responsivity measurements. A systematic measurement uncertainty analysis based on the Guide to the expression of uncertainty in measurement (GUM) as well as an extensive characterization of the measurement setups are carried out for all methods. We determine relative uncertainties of the absorption coefficient of 0.4% at 250 nm, 11% at 600 nm, 1.4% at 1000 nm, 12% at 1200 nm and 180% at 1450 nm. The data are consolidated by intercomparison of results obtained at different institutions and using different measurement approaches.

Temperature dependence of the band-band absorption coefficient in crystalline silicon from photoluminescence

This work has been supported by the Australian Research Council (ARC) and the Australian Renewable Energy Agency (ARENA).

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.

Temperature dependence of the radiative recombination coefficient in crystalline silicon from spectral photoluminescence

The radiative recombination coefficient B(T) in crystalline silicon is determined for the temperature range 90–363 K, and in particular from 270 to 350 K with an interval of 10 K, where only sparse data are available at present. The band-band absorption coefficient established recently by Nguyen et al. [J. Appl. Phys. 115, 043710 (2014)] via photoluminescence spectrum measurements is employed to compute the values of B(T) at various temperatures. The results agree very well with literature data from Trupke et al. [J. Appl. Phys. 94, 4930 (2003).] We present a polynomial parameterization describing the temperature dependence of the product of B(T) and the square of the intrinsic carrier density. We also find that B(T) saturates at a near constant value at room temperature and above for silicon samples with relatively low free carrier densities.

Micrometer-Scale Deep-Level Spectral Photoluminescence From Dislocations in Multicrystalline Silicon

Micrometer-scale deep-level spectral photoluminescence (PL) from dislocations is investigated around the subgrain boundaries in multicrystalline silicon. The spatial distribution of the D lines is found to be asymmetrically distributed across the subgrain boundaries, indicating that defects and impurities are decorated almost entirely on one side of the subgrain boundaries. In addition, the D1 and D2 lines are demonstrated to have different origins due to their significantly varying behaviors after processing steps. D1 is found to be enhanced when the dislocations are cleaned of metal impurities, whereas D2 remains unchanged. Finally, the D4 and D3 lines are proposed to have different origins since their energy levels are shifted differently as a function of distance from the subgrain boundaries.

Impact of carrier profile and rear-side reflection on photoluminescence spectra in planar crystalline silicon wafers at different temperatures

The increasing use of spectral photoluminescence as an advanced and accurate diagnostic tool motivates a comprehensive assessment of the effects of some important optical and electrical properties on the photoluminescence spectra from crystalline silicon wafers. In this paper, we present both modeling results and measurements to elucidate the effects of the internal reflectance at the planar wafer surfaces, as well as the carrier profile varying across the sample thickness due to an increased rear-surface recombination velocity, as a function of temperature. These results suggest that the accuracy of existing spectral PL techniques may be improved by using higher temperatures due to the increased effect of the carrier profile at higher temperatures. They also show that changes in the photoluminescence spectrum shape caused by the addition of a rear-side specular reflector offset those caused by changes in the carrier profile due to increased rear surface recombination, and therefore, considerable care needs to be taken when changing the rear-side optics. Finally, the possible impact of variations in the rear-side reflectance on the band-band absorption coefficient and radiative recombination coefficient, which have previously been determined using the spectral photoluminescence technique, is assessed and demonstrated to be insignificant in practice.

Dislocations in laser-doped silicon detected by micro-photoluminescence spectroscopy

We report the detection of laser-induced damage in laser-doped layers at the surface of crystalline silicon wafers, via micron-scale photoluminescence spectroscopy. The properties of the sub-band-gap emission from the induced defects are found to match the emission characteristics of dislocations. Courtesy of the high spatial resolution of the micro-photoluminescence spectroscopy technique, micron-scale variations in the extent of damage at the edge of the laser-doped region can be detected, providing a powerful tool to study and optimize laser-doping processes for silicon photovoltaics.

Light and Electrically Induced Phase Segregation and Its Impact on the Stability of Quadruple Cation High Bandgap Perovskite Solar Cells

Perovskite material with a bandgap of 1.7-1.8 eV is highly desirable for the top cell in a tandem configuration with a lower bandgap bottom cell, such as a silicon cell. This can be achieved by alloying iodide and bromide anions, but light-induced phase-segregation phenomena are often observed in perovskite films of this kind, with implications for solar cell efficiency. Here, we investigate light-induced phase segregation inside quadruple-cation perovskite material in a complete cell structure and find that the magnitude of this phenomenon is dependent on the operating condition of the solar cell. Under short-circuit and even maximum power point conditions, phase segregation is found to be negligible compared to the magnitude of segregation under open-circuit conditions. In accordance with the finding, perovskite cells based on quadruple-cation perovskite with 1.73 eV bandgap retain 94% of the original efficiency after 12 h operation at the maximum power point, while the cell only retains 82% of the original efficiency after 12 h operation at the open-circuit condition. This result highlights the need to have standard methods including light/dark and bias condition for testing the stability of perovskite solar cells. Additionally, phase segregation is observed when the cell was forward biased at 1.2 V in the dark, which indicates that photoexcitation is not required to induce phase segregation.

Growth of Oxygen Precipitates and Dislocations in Czochralski Silicon

The impact of oxygen precipitates and dislocations on carrier recombination is investigated on thick silicon slabs cut vertically from a Czochralski-grown silicon ingot. Using a combination of photoluminescence imaging, photoluminescence spectroscopy, and Fourier transform infrared spectroscopy, we investigate the impact of pre-anneal on their recombination activity. We show that the vacancy concentration during precipitate growth affects the recombination activity of oxygen precipitates. Finally, we demonstrate the impact of nonequilibrium point defect concentrations on precipitate and dislocation growth.