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1. Carrier transport in polycrystalline silicon at high optical injection: transient photoconductance vs. numerical modeling

   Anyanwu, Uchechi; Harris, Christian; Semichaevsky, Andrey (June 2017, 44th IEEE-PVSC)

   This paper discusses our experimental results obtained for several p-type polycrystalline Si wafers using transient photoconductance decay method in combination with a numerical model that accounts for the carrier density dependence of recombination lifetime. Using peak illumination intensities of up to 65 Suns, we are able to estimate the recombination rates for the various carrier recombination mechanisms separately. The model with such realistic recombination parameters produces a realistic diffusion length of minority carriers in Si at high optical injection. Our approach can be applied to the characterization of semiconductor materials for concentrator photovoltaics as well as to design of optoelectronic devices. 

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2. Exploring the practical efficiency limit of silicon solar cells using thin solar-grade substrates

   https://doi.org/10.1039/d0ta04575f  

   Augusto, A.; Karas, J.; Balaji, P.; Bowden, S. G.; King, R. R. (August 2020, Journal of Materials Chemistry A)

   null (Ed.)

   Multiple silicon solar cell technologies have surpassed or are close to surpassing 26% efficiency. Dielectric and amorphous silicon-based passivation layers combined with minimal metal/silicon contact areas were responsible for reducing the surface saturation current density below 3 fA cm −2 . At open-circuit, in passivated contact solar cells, the recombination is mainly from fundamental mechanisms (Auger and radiative) representing over 3/4 of the total recombination. At the maximum power point, the fundamental recombination fraction can drop to half, as surface and bulk Shockley–Read–Hall step in. As a result, to further increase the performance at the operating point, it is paramount to reduce the bulk dependence and secure proper surface passivation. Bulk recombination can be mitigated either by reducing bulk defect density or by reducing the wafer thickness. We demonstrate that for commercially-viable solar-grade silicon, thinner wafers and surface saturation current densities below 1 fA cm −2 , are required to significantly increase the practical efficiency limit of solar cells up to 0.6% absolute. For a high-quality n-type bulk silicon minority-carrier lifetime of 10 ms, the optimum wafer thickness range is 40–60 μm, a very different value from 110 μm previously calculated assuming undoped substrates and solely Auger and radiative recombination. In this thickness range surface saturation current densities near 0.1 fA cm −2 are required to narrow the gap towards the fundamental efficiency limit. We experimentally demonstrate surface saturation currents below 0.5 fA cm −2 on pi/CZ/in structures across different wafer thicknesses (35–170 μm), with potential to reach open-circuit voltages close to 770 mV and bandgap-voltage offsets near 350 mV. Finally, we use the bandgap-voltage offset as a metric to compare the quality of champion experimental solar cells in the literature, for the most commercially-relevant photovoltaic cell absorbers and architectures. 

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3. Wavelength scaling of the high-intensity laser pulse compression dynamics in gas-filled capillaries

   Nagar, G.; Shim, B. (November 2021, 63rd Annual Meeting of the APS Division of Plasma Physics)

   The multimodal carrier-resolved unidirectional pulse propagation equation is solved to study the wavelength-dependent (λ = 1, 2, 3 and 4 μm) spatio-temporal dynamics, particularly pulse self-compression during high-intensity laser pulse propagation in gas-filled capillaries. We find that pulse self-compression in gas-filled capillaries due to plasma is more efficient for short wavelengths in contrast to wavelength-dependent pulse self-compression in laser filamentation [1]. To explain our finding, a detailed analysis is performed by quantifying the contributions of higher-order modes and calculating the temporal delay among modes, which reveals that pulse self-compression at longer wavelengths does not occur due to larger group velocity mismatch between the fundamental and higher-order modes for longer wavelengths [2]. Our study has important implications for the various fields of high-intensity nonlinear optics in gas-filled capillaries such as supercontinuum generation and high-order harmonic generation [3]. [1] L. Bergé et al., Phys. Rev. A 88, 023816 (2013). [2] G. Nagar and B. Shim, submitted. [3] T. Popmitchev et al. Science 336, 1287 (2012). 

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4. Selective phosphorus doping of polycrystalline silicon on glass using self-assembled monolayer doping (MLD) and flash anneal

   https://doi.org/https://doi.org/10.1016/j.matlet.2021.130780  

   Glenn Packard; Carolyn Spaulding; Alex Taylor; Karl Hirschman; Scott Williams, Santosh Kurinec (August 2021, Materials letters)

   Aldo R. Boccaccini (Ed.)

   We report doping of thin (~60 nm) amorphous silicon (a-Si) on glass substrate to form n + polycrystalline silicon on glass in selective regions using Monolayer doping (MLD) via Flash Lamp Annealing (FLA). The phosphorus monolayer was formed on the exposed regions of SiO2 patterned a-Si, through functionalization with chemically bound Diethyl vinylphosphonate (DVP) dopant molecules. The samples were capped with SiO2 and annealed using a single xenon flash pulse (5.0 J/cm2, 250 μs) to simultaneously crystallize a-Si, incorporate and activate phosphorus dopants. SIMS results show an average concentration of 8x1019 cm−3 in the 60 nm of thin silicon on glass. Electrical results show a resistivity of ~6.60x10−2 Ω.cm in doped regions. N-channel field effect transistor devices are successfully demonstrated using this MLD-FLA technique. 

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5. Impact of doped barriers on the recombination coefficients of c -plane InGaN/GaN single quantum well light-emitting diodes

   https://doi.org/10.1063/5.0117318  

   Chow, Y. C.; Lynsky, C.; Nakamura, S.; DenBaars, S. P.; Weisbuch, C.; Speck, J. S. (November 2022, Applied Physics Letters)

   Differential carrier lifetime measurements were performed on c-plane InGaN/GaN single quantum well (QW) light-emitting diodes (LEDs) of different QW indium compositions as well as with and without doped barriers. Mg-doped p-type and Si-doped n-type barriers close to the QW were used to reduce the net internal electric field in the QW, thereby improving the electron–hole wavefunction overlap on the LEDs. LEDs with doped barriers show short lifetimes and low carrier densities in the active region compared to the reference LEDs. The recombination coefficients in the ABC model were estimated based on the carrier lifetime and quantum efficiency measurements. The improvement in the radiative coefficients in the LEDs with doped barriers coupled with the blueshift of the emission wavelengths indeed indicates an enhancement in wavefunction overlap and a reduction of quantum confined Stark effect as a result of the reduced internal electric field. However, doped barriers also introduce non-radiative recombination centers and thereby increase the Shockley–Read–Hall (SRH) coefficient, although the increment is less for LEDs with high indium composition QWs. As a result, at high indium composition (22%), LEDs with doped barriers outperform the reference LEDs even though the trend is reversed for LEDs with lower indium composition (13.5%). Despite the trade-off of higher SRH coefficients, doped barriers are shown to be effective in reducing the internal electric field and increasing the recombination coefficients. 

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