Enhanced Carrier Lifetime Research Articles

Doping of polycrystalline CdTe for high-efficiency solar cells on flexible metal foil

Roll-to-roll manufacturing of CdTe solar cells on flexible metal foil substrates is one of the most attractive options for low-cost photovoltaic module production. However, various efforts to grow CdTe solar cells on metal foil have resulted in low efficiencies. This is caused by the fact that the conventional device structure must be inverted, which imposes severe restrictions on device processing and consequently limits the electronic quality of the CdTe layer. Here we introduce an innovative concept for the controlled doping of the CdTe layer in the inverted device structure by means of evaporation of sub-monolayer amounts of Cu and subsequent annealing, which enables breakthrough efficiencies up to 13.6%. For the first time, CdTe solar cells on metal foil exceed the 10% efficiency threshold for industrialization. The controlled doping of CdTe with Cu leads to increased hole density, enhanced carrier lifetime and improved carrier collection in the solar cell. Our results offer new research directions for solving persistent challenges of CdTe photovoltaics.

Highly efficient photocatalysis by BiFeO3/α(γ)-Fe2O3 ferromagnetic nano p/n junctions formed by dopant-induced phase separation

A series of Bi1−x Ca x FeO3 (BCFO) nanoparticles (with x = 0.0, 0.03, 0.07, 0.10, 0.15, and 0.20) have been synthesized by sol–gel reaction. X-ray diffraction patterns establish the formation of hexagonal bismuth ferrite as the prominent phase, with a small contribution of the Bi2Fe4O9 phase (as reported by others as well) which diminishes rapidly with the increase in Ca concentration. Interestingly, above a calcium dopant concentration of about 10 % peaks of Fe2O3 (both α and γ components) are observed with a concomitant enhancement of ferromagnetism. Small contribution of the Bi6Ca4O13 phase is also noted in these samples. This phase evolution is driven by dopant-induced strain energy and increasing oxygen vacancy concentration for local charge balance. Transmission electron microscopy (with elemental scanning) and Mossbauer spectroscopy techniques bring out the evolution of nanoparticle morphology (and elemental distribution) and phase configuration, respectively. Measurements of photocatalytic activity (and photo-Fenton activity with H2O2) reveal that Ca doping at the Bi site in BFO enhances the activity significantly in the concentration regime where BFO/α(γ)-Fe2O3 phases coexist in the form of a nanocomposite. The enhancement can thus be attributed to the carrier transfer between BFO and α(γ)-Fe2O3 across nano p/n junctions leading to enhanced carrier lifetime. Importantly, the magnetization of the nanocomposite (about 16 emu gm−1 at x = 0.20) provides a convenient way to collect the photocatalyst with the help of an external magnet for reuse.

Enhancement of Minority Carrier Lifetime of Fe Contaminated Boron-Phosphorus Compensated p-Type SoG Silicon

To understand the role of deliberate phosphorus doping on the minority carrier lifetime of iron contaminated boron-phosphorus-compensated p-type solar grade silicon, a numerical study has been performed. This study confirmed that compensation results in a significant increase in bulk lifetime of minority carrier. The gain in carrier lifetime is predicted due to the shift in Fermi energy level, carriers screening and reduction in net equilibrium hole concentration. The bulk lifetime of minority carrier reaches its maximum for phosphorus concentration around 10 15 cm -3 if the boron concentrations remain fixed at 10 17 cm -3 . Full Text: PDF References M.A. Green, Solar cells- operating principles, technology and system applications (Prentice-Hall 1986). F.E. Rougieux, D. Macdonald, A. Cuevas, "Transport properties of p-type compensated silicon at room temperature", Prog. Photovolt: Res. Appl. 19(7), 787 (2011). CrossRef F.E. Rougieux, M. Forster, D. Macdonald, A. Cuevas, B. Lim, J. Schmidt, "Recombination Activity and Impact of the Boron?Oxygen-Related Defect in Compensated N-Type Silicon", IEEE Journal of Photovoltaics 1, 54 (2011). CrossRef D. Leblanc, K. Putyera, "New resistivity/dopant density model for compensated-Si", Trans.Nonferrous Met. Soc. China 21, 1172 (2011). CrossRef S. Dubois, N. Enjalbert, J.P. Garandet, "Effects of the compensation level on the carrier lifetime of crystalline silicon", Appl. Phys. Lett. 93, 032114 (2008). CrossRef D. Macdonald, A. Cuevas, "Recombination in compensated crystalline silicon for solar cells", J. Appl. Phys. 109, 043704 (2011). CrossRef J. Veirman, S. Dubois, N. Enjalbert, J.P. Garandet, M. Lemiti, "Electronic properties of highly-doped and compensated solar-grade silicon wafers and solar cells", J. Appl. Phys. 109, 103711 (2011). CrossRef D. Macdonald, PhD Thesis (Australian National University 2001). S. Rein, Lifetime Spectroscopy (Springer 2005). A. A. Istratov, H. Hieslmair, E. R. Weber, "Iron and its complexes in silicon", Appl. Phys. A: Mater. Sci. Process. 69, 13 (1999). CrossRef W. Shockley, W. T. J Read, "Statistics of the Recombinations of Holes and Electrons", Phys. Rev. 87 (5), 835 (1952). CrossRef R.N. Hall, "Electron-Hole Recombination in Germanium", Phys. Rev. 87, 387 (1952). CrossRef A. Hangleiter, R. Hacker, "Enhancement of band-to-band Auger recombination by electron-hole correlations", Phys. Rev. Lett. 65 (2), 215 (1990). CrossRef

Ultrahigh-Voltage SiC PiN Diodes with an Improved Junction Termination Extension Structure and Enhanced Carrier Lifetime

Ultrahigh-voltage SiC PiN diodes with an improved junction termination extension (JTE) structure and improved forward characteristics are presented in this paper. An improved space-modulated JTE (SM-JTE) structure was designed by device simulation, and a breakdown voltage of over 17 kV was obtained in a wider range of JTE dose with the improved SM-JTE. In addition, a lifetime enhancement process (thermal oxidation) was performed to improve the forward characteristics. The on-resistance of the SiC PiN diodes with the lifetime enhancement process was reduced to 13 mΩ cm2 at 150 °C compared with that of the SiC PiN diodes with the conventional process (32 mΩ cm2).

Growth and Characterization of Thick 4H-SiC Epilayers for Very High Voltage Bipolar Devices

To establish elemental technologies of thick 4H-SiC epitaxial growth for very high voltage bipolar devices, reduction methods for Z1/2 center, BPDs and in-grown stacking faults have been investigated. Complete elimination of the Z1/2 center in a very thick (>100 μm) epilayer and significant enhancement of carrier lifetimes are achieved by growing the epilayer under proper conditions followed by application of the post-growth process. A new method to convert remaining BPDs in an epilayer to TEDs by simple high temperature annealing is demonstrated. We also discuss the growth conditions to minimize the formation of the in-grown stacking faults during epitaxial growth under a H2+SiH4+C3H8 system.

Light induced enhancement of minority carrier lifetime of chemically passivated crystalline silicon

In this work we present light effects on chemically passivated silicon surface. Di-iodine–ethanol (I–E) mixtures were used to passivate silicon dangling bonds. The passivation quality is sensitive to both silicon surface state and light irradiation. The minority carrier lifetime values vary from 2μs for unpassivated surfaces to about 40μs for chemically passivated ones. FTIR investigations show that light irradiation catalyses the passivation effect by forming the silicon-ethoxylate group (SiOC2H5). We suggest a mechanism to explain the passivation effect based on carrier-induced dissociation of I2.

Electrical properties improvement of multicrystalline silicon solar cells using a combination of porous silicon and vanadium oxide treatment

In this paper, we will report the enhancement of the conversion efficiency of multicrystalline silicon solar cells after coating the front surface with a porous silicon layer treated with vanadium oxide. The incorporation of vanadium oxide into the porous silicon (PS) structure, followed by a thermal treatment under oxygen ambient, leads to an important decrease of the surface reflectivity, a significant enhancement of the effective minority carrier lifetime (τeff) and a significant enhancement of the photoluminescence (PL) of the PS structure. We Obtained a noticeable increase of (τeff) from 3.11μs to 134.74μs and the surface recombination velocity (Seff) have decreased from 8441cms−1 to 195cms−1. The reflectivity spectra of obtained films, performed in the 300–1200nm wavelength range, show an important decrease of the average reflectivity from 40% to 5%. We notice a significant improvement of the internal quantum efficiency (IQE) in the used multicrystalline silicon substrates. Results are analyzed and compared to those carried out on a reference (untreated) sample. The electrical properties of the treated silicon solar cells were improved noticeably as regard to the reference (untreated) sample.

Effects of GaAs(Sb) cladding layers on InAs/AlAsSb quantum dots

The structural and optical properties of InAs self-assembled quantum dots buried in AlAs0.56Sb0.44 barriers can be controlled using GaAs1−xSbx cladding layers. These cladding layers allow us to manage the amount of Sb immediately underneath and above the InAs quantum dots. The optimal cladding scheme has a GaAs layer beneath the InAs, and a GaAs0.95Sb0.05 layer above. This scheme results in improved dot morphology and significantly increased photoluminescence (PL) intensity. Both power-dependent and time-resolved photoluminescence confirm that the quantum dots have type-II band alignment. Enhanced carrier lifetimes in this quantum dot system show great potential for application in intermediate band solar cells.

Strong Carrier Lifetime Enhancement in GaAs Nanowires Coated with Semiconducting Polymer

The ultrafast charge carrier dynamics in GaAs/conjugated polymer type II heterojunctions are investigated using time-resolved photoluminescence spectroscopy at 10 K. By probing the photoluminescence at the band edge of GaAs, we observe strong carrier lifetime enhancement for nanowires blended with semiconducting polymers. The enhancement is found to depend crucially on the ionization potential of the polymers with respect to the Fermi energy level at the surface of the GaAs nanowires. We attribute these effects to electron doping by the polymer which reduces the unsaturated surface-state density in GaAs. We find that when the surface of nanowires is terminated by native oxide, the electron injection across the interface is greatly reduced and such surface doping is absent. Our results suggest that surface engineering via π-conjugated polymers can substantially improve the carrier lifetime in nanowire hybrid heterojunctions with applications in photovoltaics and nanoscale photodetectors.

Photocatalytic Hydrogen Generation Efficiencies in One-Dimensional CdSe Heterostructures.

To better understand the role nanoscale heterojunctions play in the photocatalytic generation of hydrogen, we have designed several model one-dimensional (1D) heterostructures based on CdSe nanowires (NWs). Specifically, CdSe/CdS core/shell NWs and Au nanoparticle (NP)-decorated core and core/shell NWs have been produced using facile solution chemistries. These systems enable us to explore sources for efficient charge separation and enhanced carrier lifetimes important to photocatalytic processes. We find that visible light H2 generation efficiencies in the produced hybrid 1D structures increase in the order CdSe < CdSe/Au NP < CdSe/CdS/Au NP < CdSe/CdS with a maximum H2 generation rate of 58.06 ± 3.59 μmol h(-1) g(-1) for CdSe/CdS core/shell NWs. This is 30 times larger than the activity of bare CdSe NWs. Using femtosecond transient differential absorption spectroscopy, we subsequently provide mechanistic insight into the role nanoscale heterojunctions play by directly monitoring charge flow and accumulation in these hybrid systems. In turn, we explain the observed trend in H2 generation rates with an important outcome being direct evidence for heterojunction-influenced charge transfer enhancements of relevant chemical reduction processes.

Enhancement and control of carrier lifetimes in p-type 4H-SiC epilayers

Enhancement and control of carrier lifetimes in p-type 4H-SiC have been investigated. In this study, thermal oxidation and carbon ion implantation methods, both of which are effective for lifetime enhancement in n-type SiC, were attempted on 147-μm thick p-type 4H-SiC epilayers. Effects of surface passivation on carrier lifetimes were also investigated. The carrier lifetimes in p-type SiC could be enhanced from 0.9 μs (as-grown) to 2.6 μs by either thermal oxidation or carbon implantation and subsequent Ar annealing, although the improvement effect for the p-type epilayers was smaller than that for the n-type epilayers. After the lifetime enhancement, electron irradiation was performed to control the carrier lifetime. The distribution of carrier lifetimes in each irradiated region was rather uniform, along with successful lifetime control in the p-type epilayer in the range from 0.1 to 1.6 μs.

Minority carrier lifetime enhancement in multicrystalline silicon by means of a dual treatment based on porous silicon and sputter-deposition of TiO2:Cr passivation layers

We report on a dual passivation approach of multicrystalline-silicon (mc-Si), which combines porous silicon (PS) treatment and sputter-deposition of TiO2:Cr passivation layer. At the optimal Cr content of 2at.%, the effective minority carrier lifetime was found to be enhanced significantly by more than 2 orders of magnitude (from 2 to ∼733μs). Our results demonstrate that this dual treatment not only provides strong passivation of the mc-Si substrate but decreases also the total surface reflectivity at 500nm (from 40% for untreated mc-Si samples to ∼19% for TiO2:Cr/PS treated ones).

Effect of inserting a thin buffer layer on the efficiency in n-ZnO/p-Cu2O heterojunction solar cells

Transparent conducting Al-doped ZnO (AZO)/nondoped ZnO/Cu2O heterojunction solar cells were fabricated by inserting a thin nondoped ZnO film as a buffer layer between an n+-AZO thin film and a p-Cu2O sheet prepared by thermally oxidizing a Cu sheet. The effect of inserting the buffer layer on the obtainable photovoltaic properties in n+-AZO/n-ZnO/p-Cu2O heterojunction solar cells was investigated to improve the conversion efficiency. An improvement of photovoltaic properties was obtained by optimizing the thickness as well as the deposition conditions in the nondoped ZnO thin-film buffer layer. The obtained improvement of photovoltaic properties may be attributable mainly to an increase of the barrier height formed in the np junction, resulting from the inserted buffer layer functioning as an n-type ZnO layer as well as an enhancement of carrier lifetimes near the interface between the nondoped ZnO thin-film buffer layer and the Cu2O. A high efficiency of 4.08% was obtained in an AZO/nondoped ZnO/Cu2O heterojunction solar cell fabricated using a buffer layer prepared at room temperature and an O2 gas pressure of 1.0 Pa with a thickness of 50 nm on a Cu2O sheet and measured under simulated AM1.5G solar.

Photoinduced Carrier Generation and Decay Dynamics in Intercalated and Non-intercalated Polymer:Fullerene Bulk Heterojunctions

The dependence of photoinduced carrier generation and decay on donor-acceptor nanomorphology is reported as a function of composition for blends of the polymer poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene) (pBTTT-C(14)) with two electron-accepting fullerenes: phenyl-C(71)-butyric acid methyl ester (PC(71)BM) or the bisadduct of phenyl-C(61)-butyric acid methyl ester (bis-PC(61)BM). The formation of partially or fully intercalated bimolecular crystals at weight ratios up to 1:1 for pBTTT-C(14):PC(71)BM blends leads to efficient exciton quenching due to a combination of static and dynamic mechanisms. At higher fullerene loadings, pure PC(71)BM domains are formed that result in an enhanced free carrier lifetime, as a consequence of spatial separation of the electron and hole into different phases, and the dominant contribution to the photoconductance comes from the high-frequency electron mobility in the fullerene clusters. In the pBTTT-C(14):bis-PC(61)BM system, phase separation results in a non-intercalated structure, independent of composition, which is characterized by exciton quenching that is dominated by a dynamic process, an enhanced carrier lifetime and a hole-dominated photoconductance signal. The results indicate that intercalation of fullerene into crystalline polymer domains is not detrimental to the density of long-lived carriers, suggesting that efficient organic photovoltaic devices could be fabricated that incorporate intercalated structures, provided that an additional pure fullerene phase is present for charge extraction.

Dramatic Reduction of Surface Recombination by in Situ Surface Passivation of Silicon Nanowires

Nanowires have unique optical properties and are considered as important building blocks for energy harvesting applications such as solar cells. However, due to their large surface-to-volume ratios, the recombination of charge carriers through surface states reduces the carrier diffusion lengths in nanowires a few orders of magnitude, often resulting in the low efficiency (a few percent or less) of nanowire-based solar cells. Reducing the recombination by surface passivation is crucial for the realization of high-performance nanosized optoelectronic devices but remains largely unexplored. Here we show that a thin layer of amorphous silicon (a-Si) coated on a single-crystalline silicon nanowire, forming a core-shell structure in situ in the vapor-liquid-solid process, reduces the surface recombination nearly 2 orders of magnitude. Under illumination of modulated light, we measure a greater than 90-fold improvement in the photosensitivity of individual core-shell nanowires, compared to regular nanowires without shell. Simulations of the optical absorption of the nanowires indicate that the strong absorption of the a-Si shell contributes to this effect, but we conclude that the effect is mainly due to the enhanced carrier lifetime by surface passivation.

The Effect of Nanoparticle Shape on the Photocarrier Dynamics and Photovoltaic Device Performance of Poly(3‐hexylthiophene):CdSe Nanoparticle Bulk Heterojunction Solar Cells

AbstractThe charge separation and transport dynamics in CdSe nanoparticle:poly(3‐hexylthiophene) (P3HT) blends are reported as a function of the shape of the CdSe‐nanoparticle electron acceptor (dot, rod, and tetrapod). For optimization of organic photovoltaic device performance it is crucial to understand the role of various nanostructures in the generation and transport of charge carriers. The sample processing conditions are carefully controlled to eliminate any processing‐related effects on the carrier generation and on device performance with the aim of keeping the conjugated polymer phase constant and only varying the shape of the inorganic nanoparticle acceptor phase. The electrodeless, flash photolysis time‐resolved microwave conductivity (FP‐TRMC) technique is used and the results are compared to the efficiency of photovoltaic devices that incorporate the same active layer. It is observed that in nanorods and tetrapods blended with P3HT, the high aspect ratios provide a pathway for the electrons to move away from the dissociation site even in the absence of an applied electric field, resulting in enhanced carrier lifetimes that correlate to increased efficiencies in devices. The processing conditions that yield optimum performance in high aspect ratio CdSe nanoparticles blended with P3HT result in poorly performing quantum dot CdSe:P3HT devices, indicating that the latter devices are inherently limited by the absence of the dimensionality that allows for efficient, prolonged charge separation at the polymer:CdSe interface.

Carrier Lifetime and Mobility Enhancement in Nearly Defect-Free Core−Shell Nanowires Measured Using Time-Resolved Terahertz Spectroscopy

We have used transient terahertz photoconductivity measurements to assess the efficacy of two-temperature growth and core-shell encapsulation techniques on the electronic properties of GaAs nanowires. We demonstrate that two-temperature growth of the GaAs core leads to an almost doubling in charge-carrier mobility and a tripling of carrier lifetime. In addition, overcoating the GaAs core with a larger-bandgap material is shown to reduce the density of surface traps by 82%, thereby enhancing the charge conductivity.

Behaviour of defects in semi-insulating 4H-SiC after ultra-high temperature anneal treatments

We have recently explored the nature and stability of native defects in SI 4H-SiC after post-growth anneals between 1000 and 1800 °C from combined electron paramagnetic resonance (EPR) and low-temperature infrared photoluminescence (PL) experiments. In the present work we have extended these studies to SI 4H-SiC subjected to very high post-growth anneal temperatures (1900–2400 °C) where significantly enhanced carrier lifetimes have been recently reported for such conditions. Previously, the intensities of the V C, V Si, V C– V Si and SI-5 EPR decreased monotonically with anneals from 1200 to 1800 °C. In this work, surprisingly, many of these defects reappeared after annealing at 2100 °C and above. Similar annealing behavior was observed for the IR PL lines. Many defects are present during growth and these results illustrate the effects of the post-growth anneal treatments on their concentrations.

Performance investigation of all-optical clock recovery circuit based on Fabry-Pérot filter and semiconductor optical amplifier assisted Sagnac switch

A theoretical model is developed and exploited for characterizing the behavior of an all-optical circuit that deploys a Fabry-Perot filter (FPF) and a semiconductor optical amplifier (SOA)-assisted Sagnac switch driven by an intense continuous wave (cw) holding beam to extract the clock signal from a received ultra-fast data stream. The role of each unit is thoroughly studied, and its understanding allows us to identify the critical operational parameters, which include the cw power, the energy of the data pulses, the SOA small signal gain, carrier lifetime and linewidth enhancement factor, the Sagnac loop asymmetry, and the finesse of the FPF. By means of numerical simulation, an extensive set of diagrams is derived, and from their interpretation the impact of these key factors on the amplitude modulation of the extracted clock pulses is investigated and evaluated. This process enables us to appropriately select and combine them to ensure enhanced performance concerning the defined quality metric, first at 10 Gb/s and then at 40 Gb/s. The final outcome demonstrates the technological feasibility and effectiveness of the proposed clock recovery scheme. The obtained results may be useful for theoretically addressing various all-optical signal processing tasks, whose proper execution relies decisively on clock recovery.

Effects of annealing on carrier lifetime in 4H-SiC

We present results of a thermal anneal process that increases the minority carrier lifetime in SiC substrates to in excess of 3μs, compared to the starting as-grown substrates with lifetimes typically in the &amp;lt;10ns range. Measurement of lifetimes was conducted using microwave-photoconductive decay. Electron beam induced current measurements exhibited minority carrier diffusion lengths of up to 65μm, confirming the enhanced carrier lifetime of the annealed substrate material. Additionally, positron annihilation spectroscopy and deep level transient spectroscopic (DLTS) analysis of samples subjected to this anneal process indicated that a significant reduction of deep level defects, particularly Z1∕Z2, may account for the significantly enhanced lifetimes. The enhanced lifetime is coincident with a transformation of the original as-grown crystal into a strained or disordered lattice configuration as a result of the high temperature anneal process. The operational performance of p-i-n diodes employing drift layers fabricated from the annealed high-lifetime substrates confirmed conductivity modulation in the diodes consistent with ambipolar carrier lifetimes in the microsecond range.