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

The 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.     

Fundamental materials studies of undoped, In-doped, and As-doped Hg1−xCdxTe

Variable magnetic-field Hall and transient photoconductance-lifetime measurements were performed on a series of undoped, In-doped, and As-doped HgCdTe samples grown by molecular beam epitaxy (MBE). Use of quantitative mobility-spectrum analysis (QMSA) combined with multiple carrier-fitting (MCF) techniques indicates that the majority of samples contain an interfacial n-type layer that significantly influences the interpretation of the electrical measurements. This n-type layer completely masks the high-quality electrical properties of undoped or low n-type In-doped HgCdTe, as well as complicating the interpretation of activation in As-doped p-type HgCdTe. Introduction of an intentional n-type background, typically created through doping with In to “recover” high mobility, is actually shown to increase the “bulk” layer conductivity to a level comparable to the interface layer conductivity. Photoconductance-lifetime measurements suggest that In-doping may introduce Shockley-Read-Hall (SRH) recombination centers. Variable-field Hall analysis is shown to be essential for characterizing p-type material. Photoconductance-lifetime measurements suggest that trapping states may be introduced during the incorporation and activation of As. Two distinctly different types of temperature dependencies were observed for the lifetimes of As-doped samples.     

The implementation of temperature control to an inductive‐coil photoconductance instrument for the range of 0–230°C

A new device setup for temperature and injection‐dependent lifetime spectroscopy (TIDLS) is described. It comprises two off‐the‐shelf components: a heating and cooling stage (HCS) from INSTEC and an inductive‐coil photoconductance (PC) instrument (WCT‐100) from Sinton Consulting Inc. The HCS was fitted to the WCT‐100 in a manner that circumscribes the inductive coil (the sensor) of the RF bridge circuit and controls the temperature of the wafer effectively. This setup has the advantage of requiring minor modifications to industry standard instruments while attaining a large temperature range. As experimental verification, injection‐dependent lifetimes were measured over a temperature range, 0–230°C, in three iron‐implanted silicon wafers. The measured lifetimes are consistent with the Shockley–Read–Hall equation using the impurity concentration calculated from the implant dose and the energy level and capture cross‐sections of interstitial iron from the literature. Copyright © 2008 John Wiley & Sons, Ltd.     

N+P photodetector characterization using the quasi-steady state photoconductance decay method

When a material is irradiated, it becomes more electrically conductive due to the absorption of the electromagnetic radiation. As a result, the number of free electrons and holes changes and raises its electrical conductivity. A simple but interesting phenomenon to characterise a fabricated n⁺p photodetector in order to determine its linearity (photoresponse) and photoconductance was employed. Using the transient decay when the irradiation source is switched off, the minority carrier concentration, effective lifetime and surface recombination velocity present at the surface of the detector were measured.     

A method for modeling and deciphering the persistent photoconductance and long-term charge storage of ZnO nanorod arrays

The controllability of persistent photoconductance (PPC) and charge/energy storage of ZnO nanorod arrays (NRAs) were demonstrated experimentally by tuning the nanorod diameter. The dependency of the ZnO NRAs’ photoelectric characteristics on the nanorod diameter suggests that the Debye length and photon penetration depth in ZnO could spatially partition a standalone nanorod into three different photoelectric functional regions (PFRs). Theoretically, a series of rate functions was employed to describe the different extrinsic/intrinsic carrier photogeneration/recombination dynamic sub-processes occurring in the different PFRs, associated with oxygen chemisorption/photodesorption, oxygen vacancy photoionization, and electron trapping by photoionized oxygen vacancies. On the basis of the coupled contributions of these different dynamic sub-processes in the photoelectric properties of the ZnO NRAs, a thorough-process photoelectric dynamic model (TPDM) was proposed using the simultaneous rate functions. Through solving the rate functions, the corresponding analytical equations could be employed to simulate the time-resolved PPC spectra of the ZnO NRAs, and then the quantitative parameters extracted to decipher the PPC and charge/energy storage mechanisms in the ZnO NRAs. In this way, the TPDM model provided a numerical-analytical method to quantitatively evaluate the photoelectric properties of ZnO NRA-based devices. Additionally, the TPDM model revealed how the different photoinduced carrier dynamics in the different PFRs could play functional roles in different optoelectronic applications, e.g., photodetectors, photocatalysts, solar cells and optical nonvolatile memories, and thus it illuminated a practical approach for the design of ZnO NRA-based devices via optimization of the modularized spatial configuration of the PFRs.

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N~+P photodetector characterization using the quasi-steady state photoconductance decay method

正When a material is irradiated,it becomes more electrically conductive due to the absorption of the electromagnetic radiation.As a result,the number of free electrons and holes changes and raises its electrical conductivity. A simple but interesting phenomenon to characterise a fabricated n~+p photodetector in order to determine its linearity(photoresponse) and photoconductance was employed.Using the transient decay when the irradiation source is switched off,the minority carrier concentration,effective lifetime and surface recombination velocity present at the surface of the detector were measured.     

微波光电导中的灵敏度分析

对微波光电导法测量半导体少数载流子寿命的测试系统进行灵敏度分析。利用Maxwell方程组的求解和转移矩阵的利用,可对微波光电导测试系统求出反射系数和灵敏度。讨论了灵敏度与样品的电导率、厚度的关系。通过比较一般的测试系统以及它的两种特殊情况发现：对于所有电导率的样品,在测试样品背后的合适位置放置金属反射器可增加测试系统的灵敏度。     

Temperature-dependent photoconductance of heavily doped ZnO nanowires

Ga-doped ZnO nanowires have been synthesized by a pulsed laser chemical vapor deposition method. The crystal structure and photoluminescence spectra indicate that the dopant atoms are well integrated into the ZnO wurtzite lattice. The photocurrent properties at different temperatures have been systematically investigated for nanowires configured as a three-terminal device. Among the experimental highlights, a pronounced semiconductor-to-metal transition occurs upon UV band-to-band excitation. This is a consequence of the reduction in electron mobility arising from the drastically enhanced Coulomb interactions and surface scattering. Another feature is the reproducible presence of two resistance valleys at 220 and 320 K upon light irradiation. This phenomenon originates from the trapping and detrapping processes in the impurity band arising from the native defects as well as the extrinsic Ga dopants. This work demonstrates that due to the dimensional confinement in quasi-one-dimensional structures, enhanced Coulomb interaction, surface scattering, and impurity states can significantly influence charge transport.      

Calibration of the WCT‐100 photoconductance instrument at low conductance

The WCT‐100 photoconductance instrument is used throughout the photovoltaic industry providing a relatively simple and inexpensive means to assess recombination parameters. This paper presents a method to determine its output voltage V _WCT over the conductance range, S = 0·002–20 mS, a range not easily attained by the conventional calibration method. The relationship between V _WCT and S is found to transition from being non‐linear to linear at S ∼ 1 mS, which equates to the conductance of a 300 µ m thick 30 Ω‐cm silicon wafer. For samples whose dark conductance is lower than the transition conductance, the non‐linear relationship between V _WCT and S must be taken into account to prevent a gross underestimation of recombination rate. The method we describe can equally well be used to investigate the calibration of other photoconductance instruments. It involves the simultaneous measurement of a device's electrical conductance and the instrument's output voltage under a range of steady‐state illumination intensities. Copyright © 2008 John Wiley & Sons, Ltd.