Intensity dependence of quantum efficiency and photo‐gating effects in thin film silicon solar cells

Abstract

Steady‐state photoconductivity measurements have been carried out on thin‐film silicon pin structures of i‐layer thickness typically 4 μm, where crystalline composition has been varied by adjustment of the silane concentration in the process gas. In amorphous and low‐crystallinity cells, strongly‐absorbed light incident from the p‐side at photon fluxes in excess of 1014 cm‐2 s‐1 produces strongly sub‐linear intensity dependence, ‘S’ shaped reverse current‐voltage curves and amplification of a second weakly‐absorbed beam, termed photogating. These effects are linked to the formation of space charge and attendant low‐field region close to the p‐i interface, as confirmed by computer simulation. More crystalline devices exhibit little or no such behaviour. At lower intensities of strongly‐absorbed light there is a markedly steeper increase in reverse current vs. voltage in low‐crystalline when compared to amorphous cells, particularly with light incident from the n‐side. This suggests the mobility‐lifetime product for holes is much larger in the former case, consistent with the higher hole mobilities reported in time of flight studies. Thus the prospect of composition‐dependent changes in mobility as well as defect density should be borne in mind when developing materials for application in microcrystalline silicon solar cells. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Amorphous silicon intrinsic photomixing detector for optical ranging

“…However, in case of dichromatic or multispectral illumination, the local low-field region might be offset so that the transport of electrons generated in the device front is facilitated. In result, a significantly larger amount of photogenerated electrons can drift towards the electrical contact so that collection efficiencies can exceed unity gain 27 , 29 , 33 . Such light-induced electrical field deformations enabled by photogating predominantly occur in defective materials, like a-Si:H, which exhibits a large amount of deep dangling bond states rather than localized tail states.…”

“…In a-Si:H waveguides, optical (N)AND logic gate operation in the time domain at GHz frequencies has already been demonstrated utilizing four-wave mixing Bragg scattering; a third-order nonlinear process that requires high intensity laser illumination 26 . In a-Si:H field-optimized p-i-n photodetectors and solar cells, it was found that collection efficiencies can exceed values of long-term optimized crystalline silicon photodiodes that utilize photogating in the visible range 27 – 29 . Compared to conventional high-temperature silicon manufacturing, the mature thin-film deposition technology of a-Si:H allows for ubiquitous sensor integration on top of read-out electronics at low temperatures and costs with pixel fill factors up to 30 , 31 .…”

High Responsivity and Ultra-Low Detection Limits in Nonlinear a-Si:H p-i-n Photodiodes Enabled by Photogating

“…Further experiments have been conducted to investigate and quantify the achievable bandwidth, the responsivity gain, and the detection limit at different light levels since charge carrier collection efficiencies strongly depend on the gate and the probe illumination irradiance and its ratio [13,14,16].…”

“…Previous results demonstrating photon collection efficiency gains beyond unity in a-Si:H based detectors achieve rise times ranging from 10 −5 s to 0.1 s. However, such photo-gated sensors suffer from comparatively slow decay times of the photo-gated current transients resulting in time constants exceeding seconds. Furthermore, the a-Si:H p-i-n photodetectors presented in [13,14,16], invariably require extensive light soaking prior to the measurement to establish temporarily an initial low-field region within the detector. The overall bandwidth enhancement by at least one order of magnitude reported in this work can mainly be attributed to:…”

“…A few years later, collection efficiency and photocurrent gain values up to almost 50 in the visible range have experimentally been demonstrated by Main et al [13] in comparatively thick photodetectors by applying high reverse bias voltages of -25 V. Unfortunately, absolute responsivities have not been reported impeding a quantitative performance comparison with the state-of-the-art. To achieve a sufficient device gating stability and such high optical gain values, extensive annealing and light-soaking prior to the measurements have been reported to be mandatory [13,14,16]. Initial degradation, e.g., by utilizing extensive AM1.0 illumination (AM: air mass coefficient), was required to generate an in-built low-field region in the front of the device that can be controlled and modulated by a weakly absorbed gate beam [13].…”