Light Sensing of a-Si:H P
in Diode - Wavelength and Intensity Effects
Authors: Kim Kibum , Kuo Yue 
Published: February 2020

A-Si:H pin diodes are effective photodetectors for a wide range of lights, especially in the visible wavelength region. They can be used as light sensors, optocouplers for signal communication, detectors for on-chip optical interconnect, solar cells, etc. [1-4]. For these applications, it is necessary to achieve a high light-to-electricity conversion efficiency. Although the high light intensity favors the generation of electron-hole pairs, the conversion efficiency is dependent on the spectrum, i.e., the composing wavelengths. Authors investigated electrical and optical responses of the a-Si:H pindiode to the power intensity of the red, green, and blue LEDs, separately.

The a-Si:H
pinstack was deposited by PECVD in a single chamber system using the one pump-down process at 260°C with a 50kHz RF power generator and an  automatic matching network. Deposition conditions of individual films are summarized in Table I. Both bottom and top electrodes were made of the 80 nm thick ITO deposited by sputtering. The complete diode was annealed at 200°C for 30 min in air. For the light sensing, the device was exposed to the light source from the top, which has a 2 mm diameter electrode. Red (625 nm), green (530 nm), and blue (470nm) LEDs from Thorlabs at various power densities were used as the exposure lights.  

Figure 1 shows the dependence of equivalent quantum efficiencies (EQE’s) of the a-Si:H diode at various wavelengths and power densities. The relative EQE’s are consistent with the literature report [5]. When the power intensity of the blue or green LED is increased, the EQE is decreased, which is due to the parasitic absorption of photons, loss of part of photons, or recombination of electron-hole pairs [6]. When the red LED power is increased, the EQE increases slightly and then decreases. The top n+
layer may have different absorption efficiencies at different wavelengths.

Figure 2 shows the dependence of the shunt resistance (RSH
) and series resistance (RS
) on the power density of the light. With the increase of the power intensity, the photocurrent is increased due to the generation of more electron-hole pairs, which shows as the decrease of RS
. However, the RSH
is reduced with the increase of the leakage current because of current leakage at the edge of the cell, defects presence, foreign impurities in the p-n junction, etc. [7]. Both RS
- and RSH
- power relationships follow the power law.

Figure 3 shows photocurrent density (J) vs.  open circuit voltage (VOC
) curves with the exposure light wavelength as the parameter. It was reported that the JSC
- VOC
curve followed the Arrhenius relationship with an ideality factor (n) calculated from the slope [8]. Fig. 3 confirms that the above relationship is applicable to lights of different wavelengths over a large density range. The ideality factor changes little with the wavelength, i.e., 1.58, 1.51, and 1.59 for red, green, and blue lights, respectively. An n value of 1.4 - 1.6 has been observed on other a-Si:H pin diodes. The n value is dependent on the density of recombination centers in the depletion region [9-11]. When the density of the recombination centers increases due to the increase of the density of defects or impurities, the n increases. In an ideal device, the n should be 1. In the real device, the existence of recombination centers is unavoidable because defects or impurities cannot be totally eliminated. Since all curves in Fig. 3 have similar n values, the density of recombination centers is not affected by the wavelength of the light source. The incident light wavelength and power intensity affect the EQE, RS
, RSH
, and leakage current in the a-Si:H pindiode through electron-hole and defects generations in the intrinsic a-Si:H layer and the junction region.  

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Figure 1