**Author details**

5.72mA/cm2

306 Photodiodes - From Fundamentals to Applications

cm2

**4. Summary**

12.09mA/cm2

71.41%, and efficiency of 7.49%.

top cell increased from 6.5 to 9.5 mA/cm2

nc-Si:H bottom cell, the current loss of -1.77mA/cm2

measured with solar simulator. It can be said that Jsc of triple junction solar cell is

.

. However, for the

, FF of

limited by the bottom cell and Jsc of the nc-Si:H middle cell sould be at least 5.4mA/cm2

Even though the bottom cell was composed of 3.2μm thick intrinsic layer, the Jsc was low. One of the reasons can be the absorption of incident light by the middle cell. Low haze ratio of the TCO and increased recombination of generated electron hole pair in nc-Si:H absorp‐ tion layer can also be one of the reasons. In this study, the observed Voc was 1.83V. The rea‐ son for the lower Voc is presumed to be the decrease in Voc of the microcrystalline bottom cell. In order to have initial efficiency of over 14%, Jsc of unit cells should be at least 9mA/

. For the top cell, it can be done by raising the thickness of the intrinsic a-Si:H layer to ~ 250nm or by increasing the short wavelength QE. However, for the middle and bottom cells, more efforts are needed such as light trapping, improved property of nc-Si:H and develop‐ ment of new intrinsic layer such as nc-SiGe:H [50-52] as well as improvement in the TRJ.

a-Si:H/nc-Si double and a-Si:H/nc-Si:H/nc-Si:H triple junction solar cells have been made and the effects of the top cell thickness and interlayer on the current matching and solar cell characteristics have been investigated. There is a significant impact of the multijunction cell performance on the current matching of the component cells as well as the tunnel junction in between them. When the Si:H top cell thickness was varied from 100 to 300nm, Jsc of the

10.0mA/cm2 and current matching of the multiple junction solar cell occurred around 330nm resulting in lower Jsc. For the top cell of DJ solar cells, unlike the single p-i-n type solar cell, there is no back reflector electrodes (Ag or AZO) present, because of the presence of the bot‐ tom cell. So it was difficult to raise the Jsc of this cell without increasing its i-layer thickness. The AZO inter-layer was inserted between the top and bottom cells to make the junction like a TRJ. This AZO layer may also work as a partial reflector of unabsorbed light. With a

sorption of AZO. By using textured AZO front layer electrode with high haze ratio, it was possible to develop a a-Si:H/nc-Si:H double junction solar cell with Voc of 1.424V, Jsc of

over 13%, further studies for improvements on inter-layer property, light trapping and high‐

For an a-Si/nc-Si/nc-Si triple junction solar cell, 3 units of solar cells were connected in series electrically and optically. Thus, the current matching between each unit was im‐ portant to get higher efficiency. The a-Si(150nm)/nc-Si(2.0μm)/nc-Si(3.2μm) triple junction

It may be possible to raise the Voc of the MJ cell to 1.95V by optimizing the top cell and the tunnel junction. With this, the FF is also expected to increase. To increase the stabilized effi‐

solar cell fabricated in this study, showed the Voc of 1.832V, Jsc of.773mA/cm2

, FF of 72.84%. To develop multiple junction solar cells with initial efficiency

150nm thick inter-layer, the current gain of the top cell was +1.3mA/cm2

er response of bottom cell in long wavelength range should be carried out.

. For the bottom cell, Jsc decreased from 13.0 to

occurred due to the reflection and ab‐

S. M. Iftiquar1\*, Jeong Chul Lee2 , Jieun Lee2 , Juyeon Jang1 , Yeun-Jung Lee1 and Junsin Yi1,3

\*Address all correspondence to: smiftiquar@gmail.com

1 College of Information and Communication Engineering, Sungkyunkwan University, Re‐ public of Korea

2 Korea Institute of Energy Research, Gajeong-ro, Yuseong-gu, Daejeon, Republic of Korea

3 Department of Energy Science, Sungkyunkwan University, Republic of Korea

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**Section 4**

**Circuit Applications**

**Section 4**

**Circuit Applications**

**Chapter 10**

**Noise Performance of Time-Domain CMOS Image**

Temporal noise is the main disadvantage of CMOS image sensors when compared to charg‐ ed couple devices (CCDs) sensor. The typical 3T active pixel sensor (APS) architecture presents as main noise sources the photodiode shot noise, the reset transistor and follower thermal and shot noise, the amplifier thermal and 1/f noise, the column amplifier thermal and reset noise (Zheng, 2011; Brouk, 2010; Jung, 2005; Tian, 2001; Derli, 2000; Yadid-Pecht, 1997). In order to reduce the APS noise several approaches have been proposed in the litera‐ ture. Some of these approaches are the use of high gain preamplifiers, correlated multiple sampling (CMS) and low bandwidth column-parallel single slope A/D converters (Sakaki‐ bara, 2005; Kawai, 2004; Suh, 2010; Lim, 2010; Yoshihara, 2006; Chen, 2012). However, APS in time domain has as advantage to show lower source of noise since it is composed only by a photodiode, a reset transistor and a voltage comparator. It shows as noise source only the reset transistor and the photodiode. Therefore, in principle, APS in time domain may

The only two main noise source of APS in time domain are the reset noise and the integra‐ tion noise. The source of reset noise is the incomplete reset operation. Tian et al. 2001, show that APS operates usually with incomplete reset operation. The incomplete reset operation originates a random reset voltage that varies from frame to frame as a source noise. It have been found that the reset noise is *kT/2C ph*. During the integration period, the photodiode shot noise predominates generating a integration noise that is a function of the integration time, the photocurrent and the dark current. Altought the reset noise is the same to APS in voltage domain and in time domain, the integration noise must present different behavior in both approach. We show that while the integration time increases at higher photocurrents in

> © 2012 de S. Campos et al.; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2012 de S. Campos et al.; licensee InTech. This is a paper distributed under the terms of the Creative Commons

Fernando de S. Campos, José Alfredo C. Ulson, José Eduardo C. Castanho and Paulo R. Aguiar

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/51584

presents lower overall noise.

**1. Introduction**

**Sensors**
