**Single- and Multiple-Junction p-i-n Type Amorphous Silicon Solar Cells with p-a-Si1-xCx:H and nc-Si:H Films**

S. M. Iftiquar, Jeong Chul Lee, Jieun Lee, Juyeon Jang, Yeun-Jung Lee and Junsin Yi

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/51732

## **1. Introduction**

The p-a-Si1-xCx:H alloy is popularly knows as a wide band gap semiconducting alloy. It was demonstrated in the 1980s that application of the p-a-Si1-xCx:H alloy leads to improved per‐ formance of a solar cell with better blue response of its quantum efficiency (QE) [1]. There are few other well known wide band gap alloy materials available, however one interesting advantage of the p-a-Si1-xCx:H is that both the C and Si are four fold coordinated atoms, and hence a suitably prepared material may attain wider optical gap with good stability.

The p-i-n type diodes have been widely used in photovoltaic solar (PV) energy conversion. Incident light that falls on the diode is absorbed in the intrinsic layer and electron-hole (e-h) pairs are generated, producing the PV or electrical energy, while the p-type and n-type lay‐ ers produce built-in electric field to separate the e-h pairs, created in the i-type layer. Recent‐ ly the interest on PV energy has been growing because it can provide clean energy. However, the efficiency of a solar cell is lower than that is expected, although there is a con‐ tinuous improvement in solar cell efficiency (η) throughout the history of solar cell. One of the reasons for such a low efficiency has been loss of light at the front surface of the cell due to reflection, as well as part of the low energy photons remains unabsorbed in the cell. Thus, for a maximum utilization of the incident light for PV energy conversion, the structural and material properties are expected to play some important roles. Light absorption at the doped window layer is also considered loss of light.

Another challenge is the material and interface defects that can hinder the collection of the photo generated e-h pairs [2]. This defect can exist in the material or at the interface [3,4]. Reducing such defects is also one of the priorities of a solar cell fabrication. The collection of

© 2012 Iftiquar 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. © 2012 Iftiquar et al.; licensee InTech. This is a paper 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.

the e-h pairs can also depend upon built-in electric field created by doped p-type and n-type layers [5], and thus degree and efficiency of doping of these layers are also important.

Hydrogenated nanocrystalline silicon (nc-Si:H) thin film is also known as hydrogenated mi‐ crocrystalline silicon (μc-Si:H) thin film. It is composed of amorphous phase and a few nm sized crystalline Si grains [25-27]. The p-type nc-Si:H is also a promising material for solar cell window layer [28] The nc-Si:H thin films have optical bandgap of around 1.1 eV, unlike the a-Si:H thin films that have band gap of about 1.7eV. Light induced degradation of nc-Si:H films are low [29, 30]. Having lower optical gap of this nc-Si:H films, it is possible to utilize longer wavelength radiation of solar spectra. The nc-Si:H has lower optical band gap and higher ab‐ sorption coefficient of longer wavelength light, for which it can be used in the bottom cell of a multiple-junction solar cell, preceded by a wider band gap top cell. Such a combination of amorphous and micro-crystalline cells can be called as a micromorph solar cell [31-32]. In a mi‐ cromorph solar cell, the bandgap of the top cell is ~ 1.7eV and that of bottom cell is 1.1eV. Usu‐ ally the thickness of a-Si:H top cell is thinner than a usual single p-i-n type cell. The thickness of

Single- and Multiple-Junction p-i-n Type Amorphous Silicon Solar Cells with p-a-Si1-xCx:H and nc-Si:H Films

http://dx.doi.org/10.5772/51732

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the intrinsic layer of nc-Si:H layer is almost 10 times to that in the a-Si:H top cell [20].

performance of a MJ cell remains poor.

**2.1. Deposition of Silicon Alloy Films**

**2. Experimental**

solar cell, and characterize their performance.

Furthermore, tunneling and recombination junctions (TRJs) are necessary in a MJ cell [33]. In a p-i-n type MJ cell, multiple unit cells are joined in tandem. This leads to a junction between n-type layer of the top cell to the p-type layer of the bottom one (or p-n junction), that may act as a recifying diode placed opposite to the p-i-n cell. This may be deteriorative to cell performance. Using a TRJ type p-n junction is a solution to the problem, without this the

Thus, we try to explore various aspects of fabricating single and multiple junction thin film

We prepared amorphous type p-a-Si1-xCx:H, a-Si:H, n-a-Si:H and nano-crystalline p-nc-Si:H, i-nc-Si:H, n-ncSi:H films, characterized them and applied in single junction, double junction and triple junction solar cells. We used RF PECVD, VHF PECVD, HW-CVD, for depositing silicon alloy materials, sputtering for AZO film deposition and thermal evaporation for met‐ al electrode deposition. The silane (SiH4), methane (CH4), hydrogen (H2), diborane (B2H6) (1% in H2), phosphine (PH3)(1% in H2) were the source gases for various films, where the SiH4, CH4, H2 were used for a-Si1-xCx:H alloy materials, SiH4, H2 for a-Si:H and nc-Si:H films,

The p-a-Si1-xCx:H films were deposited by 13.56 MHz RF PECVD with CH4, SiH4, H2, B2H6

films were deposited on 25mm×25mm sized Corning 1737 glass substrates and for Fourier transform infra red (FTIR) spectroscopic study we used (100) oriented p-type c-Si wafers. Later, selective samples are used for the p-type layer of the p-i-n type solar cells. Prior to film deposition the substrates were cleaned in acetone, methanol and de-ionized water. A 10-8 Torr base pressure of the reaction chamber was maintained prior to the film depositions.

C. For optoelectronic characterization, the

B2H6 was used as a p-type dopant gas and PH3 as the n-type dopant one.

source gases, at substrate temperature (Ts) of 200o

Along with its wider optical gap, the doped window layer or the top p-layer is generally made thinner as well [6], so that more light can enter into the active region of the device. However, with a thinner p-type layer the output voltage also get reduced [7,8], leading to more recombination loss of the photo generated charge carriers. Similarly, if optical gap of the window layer is high, then also the absorption loss at the p-type layer reduces. So, a wider optical gap thicker p-type layer appears to be a good option for a solar cell window layer. However, it is known that with increased carbon content within the material, optical gap increases [9,10] and the wider optical gap is usually associated to lower dark conductiv‐ ity [11], and higher activation energy (Ea). As a result the higher optical gap of the p-type layer may lead to lower output voltage from the device.

There are several different types of window layer one can use, like hydrogenated amor‐ phous silicon oxide (a-SiO:H) [12], hydrogenated amorphous silicon carbide (a-Si1-xCx:H) [1,13], hydrogenated amorphous silicon (a-Si:H) [14] etc and micro-crystalline or nano-crys‐ talline version of these materials. Out of these, wide band gap a-Si1-xCx:H and nc-Si:H mate‐ rials are two of the most promising materials.

Being amorphous in nature and containing hydrogen (H) and carbon (C) atoms in the mate‐ rial the composition and local bonding structure of the characteristic property of the materi‐ al is thus partly determined by microstructure within the material. A microstructure is a local non-uniformity of the material, and is generally used to indicate the density of SiHn or CHn type poly-hydrides in the material, where 1 ≤ n ≤ 3. Such a microstructure can also be called a void structure as well, that may be deteriorative for the material [15,16].

The carbon-silicon bonds lead to higher optical gap of the material and thus the increased carbon fraction x in a-Si1-xCx:H leads to higher optical gap of the material [1,9,17,18]. This higher optical gap results in higher optical transparency of the material, making it more suitable for a transparent window layer of a p-i-n type solar cell. It is also known that boron doping of amorphous silicon alloy material leads to reduction in optical gap [19]. Thus, a suitable boron doped p-a-Si1-xCx:H can become one of the best suited window layers.

In a multiple- junction amorphous silicon solar cell, multiple p-i-n type structures are joined in tandem [20]. The multiple junction solar cells are also known as multi-junction solar cell. The advantage of such a solar cell is that the open circuit voltage (Voc) becomes higher, and a wider spectral range of solar radiation can be absorbed in aggregate to the component cells. In this re‐ spect double (DJ) and triple junction (TJ) cells have been extensively studied in recent past [21-24]. As purpose of the DJ or TJ cell is the PV energy conversion by utilizing a wider spectral range, so tailoring of the band gap of the component cells become very important part of the design. In a suitable design, the top cell should have wider optical gap so that shorter wave‐ length light can be absorbed but the longer wavelength light will remain unabsorbed, while the middle cell should absorb the middle part and the bottom cell should absorb the longer wavelength part of the solar spectra. Thus, for the single p-i-n type cell or multiple-junction cell, a wide band gap window layer becomes a very significant component of the device.

Hydrogenated nanocrystalline silicon (nc-Si:H) thin film is also known as hydrogenated mi‐ crocrystalline silicon (μc-Si:H) thin film. It is composed of amorphous phase and a few nm sized crystalline Si grains [25-27]. The p-type nc-Si:H is also a promising material for solar cell window layer [28] The nc-Si:H thin films have optical bandgap of around 1.1 eV, unlike the a-Si:H thin films that have band gap of about 1.7eV. Light induced degradation of nc-Si:H films are low [29, 30]. Having lower optical gap of this nc-Si:H films, it is possible to utilize longer wavelength radiation of solar spectra. The nc-Si:H has lower optical band gap and higher ab‐ sorption coefficient of longer wavelength light, for which it can be used in the bottom cell of a multiple-junction solar cell, preceded by a wider band gap top cell. Such a combination of amorphous and micro-crystalline cells can be called as a micromorph solar cell [31-32]. In a mi‐ cromorph solar cell, the bandgap of the top cell is ~ 1.7eV and that of bottom cell is 1.1eV. Usu‐ ally the thickness of a-Si:H top cell is thinner than a usual single p-i-n type cell. The thickness of the intrinsic layer of nc-Si:H layer is almost 10 times to that in the a-Si:H top cell [20].

Furthermore, tunneling and recombination junctions (TRJs) are necessary in a MJ cell [33]. In a p-i-n type MJ cell, multiple unit cells are joined in tandem. This leads to a junction between n-type layer of the top cell to the p-type layer of the bottom one (or p-n junction), that may act as a recifying diode placed opposite to the p-i-n cell. This may be deteriorative to cell performance. Using a TRJ type p-n junction is a solution to the problem, without this the performance of a MJ cell remains poor.

Thus, we try to explore various aspects of fabricating single and multiple junction thin film solar cell, and characterize their performance.
