Present Status of Thin Film Solar Cells Using Textured Surfaces: A Brief Review

Abstract

This is a brief review on light trapping in Si based thin film solar cells with textured surfaces and transparent conducting oxide front electrodes. The light trapping scheme appears to be essential in improving device efficiency over 10%. As light absorption in a thin film solar cells is not sufficient, light trapping becomes necessary to be effectively implemented with a textured surface. Surface texturing helps in the light trapping, and thereby raises short circuit current density and its efficiency. Such a scheme can be adapted to single junction as well as tandem solar cell, amorphous or micro-crystalline devices. A tandem cell is expected to have superior performance in comparison to a single junction cell and random surface textures appears to be preferable to a periodic structures.

1. INTRODUCTION

Silicon based thin film solar cells is one of the most promising technologies for a global renewable energy production on a terawatt scale [1,2], as it uses abundant, nontoxic materials and well-established fabrication processes, which are feasible for a large scale production. However, it is necessary to improve device efficiency and reduce production cost for it to become a commercially attractive option. Production cost can be reduced by using thin absorber layers along with an advanced light trapping scheme [3]. Light trapping with highly textured interfaces have become a well-established regular feature for thin-film silicon solar cells [4].

For this purpose, textured substrates or superstrates have been implemented to diffusely transmit incident light to effectively elongate the optical path length inside the cell. To date, an initial efficiency of 14.8% and a stabilized efficiency of 12.5% have been realized in a-Si:H/nc-Si:H tandem solar cells with the multi-textured front electrodes, surpassing efficiencies obtained by textured ZnO [5].

The etched multi-textured substrates were proposed to improve light diffusion over a broad wavelength range because of the incoherent superposition of different scattering mechanisms caused by the combination of different feature sizes distributed over the surface and interfaces [6]. Recently, some research groups have reported that careful control of the surface texture with a high level of haze factor may be achieved, without a significant reduction of the open circuit voltage (Voc) and fill factor (FF) of a cell (where haze implies a ratio of diffused transmission to the total transmission).

In this article, we review key features of multi-textured glass substrates in solar cells for light trapping, achieving high efficiency of (a-Si:H, μc-Si:H single junction and a-Si:H/μc-Si:H tandem) thin film solar cells, back reflection and the effect on open circuit voltage, and fill factor of the device. Light trapping can be useful in enhancing device efficiency even with a thinner cell thickness, ultimately benefiting device performance in the short and long term.

 

2. THIN FILM SILICON SOLAR CELL

2.1 Amorphous silicon (a-Si) solar cell

Depending on the deposition sequence of the doped and intrinsic a-Si layers, the solar cell can be called p-i-n or n-i-p type. The p-i-n type cells are deposited on glass/TCO superstrates, where the illumination is through the glass surface and TCO is transparent conducting oxide.

Table 1 represents the as prepared or initial photovoltaic conversion efficiencies (Eff) of some of the thin film silicon solar cells. The μc-Si single junction cells show high short circuit current density but low open circuit voltage in comparison to amorphous silicon devices. However, higher surface texturing can result in a higher Jsc. An improved device performance (Voc, Jsc, FF and Eff are 0.94 V, 16.75 mA/cm2, 68%, 10.7%) was reported with amorphous silicon type solar cell [6] with quasi-periodic multi textured glass substrates over which ZnO was deposited by metal organic chemical vapor deposition (MOCVD), which showed improvement in short circuit current density due to an improved light trapping. The device was prepared on a multiscale surface texture which combine the surface textures with periods of 2.5~4 μm and 300 nm, respectively.

Table 1.Initial efficiencies of thin film silicon solar cells.

In light trapping scheme, the surface morphology is very important, which improves short-circuit current density [13]. An efficient collection of incident light in a solar cell is expected to improve optical absorption over a broad wavelength range [14], which may be helpful in reducing degradation if a thinner cell is used. One of the advantages of MOCVD is that boron doped zinc oxide (BZO) can be deposited with a high level of surface roughness, and therefore interest has grown on such a method and material [15-18].

2.2 Microcrystalline silicon (μc-Si) solar cell

Micro-crystalline or nano-crystalline silicon is a mixed-phase material that consists of small Si crystallites which are embedded into a matrix of amorphous silicon [19]. When these materials are deposited with SiF4, the amorphous matrix of the material can be significantly reduced with respect to materials grown from silane [20]. As mentioned above, micro-crystalline solar cells show high current density but are more expensive to prepare than the amorphous material. However, there are reports of improved Voc of such devices by use of passivation or a buffer layer in between the i-n interface [10], where the device parameters were Voc: 532 mV, Jsc: 26.36 mA/cm2, FF: 74.8% and Eff: 10.49%.

The BZO films can be used as the transparent front electrode layer [11], and can result in a high Voc and fill factor with high current density even though a relatively thinner active layer is used.

2.3 Tandem junction solar cell

As mentioned earlier, material dependant devices with a high Jsc comes with a reduced Voc. In a tandem solar cell, the Voc can be improved but the Jsc will remain lower. However, the advantage with a tandem device is the higher Voc compared to a single junction cell. Adapting a light trapping scheme can have an interesting impact on the Jsc of such a device. Raising the current density by surface texturing is possible and necessary in this scenario. In a multi-textured substrate, a relatively high haze ratio can be achieved over a wider wavelength range because of surface textures of multiple different sizes [28-33]. If cells are carefully fabricated, the substrates with high haze ratios can improve the current density without significant deterioration of Voc and FF [12].

 

3. LIGHT TRAPPING TECHNIQUE IN THIN FILM SOLAR CELL

In a thin film silicon solar cell, diffused transmission and scattering at interfaces between adjacent layers of different refractive indices and subsequent trapping of the incident light within the active layer is crucial to obtain high efficiency [34]. Thus, a great deal of success in light trapping comes from a classical approach with random surface texture and with larger texture sizes [35]. Some of the investigations based on the light trapping techniques using nano- to micro-meter sized surface structures are given below.

3.1 Micrometer size surface texture for light trapping in thin film solar cell

Improvements in the performance of microcrystalline silicon solar cells are related to the micro meter size textured-substrates. This type of advanced light trapping techniques are of great importance in photovoltaic technology. Many groups are aiming to raise the optical absorption in the active layer by using efficient light trapping. Micro-meter size textured superstrates can be used to achieve improvement in the haze, especially at longer wavelengths (λ > 800 nm), to enhance the long wavelength absorption in the active layer of single or tandem solar cell.

Periodic surface texture can be interesting, as indicated in Fig. 1 and Table 2. In a micro-meter sized texture-etched periodic surface texture (a period of 1.5 μm) applied in μc-Si solar cells, showed a current density of around 26 mA/cm2 and efficiency of 10.7% [26], as in Fig. 2. Here, the high current density may not only be due to periodicity of surface texturing, as microcrystalline solar cells generally shows higher current density. The periodic surface texture has limitations due to its stronger wavelength and angular dependence. Yet in some particular situations, this can give a better device performance likely due to the effect of interference.

Fig. 1.Device efficiencies for thin-film silicon solar cells of amorphous, microcrystalline silicon single junction and tandem (a-Si/μc- Si) junction solar cells [21-27]. (Reprinted with permission ① from ref. [21], ② from ref. [22], ③ from ref. [23], ④ from ref. [24], ⑤ from ref. [25], ⑥ from ref. [26], ⑦ from ref. [27]).

Table 2.Tabulated form of Jsc and device efficiency, as given in Fig. 1.

Fig. 2.Images of thin film silicon solar cell on micro-meter size surface morphology [10,26]. (Reprinted with permission, ① from ref. [26], ② from ref. [10]).

Modulated surface texture (MST) can be another useful approach. When such a surface texture was applied to a microcrystalline silicon solar cells [10], and exhibited device efficiency of 10.49%. The MST substrates show higher haze and broader angular intensity distribution. Therefore, higher red response in the external quantum efficiency (EQE) curves for solar cells were observed.

A periodic surface texture can efficiently be fabricated by a pulsed laser interference technique [36]. In addition to the capability of high Jsc of micro-crystalline solar cells, the periodic surface texture when applied to μc-Si tandem solar cell showed Jsc of 11.2 mA/cm2 [36].

3.2 Nano size texture for light trapping in thin film solar cell

Nano-meter size surface texturing in thin-film solar cells has shown improvement in cell efficiency due to increased light absorption through reduced reflection and increased light scattering across a broad wavelength range. A conversion efficiency of 10.1% was obtained with a cell deposited on nano-textured electrodes, which exhibited short circuit current density as 15.2 mA/cm2, Voc as 880 mV and the fill factor as 76% [9]. Other reports showed an improved performance of amorphous silicon solar cells because the cell was fabricated with a multi textured (a combination of micro- and nano-textured surface) superstrate. In these cells, FF and Jsc were achieved as 72.5% and 16.12 mA/ cm2, respectively [23].

Low pressure chemical vapor deposition (LPCVD) can give a high surface texture of as deposited TCO film and hence haze ratio of such a LPCVD grown TCO are high [37]. In a modified sputtering technique, like target angle variation, may also be possible to prepare a nano textured AZO film [38].

Figure 3 shows periodic nano-texture used in solar cell, with respective short circuit current density and efficiency.

Fig. 3.Initial efficiencies as a function of current density by nano size texture surface in thin film silicon solar cell [8,9,24,39-41] (Images reprinted with permission, ① from ref. [40], ② from ref. [39], ③ from ref. [9], ④ from ref. [24], ⑤ from ref. [41], ⑥ from ref. [8]).

3.3 Multi-textured surface (Micro+Nano texture) for light trapping in thin film solar cell

At micrometer and nanometer scales of surface texture, light is expected to behave differently. When the surface texture is comparable to the incident wavelength, higher scattering appears whereas for a larger surface texture light may show preferential refraction. Similar results were reported recently with experimental and numerical results for the light propagation in multiscale textured thin film silicon solar cells [13]. Nanoscale structures were shown to diffract the incident light while the micro-scale texture refracts the incident light. However, both these effects help in light trapping in a solar cell, and can be an effective processes in a multi-textured surfaces. The advantage of a multi textured surface is the randomness of surface texture further expands, and makes it easier to shift positions of the maximum haze to a suitably longer wavelength of radiation [13,22,32,42], as shown in Fig. 4.

Fig. 4.SEM images of multi-textured (micro+nano textured) substrate for thin film silicon solar cells [13,22,32,42] (reprinted with permission, ① from ref. [13], ② from ref. [42], ③ from ref. [32], ④ from ref. [22]).

However, a surface texture may be associated to texture induced defects and such texture induced defects can be reduced by a suitable film coating [43].

 

4. HIGH EFFICIENCY THIN FILM SOLAR CELL: Voc AND FF IMPROVEMENT

To improve the efficiency of thin film silicon solar cells, several additional methods can be adopted, like the insertion layer (called either buffer layer or bi-layer) between TCO and p-layer [44,45], amorphous silicon oxide layer (a-SiOx) [46], or p-type amorphous silicon carbide can also be used as high band gap materials, either in single or multi junction devices [47]. A suitable application of these materials’ device performance can be improved with an advanced carrier collection technique and preventing recombination loss at the interface [48,49]. One such method is known as “band lifting”, which is a method that can maintain the internal electric field of the intrinsic a-Si:H with a reversed band bending of p-type a-Si:H [50]. In addition, this effect becomes more pronounced when the bi-layer has a high work function. The role of the insertion layer as the high work function is crucial to improve the open circuit voltage (Voc) and fill factor (FF) for high efficiency thin film solar cells [51].

A reduction in the localized state density of p-type a-SiOx:H thin films is possible while prepared in a radio frequency plasma enhanced chemical vapor deposition (RF-PECVD) system [52]. Such a film shows a higher photoconductivity with similar optical gaps as compared to other wide bandgap a-Si materials. Application of such a material can give a further improvement in device performance with high Voc and FF of a-Si:H solar cells [52].

Recently, we achieved an efficiency enhancement due to the insertion of high work-function layers engineered in the interfaces to raise FF as well as Voc. Therefore, we were able to obtain the conversion efficiency of 10.34% with 16.14 mA/cm2 short circuit current density and 70.37% of FF [53].

We have also observed that an a-SiOx:H buffer layer can be used in the solar cell with p-a-SiOx:H window layer. This buffer layer can improve the photovoltaic performance parameters of the FF and Voc due to the reduction of the potential barrier at the TCO/p interface [46]. Furthermore, an improved TCO may also be needed as AZO type TCOs show reduced optical transmission at a longer wavelengths. An AZO/Ag/AZO type layer structure may be a better alternative in this respect [54].

 

5. CONCLUSIONS

Surface texturing is essential for improving light trapping and therefore short circuit current density in solar cells. Single textured surfaces have lower capabilities for light trapping than multi textured surfaces. Such surface texturing can be obtained by modifying the rigid supporting substrates (like glass) of the transparent conducting oxide electrode. In order to achieve higher efficiency solar cells, light trapping by surface texturing and back reflection is necessary as well as improvements in device structure.