Effects of a thermally stable chlorophyll extract from diatom algae on surface textured Si solar cells

Abstract

We present the effects of a chlorophyll extract from diatom algae as a spin-coating anti-reflection layer on surface textured silicon solar cells. The diatom extract with a refractive index value in-between Si and air can suppress the overall light reflection from the bare Si surface up to 7% over spectral regions of 350–1100 nm. Additionally, it also shows a strong photon downconversion effect within the visible light regime. Based on both optical characteristics, the short circuit current density is largely enhanced for an approximately 10% increment in the cell efficiency. Additionally, the diatom extract is also thermally stable up to 90 °C without apparent color change and any degradation of optical properties. Thus, the presented approach is simple, doable, suitable for large area application, and more importantly, it is eco-friendly.

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Introduction

Nearly full light absorption throughout the entire solar spectrum is among the key factors for developing next generation high energy conversion efficiency and low production cost silicon (Si) solar cells. Diverse surface texturing methods have been devised for enhancing light absorption, including developing various nanostructured Si arrays.^1,2 Despite the nanostructured Si arrays offering the significant benefits of light trapping, antireflection characteristics, and modulation of optical bandgap,^3–6 there is still considerable optical loss in those cells due to a large mismatch in refractive index (n) at the interface between air (n = 1.00) and Si substrate (n = 3.42). An anti-reflection (AR) coating layer with n value in-between the air and Si can partially compensate the optical loss leading to a significant improvement in light absorption.

It has been reported that materials used in AR coating can be in the form of a continuous film or a nanoparticle layer. Continuous films made of SiO₂,^7,8 Si₃N₄,^9,10 TiO₂,^7,8 Ta₂O₅,^11,12 Al₂O₃,¹¹ and MgF₂ (ref. 11) are the prime elements for AR coating. Semiconducting nanoparticles such as PbS,¹³ CdS,¹³ ZnS,¹⁴ ZnSe,¹⁵ and etc. are also potential candidates not only to serve as an AR coating layer, but also to convert the high energy photons into Si favorable spectral regime.¹⁶ Though there are many studies about the AR coating of Si solar cells, the materials used either require complex deposition systems or involve chemically toxic elements during processing. From the application perspective, it is always a goal to lower down the production cost for expending the market. Additionally, environmental safety has become one of the important issues regarding to achieve the sustainability of our planet in the past few decades. Therefore, several electronic products containing heavy metals that can cause serious pollution are prohibited from available in the market. Considering both constrains on selecting AR coating, materials with eco-friendly and simple preparation scheme are highly demanded.

Plants are the naturally grown species on earth and they gain most of their energy from sunlight through photosynthesis using chlorophyll. Chlorophyll is a green pigment and can absorb strongly the blue portion of electromagnetic spectrum. More importantly, it is abundant and readily available not only in earthy plants but also in marine forests. Diatoms are a major group of algae ubiquitous in the sea and freshwater ecosystems on earth. They are one of the most important photosynthetic organisms contributing to approximately 40% of the aquatic primary production using mostly chlorophyll a ([chl-a]).^17,18 As been reported, the integration of natural elements into device fabrication has been realized in dye-sensitized solar cells by extracting photosensitive ingredients from flowers and plants as light absorbers.^19–21 However, up to date, it is rarely seen to consider natural resources as a component in solid state solar cell fabrication.

Herein, we report an effective approach to incorporate natural resources into surface textured single crystalline Si solar cells to improve the light absorption. We extract the active ingredients from diatoms to serve as the AR coating for Si solar cells because the diatom extract with n = 2.42 is expected to partially compensate the optical loss at the interface between air (n = 1.00) and Si (n = 3.42). It is found that a thin layer of diatom extract can slightly reduce the reflectance over the entire spectrum of light from 350 to 1100 nm up to 13%, depending on the wavelength. Importantly, the diatom extract shows a strong photon down-conversion effect by converting the blue portion of the sunlight spectrum into the red one, leading to largely enhanced short circuit current and the power conversion efficiency (PCE) of the Si solar cells. In addition, concerning the stability issue, the diatom extract is also thermally stable up to 90 °C without changing its optical properties and color. Thus, the diatom extract can be one of the possible low cost, eco-friendly, and thermally stable materials for applying in solar cell fabrication. The demonstrated approach is eco-friendly and simple and can potentially compete with inorganic AR materials in aspects of environmental impact, production cost, and processing complexity. Further, it should be able to be applied to organic solar cells with transparent electrodes as well.

Experimental details

Material preparation

The marine diatoms named Thalassiosira pseudonana were cultured in the laboratory. All cultures were routinely maintained in f/2 medium at 25 °C under a 12 : 12 light : dark cycle and the flask was shook three times a day.²² The light was provided by fluorescent lamps (day-light 40 W). For the chlorophyll extraction, after cultivating for 3 weeks, those diatoms were collected by centrifuge followed by rinsed with deionized (DI) water. The obtained solids were subsequently dried in a vacuum oven overnight. The dried powders were then added into a pre-cooled ethanol solvent and the resulting solution was then kept in a fridge at −20 °C for a day for extracting chlorophyll from diatoms. A proper amount of the extracted chlorophyll was then dissolved in ethanol forming the diatom extract solution of concentration 67 mg L⁻¹. Pure chlorophyll a [chl-a] was purchased commercially from Aldrich. This pure [chl-a] was extracted from Anacystis nidulans algae.

Characterization details

The UV-vis spectra were collected by Shimadzu Model UV-2600 spectrometer while photoluminescence (PL) and photoluminescence excitation (PLE) spectra were obtained using F-7000 Hitachi spectrophotometer. The diatom extract and [chl-a] solution were prepared in ethanol with concentrations of 2.7 mg L⁻¹ and 0.5 mg L ⁻¹, respectively, for the optical measurements. The excitation wavelengths for diatom extract and [chl-a] were 435 and 420 nm, respectively. The reflectance spectra were conducted with a Perkin Elmer Model Lambda 750 spectrophotometer. The current density (J)–voltage (V) characteristics were evaluated by using a Keithley Model 2400 source meter under irradiation intensity of 100 mW cm⁻² from a calibrated solar simulator (Newport Inc.) with AM 1.5G filter. The external-quantum-efficiency (EQE) spectra were measured using a setup consisting of a Xeon lamp system, a chopper, a monochromator, a lock-in amplifier, and a standard silicon photodetector (ENLI Technology).

Results and discussion

Fig. 1(a) is the illustration of single crystalline Si solar cell structure with silver (Ag) top electrode deposited on n-side. The surface morphology of the Si solar cell is shown in Fig. 1(b), exhibiting a surface textured structure. The diatom extract layer was deposited by spin-coating the diatom extract solution at 2000 rpm on the rough silicon surface and the resulting film was dried in the air over night as the lateral SEM image shown in Fig. 1(c).

Fig. 1 (a) The illustration of Si solar cell structure. (b) The top-view SEM image of the surface morphology of the Si solar cell after removing the antireflection coating. (c) The lateral-view SEM image of the Si solar cell surface after spin-coated with the diatom extract layer.

In addition to the prominent light absorption component; i.e., [chl-a], diatoms also contain chlorophyll c [chl-c] and fucoxanthin as their light harvesting contents²³ and these two ingredients may be present in the obtained diatom extract. For a clear understanding, we also measure the properties of pure [chl-a] in parallel for comparison. Fig. 2 depicts the optical properties of the diatom extract and [chl-a] prepared in ethanol and all spectra are rescaled for clarity. As shown in Fig. 2(a), the diatom extract absorbs photons of energies in bands of blue-to-green and red regions with two prominent peaks at 430 and 663 nm and an absorption shoulder at 476 nm. [chl-a] also exhibits two strong absorption bands with prominent peaks at 430 and 665 nm in the blue and red regions, respectively. The differences in absorbance for both materials are an additional absorption shoulder and extended absorption wavelengths into the green light for the diatom extract, which can be originated from the absorption activities of [chl-c] and fucoxanthin.²³ Fig. 2 presents emission properties for both materials. The major PL peaks for the diatom extract and [chl-a] arouse from 670 and 674 nm, respectively, and both have a second emission band showing a shoulder at 720 nm (see Fig. 2(b)). It was reported that [chl-c] fluoresced at 643 nm and fucoxanthin showed no emission.²³ In our measurements, we detect no PL emission from [chl-c] for the diatom extract and this is likely due to its relatively weaker emission nature or much lower molar fraction in the sample as compared with [chl-a].²³ Further, these emission peaks located around 670 nm for both materials are excited by the same broad spectrum band of 250–470 nm (see Fig. 2(b)). Thus, the primary active content responsible for photon energy conversion effect in the diatom extract is [chl-a], which makes the diatom extract an attractive candidate for blue-to-red photon conversion. The fraction of [chl-a] in the diatom extract is determined spectroscopically to be about 70%.

Fig. 2 Optical studies for the diatom extract and [chl-a] in ethanol with concentrations 2.7 mg L⁻¹ and 0.5 mg L⁻¹, respectively. (a) UV-vis absorbance. Three major peaks for the diatom extract are at 430, 476, and 663 nm, while [chl-a] shows the similar absorption feature except a shoulder at 476 nm. The extinction coefficients for those peaks are of the order of 10⁵ M⁻¹ cm⁻¹. (b) PL and PLE properties. Both the diatom extract and [chl-a] fluoresce at 670 and 720 nm, originating from a broad spectrum band of 250–470 nm.

A layer of the diatom extract was deposited on the bare Si solar cells from a 67 mg L⁻¹ solution in ethanol by spin-coating method. The diatom extract layer having n = 2.42 determined by ellipsometry can serve as an AR coating to partially compensate the optical loss at the Si substrate and air interface. As the reflectance spectra shown in Fig. 3, bare Si shows high average reflection of ∼38% over the entire examined spectrum (350–1100 nm). However, depositing a thin diatom extract layer slightly suppresses the reflection by up to 13% as the enhancement factor shown in the inset in Fig. 3. The overall reduction in the reflected light intensity is approximately 7%.

Fig. 3 Optical reflectance spectra for the bare and the diatom coated surface textured Si solar cells.

Fig. 4 shows the best set of J–V characteristics of devices with and without the diatom extract layer. The performance parameters obtained from the J–V curves of 10 devices were summarized in Table 1. After coating with a diatom extract layer, J_sc was improved from 17.7 ± 0.23 to 19.2 ± 0.20 mA cm⁻², reflecting an averaged 8.5% enhancement compared to bare Si solar cells. However, there was no change in V_oc since the Fermi level was not affected by the diatom extract layer. FF deteriorated slightly from 65.5 ± 0.31 to 64.5 ± 0.32% because the diatom extract layer over the Si slightly increased the series resistance. Nevertheless, the overall performance of Si solar cells was improved from 6.73 ± 0.22 to 7.34 ± 0.23% by coating a diatom extract layer. The improvement is largely due to J_sc enhancement. Since there is less than 7% reduction in the reflective light over the studied range, the large increment of J_sc (∼8.5%) is not entirely coming from reduced reflected light intensity.

Fig. 4 J–V characteristics of cells with and without the diatom extract coating. The inset summarizes the performance parameters of both kinds of cells.

Table 1 Performance parameters of solar cells under AM 1.5G illumination at 100 mW cm⁻²

	Jsc (mA cm^−2)	Voc (V)	FF (%)	PCE (%)
Bare	17.7(±0.23)	0.58(±0.12)	65.5(±0.31)	6.73(±0.22)
Diatom	19.2(±0.20)	0.58(±0.13)	64.5(±0.32)	7.34(±0.23)

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In order to understand the role of the diatom extract layer, we have conducted the EQE measurement. Fig. 5 displays the EQE response in the wavelengths from 350 to 1100 nm for the solar cells with and without the diatom extract coating and the corresponding EQE enhancement factor for each wavelength is shown in the inset. The addition of the diatom extract layer results in enhanced quantum efficiency in the displayed wavelength range, which is good in accordance with the J–V result that shows an improved power conversion efficiency. By taking the ratio of the EQE values for the diatom extract-coated to the bare Si devices, the maximum enhancement factor is at ∼400 nm. According to PLE spectrum shown in Fig. 2(b), it exhibits a broad distribution in the range of 220–470 nm with a maximum at 420 nm, which is close to where the maximum enhancement of the EQE value is. Therefore, the enhanced EQE spectrum with wavelengths shorter than 470 nm can be attributed to the photon energy down-conversion effect in addition to antireflection. The diatom extract aids in effectively harvesting violet-blue light through photon down-conversion. As for the enhanced EQE values for wavelengths longer than 470 nm, because there is no PLE signal in the wavelength regime from the diatom extract layer, the photon energy down-conversion effect may have little contribution to the observed enhancement of EQE. The component [chl-a] in the diatom extract absorbs light energy, which excites electrons from ground state to excited states. The excited electrons can either relax back to ground state through intermediate levels or direct transfer to the underneath n-type Si and leave the cationic form of [chl-a⁺] behind. The former pathway emits lower energy photons while the latter case is a non-radiative process, which can lower the photon downconversion efficiency. The transferred electron can be either collected at the Ag cathode or trapped at the textured Si surface. Meanwhile, the [chl-a⁺] can be regenerated back by receiving free electrons from the underneath cathode. There is a balance among these processes and fortunately, photon down-conversion effect is still active and contributes to the proposed device structure. Thus, the enhanced J_sc value stems from the combined effects of better light trapping and photon down-conversion.

Fig. 5 EQE spectra for cells with and without the diatom extract coating. The inset shows the EQE enhancement of cell with the diatom extract coating in comparison with the bare one.

We have also fabricated device using pure [chl-a] as the AR coating and found the similar improvement of the device performance (∼10%) as those coated with the diatom extract. The result again confirms that [chl-a] is the important component playing the effect. We have not tried to tune the optimum concentration for forming the diatom extract layer. However, we believe that the light harvesting can be further enhanced with appropriate thickness for the diatom extract layer for a more effective photon down-conversion and antireflection effects. Our purpose here is to demonstrate a possible resource for inexpensive and eco-friendly AR materials.

It is noted that Si is a poor absorber due to its indirect bandgap nature and the light trapping effect due to AR coating is particularly important at long wavelengths close to the bandgap in Si solar cells. For a large area application, particularly, Si₃N₄ AR coating deposited by plasma enhanced chemical vapor deposition (PECVD)^9,10 has been the prominent material used in Si solar cell development. However, it is still relatively costly for large area processing. Semiconducting nanoparticles of II–VI group such as PbS, CdS, and so on contain toxic metal elements and others involve organic solvents during preparation. Thus, both kinds of nanoparticles are not favorable for the environmental safety. The use of extraction from naturally abundant resources is a potential step to develop low-cost, large area, and environmental friendly antireflection coating.

Concerning thermal stability issue, it is possible that the diatom extract may suffer from heat damage due to its organic base. We compare the UV-vis absorption, PL, and PLE spectra of diatom extract at room temperature and after keeping at 90 °C for 30 min. There is no apparent color change of the diatom extract after undergoing the heated temperature. Additionally, there is no change in optical properties as well (see Fig. 6). Thus, the diatom extract itself is thermally stable even up to 90 °C.

Fig. 6 Optical studies for the diatom extract in ethanol at RT and after heating at 90 °C for 30 min.

For practical application, several individual solar cells must be interconnected and laminated into a solar module, a process carried out at temperatures of up to 150 °C. Winograd et al.²⁴ has studied the thermal stability property of a [chl-a] film using X-ray photoelectron spectroscopy (XPS).²⁵ It is found that the peak intensity of the oxygen binding energy of [chl-a] remains similar up to 180 °C. Further raising up the temperature to 250 °C results in a decrease in the peak intensity and a change in the absorption spectrum of [chl-a]. Therefore, we would expect that the [chl-a] film should be stable when heated up to 150 °C for integrating into a module. The diatom extract with significant amount of active component, [chl-a], should be safe for the transformation process. Since finished modules are usually packaged before operation, the packaging step can prevent the organic [chl-a] from contacting both oxygen and moisture to slow down the degradation process of the material itself and extend the lifetime of the modules. In a word, the diatom extract containing a significant amount of [chl-a] with reasonable transmittance in visible regime and good thermal stability is another good alternative to attain low-cost, simple and eco-friendly manufacturing, in addition to its dual effects of strong photon down-conversion and antireflection properties.

Conclusion

We have successfully demonstrated a facile approach to improve the power conversion efficiency of surface textured Si solar cells by the incorporating nature resources. Based on the outstanding optical and thermal properties of the diatom extract, the device performance is enhanced due to the AR coating effect in conjunction with a second mechanism of photon down-conversion at blue spectral region, leading to an enlarged J_sc. The active content of the diatom extract is presumably shown to be [chl-a]. The presented method is simple and eco-friendly, which should be able to be applied to other types of solar cells, and open up a route for developing pollution-free optoelectronic devices.

Acknowledgements

This work is supported by Ministry of Science and Technology, Taiwan (Project no. NSC 102-2112-M-239-001-MY3).

Notes and references

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