Nanostructured Solar Cells

Nanotechnology is the new star player in solar energy technologies. Nanostructures offer remedies to the present drawbacks of conventional solar cells including high material costs and efficiency limits.

Nanostructures and nanomaterials are utilized for the improvement of thin film solar cell and dye-sensitized solar cells as well as the development of organic and carbon based solar cells, ETA, and QD solar cells.

Introduction

The most emphasized problem of modern life in the 21. century is by far the growing energy requirement of the world. The global energy consumption is estimated to double by the year 2050; the growth of any other industry is strictly related to the resources. However, this high energy demand is conventionally met with the use of fossil fuels. The disadvantages of fossil fuels including environmental pollution and resource depletion are well-disputed and undeniable. This is why researchers have steered towards new and renewable energy resources. Solar energy is considered to be the most promising solution to the clean energy requirement of the modern world. The conversion of solar energy to electricity is achieved through solar cells. We are considerably familiar with the conventional solar cells which are installed on almost every rooftop, traffic lights, and various other places we encounter on a daily basis. These are single-crystalline silicon solar cells and make up to 94% of the solar energy market. Many photovoltaic devices including these traditional solar cells have been developed over the years however; their wide-spread use is limited by conversion efficiency and cost. Traditional silicon solar cells are often referred to as first generation solar cells. With regards to first generation solar cells, high cost, and theoretical efficiency limit (which is known as the Shockley-Quiesser limit) are the two major disadvantages. The high cost of the single–crystalline silicon solar cells is due to the production of high quality Si wafers which makes up to 40-50% of the market price. The second generation thin film solar cells were developed in order to aid the high cost of traditional solar cells. 

How does Nanotechnology Enhance Solar Cell Technology?

Amorphous Si, cadmium telluride (CdTe), copper-indium-gallium-selenide (CIGS), and other compound semiconductor thin films are commonly used in second generation solar cells. However, hydrogenated amorphous silicon degenerates when exposed to sunlight hence they have a short lifetime. The CdTe and CIGS films are more efficient for the conversion of sunlight and are relatively cheap to produce, but they contain heavy metals and thus are harmful to the environment. The third generation solar cells composed of hybrid junctions, polymers, organic-inorganic hybrid assemblies, and semiconductor nanostructures are developed to improve the theoretical efficiency. The Shockley-Quiesser limit defining the theoretical efficiency of traditional solar cells was first calculated by William Shockley and Hans-Joachim Quiesser in 1981. They proposed that the maximum efficiency of a silicon solar cell would be 30% at 1.1 eV. However, later calculations have updated this limit as 33.7% at 1.37 eV. This limit is one of the most fundamental theories of solar energy systems and frequently utilized in the production of photovoltaic cells. Shockley-Quiesser limit calculations assume that; low energy photons can’t be absorbed and high energy photons can only excite one electron. However, it is important to note that this limit is only valid for conventional single p-n junction solar cells. Third generation solar cells attempt to increase the efficiency limit either by utilizing the high-energy photons more efficiently, or recovering the low-energy photons normally not converted. Nanostructures are extensively studied for this purpose. Here we will examine how nanotechnology benefits solar cells technologies and the use of nanotechnology in several different solar cells including conventional thin film solar cells, dye-sensitized solar cells, carbon and polymer based solar cells, quantum dot solar cells, and extremely thin absorber solar cells.

Employing nanotechnological solutions and nanoscale structures to the solar cell systems provide promising improvements. Nanoscale systems exhibit different properties than their bulk or thin film counterparts. High surface to volume ratio of nanostructures, quantization effects at ≈1-20 nm scale, and variety in production methods provide several benefits to solar energy systems. Nanomaterials are utilized in multiple-junction solar cells to exceed the Shockley-Quiesser limit. In theory, a finite number of junctions results in an efficiency limit of 68% at 1-sun intensity while a triple-junction solar cell based on III-V semiconductors reaches up to 34.1% efficiency. However, these high-efficiency cells are far too expensive. On the other hand, nanomaterials can be used to produce layers with different bandgaps, and the multiple junctions can be solution-processed at a significantly lower cost. Furthermore, the unique properties of nanostructures are employed to provide two major improvements to the solar energy systems; optical losses and electrical performance.

Reducing The Optical Losses

The optical losses of solar cells can be aided by either broadening the available solar spectrum to avoid transmission and thermal losses or by increasing the absorption probability of impinging photons. For this purpose; antireflective nanostructures, resonators, or quantum dots (QDs) are utilized as nanotechnology solutions. Several different nanostructures such as nanowires, nanopillars, nanodomes, and nanocones are utilized to achieve an antireflective surface morphology reducing the reflection losses. Careful optimization of the geometrical parameters plays an important role in gradually decreasing the refractive index from the semiconductor to air. A continuously graded refractive index aids the suppression of reflection. Furthermore, patterned nanostructures can be designed to capture even more light via light trapping so that less material is needed to absorb the solar flux.

Resonance absorption is a widely used phenomenon that redirects and confines light at active regions in solar cells to enhance absorption. It is achieved via dielectric and metallic resonators or photonic crystals. Metallic resonators affect the light absorption through light scattering, increased light concentration (localized plasmonic resonance, LPRs), and excitation of surface plasmon polaritons (surface plasmonic resonances, SPRs). The effect of metallic plasmonic resonance becomes even more prominent with the use of metallic nanoparticles resonators. Metallic nanoparticles induce LPRs and increase the absorption of the surrounding semiconductors. Furthermore, LPRs can be enhanced by manipulating the geometry and concentration of metal nanoparticles. The inclusion of metallic nanoparticles is much more beneficial when placed within the p-n junction. SPRs are also supported with metallic nanoparticles increasing the absorption in the semiconductor layer. One downside of metallic nanoparticles is the challenge of optimum position in the solar cell. The reflective properties of metallic nanoparticles cause optical losses if they are not positioned properly. To avoid the inherent losses associated with metallic nanoparticles and nanostructures, dielectric nanophotonic resonators utilizing Mie resonances are employed to couple leaky optical modes into the active layer.

Another nanotechnological solution in solar cells is the use of semiconductor nanocrystals such as QDs. The strong size dependence of the bandgap and the modification of the relaxation dynamics of photoexcited charge carriers are the most attractive properties of nanocrystals. Because only the radiation with higher energy than the bandgap is absorbed, narrower-bandgap materials absorb more solar photons, resulting in higher photocurrents. However, the output voltage is linearly proportional to the bandgap, and thus wider-bandgap materials allow higher voltages. The optimum bandgap for photovoltaic devices ranges between 1.2 and 1.4 eV according to Shockley-Quiesser analysis. This limitation leads to fundamental spectral losses accounting for approximately 56% of the energy loss in solar devices. With the use of nanocrystals, it is possible to broaden this bandgap due to the quantum confinement effect.

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Electrical Improvement

In addition to the improvement of light absorption, the device performance of solar cells is also affected by the electrical properties. The electrical improvement of solar cells includes promoting carrier separation, transfer, and collection. Fast and efficient charge separation, transfer, and collection are essential to achieve high power conversion efficiency. The carrier collection and transfer can be improved by manipulating the configuration and morphology of donor/acceptor surfaces. Two effective routes for the improvement of carrier collection and transfer are the control of junction interface and control of carrier collection distance.

Junction interface is important to avoid recombination during carrier transfer which reduces charge separation and transfer efficiency. A disordered junction interface increases the carrier recombination due to the discontinuous pathways for charge transport and the disorganization of the donor/acceptor energy levels. Hence, an ordered heterojunction in nanoscale improves the overall efficiency of the solar cells.

Controlling the carrier collection distance is crucial for the carrier collection efficiency of solar devices. One of the most effective methods for the control of carrier collection distance is utilizing the nanostructures with core/shell architecture. The radial nature of these nanostructures provides short and collection distances aiding the effective diffusion of carriers into depletion regions. Furthermore, core/shell structures provide large surface areas within a small volume which improves the device performance.

Nanostructures in Conventional Thin Film Solar Cells

Photovoltaic devices based on thin-film cadmium sulfide (CdS), copper indium diselenide (CIS) and cadmium telluride (CdTe) are leading contenders for large-scale production of solar cells at low cost. However, these second-generation devices require improvements to achieve higher photon conversion efficiencies and lower production costs. Nanotechnology provides promising solutions for further improvement of these devices. The most commonly studied solar devices with these materials are CdS/CIS and CdS/CdTe solar cells. In addition to these configurations Cu₂S/CdS solar cells have also been suggested however, this cell structure suffered from degradation with time.

Thin film CdS/CIS solar cells have already been developed to an efficiency of 17%. CIS is proved to be a good absorber material with a 12% conversion efficiency and a good absorption coefficient. However, these solar cells suffer from low open circuit voltage and relatively low bandgap resulting in low efficiency. Another drawback of these solar cells is the relatively thick CIS layer at several micrometers. This is why nanotechnological solutions are considered for the improvement of CdS/CIS solar cells. The bandgap and open-circuit voltage of CdS/CIS solar cells can be enhanced through quantum confinement. The thickness of the CIS layer can be reduced by employing nonporous structures. Such nanostructures would also provide an increased short circuit current improving the device performance.

CdS/CdTe solar cells are one of the most promising options due to the attractive band gap of CdTe (1.5 eV) and the compatibility of CdS and CdTe. n-CdS is found to be the best heterojunction partner p-CdTe because of the ease of CdTe deposition on CdS and desirable inter-diffusion at the interjunction. The efficiency of CdS/CdTe solar cells can go up to 16%. The limitations on the efficiency of these solar cells are the issues at the CdTe/top electrode layer interface and obtaining thinner CdS layers. Obtaining a nanocrystalline CdS layer would provide a better window material with an effective band gap higher than that of bulk CdS. Thus employing appropriate nanotechnological methods such as chemical vapor deposition would improve the device performance significantly. The most challenging problem with CdS/CdTe solar cells is the contact between the p-CdTe layer and the top electrode. The solution to this problem is suggested to be a thin layer of nanocrystalline CdTe between the bulk CdTe and top electrode since nanocrystalline CdTe provides improved surface contact. Furthermore, different surface morphologies can be employed to further improve contact between CdTe and electrode layers.

Evidently, either configuration of conventional thin film solar cells can be improved with the use of nanotechnology.

Nanostructures in Dye-Sensitized Solar Cells

Dye-sensitized solar cells based on oxide semiconductors and organic dyes or metallorganic-complex dyes have recently emerged as a promising approach to efficient solar-energy conversion with lower production costs. DSCs are advantageous in terms of their low-cost and possible high efficiency. Nanostructured DSCs are mostly based on TiO₂ and ZnO electrodes.

The most successful DSCs were obtained with the use of TiO₂ nanocrystalline film and ruthenium based dyes achieving over 11% efficiency. The high surface area of the nanocrystalline structure increases dye absorption leading to better device performance. Nanoporous TiO₂ electrodes are typically prepared by the annealing of TiO₂ nanoparticles. The efficiency of TiO₂ based nanostructured DSCs can be further improved by controlling the annealing conditions of nanoparticles and the resulting surface morphology. Different morphologies such as spherical particles, rods, wires, and hollow tubes have been utilized in TiO₂ based DSCs.

ZnO based nanostructured DSCs have recently attracted attention due to desirable properties of ZnO such as wide bandgap (3.2 eV), good carrier mobility, and ability to be doped both n and p-type. The solar cell efficiencies of ZnO based nanostructured DSCs have so far reached 5%. The relatively lower efficiency is mostly attributed to a lower number of studies and holds great potential to be improved. The most important problem with ZnO complexes is the dye absorption which requires further studies to be resolved. Amongst the several different nanostructures of ZnO, spherical nanoparticles are found to be the most desirable for high efficiency. ZnO is also suggested as a shell in the core/shell structure to decrease the recombination rate. Most commonly, ZnO shells are combined with TiO₂ or SnO₂ core materials. ZnO/ SnO₂ electrodes provide overall efficiencies up to 6.3%.

Organic and Carbon Based Nanostructured Solar Cells

Organic solar cells based on conjugated polymers have attracted attention in the last 10 years. Since photoexcitations in conjugated polymers show diffusion lengths of only around 5–20 nm, nanostructure and the control of nanophase play a crucial role in device performance and conversion efficiency. As opposed to inorganic materials, conjugated polymers can be processed under ambient conditions, employed with spin coating and 3D printing techniques as thin films of approximately 100 nm. The inherent nanosized structure of conjugated polymers enables low-cost, lightweight, and flexible structures. The efficiency of organic solar cells can rise up to ≈5% and have the potential to achieve 10% efficiencies with the manipulation of nanostructures.

Carbon based nanostructured solar cells utilize fullerene C₆₀, high fullerene C_70, and carbon nanotubes (CNTs). In addition to pristine fullerene structures at the scale of ≈100 nm, doped fullerene structures and fullerites are also utilized in solar cell technologies. Crystals and thin films of fullerene show n-type semiconductor behavior and surprisingly high photoconductivity under dark conditions. The optical and electrical properties of fullerene are mostly used in heterojunctions with conjugated polymers or other semiconductor materials. On the other hand, CNTs offer a wide range of bandgaps to match the solar spectrum, enhanced optical absorption, and reduced carrier scattering. These properties of CNTs are used in combination with fullerenes or other semiconducting materials in the hopes of improving the solar cell efficiencies. The highest overall device efficiency of carbon based solar cells is 5% however; studies show that there is room for considerable improvement.

Other Nanostructured Solar Cells

Other nanostructured solar cells include extremely thin absorber (ETA) solar cells and quantum dot (QD) solar cells. These structures represent the best features of nanostructures and hold great potential for future solar energy developments.

QD solar cells utilize semiconductor nanocrystals which have discrete electronic states and produce excited excitons upon the absorption of photons. Quantum confinement in QDs increases the bandgap of a material by 1 eV compared to bulk properties expanding the range of semiconductor materials used in photovoltaics. For example, Bulk PbS has a bandgap of 0.4 eV but PbS QDs can have bandgaps from ∼0.6 to ∼2 eV depending on their size. Furthermore, QDs enable the multiple exciton generation (MEG) which surpasses one of the limiting assumptions of the Shockley-Quisser limit. Other examples include QDs made of CdTe, copper zinc tin sulfide (CZTS), copper indium gallium selenide (CIGS), and Si. The efficiency of quantum dot solar cells have already reached 9% and expected to increase with further development.

ETA solar cell is a novel approach to solar energy technologies. This approach involves placing a thin absorber layer between p and n-type semiconductors. The incorporation of a thin absorber layer leads to reduced bulk recombination and increase the device efficiency. The carrier diffusion length requirements are relaxed in this structure. Hence, the quality requirements for the absorber material are significantly lowered. Commonly used absorbers in ETA solar cells are CdTe, CuInS₂, a-Si, CdSe, CdS, Cu(In, Ga)S_2. Current conversion efficiencies of ETA cells are around 2-5%. However, it is suggested that 15% energy conversion efficiencies are possible for ETA-solar cells with CdTe and CuInS2 absorbers having a minimum thickness of around 15 and 20 nm.

Conclusion

Here we have presented a glance into an otherwise extremely vast subject. Nanotechnology is utilized in more than one way to overcome current drawbacks of solar energy technologies including high material cost and factors limiting the device efficiencies such as the Shockley-Quiesser limit. Nanostructures such as nanowires, nanopillars, nanodomes, and nanocones and nanomaterials of semiconductors are widely utilized for the improvement of solar devices. The use of nanostructures is focused on electrical improvement and reducing optical losses, in general. High surface area and quantization effects of nanomaterials and nanostructures provide great benefits to the absorption and device efficiency of solar cells. Different nanostructures and nanomaterials are utilized in the improvement of conventional thin film solar cells and the second-generation dye-sensitized solar cells. On the other hand, exploring new nanomaterials results in various other solar cell technologies such as organic based polymer solar cells, fullerene and CNT based solar cells, QD solar cells, and ETA solar cells. Even though nanostructured solar cells are still at their infancies and have relatively lower efficiencies around 5-10%, they hold great potential for future developments. As the understanding of nanosystems improves and the number of studies increases, nanostructured solar cells will exceed their limitations and carry the solar energy systems to the next level.

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References

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