Solar energy—A look into power generation,challenges, and a solar‐powered future

Sun is an inexhaustible source of energy capable of fulfilling all the energy needs of humankind. The energy from the sun can be converted into electricity or used directly. Electricity can be generated from solar energy either directly using photovoltaic (PV) cells or indirectly using concentrated solar power (CSP) technology. Progress has been made to raise the efficiency of the PV solar cells that can now reach up to approximately 34.1% in multi‐junction PV cells. Electricity generation from concentrated solar technologies has a promising future as well, especially the CSP, because of its high capacity, efficiency, and energy storage capability. Solar energy also has direct application in agriculture primarily for water treatment and irrigation. Solar energy is being used to power the vehicles and for domestic purposes such as space heating and cooking. The most exciting possibility for solar energy is satellite power station that will be transmitting electrical energy from the solar panels in space to Earth via microwave beams. Solar energy has a bright future because of the technological advancement in this field and its environment‐friendly nature. The biggest challenge however facing the solar energy future is its unavailability all‐round the year, coupled with its high capital cost and scarcity of the materials for PV cells. These challenges can be met by developing an efficient energy storage system and developing cheap, efficient, and abundant PV solar cells. This article discusses the solar energy system as a whole and provides a comprehensive review on the direct and the indirect ways to produce electricity from solar energy and the direct uses of solar energy. The state‐of‐the‐art procedures being employed for PV characterization and performance rating have been summarized . Moreover, the technical, economic, environmental, and storage‐related challenges are discussed with possible solutions. Furthermore, a comprehensive list of future potential research directions in the field of direct and indirect electricity generation from solar energy is proposed.     

High-efficiency silicon space solar cells

SHARP's activities on Si solar cells developments and features of Si solar cells for space use in comparison with GaAs solar cells are presented. Two types of high-efficiency silicon solar cells and the same kinds of high-efficiency solar cells with integrated bypass function (IBF cells) were developed and qualified for space applications. The NRS/LBSF cells and NRS/BSF cells showed an average of 18% and 17% efficiencies, respectively, at AMO and 28°C conditions. The IBF cells have P⁺N⁺ diodes on the front surface to protect itself from reverse voltage due to shadowing. The designs and features of these solar cells are presented. The radiation tests results of these solar cells are also presented. The NRS/BSF cells showed lower degradation rate compared to conventional BSFR cells with the same thickness (100 μm). But the NRS/LBSF cells showed a higher degradation rate than the BSFR cells. The IBF cells showed almost the same radiation characteristics as the same kinds of cells without IBF. The results of radiation tests on these high-efficiency solar cells and the discussions about the radiation characteristics of them are presented. In the last section, the future silicon solar cell development plan is discussed.     

Applications of carbon materials in photovoltaic solar cells

Carbon-based photovoltaic cells (PVCs) have attracted a great deal of interest for both scientific fundamentals and potential applications. In this paper, applications of various carbon materials in PVCs, especially in silicon-based solar cells, organic solar cells and dye-sensitized solar cells, are reviewed. The roles carbon materials played in the PVCs are discussed. Further research on solar cells comprised solely of carbon is prospected.     

非晶硅薄膜电池的历史、现状和发展中的主要问题

对非晶硅薄膜太阳能电池的历史和现状进行了总结,指出了目前非晶硅薄膜太阳能电池存在的主要问题是转换效率低和严重的光致衰减效应,对解决这些问题的一些技术进行了探讨,认为非晶硅薄膜太阳能电池有很大的发展潜力,将与晶体硅和其他新兴太阳能电池三分天下。     

Super high-efficiency multi-junction and concentrator solar cells

III–V compound multi-junction (MJ) (tandem) solar cells have the potential for achieving high conversion efficiencies of over 50% and are promising for space and terrestrial applications.We have proposed AlInP–InGaP double hetero (DH) structure top cell, wide-band gap InGaP DH structure tunnel junction for sub cell interconnection, and lattice-matched InGaAs middle cell. In 2004, we have successfully fabricated world-record efficiency concentrator InGaP/InGaAs/Ge 3-junction solar cells with an efficiency of 37.4% at 200-suns AM1.5 as a result of widening top cell band gap, current matching of sub cells, precise lattice matching of sub cell materials, proposal of InGaP–Ge heteroface bottom cell, and introduction of DH-structure tunnel junction. In addition, we have realized high-efficiency concentrator InGaP/InGaAs/Ge 3-junction solar cell modules (with area of 7000 cm²) with an out-door efficiency of 27% as a result of developing high-efficiency InGaP/InGaAs/Ge 3-junction cells, low optical loss Fresnel lens and homogenizers, and designing low thermal conductivity modules.Future prospects are also presented. We have proposed concentrator III–V compound MJ solar cells as the 3rd-generation solar cells in addition to 1st-generation crystalline Si solar cells and 2nd-generation thin-film solar cells. We are now challenging to develop low-cost and high output power concentrator MJ solar cell modules with an output power of 400 W/m² for terrestrial applications and high-efficiency, light-weight and low-cost MJ solar cells for space applications.     

A technological evolution from bulk crystalline age to multilayers thin film age in solar photovoltaics

Recent advances in solar cell device technologies are surveyed, and a new trend underlying is predicted by a term “technological evolution from the bulk crystalline age to the multilayered thin film age”. In the paper, firstly, recent progress of thin film fabrication technologies for active materials of photovoltaic device are reviewed, and their significancies such as wide area, low temperature growth etc., are pointed out from currently developed live technologies. Secondly, some R & D efforts to develop the next generation type solar cells utilized by full use of multi-layers thin film growth technology are introduced together with some newly developed integrated process technology for the thin film solar cells. Then, some topics in the high cost performance multi-layers thin film solar cells are also introduced. In the final part of this paper, the current state of the art in the field of thin film solar cells and their industrialization are overviewed and the market expansion toward the 21st century is forecast, and discussed.     

Crystalline silicon thin-film solar cells on ZrSiO4 ceramic substrates

The development of a low-cost substrate is one of the major technological challenges for crystalline Si thin-film solar cells. Zirconium silicate (ZrSiO₄) ceramics is a material which can meet the demanding physical requirements as well as the cost goals. Thin microcrystalline Si films were deposited by atmospheric pressure CVD on ZrSiO₄-based ceramic substrates coated with barrier layers. The Si film was transferred into a multicrystalline grain structure by zone-melting recrystallization (ZMR). Film growth was analyzed in situ and correlated with substrate and barrier layer properties. Thin-film solar cells were fabricated from selected coarse-grained films. The best solar cell achieved an efficiency of 8.3% with a short circuit current density of 26.7 mA/cm². The effective diffusion length obtained from internal quantum efficiency measurements was about 25 μm.     

多晶硅薄膜太阳电池

对已经取得较普遍应用的Si体太阳电池来说，开发新技术以降低电池的制造成本是目前该领域最重要的努力方向之一。尽管通过优化制造工艺可以在一定程度上进一进降低单晶Si和多晶Si体太阳电池的成本，但要进一大幅度地降低Si太阳电池的成本似乎是只能依赖于新一代的多晶Si薄膜电池。多晶Si薄膜电池因其转换效率高、寿命长和工艺简化等优点而极具潜力。本文从材料制备、材料性能和有关工艺等方面对多晶Si薄膜太阳电池的发展现状作了介绍。     

Experimental evidence of very high open-circuit voltages of inversion–layer silicon solar cells

In this paper the first experimental evidence of the high V_oc-potential of inversion-layer silicon solar cells is given. Minority-carrier lifetime measurements on inversion-layer emitters have been performed and the diffused p–n contact of PN-IL silicon solar cells has been optimized for high open-circuit voltages. PN-IL silicon solar cells with open-circuit voltages of 693 mV have been fabricated on 0.2 and 0.5-Ω cm FZ p-Silicon wafers. These values are the highest ever reported V_oc's for inversion-layer silicon solar cells on p-Silicon. This demonstrates that inversion-layer silicon solar cells exhibit a similar potential for achieving high open-circuit voltages as silicon solar cells with a diffused p–n junction.     

Can we improve the record efficiency of CdS/CdTe solar cells?

Polycrystalline thin film CdTe continues to be a leading material for the development of cost effective and reliable photovoltaic systems. The two key properties of this material are its near ideal band gap for photovoltaic conversion efficiency of 1.45 eV, and its high optical absorption coefficient. Thin film CdTe solar cells are typically hetero-junctions with CdS being the n-type partner, or window layer. Efficiencies as high as 16.5% have been achieved.In this paper we make a physical analysis of the typical CdS/CdTe superstrate solar cell, and we show that present record efficiencies are very close to the practical efficiency limit for a CdS/CdTe hetero-junction cell. We show that a current estimate for the maximum efficiency of hetero-junction CdS/CdTe solar cells is around 17.5%, in contrast to old theoretical predictions, which calculate about 30% efficiencies for ideal homo-junction CdTe solar cells. This analysis explains why the record efficiency for this kind of cells has been stable for the last 10 years, going up by less than 1% from 15.8% to only 16.5%.     

Microsystems enabled photovoltaics: 14.9% efficient 14 μm thick crystalline silicon solar cell

Crystalline silicon solar cells 10-15 times thinner than traditional commercial c-Si cells with 14.9% efficiency are presented with modeling, fabrication, and testing details. These cells are 14 μm thick, 250 μm wide, and have achieved 14.9% solar conversion efficiency under AM 1.5 spectrum. First, modeling results illustrate the importance of high-quality passivation to achieve high efficiency in thin silicon, back contacted solar cells. Then, the methodology used to fabricate these ultra thin devices by means of established microsystems processing technologies is presented. Finally, the optimization procedure to achieve high efficiency as well as the results of the experiments carried out with alumina and nitride layers as passivation coatings are discussed.     

Measurement and comparison of AC parameters of silicon (BSR and BSFR) and gallium arsenide (GaAs/Ge) solar cells used in space applications

The AC parameters of silicon (BSR and BSFR) solar cells and GaAs/Ge solar cell have been measured using impedance spectroscopy. Each cell capacitance, dynamic resistance and series resistance were measured and compared. GaAs/Ge solar cell has shown only the transition capacitance throughout its operating range while silicon (BSR and BSFR) solar cells exhibited both transition and diffusion capacitance. The theoretical and experimental values of dynamic resistance were compared and found in good agreement while the diode factor in silicon solar cells varies from 2 to 1, where as in GaAs/Ge solar cell it varies from 4 to 2 to 1.     

Recent improvements and arising challenges in dye-sensitized solar cells

Work demand for energy is expected high and finding sufficient route to produce clean energy is an ever more pressing problem. Science had identified the key research challenges if solar energy is to provide a significant fraction of our energy needs. The huge gap between our present use of solar energy and its enormous undeveloped potential defines a grand challenge in energy research. One of the most attractive methods currently being developed is “dye sensitization” in solar cells in order to increase the efficiency of conversion of solar radiation into electricity. Although large improvements in present technology will be required, the review points to progress in nanoscience, in particular as a reason to be optimistic.     

Physical basis for the design of CdS/CdTe thin film solar cells

Solar cells based on polycrystalline semiconductor thin films have great potential for decreasing the cost of photovoltaic energy. However, this kind of solar cells has characteristics very different from those fabricated on crystalline silicon for which the carrier-transport and behavior is clearly known. Instead, for hetero-junction solar cells made on less known polycrystalline materials the design is almost empirical. In this work, several physical aspects related to the behavior of polycrystalline thin film solar cells will be discussed, and some considerations for an adequate design of this kind of solar cells will be made. For example, the recombination at the grain boundaries and its influence on the short circuit current as a function of the crystallite sizes on the active material is considered. Based on this, the appropriate thickness of each layer and their resistivity will be discussed. As an example, these considerations will be applied to CdS/CdTe heterojunction solar cells, taking into account typical properties of CdTe thin films used for solar cells.     

Efficiency enhancement of solar cells by luminescent up-conversion of sunlight

Significant improvements in the efficiency of solar cells by combination with luminescent up- or down-converters have recently been predicted theoretically. Here, we extend the theoretical analysis of the limiting efficiency of the up-conversion (UC)-system to realistic Airmass spectra and analyse the spectral robustness of the UC-system. We also present initial experimental results from prototypes involving bifacial silicon solar cells with UC-phosphors attached to the rear surface, and discuss the possibility of realizing efficient UC with low-band-gap solar cells in combination with a light emitting diode.     

Use of chalcopyrite semiconductors CuXSe2 (X=Al,Ga and In) in solar cells: a theoretical study

The energy of sunlight falling on surface of the earth can be directly converted into electricity by means of the solar cells. Among the various materials used for photovoltaics, the chalcopyrite compounds CuXSe₂ (X=Al, Ga, In) are very promising as semiconductors and have received much attention in the recent years. To check the applicability of these materials in solar cells, we have computed the energy bands, density of states, optical dielectric tensors, reflectivity, refraction and absorption coefficients using the full potential linearized augmented plane wave method. It is seen that the energy bandgap reduces from X=Al→In. The dielectric property of these materials is discussed in terms of interband transitions. The absorption coefficients of these materials in the region of solar radiation (0–5 eV energy) are discussed to explore their use in the fabrication of solar cells.     

Oxide/polymer interfaces for hybrid and organic solar cells: Anatase vs. Rutile TiO2

In this work, we study the effect of the transparent conducting oxide (TCO) and the polymer applied (MEH-PPV or P3HT) on the photovoltaic properties of TCO/TiO₂/polymer/Ag bi-layer solar cells. The solar cells were analyzed under inert atmosphere conditions resembling an encapsulated or sealed device. We demonstrate that the substrate applied, ITO or FTO, modifies the crystalline structure of the TiO₂: on an ITO substrate, TiO₂ is present in its anatase phase, on an FTO, the rutile phase predominates. Devices fabricated on an FTO, where the rutile phase is present, show better stability under inert atmospheres than devices fabricated on an ITO, anatase phase. With respect to the polymer, devices based on MEH-PPV show higher Voc (as high as 1 V), while the application of P3HT results in lower Voc, but higher J_sc and longer device stability. These observations have been associated to (a), the crystalline structure of TiO₂ and (b) to the form the polymer is bonded to the TiO₂ surface. In-situ IPCE analyses of P3HT-based solar cells show a red shift on the peak corresponding to TiO₂, which is not present on the MEH-PPV-based solar cells. The latter suggest that P3HT can be linked to the TiO₂ though the S-end atom, which results in devices with lower Voc. All these observations are also valid for devices, where the bare TiO₂ is replaced by an Nb-TiO₂. The application of an Nb-TiO₂ with rutile structure in these polymer/oxide solar cells is the reason for their higher stability under inert atmospheres. We conclude that the application of TiO₂ in its rutile phase is beneficial for long-term stability devices. Moreover there is an interplay between low Voc and J_sc in devices applying P3HT, since power conversion efficiency can be partially canceled by their lower Voc in comparison with MEH-PPV. These findings are important for polymer/oxide solar cells, but also for organic solar cells, where a layer of semiconductor oxides are in direct contact with a polymer, like in an inverted or tandem organic solar cells.     

The use of nickel oxide as a hole transport material in perovskite solar cell configuration: Achieving a high performance and stable device

Solar cells incorporated with organic-inorganic lead or tin halide-based perovskite materials as active light-absorber surfaces are referred to as perovskite solar cells (PSCs). This fast advancing solar technology has recorded an increase in its efficiency from 3.8% in 2009 to above 25% in recent years. The technology creates room for diverse device architectures, which enhances further development of thin-film solar cells and photovoltaics. This article reviews the use of nanocrystalline nickel oxide (NiO) film as a hole transport material in PSCs. The literature on pure nickel oxide and doped nickel oxide films has been discussed. The principle of operation, charge separation of PSCs and the various parameters that affect the efficient hole transport mechanisms, power conversion efficiency, growth mechanism, and stability of PSCs have also been discussed. Possible electron-blocking applications and future perspective of nickel oxide films have also been discussed.     

Third generation multi-layer tandem solar cells for achieving high conversion efficiencies

Different ways of connecting solar cell structures to form multi-layer tandem solar cells have been considered by re-visiting relevant device designs. It is found that the present use of a series connection or tunnel junction approach is detrimental to charge-carrier collection in the tandem cells. Each tunnel junction introduced to the solar cell structure decelerates the charge carriers and allows them to recombine at the vicinity of the tunnel junction. The adoption of parallel connections has several advantages over series connections and there is high potential for achieving enhanced efficiencies in third generation tandem solar cells. In these devices, charge carriers are continuously accelerated across the whole device and collected in the external circuit. Multi charge-carrier production and impurity photo-voltaic mechanisms are also built into this system to enhance its performance by increasing the short-circuit current density.     

Spin-coating silicon-quantum-dot ink to improve solar cell efficiency

The optical and electrical properties of quantum dots (QDs) have generated great interest in the use of QDs for photovoltaics. The optical coupling of QDs with currently manufactured solar cells is one of the most promising photovoltaic applications of QDs. Here we demonstrate that by spin-coating Si-QD ink at the solar cell surface the efficiency of screen-printed Si solar cells may be improved. The enhancement of solar cell efficiency results from the porous Si-QD film induced increase of light absorption. The solar cell efficiency may be further improved as the efficiency of down-shifting short-wavelength light to red light by Si-QDs increases.