A review of photovoltaic performance of organic/inorganic solar cells for future renewable and sustainable energy technologies
Introduction
The demand for energy is growing globally, but the primary energy sources like fossil fuels are gradually depleting. Fossil fuels also affect the air quality and public health by emitting green house gases like CO2 and other air pollutants. Based on the current economic growth figures, the world needs energy of 28 TW in 2050 and 46 TW in 2100. A significant amount of this energy should be generated by environment friendly renewable energy sources. Therefore, it is high time to explore new energy sources for future renewable and sustainable energy technologies. Solar cells have been considered as the most promising solutions for meeting the global energy needs because solar energy is the safest, clean and abundant energy source for future renewable and sustainable energy technologies. IBSC (Intermediate band solar cells) have emerged as an attractive choice for improving the energy conversion efficiency of single gap solar cells. Techniques like quantum dot (QD) nanostructures and highly mismatched semiconductor alloys can be used for improving the energy conversion efficiency of IBSCs [1]. Back junction back contacted (BJBC) solar cells are also gaining attention because of their outstanding energy conversion efficiencies which arises from the absence of optical shading and reduction of recombination losses with the use of carrier selective junctions [2]. Full area rear Al-alloyed BSF (back surface field) and PERC (passivated emitter and rear cell) are the two popular designs used for industrial mass production of c-Si (crystalline-silicon) solar cells. PERC design is gaining popularity over the years for the mass production of solar cells having power conversion efficiency (PCE) over 25% [[3], [4], [5]]. a-Si:H (amorphous hydrogenated silicon) and mc-Si (micro-crystalline silicon) materials can be used for developing Silicon (Si) SCs. The advantage of incorporating hydrogen in mc-Si and a-Si materials is that it can significantly improve the minority charge transport through the grained Si material and reduces the recombination at the grain boundary. Compared with a-Si:H material, mc-Si:H material is highly stable against optically induced degradation [6]. Because of the increasing environmental concerns on materials for photovoltaics, photovoltaic researchers are looking towards eco-friendly photovoltaic materials [[7], [8], [9]]. Point contact solar cells are most important back contacted solar cells in which the metal contacts touch the Si only in an array of points. The major advantage of this point contact solar cells over inter-digitated back contacted solar cell is that it provides high output voltage. Low cost, good reliability and high PCE are the key advantages of back contacted solar cell design [10]. The material quality of mc-Si is limited by the crystal defects and metal impurities [11,12]. Different band-gap semiconductors can be used for the effective utilization of the solar spectrum for solar energy conversion. Based on this fact, various research groups have developed hetero-junction (HJ) SCs and HJ bipolar transistor SCs [13]. In 2013, B. Endres et al. [14] demonstrated a spin solar cell based on GaAs p-n junction. Multi-junction SCs are found to be effective in achieving high PCE compared with single junction (SJ) solar cells. The PCE of SJ-SCs are constrained by the Queisser-Shockley limit [15,16]. Voc, Jsc, FF (fill factor), dark current density (J0) and PCE are the key parameters that can be used for analyzing the PV performance of SCs. The PCE of solar cells can be computed aswhere IO represents the intensity of incident light.
The dark current density (Jo) of solar cells can be expressed aswhere, ni, W, τ, Sfront and Sback represents intrinsic carrier concentration, depletion layer thickness, life time of minority carriers, velocity of surface recombination at front surface and velocity of surface recombination at back surface respectively.
The relationship between VOC and light intensity (P) is given bywhere n and C are ideality factor and a constant respectively.
The relationship between VOC, energy of charge transfer state (ECT), JSC and dark current (Jo) is given below.
The fill factor (FF) of solar cells can be computed asWhere, JMPP and VMPP are maximum power point current density and maximum power point voltage respectively. The variation in JSC and VOC due to charge density changes significantly affects the FF of solar cells. The relationship between generation rate of free charges (G), JSC and active layer thickness (L) is given by
Fig. 1 shows the device structures of single and multi-junction SCs. In 2014, Ju-Hyung Yun et al. [17] from State University of New York successfully demonstrated an incident light adjustable periodic nanolens architecture based on Si solar cells. Nanowire or nanopiller arrays have been used for photovoltaic energy harvesting applications to enhance PCE. This is due to the ability of nanowire arrays to minimize surface light reflection and improve the charge collection efficiency [[18], [19], [20], [21], [22]].
Section snippets
Silicon solar cells
The PCE of a-Si:H/mc-Si HJ-SCs can be improved by using front electron-collector in rear-emitter [23]. Since high PCE can be achieved with simple fabrication processes and cost effective materials, silicon hetero-junction (SHJ) SCs have been gaining much attention in the photo-voltaic industry. Charge recombination is the critical factor that limits the performance of solar cells and therefore, a-Si:H/mc-Si interface should be passivated to minimize the recombination of carrier. a-Si:H SCs
Perovskite solar cells (PSCs)
The PCE of flat-plate SJ solar cell is approaching to its theoretical-efficiency limit due to the rapid advancements in fabrication processes, photovoltaic materials and solar cell structures [105]. Organometal trihalide PSCs have gained tremendous attention in the PV industry due to their unique characteristics such as good flexibility, low cost, good scalability, low temperature processability and comparable photovoltaic performance with traditional thin film inorganic SCs. Even though PSCs
Dye sensitized solar cells (DSSCs)
A DSSC is basically an electro-chemical cell in which a mesoporous and spongy dye-coated semiconductor is plunged into an electrolyte [170]. Nano particle made TiO2 is the commonly used semiconductor in DSSCs. The TiO2 paste is then deposited in a TCO which act as one electrode for the DSSCs. A Monolayer of molecules called dye is then covered on the surface of a mesoporous material. The dye act as the photoactive layer in DSSCs. The electrolyte normally contains an iodide/triiodide redox pair
III-V compound semiconductor based solar cells
III-V solar-cells (III–V SCs) have been considered as the most attractive way for generating cost effective photovoltaic electricity for space and terrestrial applications [198]. Excellent PCE with high radiation resistance and lower temperature coefficients are the major advantages of III-V SCs [199]. III-V SCs can be developed as single junction, two junction, triple junction (3J) and four junction (4J) cells. Among these 4J cells are found to be more efficient compared with other
Recent developments in flexible solar cells
The demand for self-powered portable electronic devices is increasing day by day and flexible solar cells remains the most attractive solution to meet this demand. Light weight, wearability, bendability, conformability and roll to roll processing are the main advantages of flexible solar cells [231]. By 2030, solar cells are expected to account for 30% of global electrical energy generation. For future flexible photovoltaic devices, the key requirement is transparent conducting electrodes (TCE)
Recent developments in tandem solar cells
Tandem SCs can be developed in two ways and they are four terminal (4T) mechanically stacked tandem SCs and two terminal (2T) series connected tandem SCs [241]. The 4T mechanically stacked tandem SCs requires semi-transparent contacts to reduce absorption losses. The two terminal series connected tandem SCs needs the matching of band gap of top and bottom sub cells in order to minimize current losses. In order to match the band gap of bottom and top cells, in two terminal series connected
Quantum dot solar cells
Quantum dot SCs realized using semiconductor nanocrystals are emerging as one of the leading photovoltaic technologies for commercial solar-energy harvesting applications. The rapid developments in nanotechnology have made it possible to fabricate uniform quantum dot SCs from a wide range of semiconductor nanocrystals. The band gap of quantum dot is determined by its size and therefore, the main advantage of quantum dot cell is that its band gap can be precisely controlled by varying its size.
CIGS and similar materials based solar cells
The developments of economic and environment friendly materials are required for solar energy harvesting applications to generate electricity for future renewable and sustainable energy technologies. CdTe, CZTSSe (Cu2(Zn,Sn)(S,Se)4) and CIGSSe are the three commonly used semiconductor materials for thin film SCs [299]. Among these two, CZTSSe is stable, non-toxic and earth abandoned material but it offers poor PCE compared with CIGSSe material. In 2105, Bart Vermang et al. [299] introduced a
Organic solar cells
The two most important benefits of organic photovoltaic devices (OPV) based on organic semiconductors compared with inorganic photovoltaic devices are the cost effective fabrication facilities and the possibility of producing organic SCs on flexible substrates [335]. Low cost solution-processes like roll-to-roll processing, spin coating and printing methods can be used for industrial-fabrication of polymer based organic SCs. The optical absorption coefficient and band gap energy of polymers
Conclusion
According to the current economic growth figures, the world needs energy of over 30 TW in 2050 and over 50 TW in the end of this 21st centenary. A major part of this energy should be generated by environment friendly renewable energy sources. At this moment, solar cells are the largest source of renewable energy. For future sustainable and renewable energy technologies, it is essential to develop innovative and environmental friendly materials for harvesting solar energy. III-V solar cells,
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