Perovskite-based low-cost and high-efficiency solar cells 

Keywords – Perovskite Solar Cell, Hole Transport Material (HTM),Electro transport material (ETM), Fill factor(FF), Copper indium gallium selenide (CIGS) cells, open circuit voltage Voc ,short circuit current Isc, Dyesensitized solar cells (DSSC),

1. Overview :Global energy consumption has been continually increasing with population growth and fast-paced industrial development  in recent decades, which demands renewable energy sources in view of long-term sustainable development. Generating cost-effective and environmentally benign renewable energy remains a major challenge for both technological and scientific development. Solar cells based on the photovoltaic effect with the advantages of decentralization and sustainability have attracted great attention in the past 50 years. Currently, the photovoltaics market is dominated by crystalline silicon-based solar cells with a share of 89% .  This  dominance is now being challenged by the emergence of a new generation of photovoltaic cells, based on nanocrystalline materials and conducting polymer films, making solar power much more expensive in comparison with fossil fuels. A new type of solar cell made from a material called ‘perovskite’ is significantly cheaper to obtain and use than silicon. It could generate as much power as today’s commodity solar cells. Highly efficient solar cells using perovskite can be made using a simple and inexpensive technology.

1. PEROVSKITE- MATERIAL

1. PEROVSKITE-BASED SOLAR CELLS

Solar cells made up of perovskites material, have a simple architecture and can easily be produced in large quantities because the vapour deposition process used to make them is compatible with conventional processing methods for fabricating such solar cells. Prototype solar panels incorporating nanotechnology are more efficient than standard designs in converting sunlight to electricity, promising inexpensive solar power in the future.   Organo-metal-trihalide perovskite semiconductors, with the formula (CH3NH3)PbX3 — where Pb is lead and X can be iodine, bromine, or chlorine — were first employed in 2009 as the light-absorbing component. In these devices, the perovskite  materials were coated onto the surface of a film made of titanium dioxide (TiO2) nanoparticles.

         When the perovskite layer absorbs light, electrons, and holes are generated. These charge carriers are subsequently transferred to different transport materials — TiO2 for the electrons and to another material for the holes. The transport materials then carry the charges to separate electrodes, and a voltage is produced.

       Perovskite solar cells are causing excitement within the solar power industry with their ability to absorb light across almost all visible wavelengths, exceptional power conversion efficiencies as tested in lab  and relative ease of fabrication. Perovskite solar cells still face several challenges but much work is put into facing them and some companies are already talking about commercializing them in the near future.

1. CHALLENGES AHEAD

Like any other new entrant into the highly competitive solar-panel market, perovskites will have initial  difficulties in  taking over silicon solar cells. Also, since cost of silicon solar cells are falling, the financial aspect of both silicon and perovskites would have to be taken in to consideration. A major challenge in the perovskite solar cells (PSCs) is in the  field is stability; Unlike silicon cells, perovskites are soft crystalline materials and prone to problems due to decomposition of maerial over time. In a commercial context, this tends to inflate the costs of perovskite-based solar cells compared with conventional silicon cells. There have therefore been many efforts in synthesizing perovskite materials that can maintain high efficiency over long  time period. This is done by introducing different cations (positively charged ions) into the crystal structure of the perovskite. Although success has been reported in several studies, these solutions can often be difficult and expensive to implement.Thus, it is better to augment perovskites initillay rather than replace silicon solar cells to improve their efficiency. This might be an easier way to break into the solar market than trying to introduce an entirely new kind of solar cells. Quantum Dot based (QD) solar cells have shown great potential as next generation, high performance low cost photovoltaics due to the outstanding optoelectronic properties of quantum dot and their multiple excitation generation (MEG) capabilities. The recent development of organic–inorganic perovskite hetro junction solar cells has shown great future as light harvesters.      

              There is however one small challenge associated with the use of perovskite materials. The material contains small amount of lead, which is toxic. Tests will be needed to show how toxic it is. Steps can also be taken to ensure that the solar cells are collected and recycled to prevent the materials from getting into the environment .Further degradation in perovskite solar cells is a synergetic effect of exposure to humidity, oxygen, ultraviolet radiations, and temperatures.

COMPREHENSIVE   OUTLOOK

In a span of a couple of years, perovskites have demonstrated that they possess the right mix of properties to offer a solution to our energy requirements. Though they evolved out of liquid electrolyte DSSCs, they are now established as a class of their own with extensive research focus pushing the efficiency limit by 20%. Low temperature solution processing assures low per watt cost and quick energy payback times. Device architectures ranging from pin  to mesoporous to meso superstructure configuration throw wide open the possibilities for incorporation of novel materials and synthesis approaches.  Use of materials with high mobility as HTM will further improve the FF while optimization of the interfaces and selection of HTM and ETM may push forward the efficiencies to higher values. Incorporation of narrow band gap perovskites and plasmonic light harvester may broaden the spectral response with better light harvesting. Interface engineering and introduction of self-assembled layers will reduce losses and improve efficiency. Understanding of the underlying photophysical phenomenon will further help improving device structures and better selection of materials. Exploration of tandem cell configuration with perovskite based cell as the top cell will push forward the achievable efficacy limit further. With the amount of research effort under way, guided by the adherence to the issue of best practices , this technology holds great promise to addressing our energy concerns. Addressing the issues of stability and use of lead can go a long way in maturing this technology for commercial application, though in the present legal framework use of lead is not a problem as CdTe based solar cell has received wide acceptance despite Cd content. Use of lead extensively in lead acid batteries and its content at comparable levels in CIGS and silicon modules to perovskites suggest that in the short term the concern may not be pressing, but these technologies are increasingly being phased out and alternatives are explored to minimize the environmental impacts of these heavy metals. Replacement of lead with tin in perovskite solar cell is already under investigation and may offer an environment friendly alternative.

1. CONCLUSIONS AND FUTURE OUTLOOK

In summary, perovskite solar cells exhibit impressive competitiveness with other photovoltaic techniques due to their unique advantages:

(i)   Low-cost, earth abundance, and easy  preparation.

(ii)  Near-perfect crystalline structure at low  temperature

(iii)  Large charge-carrier diffusion length  for mixed-halide perovskite (CH3NH3PbI3−xClx) thin films, which is nearly 100 times higher that the other low-temperature solution-processed thin films  

(iv)    lower value of “loss-in-potential” in a solar cell, which allows the Voc of the best perovskite cells much higher than the traditional DSSCs, organic solar cells and It can even compete with crystal silicon solar cells .

(v)    Perovskite materials are better than silicon at absorbing higher-energy w.r.t. blue and green photons.

Meanwhile, perovskite materials also have some ineluctable disadvantages:

(i)      Perovskite materials are extremely sensitive to oxygen and water vapor, which reacts to break down the crystal structure and dissolves the perovskite material . Therefore  preparation of perovskite thin films requires manufacturing  in inert atmosphere.

(ii)    It is challenging to prepare large continuous films, which limits it for large scale production.

(iii)   The lead is the most-used in  perovskite solar cells which is toxic and could leach out of the solar panel onto rooftops or the soil below offering potential hazard to environment and living beings..

(iv)   Since there is a phase transition from tetragonal to cubic at 55°C, the longer-term stability of perovskite solar cells has not been verified. There are a few studies on storage lifetime but only limited on an operating cell (under illumination at the maximum power) for a sealed cell at 45°C The studies have  showed a decrease in efficiency after certain time period.

                  In view of the advantages and disadvantages addressed above, some new strategies can be forwarded to further improve the efficiency of perovskite solar cells. Research results in this field has offered many promising results towards remarkable efficiency improvement in solar cells.

Needless to say, the development of higher-efficiency solar cells will not only have a profound economic impact, but it will also represent significant social and environmental benefits. The perovskite technology would allow the mass production of solar cells with high efficiency and at relatively low temperatures, which would account for a substantial reduction of cost. In the meantime, the technology can lead to high-throughput device fabrication due to the simple deposition process required. Furthermore, flexible substrates could be used, which would allow an easier handling, transportation, installation, and building integration of these new photovoltaic devices. We believe that this new class of perovskite solar cells will find widespread applications and will eventually lead to devices that rival conventional silicon-based photovoltaics.

Presented by 

Er Tajinder Kaur  Jt Secy / Power 

BBMB Chandigarh at 

Conclave on Emerging opportunities and challenges of R&D  in Indian Power Sector

Vigyan Bhawan New Delhi in Feb 2018

References:

1. Wikipedia Perovskite solar cell, solar cells.
2. Roni Peleg, senior editor of Perovskite-info
3.  Jiandong Fan, Baohua Jia, and Min Gu ,Centre for Micro-Photonics, Faculty of Science, Engineering and Technology, Swinburne University of Technology,Hawthorn, Victoria 3122, Australia Jiandong Fan, Baohua Jia, and Min Gu,Centre for Micro-Photonics, Faculty of Science, Engineering and Technology, Swinburne University of Technology,

Hawthorn, Victoria 3122, Australia.

4. https://www.nanowerk.com/spotlight/spotid=45249.phpPerovskite solar cells - a true alternative to silicon.
5. https://www.perovskite-info.com/perovskite-solar