Can Photovoltaic Solar Cells Cap Carbon?

January 2015

The United Nations Framework on Climate Change (UNFCC) is an international treaty ratified by 195 countries. It came into effect in 1994 with the lofty goal of stabilizing greenhouse gas concentrations "at a level that would prevent dangerous anthropogenic interference with the climate system." The internationally agreed target is to limit the increase in global mean surface temperature to less than 2°C.

Achieving the 2°C limit will require that global net emissions of greenhouse gases approach zero by 2050. In turn, this will require a profound transformation of energy systems through steep declines in carbon intensity in all sectors of the world economy. Such a turnaround requires a huge increase in sustainable renewable sources of electricity generation, such as hydroelectric, wind farms, and solar cells.

Right now solar power accounts for only 1% of global energy generation, which is just a tiny step of the journey we need to make to reduce our anthropogenic carbon dioxide emissions.

To shed more light on the extent to which fundamental research on photovoltaic solar cells may contribute to meeting the UNFCC target, ScienceWatch searched the Thomson Reuters Web of Science to assess the impact of recent papers in the areas of materials science, physics, and chemistry that report technological improvements in the design and performance of solar cells.

The publication dates searched covered the years 2012 to 2014. The search was conducted on the Topic “photovoltaic solar cells,” by examining the fields Title, Abstract, Author Keywords and Keywords Plus. The sampling process produced 8,024 papers that have been cited 56,246 times in 22,046 articles.

The table displays the 10 most-cited papers on photovoltaic solar cells (PSCs) over the last two years. Collectively, these highlighted reports have garnered a total of 3,753 citations in 2,871 articles.

Highly Cited Papers on Photovoltaic Solar Cells, 2012-1014

(Listed by citations)

Rank Paper Citations
1 Z. He, et al., “Enhanced power-conversion efficiency in polymer solar cells using an inverted device structure,” Nature Photonics, 6(9): 591-5, September 2012. [S. China Univ. Technol., Guangdong] 1,090
2 L. Dou, et al., “Tandem polymer solar cells featuring a spectrally matched low-bandgap polymer,” Nature Photonics, 6(3): 180-5, March 2012. [Univ. California, Los Angeles; Natl. Renewable Energy Lab, Golden, CO] 685
3 Y. Sun, et al., “Solution-processed small-molecule solar cells with 6.7% efficiency,” Nature Materials, 11(1): 44-8, January 2012. [Univ. California, Santa Barbara] 561
4 L. Han, et al., “High efficiency dye-sensitized solar cell with a novel co-adsorbent,” Energy & Environmental Sci., 5(3): 6057-60, March 2012. [Natl. Inst. Material Res., Tsukuba, Japan; Indian Inst. Chem. Technol., Hyderabad] 229
5 A. Ip, et al., “Hybrid passivated colloidal quantum dot solids,” Nature Nanotechnology, 7(9): 577-82, September 2012.  [Univ. Toronto, Canada; King Abdullah Univ. Sci. & Technol, Thuwal, Saudi Arabia] 227
6 L. Dou, et al., “Systematic investigation of benzodithiophene and diketopyrrolopyrrole-based law-bandgap polymers designed for single junction and tandem polymer solar cells,” J. Am. Chem. Soc., 134(24): 10071-9, 20 June 2012. [Univ. California, Los Angeles]  207
7 L. Etgar, et al., “Mesoscopic CH3NH3Pbl3/TiO2 heterojunction solar cells,” J. Am. Chem. Soc., 134(42): 17396-9,  3 October 2012. [Ecole Polytech. Fed. Lausanne, Switzerland; Natl. Univ. Singapore; Hebrew Univ. Jerusalem, Israel] 198
8 J. Zhou, et al., “Small molecules based on benzo[1,2-b: 4,5-b ‘]dithiophene unit for high-performance solution-processed organic solar cells,” J. Am. Chem. Soc., 134(39): 16345-51, 3 October 2012. [Nankai Univ, Tianjin, China] 193
9 J.M. Ball, et al., “Low-temperature processed meso-superstructured to thin-film perovskite solar cells,” Energy & Environmental Sci., 6(6): 1739-43, June 2013. [Univ. Oxford, UK] 182
10 M. Wu, et al., “Economical Pt-Free catalysts for counter electrodes of dye-sensitized solar cells,” J. Am. Chem. Soc., 134(7): 3419-28, 22 February 2012. [Dalian Univ. Technol., Liaoning, China; Uppsala Univ., Sweden; Swiss Fed. Inst. Technol., Lausanne] 181
SOURCE: Thomson Reuters Web of Science

A unifying feature of these papers is the quest for higher conversion efficiency in PSCs based on organic polymers, but before looking at that in detail we’ll review what has been achieved so far with silicon.

CRYSTALLINE SILICON SOLAR CELLS

At present 90% of the global production of solar modules are made from solar cells made of crystalline silicon. Solar modules with an efficiency of about 15% currently offer the best rate of return on the investment. Bell Labs demonstrated the first silicon PSC in 1954. An early application of the new technology was the powering of satellites and spacecraft, where cost considerations were irrelevant. Large-scale applications have been with us for three decades now, a period of continuous development in the technology, matched by lowering of prices. The latest improvement in PSCs based on silicon is the silicon heterojunction cell, which achieves 20% on an industrial scale.

So what’s there not to like about silicon cells? A lot, unfortunately. The manufacturing process is expensive and requires high temperatures. These solar cells are heavy, rigid and fragile. They are suited only to flat surfaces, and therefore are limited to rooftop or field applications.

ADVANTAGES OF THIN-FILM ORGANIC PHOTOVOLTAICS

The advantage at the heart of the organic photovoltaic technology is ease of fabrication, which holds the promise of very low-cost manufacturing processes. A simple yet successful technique is the solution-processed bulk heterojunction solar cell composed of electron-donating semiconducting polymers and electron-withdrawing fullerides as active layers. The composite active layer can be prepared as a large area in a single step by using techniques such as spin-coating, inkjet-printing, spray-coating, gravure coating, and a roller-to-roller production line. These are the positive factors driving much of the research showcased in the table.

Top Paper #1describes a breakthrough in the design of a heterojunction solar cell with an inverted structure. Prior to the appearance of #1 most inverted PSCs had failed to match the performance of regular devices. Paper #1 reports an efficiency of 9.2%, which is almost there in terms of the 10% efficiency desired for commercial exploitation. This result eclipsed the previous record of 8.62% efficiency reported in paper #2. According to the authors of  #1: “We anticipate that our prototype of semitransparent PSCs will find practical applications in building windows, foldable curtains and invisible electronic circuits.”

Papers #2 and #6 report progress on tandem cell architecture, a technique of combining two solar cells with different properties in order to harvest a broader part of the solar spectrum. Paper #6 reports an 8% efficiency.

In #3, #7, and #8, one of the issues is the benefit of solution processing. In industrial and academic laboratories there is intense research activity on bulk heterojunction solar cells that require simple processing at ambient temperature. A solution-based approach is also the main message of #5 on the processing of colloidal quantum dots (CQDs), which offer a path toward high-efficiency photovoltaics. Spectral tunability of CQDs is via a quantum size effect that mediates absorption of specific wavelengths from the broad solar spectrum. CQD materials’ ease of processing derives from their synthesis, storage, and processing in solution.

 POTENTIAL OF PEROVSKITES

In 2009 perovskites joined the list of ingredients available for thin-film solar cells. The term perovskite applies to any material with the same crystalline structure as calcium titanium trioxide CaTiO3. Metal halide materials have turned out to be very efficient as the harvesting layer in thin-films PSCs. Paper #9 reports a technical advance in the processing of a lead halide perovskite: a reduction of processing temperature from 500°C to less than 150°C. Furthermore, the reported efficiency is 12.3%. And there’s plenty of scope to boost that further.

One of perovskite’s big attractions is that it joins the list of light-sensitive materials that are produced from low-temperature liquid solution, in contrast to the high-heat methods for growing silicon crystals and other solar cell materials. What’s more, applying it is just a “paint job.” But there’s a big downside: the crystals disintegrate in humid conditions—they are only stable indoors where there’s not much sunlight. It’s not going to be straightforward to fix this.

A BRIGHT FUTURE, HOPEFULLY

This ScienceWatch assessment has shown how current research on PSCs is a transformational interdisciplinary field, with great potential to benefit society and our planet. Progress is accelerating as new materials, such as perovskite, join the tool kit available in the laboratory. Design engineers need solar cells that are low mass, durable, flexible, easy to attach, preferably transparent, and definitely low cost. All of these goals seem now to be coming within reach.

Policymakers should probably take more note of where the science and technology might be in another ten years. At some point the political priorities should be to encourage much greater opportunity for solar energy harvesting and exploitation, as an affordable means of achieving the UNFCC target. The potential for developing countries is huge, particularly in Africa and rural India, where the next generation of solar panels would allow isolated communities access to sustainable electrical energy on a domestic scale.


Dr. Simon Mitton is a Life Fellow of St. Edmund’s College Cambridge. He writes on advances in physics and cosmology.

The data and citation records included in this report are from Thomson Reuters Web of ScienceTM. Web of ScienceTM is a registered trademark of Thomson Reuters. All rights reserved.