New-age thin film solar cells

For the second half of my MSc program, I am pursuing a thesis project at imec (Leuven, Belgium) on CZTSe and CIGS(e) solar cells. The absorber materials are more broadly known as Kesterite and Chalcopyrite compounds. My work deals with the fundamental science of making these quaternary compounds, as well the numerous steps required to make complete cells out of them (absorber layer formation, grain-growth, surface passivation, grid deposition and so on!)

In this blog post, I would like to present the case for thin-film solar cells.

thin_film_flexible_solar_pv_ken_fields_1
Image courtesy: commons.wikimedia.org, user: Ken Fields

As a solar cell material, Silicon is doing very well. It is the second most abundant element in the earth’s crust (in the form of silica, sand) [1]. As explained in my previous post, William Schokley and Hans-Jaochim Queisser famously calculated that an ideal semiconductor for absorbing our star’s light on the Earth’s surface, should have a band-gap between 1.2-1.34 eV. Silicon sits at a somewhat favourable 1.12eV. Thanks to the efforts of countless scientists and engineers, today we can all easily buy modules with an efficiency of 21% [2]. Silicon is also highly favoured because of the immensely vast technologies developed by the electronic industry, which uses it in every chip, for every transistor, resistor and capacitor!

It is no surprise then, that 93% of the current PV market is served by this wonder material! [2] It is present in the 1.7 million solar panels in the Solar star installation and the Tesla roof tiles that will go into production soon. Why then, does mankind search for something more? Because this is not the whole story!

1) Absorption: Silicon, with its indirect band-gap is not a good absorber. In simple terms, it means that several semiconductor materials can absorb the same light in about 1/100th material. By using silicon, we are using 100 times more material than we need to, in the form of expensive, heavy and bulky panels.

monocrystalline_solar_panel
Image courtesy: commons.wikimedia.org, user: Lamiot

Due to their crystalline nature, Silicon wafers work best under direct incident light which is difficult to track as the Sun moves in the sky. Amorphous thin-film technologies have a lesser performance drop under indirect light and can produce more energy in a day than crystalline Silicon panels with higher efficiencies! Average energy production is a better performance benchmark than peak efficiency when comparing technologies.

2) Production: Sand is abundant but SiO2 is an extremely stable compound. The extraction of Silicon is now a highly standardized process but it still remains energy intensive and expensive [3]. Alternative materials can be easier and safer to produce with the use of lesser energy and chemicals harmful to the environment. Their production can even be solution based as Perovskites cells have been demonstrated to be. [4]

3) Band-gap engineering and multi-junction cells: By using other materials, a better band-gap can also be used to approach the theoretical maximum efficiency. Some semiconductors offer ‘tunable’ band-gaps which vary according to the stoichiometric composition.

4) Uniting forces: While new technologies try to compete with Silicon, then can also instead, work with silicon (in ‘tandem’) to absorb the blue, short wavelength light that silicon might leave out. You may have noticed the blue colour of conventional solar panels (particularly the old ones).

comparison_solar_cell_poly-si_vs_mono-si
Image courtesy: commons.wikimedia.org, user: Klaus Mueller

Multi-junction solar cells can theoretically go up to 86.6% efficiency under concentrated light [5]. Such panels have already been used in space missions since the 1970s. [6]. Currently, the multi-junction domain is dominated by III-V group materials (In, Ga, As) and not Silicon. 38.9% efficiency modules are commercially available. [2]

5) Chalcopyrites: CIGS(e) and CdTe solar cells have been the champions of thin-film technologies, with record laboratory efficiencies reaching 22.6 % [7] which is greater than the multi-crystalline Si record of 21.25 % [8] and comparable to the mono-crystalline Si record of 25.6 % [2]. CIGS(e) solar cells have also exhibited high volume production and roll-to-roll manufacturing. Several challenges remain for these technologies. Perhaps that is the reason why large corporations such as TSMC solar have tried and failed to sell them. First Solar is the most dominant player in the production as well as research of CdTe solar cells. However, Te is rare and Cd is more toxic than lead or mercury, which has why this technology is getting increasingly unpopular in the European Union. One of the major drawbacks of CIGS(e) technology is the use of Indium, which has an estimated abundance of 0.049 ppm in the earth’s crust, making it rarer than even silver [9]. It’s demand has been rapidly increasing due to its wide spread use in touch screen displays and LCDs. Gallium also carries a risk of increasing costs. The economics of constituent elements could increase the cost of CIGS(e) technology as demand rises. This could stagnate or reverse the downward spiral of the prices of solar modules.

cigsdevice
Image courtesy: commons.wikimedia.org, NREL

6) Kesterite: Kesterite (CZTS(e)) solar cells seek to solve this problem of CIGS(e) technology by using earth abundant materials such as Copper, Zinc, Tin, Selenium and Sulphur. They are thus far made in the same architecture as CIGS(e) cells and follow a similar line of research. Efforts are also being made to make the production low-temperature, low-cost and with few steps. A record efficiency of 12.6 % has already been demonstrated [10]. Research in this field focuses on earth abundant materials such as copper, zinc, tin and selenium.

cztse
Image courtesy: imec

Imagine a world where solar panels are light-weight, wafer thin, flexible and cheap/easy to produce and install. They come in all shapes and sizes and can fit onto the phones in your pocket, on the top of camping tents or in the windows of your buildings and cars. The possibilities are endless and we could power our entire civilization with the infinite pool of the sun’s energy which already makes our blue planet burst with life.

Technologies take time to develop. Tracing the historical timeline of silicon solar cells, it is easy to see that the first such cell was demonstrated in 1954 with an efficiency of 6%. In 1958, Vanguard I became the first solar power satellite. It was only decades later that silicon solar cells become widely accepted. Therefore, this project is important to me because with careful scientific analysis, engineering and development, today’s 6% solar cell can change the world tomorrow.

References:

  1. Nave, R. Abundances of the Elements in the Earth’s Crust, Georgia State University
  2. Fraunhofer ISE, “Photovoltaics report,” Freiburg, updated: 6 June 2016
  3. PV-magazine, The trouble with Silicon, [online]. Available: http://www.pv-magazine.com/archive/articles/beitrag/the-trouble-with-silicon-_100001055/86/?#axzz4PDooxOvU  %5BAccessed: 6th November 2016]
  4. imec, Semi-transparent-perovskite-pv-module, [online]. Available: http://www2.imec.be/be_en/press/imec-news/semitransparent-perovskite-pv-module.html  [Accessed: 6th November 2016]
  5. Green, Martin A. (2003). Third Generation Photovoltaics: Advanced Solar Energy Conversion. Springer. p. 65.
  6. Spectrolab, [online]. Available: http://www.spectrolab.com/history.htm %5BAccessed: 6th November 2016]
  7. PV magazine, “ZSW sets new thin film world record 29th 22.6 % efficient CIGS PV cell,” ZSW, 15 June 2016. [Online]. Available: http://www.pvmagazine.com/news/details/beitrag/zsw-sets-new-thin-film-solar-world-record-with-226-efficient-cigs-pv-cell_100024995/#axzz4ClVxNMm4. [Accessed 30 August 2016].
  8. Trinasolar, “Trina Solar Announces New Efficiency Record of 21.25% Efficiency for Multi-crystalline Silicon Solar Cell,” 9 November 2015. [Online]. Available: http://www.trinasolar.com/us/about-us/newinfo_978.html. [Accessed 30 August 2016].
  9. K. Barbalace, “Periodic Table of Elements,” 1995-2016. [Online]. Available: http://EnvironmentalChemistry.com/yogi/periodic/In.html. [Accessed 30 August 2016].
  10. W. Wang , M. T. Winkler, O. Gunawan, T. Gokmen, T. K. Todorov, Z. Yu and D. B. Mitzi, “Device Characteristics of CZTSSe Thin-Film Solar Cells with 12.6% Efficiency,” Advanced Energy Materials, vol. 4, 2014.

Exceeding the Shockley-Quisser limit

After carefully reviewing the assumptions made in deriving the SQ limit (previous post), we can begin to list opportunities for exceeding the limit. All these efforts of overcoming the limit are classified as ‘third-generation’ PV cells.

1) Each incoming photon excites only one electron-hole pair: When a photon successfully excites an electron across the band-gap and into a certain region of the conduction band, the electron loses the excess energy through thermalization and falls down to the lowest energy level in the conduction band. When this lost energy is equal to or more than the band-gap energy, we are losing energy that is theoretically sufficient to excite a second electron. There are two ways to prevent this loss:

a) Multiple exciton generation: In certain materials, the phonons from the lost energy can be re-absorbed to excite another electron across the bandgap. MEG can also take place through other mechanisms. This behaviour was first exhibited in PbSe quantum dots[1] (2004). It was later also observed in quantum dots of lead, cadmium and indium compounds, silicon, single-walled carbon nanotubes, graphene, and organic pentacene derivatives[2]. Such materials show a quantum yield > 100% and upto 300%. The biggest challenge in quantum dots is the collection of these carriers before they recombine. During excitation, the electron-hole pair exists as ‘excitons’ and are bounded by coloumbic forces. Excitons have to be split into free charges to do useful work.

b) Down conversion: Certain luminescent materials can absorb high energy photons and emit a higher number of lower energy photons. These emitted photons can be energetic enough to excite electrons and thus, quantum yield > 100% can be achieved. The process is often through auger recombination and de-excitation of all excited electrons. Such luminescence has been famously exhibited in quantum dots and even has applications in ligh-emitting technologies[3]. The size of quantum-dots/nanocrystals can be tuned to obtained desired wavelengths of emitted light. Generally, smaller quantum-dots have larger band-gaps and emit light of shorter wavelengths. The drawback of using QDs is that they lead to many interfaces and hence recombinations.

800px-Quantum_Dots_with_emission_maxima_in_a_10-nm_step_are_being_produced_at_PlasmaChem_in_a_kg_scale_user_antipoff

Colloidal quatum dots produced in a kg scale at PlasmaChem GmbH

Image courtesy: commons.wikimedia.org, user: antipof

2) Photovoltaic material consists of a single band gap energy: This restriction leads to either thermalization losses (if band-gap is too low) or absorption losses (if band-gap is too high) as shown in the illustration below.

800px-ShockleyQueisserBreakdown2.svg

Image courtesy: commons.wikimedia.org, user:  Sbyrnes321

The solution to this problem is the use of multi-junction cells incorporating 2 or more different band-gap materials which can together, absorb the solar spectrum more efficiently. Multi-junction cells (upto 4 layers) are often made with III-V group elements (tunable band-gap) or amorphous silicon-microcrystalline silicon junctions (high and low band-gap respectively).

800px-StructureMJetspectre_Fraunhofer_ISE_creative_commons

Spectral utilization of multi-junction solar cells

Image courtesy: commons.wikimedia.org, Fraunhofer ISE

3) Photons with energy lower than band gap energy are not absorbed: This limit can also be addressed in two ways:

a)Intermediate band gap: Materials can be added which provide an energy level in the forbidden band-gap. Thus, electrons do not necessarily need photons of band-gap energy but can also be excited to the intermediate state with lower energy photons. From this state, they can be further excited to the conduction band with low energy photons. This behaviour has been demonstrate in InAs quantum dots embedded in a InGaAs matrix.[4]
The severe draw-back of this method is that it is similar to having defects/impurities in a materials and leads to high rates of SRH recombination.

b) Upconversion: Certain rare-earth ions can absorb low energy radiation and emit light with lesser photons, but more energy.[5] Thus, low-energy photons undergo ‘up-conversion’ and can excite electrons across a band-gap, which they otherwise could not do.

4) Illumination of the solar cell is by unconcentrated sunlight: Concentrators can be used to greatly increase the amount of incident light. This often leads to higher efficiency in a solar cell but can make the module setup more expensive.

It’s best to conclude with some perspective on this topic. While solar cells have a bad reputation for low efficiency, and the best solar cells have been pegged at 46% (4-junction cell at Fraunhofer ISE), it is also important to remember that most thermal power stations have an operating efficiency of 33% to 48%!

References:

[1] Schaller, R.; Klimov, V. (2004). “High Efficiency Carrier Multiplication in PbSe Nanocrystals: Implications for Solar Energy Conversion”. Physical Review Letters 92 (18): 186601
[2] https://en.wikipedia.org/wiki/Multiple_exciton_generation
[3] X D Pi et al 2008 Nanotechnology 19 245603
[4]Ramiro, I. et al., “Wide-Bandgap InAs/InGaP Quantum-Dot Intermediate Band Solar Cells,” in Photovoltaics, IEEE Journal of , vol.5, no.3, pp.840-845, May 2015
[5] Jan Goldschmidt et al. “Record efficient upconverter solar cell devices”, PV solar energy conference and exhibition, 2014

Shockley Queisser limit

This post offers an intuitive, non-mathematical explanation of the Shockley Queisser (SQ) limit. All research in solar cells today that is focused on energy efficiency , is based on understanding and tackling this limit. Thus a grasp of this limit can help in the analysis, critique and appreciation of research in multi-junction cells, III-V group semiconductors, quantum dot and concentrator solar cells.

The SQ limit is much like the carnot efficiency of heat engines. It puts an upper limit on the maximum achievable efficiency of a solar cell based on fundamental physics, material properties and on the nature of the incident solar radiation. For example, under the assumptions of the limit, commercially popular crystalline solar cells can never exceed an efficiency of 33% in converting sunlight to electricity on earth’s surface. Other considerations further lower this limit, but several techniques can also work around the assumptions and can thus create cells with greater efficiencies. We will go over all these aspects in steps. Some familiarity with the basic working of a solar cell is assumed on the reader’s part.

Assumptions/Conditions:

1. Each incoming photon excites only one electron-hole pair

2. Photovoltaic material consists of a single band gap energy

3. Photons with energy lower than band gap energy are not absorbed. All photons with energy greater than band gap are absorbed (infinite absorption coefficient). Any excess energy is transferred to the the generated electron-hole pair and is subsequently lost by thermal relaxation/thermalisation (collisions with molecules which generate heat)

4. Ideal, infinite mobility of charge carrier. They are able to travel to the probes regardless of where they are generated

5. Loss of electron-hole pairs is through radiative recombination only

6. Illumination of the solar cell is by unconcentrated sunlight under standard test conditions (AM 1.5, 1000 W/m2 and 300K cell temperature)

7. The sun is a black body at 6000K and the solar cell is a black body at 300K

Limiting phenomena:

Spectral mismatch: This aspect can be best illustrated by the diagram below. The sunlight reaching earth has a characteristic spectrum with intensity peaks in the visible and infra-red range. It does not have uniform irradiance across wavelengths and hence, certain wavelengths have poor irradiance even though a solar cell may be capable of absorbing them.

Solar_Spectrum

Image courtesy: commons.wikimedia.org

The wavelengths with energy lower than the band-gap energy pass unabsorbed and unutilized. The higher energy wavelengths are completely absorbed but only the ‘band-gap’ amount of energy is used to generate electricity. The excess energy is lost by electron-hole pairs to thermalisation. Thus, in both wavelength ranges we have energy which is lost and not converted to electric current. Additionally, energy lost by thermalisation heats up the solar cell, which in-turn further lowers its efficiency.

This spectral mismatch means that a band-gap of 1.2ev-1.34ev gives the highest efficiency (33.7%) for AM 1.5 solar spectrum. Any deviation from this band-gap gives a lower efficiency. Spectral mismatch accounts for about 48% of the SQ limit.

Radiative recombination: Just as an electron-hole pair can be excited by a photon, they can also recombine to emit a photon. Since this process cannot be stopped, a certain base-line recombination loss is always present in the solar cell. It is quantitatively studied in the detailed balance. It is the simplest form of recombination loss.

Black-body radiation: The solar cell is assumed to be a black-body at 300K, emitting a characteristic radiation that is an internal function of its temperature. It is in thermal equilibrium with its surroundings. This means that the molecules of a solar cell emit the same amount of energy as they thermally absorb. This represent a 7% loss of the incoming radiation which cannot be converted to electricity. This phenomena follows from Prevost’s law and Kirchoff’s theory of black-body radiation.

Voltage losses (Voc): As discussed in the previous section on spectral mismatch, only the photon energy equivalent to the band-gap energy is used to generate current via the collection of the generated electron from the conduction band and the hole from the valence band. However, the maximum voltage a solar cell can generate is rarely ever the band-gap energy voltage, but a lower value due to recombination currents. In this calculation of the SQ limit, only radiative recombination is considered. Nevertheless, this amounts to a loss in the power efficiency of a cell. This open-circuit voltage (Voc) can be up to 40% lower than band-gap voltage (eg: 1V for BG of 1.7V).

Fill factor: Unfortunately, even a cell voltage of Voc is difficult to achievable. A solar cell is rarely operated in an open cirucuit condition. This is usually operated at the point of peak power in the fill factor curve. This gives maximum power, but a voltage that is lower than Voc.

The SQ limit is determined by adding up all these loss mechanisms for a given solar cell.

Other practical losses not accounted for in this limit:

1. If other mechanisms such as shockley-read-hall (SRH) and auger recombination are considered, then efficiencies lower than the SQ limit are predicted. Both of these are non-radiative mechanisms. In indirect band gap materials such as Silicon, auger recombination is more dominant than radiative recombination, thus further reducing the limit to 29.4%. In this regard, the SQ limit is more suited for GaAs single junction cells

2. Optical losses: Reflection, shading, transmission (finite absorption coefficient)

3. Non-ideal fill factor due to series resistance in the bulk of the junction and leakage currents from shunt resistance or local defects.

By looking at the assumptions made in the derivation of this limit, it is apparent that some conditions on a solar cell can be changed in order to exceed the shockley queisser limit. In my next post, I will explain these and hence, touch upon the latest research in PV cells.

Link to the original paper: William Shockley and Hans J. Queisser, “Detailed Balance Limit of Efficiency of p-n Junction Solar Cells”, Journal of Applied Physics, Volume 32 (March 1961), pp. 510-519; doi:10.1063/1.1736034

First post: PV cells

This is a blog on photovoltaic (PV) energy, with a primary focus  on photovoltaic cells and semiconductor technology. Other topics like PV systems and global energy trends will also be discussed.

The battle of solar energy with carbon based fuels has begun and could be a long-drawn one, but there is little doubt that the infinite energy of our home star can power all our energy needs till the end of time. The Earth is a speck in comparison to the sun (0.00007% of its volume) [1] and the sun’s energy powers everything on earth in various manifestations. It is present in plants through photosynthesis, heat that living beings require, blowing winds, flowing rivers, ocean tides as well as the fossil fuels we burn for our transportation, heating and machinery.

PV cells are one way of tapping into this energy. They can convert electromagnetic radiation to electricity. Electric energy is increasing becoming the preferred form of energy for human needs as it is easy to transport and store and can offer high efficiency. In 2012, electricity powered 18% of our energy needs [2] encompassing lighting, heavy machinery, computer technology, climate control systems, transportation and so on. This share is expected to rise to 40% by 2050. PV cells have the potential to directly meet this section of our energy demand.

world_e_consumption
Image courtesy: IEA.org [2]
PV cells have come a long way from the first practical cell in 1954 [3]. Research was boosted by space applications and developments in the semiconductor industry. Commercially available multi-crystalline Si solar cell now record an efficiency of 14%-19%, and encompass 90% of the PV market. The theoretical limit on efficiency (Schokley-Queisser limit) for crystalline Si lies at 29.4%., but it can be exceeded with the use of multi-junctions, light concentrations and several other techniques. The figure below is one of the most widely used progress charts for different PV technologies. The classification is in five broad categories: Multijunction cells, single-junction GaAs, Crystalline Si cells, thin-film technologies and emerging PV technologies. The highest efficiency achieved thus far is 46% . The focus of cell research is now shifting from power efficiency to application specific utilities such as low material wastage, flexibility, weight and production cost and ease of manufacturing.

 

PVeff(rev151218)
Image courtesy: nrel.gov[4]
Additionally, PV cells have no harmful emissions or moving parts. Lifetimes can be as along as 25 years for crystalline silicon and energy payback time (EPBT)* is as low as three years [5]. All this means that the pricing of electricity through PV is now increasingly competitive. The levelised generation cost (LCOE)** for PV systems in Germany (2013) was 78-142 euro/MWh while the cheapest source, coal, was 38-80 euro/Mwh [6]. When viewed as a cost during the construction of a building, PV modules can cost 75 euro/m2 while brick-stones cost 40-70 euro/m2 and windows around 100-300 euro/m2! [7].

Swanson’s law*** predicts that the price of solar PV modules drop 20% for every doubling of  cumulative shipped volume. In the last decade, the annual growth of cell production has been greater than 40%. Thus, if current efforts continue, we can look forward to a cleaner, greener, star-powered future.


Important definitions:
* https://en.wikipedia.org/wiki/Crystalline_silicon#Energy_payback_time
** https://en.wikipedia.org/wiki/Cost_of_electricity_by_source#Levelized_cost_of_electricity
*** https://en.wikipedia.org/wiki/Swanson%27s_law



References:
1. “Solar System Exploration: Planets: Sun: Facts & Figures”. NASA. Archived from the original on 2 January 2008.

2. 2014 Key World Energy Statistics (1st January 2016) “http://www.iea.org/publications/freepublications/publication/KeyWorld2014.pdf” 

3.”April 25, 1954: Bell Labs Demonstrates the First Practical Silicon Solar Cell”. APS News (American Physical Society) 18 (4). April 2009.

4. National renewable energy laboratory (1st January 2016) “http://www.nrel.gov/ncpv/images/efficiency_chart.jpg”;

5.Vasilis Fthenakis and Erik Alsema (2005). “Photovoltaics Energy Payback Times, Greenhouse Gas Emissions and External Costs: 2004–early 2005 Status” (PDF).clca.columbia.edu. Archived from the original on 25 March 2015.

6. “Levelized cost of electricity renewable energy technologies” (PDF). FRAUNHOFER. 2013. Retrieved 6 May 2014

7. J. Werner,International symposium on Innovative Solar Cells, Tokyo, 2014 university of Stuttgart