A Review: Organic Tandem Cells

This review discusses theoretical considerations, experimental realizations, and design parameters for organic tandem photovoltaic cells. It explores the potential of renewable energy sources and the need to shift focus from non-renewable sources. The review also highlights the advancements in solar cell technology and the efficiency of organic tandem cells. It concludes with the potential for further improvements in tandem structuring and the use of plastic solar cells in manufacturing solar panels.

A Review:
ORGANIC TANDEM CELLS
Abstract
In these review theoretical considerations trying to address the would-be of tandem photovoltaic
cells, experimental realizations, basic design considerations to ensure effective and efficient
material combinations and conclusion are presented.
1. Introduction
Over 80% of the world’s consumption of energy is provided by non-renewable sources. Non-
renewable sources of energy compose mainly the fossil fuels like coal, gas and oil. In the process
of the fossil fuels producing energy for machines, motor vehicles and industrial and domestic
heating, they produce a lot of carbon dioxide which is the major of climate change. (Choy, 2012)
(Qiao, 2017) Carbon dioxide causes greenhouse effect due to destruction of ozone layer (O3). In
addition these energy sources are increasingly becoming more and more expensive. Hence, a
need for the world to shift its focus to the renewable sources of energy has become inevitable.

1.0 figure: Showing the concentration of carbon (IV) oxide overtime until 2016 (Horstmeyer,
2011)
Renewable sources of energy have thus become a major focus of worldwide new energy
strategy. This is because they represent a viable alternative in reducing overdependence on fossil
fuels, and a long term strategy in ensuring renewable sources of energy become the major source
of energy. (Rim, 2010)
There are various sources of renewable energy among the wind power, biomass, solar energy and
fuel cells. We know that renewable sources of energy have their own shortcomings and their
dependence, and total overhaul of the energy strategy won’t solve all environmental problem and
may not stabilize world climate. (Soga, 2006) But renewable energies represent viable options to
solving the present and foreseeable energy crises. The earth receives huge amounts of energy
from the sun that is approximated to 4 yottaJoules. Since the world consumption in 2013 was
0.000567 yottaJoules, it means that our consumption is dwarfed by the energy received from the
sun. Thus earth receives enough energy to fulfil world demand.
Due to increased exploration of renewable energies viability coupled with harming effects of
fossil fuels our attention has shifted to carbon free energy, photovoltaic technology. (Choy,
2012) In recent years a number of publications and journals have detailed the great strides
scientists and researchers have done in dealing with solar electricity and solar cell. (Choy, 2012)
The advancements in technology of solar cells increased steadily starting in the 1980s.This was
possibly caused by the crisis in oil industry that occurred in 1973. (Chen, 2011) (Formann, 2016)
The latest stead rise in the production of solar electricity and PV industry was witnessed in the
last decade. (Hsin-Fei, 2013)This rise might have been triggered by changing climatic conditions
caused by global warming. Since 2006 the solar technology has been advancing each year at the
rate of at least 40%.
Organic solar cells (OSC) are photovoltaic devices working on knowledge based on organic
semiconductors. OSC are still in developmental stage but are predicted to become major
candidate for PV industries. Examples include organic tandem cells and single junction cells.
Some of th e factors which favour them include ease of processing, mechanical flexibility and

low cost of large scale production
2.0 figure: showing production of PV cells since 2006 (World, 2019)
The graph above shows the impressive figures posted and recorded for years starting 2006 up to
2013. They show how the production of PV cells has been evolving greatly.
Our aim we aim to review current trends regarding organic tandem photovoltaic cells, existing as
well as discuss major experimental calculations and results and later on make some design
parameters for effective and efficient material combinations for manufacture of organic tandem
cells. (Bisquert, 2017)
2. Theoretical considerations
Organic Tandem Cells
In 1961, Shockley and Quieisser did some calculations in trying to evaluate thermodynamics
efficiency conversion of a solar energy cell. The parameters that they took into account are:
 Energy from Photons is bigger than a band packet of the proactive materials which are
absorbed and lead to contribution to conversion in photovoltaic. (Ikhmayies, 2018)
 Creation of carriers which are Hot on photon absorption decrease due to band conduction
of photoactive substances; this is known as thermalization. (Tress, 2014)
Thus the maximum efficiency is given by;
h ( EG ) =¼ JSC (EG )× VOC(EG) × FF
But Voc observed in a device differs by 0.3eV due to losses. As such Voc obeys the empirical
formula of equation;

Voc=(¼1 e ( EDonorHOMO ) − ( EAcceptorLUMO ) )−0 :3
This is the dissimilarity between the Lower Unoccupied Molecular Orbital (LUMO) (Hui Huang,
Jinsong Huang, 2014) and the Highest Occupied Molecular Orbital minus the loss.
3.0 figure: showing tandem perovskite solar cell (Bush,
2018)
The tandem cell performance was extensively observed and analysed through De Vos. The
author showed that by assembling numerous series sub-cells will allow effectiveness which
surpass Shockley and Limitation of Queisser. (Zang, 2011) This is possible if one enhances the
potential of electrochemical carrier charge extraction. (Hsin-Fei, 2013) T (Qiao, 2017)he
efficiency maximum a single organic cell such as tandem is around 40% and 49% of a tandem
consisting three band gaps with sub-cells of 1.9eV. Under the maximum possible light exposure
the efficiencies for two and three cells are 55% and 64 % respectively. (Qiao, 2017)
In another case of the tandem cell, De Vos approach tackles two other problems; the poor
mobility in charge carrier that can reduce the distance the carriers cover which in turn hinders
formation of a layer that can absorb highest light rays, and the type of light absorption in the
molecular substances which differs with that of the effective spectrum in inorganic
semiconductors. (Vayssieres, 2010)

.
4.0 figure .Atandem cell showing band diagram of (Bush, 2018)
The figure above shows a band diagram of a PV cell. It shows, the voltage crosswise of the
device is usually equal to the total of each sub device voltage across in the above connection.
(Bisquert, 2017)It means that;
VOC 1+VOC 2+VOC 3=VOCtandem
However, the current of the tandem cell relies on the fill factor (FF) of the corresponding
devices.
Efforts have been made in recent times with a sole purpose of producing solution processed
tandem solar cell. These are the strides being in ensuring that we start to mass produce organic
tandem cells. (Zang, 2011)
3. Design Rules
For us to evaluate the potential effective of the tandem solar cells that uses six variables; EQE1,
EQE2, ELUMO1, ELUMO2, EG1 and EG2 which are External Quantum Efficiency, gap band
of the donor substance and the LUMO levels.
We start with calculating R which is the increase of tandem efficiency as opposed to the best for
one cell;
R=(Emaxtandem−Max (Emaxbottom ; Emaxtop))÷( Max ( Emaxbottom ; Emaxtop ) )
The first case usually considered is of cell tandem having a band gap polymer device known as
P3HT. This device when blended with PCBM it can deliver a potential of over 4.5%. Note that

the top cell of a tandem can yield high performances alone and combining the 4.55 two devices
can allow 6.5 solar cell fabrication. (Qiao, 2017)
The second type of case is of a made up oftandem of PCPDTBT:PCBM which is the top cell
which is intergrated with a different bottom cell. Observations made show that by combining the
tandem cell with other cell can yield up to 40% efficiency. (Kumar, 2016)
5.0 figure: showing (Researchgate, 2019)
In conclusion, combining two or more devices which are in series do not always drive forward
the cell potential individually in the combination. (Hsin-Fei, 2013)But when donor substances in
solar cells which are organic can produce performances below the expectations, organic tandem
Photovoltaic cells can be preferable. (Rim, 2010)
4. Conclusion
Considerations in regard to various parameters and various research materials shows that when
trying to design organic solar cells one can combine two or additional singular cells by way of a
tandem structure. (Nelson, 2007)This also involves different approaches such as solution
processed approaches and using small molecule evaporated materials. (Formann, 2016)
Some of the most efficient organic cells have over 6.0% power conversion efficiency. And from
calculations worked out for tandem cells, 15%maximum efficiency can be achieved for
optimized materials. (Ikhmayies, 2018)

And it is hoped that further improvements will be made in tandem structuring which will enable
eventual use of plastic solar cells in manufacture of solar panels. (Soga, 2006)
References
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London: CRC Press.
Bisquert, J., 2017. The Physics of Solar Cells: Perovskites, Organics, and Photovoltaic Fundamentals.
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Researchgate, 2019. percentage increase of efficiency of tandem cell. [Online]
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