Inverted Bulk Heterojunction (BHJ) Polymer (PCDTBT-PC70BM)
Solar Photovoltaic Technology
Yogesh Kumar Dongrea*, Sanjay
Photonics Research Laboratory (PRL), S.O.S in Electronics and Photonics ,Pt. Ravishankar Shukla University, Raipur, C.G.
[Received: 28October 2021;
Revised: 12 March 2022; Accepted:13 March 2022]
Abstract. Inverted Bulk heterojunctions (Ag/MoO3/PCDTBT-PC70BM/ZnO/ITO) Organic Solar cells, based on Organic (Polymer) materials is fabricated and characterized in this work. PCDTBT-PC70BM was synthesized by chloroform, chlorobenzene and o-dichlorobenzene (organic solvent). Surface morphology of ZnO and PCDTBT-PC70BM were studied. Bulk heterojunctions of active material are formed by the mixture of PCDTBT donor and PC70BM an acceptor in a random manner. For Sufficient transportation of charge carrier (electron and hole), hole transport (HT) and electron transport (ET) layers was deposited. ZnO is used as an ETM and synthesized by using Sol-Gel technique. MoO3 thin film deposited over the active material, enhances hole transformation because of band gap tuning with Ag and active materials. Absorbance and Photoluminescence spectra of polymer material with different organic solvents were studied and results were discussed in this work. o-dichlorobenzene enhance the absorption of PCDTBT/PC70BM. At 400 nm, 90% of sun light is absorbed, and 70% absorption is figure out in 500- 550nm wavelength. The Photo-luminescence of PCDTBT/PC70BM thin film in different organic solvents is ranging from 650nm to 750nm. At 700nm, 20% is shown for chloroform, 40% for chlorobenzene and highest 80% is achieved by o-dichlorobenzene. J-V value is obtained from a solar simulator which irradiates the sun spectrum 1.5 AM, for all the devices having cell area 0.045 cm2. For concentration (1:1) ratio in different organic solvents like chloroform, chlorobenzene and o-dichlorobenzene, (3.5, 4.2, and 5.8) %, PCE were obtained respectively.
Keywords: BHJ, ETM, HTM, Sol-Gel Techniques, PCE.
Solar energy is a
one of the best promising alternative energy sources for today’s energy. Solar
cell is a device, working on photovoltaic effect convert sun energy into
electricity. The electricity generated from solar cells is green, clean and
sustainable. Now a day 90% electricity is produced by thermal power plant where
fossil fuels including Coal, Oil and Petroleum gases are used. At the time of
production of electricity, they burn and produces harmful gases like Carbon
dioxide (CO2), acids of Sulphur (S), Carbon (C) and Nitrogen (N),
the byproduct of fossil fuels is directly affected to our Environment and Human
Perera, 2018, ]. Another issues
related to fossil fuels are its availability; it is finite to be finished
within some year
and unbalanced distribution inthe earth [IEA World energy outlook, 2017)]. Requirement
of green, clean and sustainable
energy researcher look around the solar cell. Solar cells are classified by the active material which converts light
into electricity, we have 1st
to 4th generation’s solar cells, 1st generation’s cells
are mainly based on Silicon Technology, the highest reorted cell efficiency
(single crystal cell) of ~ 28%. With the advancement of Si technology single
crystalline silicon (c-Si), multi-crystalline silicon and amorphous silicon
(a-Si) were used [Kiran Ranabhat, et al. (2016),
Martin A. Green, et al.
In Second generation solar cell, hydrogenated amorphous silicon (a-Si:H),
cadmium telluride (CdTe) as well as cpper indium diselenide, (CuInSe2)
and its related alloys like Copper Indium Gallium diselenide, CuInxGa1-xSe2
(CIGS) were used in place of Si to reduce thecost of Si solar cell, with
reportedefficiencies of CIGS and CdTe 20% and 17%, respectively [R. G. Nrel, (2010), Bagher, A, M; et al. (2015)].
Because of the high processing cost of Silicon (Si) the researcher worked for
alternatives in Silicon (Si), and the thin film solar technology was invented.
Organic/polymer and dye-sensitized solar cells were fabricated in 3rd
generation. Organic photovoltaic solar cell (OPV) technology employs
semiconducting polymers as low-cost materials, ease of fabrication and tuneable
bandgap a suitable alternative to inorganic semiconductors (silicon, CdTe and
CIGS). The efficiencies for dye-sensitized and polymer single cells using
chlorinated acceptor are relatively ~ 14% and 16% reported [Sanjay Tiwari et al. (2017), Sanjay Tiwari et al. (2017)]. Combination
of Organic and Inorganic materials, represent 4th generation’s solar
cells, which is basically dealing with perovskite materials, perovskite was
discovered in 2006 by Miyasaka and his co-workers [Kojima, et al. (2006), Kojima, et al. (2009)]. This new material has a highest reported
efficiency of ~19.3% [Zho,
et al. (2014)]. The limiting factor of this technology is
un-stability in environment and hazardous because of Pb.
Materials and Device
Bulk Heterojunction (BHJ) concept with Inverted
Architecture: Low work function metals (easily transport generated charge
carrier) like calcium (Ca), barium (Ba) or aluminum (Al) were applied as
cathodes or in between cathodes and active material [Kai Wang, et al. (2016)]. Whenever
exposure of oxygen or water from the environment is subjected to electrode
caused an almost immediate oxidation of these electrodes, resulting in a fast
degradation of the power conversion efficiency of the device. By introducing
different architecture so-called inverted design,this fast electrode
degradation could be overcome [Shaheen, S. E; et al. (2001), Green,
M. A. et al. (2011)]. The low work function metals have been
replaced by transparent oxides like zinc oxide or titanium dioxide. In inverted
device design all layers can be deposited from solution and no vacuum process
is required [Green,
M. A. et al. (2011), Hou, J. et al. (2008)]. In
bulk heterojunction concept we introduce a mixture of donor usually conjugated
polymers, oligomers or conjugated pigments, and for acceptor fullerene
derivatives were used in a specific molecular ratio (Weight) and deposited to
form a blend over the surface of the ETL. A nanometers domain sizes blend film
weredeposited because of nano meter size effects blend allowing for excitons
with short lifetimes to reach an interface and dissociate due to the large
donor-acceptor interfacial area [Waldauf,
C. et al. (2006)]. The molecular arrangement (Hydrocarbon
chain) of PCDTBT donor material is shown in figure 1(a).
arrangement (Hydrocarbon chain) of PCDTBT donor material
arrangement (Hydrocarbon chain) of PC70BM (Fullerene derivatives)
process of photon absorption and exciton generation takes place on a junction,
the donor materials is responsible for that. Once the charge carriers (electron
and holes) are generated, it is easily transported to their respective layer
like (ETL and HTL) by acceptor blend. The molecular arrangement (Hydrocarbon chain) of PC70BM
(Fullerene derivatives) acceptor material is shown in figure 1(b).
Because of blend morphology of the active layer, it is
easy to harvest all the incident photon and transport the charge into electrode
[Krebs, F.C; (2008), Zhou, Y; et al. (2012)]. Here we are
combine this Bulk heterojunctions concepts with novel inverted architecture to
increases the stability and enhancement of PCE of organic solar cell. Different
device geometries of organic solar cell are described in figure 2.
Figure 2. Different device
geometries of organic solar cell.
Mostly Bilayer and Bulk-Hetrojunction are used in organic solar cells. In bilayer acceptor and donor, semiconducting polymers are used in one-by-one manner, so here two different layer of organic materials. But in case of bulk heterojunction, acceptor and donor are mixed together and deposited in one stake, so here only a single layer of organic materials. Depending on the mixing nature bulk heterojunction are categories in either ordered and disordered. Here we are combining this Bulk Hetero-with work function of ETL material [Zhou, Y; et al. (2012)]. A very high work function material is applied in between active material and top electrode to make the HTL of the device. The difference between inverted and non-inverted architecture is understand to study the below picture. Figure 3(a) show the non-inverted and figure 4(b) the inverted architecture of organic solar cell.
Figure 3.(a) Non-inverted Architecture Figure 3.(b) Inverted Architecture
BHJ device is shown
in figure 3(a & b) both having non-inverted and inverted architecture.
The devices were fabricated, consists of layers of different materials, such as
for bottom electrode a transparent oxide of indium tin (ITO), an electron
transport layer (ETL), ZnO were deposited. As the active layer the donor
polymer/acceptor molecule blend (mixture of PCDTBT-PC70BM), for hole
transport layer MoO3 and Ag for top electrode were used.
Band gap of materials used in device:
ITO (Indium Tin
Oxide) is used as a bottom electrode of the device. The work function of the
ITO is -4.7, which is less as compare to ETL material work function because of
work function suitability electron can beeasily move from ETL to electrodes. In
our work inorganic buffer layers of ZnO, work function -4.71 to -5.1, betweenthe
active layers and the electrode have been deposited using spin coater, which
act as electron-transport layer or hole blocking buffer layers. PCDTBT
(conjugated polymer) and PC70BM (Fullerene derivatives) are used as
an active material, which absorb light and generate charge carriers [White,
M. S; et al. (2006)].The work function of PCDTBT and
PC70BM are (-3.6 to -5.5), (-4.3 to -8.0) respectively. To enhance
the hole transportation to the top electrode here a very high work function
material (-6.7 to -9.71), MoO3 is deposited. The role of
the MoO3 in this device is, its block the transportation of
electron from active layer to top electrode and it’s provided a better surface
and good adhesion for Ag. Because of the high electrical conductivity Ag (-4.3)
is used as a top electrode of the device. Energy band gap of different
materials are shown in figure 4.
Figure 4. Energy band gaps
of different materials used in device.
Material/Chemicals used for fabrication of BHJ organic
fabrication of BHJ organic solar cells, varieties of materials were used. In
below table 3(a) we have given the details of materials and chemicals. In this
work ZnO is synthesized by Sol-Gel techniques and active material is
synthesized using different organic solvent with different concentration ratio.
1. Shows the Material/Chemicals used for fabrication of BHJ organic solar cell.
Characteristics of the materials
ITO coated glass substrate
Zinc Powder, HCl
Zinc Acetate (Dehydrate),
2 Methoxy ethanol
Device Fabrication and
Patterning, Cutting and Cleaning of ITO Coated Glass
ITO Coated Glass (L 50mm x W
50mm x T 1.1mm) having resistivity of 10 Ω/sq. from TechInstro is used as a
substrate, ITO Coated Glass were patterned by a mixture of Zinc powder from
Sigma-Aldrich and DI water. Here Kepton Tape was used for selective masking of
ITO. Tape were fixe within line without any air gap, and also the Zinc powder
solution was applied in unmasked region of substrate. Due to Chemical
reaction ITO were etched from unmasked region and proceed for cutting and
cleaning. The cutting of patterned substrate were done byDiamond based cutter
system with measurement unit, here 16 samples each having dimensions (L 12 mm x W 8 mm x T 1.1 mm) are made from
single substrate, so for device fabrication, we have 16 patterned substrate.
The patterned Substrate was cleaned by using DI Water, Soap solution, Acetone
and finally IPA followed one by one each for 10 Minutes in Ultra-sonicator, this cleaning process
makes chemically stable and contamination free epitaxial surfaces for the
subsequent fabrication process. After solution based cleaning, substrate were
proceeded for Plasma Ashing, where Oxygen
plasma (~ 10 minutes) is used to remove organic materials impurity and making
the surface more pure. Table 3(b) shows the processing of substrate using
pictures of each process.
Synthesization and deposition of ZnO as an ETL
Here an electron transport
layer (ETL) is deposited in between ITO and active materials, the purpose of an
electron transport layer (or of a hole blocking layer) between the active layer
and the cathode is to reduce the recombination of the free charge carriers
(electrons and holes) and transport electron easily to cathode. Zinc oxide
(ZnO) is used as an ETL materials because of work function suitability with
both active materials and cathode interlayer and also better surface
compatibility with both the active layer and the cathode, this causes ZnO
leading to less surface defects and more efficient electron transport towards
the cathode to more efficient electron extraction [Radzimska, A. K; et al. (2014)]. Apart from this the conductive nature,
transparent, and ease fabrication using spin coater as thin films (nm regime)
and generally insoluble in common organic solvents used for polymer active
layer is made ZnO has suitable candidate for ETL, here ZnO were synthesized
using sol-gel method.
synthetization of ZnO (Zinc Oxide) and deposition using LAF spin coater
ETL (Electron Transport Layer) of ZnO (Zinc Oxide)
in paste form has synthesized using Sol-Gel Techniques. Zinc Acetate
(Dehydrate) Zn(CH3COO)2. 2H2O (ZAD, 0.5 M)
(98%), Ethanolamine NH2CH2CH2OH (MEA, 0.5 M)
(98%) and 2 Methoxy ethanol anhydrous C3H8O2 (2-ME) (99.8%), all the materials were supplied
from Sigma-Aldrich. Materials were mixed according to their molecular weight. In this
chemical reactions, a base material Ethanolamine (MEA) act as a stabilizer, and initiate the growth of ZnO,
by increasing the PH value of chemical mixture[Radzimska, A. K; et al. (2014)]. Solvent 2-ME was taken in a glass bottle, and then
Zinc Acetate precursor solutions were mixed followed by Ethanolamine in appropriate
amount and after steering of 1 hour we found a transparent ZnO solution.
Further prepared ZnO solution
was applied to cleaned ITO coated glass for thin film coating, here laminar air
flow (LAF) Programmable Spin coater was used for 60 seconds at 2000 rpm. After
deposition of ZnO (~ 15 nm, thin), samples were proceeding for annealing at 2000C
for 10 minutes at hot plate.
Process of preparation and deposition of ZnO is figure out in a table 3(c).
2. Process of preparation and deposition of ZnO
Figure 5.(a) Cleaned ITO Coated Glass before ZnO
deposition, 6(b) Prepared ZnOsolution
and 6(c) ITO Coated Glass, after
Organic active material (PCDTBT/PC70BM)
preparation with organic solvent and deposition
PCDTBT conjugated polymers and PC70BM
fullerene derivatives are used as an active layer material. The precursor
mixture solutions of the PCDTBT and PC70BM were prepared with
organic solvent (3 wt%) chloroform, dichlorobenzene and o-dichlorobenzene with
a ratio of 1:1:1 respectively, Organic chemicals were supplied from Sigma-Aldrich. The weight
measurement has done in Weight measurement machine and for liquid we have used
a syringe, all the materials were mixed according to their molecular weight and kept for 12 hours steering for proper mixing at 600Ctemperature.After
that it has spin coated on the ZnO coated substrate at 1000 rpm for 1 minute
and annealed at 130°C for 10 minutes, the thickness of deposit PCDTBT/PC70BM
is around 120 nm, all the procedures were done in Glove Box. Synthetization of
PCDTBT/PC70BM with their appropriate organic solvent (o-dichlorobenzene,
dichlorobenzene and Chloroform) and deposition over the surface of ZnO.
Figure 6. (a) ITO Coated Glass after ZnO deposition,
6(b) PCDTBT and PC70BM
Mixture and 6(c) Deposited thin film of
PCDTBT and PC70BM over ZnO surface
Hole Transport (HT) material and Top Electrode Silver (Ag)
MoO3 thin film (approximately 12 nm) were
deposited over the active material using Thermal Evaporator at deposition rate
0.1Å/Sec, here Steel mask is used for selective evaporation. High transmittance
in the visible region, excellent ambient stability and very high work function
(5.3, 5.68, and 6.86 eV) tuning with PCDTBT/PC70BM we were used MoO3
for HTL material.After
deposition of HTL, we have deposited Silver (Ag) electrode (100 nm thin)
using the same mask at 0.5 Å/sec deposition rate.
Figure 7. (a) Pictures of Steel mask for one
(1) sample containing eight (8) devices and (b) Deposited thin film of Ag over
MoO3, over the PCDTBT and PC70BM.
Legging and Encapsulation of the device
One of the major challenges associated with the micro devices is there electrical interconnects, in this work one sample containing eight devices, and contacts are associated in bottom and top only. Aluminium (Al) legs were used to connect both the top (Ag) and bottom (ITO) contact. Proper matching of Al legs to ITO and Ag is required otherwise devices are short or no connection. Once the legging done epoxy (Glue) is used to stick the device with transparent glass for encapsulation of devices.
Figure 8.(a & b)
pictures of aluminium legs with fixed point and connector for top contact (Ag)
and bottom contact (ITO), with fabricated devices Top View, legs on the
In this work Zeta 3D
Microscope was used to study the deposited surface of ZnO and Active materials,
because for fabrication and better performance of the devices, we require
smooth and uniform deposited surface. Here ZnO surface is same for the entire different
layer. From the microscopic study ZnO is deposited very uniformly over the ITO,
but the variation is observed in PCDTBT/PC70BM layer, due to
variation in concentration. Figure
9.(a) ZnO surface, deposited on the ITO coated glass, (b) 3D View of ZnO surface,
deposited on the ITO coated glass, (c) Surface of PCDTBT/PC70BM in
chloroform over ZnO, (d) 3D View of PCDTBT/PC70BM chloroform Surface over ZnO, (e) Surface of
PCDTBT/PC70BM in chloroform over ZnO, (f) 3D View of PCDTBT/PC70BM chloroform Surface over ZnO, (g) Surface of
PCDTBT/PC70BM in o-dichlorobenzene over ZnO, (h) 3D View
of PCDTBT/PC70BM Surface in o-dichlorobenzene over ZnO.
Absorption Curve with Different Organic Solvents
The absorbance of PCDTBT/PC70BM thin film deposited
in different organic solvents are shown in below figure 10 (a), it shows
o-dichlorobenzene enhance the absorption of PCDTBT/PC70BM. At 400
nm, 90% of sun light is absorbed, and 70% absorption is figure out in 500-
550nm wavelength. For chloroform and chlorobenzene absorption wavelength are
same but the absorption coefficient is low respectively (80, 70) at 400nm and
(60, 50) at 500- 550nm.
Figure 10.(a) PCDTBT/PC70BM Absorption Curve
PCDTBT/PC70BM Photoluminescence curve with
different organic solvents
The luminescences of PCDTBT/PC70BM thin
film in different organic solvents are ranging from 650nm to 750nm. At 700nm,
20% is shown for chloroform, 40% for chlorobenzene and highest 80% is achieved
by o-dichlorobenzene. Figure 10.(b) PCDTBT/PC70BM
Characterization of Devices
J-V value is obtained from a solar simulator which
irradiates the sun spectrum 1.5 AM, for all the devices having cell area 0.045
cm2. Both the value of Voc and Jsc tends to increase the fill factor
of the devices which leads to increase the overall power conversion efficiency
of the devices. For concentration (1:1) ratio in different organic solvents
like chloroform, chlorobenzene and o-dichlorobenzene, (3.5, 4.2, and 5.8) %,
PCE were obtained respectively. The device performance data is calculated from
J-V curve obtained by Solar Simulator. The optimal thickness of ZnO is 15 nm
and MoO3 12 nm. This deviceshows a maximum PCE 5.8%, with a short
circuit current Jsc 8.9mA cm-2, an open-circuit voltage Voc = 0.86
V, and a fill factor FF = 0.57.Figure 11.Plot between short circuit current Vs voltage
Internal quantum efficiency (IQE) and external quantum
efficiency (EQE) of PCDTBT/PC70BM
The Internal and External quantum efficiency of PCDTBT/PC70BM polymer solar cell
is calculated from the solar simulator data (J-V curve), absorption and
photoluminescence spectra of PCDTBT/PC70BM material and shown in
figure 12 (a&b).Figure 12. (a) IQE of PCDTBT/PC70BM Figure 12.(b) EQE of PCDTBT/PC70BM
Results and Conclusion
Here, we have successfully fabricated and
characterized an inverted PCDTBT/PC70BM organic solar cell device in
order to understand the effects of different organic solvents. Results suggest
that the power conversion efficiency is better with o-dichlorobenzene (around
5.8%) as compared to chlorobenzene and chloroform. Here HTL and ETL materials
(MoO3 and ZnO) were used to enhance the charge transportation of the device.
Incident photon absorption (photon trapping) is increased by using Inverted
architecture. Apart from this cleaning and softly handling of the process is
also required for efficiency enhancement.
The author would like to thank NCPRE-PUMP
(INUP), IIT Bombay for their support (Materials and Instruments) and research
platform provided to me. The Fabrication work is done in Nano-Electronics lab
IIT Bombay and PRL, Raipur. I am very much, thank full to my supervisor Prof.
Sanjay Tiwari to provide me an opportunity to work at IIT Bombay and Photonics
Research Laboratory, Raipur, and also their moral and technical support for completion
Perera, (2018). Pollution from Fossil-Fuel Combustion is the Leading
Environmental Threat to Global Pediatric Health and Equity: Solutions Exist,
Int. J. Environ. Res. Public Health, 15(1):16.
IEA. World energy outlook (2017).
Ranabhat, et al. (2016). An introduction to solar cell technology, Journal of
Applied Engineering Science, 14(2016)4, 405, 481 – 491.
A. Green, et al. (2014). Solar cell efﬁciency tables (version 44),
Progress In Photovoltaics: Research and Applications Prog. Photovolt: Res.
R. G. Nrel, AUGUST (2010)
Renewable Energy Data Book, 2010, pp. 1–132. August 2010.
A, M; et al. (2015). Types of Solar Cells and Application American Journal of
Optics and Photonics. Volume
3, Issue 5, October 2015, Pages: 94-113.
Tiwari, J. V. Yakhmi, Sue Carter and J. Campbell Scott, 2017, Handbook of Ecomaterials, “Optimization of Bulk Heterojunction Organic
Photovoltaic Devices”, published by Nature Springer, pp 1103-1138.
Tiwari, J. V. Yakhmi, Sue Carter and J. Campbell Scott, 2017, “Advances in
polymer based photovoltaic cells: Review of pioneering Materials, Design and
Device Physics”, Handbook of Ecomaterials,
published by Nature Springer.
et al. (2006). Novel photoelectrochemical cell with mesoscopic electrodes
sensitized by lead-halide compounds (2). ECS Meeting Abstracts, Volume
MA2007-02, B8 - Next Generation
Photovoltaics and Photoelectrochemistry
et al. (2009). Organometal halide perovskites as visible-light sensitizers for
photovoltaic cells, J. Am. Chem. Soc., vol. 131, pp. 6050–6051.
Zho, et al. (2014).
Interface engineering of highly efficient provskite solar cells, Science (80),
Kai Wang, et al. (2016). Inverted organic photovoltaic cells, Chemical
Society Review, 45, 2937-2975
Shaheen, S. E; et al.
(2001). 2.5% Efficient organic plastic solar cells, Appl. Phys. Lett. 78,
Green, M. A. et al. (2011).
Solar cell efficiency tables (version 37), Prog Photovolt Res Appl, https://doi.org/10.1002/pip.1088.
Hou, J. et al. (2008). J.
Am. Chem. Soc.130, 48, 16144–16145.
C. et al. (2006). Highly efficient inverted organic photovoltaics using
solution based titanium oxide as electron selective contact. Appl Phys Lett. 89,
F.C; (2008). Air stable polyer photovoltaics based on a process free from
vacuum steps and fullerenes. Solar Energy Mater Solar Cells Vol. 92, No.
7, 2008, pp. 715-726. doi:10.1016/j.solmat.2008.01.013.
Y; et al. (2012). A universal method to produce low work function electrodes
for organic electronics, Science 2012 Apr 20; 336(6079): 327-32. doi: 10.1126/science.1218829.
M. S; et al. (2006). Inverted bulk heterojunction organic photovoltaic device
using a solution-derived ZnO underlayer, Appl. Phys. Lett. 89,
A. K; et al. (2014). Zinc Oxide from Synthesis to Application: A Review”,
Materials, Materials, 7(4), 2833-2881.