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9-28-2012
Achieving Fill Factor Above 80% in Organic Solar
Cells by Charged Interface
Biswajit Ray
Purdue University - Main Campus, ray0@purdue.edu
Muhammad A. Alam
Purdue University, alam@purdue.edu
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Ray, Biswajit and Alam, Muhammad A., "Achieving Fill Factor Above 80% in Organic Solar Cells by Charged Interface" (2012). Birck
and NCN Publications. Paper 887.
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IEEE JOURNAL OF PHOTOVOLTAICS
1
Achieving Fill Factor Above 80% in Organic Solar
Cells by Charged Interface
Biswajit Ray, Student Member, IEEE, and Muhammad Ashraful Alam, Fellow, IEEE
Abstract—Poor carrier mobility in organic semiconductors and
high free carrier recombination at the polymer interfaces hinder efficient collection of photogenerated carriers in organic photovoltaic
(OPV) devices. This inefficiency in charge collection is reflected in
the low fill factor (FF) of the OPV cells (FFO P V ∼ 65%) compared with its inorganic counterparts (FF ∼ 85%). In this paper,
we show that the FF of the OPV devices made of the conventional
low-mobility materials can be radically improved (>80%) by introducing a “fixed charge layer” at the donor–acceptor interfaces.
We find that the fixed charges prevent free-carrier accumulation
at the interface and, hence, minimize charge loss due to interfacial recombination. We use detailed device simulation to estimate
the fixed charge density required for significant performance gain
in typical P3HT:PCBM cells. We conclude by suggesting several
strategies for introducing such charged interfaces within the conventional OPV device structure.
Index Terms—Bulk heterojunction (BHJ), fill factor (FF), interfacial charge recombination, organic photovoltaic (OPV) cell.
I. INTRODUCTION
HE efficiency of organic solar cells, which are made of
conjugated polymer (donor, D) blended with fullerene
derivatives (acceptor, A), has increased from 1% to more than
10% over the last decade (see Fig. 1) [1]–[8]. The tremendous efficiency enhancement mirrors the development of new
semiconducting polymers with lower optical gap and use of
new acceptor molecules that improve band alignment [9]. The
lower optical gap enhances the photon absorption and, hence,
the short-circuit (SC) current density JSC . Improved band alignment optimizes the HOMOD -LUMOA gap at the D–A interface,
and thereby improves the open-circuit voltage VOC . Unfortunately, despite these material innovations, the fill factor (FF)
has remained stagnated at 0.5–0.7 (see Fig. 1), much lower than
the inorganic counterparts (∼0.85) [6]. This gap suggests an
opportunity to further improve efficiency by engineering the FF
in future organic photovoltaic (OPV) designs.
The FF of a solar cell reflects the efficiency of charge collection before they recombine inside the cell. The efficacy of charge
T
Manuscript received May 22, 2012; revised July 20, 2012; accepted August
19, 2012. This work is based upon work supported as part of the Center for
Redefining Photovoltaic Efficiency Through Molecule Scale Control: an Energy
Frontier Research Center funded by the U.S. Department of Energy, Office of
Science, Office of Basic Energy Sciences under Award DE-SC0001085, and
by the Network of Computational Nanotechnology under National Science
Foundation Award EEC-0228390.
The authors are with the School of Electrical and Computer Engineering,
Purdue University, West Lafayette, IN 47906 (e-mail: biswajit.025@gmail.com;
alam@purdue.edu).
Color versions of one or more of the figures in this paper are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/JPHOTOV.2012.2216508
Fig. 1. Evolution of the performance of polymer: fullerene-based BHJ-OPV
cells. All the PV performance metrics (efficiency η, SC current density JS C ,
open-circuit voltage V O C , and FF) are plotted along with the names of the
absorbing polymers (in x-axis). Clearly, JS C and V O C have improved over the
years, but FF has not, and it remains low (<0.7), even in the high-efficiency
cells.
collection depends on three factors: the mobility of the charge
carriers, the internal electric field sweeping the carriers to their
respective contacts, and the carrier recombination rate. Since the
carrier mobility in the organic semiconductors is poor (∼10−5 –
10−3 cm2 /V·s), the charge carriers tend to pile up at the D–A
interface, especially at the maximum power condition when the
internal field in the device is low. The charge pileup leads to high
recombination loss lowering the FF. A characteristic feature of
the organic heterojunction-based devices (e.g., OPV, OLED) is
that the most of the charge recombination takes place at the
D–A interfaces because the energy barrier of the heterojunction
[see Fig. 2(b)] confines the holes to the donor and electrons to
the acceptor materials [see Fig. 2(c)]. Therefore, carriers can
recombine only at the D–A interface [see Fig. 2(d)].
The fundamental requirement for achieving high FF in
the OPV cells is to minimize the interfacial recombination.
One approach for lowering the recombination loss is the enhancement of the carrier mobility in the organic semiconductors [see Fig. 2(d)]. In principle, the mobility of the organic
semiconductors can be improved in several ways [10]–[13].
For example, Chu et al. [11] had demonstrated increased crystallinity and molecular packing (hence, higher mobility) of the
2156-3381/$31.00 © 2012 IEEE
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2
IEEE JOURNAL OF PHOTOVOLTAICS
Fig. 2. OPV operation and device physics. (a) Schematic of planar hetero-junction (PHJ) cell. The various steps in the operation of the cell are denoted by arrows.
(b) Energy band diagram with the different carrier fluxes (arrows) at the interface. (c) Carrier density [electrons (black) and holes (red)] across the device. Carriers
pile up at the interface in low-μ materials. (d) Recombination density across the device. Clearly, most of the charge recombination takes place at the interface.
Recombination is higher for low (dashed line) mobility materials. For clarity, we assume μ e = μ h = μ and L D = L A = 50 nm. All other simulation parameters
are given in Table II.
phase-segregated organic film with controlled thermal annealing. Several other techniques such as antisolvent crystallization
with inkjet printing [10], percolation doping of the polymer film
by carbon nanotubes [13]–[15], addition of small amount of additives in the active blend [12], etc., are utilized to improve the
mobility in the organic semiconducting materials. In practice,
however, the mobility-gain in the OPV materials has been limited. Moreover, the requirement of solution processing precludes
many film growth techniques that may otherwise improve the
material mobility.
The objective of this paper is to suggest an alternative approach based on “charged interface (CI)” for improving the FF
of OPV cells fabricated from the usual organic semiconductors
characterized by poor mobility. We show that if a fixed charge
layer is embedded at the D–A interface, the interfacial recombination can be suppressed dramatically, which would in turn
significantly improve the FF. Using detailed numerical simulation, we find that the fixed charges create a local electric field
at the interface, which prevents the carrier pileup and thereby
reduces the interfacial recombination. Indeed, there have been
several recent works on the tuning of the D–A interfaces by
adding a thin interlayer (ferroelectric dipole [16], [17], cascade
energy level alignment [18], [19]) in planar heterojunction OPV
cells, which have shown improved device performance. It is
shown that most of the performance gain is derived from higher
Vo c due to the increase in the cross gap at the D-A interface in
the presence of the dipole layer. The proposed concept of CI
is considerably different than these interlayer techniques as we
show that even if the D-A cross gap remains unchanged with the
CI, the performance gain comes from higher FF due to better
device electrostatics. In the following sections, we first illustrate
the physics behind the concept of CI by numerical simulation
and then quantify the charge density required for significant
performance improvement. We also discuss strategies for introducing such fixed charges at the interface of bulk heterojunction
(BHJ) OPV cells.
II. DEVICE OPERATION AND MODEL SYSTEM
Fig. 2(a) illustrates the four well-known stages of photovoltaic operation in a planar heterojunction (PHJ) OPV cell:
1) photon-induced exciton generation in organic semiconductor, 2) exciton diffusion to the D/A interface, 3) formation of
a charge transfer (CT) state, and 4) dissociation of CT state
into free electrons and holes that are subsequently swept toward
the respective electrodes by the built-in field. The device operation is further explained in Fig. 2(b) with the energy-band
diagram, where the arrows indicate the direction of various current fluxes within the cell. As evident from the band diagram,
D–A heterojunction plays several critical roles in OPV operation. For example, the heterojunction helps to dissociate excitons
into free charge carriers so that electrons are transferred to the
acceptor and holes remain in the donor [see Fig. 2(c)]. The large
energy barrier offered by the heterojunction ensures unipolar
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RAY AND ALAM: ACHIEVING FILL FACTOR ABOVE 80% IN ORGANIC SOLAR CELLS BY CHARGED INTERFACE
TABLE I
EQUATIONS FOR CARRIER TRANSPORT
carrier transport with hole and electron currents confined exclusively within donor and acceptor materials, respectively. In
other words, the carrier density in the donor is predominantly
holes and in the acceptor it is electrons. Due to the absence of
the minority carriers, recombination density in the pure donor
or acceptor phase is very low, and most of the recombination
takes place only at the D–A interface [see Fig. 2(d)].
The transport of carriers (excitons, electrons, and holes) is
modeled by the generalized drift-diffusion formalism, described
in detail in our previous publications [20]–[23]. For completeness, we offer the following summary. Optical absorption in the
different layers of the OPV cell is calculated by the transfer
matrix method (TMM) [24] based solution of Maxwell’s equations with the input of AM1.5 illumination. The materials in the
different layers of the cell are characterized by the complex refractive indices obtained from measurements [25]. It is assumed
that all the absorbed photons create excitons, which diffuse to
the D-A interface and dissociates into electron and holes at the
heterojunction. Thus, charge generation is computed only at the
D–A interfacial nodes, and its rate is determined from the excition diffusion length (Lex ) and the optical absorption profile.
The electron and hole transport inside the cell morphology is
simulated by self-consistent solution of Poisson and continuity equations. The charged carrier recombination term in the
continuity equations is implemented by the bimolecular recombination [26]. In this analysis, we assume both the donor and
the acceptor semiconductors are defect free. We claim no new
contribution to model development; rather, we use the wellcalibrated and well-tested model equations (see Table I) and
simulation parameters (see Table II) to explore the implications
of CI on the performance of organic solar cells.
TABLE II
SIMULATION PARAMETERS
3
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IEEE JOURNAL OF PHOTOVOLTAICS
charge recombination(Jrec ). Using the flux balance condition at
the interface as shown in Fig. 2(b), one can write the following:
Jq = qηex (V )Jex = Jph (V ) + Jrec (V ).
Fig. 3. Theory of photocurrent. (a) Voltage dependence of various current
fluxes [shown by arrows in Fig. 2(b)] which construct the current–voltage (J–
V) characteristics. The current components are charge generation rate at the
interface (Jq = qη e x Je x ), generation dependent recombination current Jre c ,
dark or injection current Jd a rk , and the photocurrent Jp h . In the above plot,
Jp h = |Jq | − |Jre c |, with the assumption of ηe x = 1. (b) Total current defined
as Jlig ht =Jd a rk +Jp h is plotted for low (dashed) and high (solid) mobility
materials. Clearly, a very high mobility (>10−2 cm2 /V·s) is needed to achieve
FF above 80%. The simulation parameters are given in Table II.
III. RESULTS AND DISCUSSION
To explain the role of CI on the solar cell performance, we
first develop an analytical theory of photocurrent in the organic
planar heterojunction (PHJ) cell. Then, we use the detailed numerical results to illustrate the effect of CI on the output characteristics of both PHJ and BHJ devices. As a specific illustrative
example, we calculate the charged density required for significant performance gain in the conventional P3HT: PCBM based
BHJ-OPV cells.
A. Analytical Theory of Photocurrent in Organic Photovoltaic
Photocurrent in a solar cell is defined as Jph = Jlight − Jdark ,
where Jlight is the total current under light illumination, and
Jdark is the current under dark conditions. The voltage dependence of the photocurrent in organic solar cell is generally modeled by the field-enhanced separation of CT excitons into free
carriers [27]. The free charges generated after the dissociation of
CT excitons can again recombine, further reducing the photocurrent with the applied bias. Indeed, recent experimental findings
show that charge generation is nearly field independent and the
voltage-dependence of the photocurrent (and, hence, the FF) is
essentially dictated by the recombination of the free charges at
the D–A interfaces [28]–[33]. In the following paragraph, we
model the voltage dependence of photocurrent arising from the
recombination of the free charges.
The various current fluxes operating in an organic solar cell
are shown by arrows in the energy-band diagram of PHJ-OPV
cell in Fig. 2(b). The voltage dependence for each of these current components is depicted in Fig. 3(a). The exciton diffusion
flux Jex at the D–A interface is determined by the photon absorption and the exciton diffusion length Lex . Since excitons
are charge neutral, Jex is independent of voltage. The charge
generation rate Jq depends on the CT exciton dissociation probability ηex by the bulk electric field, i.e., Jq = qηex (V )Jex . The
photocurrent Jph (V ) collected at the contact will be smaller in
magnitude than this charge generation rate due to the interfacial
(1)
The photocurrent Jph is primarily drift current, and it is
proportional to the mobility μ of the charge carriers, i.e.,
Jph ∼ qμEnI . Here, nI is the electron density (or hole density if calculated in donor side) at the interface; E is the electric field at the interface, which is approximately given by
E = (Vbi − V )/Tfilm , where Vbi is the built-in potential, and
Tfilm is the thickness of the active layer. Photocurrent calculated
at the interface remains same throughout the device since the
recombination in the bulk is negligible.Therecombination at
the interface can be written as Jrec = q τne If f Wint , where τeff
is the effective recombination time determined by the recombination mechanism (e.g., bimolecular or SRH). Equation (1) can
now be rewritten as
μτef f E
Jph (V ) =
Jq = ηrec (V )ηex (V )Jex . (2)
μτef f E + Wint
Equation (2) above clearly shows that even if the dissociation of
CT excitons is perfectly efficient (i.e., ηex = 1, which is indeed
the case as confirmed by many recent experiments [28]–[33]),
the voltage dependence in the photocurrent will still arise due
to the charge recombination factor given by ηrec . Equation (2)
also illustrates the fact that if the μτ product is high, the recombination factor would approach unity, making the photocurrent
(almost) independent of applied voltage, which will in turn result
in higher FF. With typical mobility values, however, the freecarrier recombination loss lowers the photocurrent significantly,
making the OPV FF poorer compared to other PV technologies.
A very high charge mobility (> 10−2 cm2 /V·s) is required to
achieve FF above 80%, as shown in Fig. 3(b). In the following
sections, we demonstrate that even if the material mobility is
low (∼10−4 cm2 /V·s), we can still achieve such high FF by the
concept of the CI. We find that the presence of the fixed charges
at the interface depletes the interface of free carriers and, hence,
reduces the interfacial recombination.
B. Illustration of the Concept of Charged Interface
In this section, we describe the physics behind the concept
of CI using the PHJ as the test structure. As shown in Fig. 4(a),
let us assume a layer of fixed charges with uniform density are
embedded across the D–A heterojunction. The positive part of
the charged layer needs to be in the acceptor and the negative
part in the donor side. The device I − V characteristics of such
a structure, obtained by numerical simulation, are shown in
Fig. 4(b) as a function of charge density of the dipole layer
with the layer thickness WQ = 4 nm. For the device simulation,
we assume perfect dissociation of CT excitons, i.e., ηex = 1 in
consistent with many recent experiments [28]–[33]. However,
even if ηex < 1, the CI will assist in exciton dissociation [by
enhancing the interfacial electric field, see Fig. 5(a)], and hence,
the exciton dissociation efficiency will be closer to unity in the
presence of CI. The other simulation parameters are summarized
in Table II. Fig. 4(b) clearly shows that the charge layer at the
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RAY AND ALAM: ACHIEVING FILL FACTOR ABOVE 80% IN ORGANIC SOLAR CELLS BY CHARGED INTERFACE
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Fig. 4. Effect of CI. (a) Schematic of PHJ cell with fixed charges at the
interface. Uniform charge density (Q in number/cm3 ) is assumed with the
thickness of charge layer, which is denoted as WQ . (b) Effect of CI on the
current–voltage characteristic is shown. From the plot, it is clear that the main
effect of the CI is the improvement in FF with slight improvement in JS C .
Fig. 6. Analysis of the CI concept at the maximum power point condition.
(a) Electric field profile, (b) energy band diagram, (c) carrier density, and
(d) recombination density are plotted across the device [from top electrode
(ITO) to bottom electrode (Al)] in the presence (dashed lines) and absence
(solid line) of CI.
Fig. 5. Analysis of the CI concept at the SC condition. (a) Electric field profile,
(b) energy-band diagram, (c) carrier density, and (d) recombination density are
plotted across the device [from top electrode (ITO) to bottom electrode (Al)] in
the presence (dashed lines) and absence (solid line) of CI.
interface improves the solar cell performance by significantly
increasing the FF (along with small improvement in JSC ) of the
device. The effect of CI on each of the solar cell performance
metrics is explained in details in the following paragraph.
Effect of CI on SC current: The fixed charges at the interface
alter the device electrostatics as illustrated in Fig. 5(a)–(d). All
the plots in Fig. 5 correspond to the SC condition. Fig. 5(a)
demonstrates the enhancement of electric field around the interface due to the fixed charges. The charges do not change
the optical gap or energy bandgap as shown in Fig. 5(b). However, energy bands bend slightly steeper near the interface in
the presence of the fixed charges. In Fig. 5(c), we compare the
free-carrier distribution in the presence and the absence of the
fixed charges, respectively. The enhanced electric field at the
interface pushes the free carriers away from the heterojunction
and, thus, depletes the interfacial region of free carriers. Since
free carriers mostly recombine only at the heterointerface, such
re-distribution of free carriers significantly reduces recombination loss, as shown in Fig. 5(d). It is important to point out here
that at the SC condition, the recombination loss is minimum,
because the built-in field is high enough to sweep out the photogenerated carriers. Thus, it is expected that the improvement
in JSC with CI is not very significant.
Effect of CI on FF: At the maximum power point condition,
the built-in field is much weaker than the SC condition. Thus,
recombination loss is considerably higher at this bias condition
as shown in Fig. 6(d). Any small enhancement in the interfacial
electric field will significantly reduce the free-carrier recombination and, hence, improve the performance. This is illustrated
in Fig. 6, where all the plots correspond to the maximum power
point biasing condition. The electric field [see Fig. 6(a)] is lower,
and the energy bands are flatter [see Fig. 6(b)] than the SC condition. The free-carrier density [see Fig. 6(c)] is significantly
higher especially at the interface. Similar to the SC condition,
in Fig. 6(c) and (d), we find that fixed charges deplete the interface of free carriers and, thus, reduce the recombination loss.
However, the magnitude of reduction in recombination loss is
much higher at the maximum power point, and, hence, FF is
significantly improved.
Effect of CI on open-circuit voltage: In our previous publication [22], we have shown that open-circuit voltage mainly
depends on the fundamental material properties such as the
crossgap at the heterojunction, density of states, and the
recombination coefficient (γ). Since none of these material
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6
Fig. 7. Effect of CI on the BHJ cells. (a) Typical morphology of the BHJ cell.
(b) Equivalent geometric transform for the analysis of BHJ cells (details given
in [22]). We use the same volume fraction and same average cluster size (W D )
for the equivalent transform. (c) Simplified BHJ-OPV structure used for simulation. Uniform charge layers are assumed across the D–A interface with thickness
of the charge layer W Q . (d) Effect of the CI on the I–V characteristics. The symbols in the plot are the measured J–V data for the high-efficiency P3HT: PCBM
cell [7]. For fitting the measured data, we assume μ e = 6 × 10−2 cm2 /V s and
μ h = 3 × 10−2 cm2 /V s. All other simulation parameters are in Table II.
properties is affected by the fixed charge layer (as assumed
in this analysis), we find that VOC is almost unaffected by this
CI concept. Depending on the implementation strategies, as
discussed in Section IV (e.g., heavy doping or additives), the
effective bandgap of the semiconductor might change, which
can affect the open-circuit voltage. In this analysis, however, we
only focus on the electrostatic effect of the fixed charges.
C. Charged Interface in Bulk Heterojunction -Organic
Photovoltaic
In this section, we explore the relevance of the concept of
CI for the BHJ-OPV cell. Although BHJ morphology appears
to be very complex [see Fig. 7(a)], it can be accurately analyzed with a suitable geometrical transform based on average
domain width WD as shown in Fig. 7(b). The verification and
details of this approach are described in our previous work [16].
In this paper, we explore the effect of CI on the performance
of BHJ-based OPV devices by solving the transport problem
self-consistently on the simplified BHJ structure, as shown in
Fig. 7(c). Similar to the analysis done for PHJ structure, here,
we also assume a fixed charge layer with uniform charge density
Q, and layer width WQ at the D–A interface [see Fig. 7(c)]. The
corresponding I–V characteristics are plotted in Fig. 7(d) as a
function of charge density. The symbols in the figure represent
the measured data for the current-voltage characteristics of the
P3HT:PCBM based BHJ cell. We first choose the simulation
parameters (see Table II) to fit the data and use the same set
of parameters for evaluating the effect of CI. The figure clearly
IEEE JOURNAL OF PHOTOVOLTAICS
Fig. 8. Effect of CI on the performance metrics of classical P3HT: PCBMbased BHJ-OPV. (a) Efficiency, (b) SC current, (c) open-circuit voltage, and (d)
FF. Dashed lines represent performance without charge. Symbols (circle [7],
square [8], and triangle [3]) are the experimental record efficiencies for P3HT:
PCBM-based BHJ cells.
shows significant improvement in FF (from 55% to 82%) and
slight improvement in JSC with the increasing density of the
fixed charges.
For precise quantitative estimates of the effect of the charged
layer, we plot all the four performance metrics of BHJ cell as a
function of charge density and the width of the charged layer in
Fig. 8(a)–(d). The symbols in the figure are the corresponding
performance metrics of the high-efficiency P3HT:PCBM-based
BHJ cells reported by various groups [3], [7], [8]. Fig. 8. clearly
shows that a very thin layer of charges (WQ ∼ 4 nm) with density, Qfix = 1019 /cm3 can enhance the FF of BHJ cells made
of conventional materials more than 80%. The figure, however,
shows no improvement in VOC with CI because we assume that
the charged layer only alters the device electrostatics and not the
optical gap at the interfaces. Here P3HT: PCBM system is used
as an illustrative example. The CI concept is, however, generic
and similar performance gain is expected for other OPV semiconductors if we appropriately design the fixed charge layer,
e.g. charged layer width (WQ ) and the charge density (Qf ix ).
IV. POSSIBLE STRATEGIES FOR EMBEDDING FIXED CHARGES
The buildup of fixed space charges at the interface or junction
of an electronic device is a common phenomenon, particularly
in the inorganic semiconductors. e.g., in the PN junction diodes.
Even in organic semiconductor-based devices (e.g., LED, photodiodes, etc.), the presence of the charged layer is observed
at the heterointerfaces of the organic layers, especially if one
of the semiconductors is molecularly doped [34]–[37]. From
the context of organic solar cells, recently, it has been reported
that doping of the organic layer (cyanine) improves the device
performance [38]. Nalwa et al. [40] have recently demonstrated
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RAY AND ALAM: ACHIEVING FILL FACTOR ABOVE 80% IN ORGANIC SOLAR CELLS BY CHARGED INTERFACE
improvement in FF by doping the P3HT layer with ferroelectric dipoles. They have shown localized enhancement in
electric field after doping, which causes the FF improvement.
Thus, molecular doping of the donor and acceptor layers is a
promising method for the formation of CI. The other promising approach for embedding the CI in the BHJ cells involves
mixing a small amount of appropriate additives in the absorbing
materials. For example, Chen et al. [12] has recently shown that
adding a small amount of a polymeric additive (PFLAM) in the
typical P3HT:PCBM cells improves the FF of the device. Lobez
et al. [39] has recently proposed many side chain functionalized
polythiophene additives, which improve the OPV performance.
Similarly, treating the polymers with either electrophiles or nucleophiles in the solution can introduce fixed charges in the
interfaces as discussed by Liang et al. [40]. Recently, many
groups have shown that the incorporation of a thin ferroelectric
film [16] or a surface-segregated monolayer (SSM) of fluorinated compound between the D–A layers [17] can set up an
interfacial dipole moment, which not only increases the interfacial field but alters the optical gap at the interface as well. The
fabrication technique (contact film transfer) associated with the
SSM formation is currently restricted to PHJ cells; however,
given the potential improvement in FF, a generalization of the
approach should be explored for BHJ-OPV.
V. CONCLUSION
In summary, we have demonstrated that it is possible to realize very high FF (>0.80) in low-mobility organic solar cells
by incorporating a fixed charge layer at the D–A interface. The
fixed charges at the interface create an interfacial field that depletes the interface of free carriers and, hence, reduces the carrier recombination. We have provided a quantitative estimate of
the charge density required for high-FF cells and discussed the
potential strategies for the implementation of the concept for
conventional OPV devices. In general, this paper motivates the
future OPV material design based on the formation of the dipole
layer at the D–A molecular interfaces.
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IEEE JOURNAL OF PHOTOVOLTAICS
Biswajit Ray (S’12) received the B.Tech degree in
electrical and electronics engineering from the National Institute of Technology, Trichy, India, in 2006
and the M.Sc.(Engg.) degree from the Indian Institute
of Science, Bangalore, India, in 2008. Since 2008, he
has been working toward the Ph.D. degree with the
School of Electrical and Computer Engineering, Purdue University, West Lafayette, IN.
His research interest includes semiconductor device physics and electronic transport in organic and
amorphous materials.
Mr. Ray received the Technoinventor Award in 2009 from the Indian Semiconductor Association for his master’s thesis. He received the Best Poster Award
in the area of Organic Photovoltaics at the 38th IEEE Photovoltaic Specialist
Conference, 2012.
Muhammad Ashraful Alam (M’96–SM’01–F’06)
received the Master’s and Doctoral degrees from
Clarkson University, Potsdam, NY, and Purdue University, West Lafayette, IN, in 1991 and 1995,
respectively.
He is currently a Professor of electrical and computer engineering and a University Faculty Scholar
at Purdue University, where his research and teaching focus on physics, simulation, characterization,
and technology of classical and emerging electronic
devices. From 1995 to 2001, he was with Bell Laboratories, Murray Hill, NJ, as a Member of Technical Staff in the Silicon ULSI
Research Department. From 2001 to 2003, he was a distinguished member
of technical staff at Agere Systems, Murray Hill. During his time in industry,
he made important contributions to reliability physics of electronic devices,
MOCVD crystal growth for optoelectronic integrated circuits, and performance
limits of directly modulated semiconductor lasers. After joining Purdue University in 2004, his research has broadened to include nanocomposite flexible
electronics, organic solar cells, and performance limits of nanobiosensors. He
has published over 150 papers in international journals and has presented many
invited and contributed talks at international conferences.
Dr. Alam is a Fellow of the American Physical Society and the American
Association for the Advancement of Science. He also received the 2006 IEEE
Kiyo Tomiyasu Award for contributions to device technology for communication systems.