1764 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 16, NO. 6, NOVEMBER/DECEMBER 2010
Charge Carrier Transport, Recombination, and
Trapping in Organic Solar Cells Studied
by Double Injection Technique
Gytis Juˇ ska, Kristijonas Geneviˇ cius, Nerijus Nekraˇ sas, and Gytis Sliauˇ zys
Abstract—In this paper, we demonstrate the possibilities of the
double injection (DoI) current transient technique for the study of
ganic solar cells (OSC). Numerically calculated DoI current tran-
sients were compared with the experimentally obtained current
transients in regioregular poly(3-hexylthiophene) (RR-P3HT) and
From coefficient of reduced Langevin recombination dependence
on the density of charge carriers in the annealed PCBM/RR-P3HT
bulk heterojunction, 2-D Langevin recombination was experimen-
tally confirmed. Trapping of the electrons was observed in the
samples of TiO2/RR-P3HT and degraded blends of PCBM/RR-
P3HT. The two injecting voltage pulses with delay between them
on charge carrier’s transport.
Index Terms—Double injection (DoI), organics, recombination,
solar cells, transport, trapping.
applications. Low mobility of charge carriers and fast bimolec-
ular Langevin recombination are typical in organic materials
and they are one of the factors limiting solar energy conver-
sion in organic solar cells (OSC). Langevin recombination is
determined by the probability for the charge carriers to meet in
the coordinate space for low mobility materials, due to small
mean-free path or hopping distance of charge carriers in com-
parison with the Coulomb radius. However, it could be reduced
by changing nanomorphology of the structure .
Charge carriers’ trapping is a limiting factor for practical ap-
plications also. The trapping is caused by the impurities and
also by disorder that could be reduced by means of technologi-
cal process. Achieving more than 6% conversion efficiency 
In this paper, we propose a convenient and simple technique
for charge carriers’ transport, trapping, and recombination stud-
ies in OSC using analysis of double injection (DoI) current
transients. There are several advantages of DoI in comparison
RGANIC semiconductors, because of the cheap and sim-
ple technology, are alternative materials for solar cell’s
Manuscript received November 12, 2009; revised January 4, 2010 and
January 20, 2010; accepted January 21, 2010. Date of publication March
1, 2010; date of current version December 3, 2010.
ported by the Lithuanian State Science and Studies Foundation under Contract
The authors are with the Department of Solid State Physics, Vilnius Uni-
versity, 10222 Vilnius, Lithuania (e-mail: email@example.com; kristijonas.
firstname.lastname@example.org; email@example.com; firstname.lastname@example.org).
Digital Object Identifier 10.1109/JSTQE.2010.2041752
This work was sup-
with other techniques: simple experimental equipment (only
pulse generator and oscilloscope are needed); it is possible
to investigate the dependence of recombination parameters on
electrical field; determination whether Langevin recombination
is dominant or reduced bimolecular is very simple; possible
to test which carriers are trapped—fast or slow; mobilities of
the electrons and holes could be determined in few ways. Nu-
merically calculated DoI current transients were compared with
the experimentally obtained current transients in heterojunction
of TiO2 and regioregular poly(3-hexylthiophene) (RR-P3HT)
and its blends with [6,6]-phenyl-C61-butyric acid methyl ester
Charge carriers injection into the material could be split and
analyzed in two cases: injection into the semiconductor, when
dielectric relaxation time is shorter than transit time of faster
charge carrier’s (τσ? ttr), and injection into the insulator,
when τσ? ttr. The case of charge carrier’s injection into
the semiconductor is applicable for relatively thick and conduc-
tive layers. The possibilities of this method for investigation of
thick organic layers are presented in –. For OSC and other
thin structures, let us analyze the injection into the insulator as
Forward direction voltage U pulse is applied to the OSC
of thickness d with hole and electron injecting electrodes and
current transients are observed. Until the moment when charge
where the current transient is the same as the space-charge-
μn+ μp. Initial current
where εε0represents dielectric permittivity.
In the case of Langevin recombination, charge carriers re-
combine completely in the interelectrode distance, so current
transients look exactly as in the case of the SCLC (see Fig. 1).
When bimolecular recombination is reduced, charge carriers
miss each other—charge is neutralized, therefore, the densities
tributions of electrons and holes densities and electrical field for
1077-260X/$26.00 © 2010 IEEE
JUˇSKA et al.: CHARGE CARRIER TRANSPORT, RECOMBINATION, AND TRAPPING IN ORGANIC SOLAR CELLS STUDIED1765
Langevin recombination and reduced Langevin recombination β = 10−3βL;
derivative of DoI current when β = 10−3βL(dashed line). The mobilities of
n?(thin solid line), and holes density p’ (thin dashed line) in the case of DoI
at the different time moments: (a) tsc—meeting time of electron and holes,
(b) tsl—transit time of slower charge carriers (holes), (c) saturated distributions
of electrical field and charge carrier densities. Calculations were made for
β/βL= 10−3and μn/μp= 10. Densities of charge carriers n?and p?were
normalized to amount of charge carriers in the electrode CU/ed.
Distributions of electrical field E (thick solid line), electrons’ density
the different time moments are shown: a) at the charge carriers
meeting time tsc; b) at the transit of slower charge carriers tsl;
and c) at the current saturation. It could be noticed that after
tsl, the distribution of electrical field remains the same and only
densities of charge carriers increase.
In an insulator after tsl, n ≈ p [see Fig. 2(c)], the continuity
dt= G −n
where β represents the bimolecular recombination coefficient.
In the case of DoI, after the transit of slower charge carriers
the rate of DoI
From (3) and (5), it follows that after the ambipolar transit
time tambuntil recombination is revealed, the charge carrier’s
density [see Fig. 2(b) and 2(c)], and therefore, current will grow
j(t) ≈e(μn+ μp)U
and due to recombination current saturates at time tr.
In the case of monomolecular recombination time tr= τ and
precise calculation gives the same dependence with the differ-
ence by the factor 7 .
In the case of reduced Langevin bimolecular recombination
(β ? βL) 
where βL= e(μn+ μp)/εε0 is the Langevin recombination
From current saturation time trand value js, it is convenient
to estimate the coefficient of bimolecular recombination
In the case when bimolecular recombination coefficient β
depends on charge carrier density β(n) = γnα, which is typical
for some OSC , 
dt= G − γn2+α
and in the same way as in 
2+ α and tr∼ U−2+ 2α
1766 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 16, NO. 6, NOVEMBER/DECEMBER 2010
are deep trapped. Density of the trapping states Ntis normalized to amount of
the charge carriers in electrode CU/ed.
Calculated DoI transients, in the case when the faster charge carriers
In Fig. 1, examples of numerically calculated DoI current
transients are presented. The diffusion was not taken into ac-
count. The continuity and Poisson equations have been used in
numerical calculations. From DoI current transient, it could be
determined whether there is a Langevin or reduced recombi-
nation: in case reduced Langevin recombination ratio js/j0is
more than 2. The recombination law and value can be estimated
from js and tr [see (7), (10), and (12)]. The values of faster
charge carrier’s mobilities can be estimated, according to (1)
and (2), and slower—from (4), where tslis estimated from dj/dt
maximum or, in the case when charge carrier’s mobilities differ
more than three times, from the time when j/jSCLC≈ 5.
In the case when the faster charge carriers are trapped in
the deep states, DoI current decreases until trapping states are
filled, and the rise of DoI current is delayed (see Fig. 3) .
Moreover, a current cusp originates, which is determined by the
transient time of the slower charge carriers. The density of trap-
ping states could be approximately estimated by the integration
of the current until it starts to rise.
In the case when slower charge carriers are trapped into the
deep trapping states, the rise of the current is also delayed, but
current decrease is absent, because it is caused by fast charge
carriers SCLC (see Fig. 4). Two voltage pulses with delay be-
tween them tU could be used for qualitative estimation of the
influence of trapping on charge carriers’ transport properties. In
the case when trapping is present, the first voltage pulse fills the
traps and, if thermal release during delay time tUis negligible,
by traps. Thermal release from the deep trapping states could
be observed by varying delay time tUbetween voltage pulses.
Sandwich-type samples were fabricated as follows: 10 Ω/sq.
indium tin oxide coated glass substrates were coated with a
top of it, a solution of P3HT:PCBM with a mass ratio 1:0.8 was
deposited by the same technique. Finally, 100 nm of aluminum
to amount of the charge carriers in electrode CU/ed.
Numerically calculated DoI transients, in the case when the slower
were thermally evaporated and the structure was encapsulated
under an inert atmosphere. In order to minimize the inuence of
contact resistance (10 Ω) to the DoI current, the sample was
made thicker than a typical solar cell d ≈ 1.2 μm. The dielectric
relaxation time in the sample was tσ> 30 μs and it was longer
than ttr< 3 μs at room temperature for the used voltages, thus
corresponding to DoI into the insulator.
Thin TiO2 films were made by ITO substrate dipping into
the solution of titanium salt. Thick TiO2films were made by
doctor blading Solaronix Ti-Nanoxide T20/SP titania paste on
ITO substrate. Samples were dried at room temperature, and
annealed at 450◦C for 15 min in the air. TiO2films were trans-
from dichlorobenzene solution on top of TiO2. Semitransparent
gold electrode was thermally evaporated and the structure was
encapsulated under inert atmosphere.
The experimental setup consisted of a digital memory oscil-
loscope and an arbitrary waveform generator.
In the current transient in pure RR-P3HT, only SCLC of in-
jected holes is observed [see Fig. 5(a)]. The mobility of holes
it corresponds to the value obtained by the time-of-flight (TOF)
technique. Since current after transit time ttris not decreasing,
the trapping of holes is not significant [compare with calculated
SCLC with trapping, see Fig. 5(b)]. It is worth noticing that
RR-P3HT used in PCBM/RR-P3HT blends and pure RR-P3HT
samples were obtained from different producers and the differ-
ence in hole mobilities in those samples could be caused by
In Fig. 6, DoI initial and saturated current volt–ampere char-
acteristics are presented in PCBM/RR-P3HT. From initial cur-
rent, using (2), the sum of mobilities’ dependence on electric
field was calculated; in Fig. 7, this dependence is shown and
compared with electrons mobility measured by TOF. The mo-
bility estimated from DoI slightly exceeds the value obtained
from TOF; the difference could be explained by the influence
JUˇSKA et al.: CHARGE CARRIER TRANSPORT, RECOMBINATION, AND TRAPPING IN ORGANIC SOLAR CELLS STUDIED1767
calculated with different trapping times and without trapping.
Monoinjection current transients (a) in RR-P3HT, (b) numerically
recombination time tron voltage in PCBM/RR-P3HT blend.
Dependencies of DoI initial current j0, saturation current js, and
of the diffusion, which was not calculated in the DoI model
or by the filling shallow trapping levels at high charge carrier
densities. It could be seen that the initial current dependence on
voltage is little bit steeper than (2), thus predicting that it is the
result of the mobility dependence on electric field.
The value of saturated current significantly exceeds SCLC
tion is reduced with respect to the typical for organic materials’
Langevin recombination. The saturated current jsdemonstrates
weaker dependence on the applied voltage than j0and satura-
tion time dependence is stronger than U−1and it shows that the
coefficient of bimolecular recombination depends on the den-
sity of charge carriers. Such recombination process is observed
after OSC annealing, when lamellar structure is formed. In ,
it was proposed that the reduction of Langevin recombination
could be determined by the 2D lamellar structure in RR-P3HT.
The spacing between lamellas evaluated from X-ray studies is
determined by TOF and DoI transient techniques.
= 1.3 μm). 1— freshly made sample; 2—degraded sample. The arrows indicate
transient time of the slower charge carriers.
DoI current transients in blend of PCBM and RR-P3HT (U = 4 V, d
l = 1.6 nm and mobility across and along the lamellar structure
differs more than 100 times . In this case, according to ,
α = 0.5, and according to Monte Carlo simulations , α =
0.43. The results obtained by different experimental techniques:
the photo generated charge extraction in a linearly increasing
voltage (photo-CELIV), integral TOF , , transient photo-
voltage and transient absorption spectroscopy  correspond
to 2-D Langevin recombination. In case α = 0.5, from (12), it
follows that tr∼ U−6/5and js∼ U9/5thus confirming the 2D
Langevin recombination process (see Fig. 6).
In freshly made and encapsulated in the inert atmosphere
sample of PCBM and RR-P3HT blend, the DoI current tran-
sient does not depend on the delay time between two voltage
pulses tU, so trapping is not observed. However, after several
months of degradation, trapping of faster carriers—electrons
appears (compare Figs. 3 and 8). Increased transient time and
reduced saturation current are also observed. Due to the trap-
ping, electron’s mobility was reduced from 10−2cm2/V.s down
to 10−3cm2/V.s, holes’ mobility from 2×10+3cm2/V.s down
to 2 × 10−4cm2/V.s. The reduction of the mobility of charge
carriers and the appearance of charge carriers trapping could be
caused by the structural changes in the sample.
In RR-P3HT structure with TiO2layer, the current transient
with long delay before sharp current rise is observed. This is
1768 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 16, NO. 6, NOVEMBER/DECEMBER 2010
TiO2layer, delay time between the first (1) and the second (2) voltage pulses
td= 100 ms, U = 4 V.
DoI current transients in the sample of RR-PH3T with highly porous
caused by the small mobility of the electrons and also by trap-
ping phenomena. In Fig. 9, an experimental example of the
response to two sequent voltage pulses is presented, and the dif-
tdelbefore the current rise are observed. The electron mobility
estimated from DoI is μn= 3 × 10−7cm2/V.s and it is in good
agreement with TOF results. Experimentally, it was observed
that js/j0? 2, which indicates reduced Langevin recombina-
tion (Fig. 1). The recombination coefficient in this structure is
many times smaller than Langevin recombination coefficient
also. From comparison of the first and second impulse delay
times tdel, the influence of deep trapping could be evaluated
(see Figs. 4 and 9).
for the investigation of the charge carriers’ mobility, recombi-
nation, and trapping in thin organic films. The 2-D Langevin re-
combination was confirmed in the blends of PCBM/RR-P3HT.
Numerical calculations demonstrated how charge carriers’ trap-
ping changes current transients; this could be used for deter-
mination as to which charge carriers are trapped: faster ones
or slower ones. It was shown experimentally that in a degra-
dated blend of PCBM/RR-P3HT, drift mobilities of electrons
and holes are reduced. The way of evaluation of the influence of
voltage pulses was presented.
The authors would like to acknowledge G. Dennler from
Konarka, Austria, and R.¨Osterbacka from ˚ Abo Akademi Uni-
versity for the supplied samples.
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Gytis Juˇ ska was born in Kaunas, Lithuania, on April
3, 1942. He received the Physicist Diploma in 1964,
the Ph.D. degree in physics in 1971, and the Habil-
itation in 1990, all from Vilnius University, Vilnius,
Lithuania, where he has been the Head of the Depart-
ment of Solid Electronics since 1999.
In 1962, he joined the physics faculty of Vilnius
University; became Docent in 1981 and Professor in
1991. He has authored or coauthored more than 300
publications and conference papers. His current re-
search interests include charge carriers’ transport in
disordered materials—mainly in thin-film solar cells.
Kristijonas Geneviˇ cius was born in Vilnius,
Lithuania, on April 3, 1974. He received the
Master’s degree in radiophysics and electronics in
1998 and the Ph.D. degree in physics in 2003 from
Vilnius University, Vilnius, where he is a Senior Re-
search Associate in the Department of Solid State
Electronics since 2006.
From 2004 to 2005, he was a Postdoctoral Re-
searcher at the Merck Chilworth Technical Center.
His current research interests include the field of
transport studies in amorphous, inorganic, and or-
ganic materials, organic solar batteries, and field effect transistors.
JUˇSKA et al.: CHARGE CARRIER TRANSPORT, RECOMBINATION, AND TRAPPING IN ORGANIC SOLAR CELLS STUDIED1769 Download full-text
Nerijus Nekraˇ sas was born in Vilnius, Lithuania, on
January 7, 1975. He received the Master’s degree in
2000 and the Ph.D. degree in physics in 2005 from
Vilnius University, Vilnius, where he is a Senior Re-
search Associate since 2006.
port in thin-film solar cells.
Gytis Sliauˇ zys was born inˇSirvintos, Lithuania,
on April 9, 1977. He received the B.S. degree in
physics in 2004 and the Ph.D. degree in technologi-
calsciences in2009fromVilniusUniversity, Vilnius,
Lithuania, where he is a Postdoctoral Researcher at
the Physics Institute since 2009.