Supplementary Material
Polymer defect states modulate open-circuit voltage in bulkheterojunction solar cells
Teresa S. Ripolles, Antonio Guerrero, and Germà Garcia-Belmonte*
Photovoltaic and Optoelectronic Devices Group, Departament de Física, Universitat
Jaume I, ES-12071 Castelló, Spain
1. Experimental method
1.1. Device fabrication
P3HT (Aldrich), PCDTBT (1-st Material), PC60BM (Nano-C, 99 %), PC70BM (Solenne, 99
%), PEDOT:PSS (CLEVIOS P Al 4083), chlorobenzene (Aldrich, ≥99.5 %), o-dichlorobenzene
(Aldrich, 99.9 %), Ca (Aldrich, 99.995 %) and Ag (Aldrich, 99.99 %) were used as received
without further purification. The active layer films were prepared from the P3HT:PC 60BM
solution (1:1 by weight) in o-dichlorobenzene (34 mg mL-1) and were stirred at 90 ºC for 2 hours
until complete dissolution. The concentration of the pure P3HT solution was 17 mg mL-1 in odichlorobenzene solvent. The PCDTBT:PC70BM active layer films with a solvents mixture
chlorobenzene:o-dichlorobenzene were prepared a ratio of 1:1which concentration were 7 mg
mL-1. All manipulations were carried out in a glove box under a nitrogen atmosphere unless
otherwise stated.
The polymer photovoltaic device in this study consists of P3HT:PC60BM blend thin films in
a sandwich structure between transparent anode indium-tin-oxide (ITO) and the selective Ca/Ag
cathode electrode. The organic solar cells devices were prepared on a glass substrate UVO
cleaned for 5 min under atmosphere ambient. PEDOT:PSS was deposited by spin coating at
5500 rpm for 30 s, then 3000 rpm for 30 s in air. The thickness of the PEDOT:PSS layer was
about 35 nm. A thermal treatment was realized at 130 ºC for 10 min in the atmosphere and this
treatment was repeated into the glove box to completely remove traces of water from the
PEDOT:PSS. The P3HT:PC60BM active layer films were deposited by spin coating in the glove
box as detailed in previous publications.1 Some samples were slowly dried as stated in the main
text. In those cases, spin coating was carried out at 900 rpm for 10 s and then substrates were
introduced into a petri dish and were covered for 2 h (Devices 1 and 2). The remaining samples
were completely dried at 3500 rpm for 2 min. These are denoted in the main text as fast drying
devices. Films were heated at the temperatures stated in the Table 1 for 10 minutes. These
annealing treatments were carried out either to remove traces of solvent (up to 90 ºC) or to
change the photoactive layer morphology (130 ºC).2 The active layer thickness of the films was
approximately 300 nm. The P3HT active layer film was spin coasted at 1000 rpm for 10 s and
was completely dried after 1 h 30 min into a close petri dish. After the dry treatment, the P3HT
film was heated into the glove box at 130 ºC for 5 min. The PCDTBT:PC70BM sample (1:1)
were spin coasted at 1000 rpm for 1 min and then annealed at 70 ºC for 30 min.3 Finally, the
cathode electrode was deposited (5 nm Ca /100 nm Ag) by thermal evaporation for all samples
at a base pressure below 3x10-6 mbar. The top Ca/Ag electrodes were then encapsulated with
epoxy and a glass slide before testing. An additional thermal treatment at 130 ºC for 10 min was
carried out for Device 2 noted as post-cathode annealing in the main text. The final device active
areas were either 0.09 or 0.25 cm2.
1.2. Characterization techniques
The optical absorption spectra of the active layers were recorded by a Cary 500 UV-Vis
Varian spectrophotometer. Absorption of Device 2 (post-cathode annealed) was measured by
peeling off the contact using adhesive tape. The crystalline structures of these active layer films
were determined using a thin film X-ray diffractometer (D4 Endeavor, Bruker-AXS) with Cu Kα
radiation. The diffraction angles used for the measurements range is 3º<2<8º for
P3HT:PC60BM films and 3º<2<30º range PCDTBT:PC70BM, respectively. Film thickness was
determined by a VEECO DEKTACK 6M Stylus Profiler. The current density-voltage
measurements were carried out under illumination using an Abet Sun 2000 solar simulator with
an air mass (AM) 1.5G filter. The simulated light intensity was adjusted to 1000 W m-2 by using
a NREL-calibrated Si solar cell. The Capacitance-Voltage data were measured using a Gamry
Instruments (Reference 3000) in dark conditions at 100 Hz for P3HT-polymer devices and at
1000 Hz for PCDTBT-polymer devices.
2. XRD Spectra
The X-ray diffraction spectra are carried out to analyze the crystallinity of the films. Fig. SI
a and b shown the spectra for P3HT:PC60BM and PCDTBT:PC70BM, respectively. The XRD
spectra of the films prepared in the configuration glass/ITO/blend by using different processing
conditions are compared. A single crystalline peak is observed for all P3HT:PC60BM samples at
about 5.4º. The corresponding lattice constant d=1.61 nm was calculated using Bragg’s law. 4
This diffraction peak represents the P3HT crystallites with a-axis orientation that corresponds
with the packing of the alkyl chains of the P3HT. On other hand, b- or c- axis orientations
would provide information of the packing of the thiophene rings (peak at ~23º found in pure
P3HT).5 However, these were not observed in our blends and for this reason the range of angles
shown in the Fig. SI a is limited to 3° < 2𝜃 < 8° . The relative diffraction intensity depends on
the processing conditions of the blends. The films prepared under controlled drying conditions
present higher intensity of the peak at 5.4° implying that the a-axis crystallites are formed with a
high crystalline volume density. The others fast dried films present a temperature-dependence.
On the one hand, the angle shifts from low 2𝜃 values for films not thermally treated to higher
values for annealed samples. As an approximation, for amorphous or semicrystalline material
the size of the P3HT crystallites L can be obtained using Scherrer’s relation
L
0.9
 2 cos 
(SI 1)
where Δ2 is the full width at half maximum of the peak, λ is the wavelength and θ is the Bragg
angle. The crystalline sizes (L) for the P3HT:PC60BM active layer films with different drying
and heating treatments are summarized in Table SI1 . These estimated values correspond to the
domain size along the a-axis. The lowest P3HT-size domain corresponds to the fast dried films
not thermally treated with values of 14 nm. According to the XRD data upon annealing the
domain size increases slightly up to about 18 nm. Still higher domain size is observed for an
annealed sample that has been slowly dried (Sample 2). These results is the combined result of
having thermal diffusion of PC60BM molecules into larger PC60BM aggregates and the
rearrangement of P3HT to improve the packing.6 Note that the crystallinity monitored by using
the XRD diffraction pattern is limited to the a-axis, packing of the alkyl chains of the P3HT
crystallites. However, the XRD diffraction patterns corroborate that P3HT:PC60BM blends
provide semicrystalline films.
Alternatively, the highly amorphous nature of PCDTBT is manifested in Fig. SI b. In this graph
the
XRD
spectra
between
3° < 2𝜃 < 30°
is
shown
for
the
configuration
glass/ITO/PCDTBT:PC70BM. Active layer sample shows only the base line indicating poor
crystallinity of the blend composed of PCDTBT:PC70BM.
Fig. SI 1 XRD spectra of a semicrystalline blend, P3HT:PC60BM (a) and amorphous blend,
PCDTBT:PC70BM (b).
Table SI1 Calculated Domain size based on the diffraction peak along the a-axis in a controlled
morphology of the P3HT:PC60BM blends.
2θ (º) L (nm)
Device
Active layer thickness (nm)
1
345
5.5
14
2
360
5.4
21
3
288
4.8
14
4
271
4.9
18
5
310
5.1
18
3. Mott-Schottky measurements
A P3HT-based solar cell is analyzed by capacitance- voltage measurements without any
further acceptor fullerene material to corroborate the polymer effect. The Mott-Schottky plot at
100 Hz and under dark conditions for the P3HT-based solar cells is shown in
Fig. SI . From this curve the number of impurities is calculated being n approximately
2.2×1016 cm-3.
Fig. SI 2. Mott Schottky plot for the P3HT-based solar cell under dark conditions and a constant
frequency at 100 Hz.
An amorphous blend is also studied here such as PCDTBT:PC70BM blend. The MottSchottky measurement is represented in Fig. SI . From this capacitance measure, the number of
impurities is calculated using the Eq. 2, 1.4×1017 cm-3.
Fig. SI 1 Mott Schottky measurement for PCDTBT:PC70BM (1:1) device under dark conditions
and a constant frequency at 1000 Hz.
References
1
A. Guerrero, T. Ripolles-Sanchis, P. P. Boix, and G. Garcia-Belmonte, Org. Electron.
13 (11), 2326 (2012).
2
Sujuan Wu, Jinhua Li, Qidong Tai, and Feng Yan, J. Phys. Chem. C 114 (49), 21873
(2010).
3
Antonio
Guerrero,
Bernhard
Dörling,
Teresa
Ripolles-Sanchis,
Mahdieh
Aghamohammadi, Esther Barrena, Mariano Campoy-Quiles, and Germà GarciaBelmonte, ACS Nano 7 (5), 4637 (2013).
4
P. Vanlaeke, A. Swinnen, I. Haeldermans, G. Vanhoyland, T. Aernouts, D. Cheyns, C.
Deibel, J. D'Haen, P. Heremans, J. Poortmans, and J. V. Manca,
Solar Energy
Materials and Solar Cells 90 (14), 2150 (2006).
5
B. F. Xue, B. Vaughan, C. H. Poh, K. B. Burke, L. Thomsen, A. Stapleton, X. J. Zhou,
G. W. Bryant, W. Belcher, and P. C. Dastoor, Journal of Physical Chemistry C 114
(37), 15797 (2010).
6
Uladzimir Zhokhavets, Tobias Erb, Gerhard Gobsch, Maher Al-Ibrahim, and Oliver
Ambacher, Chemical Physics Letters 418 (4–6), 347 (2006).