Bulk Heterojunction Solar Cells with
Internal Quantum Efficiency Approaching 100%
Sung Heum Park, Anshuman Roy, Serge Beaupré, Shinuk Cho, Nelson Coates, Ji Sun Moon, Daniel Moses, Mario Leclerc, Kwanghee Lee and Alan J. Heeger
1.Temperature dependence of device
Figure S1 shows the current density versus voltage characteristics (J-V) of a device after being annealed at different temperatures. When we anneal the devices at 100o C and 120o C, the performance decreases with reduced fill factor, short-circuit current and open-circuit voltage. Therefore the use of thermal annealing is eliminated as a strategy for improving device performance in our experiments.
Figure S1. Current-voltage graph of device with annealing temperature
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doi: 10.1038/nphoton.2009.69
2.The role of TiO x as a hole blocking layer in BHJ solar cell
For the PCDTBT co-polymer, we carried out J-V measurements with the cell in the dark and measured the dark current of solar cells made blends of PCDTBT:PC 70BM with and without TiO x . Figure S2 shows the “dark” current density versus bias voltage. The device with TiO x layer shows better rectification with smaller leakage current compared with the device without TiO x layer. Especially, under negative bias of -2 V , the solar cell made with TiO x shows a much smaller current density compared with solar cell made without TiO x . Since holes would be injected from the Aluminum electrode to the TiO x layer under negative bias, the smaller current density reconfirms the hole blocking effect of TiO x .
(a)(b)
Figure S2. Dark current characteristics of the device with and without TiO x layer.
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4
6
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12
C u r r e n t
D e n s i t y (m A /c m 2)Voltage (V)
1E-51E-4
1E-3
0.010.1110100C u r r e n t D e n s i t y (m A /c m 2)Voltage (V)
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doi: 10.1038/nphoton.2009.69(a: linear and b: the absolute value of current density on log scale)
3.The role of TiO x as a hole blocking layer in field effect transistor
In order to simply clarify the role of the TiO x layer as a hole blocking and electron transporting layer, bipolar field-effect transistors (BiFETs) using bulk heterojunction (BHJ) mixture comprised of PCDTBT and PC 70BM (1:4 ratio) covered with TiO x on top of the active layer were fabricated. For the device without TiO x layer,the transport curve is typical of BHJ BiFETs as shown in Figure S3.The current enhancement with negative gate bias (V gs ) indicates hole transport (p-type mode), while the current enhancement with positive V gs indicates electron transport (n-type mode).For the device with TiO x layer, however, because of the large energy barrier between the Al electrode (4.3 eV) and the top of the valance band of TiO x (8.1 eV), hole carriers are blocked by the TiO x layer; with the TiO x layer in place, there is no indication of hole injection into the p-type PCDTBT channel as a function of the applied gate bias.Furthermore, because of the small barrier at the TiO x /Al interface and the good electron transport nature of TiO x , the device with the TiO x layer exhibits even better n-type performance compared to the device without the TiO x layer.
Figure S3, FET behavior of PCDTBT:PC 70BM devices with and without TiO x .
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101010101010|
I d s
| (A )V gs (V)
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4.The current characteristics under negative bias
The 100% internal quantum efficiency (IQE) implies that every absorbed photon
results in a separated pair of charge carriers and that all photogenerated carriers are
collected at the electrode. Therefore, if IQE of device is 100%, in principle, it should
not be possible to increase the photocurrent by applying a voltage in reverse bias. Since
the IQE of our device is around 90% over a significant part of the spectrum, however, it
is still possible to obtain an enhanced photocurrent under negative bias condition due to
IQE loss of ~10%. Figure S4 (a) shows J-V characteristics from -4V to 1V. Under
negative bias, the device exhibits an enhancement in current (red-line). Because the total
light-induced current (red-line) under negative bias includes the current injected from
electrode together with the photocurrent, we simply subtract dark current (black-line)
from light current (red-line) to get photocurrent (blue-line). As shown in Figure S4 (a),
the J-V curve of the calculated photocurrent (blue line) is flatter below 0V compared
with that of light current (red-line) and becomes even flatter below -1V (see Figure S4
(b)). In order to confirm the photocurrent below 0V, we have also measured IPCE of
device. Figure S4 (c) shows the IPCE of the device under negative bias. The spectral
shape and intensity in the IPCE are slightly different with those of the device in Figure 6
(a) in text. Note that because the device is fabricated from a different batch of polymer
with higher molecular weight, we expect that the total absorption of the device referred
to in Figure S4 is higher than that of the device in Figure 5 (a) in text. The IPCE is
slightly increased under negative bias condition and is approaching saturation. Once
again, the calculated photocurrent obtained by folding the IPCE spectrum with the AM
1.5 spectrum matches the experimentally obtained photocurrent (blue line) in Figure S4
(b).
(b)(c)Figure S4, (a) Current density versus voltage (J-V) under dark (black-line) and
light (red-line), and calculated photocurrent (blue-line). (b)
Voltage (V)
C u
r r
e n t
D
e n s
i t
y
(m A
/
c
m 2)Voltage (V)
I P C E (%)
Wavelength (nm)
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Magnified J-V and (c) IPCE spectrum of device under negative bias.
5.Lifetime measurement of the device measured at NREL.
Figure S5 shows the device performance as a function of the number of days after
fabrication and encapsulation using a cover glass and UV-curing epoxy on top of the
aluminum electrode. The device performance was monitored for a week before shipping
to NREL, and showed slight degradation with a reduced power conversion efficiency
from 6.1% to 6.0%. After 10 days from the day shipped, the device performance was
measured at NREL, where the device had a certified efficiency of 5.96%. After being
returned from NREL, the device was measured again at our lab and we obtained 5.8%
efficiency. The observed decrease in power conversion efficiency is likely due to
imperfect encapsulation.
Figure S5. Device performance variation during 40 days
6.The synthesis of TiO x
The TiO x material is synthesized using sol-gel chemistry. A schematic drawing of the apparatus is shown in Figure S6 (a). Prior to usage, the three-necked flask (100 ml)
was heated at 120?C with flowing dry N2 to remove any moisture from inside flask. The
sol-gel procedure starts with the injection of the precursor, titanium(IV) isopropoxide
(Ti[OCH(CH3)2]4, Aldrich, 99.999%, 5mL), followed by injection of 2-methoxyethanol
(CH3OCH2CH2OH, Aldrich, 99.9+%, 20mL) and ethanolamine (H2NCH2CH2OH,
Aldrich, 99+%, 2mL) into the three-necked flask connected with a water condenser and
nitrogen gas inlet/outlet, one by one at room-temperature (RT). Starting materials must
be injected in this order. After one hour stirring at RT, the mixed solution was heated at
80?C for an hour (using a silicon oil bath), followed by heating to 120?C for one hour.
During all procedures, the inside of the flask must be under dry N2 atmosphere and the
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mixed solution must be stirred continuously (magnetic stirring at 600-800 rpm). After
heating at 120?C for one hour, the solution transformed to a low-density gel with dark
wine color. As a final step after cooling to room temperature, 10 ml of alcohol
(methanol, ethanol or isopropanol) was injected to extract the final transparent (see
Figure S6 (b)) TiO x sol-gel product. To apply on the device, the TiO x sol-gel product
was diluted by 1:200 in methanol (CH3OH). When the TiO x sol-gel (diluted in
methanol) is taken out into air, hydrolysis and condensation start immediately resulting
in the formation of Ti-O-Ti linkages.These reactions occur very slowly because the
alcohol solution inhibits contact of the TiO x with moisture. The hydrolysis and
condensation are significantly accelerated, however, after spin casting to form thin solid
films.
Figure S6. Preparation of the TiO x precursor solution. a. Schematic drawing of the sol-
gel processing apparatus. b.Photo of the TiO x sol-gel precursor diluted by 1:100 in
methanol (CH3OH). The solution is transparent and colorless.
7. X-ray diffraction spectrum of PCDTBT polymer
Figure S7 shows the x-ray diffraction (XRD) data of PCDTBT polymer. The XRD data
shows that PCDTBT is typically amorphous.
Figure S7. X-ray diffraction data of PCDTBT