ELECTROCHEMICALLY GATED ORGANIC
PHOTOVOLTAIC WITH TUNABLE CARBON
NANOTUBE CATHODES: SUPPLEMENTAL INFO
Alexander Cook, Jonathan Yuen, and Anvar Zakhidov
Physics Department and Alan G. MacDiarmid Nanotech Institute, University of Texas at Dallas,
Richardson, TX, 75080
IV CURVES BEFORE AND AFTER IONIC LIQUID APPLICATION
Applying an ionic liquid to the CNT in our devices has a small but notable impact on the IV
characteristics, already at zero gate voltage. In Figure 1 we show that after the application of the
ionic liquid, the IV curve (red) shows some small hysteresis and a generally higher resistance, as
compared to the IV curve (black) without ionic liquid. This is a different device from that depicted
in the text of the main paper which ultimately showed less photovoltaic performance as indicated
by the IV curve (blue) at a gate voltage of 1.5V. The small increase in resistance is likely due to the
ionic liquid wetting the carbon nanotubes and partially separating CNT bundles from each other
and thus decreasing tube-to-tube contacts, leading to suppressed transport of charge carriers.
Figure 1: In this figure we see IV curves for a separate device before and after application of ionic
liquid as well as when the gate is charged to 1.5V. The red curve after application of ionic liquid is
for the zero gate voltage, i.e. at 0 V. Blue curves at 1.5 V gating show slightly hysteretic behavior
indicated by arrows.
CHARGING DATA & CALCULATIONS
Symmetric supercapacitive cells were fabricated from MWCNT and DEME-BF4 ionic liquid to
estimate the capacitance and to measure the charging characteristics in the ionically gated OPV
devices. This data was further used to estimate the quantity of power needed to charge and
maintain charge in these devices. Measurements were also taken in full devices and the results
agree well.
Current in microAmps
3.5
3
2.5
2
1.5
𝐼 = 2.2 x 10−7 x 𝑒 −2.7 𝑥 10
R2 = 0.77
1
−3
𝑥𝑡
0.5
0
0
100
200
300
Time in Seconds
400
500
600
Figure 2: shows the current versus time of a MWCNT-DEME:BF4 supercapacitive cell charged at
1.5V for ten minutes. The Fit shows that the data does not agree with the standard exponential
charging of typical capacitors.
The Current from figure 2 was integrated over time to find the total charge and then the total
charge was divided by the voltage to get the capacitance. These values are summarized in table I.
Additionally, the final current was subtracted from the earlier current values to arrive at capacitor
values less the leakage current. The equations used are depicted below. The leakage current is
about 55nA.
𝑡2
𝑛−1
𝑄 = ∫ 𝐼(𝑡) 𝑥 𝑑𝑡 ≈ ∑(𝐼𝑖+1 + 𝐼𝑖 ) 𝑥 (𝑡𝑖+1 − 𝑡𝑖 )
𝑡1
𝑖=0
𝐶=
𝑄
𝑉
𝐸 =𝑄𝑥𝑉
𝑃 = 𝐼𝑙𝑒𝑎𝑘𝑎𝑔𝑒 𝑥 𝑉
Table I: Depicts the results of the calculations outlined in the preceding formulas using the data
depicted from Figure 1.
Normal
Minus I(10min)
Charge (μC)
70.8
38.1
Capacitance (μF)
47.2
25.4
Energy (μJ)
106.2
57.2
Average Power to charge (nW)
177.0
95.3
Maintenance Power (nW)
81.99
N/A
The Electrode area is approximately 0.5cm2, therefore, the energy per unit area to charge the CNT
electrode of this cell is about 0.2mJ/cm2, or 2J/m2. AM1.5G solar light contains approximately
100mW/cm2 or 1kW/m2 of power. At 3.3% efficiency, our solar cell would generate about
3.3mW/cm2 or 33W/m2 of power, one hour of operation corresponds to: 33Wh/m2 = 33W x 3600
sec/m2 = 12kJ/m2. The time it takes the OPV to generate energy equal to that used in the initial
charging is negligible: 2J/m2 / 33W/m2 = 70ms. Secondly, we need to consider the efficiency of the
OPV less the maintenance power. In this case the efficiency would be given by:
𝑛=
𝑃𝑜𝑢𝑡
𝑃𝑖𝑛
=
𝑉𝑂𝐶 𝑥 𝐽𝑆𝐶 𝑥 𝐹𝐹
𝑃𝑆𝑜𝑙𝑎𝑟 + 𝑃𝑀𝑎𝑖𝑛𝑡𝑒𝑛𝑎𝑛𝑐𝑒
The power required to maintain charge is about 0.16uW/cm2 or 1.6mW/m2, one hour of
maintenance corresponds to: 1.6mWh = 1.6mW x 3600sec/m2 = 5.8J/m2. However, the
maintenance charging power is about 630,000 times smaller (1kW/m2 versus 1.6mW/m2) than the
power of incoming solar radiation, and ~ 21,000 times smaller than power generated by 3.3% OPV
device and therefore is inconsequential when considering the overall efficiency of OPV.
DISCHARGING
The discharging characteristics of the device were measured by physically disconnecting the device
from the power supply and measuring the Voc and Jsc (in two different measurements) as the
supercapacitor discharged. The data is show below. The devices were continuously illuminated and
measured during the discharge period though as the gate was physically disconnected, there is not
data on the decay in gate voltage. The experiment was run twice, once for Jsc and once for Voc. In
both cases the experiment was cut off when the values fell to approximately 25% of the original
value.
Figure 3: This depicts the discharge behavior of an ionic OPV device after the device’s gate electrode
is physically disconnected from the power supply.