Supporting information
Analysis of Charge Transfer and Recombination for the P3HT:PCBM Organic Solar
Cells with Iron Oxide Nanoparticles in Various Layers
Eung-Kyu Park1, Ji-Hwan Kim1, Hyeong-Jun Cho1, Dong-Hoon Lee1 and Yong-Sang Kim1,a)
Figure S1. (a) HR-TEM image of iron oxide NPs and (b) size distribution.
Figure S1 shows the HR-TEM images of iron oxide NPs obtained from Sigma Aldrich. The
average size of iron oxide nanoparticle was about 41nm.
Figure S2. J-V characteristic of organic solar cells with different concentration of iron oxide
NPs (a) active layer (b) interface layer (c) PEDOT:PSS layer
Figure S1 shows the measured J-V characteristics of organic solar cells. All the devices have
similar mechanism of surface plasmonic effect (Fig. S8(a)). This higher concentration of
Fe2O3 NPs increases the exciton generation. A blockage in hole transfer is seen even though a
high number of excitons are generated leading to decreased current density. This change in
energy level in Fe2O3 NPs devices prevents the transport of holes from P3HT:PCBM to silver
electrode.
Figure S3. Tapping mode AFM topography (top) and phase images (bottom) of P3HT:PCBM
(a, c) and P3HT:PCBM:NPs (b, d) based active layer.
The P3HT:PCBM active layer without iron oxide NPs has an RMS surface roughness of 0.37
nm and the iron oxide NPs blended active surface has a roughness of 0.41nm. The phase
images shown in Fig S3(c, d) reveal the uniform distribution of active layer and
interpenetrating networks, which are profitable for charge generation and transportation.
Figure
S4.
AFM
images
P3HT:PCBM/PEDOT:PSS
of
three
(b)
different
layers
of
iron
oxide
NPs
P3HT:PCBM:NPs/PEDOT:PSS
(a)
(c)
P3HT:PCBM/NPs/PEDOT:PSS (d) P3HT:PCBM/PEDOT:PSS:NPs.
As shown in figure S4, the P3HT:PCBM active layer without iron oxide NPs has an RMS
surface roughness of 1.66 nm and the iron oxide NPs blended active, interface and
PEDOT:PSS layer has an RMS surface roughness of 2.55, 1.19 and 5.68 nm, respectively.
Figure S5. Nyquist plots of the organic solar cells with iron oxide NPs in different layers.
(Inset: equivalent circuit model)
The Nyquist plots has a semicircle in the complex plot of the real part vs. the imaginary part
of the impedance. In general, the semicircle on Nyquist plot is associated with the charge
transfer across the electrode interface. Moreover, it is also found that the diameter size of
semicircles depends strongly on the electron transfer and surface roughness (Figure S4).
Figure S6. J-V characteristics of organic solar cells as a function of incident light intensity (a)
P3HT:PCBM (b) P3HT:PCBM:NPs (c) P3HT:PCBM/NPs (d) P3HT:PCBM/PEDOT:
PSS:NPs.
The SRH recombination due to trap states can be clearly observed at low light intensities,
where the cell efficiency becomes highly dependent on the bias and light intensity. The
charge collection probability becomes dependent on the incident light intensity. At open
circuit voltage (Voc), the photocurrent is zero and all photo generated carriers recombine
within the cell. The bimolecular recombination is the recombination of mobile electrons and
holes at the interface of the donor/acceptor heterojunction.
Figure S7. Incident photon conversion efficiency (IPCE) of inverted solar cells with iron
oxide NPs in different layers.
Fig. S7 shows the measured incident photon conversion efficiency (IPCE) (K3100,
McScience) of OSCs. All the cells show a broad IPCE spectra in the 300-700 nm range. The
device with NPs as an interface layer has the highest photon harvesting and charge collection
within the cell. The IPCE data support the electrical analysis and UV-visible data
corresponding to Fig. 4 and S8.
Figure S8. Iron oxide NPs in three different layers of UV-Visible (a) absorption spectra (b)
reflectance (c) transmittance. (d) Photoluminescence spectra with iron oxide NPs in three
different layer. (Excitation source: 495nm)).
The UV/visible (Lamda35, Perkinelmer) spectra of the pristine device and device with Fe2O3
NPs in different layers, after annealing at 160 ºC for 10 min is shown in Fig. S8 (a-c). The
black line represents the control device (ITO/ZnO/P3HT:PCBM/PEDOT:PSS) without any
mixed NPs; in comparison, device with iron oxide NPs shows more absorbance. Fig. S8((a)
inset) shows the schematic representation of light trapping through forward scattering as a
result of NPs induced LSPR. Fig. S8(b, c) shows the reflectance and transmittance spectra of
the devices fabricated with and without iron oxide NPs in device. The lower reflectance and
transmittance of the device with NPs indicates stronger absorption of the incident light.
Fig. S8(d) shows the photoluminescence (PL) measurements of the control and NPs blended
devices. The PL emission wavelengths of P3HT is 495 nm. The LSPR enhanced the
excitation rate and increased the density of photogenerated excitons in P3HT, thereby
enhancing the PL intensity. Therefore, we confirm the LSPR effect, induced by the presence
of the iron oxide NPs enhanced the light collection in the active layer of OPVs. A 40%
increase in absorbance (wavelength: 500nm) and PL (wavelength: 650nm) was obtained due
to the addition of NPs in interface layer. The optical path length increased because of the
LSPR and high-angle scattering.