Supplementary Materials
Self-organized Phase Segregation between Inorganic Nanocrystals and PC60BM for Hybrid HighEfficiency Bulk Heterojunction Photovoltaic Cells
Sai-Wing Tsang,† Huiying Fu, † Jianying Ouyang,§ Yanguang Zhang, † Kui Yu, § Jianping Lu, †,* and Ye
Tao†,*
†
*
Institute for Microstructural Sciences (IMS), §Steacie Institute for Molecular Sciences (SIMS), National
Research Council of Canada, Ottawa, Ontario, Canada K1A 0R6
To whom correspondence should be addressed: email, Ye.Tao@nrc-cnrc.gc.ca ; Jainping.Lu@nrc-
cnrc.gc.ca
1
1. Synthesis of nanocrystals:
PbO (222.8 mg, 1 mmol), OA (5.630 g, 20.0 mmol) and ODE (40.0 ml) were loaded into a 100
ml three-necked flask. The mixture was degassed and heated up to 100 °C for 1 h under vacuum to form
lead oleate and to dry the solution. The solution was cooled down to 65 °C and (TMS)2S (105 μl, 0.5
mmol) in ODE (2.5 ml) was swiftly injected into the flask under vigorous stirring with the protection of
dry nitrogen. The reaction mixture was kept at 64-65 °C for 11 min and quenched by pouring the reaction
solution into cold anhydrous 2-propanol (100 ml) in a dry-ice bath. The products were centrifuged and a
mixture of PbS nanocrystals and unreacted lead oleate was obtained. A 5 ml of anhydrous hexanes was
added to the mixture to dissolve the nanocrystals. The undissolved lead oleate was removed by
centrifuging at 4000 rpm for 3 min. The nanocrystals were precipitated by adding 5 ml of acetone which
was dried with molecular sieves into the obtained black hexanes solution. The PbS nanocrystals were
collected by flocculating the materials with a mixture of acetone and hexanes (1:1) and centrifuging twice.
The nanocrystals were dried with a stream of nitrogen and dissolved in TCE for photophysical
characterization. Figures S1 (a) shows the absorption spectrum of the PbS NCs in the solution used for
this study. The first excitonic transition peak E1 is at 1.24 eV. Figure S1 (b) and (c) show the TEM images
of a monolayer of PbS NCs before and after cross-linking respectively. The size of the NCs is around 3.5
nm. The inter-NC spacing is reduced from around 3 nm to 0.6 nm after cross-linking which is consistent
with the sizes of OA and BDT molecules.
2
Figure S1. (a) Optical absorption spectrum of the PbS-OA NCs in solution, the first excitonic transition
peak E1 is located at 1.24 eV. The TEM images of a monolayer of PbS NCs (b) before and (c) after crosslinking. The scale bar is 50 nm.
2. Thin film characterizations and preparation:
The transmission electron microscopy (TEM) samples were prepared with a similar fashion to the
device fabrication. A monolayer of PbS-OA NCs was spun on a copper grid. The grid was then immersed
into the cross-linker solution for surface ligand exchange and NC bridging processes. The NCs were
probed by a JEOL JEM-2100F field emission source TEM operating at 200kV. The cross section images
3
of the devices were probed by a Hitachi S-4700 field emission scanning electron microscope (FE-SEM)
with a 3 kV acceleration voltage. Prior loading to the SEM station, a thin layer of Pt was deposited onto
the sample, and then a trench was sputtered with a focus ion beam. The morphology and phase images of
the composite films were determined by an atomic force microscope (AFM) in tapping mode, where
indium-tin-oxide (ITO) coated glasses were used as substrates. Details of the film preparation are
discussed below.
3. Device measurement
The photovoltaic responses were characterized under one sun of simulated Air Mass 1.5 Global
(AM 1.5G) solar illumination (100 mWcm-2, Sciencetech Inc., SF150) at room temperature in a nitrogen
filled glovebox. The light intensity was calibrated with a power meter (Gentec Inc., UP19K-30H-H5), the
detector was covered with an aperture similar to the device active area. In calculating the PCE, the short
circuit was further calibrated using the external quantum efficiency (EQE) data. After the J-V
measurement, the sample was exposed in air for the EQE and reflectance measurements. The film
thicknesses were measured by a Dektak profilometer.
4. Size dependent NC photovoltaic cells:
Figure S2 shows power conversion efficiency (PCE) of Schottky type photovoltaic cells
fabricated with different sizes of PbS NCs under one sun of simulated Air Mass 1.5 Global (AM 1.5G)
solar irradiation. The first excitonic transition peaks E1 of NCs varies from 0.8 eV to 1.5 eV and the
devices have a general structure of ITO/PbS-BDT (90-120 nm) / LiF (1 nm) / Al. Details of the
preparation of cross-linked PbS-BDT layer is described in the experimental section in the manuscript. For
each NC size, the device is optimized with the thickness of the cross-linked NC layer from 70 nm to 150
nm. As shown in Figure S2, the PCE is strongly depended on the size of NCs. The PCE is over 3.0 % for
E1 in a range of 1.1 eV to 1.3 eV and a maximum of 3.5 % is obtained at 1.15 eV. The optimal bandgap
4
obtained in the PbS nanocrystal PV cells agrees with that predicted from conventional bulk
semiconductors. The size dependent nanocrystal PV cell performance is also possibly due to the quantum
size effect on the optoelectonic properties of the nanocrystals. Details of the analysis are out of the scope
of this manuscript and will be discussed elsewhere.
Figure S2. Power conversion efficiency (PCE) of Schottky type photovoltaic cells fabricated with
different sizes of PbS NCs under one sun of simulated Air Mass 1.5 Global (AM 1.5G) solar irradiation.
E1 represents the first excitonic transition peak of the NCs measured in solution.
Figure S3 shows the Schottky type PV response with the PbS NCs used to fabricate the hybrid
heterojunction devices as presented in the manuscript. Figure S3a shows the current density – voltage (JV) characteristic of the device under one sun of simulated AM 1.5G solar irradiation. The device has a
short circuit current (JSC) of 10.52 mA/cm2, an open circuit voltage (VOC) of 0.56 V, a fill-factor (FF) of
53 %, a series resistance (RS) in dark condition of 8.2 cm2, and therefore an overall PCE of 3.1% is
5
obtained from J - V measurement. After calibrated with the EQE data as shown in Figure S3b, the PCE of
the device is 3.2%.
Figure S3. (a) Current density – voltage (J - V) characteristic of a Schottky type PV cell ITO / PbS-BDT
(110 nm) / LiF(1nm) / Al under one sun of simulated AM 1.5G solar irradiation. (b) The measured
external quantum efficiency (EQE) of the device.
It is worth noting that the bilayer heterojunction device fabricated with same batch of NCs
presented in the manuscript has a lower short circuit current (JSC) than the Schottky type device. Although
it has been shown that there is substantial charge transfer between the PbS NCs and PC60BM,[Ref. 8] the
reduction in JSC of the bilayer device suggests a weaker depletion region in the PbS-BDT/PC60BM/LiF:Al
device. Similar phenomenon is observed in other batches of NCs with PC60BM. It suggests that high
efficiency in the hybrid heterojunction device can be achieved with another electron acceptor material
which has better energy matching with the NCs.
6