Supporting Information
Colored ultrathin hybrid photovoltaics with high quantum efficiency
Kyu-Tae Lee†, Jae Yong Lee†, Sungyong Seo, and L. Jay Guo
†
These authors contributed equally to this work.
Department of Electrical Engineering and Computer Science,
The University of Michigan, Ann Arbor, Michigan 48109, USA
Figure S1. a) AFM images of Ag films on Ge, PTCBI, and Si substrates, b) refractive
indices of PTCBI and a-Ge, c) Absorption spectrum of wetting layers, d) Calculated
transmission spectrum of 10 nm Ag layer on wetting layers.
In terms of optical performance, the rough surface morphology can induce the scattering
that leads to broad transmission or reflection spectrum, which is typically not a desirable
property. Moreover, it is observed that the sheet resistance of the non-flat metal surface is
high, yielding the poor electrical conductivity of the electrode. In this regard, it is
important to study a surface morphology of the thin Ag film as ~20 nm of Ag layer in
anode is employed in our design. It is well-known that the thin Ag film deposited on the
germanium (Ge) shows little surface roughness, small grain-size distribution, and low
sheet resistance, which is attributed to the fact that the Ge wetting layer can reduce the
mass transportation and surface diffusion of Ag. Due to the transparency of the Ge
material in infrared (IR) regions (> 2 µm), there was no issue when depositing the thin
Ag film on Ge wetting layer. However, the Ge has a very significant absorption in the
visible ranges owing to the presence of the bandgap of Ge (0.66 eV) in the near IR.
Considering that even 1 nm of Ge can generate a large loss, there is a critical need to find
a material that can make the surface of thin Ag film smooth with an insignificant
absorption.
We performed AFM measurements on the deposited thin Ag film on PTCBI, Ge, and
silicon substrates. It is found that the optically thin Ag film on PTCBI layer can be fairly
even that is comparable to what the Ge provides as described in Figure S1a. The average
root-mean-square (RMS) value of 10 nm Ag on the silicon substrate is 8.40 nm as a
reference. On the other hand, the RMS values of surface roughness of 10 nm Ag on 2 nm
Ge and 2 nm PTCBI are 0.235 nm and 0.213 nm, respectively. This lower RMS value
implies that the scattering loss is significantly reduced.
Another important feature of the PTCBI wetting material is much lower absorption than
the absorption of Ge layer. In Equation S1, it is obvious that the light absorption is
proportional to the real (n) and imaginary parts (k) of refractive index of material. This
means that a material with higher values of complex refractive index will suffer from the
large absorption loss. The n and k values of PTCBI and Ge were measured by an
ellipsometer and demonstrated in Figure S1b. It is evident that the n and k values of Ge
material are much higher than those of PTCBI, suggesting the large light absorption of
Ge. In Figure S1c, the calculated absorption spectra in 5 nm PTCBI and Ge are
illustrated showing that the absorption of PTCBI is ~3 % whereas the absorption of Ge is
~55 % at 425 nm. Additionally, the transmission of 10 nm Ag on 5 nm PTCBI and 5 nm
Ge on glass substrates is simulated as exhibited in Figure S1d. At 400 nm, the
transmission in the case of Ge is only ~36 %, while the transmission of PTCBI case is
~81 %. As is observed in the figure, generally there is a negligible difference between 10
nm Ag films on PTCBI and glass substrates. From these characteristics (1. low surface
roughness 2. trivial absorption loss), it turns out that the PTCBI can be a great alternative
to the Ge as a wetting layer in the visible ranges.
Light Absorption 
2c 0 kn

E ( x,  )
2
(1)
Figure S2. Calculated (a-c) and measured (d-f) angular dependences for s-polarization.
We studied an angle dependence of our colored PVs for s-polarization shown in Figure
S2. 27, 18, and 10 nm of a-Si layers are used to create cyan, magenta, and yellow (CMY)
colors. Angle resolved reflection spectra were simulated (Figure S2a-c) by using a
transfer matrix method and measured (Figure S2d-f) by a variable angle spectroscopic
ellipsometer (VASE, J. A. Woollam) for angles of incidence ranging from 15° to 60°.
The simulation results show a good agreement with the experiment data. It is clear to see
that a resonance remains at the same level over large incident angles. This angle
insensitive property is due to the trivial phase shifts accumulated during the propagation
compared to the reflection phase shifts at the interface between a-Si and metal.
Table S1. Summary of power conversion efficiency of each colored cell.
Cyan device
Magenta device
Yellow device
Efficiency (%)
1.92
2.80
2.36
Jsc (mA cm-2)
4.89
6.79
6.50
Voc (V)
0.62
0.65
0.60
FF (%)
63
64
61
The performance of devices is characterized by current density-voltage (J-V)
measurement. Circular-shaped devices with a 1 mm diameter are tested under
illumination by AM1.5 simulated sunlight (100 mW cm-2). The intensity of the light is
uniformly distributed throughout the cell area by an optical setup. A Keithley 2400 is
used for the data acquisition of current and voltage values in J-V. Short circuit current
density (Jsc), open circuit voltage (Voc), and fill factor (FF) are determined using the J-V
data. The above three power conversion efficiency (PCE) determining values are the
average of nine devices per a substrate. All the devices showed Voc and FF values, larger
than 0.6 V and 60 %, respectively.
Figure S3. Calculated absorption spectra in a-Si layer depending on the Ag layer
thickness of the DMD anode for yellow, magenta, and cyan.
The top metal layer thickness is related to Q-factor of the optical resonance and will
affect the amount of light absorbed by the a-Si layer. Figure S3 shows the influence of
the thickness of Ag film on the absorption in a-Si layer, which is directly related to the
photocurrent Jsc. As can be seen in those spectra, the resonance gets sharp as the Ag
thickness increases because of the increased reflectance of the DMD layer in the FabryPerot cavity. This results in lower power conversion efficiency (PCE) since the
absorption in a-Si layer is reduced. Since light cannot transmit through the solar structure
and most of incident light is absorbed by the photoactive layer, the complementary
spectrum of the absorption in a-Si is essentially the reflection profile of the structure.
Even though increasing the Ag thickness decreases the PCE, a resonance behavior
becomes more pronounced, yielding a narrow bandwidth (i.e. high color purity). In this
work to clearly demonstrate the photon recycling scheme, in our device design we
focused on attaining a sharp resonance so we used 23 nm of Ag film by sacrificing the
power conversion efficiency to some extent. We calculated photocurrent Jsc for different
Ag film thicknesses and the results are summarized in the Table S2. As expected the
power conversion efficiency can be improved by simply reducing the thickness of Ag
layer (e.g. Ag=10 nm). Thinner and continuous Ag can be obtained, e.g. by using our
recent finding that adding small amount of Al during Ag deposition can produce an ultrathin (~7 nm), smooth and low loss film.
Table S2. Summary of calculated Jsc values as a function of the Ag layer thickness in
DMD anode with 20 nm WO3 for CMY-colored devices.
WO3=20 nm
Ag thickness (nm)
Jsc (mA cm-2)
Cyan
Magenta
Yellow
10
5.78
8.40
8.66
15
5.54
8.09
7.85
20
5.30
7.59
7.17
23
4.97
6.92
6.68