Colored ultrathin hybrid photovoltaics with high quantum efficiency
Inorganic photovoltaic (PV) devices use thick semiconductor layers to completely absorb a wide range of light wavelengths with high efficiency. However, these thick semiconductor films are often esthetically unappealing when installed, as is usually the case, on rooftops. To address this issue, colored PV cells have recently gained considerable interest owing to their ability to be integrated into the inner and outer surfaces of architectural structures, which enables the generation of electric power from a large area of buildings.
We have manufactured dual-purpose PV cells that use ultrathin dopant-free amorphous silicon (a-Si) to produce transmissive or reflective colors. Moreover, these colors have the desired property of being independent of both the polarization and the angle of the incident light. We achieved this by using a strong resonance effect in the ultrathin semiconductor in the visible wavelength range.1–3 The ultralow thickness of the a-Si layer makes the shift in the phase of propagated light nearly negligible; an additional effect is that the phase of light reflected at the semiconductor-metal boundary is cancelled. Both of these features contribute to the colors being angle- and polarization-independent.4 Given that the semiconductor film is ultrathin, it also helps suppress electron-hole recombination, such that most photons absorbed in the photoactive layer take part in generating electric power.
Figure 1(a) shows the structure of an a-Si PV device for producing reflective colors. We used a dielectric-metal-dielectric (DMD) multilayer electrode as a transparent anode and tungsten trioxide and silver (Ag) for the dielectric and metallic layers, respectively. We used vanadium pentoxide, which is a dielectric material with a high work function, as an efficient hole-transporting layer to form an interface with the a-Si photoactive layer. On the cathode side, indene-C60 bisadduct (ICBA) formed an interface with the a-Si layer to function as an efficient electron-transporting layer, as the edge of the conduction band of a-Si is well aligned with the lowest unoccupied molecular orbital of ICBA. We spin-cast a bisadduct fullerene surfactant on top of the ICBA to reduce the work function of the Ag cathode, forming an ohmic contact, and completed the device by depositing optically thick Ag as the cathode. Figure 1(b) presents reflection spectra (cyan, magenta, and yellow colors) of the proposed colored PV cells at a normal angle, showing fairly good agreement between simulated and experimental data.
The non-trivial shift in the phase of reflected light—neither 0 nor π—that occurs at the interface between the light-absorbing semiconductor and the metal contributes strongly to an interference effect in the Fabry-Pérot cavity that is formed by ultrathin a-Si sandwiched between two semitransparent metal reflectors. A major advantage of this Fabry-Pérot cavity design over the conventional design, which uses an optically transparent medium, is that the resonance is much more insensitive to the angle of incident light. This feature can be attributed to the reduced phase shift of propagated light and the effect that cancels the phase of light as a result of reflection of light from the semiconductor-metal interface.4–6
Using this approach, customized colors and patterns can be made, such as the power-generating University of Michigan logo shown in Figure 1(c). Figure 1(d) illustrates the current density-voltage characteristics of reflective colored PV cells. Interestingly, we achieved a power conversion efficiency (PCE) of ∼3% using a reflective colored PV device with a photoactive layer only 18nm thick, which is more than fifteen times thinner than traditional a-Si PV cells with PCE values of ∼10%. For the generation of color, ultrathin a-Si is needed; traditional doped regions cannot be used, as these are typically 40–50nm thick. Instead, we adopted the charge transporting/blocking interfacial layers commonly used in organic solar cells to extract photogenerated electrons and holes. This approach enables an intrinsic ultrathin a-Si layer to be used. The thickness of this layer is so low—much less than the carrier diffusion length—that the majority of photogenerated carriers can reach the electrodes with negligible charge recombination, which leads to a very high internal quantum efficiency.
Using a semitransparent DMD electrode at the cathode enables the construction of transmissive colored PV devices. Figure 2(a) shows simulated and measured transmission spectra of red-, green-, and blue-colored PV cells; Figure 2(b) shows a semitransparent power-generating US flag. The density-voltage characteristics of transmissive colored PV cells are illustrated in Figure 2(c).
In summary, we have developed a dual-purpose PV device that can generate electric power and simultaneously produce vivid colors that are independent of the incident angle and polarization of light by integrating an optical resonance cavity into the design of the solar cell. We expect that the PCE could be increased by adopting devices with different structures.7–10 We could also extend the design principle to other PV material systems, including perovskite11 and organic semiconductors.12
University of Michigan
