A minimalist molecular coating enables organic devices to generate power and detect light indoors, resolving a materials interface problem while improving efficiency, scalability, and cost.
(Nanowerk Spotlight) Indoor electronics are becoming more intelligent, more connected, and more invisible. Smart sensors track motion, light, temperature, and biometrics in homes, factories, and wearables. Often, they operate without anyone noticing they’re there. But these systems have a problem. They rely on energy sources that don’t scale well. Batteries wear out. Wires don’t reach. Light is everywhere indoors, but turning that into reliable, self-sustaining power while still leaving room for sensing has remained a technical bottleneck.
The most promising materials for solving this are organic semiconductors. These are flexible, lightweight, and capable of harvesting and responding to visible light. In principle, they can be used to build devices that both generate power from ambient illumination and detect optical signals. That is exactly what indoor systems need: a single film that powers itself while sensing its environment.
But in practice, these two functions pull in opposite directions. Organic photovoltaics need to move electrical charges quickly and with minimal resistance. Photodetectors need to suppress noise and limit charge movement to pick out weak signals. When combined in one structure, the device ends up compromising both roles.
This limitation has not just slowed progress. It has split it. Two functions that could, in theory, be combined into a single organic layer have remained siloed into separate devices. Each one adds cost, complexity, and space. The result is a missed opportunity in materials design, especially for compact, low-power systems that must perform reliably under soft indoor lighting.
Despite its minimal design, BPA creates a stable and uniform interface that supports both efficient charge extraction and low-noise light detection. It offers a practical solution to the conflicting electrical requirements of dual-function organic devices.
a Device architecture, b energy-level diagram and the chemical structures of 2PACz and BPA, c optical properties. (Image: Reprinted with permission by Wiley-VCH Verlag) (click on image to enlarge)
The molecule itself is small. It contains only a benzene ring and a phosphonic acid group. When deposited onto indium tin oxide (ITO), a common transparent electrode, it forms a single layer of molecules that binds chemically to the surface. This anchoring alters the energy levels at the interface, which improves the movement of positive electrical charges (holes) in the photovoltaic mode and suppresses background current in the detector mode. In conventional devices, these two requirements usually compete. BPA allows them to coexist.
In indoor solar cell testing under 1000 lux LED lighting at a color temperature of 2700 Kelvin, BPA-based devices achieved a power conversion efficiency of 28.6 percent. For comparison, devices using the widely adopted PEDOT:PSS reached 24.9 percent, while those using 2PACz, another self-assembled monolayer, reached 27.5 percent. These results show that BPA matches or exceeds the performance of the best alternatives while using a simpler molecular structure and avoiding some of their known limitations.
Critically, this efficiency was preserved when the devices were scaled up. BPA-based solar cells maintained 93 percent of their original performance when their surface area increased more than 200 times. By contrast, devices using PEDOT:PSS and 2PACz showed efficiency drops of more than 10 and 13 percent, respectively. These losses are attributed to uneven surface coverage and interfacial instability in the control materials. BPA formed a more uniform coating that maintained its electrical and structural properties at larger scales.
Photodetector performance was tested using the same device structure, operated in a self-powered state with no external voltage. In this mode, the BPA-based device achieved a noise equivalent power (NEP) of 584 femtowatts at a wavelength of 730 nanometers and a bandwidth of 1 hertz. NEP is a measure of the minimum optical power a detector can distinguish from electrical noise. A lower NEP means higher sensitivity. The BPA-based detector also reached a specific detectivity (D*) of 5.41 × 10¹¹ cm·Hz⁰·⁵·W⁻¹, outperforming devices based on PEDOT:PSS or 2PACz.
These results are attributed to BPA’s influence on molecular packing and surface uniformity. The molecule promoted tighter stacking of the light-absorbing material, improving charge transport. It also reduced surface roughness compared to other monolayers. Measurements using atomic force microscopy and grazing-incidence wide-angle X-ray scattering confirmed more ordered and denser molecular arrangements, which reduce recombination losses and background noise.
Response speed was also improved. The BPA-based photodetector reached its peak current faster than the other devices, with a rise time of 2.76 microseconds. Its 3 dB bandwidth was measured at 103 kilohertz, indicating suitability for real-time light detection. Although its fall time was slower, at approximately 3.09 microseconds, this was likely due to minor capacitive effects or delayed recombination. Crucially, the device remained stable over 30,000 switching cycles and showed consistent performance.
The improvements in efficiency and sensitivity are only part of the story. BPA also brings a major cost advantage. Because of its simplified structure and use of inexpensive starting materials, the molecule is cheaper to synthesize and easier to deposit. The cost per square centimeter of BPA was calculated at $0.042, compared to $0.135 for PEDOT:PSS and $0.351 for 2PACz.
Taking both cost and output into account, BPA-based devices produced 19.25 milliwatts per dollar. This is nearly nine times higher than the value for 2PACz-based devices. The cost-performance ratio makes BPA a strong candidate for commercial use in settings where material price and process complexity are limiting factors.
Stability testing under continuous indoor LED lighting showed that BPA devices retained 86.9 percent of their original efficiency after 1000 hours. Devices made with PEDOT:PSS dropped to 68.7 percent, and those with 2PACz to 79.2 percent. The stability of BPA is linked to its strong chemical bonding with the ITO surface and its resistance to environmental degradation. This helps the device maintain performance even in the presence of humidity and oxygen.
The potential applications are wide-ranging. BPA could enable self-powered sensors that operate under ambient indoor light without needing to be recharged or replaced. These might include motion detectors in building automation systems, wearable monitors that track health data using ambient light, or small-scale surveillance devices in industrial settings.
Because BPA is deposited as a single molecular layer and is compatible with flexible substrates, it also supports bendable electronics. Mechanical stress tests were not part of this study, but the thinness of the material suggests possible advantages in future flexible systems.
By addressing a long-standing interfacial mismatch in organic electronics, BPA provides a practical way to combine two previously incompatible functions in one material system. It delivers efficiency, scalability, and low cost, not through complexity but through deliberate simplification. The design shows that sometimes, the smallest change at the molecular level can solve the biggest bottlenecks in device integration.
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