A team from the University of Nebraska-Lincoln, working with researchers at the National Institute of Standards and Technology (NIST; Boulder, CO), has shown that electron-trapping defects that are typically problematic in solar cells can be an asset when engineering sensitive light detectors.1
For the past few years, researchers have been studying films of organometal trihalide perovskites (OTPs) for use in solar cells because they have several attractive qualities, including the fact that they are solution-processable. The new work suggests that certain defects in OTPs, such as those created by groups of lead (Pb) cations, could be harnessed to make light sensors that consume very low power.
When a photon is absorbed by a light-sensitive material, it transfers energy to a negatively charged electron, which goes to an excited state and leaves behind a positively charged hole. For proper photovoltaic or photodetector operation, these oppositely charged carriers need to drift in opposite directions towards different electrodes, but material defects that "trap" either electrons or holes reduce carrier mobility and degrade the device's performance.
The team found that although trap states in the bulk material are bad for solar cells, surface defects and traps close to the electrodes in OTPs can be engineered to boost their light-detecting performance.
Traps near the electrodes are best
"One way to increase a detector's sensitivity to light is to apply a voltage to it," says NIST's Andrea Centrone. "Traps located near the electrodes lower the energy barrier for injecting electrons into the material. In our devices, lowering the barrier effectively multiplies the material's light sensitivity up to 500 times when we apply the right voltage."
Using photothermal induced resonance (PTIR), the NIST researchers studied the surface decomposition of OTP films at the nanoscale during curing. PTIR combines the spatial resolution of atomic-force microscopy with the chemical specificity of infrared spectroscopy.
The data they gathered suggest that during curing, metal ion clusters are left on the OTP film surface. These ion clusters could act as charge traps, enabling the increased sensitivity. The researchers believe that PTIR characterization will provide important information to link the nanoscale properties of OTP films to the macroscale properties of OTP devices, which may allow the engineering of more efficient OTP-based light detectors and solar cells.
Source: http://www.nist.gov/cnst/novel-light-detecting-041615.cfm
REFERENCE:
1. R. Dong et al., Advanced Materials, Volume 27, Issue 11, pages 1912–1918, March 18, 2015; doi: 10.1002/adma.201405116
