Traditional cameras acquire a 2D scene; while range imagers use time-delay information from an active laser pulse source to obtain 3D information. The approach has applications in user authentication, augmented reality, and self-driving cars. Early applications such as geospatial imaging pioneered key elements of the technology; today automotive sensors are a more widely-used application of LIDAR.
Even broader applications of 3D imaging will rely on further increases in performance (both in signal-to-noise and in temporal resolution), as well as the use of practical, processible direct-bandgap materials.
Traditionally, silicon photodetectors have been used for LiDAR, but their low near-infrared absorption coefficient limits their performance. Their weak absorption coefficient at LiDAR-relevant wavelengths (~905 nm) demands thick devices to absorb 90% of incident light. Silicon devices must, however, be thinner than 3 μm in order to achieve 2 GHz bandwidth (ideal speed for LiDAR). This trade-off limits its detection efficiency * speed combination (Fig. 1a).
Perovskites are solution-processed materials that form remarkably high-quality crystalline thin films even when fabricated at low temperatures onto a wide variety of substrates. We focused in on mixed-Pb/Sn perovskite thin films since these unite high carrier mobility with high direct-gap absorption at 940 nm.
Early in our project, we realized that transport layers would need attention if they were to shuttle carriers rapidly between contacts and active layers (Fig. 1b). The high crystallinity and mobility of NiOx make it a contender as a promising hole transport layer (HTL). In our initial studies, we discovered a chemical incompatibility, though, between known anti-oxidation strategy for PbSn perovskites and NiOx solgel.
Figure 1. LiDAR with perovskites | a. Performance modeling of silicon p-i-n photodetector. b. Modeling of combined speed-efficiency characteristics for PbSn perovskites compared to silicon. c. Schematic showing the Sn metallic wire reducing precursor process. d. Performance of silicon and solution-processed photodetectors for wavelength detection > 850 nm. e. The responsivity plot of PbSn devices compared with commercial fast and slow silicon photodetector. f. Schematic of ToF measurement. g. The response of PbSn PD at the indicated position of the moving mirror. h. Contour plot of data shown in panel (g) vs. the travel distance of light with respect to the reference. i. 3D visualization of estimated depth using peak position of PbSn photodetector response. Each pixel is an individual measurement.
Writing in Nature Electronics (https://doi.org/10.1038/s41928-022-00799-7), we describe a method to create fast and efficient solution-processed photodetectors by overcoming the stated barrier. We developed a strategy that inhibits the oxidation of perovskites during the fabrication process without adversely affecting the NiOx and its interface with perovskite. This approach removes oxygen from the solution, converts unwanted species, and avoids leaving a detrimental residue behind (Fig. 1c).
The resulting devices exhibit a combined efficiency and speed that is >2x higher than in fast silicon photodetectors and >100 times higher than in the previously reported solution-processed photodetectors (Fig. 1d-e). To illustrate the potential of the approach, we showcase that these developed photodetectors can measure the distance of the object with sub-millimeter accuracy.
Figure 1f shows the schematic of our time-of-flight (ToF) setup. The mirror on the stage is moved based on the pre-defined map and the distances (depth) are estimated using the speed of light (Fig. 1g-h). We then reconstructed a 3D image using these individual measurements (Fig. 1i).
To extend the use of perovskites photodetector to long-range detection and ranging applications, a degree of gain is required. Further investigation is needed to trade off response time against gain.