There are many alternative semiconductors to silicon, some of which are more promising for applications in optoelectronics and photonics.
Perovskites are one set of materials that provide some advantages over silicon in optoelectronic devices.
Perovskites are normally discussed for their use in solar cells, but they are also useful in photodetectors.
Perovskite optoelectronic materials are named after crystallites with a perovskite crystal structure
It’s not often that the broader optoelectronics industry starts talking about alternatives to silicon, but that is happening now with perovskites. This class of materials is named after the crystal structure of a particular mineral, yet it encompasses a range of available optoelectronic materials. This set of materials may play a vital role in making the cost of solar cells more competitive, as well as enabling unique devices like flexible, printable solar cells.
The story around perovskites doesn’t end at solar cells. Photodetectors are one device that can make use of exotic optoelectronic materials like perovskites. For systems designers, chip designers, and board designers, it helps to understand the current progress in perovskite photodetectors and how these devices fit into the current electronics landscape. As we’ll see, commercialization is still far off, but designers can still make progress integrating perovskite material platforms into conventional electronics.
An Overview of Perovskites
Perovskites are a class of optoelectronic materials with spectral responsivity spanning across the visible range and into infrared wavelengths. They have unique properties that make them very useful in a variety of applications, principally their high absorptivity. This is very important in devices like solar cells and photodetectors. A perovskite film only a few hundred nm thick can provide very high absorption in the NIR and visible ranges. Aside from high optical absorption, there are many other properties that make perovskites desirable in optoelectronic systems.
The Characteristics of Perovskites
As was mentioned above, there is no single perovskite material, but their common chemical structure gives these materials some common properties.
Chemical formula: All perovskites have the general chemical formula ABX3, where A and B are cations (commonly a metal or organic polymer) and X is an anion.
Structure: These compounds have a common chemical structure, commonly visualized as a cation (A or B) surrounded by an ionically bonded lattice system (B or A + X). This allows some simple methods like ion exchange to be used to engineer perovskites.
Doping effects: Perovskites can be doped in solution to produce n-type or p-type behavior. Some cations that are expected to produce p-type behavior actually produce anomalous n-type behavior.
Deposition: Perovskite thin films are commonly deposited in solution or grown epitaxially.
Heterogeneity: Because different perovskite thin films have similar crystal structures, heterojunction devices can be fabricated without excessively complex processes.
Tunable direct bandgap: This is the major reason perovskites are desirable over silicon as photodetectors at a range of wavelengths. The bandgap is direct and tunable through engineering the chemical structure.
Carrier mobility: Perovskites tend to have carrier mobility that is competitive with doped silicon, giving high responsivity and response time.
Compared to silicon photodetectors, perovskite films used in photodetectors can be produced at much lower temperatures, both in solution and epitaxially. Solution processing at low temperatures is a much simpler route for fabricating high-efficiency photodetectors compared to standard processes used for silicon and other materials.
The Structure of Perovskite Photodetectors
There are many possible structures for perovskite photodetectors. These thin film devices resemble their Si counterparts, but they have a stacked structure rather than the typical planar structure on an Si wafer.
The image below shows a typical structure of a perovskite photodetector. This structure is rather simple, as it’s built on a glass or quartz substrate with a transparent conductor (FTO, ITO, etc.) as the back electrode and an opaque conductor (Al, Au, Cu, etc.) as the top electrode. Light will pass through the transparent substrate and will be absorbed in the ultra-thin perovskite layer.
Perovskite photodetector structure
On each side of the perovskite layer, there is a p-type or n-type layer providing charge transport. These layers are intended to provide transport across the heterojunction structure. Note that one of these layers will need to be a transparent thin-film solution-processable semiconductor. PEDOT:PSS is one popular p-type option. Finally, the electrode materials must be chosen so that they form Ohmic contacts with the buried semiconductors.
This standard stacked structure is also used in solar cells. The difference between these devices comes in how they are integrated into a broader optoelectronic system.
Integrating Perovskite Photodetectors Into Electronics
If you happen to be designing systems that interface with perovskite photodetectors or you need to design a device that integrates a perovskite photodetector, your electronics design software is already prepared to handle a physical layout. CAD tools can address the electrical and mechanical layout of nearly any electronic system as long as components are defined in 2D and 3D views. This includes small perovskite photodetectors and any other small-scale perovskite device.
The simulation and evaluation portions of perovskite photodetectors and related devices are still challenging areas. Simulations fall into three areas:
Material structure simulations: This is probably the most difficult part of perovskite design. These simulations examine how the important optoelectronic properties relate to chemical and crystal structure using density functional theory.
Device-level simulations: Once material properties have been examined, device simulations can be performed with a 3D electromagnetic field solver application. The device-level behavior is normally investigated in terms of material properties and physical layout.
Circuit-level simulations: For perovskite devices, these simulations are photodiode simulations with a SPICE package. Photodetectors operating in the photovoltaic or photoconductive mode can be evaluated alongside supporting circuitry.
There are some important points to evaluate in device-level simulations and in circuit-level simulations: spectral responsivity, linearity, and response time.
The other major point to examine in a perovskite photodetector is the spectral responsivity, which will determine the sensitivity of the device to different wavelengths of light. There are several factors that affect the responsivity, namely the chemical structure and the dielectric constant. Electronics design tools can’t simulate responsivity directly, but responsivity can be used in SPICE models to design supporting circuitry and examine the response time.
Just like standard photodiodes, perovskite photodetectors can be operated in photovoltaic or photoconductive mode. The delineation between each mode can be seen in the load line for a photodetector circuit. The load line can be simulated with a DC sweep for various values of supporting components and bias. Follow the same procedure you would follow for a silicon photodiode when simulating a perovskite photodetector’s load line.
Response Time as a Function of Structure
The response time is one important parameter for high-speed photodetectors. This metric tells you how fast the device can respond to an input light pulse, which will be a function of the device structure.
Like other heterojunction semiconductor devices, the stacked structure creates some equivalent capacitance, and the device capacitance and resistance across the junction will create an equivalent RC circuit. This circuit will have some RC time constant that dictates how the device responds to input light, as shown in the example results below.
Example results showing responsivity (left), linearity (center), and response time (right). [Source]
No matter which unique material platform you want to use for electronics, you’ll need PCB design and analysis software to build your perovskite photodetector systems. Cadence provides powerful software that helps automate many important tasks in systems analysis, including a suite of pre-layout and post-layout simulation features to evaluate your system.