The development of perovskite-based solar cells has witnessed significant improvements over the past two decades. This development has triggered more research to increase the power conversion efficiency (PCE) to 25%, thus increasing the photovoltaic performance of perovskite solar cells.

The development of solid-state perovskite solar cells began in 2012. Ever since, researchers have continued to work on the device physics, process engineering, material chemistry, and structural design of solid-state perovskite solar cells.

Perovskite-based solar cells are preferred over other types of solar cells due to their low cost of materials and processes as well as their high efficiency. However, perovskite solar cells are relatively new and there are a few structural design challenges that affect their performance with regards to their efficiency.

This article discusses these challenges and offers future research directions for enhancing the efficiency of perovskite-based solar cells.

Defect Structure

So far, the methylammonium lead triiodide (MAPbI3) perovskites and their associated materials are produced via a low-temperature solution process. This process generates various defects in the perovskite including grain boundaries (GBs), surfaces, interfaces, and thermodynamically stable point defects.

In general, an open-circuit voltage (Voc) defect develops from the non-radiative recombination of a trap-assisted electron-hole. In perovskite solar cells, the Voc decreases gradually compared to other inorganic semiconductors. Carrier scattering and non-radiative recombination are frequently caused by defects, which create deep-level states in the bandgap.

Many structural defects in perovskites, such as GBs and surfaces take place in deep-level states. Normally, deep-level defects that have large energy of formation are rare in MAPbI3. However, the bandgap shallow trap states are in abundance, leading materials that can tolerate defects. The ionic characteristics and strong Pb lone-pair s-halogen p anti-bonding coupling have been reported as the reason for these unique defect properties.

The most pronounced defects in MAPbBr3 and MAPbI3 perovskites are halide and methylammonium (MA) interstitials. That is, Ii−/Bri– and MAi+. MAi does not create extra gap states because it is not covalently bonded to the Pb−I framework. The lower electronegativity of I in comparison to Br is the reason for the high stability of positive I interstitial.

Recent studies have focused on enhancing perovskite production technologies. These studies show that perovskite semiconductors have significant tolerance to defects. The traditional semiconductor technologies center on producing perfect crystals to reduce defects. A prominent example is the single-crystalline Si, which has remarkably low defect densities. The case of producing MAPbI3 by solution processing at a temperature close to room temperature is different. This is due to the inevitable high defect densities that can negatively affect its performance.

Researchers at SOLRA, a spin-off from the Hebrew University, developed a fully-printable perovskite-based solar cell. The technology used by SOLRA allows improved scalability and stability of the perovskite-based solar cell to harvest sunlight indoors and outdoors. Another company that researches and produces perovskite-based solar technology is a US-based company known as CubicPV.

Microstructure

Generally, highly efficient perovskite-based solar cells are synthesized via simple solution methods. Through this method, low-cost photovoltaic devices can be produced. The spinning of the preceding solution often leads to the production of polycrystalline thin films on transparent, conducting substrates.

Parts of the polycrystalline thin-film photovoltaics are perovskite-based solar cells, CdTe, Cu(In1−xGax)Se2 (CIGS), and polycrystalline Si. The microstructure of the thin-film solar cells determines the PCE and stability of solar cells. In polycrystalline functional materials, the grain boundaries present a critical defect. Grain boundaries can impact the electrical, chemical, and mechanical properties of perovskites.

The impact of grain boundaries on the electrical properties is very important because grain boundaries modify the charge-carrier transport properties, and consequently, the cell’s performance. Grain boundaries can cause segregation and/or depletion of impurities, broken and/or incomplete bonds, causing dangling bonds, and lattice discontinuity.

At the grain boundaries, the bond distances, distorted bond angles, vacancies, interstitials (misplaced atoms), dislocations, and defects act as trap states and/or recombination centers for charge carriers. In polycrystalline thin-film photovoltaics, the grain boundaries could be detrimental or benign to the performance of the cell depending on their centers and/or trap sites.

Research is also ongoing for new perovskite-based solar cell designs and approaches to encapsulate the cells to protect them from very harsh environments. P3C, an India-based startup develops new generation perovskite-based solar cells using nanotechnology-based products. The company is designing and developing its machinery technology for independent production.

Crystal Structure

The crystal structure of the MAPbI3 perovskite is ABX3 for a simple cubic lattice. In principle, a stable perovskite crystal can be formed by the combination of compatible ions. Here, the octahedral factors and tolerance are met within an empirically defined range, which depends on the constituents’ ionic radii. In a simple MAPbI3 perovskite lattice, the MA ions are found between building blocks of the octahedral, and the Pb ions are centered in the octahedral.

The MAPbI3 transforms from the tetragonal phase to the cubic phase below 42 ℃ – 57 ℃ and changes to the orthorhombic phase from the tetragonal phase at approximately -113 ℃. In assessing the charge-carrier dynamics, the above transitions need to be seriously considered. This is due to the possibility of these transitions altering the material’s electronic band structure and then its optoelectronic properties.

Hybrid perovskites demonstrate transitions in the crystallographic phase with different pressures and temperatures. Recent studies using the measurements of heat capacity along with temperature-dependent X-ray diffraction (XRD) show that the CsPbBr3, FAPbBr3, MAPbCl3, MAPbBr3, and MAPbI3 perovskites transit to the tetragonal phase from the cubic phase at temperatures of 130 ℃, -35 ℃, -95.8 ℃, -36.9 ℃, and 57.8 ℃, respectively. The orthogonal and tetragonal phases stabilize with a decrease in temperature with the accompanying MA molecular ions ordering. Studies using nuclear magnetic resonance (NMR) reveal that the reorientation of MA+ is the reason for the structural transition to the tetragonal phase from the cubic phase.

Despite the dependence on the temperature of the phase transitions, hybrid perovskites demonstrate crystallographic phase transitions when pressure is applied. Recent studies indicate that, at ambient pressure and temperature, the MAPbI3 crystal structure is tetragonal (14/mcm). The structures also have a distorted orthorhombic phase with the Imm2 space group when the pressure is higher than 0.3 GPa.

Future Research

There have been many studies on the unique characteristics of perovskite-based solar cells to help explain their optoelectronic properties. However, further research is still required to understand certain perovskite properties like the grain boundaries effect, large polaron formation, and Rashba splitting.

To further enhance the performance of perovskites, research is required to understand the nature of perovskite materials’ defects. Also, understanding and controlling the defects to subdue the non-radiative charge recombination are vital for improving the efficiency of perovskite-based solar cells. Further research is needed to clarify the local intrinsic properties of grain boundaries.

Most of the fundamental studies on perovskite-based solar cells have concentrated on MAPbI3. Therefore, future research needs to extend to the design of more ideal perovskite materials. Moreover, the defect densities are still high in solution-processed films, making the development of efficient methods to passivate the surface/grain boundaries and bulk defects very important to attain high efficiencies above 20%.