Introduction
Stanford ENERGY, Video by Mark Shwartz/Source/CC BY 2.5.
Solar cells based on metal halide perovskite continue to approach their theoretical performance limits. Mastering material properties and addressing stability may allow this technology to bring profound transformations to the power industry. High-quality optoelectronic properties – unexpected for materials are made from solution and at low temperatures. They are easily employed as absorber layers in highly efficient PVs.
Perovskite Structure
Has a 3D crystal structure, 3 ions, ABX3. For all inorganic compounds, Caesium is the A-cation, either lead or tin are B cation, halides of chlorine, bromine, or iodine as X anion.
‘Hybrid’ perovskites show the most promise. Hybrids have methylammonium (MA)/formamidinium (FA) as A cation. Most efficient and stable compounds have multiple mixtures of ions at A and X sites. Various combinations are the key.
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Benefits of Perovskite Solar Cells
Single junction cells are approaching Photoelectric Conversion Efficiency (PCE) of up to 23%. Not only efficiency, but real-world applications also need other properties like low costs, high stability, and independence from rare elements.
Lead and iodine are low-cost chemicals. Their Long-term stability is a concern. PCE above 20% is routine in perovskites. Make changes in materials composition, processing conditions, and device architectures are to be made.
The external radiative efficiency (ERE) of isolated perovskite films is as high as 70%, so there is a possibility of achieving high ERE in complete solar cells. When the perovskite absorber layer is sandwiched between charge-selective contacts in a PV device, further recombination losses are introduced.
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Ways to Improve Perovskite Solar Cells
Various methods exist to reduce recombination losses in perovskites such as molecular passivation of perovskite surface or reducing the interfacial contact between the charge extraction layers and perovskite absorber.
Stanford ENERGY, Video by Mark Shwartz/Source/CC BY 2.5.
Our present understanding of recombination at these heterointerfaces is limited, and there remains more than 100mV to be gained in V0C by minimizing these losses.
Several optoelectronic properties indicate that the average quality of metal halide perovskites is already high though they are deposited through the solution process and have a high density of crystalline defects, so these are called defect tolerant.
Minimization of defects due to material improvements up to thermodynamic efficiency limits, solutions usually contain A, B, and X precursors, incorporated into colloids, dissolved ions, and complexes, along with additives whose role in chemical reactions and quality needs further understanding.
Crystallization proceeds through the intermediate crystalline precursor phase.
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Efficiency
Efficiency boost will be obtained if we move to advanced concepts beyond single junction PV, methods include multi-exciton generation, singlet fission, hot-carrier collection, and intermediate band gap cells. These processes have been reported in perovskite films, they have not been made into devices.
To move to higher efficiency employ multi-junction concepts in which perovskite films are combined with silicon or other materials to expand absorption spectral range, and convert solar photons into electrical potential energy at a higher voltage.
Tuning optical bandgap ideally from 2eV to 1.2eV. Tuning has been demonstrated by changing chemical composition and leveraging quantum confinement. Cells with a bandgap from 1.5eV-1.7eV deliver high efficiency and stability.
Perovskites are held together by ionic bonding, this may be why it gives high tolerance to crystal defects and high crystalline films can be easily fabricated at low temperature.
Ionic bonding in perovskites also results in low decomposition temperatures making them intrinsically less thermally stable than silicon.
To form a stable octahedral framework in the 3D perovskite structure, use organic cations in the A-site which can withstand much lower temperatures than their inorganic counterparts since they are larger than inorganic ions.
Few potential routes have emerged to stabilize this framework with inorganic ions. Aging causes electronic degradation in silicon arising from the ‘un-passivation’ of the dangling bonds on the surface or at grain boundaries.
PbI2 is a degradation product, even though a large fraction of this is present, perovskites show high efficiency. Present generation perovskite solar cells are more stable due to the substitution of ions and improved contact materials, with appropriate encapsulation that may already be close to real-world deployment.
International Electrotechnical Commission (IEC) 61215 standard specifications for solar cells. To make technology profitable it is important to have a good level of certainty that the product will last 25 years. This needs us to go beyond IEC 61215 standard and understand potential failure modes if they exist.
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