Solar cells are marketed as a promising technology for generating clean energy.
The Role of Price in Innovation:
A decade ago, efficient, inorganic silicon solar cells were commercialized. Despite its potential, a significant bottleneck remained – price. It’s overall cost was significant in comparison to electricity. This led to an advent of other solar cell technologies based on thin films of CdTe, CIGS, low band gap organic polymers and semiconducting dyes. All these solar cell technologies showed significant cost reduction compared to silicon solar cells, but factors such as toxicity, use of rare elements and low device efficiency were enough to supplement with alternative photovoltaic materials.
Unique Characteristics of Perovskite Solar Cells:
Organic-inorganic hybrid perovskite offered excellent properties suitable for photovoltaic application. Perovskite are organic and inorganic hybrid materials with a formula ABX3, where A is an organic cation, B is a metal ion and X a halide ion. The widely researched perovskite is synthesized from a combination of methylammonium iodide (CH3NH3I) and lead iodide (PbI2) which results in methylammonium lead triiodide (CH3NH3PbI3). Some of the important characteristics of these perovskites which makes it highly desirable as a photovoltaic material is its high absorption coefficient, low energy tunable bandgap, shallow defects, low exciton binding energy, high charge carrier diffusion length, balanced electron–hole transport and easy low cost solution processing.
The Rise in Photovoltaic Power Conversion:
These unique characteristics drew greater interest in perovskite that has propelled enormous work done in a short span of four years. In turn, the photovoltaic power conversion efficiency of these organic-inorganic hybrid perovskite solar cells increased from 6% to more than 22%. The rise in efficiency has been achieved by continuous improvement in device layers and architecture, perovskite growth techniques and use of mixed halide perovskite with enhanced electronic properties.
Toxicity and Other Challenges with Perovskite Solar Cells:
Despite having potential to replace silicon based solar cell, with low cost fabrication and high device efficiency, perovskite solar cells also faces some major challenges. Material toxicity, device hysteresis, and perovskite material stability are major challenges which need to be overcome for it be commercialized on a wide scale. Use of toxic lead in perovskite is a matter of environmental concern. Ongoing efforts aim to find a replacement for toxic lead.
Apart from toxicity, long term stability of perovskite solar cells is a major bottleneck in achieving its high potential. These high efficiency organic-inorganic hybrid perovskite solar cells are highly sensitive to moisture in ambient air. Moisture leads to a domino effect of decomposition of perovskite film causing material degradation, thus losing photovoltaic property.
Temperature is also an important factor for perovskites as it leads to decomposition at high temperatures (>90 ºC). Hysteresis phenomenon in perovskite material is another critical issue, which has been studied and addressed widely. Various methods have been adopted to reduce the hysteresis behavior in these devices.
Perovskite Solar Cells in Devices:
Even with critical issues, perovskite holds promise to be used in portable device applications such as cell phones, watches, cameras, which usually last for couple of years. Perovskite solar cells can meet the energy requirement with its high power conversion efficiency along with decent device stability for a couple of years with efficient encapsulation techniques.
Perovskite Solar Cells in Space:
It also holds promise in space application where moisture is absent, whereby producing power by low cost installation and long term stability (however, the effect of radiation in space needs to be considered and is an area of current study). With all these ground challenges and future promises, perovskite photovoltaic researchers are still doing ground-breaking research to bring this low cost solar cell technology to market.
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