This entry was written in part by Victoria González
Efficient solar energy conversion using solar cells requires materials that absorb in a broad spectral range, from visible to near infrared, to harvest most of the solar photons, as well as with the capability to convert effectively the incident light into free charges that produce electrical current and voltage. High crystalline materials, with good carrier mobility reduce the energetic costs associated to the electron/hole separation and charge extraction. However these technologies stumble with high cost fabrication procedures, that make use of vacuum based or high-temperature processing, limiting its commercial application. Issues of cost and fragility have impeded the availability of low cost solar energy that is much needed to replace fossil fuels as the primary energy source.
On the other side, third generation of photovoltaic devices, dye sensitized solar cells (DSCs) and organic photovoltaics, need simpler procedures what reduces significantly the cost of manufacturing processes. But these technologies employ a combination of amorphous and disordered materials, so the energy costs associated to extract free charges represent a fundamental loss and a main limitation to high efficiency systems.
Currently, the recently emerged perovskite structure based materials is revolutionizing the photovoltaic field, and, as very recent studies demonstrate, they can reach a trade off between broad spectral absorption range, as well as high crystalline that induces efficient charge extraction reducing energetic costs, with simple and low cost fabrication techniques.
Mitzi and co-workers have reported the synthesis of several organometalic perovskites based in lead halide materials. Different structures 2D and 3D spatial distributions were developed and its suitable opto-electronic proprieties were exploited in the fabrication of transistor and light emitting diodes (D.B. Mitzi et al., Nature, 1994, 369,467; K. Chondroudis, Chem Mat., 1999, 11, 3028). More recently, Miyasaka et al. (A. Kojima, J. Am. Chem. Soc., 2009, 131 (17), 6050) demonstrated the potentiality of methyl ammonium lead halide perovskites in sensitized solar devices based on TiO2 mesostructure photoanodes. Although it was during last year, 2012, when the boom of perovskites materials took place and the different contributions of Nam-Gyu Park, Michael Grätzel, and Henry Snaith have highlighted the great potential of these materials towards the development of efficient solid state photovoltaic devices (H.S. Kim, Sci. Rep. 2012, 2, M.M. Lee, Science, 2012, 338, 643; J. Ball, Energy & Environmental Science, 2013, DOI:10.1039/C3EE40810H). Two weeks ago, a conference report by Michael Grätzel showed a 14.14% efficient certified cell of this class, which put the perovskite solar cells ahead of several traditional technologies.
On wednesday 28 at EMRS meeting in Strassbourg, Henry Snaith reported a major step in the evolution of the perovskite solar cells. He reported the construction of a 15% cell with thin-film structure (TIO2/CH3NH3PbI3/OMeTAD) . For the crystal growth of vapor deposition of CH3NH3PbI3 were employed spin coating and vapor deposition techniques. Higher crystallinity was obtained for films grown by vapor deposition technique that has provided the record efficiency of 15.35% (Jsc= 21 mA/cm2, Voc= 1.07 and FF= 0.67) for this material. This result highlights the potentiality of these materials that have emerged as a competitive technology in thin solar cell fabrication, which can be processed with solution-based techniques at low-temperatures, leading to the least expensive technology.
The new result by Snaith is also significant in that there seems to be no nanostructure assisting charge separation in the absorber film, but just a homogeneous film with selective contacts, the TiO2 compact layer for electrons and organic hole conductor Spiro-OMeTAD and gold contact for holes. Thus the perovskite solar cells start to move to the class of ordinary thin films, as amorphous silicon, CdTe, CIGS, and other, that already form competitive technologies in the market. Nonetheless, the organic hole conductor is confirmed as a significant component for high efficiency photovoltaic action, thus the perovskite solar cell still remains into the family of hybrid organic-inorganic solar cells.
A range of significant questions open up right now, concerning the significance of crystallinity, the relationship between efficiency and stability with film deposition method, that will impact the cost effectiveness of the technology. No doubt these issues will be addressed by a broad research community, and probably this will happen rapidly. Snaith commented that with the ramping up from 10 to 15% in six months, clearly 20% could be on sight ahead, then perovskite solar cell will emerge as a real major player for the cheap production of solar energy on a real large scale. He’s probably right.