Organometal trihalide perovskite based solar cells have exhibited the highest efficiencies to-date when incorporated into mesostructured composites. However, thin solid films of a perovskite absorber should be capable of operating at the highest efficiency in a simple planar heterojunction configuration. Here, it is shown that film morphology is a critical issue in planar heterojunction CH3NH3PbI3-xCl x solar cells. The morphology is carefully controlled by varying processing conditions, and it is demonstrated that the highest photocurrents are attainable only with the highest perovskite surface coverages. With optimized solution based film formation, power conversion efficiencies of up to 11.4% are achieved, the first report of efficiencies above 10% in fully thin-film solution processed perovskite solar cells with no mesoporous layer. © 2013 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim.

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Morphological control for high performance, solution-processed planar heterojunction perovskite solar cells

The power conversion efficiency of hybrid solid-state solar cells has more than doubled from 7 to 15% over the past year. This is largely as a result of the incorporation of organometallic trihalide perovskite absorbers into these devices. But, as promising as this development is, long-term operational stability is just as important as initial conversion efficiency when it comes to the development of practical solid-state solar cells. Here we identify a critical instability in mesoporous TiO₂-sensitized solar cells arising from light-induced desorption of surface-adsorbed oxygen. We show that this instability does not arise in mesoporous TiO₂-free mesosuperstructured solar cells. Moreover, our TiO₂-free cells deliver stable photocurrent for over 1,000 h continuous exposure and operation under full spectrum simulated sunlight.

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Overcoming ultraviolet light instability of sensitized TiO 2 with meso-superstructured organometal tri-halide perovskite solar cells

The study of the photophysical properties of organic–metallic lead halide perovskites, which demonstrate excellent photovoltaic performance in devices with electron- and hole-accepting layers, helps to understand their charge photogeneration and recombination mechanism and unravels their potential for other optoelectronic applications. We report surprisingly high photoluminescence (PL) quantum efficiencies, up to 70%, in these solution-processed crystalline films. We find that photoexcitation in the pristine CH3NH3PbI3–xClx perovskite results in free charge carrier formation within 1 ps and that these free charge carriers undergo bimolecular recombination on time scales of 10s to 100s of ns. To exemplify the high luminescence yield of the CH3NH3PbI3–xClx perovskite, we construct and demonstrate the operation of an optically pumped vertical cavity laser comprising a layer of perovskite between a dielectric mirror and evaporated gold top mirrors. These long carrier lifetimes together with exceptionally high...

http://pubs.acs.org/doi/pdf/10.1021/jz5005285

High Photoluminescence Efficiency and Optically Pumped Lasing in Solution-Processed Mixed Halide Perovskite Semiconductors

Organometal halide perovskite solar cells have evolved in an exponential manner in the two key areas of efficiency and stability. The power conversion efficiency (PCE) reached 20.1% late last year. The key disquiet was stability, which has been limiting practical application, but now the state of the art is promising, being measured in thousands of hours. These improvements have been achieved through the application of different materials, interfaces and device architecture optimizations, especially after the investigation of hole conductor free mesoporous devices incorporating carbon electrodes, which promise stable, low cost and easy device fabrication methods. However, this work is still far from complete. There are various issues associated with the degradation of Omh-perovskite, and the interface and device instability which must be addressed to achieve good reproducibility and long lifetimes for Omh-PSCs with high conversion efficiencies. A comprehensive understanding of these issues is required to achieve breakthroughs in stability and practical outdoor applications of Omh-PSCs. For successful small and large scale applications, besides the improvement of the PCE, the stability of Omh-PSCs has to be improved. The causes of failure and associated mechanisms of device degradation, followed by the origins of degradation, approaches to improve stability, and methods and protocols are discussed in detail and form the main focus of this review article.

Organometal halide perovskite solar cells: degradation and stability

Solution-processed hybrid organic–inorganic perovskites (HOIPs) exhibit long electronic carrier diffusion lengths, high optical absorption coefficients and impressive photovoltaic device performance. Recent results allow us to compare and contrast HOIP charge-transport characteristics to those of III–V semiconductors — benchmarks of photovoltaic (and light-emitting and laser diode) performance. In this Review, we summarize what is known and unknown about charge transport in HOIPs, with particular emphasis on their advantages as photovoltaic materials. Experimental and theoretical findings are integrated into one narrative, in which we highlight the fundamental questions that need to be addressed regarding the charge-transport properties of these materials and suggest future research directions. The charge transport properties of hybrid organic—inorganic perovskites, which can explain their excellent photovoltaic performance, are reviewed through an integrated summary of experimental and theoretical findings. The potential origins of these properties are discussed and future research directions are indicated.

Hybrid organic—inorganic perovskites: low-cost semiconductors with intriguing charge-transport properties

Lithium salts have been shown to dramatically increase the conductivity in a broad range of polymeric and small molecule organic semiconductors (OSs). Here we demonstrate and identify the mechanism by which Li+ p-dopes OSs in the presence of oxygen. After we established the lithium doping mechanism, we re-evaluate the role of lithium bis(trifluoromethylsulfonyl)-imide (Li-TFSI) in 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenyl-amine)9,9′-Spirobifluorene (Spiro-OMeTAD) based solid-state dye-sensitized solar cells (ss-DSSCs). The doping mechanism consumes Li+ during the device operation, which poses a problem, since the lithium salt is required at the dye-sensitized heterojunction to enhance charge generation. This compromise highlights that new additives are required to maximize the performance and the long-term stability of ss-DSSCs.

James Kirkpatrik

Papers

Lithium salts as “redox active” p-type dopants for organic semiconductors and their impact in solid-state dye-sensitized solar cells