Consolidation of Perovskite Solar Cells

This month a very interesting article in the Journal of Physical Chemistry Letters has been published by Henry Snaith, one of the instigators of the new Perovskite Solar cells that have already consolidated as one of the most promising and feasible low-cost photovoltaic technologies.

(Perovskite solar cells from Swansea Univeristy. Picture by Matt Carnie)

In the article he describes the evolution from the initial liquid dye-sensitised solar cell, to the solid state device using OMeTAD, then the extremely thin absorber (ETA) approach and finally the Perovskite solar cell or also known as Meso-Superstructured Solar Cell (MSSC).

One of the most impressive characteristics of the perovskites is that it groups all the three essential mechanisms for the photovoltaic conversion of energy into a single material, i. e. the perovskite is able to absorb the light, accumulate the separated carriers and transport both electrons and holes to the external contacts.

The most recent improvements in efficiency already reach 15% efficiency, using different configurations, a planar thin-film device (reported by Liu et. al.) and a nanostructured approach (reported by Burschka et al.). Snaith also remarks in the article that efficiencies of 20% or 25% could be also possible in the near future, reaching the most efficient current photovoltaic technologies.

Thermoelectric Organic Semiconductors Reveals New Trends and Improve Efficiency

A recent paper appeared in Nature Materials has revealed new trends in thermoelectricity existing in organic semiconductors which significantly differs from the behaviour widely observed in inorganic semiconductors. The increase of the carrier concentration n in inorganic semiconductors leads to the increase of electrical conductivity σ, but decreases the Seebeck coefficient S. Since the performance of thermoelectric materials is determined by the figure of merit Z=σS^2/κ, where κ is the thermal conductivity, a compromise between all these three parameters needs to be reached. For this reason highly doped semiconductors are the materials that better achieve this trade-off as observed in the classical figure below.

However, Kim et al. at the University of Michigan have proved that this is not the case in organic semiconductors, where a reduction of the dopant concentration leads to an improvement in Z due to the simultaneous increase of S and σ. They attribute this feature to the major influence of the mobility μ rather than the n in the electrical conductivity which is proportional to the product of both (σ=qnμ). In other words, the mobility enhancement (that the authors attribute to a decrease in the tunnelling distances) overwhelms the reduction in carrier concentration. The opposite occurs in inorganic semiconductors where the changes in μ are small relative to changes in n, and S and σ have opposite dependences on n.

This provides a new strategy to optimise the thermoelectric properties of polymer/organic thermoelectrics. Applying this to PEDOT:PSS, a outstanding value of ZT=0.45 at room temperature has been achieved by dedoping the polymer, which represents another significant enhancement in the efficiency of organic thermoelectrics in a short time.

Impressive Electrochromic Device

Llordés et al. at Lawrence Berkeley National Laboratory (USA) have reported these days in Nature an  outstandng electrochromic device capable to block near-infrared and visible light selectively and independently by varying the applied electrochemical voltage over a range of 2.5 volts. This material demonstrates a novel optical switching behaviour that will enable the dynamic control of solar radiation through windows.

The change in transmittance is achieved at different voltages applied to the device (left graph). The right picture shows the device at fully transparent state (pictures from Nature).

They achieve this by incorporating tin-doped indium oxide (ITO) nanocrystals into niobium oxide glass. This synthetic strategy is called "nanocrystal-in-glass" and nanocrystals become covalently bonded into the amorphous material, resulting in a new amorphous structure. The nanocrystal-in-glass film switches progressively between three optical states: 

• Fully transparent (at 4V versus Li)
• Selective blocking of near-infrared (around 2.3 V)
• Simultaneous blocking of visible and near-infrared (1.5 V)

Thus, solar radiation can now be dynamically modulated with spectral selectivity. For example, we can select reducing the infrared part during hot days (reducing the air conditioning consumption). Both infrared and visible can be cut down in very hot days to gain also privacy and reduce heat input further, or we can also leave it transparent during the winter. Additionally the new material is also highly stable to swith on/off cycling.

Perovskite Solar Cells Reach 15% Efficiency

Perovskite Solar Cells experienced last year a significant development, mainly due to their high efficiency and the ability of being processed from solution (See link). This month a significant step forward has been made by achieving a new high efficiency of 15.4%. This has been announced by Oxford Photovoltaics, the spin-off company devoted to the development and scale-up of this novel technology.

 

 (Dr. Henry Snaith from Oxford University holding a perovskite solar cell)

At EMRS meeting in Strassbourg last month, Henry Snaith reported this achievement. Surprisingly the new cell was constructed as a homogeneous thin film structure, i. e. there is no nanostructured material in it. The cell is formed by TiO2 compact layer as blocking layer, the CH3NH3PbI3 perovskite acting as absorber and electron conductor and the organic Spiro-OMeTAD for hole conduction with gold contacts on top. The record cell showed an efficiency of 15.35% (J_sc= 21 mA/cm², V_oc= 1.07 and FF= 0.67) (See link).

This positions these cells as a highly competitive technology able to provide very low-cost devices, which is one of the most demanded requirements for solar energy to be successful. However, very few things have been reported regarding lifetime and stability which is also very relevant for a market success. Let´s see how these very promising technology evolves and if it will be able to meet all the needs of the market, which is not something easy to achieve.

Polymer-based/Organic Thermoelectrics

A lot of attention has been paid recently to the hardly explored use of conducting polymers as thermoelectric (TE) materials. As it is well-known, a good TE material should have a high figure of merit (ZT), that is, high electrical conductivity and Seebeck coefficient and low thermal conductivity. Thermal conductivity of polymers typically lies in the range of 0.1-1 W/mK, around an order of magnitude lower than the best inorganic materials. However, the electrical conductivity of polymer TEs is not as good and usually extends in a very broad range (from 10e-8 S/cm to 104 S/cm). On the other hand, the Seebeck coefficient ranges from 10 μV/K to 1000 μV/K and, as in many materials, it is not easy to achieve very high electrical conductivities without strongly decreasing the Seebeck coefficient.

(Picture of a P3HT/CNT composite TE from Bounioux et al.)

Last year, Xavier Crispin and co-workers from Linköping University in Sweden achieved the polymer-based TE material with the highest ZT (0.25) at the moment. Their results were published in Nature Materials. They were able to provide PEDOT with an electrical conductivity over 1000 S/cm by replacing the PSS anion with tosylate. Additionally, this week another breakthrough has been reported in Nature where it has been shown that the carrier mobility in conducting polymers can be significantly increased (up to 8.5 cm2/Vs) by covering the delocalised π chain with macrocycles, rising another promising opportunity for more improvement of electrical conductivity.

Another approach that is currently being intensively researched is the preparation of polymer-based nanocomposites, where the polymers are complimented with the properties of nanofillers such as carbon nanotubes. One of the main achievements in this strategy has been reported by See et al. who prepared composites of PEDOT:PSS with Te nanorods, which resulted in a figure of merit ZT=0.1.

If you want to gain more insight into this topic, some recent reviews has been published by He et al. and  Bubnova et al.

Top Ten Emerging technologies in 2013

Recently, the World Economic Forum has published a relation of the the top 10 most promising technologies that can help to deliver sustainable growth in the near future. Electric vehicles powered by magnetic fields under the roads, 3D printing, efficient water purification, organic electronics and photovoltaics are among the selected technologies. To see the whole list and a brief summary of their relevance, click here.