Emerging PV technologies promise higher efficiency and lower cost
*by Mike Rycroft, NOW Media*
The cost of the photovoltaic (PV) panels is a critical factor in the roll-out of utility-scale solar power systems. The reduction in the price-per-watt of silicon solar panels, based on manufacturing and material costs, appears to have reached a plateau and any further reductions can only be achieved by increasing the efficiency of the solar module itself. An increase in efficiency of a few percentage points in can make a huge difference in output and cost reduction.
Crystalline silicon (CSI) and thin-film technologies dominate the market. Material costs are estimated to comprise 95% of the cost of CSI PV and cannot be reduced without compromising quality. No further cost reduction for thin film is anticipated either. Advanced versions of CSI, with higher efficiencies, have been developed but as cost/watt is the driving factor, high efficiency at a higher cost is not popular in the market.
There are, however, some third-generation emerging technologies which promise to deliver higher efficiency at a lower price. Many of these technologies are still at laboratory stage but show the potential to achieve commercial realisation. Fig. 1 shows the status of emerging PV technologies as at 2019. It is important to note these are research cells and are not yet commercially available. Fig. 1: Status of energy technologies (NREL).
This article will discuss a few of these new technologies.
Dye sensitised solar cells (DSSC)
The basic structure of the cell is shown in Fig. 2. The cell consists of a transparent glass or plastic conductive front layer, followed by a layer of wide band gap (WBG) nanocell-based semiconductor material such as titanium dioxide or zinc dioxide, which contains the sensitising dye. This layer is followed by electrolyte which is sandwiched between the front layer and the transparent conductive back plane. Fig. 2: Dye sensitised solar cell structure (AERE-IDCOl).
Dye sensitisers serve as the solar energy absorber in DSSCs, and their properties have a great effect on the light harvesting efficiency, and the overall photoelectric conversion efficiency. The sensitiser absorbs light and injects electrons into the conduction band of the semiconductor, which are returned via the load and the electrolyte. The spectral response of the dye can be wider than that of conventional PV cells.
DSCC can be rigid or flexible. Flexible DSCCs can be manufactured in a continuous roll process. Although efficiency is rather low (≈13%), the DSCC can be used in applications which require flexibility and transparency, such as BIPV, and small solar powered appliances. The cost of DSSC is reported to be significantly lower than conventional PV, which could lead to a lower overall cost/watt.
Organic solar cells (OPV)
The OPV is based on semiconductor organic materials. These materials are efficient light absorbers and their properties can be tuned through chemical synthesis. A typical OPV has a layered structure involving a substrate, transparent bottom electrode, photoactive layer and top metal electrode (Fig. 3). Fig. 3: Organic solar cell structure (Cambridge University).
The photoactive layer consists of two active organic semiconductors, the donor and the acceptor, which may be separate, or homogeneous forming heterojunction interfaces throughout the bulk of the photoactive layer. Photon absorption by a molecule in the active layer results in an electron-hole pair. At the interface between acceptor and donor semiconductors, a charge-transfer state is formed whereby the electron and hole exist on different molecules. This is followed by charge transport and charge extraction at the electrodes of the solar cell.
The fabrication of complete solar cells can be done almost entirely from solution, using scalable processing methods such as offset printing. This is expected to result in lower manufacturing costs. The power conversion efficiency (PCE) of OPVs is approaching 17%. OPVs can also be manufactured on flexible substrate, opening possibilities for BIPV and other applications.
Organic tandem cells
These are similar in construction and operation to OPVs, but contain multiple cells in the same structure. The spectral response of the two cells is tuned to cover different parts of spectrum and hence give a wider spectral coverage than is possible with a single cell (see Fig. 4). An efficiency of 17% has been claimed with a future target of 25%. Fig. 4: Tandem OPV structure .
Perovskite PV cells
Perovskite is the term for a mineral crystal structure first found in the Ural Mountains in 1839, most commonly a calcium titanium trioxide mineral, (CaTiO3) and is applied to anything that adopts this same structure. The cells are made from a synthetic substance having the same crystal structure as the mineral but consisting of different elements. Small but vital changes to the material allow it to absorb sunlight very efficiently. The material is also easy to fabricate using liquids which could be printed on substrates like ink in a printing press or made from simple evaporation. These properties suggest an easy, affordable route to cheaper solar cells.
By altering the basic composition, it is possible to tune the perovskite material to respond to different parts of the sun’s spectrum. This means that the material can be changed by deliberately introducing impurities, and in such a way that it can be used in multi-junction solar cells which have ultra-high efficiencies. A common material is (CH3NH3) PbI3.
Perovskite’s conversion efficiency has grown to about 25%, close to that of crystalline silicon. The theoretical maximum efficiency of a perovskite-based solar cell is calculated to be 31%. Multi-junction cells, based on perovskites, could attain still higher efficiencies.
Unlike silicon-based PV cells, perovskite cells are soluble in a variety of solvents which makes them easy to spray onto surfaces, ink or paint. This potentially makes the cells much cheaper to manufacture and means that the light-gathering film can be attached to flexible materials, opening up a range of new applications. Perovskite materials are cheap and easy to manufacture. The structure of perovskite crystal and of a perovskite solar cell is shown in Fig. 5. Fig. 5: Perovskite PV crystal structure (ossila.com).
In Fig. 5, A = An organic cation-methylammonium (CH NH ) or formamidinium (NH CHNH); B = A big inorganic cation – usually lead (II) (Pb); X = A slightly smaller halogen anion – usually chloride (Cl) or iodide (I) Fig. 6: Perovskite PV cell (ossila.com).
Perovskite Si tandem cells (PSi)
Perovskite materials have a wide bandgap. This creates an opportunity to pair them up with low bandgap photovoltaic technology, which will result in improved efficiency and will matter in a highly competitive market where system costs depend on efficiencies. In addition, perovskite solar cells offer additional attributes like flexibility, semi-transparency, thin-film, light-weight, and low processing costs. PSI cells consist of a conventional silicon bottom cell, combined with a perovskite top cell (see Fig. 7). Fig. 7: *Silicon perovskite tandem cell (Hossein ).*
The perovskite cell is configured to respond to the part of the solar spectrum not covered by the silicon cell. The combination has achieved efficiencies of the order of 28% and is expected to achieve 30% .
Inorganic cells (Kesterite)
The Kesterite structure semiconductor (Cu2ZnSn(S,Se)4) is a promising light absorbing material which could dominate the next generation of thin film solar cells, owing to its low cost, non-toxic, and earth-abundant source materials. Kesterite is being considered as an alternative to perovskite and other thin-film technologies such as CdTe, CIGS and CdS because of the absence of toxic materials in its composition. Kesterite is also easy to manufacture. Fig. 8: *Kesterite (Cu ZnSnS shown here) has a tetragonal crystal structure. Cu = red, Zn = green, Sn = blue, S = orange (Peplow: Chemical and engineering news).*
Quantum dot PV devices
A quantum dot solar cell (QDSC) is a solar cell that uses quantum dots of semiconducting material as the photovoltaic material. Quantum dots have bandgaps which are adjustable through a wide array of energy levels by changing the size of the dots, and this allows adjustment of the spectral response of the QDSC. Current efficiency is approaching 17%.
Typical quantum dot materials would be GaAs, PbS, PbSe, CdS, CdSe, CdTe, and ZnSe, and recently perovskite is being considered for QDSC. The material requirements of QDSC are much lower than other PV devices, and the manufacturing process is simple. QDSC can be manufactured on flexible substrates. Quantum dots have the potential to dramatically increase the efficiency of PV in converting sunlight into energy, because of their ability to generate more than one bound electron-hole pair, or “exciton”, per incoming photon. Today’s solar cells produce only one exciton per incoming photon. This “multiple exciton generation” (MEG) effect is expected to yield efficiency figures of the order of 60%. Fig. 9: *Quantum dot solar cell structure (Alternate energy mag).*
 Z Shi, et al: Sustainable Energy & Fuels
 M Hossein, et al: Nano-Micro Lett. (2019)
 L Calderone , “*Quantum dot solar cells are coming*”, Alternate energy mag., 22 May 2018.
 “*Perovskites and Perovskite Solar Cells: An Introduction*”, Ossilla.com
 M Peplow: “*Kesterite solar cells get ready to shine*”, C&EN, Volume 96 Issue 7.
Send your comments to email@example.com