Single junction efficiency limits

Ideally, all photons with a wavelength of about 900 nm or shorter should be harvested and converted to electric current. This limit is derived from thermodynamic considerations showing that the conversion efficiency of any single-junction photovoltaic solar converter peaks at approximately 33% near the threshold energy of lAeV.1 2 There are numerous ways to convert the solar radiation directly into electrical power or chemical fuel. The silicon solar cell is the most efficient in this respect. Nevertheless, the capital cost of such devices is not attractive for large-scale applications. 

As mentioned in Section 9, the highest conversion efficiency observed to date for an amorphous silicon solar cell is 10.1 % (Catalano et al, 1982). The theoretical limit for the conversion efficiency of a single-junction a-Si H cell can be estimated to be —20%. This follows from an upper limit of — 22 mA cm-2 for JK as determined from optical absorption data (optical path length — 2 fim), and from upper limits of —1.0-1.05 V for and -0.86 for the fill factor (Tiedje, 1982). 

Overall, the key condition for PEC hydrogen production is that the quasi-Fermi levels under solar illumination generate sufficient useable photopotential to drive the redox reactions for water splitting after all the solid-state, interfacial and solution overpotential losses have been taken into accoxmt. This challenge has severely limited efficiency in all single-junction PEC devices to date. 

Fundamental thermodynamic limits for the conversion efficiency of single- and multijunction semiconductor PEC devices have also been established in numerous studies [74-81]. The thermodynamic single-junction limit under ideal conditions, i.e., including thermalization losses but with no overpotential losses, has been... 

Fig. 7.15 Maximum achievable photocurrent density levels and potential solar-to-hydrogen conversion efficiencies for single-junctions as a function of bandgap based on optical absorption limits. Highlighted are the high-bandgap materials that have demonstrated spontaneous PEC water splitting but at correspondingly low efficiencies...

As with the single-junction PV or PEC devices, efficiency bounds can be placed on multijunction configurations based on optical absorption limits. Figure 7.23 is a two-dimensi(Mial extension of Fig. 7.15 for tandem devices, where maximum photocurrent, and the corresponding STH levels for PEC devices, are calculated as a function of both top- and bottom-junction bandgaps. The assumptions included in the derivation of this graph are as follows ... 

As discussed in Sect. 7.3.1, fundamental thermodynamics limits conversion efficiency for both single- and multijunction semiconductor PEC devices have also been established in numerous studies. The thermodynamic tandem limit under ideal conditions, i.e., including thermalization loss, but with no overpotential loss, has been calculated at approximately 40% STH. This is consistent with the optical limits shown in Fig. 7.23. It is also substantially higher than the 30% STH limit established for the single-junction case. Moreover, employing advanced multi-exciton device schemes which eliminate part of the thermalization loss, the theoretical tandem limit is further increased to 46% STH [58]. 

The nontraditional approaches in photoelectrochemistry encompass systems and effects where the conversion efficiency exceeds that of the single junction Shockley-Queisser limit (defect level absorbers, multiple exciton generation, singlet fission). In a second approach, the use of hot electrons for initiating electrochemical reactions which otherwise would necessitate large overvoltages is attempted. Besides the direct use of non-thermalized hot electrons from the absorber surface [107], recently, contributions from the decay of surface plasmons have been investigated, too [108, 109]. The third approach is based on... 

OPV materials that can operate with a band gap of 1.1-1.3 eV have, so far, not been made. Several successful donor polymers have been synthesized that absorb light to energies as low as 1.3 eV. But the most commonly used acceptor, PCBM, has a band gap of 1.75 eV, which is ultimately the limiting factor for efficiency [9]. Several different electrical device models have been used to calculate the maximum possible PCE of an OPV device [9-11]. All three models give a maximum PCE for a single junction device of 10-11%. The model by Veldman et al. predicts the maximum possible Voc to be [9] ... 

The first generation was based upon thick Si-wafers, which could be etched, texturing the surface and enhancing absorption of photons. Efficiency of the laboratory-scale devices approached the theoretical limit for single junction PV of 31%. 

The great energy consumption, limited recources of traditional fuels and environmental problems have lead to intensive research on the conversion of solar energy during the last fifteen years. Conversion into electrical energy has been realized in technical devices consisting of pn-junction photovoltaic cells. Efficiencies of up to 20 % have been obtained with single crystal devices and around 9 % with polycrystalline or amorphous layers. 

As it has been described in various other review articles before, the conversion efficiencies of photovoltaic cells depend on the band gap of the semiconductor used in these systems The maximum efficiency is expected for a bandgap around Eg = 1.3eV. Theoretically, efficiencies up to 30% seem to be possible . Experimental values of 20% as obtained with single crystal solid state devices have been reported " . Since the basic properties are identical for solid/solid junctions and for solid/liquid junctions the same conditions for high efficiencies are valid. Before discussing special problems of electrochemical solar cells the limiting factors in solid photovoltaic cells will be described first. 

Water is involved in most of the photodecomposition reactions. Hence, nonaqueous electrolytes such as methanol, ethanol, N,N-d i methyl forma mide, acetonitrile, propylene carbonate, ethylene glycol, tetrahydrofuran, nitromethane, benzonitrile, and molten salts such as A1C13-butyl pyridium chloride are chosen. The efficiency of early cells prepared with nonaqueous solvents such as methanol and acetonitrile were low because of the high resistivity of the electrolyte, limited solubility of the redox species, and poor bulk and surface properties of the semiconductor. Recently, reasonably efficient and fairly stable cells have been prepared with nonaqueous electrolytes with a proper design of the electrolyte redox couple and by careful control of the material and surface properties [7], Results with single-crystal semiconductor electrodes can be obtained from table 2 in Ref. 15. Unfortunately, the efficiencies and stabilities achieved cannot justify the use of singlecrystal materials. Table 2 in Ref. 15 summarizes the results of liquid junction solar cells prepared with polycrystalline and thin-film semiconductors [15]. As can be seen the efficiencies are fair. Thin films provide several advantages over bulk materials. Despite these possibilities, the actual efficiencies of solid-state polycrystalline thin-film PV solar cells exceed those obtained with electrochemical PV cells [22,23]. 

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