Abstract
The ShockleyQueisser limit describes the maximum solar energy conversion efficiency achievable for a particular material and is the standard by which new photovoltaic technologies are compared. This limit is based on the principle of detailed balance, which equates the photon flux into a device to the particle flux (photons or electrons) out of that device. Nanostructured solar cells represent a novel class of photovoltaic devices and questions have been raised about whether or not they can exceed the ShockleyQueisser limit. Here we show that singlejunction nanostructured solar cells have a theoretical maximum efficiency of ∼42% under AM 1.5 solar illumination. While this exceeds the efficiency of a nonconcentrating planar device, it does not exceed the ShockleyQueisser limit for a planar device with optical concentration. We consider the effect of diffuse illumination and find that with optical concentration from the nanostructures of only × 1,000, an efficiency of 35.5% is achievable even with 25% diffuse illumination. We conclude that nanostructured solar cells offer an important route towards higher efficiency photovoltaic devices through a builtin optical concentration.
Introduction
In 1961, Shockley and Queisser developed a theoretical framework for determining the limiting efficiency of a single junction solar cell based on the principle of detailed balance equating the incoming and outgoing fluxes of photons for a device at opencircuit conditions^{1}. This model incorporates various light management and trapping techniques including photon recycling, optical concentration and emission angle restriction^{1,2,3}. It was recently suggested that a nanowire solar cell could exceed the ShockleyQueisser (SQ) limit based on its geometry^{4}; however, without exploiting 3rd generation photovoltaic (PV) concepts that break the assumptions of Shockley and Queisser (e.g. multiexciton generation, hot carrier collection, etc)^{5,6,7}, even nanowire solar cells should be bounded by the SQ limit. Here we show that for any nanostructured solar cell (e.g. composed from wires, cones, pyramids, etc.), the limiting efficiency is identical to that of a planar solar cell with concentrating optics and that the improvement results strictly from an increase in the opencircuit voltage. This formalism leads to a maximum efficiency of ∼42% for a nanostructured semiconductor with a bandgap energy of ∼1.43 eV (e.g. GaAs) under AM 1.5G illumination^{8}.
The SQ limit is reached by applying the principle of detailed balance to the particle flux into and out of the semiconductor^{1}. For every above bandgap photon that is absorbed by the semiconductor, one electronhole pair is generated. The maximum possible efficiency is achieved when nonradiative recombination is absent and all generated carriers are either collected as current in the leads or recombine, emitting a single photon per electronhole pair. The total generated current is:
where q is the charge of an electron and N_{abs} and N_{emit} are the numbers of photons per unit time that are absorbed or emitted by the photovoltaic device, respectively. These rates can be calculated as^{2}:
where σ_{abs}(θ, ϕ, E) is the absorption crosssection, F(E, T, V) is the spectral photon flux and θ_{max} is the maximum angle for absorption (for N_{abs}) or emission (for N_{emit}). For a bulk planar cell, the absorption crosssection is given by σ_{abs}(θ, ϕ, E) = A_{cell} × a(θ, ϕ, E), where A_{cell} is the top illuminated surface area of the cell and a(θ, ϕ, E) is the angle dependent probability of photon absorption for incident photons of energy E. In the simplest case, a(θ, ϕ, E) is a stepfunction going from 0 (for E < E_{g}) to 1 (for E ≥ E_{g}). The spectral photon flux can be obtained from the generalized Planck blackbody law^{9}:
where h is Planck’s constant, k_{b} is Boltzmann’s constant, c is the speed of light, n is the refractive index of the surroundings, which is usually taken to be vacuum (n = 1) and qV characterizes the quasiFermi level splitting when describing emission from the cell. The incoming flux from the sun can be obtained from experimental data (e.g. AM 1.5 solar spectrum) or from the blackbody expression above with V = 0 and where θ_{max} = θ_{s} = 0.267° is the acceptance halfangle for incident light from the sun at temperature T = T_{s} = 5760 K. The outgoing flux from the cell is given by Eq. (2) for a cell temperature T_{c} = 300 K, operating voltage V and emission halfangle θ_{max} = θ_{c} = 90°. At opencircuit conditions, there is no current extracted and the current balance equation becomes
where the middle term corresponds to absorption due to emission from the ambient surroundings, also at T = 300 K; however, this term is much smaller than the flux from the sun. Thus, the light generated current is given by I_{L} = qN (θ_{s}, T_{s}, V = 0) and the dark current, in the radiative limit, is given by , where I_{R} is the reverse saturation current. Solving Eq. (4) for the voltage yields the common expression for the opencircuit voltage^{1,8}:
which is valid for both bulk planar solar cells and nanostructured solar cells with the appropriate absorption crosssections as described in the next section.
Results
Nanostructured solar cells with builtin optical concentration
To achieve the maximum efficiency, we need to increase the light generated current compared to its bulk form or reduce the reverse saturation current to increase V_{oc}. For any absorbing structure, Eqs (2, 3, 4, 5) can be used to determine the resulting V_{oc} numerically; however, for the limiting case, we will consider a simple analytical expression. For maximum V_{oc}, we want the absorption crosssection to be maximized for angles near normal incidence up to an angle θ_{m} (where θ_{s} ≤ θ_{m}) and minimized for all other angles θ_{m} ≤ θ ≤ θ_{c}, where θ_{m} is some angle defined by the structure. We can define this piecewise function for the absorption crosssection as: σ_{abs}(θ : 0 →θ_{m}) = σ_{max} and σ_{abs}(θ : θ_{m} →θ_{c}) = σ_{min}, which allows us to perform the solid angle integration to determine the light and dark currents:
where σ_{abs} = 0 for E < E_{g}, I_{L,0} is the light generated current for an ideal bulk cell of area A_{cell}, and
where I_{R,0} is the reverse saturation current for a bulk cell. Substituting these expressions into Eq. (5), we have
where
Thus, the opencircuit voltage for a nanostructured device takes on the same form as the opencircuit voltage for a macroscopic concentrating system, where X is the concentration factor^{8}. For maximum concentration, we consider the limit as θ_{m }→θ_{s} and σ_{min} → 0, yielding
which is the same as the maximum concentration factor that is obtained for a macroscale concentrator and results in a maximum solar energy conversion efficiency of ∼42%. For practical devices it is reasonable to assume a minimum value of σ_{min} corresponding to the geometric crosssection of the device, σ_{min} →σ_{geo}. For this case and with cos(2θ_{m}) = cos(2θ_{s}) ≈ 1, we get X = σ_{max}/σ_{geo} and the opencircuit voltage reduces to:
Finally, the power conversion efficiency is given by η = I_{L}V_{oc}FF/P_{in}, where FF is the fillfactor, which can be obtained from the I−V characteristic defined by Eq. 1 and P_{in} is the incident power from the sun. We note that the area used to calculate P_{in} is determined by the illumination area and not the geometric crosssection, which would lead to undercounting the number of incident photons. In general, optical concentration can be achieved using lenses, mirrors, or unique optical nanostructures (see Fig. 1(a)). A nanostructured solar cell can result in optical concentration that is similar to the concentration obtained using lens or parabolic mirrors but relies on the wave nature of light. Fig. 1 (b) shows the power conversion efficiency of recently reported vertically aligned nanowirebased PV cells^{4,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24}. The optical and geometrical crosssections are extracted from the current density data and from the geometrical information provided within the references. The vast majority of the experiments are focused on Si, GaAs and InP radial or axial junction nanowire arrays fabricated with various techniques, such as MBE, MOVCD, reactiveion etching, etc. Generally, is found to fall in the range of 1–10 for these structures; however, the actual concentration factor is likely significantly smaller if σ_{min} > σ_{geo}. Additionally, the reduced efficiency in these nanowire structures compared to the theoretical limit is due to significant surface recombination and device and material constraints that could be improved with further experimental development.
The effect of entropic losses on V_{oc}
Next we consider an alternative, but equivalent, approach to understanding the maximum efficiency of a nanostructured PV device by considering the energetic and entropic loss mechanisms^{25,26,27}. The generalized Planck equation can be used to determine the opencircuit voltage of a solar cell operating at the maximum efficiency limit^{25,28,29}:
where γ_{s} and γ_{c} are blackbody radiation flux terms that depend on E_{g}, T_{s} and T_{c}. The first term represents a voltage drop related to the conversion of thermal energy into work (sometimes called the Carnot factor). The second term occurs from the mismatch between Boltzmann distributions at T_{c} and T_{s}^{30}. The third term is the voltage loss due to entropy generation as a result of a mismatch between the absorption solid angle and the emission solid angle of the cell. This third term represents a voltage drop of ∼0.28 V, which can be recovered if Ω_{emit} = Ω_{abs}. Modification of the directionality of absorption and emission to improve the opencircuit voltage of a solar cell is wellknown^{31,32,33} and has recently been shown in experiments^{34,35}.
The most common way to recover the entropy loss due to the mismatch between the absorption and emission solid angles is through optical concentration (Fig. 2(a)). For a planar solar cell without optical concentration, the absorption solid angle corresponds to the sun’s angular extent, i.e. Ω_{abs} = 2π(1 − cos(θ_{s})) = 6.82 × 10^{−5} sr. However, emission from the cell occurs over Ω_{emit} = 4π. The addition of a back reflector reduces the emission solid angle to Ω_{emit} = 2π, resulting in a slight voltage improvement^{2}. For more substantial voltage improvements, optical concentration is necessary. Optical concentration enables the absorption solid angle to exceed the sun’s solid angle and approach the cell’s emission solid angle (Fig. 2(a)), which could largely increase the V_{oc}.
Properly designed photovoltaic nanostructures can have the same effect, reducing the entropy generation by either increasing Ω_{abs} or by reducing Ω_{emit} in an attempt to achieve Ω_{emit} = Ω_{abs} (Fig. 2(b)). From a device pointofview, Ω_{abs} is related to the light generated current density, J_{L} = I_{L}/A and Ω_{emit} is related to the reverse saturation current density, J_{R} = I_{R}/A. Because the V_{oc} depends on their ratio (see Eq. 5), increasing Ω_{abs} will have the same affect as decreasing Ω_{emit}. Thus, the voltage improvement can equivalently be seen from the thermodynamics of reduced entropy generation or from the device aspects of the pn junction.
According to Kirchhoff’s law, the emissivity and absorptivity of a solar cell are equal in thermal equilibrium^{2,36}. For a standard cell without back reflector, the device can absorb the incident power from all directions and hence will emit in all directions (Fig. 3(a)). The addition of a back reflector reduces both absorption and emission from the back surface (Fig. 3(b)); however, this has no effect on the absorption of the incident solar power because no illumination is coming from the back. Thus, I_{L} is unaffected by the addition of the back reflector but I_{R} is reduced (note: technically I_{L} could be slightly increased due to an increased path length in thin or low absorption materials, resulting in a small increase in V_{oc}). An ideal nanostructure would allow for absorption only over the range of angles corresponding to the incident illumination of the source, i.e. the sun (Fig. 3(c)). The currentvoltage characteristics for these devices show that a back reflector yields a ∼2% increase in efficiency over the traditional planar device and an ideal nanostructure yields a ∼11% improvement, resulting in a ∼42% efficient device.
Effect of diffuse illumination
While the maximum power conversion efficiency is achieved with 100% direct illumination (i.e. the incident light is completely within the solid angle defined by θ_{s}), an efficiency of ∼38% can be achieved when 25% of the incident illumination is diffuse (Fig. 4), which is typical of many geographic regions. Incident illumination on earth contains both direct and diffuse components (due to scattering of the incident light). Using traditional macroscopic concentrating optics, light is concentrated for all wavelengths and only the direct components can be used. Alternatively, nanostructures typically have a wavelengthdependent response and may only be able to concentrate light over a particular bandwidth, e.g. from the semiconductor bandgap energy (E_{sc}) to some cutoff energy (E_{cutoff}). This limited bandwidth for concentration is beneficial when the illumination is not 100% direct, because the diffuse components that lie outside this range can still be collected.
Figure 4 shows that efficiencies >35% can be achieved even when the illumination contains a significant fraction of diffuse light. The nanostructures depicted in Fig. 4 are able to concentrate the incident light from E_{sc} to E_{cutoff} and are unable to concentrate light with energies >E_{cutoff}, which corresponds to absorption of diffuse light in that bandwidth. For E_{cutoff} = 1.74 eV, X = 1,000 and 25% diffuse illumination, the nanostructured devices reach an efficiency of 35.5%.
Numerical simulation of nanowire PV
While the above discussion is general and provides the limiting efficiency of any nanostructured solar cell (e.g. wires, cones, pyramids, etc.), explicate cell architectures can be studied via numerical simulation. There are no implicit assumption about the directionality of the absorption or emission; these quantities are numerically calculated directly for each structure. We have simulated a bulk (80 μm thick) GaAs solar cell and a nanowire solar cell with the same thickness (with periodicity of 300 nm and radius of 75 nm) using the S4 simulation package^{37} to obtain the absorption profiles. We then solved the detailed balance expression numerically^{38,39}. A similar method was recently used to calculate the detailed balance efficiency for an InP nanowire array and an efficiency improvement of 1.5% was reported compared to a bulk device^{40}. For simplicity, we used the blackbody spectrum in the following calculations. The nanowires are embedded within a material with an index of refraction of n = 2.66 and both the nanowire and planar structures are coated with a doublelayer antireflection coating (a 52 nm layer with n = 2.66 and a 98 nm layer with n = 1.46). The antireflection coating is designed to maximize the efficiency of the bulk GaAs cell. The integrated short circuit current density is almost identical for both cases (<1% difference); however, the emitted power density is significantly different. Because a large amount of the radiated power is near the bandgap, the lower absorption rate near the bandgap that occurs with the nanowire structure leads to a decrease in emission. This effect is demonstrated in Fig. 5(d), where the bulk cell has a higher reverse saturation current density compared to the nanowire cell with same thickness. The reverse saturation current of the nanowire cell decreases by 3.46% and the absorption increases by 0.38%. As a result, the V_{oc} increases by 1 mV due to these combined effects in the nanowire device and thus, the nanowire solar cell has a slightly higher efficiency than the bulk device (28.22% vs. 28.09%).
Ideally, an optical structure should be designed to minimize absorption for angles greater than θ_{s}, particularly near the semiconductor bandgap, which is where the emission is peaked. To emphasize this effect, we consider a smaller radius nanowire (40 nm), which will have increased optical concentration. In order to minimize the loss in photogenerated current, the periodicity is decreased to 200 nm and the nanowire length is set to 2 μm, which is a reasonable thickness for a GaAs cell. Figure 5(c) shows this device whose absorption near the bandgap is limited so that the reverse saturation current density is one order of magnitude smaller than that of the bulk cell (Fig. 5(d)). This nanostructuring leads to the reverse saturation current decreasing from 8.751 × 10^{−18} to 9.946 × 10^{−19} A/m^{2}. Although the absorption is also decreased (J_{L} decreased from 362.68 to 237.55 A/m^{2}), the V_{oc} is increased from 1.169 V to 1.214 V, showing an improvement of 45 mV in V_{oc}. This result suggests that nanostructures that incorporate more complexity may yield higher V_{oc}’s without loss in I_{L}.
Discussion
While the overall performance of nanostructured solar cells is still bounded by the SQ limit, one must consider the builtin optical concentration when applying this theory. Recently an InP nanowire solar cell was found to have a V_{oc} in excess of the record InP planar device^{21,41}. This improvement is likely the result of the builtin optical concentration, which leads to higher carrier densities and hence a higher V_{oc}. Although the best devices to date are <14% efficient^{4,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24}, there is great potential for improvement, which could allow nanowire solar cells to exceed 40% solar power efficiency. Here we have shown that besides the possibility of improved carrier collection that has been previously reported^{42,43,44}, another key advantage of nanostructured solar cells over planar ones is that the optical concentration is already builtin, yielding the possibility of higher efficiencies than planar devices.
The main limitations for exploiting these concepts in practical devices lie in minimization of nonradiative recombination and achieving appropriate optical design. Minimizing both surface and bulk nonradiative recombination is important for all PV technologies and great strides have been achieved recently. GaAs has been shown to have an internal luminescence efficiency of >99%, leading to solar cells that operate in the radiative limit^{45,46}, a key requirement for exploiting the concepts discussed in this manuscript. For nanostructured PV, nonradiative recombination is likely dominated by surface recombination. InP has shown excellent promise for nanostructured PV with unpassivated nanowire structures yielding surface recombination velocities as low as 170 cm/s^{47,48}. Finally, implementation of high quality optical structures with the appropriate angular and frequency dependence may be further guided by concepts from metamaterials, metasurfaces and transformation optics, which have previously yielded broadband angular selectivity^{49,50}.
In conclusion, we have used the principle of detailed balance to determine the maximum efficiency for nanostructured photovoltaic devices. Because the principle of detailed balance requires knowledge of the absorption within the structure rather than the detailed geometry or arrangement, any specific nanostructure (regardless of configuration) will be bounded by this limit. The role of the geometry, period, disorder, etc. are all included by considering the absorption spectrum. The ideal nanostructured devices result in an efficiency of 42%, which is equivalent to the result of Shockley and Queisser when considering full optical concentration. This improvement comes strictly from an improvement of the opencircuit voltage and not from an improvement in the current. We have assumed that the cell is limited by radiative emission and is under direct illumination in order to achieve the maximum efficiency limit. As with other forms of optical concentration, the efficiency is reduced if part of the incident illumination is diffuse (e.g. if 25% of the incident light is diffuse, the maximum efficiency is reduced to 38%). For future nanostructured devices to take advantage of these benefits, high quality surface passivation and reduced nonradiative recombination are needed. From an optical design pointofview, nanostructures should be created that have limited absorption for angles and wavelengths that do not match the incident illumination. When this condition is achieved, new high efficiency nanostructured PV devices will be possible.
Additional Information
How to cite this article: Xu, Y. et al. The generalized ShockleyQueisser limit for nanostructured solar cells. Sci. Rep. 5, 13536; doi: 10.1038/srep13536 (2015).
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Acknowledgements
Authors acknowledge the University of Maryland for startup funds to initiate this project and support by the National Science Foundation under Grant CBET1335857.
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Y.X. and J.N.M. conceived the project and performed analytical calculations. Y.X. and T.G. performed numerical calculations. J.N.M. supervised the project. All authors reviewed the manuscript.
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Xu, Y., Gong, T. & Munday, J. The generalized ShockleyQueisser limit for nanostructured solar cells. Sci Rep 5, 13536 (2015). https://doi.org/10.1038/srep13536
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