above that from a singe junction solar cell. While there are a number of advanced concept approaches, a feature of theses is that they have focused primarily on ...
HYRBID ADVANCED CONCEPT SOLAR CELLS
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1 2 1 1 1 Christiana B. Honsberg ,Stephen Bremner , Jongwon Lee , Adam Bailey and Som Dahal School of Electrical, Computer and Energy Engineering, Arizona State University, Tempe, Arizona, USA 2 School of Photovoltaic Engineering, University of New South Wales, Sydney, Australia
ABSTRACT Advanced concept or third generation solar cells rely on modification to the basic process of absorption /recombination or collection to increase the efficiency above that from a singe junction solar cell. While there are a number of advanced concept approaches, a feature of theses is that they have focused primarily on inclusion of a single type of physical mechanism. Such single mechanism advanced concept solar cells are in general limited in efficiency to the equivalent tandem solar cell which has the same number of processes. This paper demonstrates that solar cells using combinations of different approaches have higher thermodynamic efficiencies. Some combinations demonstrate higher efficiencies that either of the constituent approaches, and, significantly, even exceed the efficiency of the “equivalent” tandem structure. Moreover, such advanced concept hybrid structures have benefits other than efficiency increases, including reduced sensitivity to materials or to physical processes which are difficult to implement, or to non-idealities in the solar cell structure. INTRODUCTION Advanced concept or third generation solar cells offer the advantage of thermodynamically exceeding the ShockleyQueisser (SQ) efficiency limit. Despite their ability to theoretically exceed the single junction limit, they still have several limitations in efficiency. First, even theoretically, advanced concept approaches are limited by the “equivalent” tandem, where equivalent means a tandem with the same number of junctions as the advanced concept solar cell has distinct physical processes. For example, the intermediate band solar cell, with three absorption/emission processes has a similar efficiency (but slightly lower) than a three-junction tandem solar cell. A multiple carrier or multiple exciton solar cell with M=2 (the maximum number of electron/hole pairs generated) has an efficiency below that of a two junction tandem for one sun and nearly identical under maximum concentration. Hot carrier or thermophotonic solar cell, which essentially allow nearly all energy transitions, come closest theoretically to achieving the infinite-tandem solar cell efficiency. A further issue with advanced concept approaches is that as non-idealities are included, the efficiency limit of nearly all approaches is approximately 50-55%, or about what is expected from an optimized three-junction tandem. For example, the intermediate band approach, under
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maximum concentration has an efficiency limit of 63%. However, finite concentration, inclusion of effects such as limited spectral selectivity, finite width of energy bands and the availability of ideal band gaps makes efficiencies of about 50%. While the IB approach can be extended to include additional intermediate bands [1] to reach higher efficiencies, but even theoretically the ability to include more than a single intermediate band encounters difficulties in the ability to balance the rates given the solar spectrum. Similarly, assuming infinite concentration and an “infinite M” value (the maximum number of electron/hole pairs which can be generated from a high energy photon), the maximum efficiency is over 80%. However, given limited concentration, limited M values, and threshold energies above mEG (where m is the number of electron/hole pairs generated) further reduces efficiency to the order of 50% [2]. Other approaches such as thermophotonic and hot carrier devices also show efficiencies on the order of 50% with the inclusion of non-idealities [3,4]. Overall, despite the thermodynamic promise of advanced concept solar cells approaches involving a single concept have efficiencies of near 50%, about the theoretical efficiency of a three-junction tandem solar cell. This paper shows that by combing different advanced concept approaches or by including conventional solar cells, both practical and theoretical advantages can be achieved. The paper illustrates two main types of advantages; (1) Where, even in relatively simple structures, the efficiency of the combination is higher than that of its constituent parts, or the efficiency of the combination achieves an efficiency advantage; (2) Where the combination incurs practical benefits, such as a broader range of materials or easier implementation of approaches, or reduced impact of non-idealities, often with the added benefit of higher efficiency. INCREASED EFFICIENCY OF HYBRID APPROACHES A key reason to examine a hybrid advanced solar cell is that it has higher efficiency than either of the component efficiencies. The simplest structure (although not with the highest efficiency impact) to demonstrate this is shown in Figure 1. It consists of single junction solar cell in series or independently connected to an intermediate band (IB) solar cell modified to bar any transitions between the valence band (VB) and conduction band (CB). This is thermodynamically consistent since barring both absorption and recombination processes does not
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introduce a thermodynamic discrepancy. The efficiency of this combination is marginally higher than that of the intermediate band (64.1% as compared to 63.2%), despite the fact that the solar cell consists of the same number of absorption/emission processes as the intermediate band solar cell. The model for this case treats the single junction as conventional in detailed balance. The IB detailed balance calculations are modified from the conventional IB equations by eliminating both the recombination and absorption between the conduction and valence band. The two cell structures are then connected in parallel. A parallel connection is chosen since this most closely approximates the conventional IB approach and allows efficiency comparisons. More significant that the increased configuration compared to the IB approach is that the efficiency is also marginally higher than a three junction tandem. This shows that by appropriate choice of hybrid combinations, the theoretical efficiency can be increased, without increasing the number of physical processes or materials needed. This results in the first advanced concept structure where, using the same number of physical process or absorption/recombination processes, the efficiency exceeds that of an equivalent tandem.
a hybrid approach based on thermal/conventional combinations, which, in a single solar cell structure, allows a theoretical efficiency limit of close to the thermodynamic maximum efficiency of 70% at one sun, substantially higher than other approaches, including IB (47%) MEG (46%), hot carrier solar cell (54%). Intermediate band hybrids Intermediate band solar cells give a detailed balance efficiency similar to that of an ideal triple junction solar cell. Thus, a key advantage is not the efficiency as such, but the fact that this efficiency can be reached in a single solar cell structure Hybrid intermediate band structures can be used to both increase the efficiency of advanced concept approaches as well as to mitigate non-ideal effects. The conceptually most straight forward intermediate band hybrid is a tandem with two intermediate bands. The results of the intermediate band tandem are shown in Table 1 and Table 2 below. Table 1 shows the efficiency and optimum band gaps for a 5J and 6J tandem, under one sun and maximum concentration, while Table 2 shows the optimum for tandem intermediate band. Table 1: Efficiency and band gpas for a 5J and 6J tandems N 5 6
C 1 1
Eg1 0.66 0.45 0.61 0.40
Eg2 1.07 0.88 0.96 0.78
Eg3 1.50 1.34 1.33 1.17
Eg4 2.03 1.88 1.74 1.60
Eg5 2.79 2.66 2.26 2.12
Eg6
eff
3.00 2.87
56.0 72.0 58.0 74.4
Table 2: Band gaps and efficiency of tandem IB. Con ECI1 EIV1 Eg1 ECI2 EIV2 Eg2 One 0.54 1.02 1.56 1.57 2.13 3.7 1000 0.44 0.88 1.32 1.33 1.89 3.22 10000 0.4 0.82 1.22 1.23 1.79 3.02 Max 0.4 0.82 1.22 1.23 1.79 3.02
Figure 1: Schematic of a simple hybrid solar cell. OTHER BENEFITS OF ADVANCED CONCEPT HYBRID SOLAR CELLS While a hybrid solar cell may have a higher efficiency than an equivalent solar cell using the same number of materials or physical processes, hybrid solar cells may have additional practical benefits. Several examples are used to illustrate this point, including intermediate band hybrids (including approaches with hot carriers, tandem combinations of intermediate band solar cells, and IB solar cells with multiple carrier generation). The final example is
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Eff 53.86 64.83 68.52 70.83
The results in Tables 1 and 2 show that a IB tandem can achieve an efficiency similar to a 5J tandem. The efficiency are not those of 6-junction tandem, as mght be expected to the overlap of the band gaps in the two devices. However, the band gaps are substantially different than in a single IBT. For the lower band gaps, the tandem makes the material problem substantially easier by reducing the conduction band offset required to achieve these results. The materials for the higher band gaps devices are more difficult in both cases due to the need for high band gaps. In the IB tandem, the higher band gaps are suitable from the AlGaN material system; this material system also exhibits a low valence band offset. An additional approach for an intermediate band tandem is a hybrid between an IB and a multiple exciton device. The hybrid transistions are shown in Figure 2 and the results for maximum concentration in Table 3. The results use M=2, such that the MEG events are only used to promote
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the carriers to a higher energy level. Even with a very moderate assumption for M, the efficiency of the hybrid increase. A potentially more important result, however, is that the optimum band gaps decrease, increasing the viability of the materials for the hybrid.
The energy extracted from these hot carriers is transferred to the carriers in the bottom of the QD, and enough energy is extracted so that they can escape.
Figure 2: Possible transitions of carriers of in MEG/IB solar cell. Maximum ECI EIV Maximum Concentrtiton Efficiency (%) IB 0.72 1.25 63.17 MIB 0.6 1.1 65.07 Table 3: The maximum efficiency and its optimum conditions (maximum concentration). Although the efficiency increase is modest, this shows that even with small M values that can more readily experimentally realized, there is both an efficiency and a practical advantage. One practical advantage is the trend towards lower band gap when MEG processes are included. This is important for nano-structured implementations of IB approaches, since a limitation in the implementation of a nano-structured IB approach is realizing a material system with both low valence band offset and high conduction band offset (or, theoretically, the reverse). The lower requirement of the valence band offset in the hybrid approach makes it easier to implement. Further, it provides a benefit for MEG processes by allowing the realization of a low effective band gap in a solar cell without introducing the disadvantages of high doping and difficulty in making pn junctions in very low band gaps. Other practical benefits [5,6] are that the inclusion of MEG processes provide an additional avenue for extracting carriers from the IB, such that even with MEG recombination processes dominating, the IB/MEG device outperforms a two junction tandem or improves efficiency when the band gaps in the IB are non-ideal.
Figure 3: Schematic of a hot carrier/IB hybrid. The transitions in the intermediate band (IB) are modified such that the only transitions from the conduction band to valence band (CB-VB) and from the (VB-IB) are present. To extract carriers from the IB, hot carrier processes are used. Carriers crossing the QD are hot in the QW. Energy is extracted from these hot carriers, and as a result, the average energy of these carriers is reduced. The energy extracted from these hot carriers is transferred to the carriers in the bottom of the QD, and enough energy is extracted so that they can escape. The efficiency of this process is higher than a conventional IB or a thermal converter at one sun. This is physically due to the fact that a hot carrier process is more efficient than a radiative/recombination transition when carriers are subject to thermalization. In the intermediate band processes, there is a broad distribution of carrier temperature in the CB, and hence the efficiency of the hot carrier process is higher than that for a radiative transition. The efficiency of the hybrid continues to increases as the concentration increases, but since a hot carrier solar cell by itself increases its efficiency radiply with concentration, the efficiency advantage undergoes a peak at concentrations between 200X-500X.
A key issue in an IB solar cell is the excitation of carriers from the intermediate band (or isolated energy level). One approach to mitigate this issue is to use thermal escape rather than a radiative process to extract the carriers. The concept is shown in Figure 3. The transitions in the are modified such that the only transitions from the conduction band to valence band (CB-VB) and from the (VB-IB) are present. To extract carriers from the IB, hot carrier processes are used. Carriers crossing the QD are hot in the QD, and energy is extracted from these hot carriers.
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Figure 4 Efficiency vs ECI with one sun and maximum concentration An additional advantage of the hybrid hot carrier/IB approach is that carrier extraction may prove easier in a QD or QW implementation of IB than in a conventional hot carrier or in a conventional IB solar cell. There are several reasons for this. First, compared to radiative emission, the hot carrier/IB is less sensitive to values of the IB-CB band gap, which simplifies the materials selection. Further, the conditions for radiative emission are relatively stringent, requiring that the absorption coefficient of the IB-CB process is similar compared to that of both the CB-VB and to the VB-IB and also that there is a large density of states in the IB. Both of these conditions are relaxed in the hybrid. Compared to a hot carrier only device, the hybrid also offers benefits. Because the hybrid structure inherently consists of QD or QW separated by bulk regions in which hot carriers are not necessary, hot carrier collection must occur only in the nano-structures. Rapid collection in such nanostructures has already been shown, increasing the potential of implementing a hot carrier/IB hybrid. THERMAL HYBRIDS The previous examples of hybrid advanced concept solar cells show efficiency increases compared to existing advanced concept concepts, particularly under moderate concentration. However, hybrid advanced concept structures can also achieve, in a single structure, efficiencies close to the maximum thermodynamic efficiency, even under one sun conditions. Figure 4 shows the comparison, at one sun of a conventional solar cell, a hot carrier solar cell, and a combination of hot carrier or thermal-type conversion with a conventional solar cell approach [9].
Figure 5: Thermal hybrid (red) compared to other approaches. The IB and MEG are shown as a constant dashed line, since their efficiency depends not on a single band gap. CONCLUSION AND DISCUSSION Advanced concept hybrid solar cells consisting of combinations of advanced concept or conventional solar cell approaches offer multiple advantages. In terms of efficiency, they allow efficiencies higher than either of the constituent components, including, uniquely, solar cell approaches which allow efficiencies close the thermodynamic maximum under one sun. In addition, they allow other benefits, ranging from relaxing material constraint, to addressing conceptual difficulties in achieving processes such as rapid carrier collection in hot carrier devices or emission from the IB to the CB in intermediate band solar cells. REFERENCES [1] W. Shockley and H.J Queisser. "Detailed Balance Limit of Efficiency of p-n Junction Solar Cells", J.Appl.Phys.32,1961, pp.510-519. [2] J.H. Werner et al., “Novel optimization principles and efficiency limits for semiconductor solar cells”, Phys. Rev. Lett, 72, 1994, pp. 3851-3854. [3] A. Luque and A. MartiI., “Increasing the Efficiency of Ideal Solar Cells by Photon Induced Transitions at Intermediate Levels”, Phys. Rev. Lett. 78, 1997, pp 50145017. [4] A. Luque et al, "Impact-Ionization-Assisted Intermediate Band Solar Cell", IEEE Tranctions on Electron Devices",50, 2003, pp. 447-454. [5] M. Ley et al, "Thermodynamic Efficiency of an Intermediate Band Photovoltaic Cell with Low Threshold Auger Generation", J. Appl. Phys, 98, 2005, pp 1-5.
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