May, 24 2013
Index ¦ Research Lines ¦ Fundamental Studies ¦ Group Activity
In a conventional solar cell, the value of the forbidden band determines both current and voltage. High values produce low currents (just a few photons are absorbed) and high voltages, and vice versa. There is an optimum which is theoretically oriented towards the silicon forbidden band (for isotropic solar illumination) .
Members of the team1 proposed in 1997 the convenience of placing an intermediate band (IB) allowed in the middle of the semiconductor forbidden band. According to a procedure patented by this team, the solar cell is formed up by placing the intermediate band material between two ordinary semiconductors: one of them n-type for contacting the conduction band (CB) and the other, p-type, for contacting the valence band (VB). The intermediate band is therefore isolated from the metallic contacts.
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In this cell, as well as the ordinary process of electron pumping from the VB to the CB by means of a photon with sufficient energy, we must consider the pumping process to be effected in two phases: one from the VB to the IB with a lower energy photon, followed by the pumping from the IB to the CB by means of another lower energy photon (Fig 1). Maximum photovoltaic conversion efficiency is 63.2 % to be compared to the 40.7 % limit shown by single gap cells, or the 55.4 % shown by the two cells multi-junction. Obviously, the optimum value of the forbidden band is not produced by silicon (1.1 e.V.), but it is located 1.95 e.V. with the forbidden sub-bands of 0.7 and 1.2 e.V.approximately.
Certainly, the undeniable convenience of this cell could be marred by the difficulty to obtain intermediate band materials, as some prestigious researchers have anytime forecasted. The answer is, in fact, that intermediate band materials do exist and can be obtained in different ways.
Our team has already theoretical and experimentally studied the practical implementation of intermediate band solar cell by means of quantum dot technology (see Figure 2)2,3. With this approximation, the intermediate band emerges from the energetic levels of the electrons confined into the dots. Intermediate band quantum dot solar cells have already been manufactured, achieving up to 9% efficiencies. Nevertheless, as already mentioned, the potential is very high and the obtained low efficiencies are mainly due to the low absorption of the intermediate band on account of the low density of the absorption centres (lower 1017 cm-3) of the quantum dot system. As a matter of fact, these cells must be considered as research test structures and not yet as devices of a practical interest.
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From a researching point of view, the production of photocurrent as a consequence of the absorption of photons, and therefore transitions from the VB to the IB, has already been experimentally proven4. Recent results proving the absorption between the IB and the CB using FTIR are still unpublished.
On the other hand, the production of high voltages depends on the effects of three different electrochemical potentials (or quasi-Fermi levels), one for the VB, another one for the CB and the third one for the IB.
We have recently demonstrated the mentioned separation5 in a paper published in the Virtual Journal of Nanoscale Science & Technology, August 29, 2005. The importance of these results has been noticed by researchers in USA6,7,8, Japan9 and Australia10 where, as a result of our proposal in 1997 —with more than 130 citations according to the Science Citation Index— and subsequent works, research lines on the intermediate band solar cells are being established.
It is, therefore, the intention of our research consortium to join forces keep this leading position. On the other hand, it is well known the fact that impurities produce deep levels on the semiconductors, near the forbidden band. Quantum calculations reproduce this situation for many impurities11 with not very high concentrations. When the concentration of impurities arise the IB, it tends to enter either the CB or the VB. An exception to be mentioned is Ti on a GaAs or PGa12 matrix or keeping the intermediate band more easily separated due to its s2d2 configuration.
However, on account of thermodynamic reasons, Ti could not be properly incorporated to the matrixes in the desired concentrations13. Obviously, the higher the concentration of impurities, the higher the obtained absorption, which is the aimed characteristic. These levels are known as centers of non radiative recombination (SRH recombination) but under certain circumstances we have theoretically predicted —according to a still unpublished but already patented14 work— this recombination would disappear if a concentration producing the Mott transition were produced, i.e.: from wave functions of the impurities located on them to functions distributed on the semiconductor —which is produced at concentrations of around 6×1019 cm-3–.
The success of the research on intermediate band materials would constitute a full-scale scientific achievement, influencing on the substantial reduction of photovoltaic solar energy costs.
[1] A. Luque y A. Martí, Physical Review Letters, 78(26):5014–5017, 1997
[2] A. Martí, L. Cuadra y A. Luque, IEEE Trans. Elec. Dev., 48(10):pp. 2394–2399 (2001).
[3] A. Martí, L. Cuadra and A. Luque, IEEE Trans. Elec. Dev., 49(9):pp. 1632–1639 (2002)
[4] A. Luque, A. Martí, C. Stanley, N. López, L. Cuadra, D. Zhou and A. Mc- Kee, Journal of Applied Physics, 96(1):pp. 903–909 (2004).
[5] A. Luque, A. Martí, N. López, E. Antolín, E. Cánovas, C. Stanley, C Farmer, L.J. Caballero, L. Cuadra and J.L. Balenzategui. Appl. Phys. Lett. 87, 083505 (2005).
[6] K. M. Yu, W. Walukiewicz, J. Wu, W. Shan, J. W. Beeman, M. A. Scarpulla, D. Dubon y P. Becla, Physical Review Letters, 91(24):pp. 24603–1 (2003).
[7] A. G. Norman, M. C. Hanna, P. Dippo, D. H. Levi, R. C. Reedy, J. S.Ward y M. M. Al-Jassim, in “Proc. of the 31st IEEE Photovoltaics Specialists Conference”, (2005).
[8] L Nathan et al. “Basic Research needs for solar energy utilisation” The Office of Science, Department of Energy,Washington 2005.
[9] Y. Okada, N.Shiotsuka, H. Komiyama, K.Akahane, N.Ohtani, 20th European Photovoltaic Solar Energy Conferenceand the Exhibition, 2005.
[10] M. A. Green, Prog. in Photov, Research and Applications, 9(2):pp. 137–144 (2001).
[11] P. Palacios, J. J. Fernández, K. Sánchez, J. C.Conesa, and P. Wahnón, Phys. Rev. B, 73, 2006, pp. 085206.
[12] P. Wahnón and C. Tablero, Physical Review B. Condensed Matter, 65(165115):pp. 1–10 (2002)
[13] P. Wahnón, P. Palacios, J.J. Fernández & K. Sanchez J.C. Conesa 20th European Photovoltaic Solar Energy Conference and the Exhibition, 2005.
[14] A Luque, A Martí, E. Antolín, C. Tablero “Método Para La Supresión de la Recombinación No-Radiativa en Materiales Dopados con Centros Profundos”. Solicitud de Patente Española P200503055, (2005).
