Multiband Solar Cell

ORIGINAL: LBNL

Ene 28, 2011

It was recognized over thirty years ago that the introduction of states in a semiconductor band gap presents an alternative to multijunction designs for improving the power conversion efficiency of solar cells. The intermediate band acts as a “stepping stone,” allowing absorption of photons at three different energy levels, corresponding to the three different band gaps. In particular, low-energy photons are captured that would pass through a conventional solar cell (Fig. 1). Detailed theoretical calculations indicated that a single junction cell with a properly located band of intermediate states could achieve power conversion efficiencies up to 62% - i.e. higher than those for optimized double-junction tandem cells (Fig. 2). Even higher efficiencies of up to 71.7% were predicted for materials with two bands of intermediate states. 

Figure 1 A schematic of an intermediate band solar cell

Until very recently, practical realization of semiconductors with this multi-band structure had not been achieved. We have designed and synthesized a new class of semiconductor alloys with an intermediate band within the energy gap. The design of our material is based on the recently introduced band anticrossing (BAC) model of highly mismatched semiconductor alloys (HMAs). Group III-N-V alloys in which group V anions are partially replaced with N or group II-O-VI alloys in which column VI element is replaced with O are the well known examples of the HMAs. The electronic structure of the HMAs is determined by the interaction between localized states associated with N or O atoms and the extended states of the host semiconductor. As a result the conduction band splits into two subbands (E- and E+) with non-parabolic dispersion relations.

In most instances, e.g. N in GaAs or O in CdTe, the localized states are located within the conduction band and consequently a relatively wide lower subband is formed. A narrow band can be formed only if the localized states occur well below the conduction band edge. 

Figure 2 Comparison of the maximum power conversion efficiency of an intermediate band solar cell, a single gap solar cell, and a double junction tandem cell. The higher gap values for the tandem cell and the IBSC are indicated on the plot. (after Luque et. al. PRL, 78, 5014(1997)) 

In this case the E- subband states are of highly localized character and the E+subband states become more extended. This situation occurs for O in ZnTe, MnTe, MgTe. Figure 3 illustrates the formation of a narrow intermediate band by the incorporation of O into ZnTe. In addition to the O content, Mn alloying can also be used to adjust the energy level positions. Thus, band locations corresponding to those that are optimal for a multiband solar cell can be produced in ZnTeO and ZnMnTeO. With multiple band gaps that fall within the solar energy spectrum, Zn1-xMnxOyTe1-y is well suited for the proposed high efficiency intermediate band solar cells (IBSCs). This new II-VI oxide multi-band semiconductor was synthesized using the combination of oxygen ion implantation and pulsed laser melting. This highly non-equilibrium technique allowed for the synthesis of ZnMnOTe alloys with up to 3% of Te replaced with O atoms. Fig. 4 shows PR spectra from a Zn0.88Mn0.12Te substrate and two Zn0.88Mn0.12Te samples implanted with 3.3% of O followed by PLM with laser energy fluence of 0.15 and 0.3 J/cm2. Two optical transitions occurring at energies distinctly different from the fundamental band gap transition EM (=2.31 eV) of the Zn0.88Mn0.12Te matrix can be clearly observed at ~1.8 and 2.6 eV from the samples after PLM.

Fig. 3. Band anticrossing and formation of an intermediate band in Zn1-yMnyTe1-xOx.

Fig. 4. Photomodulated reflectance (PR) spectra obtained from Zn0.88Mn0.12Te samples as-grown and implanted with 3.3% O+followed by PLM with energy fluence of 0.15 and 0.3 J/cm2

These two optical transitions can be attributed to transitions from the valence band to the two conduction subbands, E+ (~2.6 eV) and E- (~1.8 eV) formed as a result of the hybridization of the localized O states and the extended conduction band states of ZnMnTe.

Our results from optical transitions clearly demonstrate that three types of optical transitions are possible in this band structure; (1) the transitions from the valence band to the E+subband with the absorption edge at EV+=E+(k=0)-EV(k=0)=2.56 eV, (2) transitions from the valence band to E- subband with the edge at EV-=E-(k=0)-EV(k=0)=1.83 eV and (3) the low energy transitions from E- to E+ with the absorption edge atE_+=E+(k=0)-E_(k=0)=0.73 eV. The three absorption edges span much of the solar spectrum, demonstrating that these alloys are good candidates for the multi-band semiconductors envisioned for high efficiency photovoltaic devices.

Detailed balance calculations of the power conversion efficiency for a intermediate band solar cell based on this material is shown in Fig. 5. Even for this non-optimal band gap configuration we calculate a power conversion efficiency of 45%, which is higher than the ideal efficiency of any solar cell based on a single junction in a single-gap semiconductor and is comparable to the efficiency of double-junction cells.

The potential technological importance of the multiband semiconductors raises the question if they can also be realized in group III-Nx-V1-x HMAs as well. In most III-V compounds the localized N level lies above the conduction band edge. An exception is the GaAs1-yPy alloy system in which N-level falls below the conduction band edge for y>0.3. Consequently the anticrossing interaction of the N states with the extended conduction band states in these GaAsP alloys is expected to result in the formation of a narrow band of intermediate states. Recently we have also synthesized GaNxAs1-yPy with y=0 to 0.4 using N+-implantation followed by PLM and RTA techniques. With an implanted N concentration of 2%, the N concentration incorporated in the As sublattice amounts to about 1% and 0.3% for films with y≤0.12 and y>0.12, respectively. GaNxAs1-yPy with y>0.2 clearly shows strong optical transitions corresponding to both the lower (E-) and upper (E+) conduction subbands. GaNxAs1-yPy alloys with y>0.3 have a three band structure making them suitable for testing the theoretical predictions of the highly-efficient intermediate band solar cell concept. Theoretical ideal efficiency for IBSC using the GaN0.02As0.58P0.4 is calculated to be >55%.

Fig. 5: The calculated power conversion efficiency for a solar cell fabricated from a Zn0.88Mn0.12OxTe1-x alloy as a function of O content. The solid line is an empirical polynomial fit of the calculated data. 

Selected References:

• A. Luque, A. Marti., Phys. Rev. Lett., 78, 5014 (1997).
• K. M. Yu, W. Walukiewicz, J. Wu, W. Shan, and J. W. Beeman, M. A. Scarpulla, O. D. Dubon, and P. Becla, “Diluted II-VI Oxide Semiconductors with Multiple Band Gaps,” Phys. Rev. Lett. 91, 246203 (2003).
• W. Shan, K. M. Yu, W. Walukiewicz, J. Wu, J. W. Beeman, and J.W. Ager III, M.A. Scarpulla, O.D. Dubon, and E. E. Haller, “Effects of Pressure on the Band Structure of Highly Mismatched Zn1-yMnyOxTe1-x Alloy,” Appl. Phys. Lett. 84, 924 (2004).
• K. M. Yu, W. Walukiewicz, J.W. Ager III, D. Bour, R. Farshchi, O. D. Dubon, S. X. Li, I. D. Sharp, and E. E. Haller, “Multiband GaNAsP Quaternary Alloys,” Appl. Phys. Lett. 88, 092110 (2006).
• K. M. Yu, W. Walukiewicz, M. A. Scarpulla, O. D. Dubon, W. Shan, J. Wu, J. W. Beeman, and P. Becla, “Synthesis and Properties of Highly Mismatched II-O-VI Alloys,” invited paper, pres. at E-MRS 2004 SPRING MEETING, Symposium M: Dilute nitride and related mismatched semiconductor alloys, Palais de la Musique et des Congres, Strasbourg, France, May 24-28, 2004. IEE Proceedings-Optoelectronics 151 (5): 452-459, Oct. 2004 (IEE-Inst. Elec. Emg., Michael Faraday House, Six Hills Way, Stevenage, Hertford, SG1 2AY, ENGLAND).

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ORIGINAL: Ecoosfera

Nuevos paneles solares capturan más energía de los rayos solares no visibles

En un invento que podría revolucionar a corto plazo la generación de energía alternativa a través del sol, un grupo de investigadores del Lawrence Berkeley National Laboratory anunció la creación de páneles solares que cargan energía aún en la oscuridad. La relevancia de este invento resulta evidente ya que permitiría eficientar radicalmente la creación de energía aprovechable a través del sol.

Actualmente uno de los grandes problemas de la energía solar es que depende de las horas de sol a las que esten expuestas los páneles. Esto quiere decir que en latitudes del norte, donde existen temporadas en las que el sol escacea, a veces resulta poco rentable la instalación de una infraestructura. Lo mismo sucede en días nublados y en promedio la mitad del tiempo no estarás produciendo energía por medio de esta vía.

Pero estas nuevas celdas solares permitirán la generación de energía aun durante la noche ya que están diseñadas para aprovechar casi la totalidad del espectro del sol, incluso cuando este no es visible en un lugar durante la noche. Pero el otro aspecto fundamental para considerar esta invención como algo energéticamente esperanzador es el hecho de que su costo será bastante accesible lo cual permitirá que su aprovechamiento se popularize y en este sentido es un real candidato a mejorar las expectativas frente a las energías alternativas con miras a emanciparnos de nuestra histórica dependencia de los hidrocarburos