1. Introduction
Tandem solar cells, which consist of several sub-cells with different energy band gap energies, enable conversion efficiency to be increased. As one suitable candidate for the top cell material, GaInP has been proved to be the best option owing to its good material quality and appropriate band gap energy[1-4]. From the modeling at a given band gap energy of 1.42 eV of GaAs, a potential efficiency as high as 36% of GaAs/GaInP dual-junction at AM 1.5G light intensity is expected[5]. For GaInP/GaAs dual-junction solar cells (SCs), since 1997 a record efficiency of 30.2% has not been beaten, though plenty of novel concepts have been employed to improve the efficiency. The metal-organic chemical vapor deposition (MOCVD) technique is generally used for epitaxial growth. As one of the most important epitaxial techniques, molecular-beam-epitaxy (MBE) has shown its unique advantages for basic research[6, 7] However, the performance of the earlier GaAs SCs grown by MBE is worse than those obtained by MOCVD growth because of its low growth temperature and the presence of isolated defects[8, 9]. With the successful development of control and operation of the phosphorous source, high quality phosphide-related material has been obtained by MBE growth[10, 11]. However, research about solar cells using MBE is scarce. Very recently, we reported that MBE-grown phosphide-contained Ⅲ-Ⅴ compound semiconductor solar cells can be quite comparable to those grown by MOCVD[12]. In this paper, we report the initial result of GaInP/GaAs dual-junction SCs with an efficiency of 27% grown by MBE. The energy loss mechanism of our GaAs/GaInP tandem dual-junction SC is also discussed on the basis of the optical and electrical characteristic of devices.
2. Experiment
The epitaxial growth of the SC material was performed in a Veeco GEN20A dual-chamber all solid-state MBE machine with a valved phosphorous cracker cell and a valved arsenic cracker cell. The typical growth rate of GaAs and GaInP is 1
Figure 1 shows the scanning electronic microscopy (SEM) image in the cross section of our GaAs/GaInP dual-junction solar cell, and the right part shows the corresponding layers within the structure. The GaAs bottom cell consists of a p+-GaInP back surface field (BSF) layer, a p-GaAs base layer, an n+-GaAs emitter, and an n+-GaInP window layer. The GaInP top cell consists of a p+-AlGaInP/p+-GaInP BSF layer, a p-GaInP base layer, an n+-GaInP emitter and an n-AlInP window layer. A clear interface between the As and P can be observed, indicating a good switch of V-group As and P. The GaInP top cell and the GaAs bottom cell are electrically and optically connected by a GaAs tunnel junction.
3. Results and discussion
Figure 2 shows the current-voltage
The operation of all the solar cell devices includes the processes of photon absorption, carrier separation, transport and collection, therefore many parameters may affect the performance improvement of the device. If only taking into account the material quality, the most important point is to lower the recombination, including the radiative and nonradiative recombination of the material, interface recombination and to increase the carrier mobility. Benefitting from high purity material growth, high performances of the GaAs and GaInP single junction SCs have been obtained. It is important to note that the performance of our GaAs single-junction is comparable to the best efficiency reported at the end of 2010[14]. In addition, the GaInP single junction SC also reaches a high level. However, the GaAs/GaInP tandem dual-junction SC is almost 3% lower than the best reported value. The short circuit current of the dual-junction solar cell is almost equal to the GaInP single junction. While the open voltage of the tandem SC is only 2.31 V, which is about 100 meV less than the sum of the single top GaInP (1.37 V) and bottom GaAs (1.045 V) SCs. The lower than expected efficiency is due to the decreased open voltage. In contrast to GaAs and GaInP single junction SCs, the performance of the tunnel junction connected to the top and bottom cells is significant to the tandem multi-junction SC. Since the device structure within the dual-junction is almost identical to the respective cell of the top and bottom ones, it is reasonable to blame the decrease of the total voltage drop resulting from the energy loss of the incorporation of the tunnel junction. Figure 4 presents the
In the device, an AlGaInP layer is used as the back surface field layer of the GaInP SC, and it plays also a role as the barrier of the tunnel junction. Normally, a highly doped p-n junction is needed to form a tunnel junction. In our case, a highly-doped beryllium GaAs layer (4
4. Summary
In summary, owing to the high purity and high material quality grown by MBE, the high performance of a GaInP/GaAs dual-junction SC with an efficiency of 27% has been obtained. The energy loss mechanism of our GaAs/GaInP tandem dual-junction solar cells is discussed. It is demonstrated that the MBE-grown phosphide-contained Ⅲ-Ⅴ compound semiconductor solar cell is very promising for achieving high energy conversion efficiency.