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The effect of ultra-thin inserting layer (UIL) on the photovoltaic performances of InGaN/GaN solar cells is investigated. With UIL implemented, the open-circuit voltage was increased from 1.4 V to 1.7 V, short-circuit current density was increased by 65% and external quantum efficiency was increased by 59%, compared to its counterparts at room temperature under 1-sun AM1.5G illumination. The improvements in electrical and photovoltaic properties are mainly attributed to the UIL which can boost the crystal quality and alleviate strain. Moreover, it can act as a transition layer for higher indium incorporation and an effective light sub-absorption layer in multiple quantum wells.
©2013 Optical Society of America
1. Introduction
Nowadays, group-III-nitrides have been demonstrated to be a promising material system for photovoltaic applications. The properties of III-nitrides include large carrier mobility, high drift velocity, strong optical absorption near the band edge, and high resistance to radiation compared to other photovoltaic materials, making them ideal for the development of photovoltaics [1–5]. These characteristics enable the InGaN solar cell to operate in severe environments such as the desert or space, in which the performances of Si-based cells are degraded. Furthermore, InGaN-based solar cells with a direct energy bandgap cover nearly the entire solar spectrum (from 0.65 to 3.4 eV), which attracts great attention from researchers [6–8]. An theoretical calculation has indicated that solar cells with a solar energy conversion efficiency larger than 50% can be fulfilled by the adjustment of the Indium composition in InGaN-based multi-junction solar cell [9, 10]. However, due to the currently remaining problems to be solved in the growth of InGaN layer with high In composition, the realization of high-efficiency multi-junction solar cell is highly challenging. On the other hand, to enhance the performance of InGaN-based solar cells, it is necessary to carry out in-depth study and optimization of its structure. In recent years, many studies have been carried out on the design of active region structure in InGaN-based solar cells [1, 3, 11, 12], Chen et al have fabricated InGaN-based solar cells with approximately 400nm p-i-n InGaN layer. However, one of the most serious issues preventing further improvements is the difficulty in fabricating thick InGaN films with high crystalline quality. To solve the problem, some groups have suggested that the adjustment of multiple quantum well (MQW) region will result in improved performance. For example, optimization of barrier thickness [13], adjustment of MQW period number [14] adoption of the quantum well GaN cap layers [15] and the doping on both ends of MQWs [16]. All of which contribute to the improvement of solar cell performance. In this letter, an ultra-thin layer inserting methodology was adopted for In0.2Ga0.8N/GaN MQW based solar cells in which a 1 nm In0.05Ga0.95N ultra-thin layer was inserted between quantum barrier and quantum well layer. It is demonstrated that the use of UIL is beneficial to enhance the photovoltaic performance of the solar cells.
2. Experimental details
InGaN/GaN MQWs solar cells were grown by metal-organic chemical-vapor deposition (MOCVD) on a (0001) sapphire substrate. Trimethylgallium (TMGa), trimethylindium (TMIn), and ammonia (NH3) were used for the Ga, In and N sources. Silane (SiH4) and bis-cyclopentadienyl magnesium (Cp2Mg) were used for n-type and p-type doping, respectively. Figure 1 shows a cross-sectional schematic of the epitaxial structure of the devices. The solar cell reference structure (labeled as sample A as shown in Fig. 1(a)) consisted of a 1 μm thick unintentionally doped GaN layer, a 2 μm Si-doped n-GaN layer ([Si] = 5 × 1018 cm−3), a 20 period InGaN/GaN MQWs active region with 8 nm GaN barriers (820 °C, 400mbar) and 3 nm In0.2Ga0.8N well (730 °C, 400mbar), a 0.15 μm thick Mg-doped p-GaN layer ([Mg] = 5 × 1017 cm−3). Another epitaxial structure (labeled as sample B as shown in Fig. 1(b)) was composed of the same structure except that a 1nm thick In0.05Ga0.95N (765 °C, 400mbar) was inserted between each period of In0.2Ga0.8N/GaN MQWs.
Fig. 1 (a) Cross-sectional schematic of the reference device structure (b) cross-sectional schematic of the improved device structure (c) an optical microscopy image of a fabricated device
Following the MOCVD growth, structural characteristics of the samples were analyzed by a Bede D1 high-resolution x-ray diffractometer (HRXRD). Afterwards, the samples were processed into solar cell chips using standard contact lithography and reactive ion etching procedure. The area of the device was of 1 × 1 mm2, and the configuration of the electrodes is indicated in Fig. 1(c). The metal for n-type and p-type ohmic contacts was Ti/Al/Au and Ni/Au, respectively. No current spreading layer and antireflection layer were used for the devices. The measurements of external quantum efficiency (EQE) were performed under monochromatic illumination by a Xe lamp coupled to a monochromator and the incident light intensity was calibrated with a reference silicon photodetector. J-V curves were measured with a Keithley 2400 source meter. The devices were illuminated by an Oriel Xe lamp with an equivalent AM1.5G illumination intensity of 1 sun.
3. Results and discussion
Figure 2 depicts XRD 3-axis omega/2-theta scans taken across the (0002) reflection for samples A and B. One can see that more obvious and much sharper satellite peaks are observed in sample B compared to that in sample A, which signify steeper and flatter interface in the MQW region of sample B. On the other hand, the full-width at half-maximum (FWHM) of GaN (0002) rocking curves in sample A and sample B are 215 arcsec and 188 arcsec, respectively. It is considered that the crystal quality of subsequently grown p-GaN layer could be improved by the insertion of UIL. Thus, the overall crystal quality of InGaN/GaN solar cell could be ameliorated by the insertion of UIL.
Fig. 2 Omega /Two-Theta XRD rocking curves of InGaN solar cell with and without ultra-thin inserting layer.
Figure 3 shows an XRD reciprocal space map taken around the asymmetric (105) reflection for sample A and B. Generally, the vertical alignment of the MQW peaks with the GaN peak indicates that the InGaN/GaN MQW was coherently strained to the GaN template (vertical dotted line is shown in Fig. 3). It is shown in Fig. 3(a) that there is a lateral shift of MQW satellite peak with respect to the GaN template, which indicates that sample A experiences a partial relaxation of strain. As for sample B with the adoption of UIL, the lateral shift of MQW satellite peak in Fig. 3(b) is obviously decreased. Therefore, the implantation of UIL can alleviate the accumulation of strain caused by the large lattice mismatch of the InGaN and GaN. In addition, one can see that the InGaN peak of sample B moved further down from GaN peak. This result indicates that higher average In content in sample B compared to sample A. This is due to the fact that the UILs contribute to increase the average In content of the QWs [15].
Fig. 3 Reciprocal space map around the asymmetric (105) reflection of InGaN/GaN solar cell sample A (a) and sample B (b).
Figure 4 shows the normalized EQE as a function of excitation wavelength for sample A and sample B. In fact, the EQE value of sample B with the change of wavelength was consistently higher than that of sample A within the wavelength range we measured. The maximum EQE of sample B is approximately 1.5 times as high as that of sample A at a wavelength of 375 nm. These results indicated that sample B exhibited better EQE performance. Please note that even in the peak EQE normalization, sample B in the short wavelength (300 nm to 375 nm) consistently shows a higher EQE compared to sample A, indicating that UIL contributes to short wavelength region absorption, since the band gap of In0.05Ga0.95N is larger. Generally, the emission wavelength of InGaN/GaN MQWs region is longer than the peak wavelength of EQE [17]. This is due to the fact that in emission process carriers are relaxed to lower energy states before being recombined radiatively, whereas electrons (holes) in absorption process can be excited to higher states as long as the absorbed photons’ energy matches with the energy level separation. The emission wavelength of In0.05Ga0.95N corresponding to the wavelength of EQE is approximately located between a range from 360 nm to 375 nm. Hence, the absorption improvement of sample B in short wavelength range is attributed to the insertion of low In content InGaN layer which boost the absorption in short wavelength. On the other hand, spectral response of sample A could be approximately 470 nm while that of sample B can be extended to longer wavelength (480 nm) in the long wavelength direction which suggests that sample B has a higher In content. In the normalized EQE figure, sample B in the long wavelength (390 nm to 460 nm) consistently shows a extra higher EQE compared to sample A which probably indicates that the increase of In content in sample B. The result is in agreement with reciprocal space map results. Therefore, the absorption improvement of sample B in long wavelength range is ascribed to higher In content of sample B. This is probably because the UIL transition layer with low In content inserted between quantum barrier layer and quantum well layer will help to increase the In content in InGaN well. Besides, the UIL with low In content grown after quantum well layer also contributes to improved crystal quality of InGaN quantum well. Above these results are indicated that the absorption in short wavelength could be increased and the absorption in long wavelength could be ameliorated due to the insertion of UIL. The response in the shorter wavelength region (< 300 nm) is limited by the use of p-GaN window [18] and can be improved if a larger band gap material such as p-AlGaN or p-InAlGaN is incorporated.
Table 1 illustrates the photovoltaic characteristics of the two experimental samples measured at room-temperature. The device with UIL demonstrated higher fill factor (FF) of 56.8% compared to sample A (51.9%). The maximum EQE of sample B (21.9%) is higher than that of sample A (15.7%). Figure 5 shows the current density versus voltage characteristics for devices with and without UIL under concentrated 1 sun AM1.5G illumination. Short circuit current density for device with UIL (0.43 mA/cm2) was 65% higher than that of without UIL (0.26 mA/cm2). Likewise, open circuit voltage for sample B was 22% higher than that of sample A, enhanced open circuit voltage from 1.4 V to 1.7 V. It suggests that solar cell with UIL is beneficial to improve the photovoltaic performance.
Table 1. The summary of the photovoltaic property of the two samples
Fig. 5 Illuminated J-V curves from devices with and without ultra-thin inserting layer under 1sun AM1.5G equivalent illumination.
Figure 6 shows the JV characteristics for the MQW solar cells under dark condition. The JV characteristic of sample A displayed larger leakage current though, compared to sample B, impacting the overall efficiency of devices. The leakage current is bias dependent and possibly occurs when carriers tunneling via defect levels, such as threading dislocations, V defects [19]. It suggests that the density of defects in sample A is higher than that in sample B, which is consistent with the decrease of JSC in sample A. This is due to fact that JSC is very much influenced by defects, the presence of defects possibly results in the formation of a leakage current path through the junction [13]. Thus, high density defects can lead to the decrease of JSC. The electricity performances of two samples indicate that the insertion of UIL help to improve the crystal quality and alleviate the accumulation of high strain, which is in agreement with the XRD rocking curves and reciprocal space map observations. This could be due to the fact that an ultra-thin inserting layer can moderate the accumulation of high strains in MQWs and avoid local strain-derived relaxations with the increase in the number of the MQW period. With the reduction of the dislocations, the nonradiative recombination of photogenerated carriers at the traps is decreased. Moreover, the adoption of UIL can effectively prevent the destruction of InGaN layer caused by ramping temperature and incorporate higher In content which consistent with reciprocal space map and EQE results.
4. Conclusions
In conclusion, a 1 nm In0.05Ga0.95N ultra-thin layer was inserted between GaN barriers and In0.2Ga0.8N wells layer to improve the photovoltaic properties of the InGaN-based solar cells. With the adoption of UIL, the open-circuit voltage was increased from 1.4 V to 1.7 V, short-circuit current density and external quantum efficiency (EQE) was increased by 65% and 59% compared to reference structure. The improvement on electrical and photovoltaic properties is attributed to the UIL which not only effectively boost the crystal quality and ameliorate strain, but also can be regarded as a transition layer to incorporate higher In content and a light sub-absorption layer in MQWs.
Acknowledgments
This work was supported by the National Nature Science Foundation of China (Grant No. 51172079), the Science and Technology Program of Guangdong province, China (Grant Nos. 2010B090400456, 2009B011100003, and 2010A081002002), and the Science and Technology Program of Guangzhou City, China (Grant Nos. 2010U1-D00191 and 11A52091257).
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