# Carrier Dynamics in InGaN/GaN on the Basis of Different In Concentrations
## 1 Introduction  
The use of photovoltaic devices is attractive due to the high global energy demands and the abundance, cleanliness, and renewability of solar energy. Among semiconductors, group III nitrides feature excellent potential as replacements for traditional photovoltaic absorption materials in high-efficiency solar-cell production [1; 2; 3; 4; 5; 6]. The direct energy gap of the In\({}_{\mathrm{x}}\)Ga\({}_{\mathrm{1-x}}\)N system can be adjusted by optimizing In composition to absorb the entire solar spectrum [7]. Excellent physical features, such as high mobility, drift velocity, radiosity, thermal conductivity, and high-temperature resistance, result in prolonged lifetime and strong light absorption, rendering InGaN alloys as possible substitutes for traditional solar cell absorbents [7; 8]. Si solar cells may be affected by unstable operation, whereas InGaN-based solar cells possess the potential to operate smoothly in harsh environments [5]. [PERSON] et al. calculated the conversion efficiency of group III nitride photovoltaic devices in 2007 [9] and recorded the efficiency of a single In\({}_{0.65}\)Ga\({}_{0.35}\)N battery at a value of up to 20%. InGaN/GaN, as a battery interface, increases efficiency up to 35.1% [10]. Under a similar operating background, [PERSON] et al. [11] improved the efficiency of InGaN/Si series solar cells by 35% using two InGaN junctions. Theoretically, the efficiency of multi-junction solar cells increases with additional junctions, whereas that of a series of solar cells containing eight stacked layers can reach 56% [2]. Although these characteristics present broad prospects, the performance of solar cells using InGaN must be considerably improvedowing to their low conversion efficiency [2; 12]. The performance of InGaN solar cells is hindered by structural degradation of thick InGaN layers. Alloys possess a high In content (\(>\)10%) owing to the dislocation and relaxation caused by inhomogeneity and vacancy-related defects [13; 14]. The limitations of a thick InGaN layer are overcome by the use of multiple quantum wells (MQWs) or a superlattice absorbing layer in solar cells to limit the accumulation of stress and control defect formation. Compared with solar cells manufactured using a thick InGaN-absorbing layer [2; 15; 16], MQW structures improve the crystallinity and performance of solar cell devices. The performance of InGaN MQW solar cells can be further improved by increasing the number of quantum wells and thickness of the active region [13; 17; 18]. However, the loss of collecting carriers and the increase in MQW region composition result in the saturation of a large number of quantum wells by short-circuit currents [19]. Therefore, photon absorption in InGaN MQW active regions for photovoltaic applications can be strengthened by developing new structures that can achieve large InGaN active regions. Nonlinear four-wave mixing (FWM) technology is becoming a general tool for describing a large number of crystals and micron-scale semiconductor and nanostructures. FWM can quickly and reliably evaluate new photoelectric materials and related technologies. This technology enables multiple electrical parameter measurement of semiconductors in an all-optical manner on the basis of correlations between electrics and optics. The unique advantage of FWM is its capability to directly analyze transmission of carriers by changing the interval of optical interference patterns. Previous studies on InGaN materials using FWM technology were carried out based on heterostructure and single quantum well samples [20; 21; 22]. [PERSON] et al. [20] have proven that the considerable decrease in external quantum efficiency (n\({}_{\text{text{text}}}\)) of light-emitting diode in InGaN refers not to the increase in non-radiative composite centers but the carrier delocalization caused by rapid diffusion.