Enhanced light absorption of kinked nanowire arrays for high-performance solar cells

Semiconductor nanowire (NW) array has been considered as one of the most promising solar cell structures due to the excellent properties including strong light trapping, weak emission of photons, as well as the ability to combine different materials to extend the absorption spectrum [1], [2], [3]. Owning to the direct and tunable band gaps, III–V NWs have attracted particular attention. Up to date, NW array solar cells based on different III–V materials, such as GaAs, InP, InAs, and GaAsP, have been demonstrated, which exhibited exciting performance at a low filling ratio [4], [5], [6], [7], [8], [9]. A typical III–V NW array solar cell is built by growing vertical-aligned uniform cylindrical NW p(i)n junctions on a homogeneous or heterogeneous substrate. However, recent studies have shown that the uniform cylindrical NW array is probably not the optimal structure for high-performance solar cells. On one hand, the absorption of the uniform cylindrical NW array is still limited by the reflection at the top interface and transmission into the substrate [10], [11]. On the other hand, the vertical configuration typically leads to strong absorption in the highly doped top layer, resulting in significant recombination loss. Hence, tailoring the shape, diameter, orientation, and distribution of NWs has been considered as a critical technique to achieve high efficiency solar cells and attracted increasing attention in recent years [12], [13], [14], [15], [16].

From the point of view of device quality, another factor that affects the performance of traditional III–V NW solar cells is the crystal quality of NWs. It is known that <111> is a thermodynamically favorable growth direction for most III–V NWs. However, for <111> III–V NWs, stacking faults (SFs) are commonly generated perpendicular to the growth direction, which are expected to affect the performance of NW devices. Studies have shown that stacking faults may decrease the carrier lifetime and carrier mobility in NWs, leading to degradation of optoelectronic device performance [17], [18], [19]. Moreover, the stacking faults may be responsible for non-ideal diode behavior in photovoltaic devices [20]. So far many efforts have been done to suppress the stacking faults in NWs including controlling the growth conditions, particle size, or doping [21], [22], [23], [24]. It is worth noting that some non-<111>-oriented NWs exhibit intrinsically stacking-faults-free crystal structure, which are particularly promising in high-performance NW solar cells [25], [26], [27].

Hence to achieve an ideal NW solar cell, both the array structure and crystal quality should be optimized to obtain maximum absorption and excellent electrical properties. In recent years, a self-catalyzed kinked InP NW array has been demonstrated [28]. Unlike the vertical-aligned NW, the kinked NW is composed of a short vertical <111>-oriented NW and a long tilted <110>-oriented NW. As the <110> NW exhibits stacking-faults-free pure zinc blende structure, the device performance is expected to be significantly improved [28], [29], [30]. In addition, the kinked shape may lead to an enhancement of absorption due to the strong scattering effect [31]. Although single kinked NWs have shown advantages in high performance field effect transistors and photodetectors, solar cells based on the kinked NW array have not been studied yet [29], [30]. In this work, a coupled three-dimensional (3-D) optoelectronic simulation is presented to investigate the photovoltaic performance of kinked InP NW array solar cells. The results show that the kinked morphology significantly enhances the absorption by increasing the optical path, exciting more resonance modes, and reducing the transmission into the substrate. The kinked morphology also increases the absorption in the intrinsic region by suppressing the absorption loss in the top doped region. For short NWs with a total length of 1–1.5μm, the kinked structures exhibit remarkable efficiency exceeding 14%, significantly higher than the vertical counterparts. NWs with different kinked angles are studied, and a moderate kinked angle is beneficial for obtaining optimal performance.

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