Challenges and Prospects of Nanopillar-Based Solar CellsZhiyong Fan1,2,3, Daniel J. Ruebusch1,2,3, Asghar A. Rathore1,2,3, Rehan Kapadia1,2,3, Onur Ergen1,2,3, Paul W. Leu1,2,3, and Ali Javey1,2,3（ ）1 Department of Electrical Engineering and Computer Sciences, University of California at Berkeley, Berkeley, CA 94720, USA2 Berkeley Sensor and Actuator Center, University of California at Berkeley, Berkeley, CA 94720, USA3 Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USAReceived: 13 September 2009 / Revised: 26 September 2009 / Accepted: 26 September 2009©Tsinghua University Press and Springer-Verlag 2009. This article is published with open access at Springerlink.com00829Nano Res (2009) 2: 829 843DOI 10.1007/s12274-009-9091-yReview ArticleAddress correspondence to [email protected] and device architecture innovations are essential for further enhancing the performance of solar cells while potentially enabling their large-scale integration as a viable source of alternative energy. In this regard, tremendous research has been devoted in recent years with continuous progress in the fi eld. In this article, we review the recent advancements in nanopillar-based photovoltaics while discussing the future challenges and prospects. Nanopillar arrays provide unique advantages over thin fi lms in the areas of optical properties and carrier collection, arising from their three-dimensional geometry. The choice of the material system, however, is essential in order to gain the advantage of the large surface/interface area associated with nanopillars with the constraints different from those of the thin fi lm devices.KEYWORDSNanopillar-based photovoltaics, solar cells, nanowires (NWs)IntroductionSemiconductor nanowires (NWs) exhibit unique electrical, optical, and mechanical properties arising from their miniaturized dimensions and single-crystalline structures with tunable atomic composition. In the past decade, they have been extensively investigated as the building blocks for various technological applications such as electronics [1 7], optoelectronics [8 10], and sensors [11 16]. Recently, as an emerging field, NWs have been utilized for energy harvesting devices, for instance, to convert thermal [17, 18], mechanical [19], and solar energy into electricity [20 33]. In this review article, we summarize the continuous progress of NW-based photovoltaics (PVs) while discussing the challenges and prospects associated with their integration for effi cient and affordable solar cell modules.1) Device structuresVarious PV device structures based on vertically oriented NW (i.e., nanopillar) arrays can be envisioned as illustrated in Fig. 1, some of which have already been demonstrated experimentally. Specifically, three main device structures have been proposed and explored, including nanopillar (NPL) arrays with radial [21, 22] (Fig. 1(a)) and axial (Fig. 1(b)) junctions as well NPL arrays embedded in thin fi lmsNano Research830Nano Res (2009) 2: 829 843of an absorber layer (Fig. 1(c)) [20, 28, 33]. Each of these device structures has its own advantages and disadvantages. The device structures utilizing radial NPL junctions (i.e., core/shell) or NPL collectors embedded in an absorber film provide significant enhancement of the junction area with reduced minority carrier collection lengths. On the other hand, the NPL axial and radial junctions provide a three-dimensional (3-D) geometric confi guration for reduced surface optical reflection and enhanced absorption. The enhanced carrier collection and optical absorption can in principle enable more effi cient PVs as compared to planar structures. However, the surface and the interface area enhancement also result in an increase in surface/interface recombination events. In that regard, only material systems with low surface recombination velocities are desirable for the NPL confi guration.2) Reduced optical reflection and enhanced absorptionThe effi ciency of a PV cell depends on the probability of an incident photon being absorbed, and the subsequent collection of the generated carriers. Nanowire arrays can potentially provide a unique advantage due to their anti-reflective and light trapping properties. Silicon NW arrays of only several microns in length have been noted for their strong broadband optical absorption and dark visual appearance [25, 34]. Numerical studies on the opticalproperties of disordered NW arrays by diffuse scattering [35] and ordered vertical arrays by specular reflection [36] have demonstrated that NW arrays have distinct absorption spectra from their thin fi lm counterparts. The ordering of the NW arrays may be used as light trapping schemes analogous to random surface texturization or periodic grating couplers in thin fi lms [37]. NW arrays can be used to redistribute the absorption spectrum from regions where absorption is not needed to spectral regions where absorption enhancement would lead to enhanced photocurrents.The NW array material, diameter, length, and pitch can all be used to tailor the absorption spectrum of the NW solar cell. Parameters such as tapering of NWs may further improve upon the anti-reflective and enhanced absorption properties [36]. In our own studies, the exposure length of CdS NPL arrays out of anodic aluminum membranes was found to decrease the optical reflection as shown in Fig. 2 [20]. These various experimental and computational studies suggest that NPL arrays can be designed and optimized for PV cells by reducing the refl ection and enhancing the optical absorption over specific spectral regions.3) Enhanced carrier collection effi ciencyNPL-based solar cells introduce important changes in the dynamics of carrier collection efficiency as compared to their thin film counterparts. Among other factors, high efficiency in traditional thin film Figure 1 Nanopillar solar cell architectures. Three commonly explored PV device structures include: (a) NPL radial junctions; (b) NPL axial junctions; (c) NPL arrays embedded in thin fi lm of an absorber831Nano Res (2009) 2: 829 843solar cells requires efficient light absorption and carrier collection [38]. Yet, absorption and collection are in competition in thin film devices [39, 40]. A thin film device must be thick enough for nearly all of the incident photons to be absorbed. Thus, ideally the thickness of the absorber film is tailored to the characteristic absorption length.