REVIEW Piezo-Phototronic Effect www.afm-journal.de Recent Progress in Piezo-Phototronic Effect Enhanced Solar Cells Laipan Zhu and Zhong Lin Wang* (S–Q) limit) indicates that an ultimate For third generation semiconductors, for example, wurtzite ZnO and efficiency of 33.7% can be realized at an GaN, a piezopotential will be induced in these non-centrosymmetric optical absorption band edge of 1.34 eV structures through applying a stationery deformation. The synchronous (Figure 1a).[3,4] However, on account of the occupancy of semiconductor engineering, piezoelectric property, and optical inefficient optical absorption and carrier accumulation, the measured short circuit excitation processes in these materials brings about outstanding device current (Jsc) is much lower than the pre- performances, such as promoting carrier creation, transfer, separation, dicted value. The open circuit voltage (Voc) or suppressing carrier recombination. Enormous research interests have is evidenced as well to be lower than the been sparked in this emerging field known as the piezo-phototronic S–Q limiting value due to recombination effect. This manuscript reviews the fundamental research progress in mechanisms, such as Auger, interface/ surface, band tail recombination, etc.[5–7] enhanced photovoltaic efficiency by this effect, which not only provides Moreover, because of the existence of a comprehensive coverage of the theoretical and experimental works series and shunt resistances, the fill factor illustrating the basic physics for understanding the enhanced solar cells when (FF) will also be suppressed.[8] From the an external mechanical strain is applied, but also gives new insight into above, for a given bandgap, the practical designing high-performance solar cells. efficiencies are usually significantly lower than that obtained from the S–Q limit. Light management and carrier manage- ment are two main approaches to promote 1. Introduction the performance of the solar cells (Figure 1b), for instance, seeking for highly efficient light absorption materials, opti- To cut carbon emissions and protect the atmospheric environ- mizing device structures, and even utilizing new photovoltaic ment, fossil fuels are urgent to be replaced by renewable energy mechanisms.[4,9–11] technologies.[1,2] Among them, the photovoltaic (PV) technology Recently, an increasing interest in piezotronics and piezo- has caused widespread concern to become a green energy phototronics accompanied by novel physical science and source.[3–5] Conversion efficiency η( ), the most important unprecedented device applications has been sparked.[12–17] A parameter to evaluate a solar cell performance, represents how static force induced piezopotential in the wurtzite structured much the incident solar power is converted into the maximum crystal has appeared due to the lack of central symmetry. The output power. Recently, the calculated world record-efficiencies generation of piezopotential influences largely on the transport regarding to different widely used PV technologies are of electrons and holes at the interface/junction, which can counted (Figure 1a). Theoretical analysis (Shockley–Queisser find applications in flexible wearable devices, human interface technologies, sensors, etc.[13] Coupling the light excitation, the piezo-phototronic effect can also have a huge positive impact Dr. L. Zhu, Prof. Z. L. Wang [18,19] on the optoelectronic devices, for example, optical sensors, CAS Center for Excellence in Nanoscience [20,21] [22] Beijing Key Laboratory of Micro-nano Energy and Sensor light emitting diodes (LED), photoelectric switches, and Beijing Institute of Nanoenergy and Nanosystems even spintronic devices.[23] Recently, numerous researchers Chinese Academy of Sciences have demonstrated a strengthened performance of solar cells Beijing 100083, P. R. China with the help of piezo-phototronic effect.[14,24,25] E-mail: [email protected] This paper gives a brief review of recent advances in Dr. L. Zhu, Prof. Z. L. Wang School of Nanoscience and Technology improved photovoltaic technologies via the piezo-phototronic University of Chinese Academy of Sciences effect. First, the basic theory and working principles of the Beijing 100049, P. R. China effect to boost the performance of several typical solar cells Prof. Z. L. Wang are presented. Then, the latest researches of piezo-phototronic School of Material Science and Engineering effect enhanced solar cells with different nanoscale dimensions Georgia Institute of Technology are reviewed from both theoretical and experimental aspects, Atlanta, GA 30332, USA highlighting the advantages and scientific significances of The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adfm.201808214. each approach. At last, a brief summary and some objective perspectives are provided. This review not only wants to DOI: 10.1002/adfm.201808214 present deliberately an extensive summary to the theoretical Adv. Funct. Mater. 2018, 1808214 1808214 (1 of 18) © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.advancedsciencenews.com www.afm-journal.de and experimental works in order to illustrate the basic physics for understanding this effect enhanced PV performance, but Laipan Zhu received his also lays a solid foundation for designing and fabricating future B.S. degree in Physics advanced PV devices. Department from Shandong University in 2010 and his Ph.D. in Institute of 2. Theory of Piezo-Phototronics Semiconductors, Chinese Academic of Sciences in 2.1. Piezoelectricity, Piezopotential, and Piezo-Phototronics 2015. Now he is an associate professor in Prof. Zhong One of the common characteristics of the third generation Lin Wang’s group in Beijing semiconductors is that most of them have wurtzite struc- Institute of Nanoenergy ture. The breaking of cubic symmetry leads to new effects in and Nanosystems, Chinese conjunction with conventional semiconductor physics. Here, Academic of Sciences. His main research interests the wurtzite-structured ZnO is chosen as a typical example. have been focused on the field of piezotronics, piezo- The hexagonal-structured wurtzite crystal usually possesses phototronics, and spintronics. a large anisotropic property. The lack of center symmetry in the crystal gives rise to piezoelectricity. Simply, the centers of Zhong Lin Wang is the positive Zn2+ cations and negative O2− anions are overlapped Hightower Chair in Materials with each other. Therefore, no polarization can be detected in Science and Engineering and the crystal without applied strain. However, the two centers Regents’ Professor at Georgia shift reversely with a strain applied at the top of a tetrahe- Tech, the Chief Scientist dron, leading to a polarization (Figure 2a). Figure 2b shows and Director of the Beijing the perfectly aligned ZnO nanowires (NWs) grown through Institute of Nanoenergy solution processes. A macroscopic potential is induced by and Nanosystems, Chinese a continuous overlay of the dipole polarization in the ZnO Academy of Sciences. His crystal, which is the so-called piezopotential.[12,13,26] The inner discovery and breakthroughs piezopotential keeps existence with the presence of stress, in developing nanogenerators because it is created by the ionic charges. The applied strain and self-powered nanosys- and the doping density (i.e., the doping-induced carriers can tems establish the principle and technological road map screen the piezoelectric charges) result in the magnitude of for harvesting mechanical energy from environmental and the piezopotential. biological systems for powering personal electronics and A piezopotential distribution can be simulated regard- future sensor networks. He coined and pioneered the field less of the doping in ZnO NW.[27–29] As shown in Figure 2c, of piezotronics and piezo-phototronics. an overall potential drop of ≈0.4 V is created with positive potential at +c side (Figure 2c).[27] The introduction of this Figure 1. Comparison of different kinds of PV technologies. a) World record-efficiencies of different PV technologies compared to the theoretical S–Q efficiency limit as a function of bandgap. b) Comparison of the ratio (Jsc/JSQ) versus (Voc × FF/VSQ × FF) for different kinds of record-efficiency cells. The two arrows indicate how much a specific PV material needs to improve regarding to light management and carrier management. Colors with red, green, and blue regions correspond to cells achieving <50%, 50%–75%, and >75% of their S–Q efficiency limit, respectively. Reproduced with permis- sion.[4] Copyright 2016, AAAS. Adv. Funct. Mater. 2018, 1808214 1808214 (2 of 18) © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.advancedsciencenews.com www.afm-journal.de and an improved efficiency of InxGa1−xN quantum well solar cell by an applied strain has indeed been observed.[38–42] 2.3. Piezo-Phototronic Effect on Metal–Semiconductor Contact Schottky barrier is derived from a contact between metal and semiconductor, which possesses rectifier properties.[43,44] The height of Schottky barrier is usually not depended on the dif- ference of work functions between the semiconductor and the metal.[44–48] For a semiconductor with good piezoelectric property, thanks to the partial shielding of piezocharges, the remnant piezocharges can affect tremendously the nature of Schottky barrier. Due to the positive piezoelectric polari- zation under a tensile strain, the Schottky barrier height (SBH) is decreased, and the space charge zone