The organic-inorganic hybrid lead trihalide perovskites have been emerging as the most attractive photovoltaic materials. As regulated by Shockley-Queisser theory, a formidable materials science challenge for improvement to the next level requires further band-gap narrowing for broader absorption in solar spectrum, while retaining or even synergistically prolonging the carrier lifetime, a critical factor responsible for attaining the near-band-gap photovoltage. Herein, by applying controllable hydrostatic pressure, we have achieved unprecedented simultaneous enhancement in both band-gap narrowing and carrier-lifetime prolongation (up to 70% to ∼100% increase) under mild pressures at ∼0.3 GPa. The pressure-induced modulation on pure hybrid perovskites without introducing any adverse chemical or thermal effect clearly demonstrates the importance of band edges on the photon-electron interaction and maps a pioneering route toward a further increase in their photovoltaic performance. . The remarkable photovoltaic performance is attributed to its strong and broad (up to ∼800 nm) light absorption (10), as well as the long diffusion lengths facilitated by its extraordinarily long carrier lifetimes (∼100 ns in thin film) despite its modest mobility (11,12,15,16). To further approach the Shockley-Queisser limit (17, 18), it is highly desirable to tune the crystal structure of perovskite in the way that can synergistically narrow down the band gap for broader solar spectrum absorption (10) and prolong carrier lifetime for greater photovoltage (7,11,12,15,16). However, compositional modification suffers from challenges, such as the largely shortened carrier lifetime (∼50 ps), and thus considerable loss of photovoltage upon the replacement of Pb by Sn (5, 19), or the largely widened band gap, and thus low photocurrent when I is substituted with Br or Cl (16). It also has been demonstrated that using formamidinium (FA) cations instead of MA cations in organic-inorganic perovskite materials narrows down the band gap; however, a shorter carrier lifetime is generated inevitably (20). In fact, to date, there is no reported method for simultaneously achieving band-gap narrowing and carrier-lifetime prolongation for MAPbI 3 .Nonetheless, the chance is to rescrutinize the band structure of MAPbI 3 . The relatively long carrier lifetimes of 10 2 to ∼10 3 ns observed in MAPbI 3 single crystals originate from their unique defect physics (21). First-principles calculations demonstrated that the readily formed point defects such as interstitial MA ions and/or Pb vacancies create shallow states with trap energy less than 0.05 eV below the conduction band minimum (CBM), or above the valence band maximum (VBM), rather than detrimental deep traps at the middle of the forbidden zone, which typically lead to nonradiative recombination (21). The uneven distribution of the trap states has been identified further by in-depth electronic characterization of MAPbI 3 perovskite single crystals, concluding that the traps are close to the conduction an...

A new technique has been developed for optical studies of amorphous solids to very high pressures. Raman spectra of Si02 glass measured at 8 GPa indicate a significant reduction in the width of the Si-0-Si angle distribution, which has been associated with a number of anomalous properties of silica glass under ambient conditions. Between 8 and -30 GPa irreversible changes in the Raman spectrum occur that are consistent with a shift in ring statistics in densified glass. The spectra suggest a breakdown in intermediate-range order at higher pressure. PACS numbers: 78.30.6t, 61.40. +b, 62.50.+p The structure and properties of amorphous solids are of widespread interest because of their obvious importance in the design and synthesis of new materials. ' The structure of these materials, including the degree of short-range and intermediate order and the dependence of these quantities on pressure and temperature, are also of fundamental concern. The detailed structure of vitreous silica, a tetrahedral oxide glass, has in fact been the subject of much recent controversy. 2 'In the case of crystalline solids, the application of diffraction and spectroscopic techniques at high pressure for determining the effect of pressure on structure and for constraining assignments for phonon spectra has long been recognized.Similar studies of amorphous materials at high pressure, however, have been unsuccessful. 9 The problem with such measurements arises from limited access to samples in high-pressure devices, strong spurious scattering from the apparatus which interferes with the signal from the sample, and the generally weak and/or broad absorption or scattering cross sections of amorphous solids in comparison to those of crystals. In the present study we show that high-quality Raman spectra of weakly scattering amorphous solids at pressures of &30 GPa (&300 kbar) can be obtained with a sensitive micro-optical technique. We report the first Raman spectra of Si02 glass measured in situ at high pressure and find dramatic effects of pressure on its structure and vibrational properties.In addition to the controversy surrounding the structure of silica glass, there is fundamental interest in this material because it has a number of unusual properties, including anomalous behavior at high pressure. 9 " On compression, the bulk and longitudinal moduli of silica glass decrease, in contrast to the increase which is observed in most solids, and pass through a minimum at -2 GPa. 9'o In addition, silica glass can be densified or compacted by static high pressures (i.e. , &10 GPa), shock loading, and neutron irradiation.On the basis of Brillouin scattering measurements performed at high pressure and Raman measurements of the material quenched from high pressure, Grimsditch9 proposed that a new type of polymorphism between amorphous states occurs on static compression. However, the structural basis of such a transformation is unknown, as Raman spectra could not be measured in situ at high pressure because of strong interference from fluoresc...