Reduced Dimensionality in Drift-Diffusion Models of Back-Contact Solar Cells and Scanning Photocurrent Microscopy

Solar cells are three-dimensional objects frequently modeled as being one-dimensional for convenience. However, for more complex designs of solar cell or if the cell is only illuminated at one point, one-dimensional modeling is insufficient. Here, some conditions for reducing the complexity of multidimensional drift-diffusion simulations are investigated in realistic situations for a back-contact perovskite solar cell. The analysis investigates under what situations we may neglect vertical carrier density variation and approximate extraction currents to be linearly dependent on the vertically averaged carrier concentration. Analytic expressions for the linear relationship in both the low and high extraction velocity regimes are demonstrated, and the conditions where these approximations break down are investigated. It is shown that recombination is usually accurately modeled using only vertically averaged carrier concentrations when the distance between electrodes is many times the height and when less than half the charges that are generated recombine, although edge effects around the onset of electrodes are noted. These findings are then applied to a problem that often emerges in scanning photocurrent microscopy, a point-excited film with a laterally offset electrode. It is demonstrated that we expect the current recorded in this case to decay exponentially with the distance between excitation and electrode, with a decay constant that can be related to device parameters. The characteristic equilibration time for the system to reach this current, which can be extracted from the phase delay in a lock-in amplifier measurement, is demonstrated to increase linearly with distance. It is shown that information about the diffusion and recombination rates can be extracted from a wide variety of planar systems.This work was supported by the UK government through the EPSRC

Probing the switching mechanism in ZnO nanoparticle memristors

We investigate the resistance switching mechanism in memristors based on colloidal ZnO nanoparticles using electroabsorption (EA) spectroscopy. In this EA experiment, we incorporate a small amount of low-bandgap polymer, poly(9,9-dioctylfluorene-cobenzothiadiazole) (F8BT), as a probe molecule in ZnO-nanoparticle memristors. By characterizing this polymer, we can study the change of built-in potential (VBI) in the device during the resistance switching process without disturbing the resistance state by the EA probe light. Our results show that VBI increases when the device is switched to the high resistance state, suggesting a shift of effective workfunction of the electrode. Thus, we attribute the resistance switching to the field-dependent migration of oxygen vacancies associated with the adsorption and desorption of oxygen molecules at the Al/ZnO interface. This process results in the modulation of the interfacial injection barrier which governs the resistance state of the device.This work was supported by the Engineering and Physical Sciences Research Council [Grant Number EP/G060738/1]Copyright 2014 American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics. This article "Probing the switching mechanism in ZnO nanoparticle memristors" has been accepted by Journal of Applied Physics. After it is published, it will be found at http://scitation.aip.org/content/aip/journal/ja

Spin signatures of exchange-coupled triplet pairs formed by singlet fission

We study the effect of an exchange interaction on the magnetic-field-dependent photoluminescence in singlet fission materials. We show that, for strongly interacting triplet exciton pairs (intertriplet exchange interaction greater than the intratriplet spin-dipolar interaction), quantum beating and magnetic-field effects vanish apart from at specific magnetic fields where singlet and quintet levels are mixed by a level anticrossing. We characterize these effects and show that the absence of a magnetic-field effect or zero-field quantum beats does not necessarily mean that fission is inoperative. These results call for a reconsideration of the observations that are considered hallmarks of singlet fission and demonstrate how the spin coherence and exchange coupling of interacting triplet pairs can be measured through magneto-photoluminescence experiments.Engineering and Physical Sciences Research Council (Grant ID: EP/G060738/1)This is the author accepted manuscript. The final version is available from the American Physical Society via http://dx.doi.org/10.1103/PhysRevB.94.04520

Critical light instability in CB/DIO processed PBDTTT-EFT:PC<inf>71</inf>BM organic photovoltaic devices

Organic photovoltaic (OPV) devices often undergo ‘burn-in’ during the early stages of operation, this period describing the relatively rapid drop in power output before stabilising. For normal and inverted PBDTTT-EFT:PC71BM OPVs prepared according to current protocols, we identify a critical and severe light-induced burn-in phase that reduces power conversion efficiency by at least 60% after 24 hours simulated AM1.5 illumination. Such losses result primarily from a reduction in photocurrent, and for inverted devices we correlate this process in-situ with the simultaneous emergence of space-chare effects on the μs timescale. The effects of burn in are also found to reduce the lifetime of photogenerated charge carriers, as determine by in-situ transient photovoltage measurements. To identify the underlying mechanisms of this instability, a range of techniques are employed ex-situ to separate bulk- and electrode-specific degradation processes. We find that whilst the active layer nanostructure and kinetics of free charge generation remain unchanged, partial photobleaching (6% of film O.D.) of PBDTTT-EFT:PC71BM occurs alongside an increase in the ground state bleach decay time of PBDTTT-EFT. We hypothesise that this latter observation may reflect relaxation from excited states on PBDTTT-EFT that do not undergo dissociation into free charges. Owing to the poor lifetime of the reference PBDTTT-EFT:PC71BM OPVs, the fabrication protocol is modified to identify routes for stability enhancement in this initially promising solar cell blend.The authors would like to thank SABIC for partially funding this research. PEH, EC, RHF and NCG thank the EPSRC for funding through the Supergen Supersolar Consortium (EP/J017361/1). PEH also thanks CKIK for additional funding. KD thanks the Gates Cambridge Scholarship fund. MAJ thanks Nyak Technology Ltd for PhD scholarship funding. AJP thanks David Lidzey (University of Sheffield) for use of a sample chamber for X-ray scattering measurements and Adam Brown (University of Cambridge) for UPS measurements.This is the final version of the article. It first appeared from Elsevier via http://dx.doi.org/10.1016/j.orgel.2015.12.02

Oxygen Degradation in Mesoporous Al<inf>2</inf>O<inf>3</inf>/CH<inf>3</inf>NH<inf>3</inf>PbI<inf>3-</inf><inf>x</inf>Cl<inf>x</inf> Perovskite Solar Cells: Kinetics and Mechanisms

The rapid pace of development for hybrid perovskite photovoltaics has recently resulted in promising figures of merit being obtained with regard to device stability. Rather than relying upon expensive barrier materials, realizing market-competitive lifetimes is likely to require the development of intrinsically stable devices, and to this end accelerated aging tests can help identify degradation mechanisms that arise over the long term. Here, oxygen-induced degradation of archetypal perovskite solar cells under operation is observed, even in dry conditions. With prolonged aging, this process ultimately drives decomposition of the perovskite. It is deduced that this is related to charge build-up in the perovskite layer, and it is shown that by efficiently extracting charge this degradation can be mitigated. The results confirm the importance of high charge-extraction efficiency in maximizing the tolerance of perovskite solar cells to oxygen.This work was supported by SABIC and by the EPSRC, including by the Supergen Supersolar Consortium (EP/J017361/1) and the European Union Seventh Framework Program [FP7 2007-2003] under grant agreement 604032 of the MESO project. GE is supported by the EPSRC and Oxford Photovoltaics Ltd. through a Nanotechnology KTN CASE award. JW acknowledges the Swire Educational Trust for supporting his D.Phil. study at Oxford. We thank Sian Dutton (University of Cambridge) for access to XRD facilities and Felix Deschler (University of Cambridge) for helpful discussions.This is the final version of the article. It first appeared from Wiley via http://dx.doi.org/10.1002/aenm.20160001

Perovskite/Colloidal Quantum Dot Tandem Solar Cells: Theoretical Modeling and Monolithic Structure

Metal-halide perovskite-based tandem solar cells show great promise for overcoming the Shockley-Queisser single-junction efficiency limit via low-cost tandem structures, but so far they employ conventional bottom-cell materials that require stringent processing conditions. Meanwhile, difficulty in achieving low-bandgap (11% absolute gain) to the ultimate efficiency via photon recycling. We report initial experimental demonstration of a solution-processed monolithic perovskite/CQD tandem solar cell, showing evidence for subcell voltage addition. We model that a power-conversion efficiency of 29.7% is possible by combining the state-of-the-art perovskite and CQD solar cells

Charge Generation and Electron-Trapping Dynamics in Hybrid Nanocrystal-Polymer Solar Cells

We investigate the charge-trapping dynamics in hybrid nanocrystal-polymer systems and their effect on performance in photovoltaic devices. Employing various steady-state spectroscopy techniques and ultrafast, three-pulse transient absorption methods, we identify the depth of electron trap states in the nanocrystal band gap and measure their population dynamics. Our findings show that photogenerated electrons are trapped at midgap states on the nanocrystal within hundreds of picoseconds. The trapping of the majority of charge carriers before charge extraction results in a lowering of the quasi-Fermi level of the electrons which limits the device open-circuit voltage, thereby underlining the significance of these processes in conjugated polymer/nanocrystal hybrid photovoltaics.Engineering and Physical Sciences Research Council (Grant IDs: EP/M005143/1, EP/G060738/1, EP/G037221/1), Worshipful Company of Armourers and Brasiers (Gauntlet Trust award), German National Academic Foundation (Studienstiftung)This is the final version of the article. It first appeared from the American Chemical Society via http://dx.doi.org/10.1021/acs.jpcc.6b0759

Electroluminescence from Organometallic Lead Halide Perovskite-Conjugated Polymer Diodes

Organometallic lead perovskite-based solar cells can be converted to light-emitting diodes by engineering the current density. Diodes are fabricated with adjacent perovskite and conjugated polymer layers using orthogonal solvents. Under forward bias, these devices show simultaneous emission from both the luminescent conjugated polymer and the perovskite, providing direct information on electron and hole recombination as a function of device architecture and bias voltage.We gratefully acknowledge funding from the Engineering and Physical Sciences Research Council (EPSRC). A.K. acknowledges NRF-Singapore for a scholarship.This is the final version of the article. It first appeared from Wiley via http://dx.doi.org/10.1002/aelm.20150000

Tuning the role of charge-transfer states in intramolecular singlet exciton fission through side-group engineering

Understanding the mechanism of singlet exciton fission, in which a singlet exciton separates into a pair of triplet excitons, is crucial to the development of new chromophores for efficient fission-sensitized solar cells. The challenge of controlling molecular packing and energy levels in the solid state precludes clear determination of the singlet fission pathway. Here, we circumvent this difficulty by utilizing covalent dimers of pentacene with two types of side groups. We report rapid and efficient intramolecular singlet fission in both molecules, in one case via a virtual charge-transfer state and in the other via a distinct charge-transfer intermediate. The singlet fission pathway is governed by the energy gap between singlet and charge-transfer states, which change dynamically with molecular geometry but are primarily set by the side group. These results clearly establish the role of charge-transfer states in singlet fission and highlight the importance of solubilizing groups to optimize excited-state photophysics.S.L. thanks AGS(O) Scholarship support from A*STAR Singapore. J.W. acknowledges financial support from MOE Tier 3 grant (MOE2014-T3-1-004). This work was supported by the Engineering and Physical Sciences Research Council, U.K. (Grant numbers EP/M005143/1 and EP/G060738/1). D.H.P.T. and N.D.M.H. acknowledge the Winton Programme for the Physics of Sustainability. K.C. and J.M.H. acknowledge support from a Rutherford Discovery Fellowship to J.M.H

Size-Dependent Photon Emission from Organometal Halide Perovskite Nanocrystals Embedded in an Organic Matrix

10.1021/jz502615eJournal of Physical Chemistry Letters63446-45