Enhancing photoluminescence yields in lead halide perovskites by photon recycling and light out-coupling

In lead halide perovskite solar cells, there is at least one recycling event of electron-hole pair to photon to electron-hole pair at open circuit under solar illumination. This can lead to a significant reduction in the external photoluminescence yield from the internal yield. Here we show that, for an internal yield of 70%, we measure external yields as low as 15% in planar films, where light out-coupling is inefficient, but observe values as high as 57% in films on textured substrates that enhance out-coupling. We analyse in detail how externally measured rate constants and photoluminescence efficiencies relate to internal recombination processes under photon recycling. For this, we study the photo-excited carrier dynamics and use a rate equation to relate radiative and non-radiative recombination events to measured photoluminescence efficiencies. We conclude that the use of textured active layers has the ability to improve power conversion efficiencies for both LEDs and solar cells.We acknowledge financial support from the Engineering and Physical Sciences Research Council of the U.K. (EPSRC). J.M.R. and M.T. thank the Winton Programme for the Physics of Sustainability (University of Cambridge). L.M.P.-O. thanks the Cambridge Home European Scheme for financial support. L.M.P.-O. and J.P.H.R. also thank the Nano Doctoral Training Center (NanoDTC) of the EPSRC for financial support. M.A.-J. thanks Nyak Technology Limited for a PhD scholarship. F.D. acknowledges funding from a Herchel Smith Research Fellowship

Efficient singlet exciton fission in pentacene prepared from a soluble precursor

Carrier multiplication using singlet exciton fission (SF) to generate a pair of spin-triplet excitons from a single optical excitation has been highlighted as a promising approach to boost the photocurrent in photovoltaics (PVs) thereby allowing PV operation beyond the Shockley-Queisser limit. The applicability of many efficient fission materials, however, is limited due to their poor solubility. For instance, while acene-based organics such as pentacene (Pc) show high SF yields (up to 200%), the plain acene backbone renders the organic molecule insoluble in common organic solvents. Previous approaches adding solubilizing side groups such as bis(tri-$\textit{iso}$-propylsilylethynyl) to the Pc core resulted in low vertical carrier mobilities due to reduction of the transfer integrals via steric hindrance, which prevented high efficiencies in PVs. Here we show how to achieve good solubility while retaining the advantages of molecular Pc by using a soluble precursor route. The precursor fully converts into molecular Pc through thermal removal of the solubilizing side groups upon annealing above 150 °C in the solid state. The annealed precursor shows small differences in the crystallinity compared to evaporated thin films of Pc, indicating that the Pc adopts the bulk rather than surface polytype. Furthermore, we identify identical SF properties such as sub-100 fs fission time and equally long triplet lifetimes in both samples.M.T. thanks the Gates Cambridge Trust and the Winton Programme for the Physics of Sustainability for funding. A.H.K. acknowledges the Cambridge Nehru Bursary, the Cambridge Bombay Society, a Trinity-Henry Barlow- and Haidar Scholarship as well as Rana Denim Pvt. Ltd. for financial support. K.B. and J.N. would like to thank Dr. Tom Arnold and Jakub Rozboril for assistance during the beam time at Diamond Light Source. Financial support for K.B. from Diamond Light Source, Swiss Light Source, and the German Research Foundation (Grant No. BR 4869/1-1) is gratefully acknowledged. M.L.B. is a research fellow of Christ’s College, Cambridge. This work was supported by the Engineering and Physical Sciences Research Council (Grant Nos. EP/M005143/1, EP/G060738/1 and Cambridge NanoDTC EP/G037221/1, EP/L015978/1)

Long-Range Charge Extraction in Back-Contact Perovskite Architectures via Suppressed Recombination

Metal-halide perovskites are promising solution-processable semiconductors for efficient solar cells and show unexpectedly high diffusion ranges of photogenerated charges. Here, we study charge extraction and recombination in metal-halide perovskite back-contact devices, which provide a powerful experimental platform to resolve electron- or hole-only transport phenomena. We prepare thin films of perovskite semiconductors over laterally-separated electron- and hole-selective materials of SnO¬2 and NiOx, respectively. Upon illumination, electrons (holes) generated over SnO¬2 (NiOx) rapidly transfer to the buried collection electrode, leaving holes (electrons) to diffuse laterally as majority carriers in the perovskite layer. Under these conditions, we find recombination is strongly suppressed. Resulting surface recombination velocities are below 2 cm s-1, an order of magnitude lower than in the presence of both carrier types, and approaching values of high-quality silicon. We find diffusion lengths of electrons and holes exceed 12 µm in our horizontal polycrystalline device, an order of magnitude higher than reported in vertically stacked architectures. We fabricate back-contact solar cells with short-circuit currents as high as 18.4 mA cm-2, reaching 70% external quantum efficiency

Maximizing and stabilizing luminescence from halide perovskites with potassium passivation

Metal halide perovskites are of great interest for various high-performance optoelectronic applications. The ability to tune the perovskite bandgap continuously by modifying the chemical composition opens up applications for perovskites as coloured emitters, in building-integrated photovoltaics, and as components of tandem photovoltaics to increase the power conversion efficiency. Nevertheless, performance is limited by non-radiative losses, with luminescence yields in state-of-the-art perovskite solar cells still far from 100 per cent under standard solar illumination conditions. Furthermore, in mixed halide perovskite systems designed for continuous bandgap tunability2 (bandgaps of approximately 1.7 to 1.9 electronvolts), photoinduced ion segregation leads to bandgap instabilities. Here we demonstrate substantial mitigation of both non-radiative losses and photoinduced ion migration in perovskite films and interfaces by decorating the surfaces and grain boundaries with passivating potassium halide layers. We demonstrate external photoluminescence quantum yields of 66 per cent, which translate to internal yields that exceed 95 per cent. The high luminescence yields are achieved while maintaining high mobilities of more than 40 square centimetres per volt per second, providing the elusive combination of both high luminescence and excellent charge transport. When interfaced with electrodes in a solar cell device stack, the external luminescence yield—a quantity that must be maximized to obtain high efficiency—remains as high as 15 per cent, indicating very clean interfaces. We also demonstrate the inhibition of transient photoinduced ion-migration processes across a wide range of mixed halide perovskite bandgaps in materials that exhibit bandgap instabilities when unpassivated. We validate these results in fully operating solar cells. Our work represents an important advance in the construction of tunable metal halide perovskite films and interfaces that can approach the efficiency limits in tandem solar cells, coloured-light-emitting diodes and other optoelectronic applications.M.A.-J. thanks Nava Technology Limited and Nyak Technology Limited for their funding and technical support. Z.A.-G. acknowledges funding from a Winton Studentship, and ICON Studentship from the Lloyd’s Register Foundation. This project has received funding from the European Union’s Seventh Framework Programme (FP7/2007-2013) under REA grant agreement number PIOF-GA-2013-622630, the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement number 756962), and the Royal Society and Tata Group (UF150033). We thank the Engineering and Physical Sciences Research Council (EPSRC) for support. XMaS is a mid-range facility at the European Synchrotron Radiation Facility supported by the EPSRC and we are grateful to the XMaS beamline team staff for their support. We thank Diamond Light Source for access to beamline I09 and staff member T.-L. Lee as well as U. Cappel for assistance during the HAXPES measurements. S.C., C.D. and G.D. acknowledge funding from the ERC under grant number 25961976 PHOTO EM and financial support from the European Union under grant number 77 312483 ESTEEM2. M.A. thanks the president of the UAE’s Distinguished Student Scholarship Program, granted by the Ministry of Presidential Affairs. H.R. and B.P. acknowledge support from the Swedish research council (2014-6019) and the Swedish foundation for strategic research. E.M.H. and T.J.S. were supported by the Netherlands Organization for Scientific Research under the Echo grant number 712.014.007

High-efficiency perovskite–polymer bulk heterostructure light-emitting diodes

Perovskite-based optoelectronic devices have gained significant attention due to their remarkable performance and low processing cost, particularly for solar cells. However, for perovskite light-emitting diodes (LEDs), non-radiative charge carrier recombination has limited electroluminescence (EL) efficiency. Here we demonstrate perovskite-polymer bulk heterostructure LEDs exhibiting record-high external quantum efficiencies (EQEs) exceeding 20%, and an EL half-life of 46 hours under continuous operation. This performance is achieved with an emissive layer comprising quasi-2D and 3D perovskites and an insulating polymer. Transient optical spectroscopy reveals that photogenerated excitations at the quasi-2D perovskite component migrate to lower-energy sites within 1 ps. The dominant component of the photoluminescence (PL) is primarily bimolecular and is characteristic of the 3D regions. From PL quantum efficiency and transient kinetics of the emissive layer with/without charge-transport contacts, we find non-radiative recombination pathways to be effectively eliminated. Light outcoupling from planar LEDs, as used in OLED displays, generally limits EQE to 20-30%, and we model our reported EL efficiency of over 20% in the forward direction to indicate the internal quantum efficiency (IQE) to be close to 100%. Together with the low drive voltages needed to achieve useful photon fluxes (2-3 V for 0.1-1 mA/cm2), these results establish that perovskite-based LEDs have significant potential for light-emission applications

Long-Range Charge Extraction in Back-Contact Perovskite Architectures via Suppressed Recombination

Metal-halide perovskites are promising solution-processable semiconductors for efficient solar cells with unexpectedly high diffusion ranges of photogenerated charges. Here, we study charge extraction and recombination in metal-halide perovskite back-contact devices, which provide a powerful experimental platform to resolve electron- or hole-only transport phenomena. We prepare polycrystalline films of perovskite semiconductors over laterally separated electron- and hole-selective materials of SnO2 and NiOx. Upon illumination, electrons (holes) generated over SnO2 (NiOx) rapidly transfer to the buried collection electrode, leaving holes (electrons) to diffuse laterally as majority carriers in the perovskite layer. Under these conditions, we find recombination is strongly suppressed. Resulting surface recombination velocities are below 2 cm s−1, approaching values of high-quality silicon. We find diffusion lengths exceed 12 μm, an order of magnitude higher than reported in vertically stacked architectures. We fabricate back-contact solar cells with short-circuit currents as high as 18.4 mA cm−2, reaching 70% external quantum efficiency. Metal-halide perovskites are promising sustainable low-cost materials for optoelectronic devices such as solar cells and LEDs. To optimize performance in these applications, a detailed understanding of charge transport characteristics and the influence of interfaces, such as grain boundaries, is vital. At present, a wide range of transport parameters have been reported, often via indirect measurements, since direct measurement has proven challenging. Here, we demonstrate an approach based on measurements in a back-contact geometry that is capable of probing electron and hole transport mechanisms separately. Such insights are not typically accessible in vertical architectures. We demonstrate a back-contact perovskite device, which we find to operate by majority-carrier diffusion and find that charges diffuse remarkable distances in such scenarios. Diffusion over remarkably long distances over electron-extraction electrodes enables efficient charge collection in short-circuit conditions. A detailed understanding of charge transport is vital to maximize the efficiencies of optoelectronic devices. Using a back-contact architecture, the authors probe transport of electrons and holes separately in polycrystalline hybrid perovskite thin films. Isolating photoexcited charge carriers in separate regions of the device leads to long diffusion ranges of carriers. The authors demonstrate a back-contact perovskite solar cell that operates on majority-carrier diffusion. These results highlight electrode interfaces as limiting aspects of current back-contact architectures, indicating opportunities for improvement

Research data supporting "Efficient singlet exciton fission in pentacene prepared from a soluble precursor"

Underlying datasets for all figures in the Manuscript and the SI. Data comprises results from absorption-, photoluminescence-, ellipsometry-, transient spectroscopy-, FTIR-, X-ray and atomic force microscopy measurements