Kwanghee Lee

Introduction to the Issue on Next-Generation Organic and Hybrid Solar Cells

The 41 papers in this special issue focus on next-generation organic and hybrid solar cells.

Understanding the role of titanium dioxide (TiO 2 ) surface chemistry on the nucleation and energetics of hybrid perovskite films (Conference Presentation)

We demonstrate how amino-terminated silane monolayers alter the chemical and energetic composition of the TiO2 surface, which controls the interfacial nucleation, growth and energetics of device-relevant, hybrid perovskite (PVSK) thin films. The surface chemistry and energetics of compact TiO2 thin films are modified with a 3-aminopropyltriethoxysilane (APTES) monolayer that can either weakly coordinate Pb2+ ions (–NH2/free base form) or act as a surrogate organic cation (–NH3+/acid form) at the TiO2/PVSK interface, providing for significant differences in the nucleation free energy for the PVSK active layer as a function of NH3+/NH2 ratio. XPS spectra of amine-modified TiO2 surfaces (N 1s core level) demonstrate that we can achieve NH3+/NH2 ratios of between 3:1 and 1:3 depending upon subsequent acid and base treatment, respectively. Methylammonium lead triiodide (MAPbI3) films are incrementally co-evaporated on TiO2, TiO2/APTES-NH3+ and TiO2/APTES-NH2 interfaces, and the chemical composition, growth dynamics and energetics are systematically investigated using in situ X-ray photoelectron spectroscopy (XPS) and UV photoelectron spectroscopy (UPS). The XPS and UPS results reveal that initial nucleation and subsequent growth of the MAPbI3 PVSK film strongly depends on the chemical functionality of the TiO2 surface. The evaporated films display island-like growth on the bare TiO2 surface, which hinders nucleation of the PVSK phase until ca. 15 nm of precursor material is deposited. Conversely, film growth is more layer-by-layer on the amine-modified TiO2 interfaces, which promote nucleation of the PVSK phase within the first ca. 5 nm of deposition. In addition to vacuum evaporated thin films, we show how these TiO2 surface modifications control the morphology and crystallinity of solution-processed PVSK films based on formamidinium and methylammonium organic cations. These studies elucidate the role of TiO2 surface chemistry on the formation mechanism of hybrid PVSK active layers and the interfacial and bulk energetics, which have significant consequences related to the processing and operation of next-generation optoelectronic device platforms.

Mapping structural properties of lead halide perovskites by scanning nanofocus x-ray diffraction (Conference Presentation)

To date, the most widely employed technique to investigate the crystal structure and morphology of perovskite films have been SEM and X-ray diffraction. Most perovskite films are highly polycrystalline and thus traditional imaging techniques cannot easily isolate single crystallites or determine the statistics of lateral grain size or crystal size and morphology. A further limitation is that both SEM and other scanning probe microscopy techniques are surface-sensitive, with the bulk of the material being largely inaccessible using conventional imaging methods. nnTo gain better insight into a perovskite film, we scan an X-ray beam focused to 400 nm across a sample, while simultaneously recording wide-angle X-ray scattering patterns. Such scanning nanofocus x-ray diffraction (nXRD) techniques have been employed to resolve local variations in structure across a broad range of materials and research fields, however this is the first time they have been applied to a perovskite film.nnWe describe a method to in which each scattering pattern is analysed, allowing a classification to be made of individual perovskite grains according to a specific Miller plane. Using this method, we show that nXRD is able to resolve the extent of individual perovskite grains buried within a polycrystalline film (grain segmentation). We then use nXRD and SEM to demonstrate that the perovskite film coverage can be controlled by varying the temperature of the substrate during spin-coating. We conclude by showing that solar cells with a perovskite layer cast on a substrate held at a relatively high temperature result in devices with higher PCE.

Lead halide perovskites of different dimensionalities: growth, properties, and applications in optoelectronics (Conference Presentation)

Three-dimensional (3D) lead halide perovskites exhibit impressively long carrier diffusion lengths and low trap densities, while reduced-dimensionality perovskites, such as two-dimensional (2D) and zero-dimensional (0D) perovskite derivatives, possess large exciton binding energies and high photoluminescence quantum yields. These characteristics make the diverse class of perovskite materials ideal for photovoltaics, photodetection, and light emission. Here we discuss our latest advances in growing and understanding the properties of monocyrstalline 3D perovskites, as well as 0D and 2D perovskite derivatives. We also demonstrate the integration of these materials in a range of optoelectronic applications including: monocrystalline perovskite solar cells; simultaneously fast and sensitive photodetectors that can operate in both broadband and narrow-band regimes; and efficient light-emitting diodes. Thus, in these device prototypes, we showcase the importance of crystallinity, dimensionality, and composition for tailoring materials properties, and realizing novel and efficient perovskite optoelectronics.

Towards a reliable measurement protocol for perovskite solar cells (Conference Presentation)

The rapid rise of power conversion efficiency of metal-halide perovskite solar cells beyond 20 % has drawn huge attention. Recent certified efficiencies, however, have been marked as “unstabilized” as perovskite solar cells tend to show a hysteretic behaviour during current density-voltage (J-V) measurements. This leads to deviating results for varying scan parameters and conditions, which challenges reliable and comparable results. In particular, the extent of this behaviour is highly dependent on device preparation method, architecture, device history, and more importantly on measurement preconditions and scan rate. Thus, the demand for reliable stabilized values arises which are reproducible and comparable among different laboratories. Here we introduce an adaptive tracking of the maximum power point and the open circuit voltage. We compare these values with device characteristics derived from standard J-V measurements. Furthermore, we discuss the challenges of a correct efficiency determination and provide the algorithms for easy implementation in existing measurement systems.

2D compact model to characterize phase separation in organic solar cell bulk heterojunctions (Conference Presentation)

The phase separation between donor and acceptor molecules within the active layer of an organic solar cells dictates the morphology and hence is key to the recombination rate and ultimately the performance of the organic solar cell. Molecular dynamics (MD) simulation is a suitable technique to understand this phenomenon; however, conventional all-atom MD simulations cannot reach the appropriate length and time scales to compare with macroscopic observation. Even with the many available coarse-grained MD models, it is difficult to reach these scales. Therefore, we introduce here a 2D compact model to overcome this challenge, built by multiscale coarse-graining. First, we simulate systems including conjugated polymers, fullerenes, and organic solvents using all-atom MD to extract information about molecular conformation and packing. This includes an analysis of polymer solution behavior, fullerene clustering, and binary and tertiary mixing properties. These results are then used to systematically parameterize the molecules used in 2D coarse-grained MD simulations. The 2D simulations probe experimentally relevant length scales that were previously intractable to sample by other MD simulation methods. Using this model, we explore ternary systems including polymer, fullerene, and solvent molecules to investigate the phase separation process between polymer donors and fullerene acceptors. In this scheme, we additionally introduce explicit solvent evaporation to emulate realistic processing conditions. We quantify phase separation domain sizes that are comparable to experimentally observed values from resonant soft x-ray scattering. In addition, we extend this framework to other chemical species to demonstrate the flexibility of the approach.

Self-doped n-type interfacial materials for high-performance polymer/perovskite solar cells (Conference Presentation)

The performance of polymer/perovskite solar cells (PSCs/PVKSCs) is highly dependent on the interfacial contact between the active layer and electrodes. Water/alcohol soluble interfacial materials, which enable orthogonal processing of high-performance multi-layer PSCs can greatly improve the interfacial contact as well as device performance. Traditional interfacial materials are not compatible with the large-area processing of PSCs using roll-to-roll techniques. Here, we present a series of self-doped interfacial materials with controlled doping properties and high mobility for the interface optimization of PSCs. Self-doped interfacial materials containing n-type conjugated backbone and polar side chains are prepared. It was shown that neutral amino groups undergo photo-induced doping process while the bromide-quaternized groups employ a self-doped mechanism.1 Further study on the counterions of the self-doped interfacial materials shows that the counterions can also induce different self-assembling and doping behavior with different strength, leading to varied charge-transporting properties. 2 More importantly, these self-doped interfacial materials can enable high-performance (>10%) PSCs and still work efficiently in varied thickness, which match well with the requirement of the fabrication of large-area PSCs. Based on the development of self-doped interfacial materials, a high-performance interconnecting layer for tandem solar cells was also developed, which can boost the power conversion efficiency (PCE) of tandem solar cells to 11.35%.3 Moreover, these interfacial materials can passivate the surface traps of perovskite to improve the electron extraction properties of PVKSC, leading to high-performance PVKSCs even when the thickness of interfacial material is more than 200 nm.4 The successful development of self-doped interfacial materials offers a better processing window for potential fabrication of PSCs/PVKSCs using large-area processing method.nn1 J. Am. Chem. Soc. 2016, 138, 2004-2013.n2 Mater. Horiz. 2017, 4, 88-97.n3 Adv. Mater. 2016, 28, 4817-4823.n4 Adv. Energy Mater. 2016, 1501534.

Impact of exciton transfer dynamics on charge generation in polymer/nonfullerene solar cells (Conference Presentation)

The initial steps in organic photovoltaic cell (OPV) operation involve the formation of neutral excitons through photo absorption, exciton diffusion to and separation into free charges at the donor acceptor interface.1, 2As the usable solar spectrum spans a large range from the visible to the infra-red (IR), an obvious direction for improved light harvesting is to synthesize donor and acceptor materials with complementary absorption. In such devices, specifically those involving polymer donors and small molecule acceptors, both charge transfer from donor and acceptor moieties, and energy (exciton) transfer from high band gap to low band gap material are possible. Here we show that when charge and exciton transfer processes are present, the co-existence of excitons in both domains can cause a loss mechanism. Charge separation of excitons in a low band-gap polymer is hindered due to exciton population in the larger band-gap acceptor domains. Our results further show that excitons in the lower bandgap material should have a relatively long lifetime compared to the transfer time of excitons from the higher band gap material, in order to contribute to the charge separation. These observations provide significant guidance for design and development of new materials in OPV applications.

A new series connection architecture for large area printed organic solar cell modules (Conference Presentation)

The fabrication of organic photovoltaic modules via printing techniques has been the greatest challenge for their commercial manufacture. Current module architecture, which is based on a monolithic geometry consisting of serially interconnecting stripe-patterned sub-cells with finite widths, requires highly sophisticated patterning processes that significantly increase the complexity of printing production lines and cause serious reductions in module efficiency due to so-called ‘aperture loss’ in series connection regions. In this study, we demonstrate an innovative module structure that can simultaneously reduce both patterning processes and aperture loss. By using a charge recombination feature that occurs at contacts between electron/hole transport layers, we devise a series connection method that facilitates module fabrication without patterning the charge transport layers. With the successive deposition of component layers using slot-die and doctor-blade printing techniques, we achieve a high module efficiency reaching 7.5% with area of 4.15 cm2.

Transport and recombination considerations for charge generation efficiency in organic solar cells (Conference Presentation)

The efficiency with which photogenerated free carriers are extracted in excitonic solar cells is governed by the interactions between a number of material specific and device level properties and parameters: recombination dynamics, faster and slower carrier mobilities, contact potentials, etc. This complexity is inherent in disordered excitonic semiconductors, and understanding the underlying structure-property relationships is an ongoing endeavour in the field of organic solar cells. Questions such as – do higher mobility semiconductors necessarily deliver improved charge collection; how does supressed bimolecular recombination lead to fill factor maintenance especially at higher input irradiances [1]; or, how do we achieve high fill factors and efficiencies in thick junctions more suited to viable manufacturing [2]?nIn my talk I will examine some of the latest thinking in this area [3], and present data from a suite of emerging experimental techniques such as intensity dependent photocurrent, resistant dependent photovoltage and related extraction methodologies. I will also discuss recent findings on the interrelationship between the efficiency with which free charges are extracted and charge separated state formation – the two possibly connected through entropic considerations [4].n[1] “The impact of hot charge carrier mobility on photocurrent losses in polymer-based solar cells”, B. Philippa, M. Stolterfoht, P. L. Burn, G. Juska, P. Meredith, R. D. White & A. Pivrikas, Scientific Reports, 4, 5695 (2014).n[2] Efficient, large area, and thick junction polymer solar cells with balanced mobilities and low defect densities. A. Armin, M. Hambsch, P. Wolfer, H. Jin, J. Li, Z. Shi, P. L. Burn, and P. Meredith, Advanced Energy Materials, 5, 1401221 (2015).nn[3] “Impact of charge transport on current-voltage characteristics and power conversion efficiency of organic solar cells”, U. Wurfel, D. Neher, A. Spies & S. Albrecht, Nature Communications, 6:6951 (2015).n[4] “Slower carriers limit charge generation in organic semiconductor light harvesting systems”, M. Stolterfoht, A. Armin, S. Shoaee, I. Kassal, P.L. Burn & P. Meredith, Nature Communications, 7, 11944 (2016).