Session M10: Recent Advances in Solar Photovoltaics and Energy Conversion: Materials and Devices

Enhanced performance and stability of ternary organic solar cells based on fullerene acceptors and PEIE dipole interfacial transport layer

The limiting stability of organic solar cells (OSCs) is the main barrier to commercial use. Ternary blend strategy and usage of the dipole interfacial transport layer contribute to achieving the required device stability and improving the performance of OSCs. The fullerene acceptors have played a key role in the bulk heterojunction OSCs due to advantages such as ultrafast photo-induced electron transfer from polymer and strong absorption in the visible region. In this presentation, we demonstrate the efficiency of ternary OSCs having PC61BM into binary blend PTB7-Th:PC71BM on a low temperature processed dipole interfacial polyethylenimine ethoxylated (PEIE) electron transport layer (ETL). The Impedance measurements confirm that ternary blend OSC device has faster charge transfer and high recombination resistance at the interface than binary blend OSC devices. Therefore, a high PCE of 7.8 % with 5 % increased current density (J_sc) due to complementary absorption of PC61BM was achieved for the ternary blend, which is 10% higher than the binary device. The stability of encapsulated OSCs was tested after storing these in the ambient environment. The ternary and binary OSCs retained almost 90% and 85% of their initial performances after 60 days, respectively.   

Manipulating the Dimensionality and Ionic Conductivity of CH3NH3PbBr3 Thin Film by Graphene quantum dots

Metal Halide perovskites have attracted immense attention owing to their diverse applications in the fields of photovoltaics, optoelectronics, and energy storage. Ion migration is one of the interesting and mysterious processes in these materials. The facile manipulating of ionic- and electronic conductivities is one of the key features to make these materials a versatile candidate for next-generation energy conversion and storage applications. The electronic charge transport in perovskite materials is coupled with ion migration. In this work, we have reported a very facile strategy to control the dimensionality and ionic conductivity of the perovskite materials by introducing Sulphur doped graphene quantum dots (SGQDs) in methylammonium lead tri-bromide. We have performed impedance spectroscopy on an ITO/TiO2/perovskite/electrolyte device structure to demonstrate that the ionic conductivity is strongly dependent on the applied bias in SGQDs doped perovskite materials. SGQDs doped perovskites with lower ion migration as well as weaker electron-ion coupling compared to pristine perovskites. The SGQD acts like a filter for the ionic current while electronic transport is unaffected.   

Probing Halide Perovskites' Degradation from the Macro- to the Nanoscale

At the macroscale, we interrogate the individual and combined effects of extrinsic (humidity and oxygen) and intrinsic (light, bias, and temperature) stressors on halide perovskite materials by implementing photoluminescence (PL) under environmentally controlled conditions. We unravel the effects of distinct humidity levels on the charge carrier recombination in CsxFA1−xPb(IyBr1−y)3 perovskites through in situ PL, where we temporally and spectrally measure light emission within loops of critical relative humidity (rH) levels. Our results show that the Cs/Br ratio strongly affects the spectral stability of light emission hysteresis, as well as its extent. The photo-emission dynamics in metal halide perovskites with both I and Br is also interrogated by environmental PL, where we find that the presence of Br suppresses hysteresis. The isolated effects of both temperature and photon excitation energy are also investigated. At the nanoscale, we use advanced scanning probe microscopy methods to image the electrical behavior (photovoltage and photocurrent) of a series of perovskites. Here, we resolve ion motion in real-time.    

Highly Mobile Excitons in Single Crystal Methylammonium Lead Tribromide Perovskite Microribbons

Excitons are often given negative connotation in solar energy harvesting in part due to their presumed short diffusion lengths. We investigate exciton transport in single-crystal methylammonium lead tribromide (MAPbBr3) nanoribbons via spectrally, spatially, and temporally resolved photocurrent and photoluminescence measurements. Distinct peaks in the photocurrent spectra confirm exciton formation and allow for accurate extraction of the low temperature exciton binding energy (39 meV). Photocurrent decays within a few um at room temperature, while a long-range photocurrent component appears at lower temperatures (100 um below 140 K). Carrier lifetimes of 1.2 us or shorter exclude the possibility of the long decay length arising from slow trapped-carrier hopping. We attribute the long-distance transport to high-mobility excitons. The observation of high-mobility excitons in halide perovskites implies exciton transport does not limit the energy conversion process and may open up new opportunities for novel exciton-based photovoltaic applications.   

Effect of negative capacitance behavior on the performance of encapsulated MAPbI3 perovskite solar cell at a different aging time

Negative capacitance behavior in the low-frequency region is a familiar feature in perovskite solar cells. In this study, we have investigated the negative capacitance behavior of encapsulated MAPbI₃ perovskite solar cell at different aging times (15 days, 30 days, and 51 days) with impedance measurement technique.  For the study, we have used impedance spectroscopy to analyze the response of MAPbI₃ perovskite solar cell at different bias voltages. The origin of this kind of behavior is still under debate. Here, we have used the surface polarization model (SPM) to explain this behavior. An equivalent circuit based on the SPM was implemented to model the impedance spectra and a kinetic relaxation time of 0.96 s was observed. The performance of solar cell slowly decreases with time. However, the power conversion efficiency remains around 10% even after 51 days of encapsulation. We found that negative capacitance behavior is directly related to the aging of the solar cell. This aging of cell leads to the formation of traps states and/or the accumulation of charges or ions at the interface. To understand the nature of ionic movement along with finding activation energy and capture cross-section, we performed the deep level transient spectroscopy (DLTS) measurement. Overall, our study may provide valuable information about the mechanism inside the MAPbI3 perovskite solar cell that has a direct effect on its performance.   

Tetra(phenoxy)perylenediimide Polycrystalline Films: Symmetry-Breaking Charge Separation in the Solid State for Open-Circuit Voltage Enhancement in Organic Photovoltaics

The light-absorbing layer in an organic photovoltaic (OPV) device is made of separate electron donor and acceptor molecules. Finding suitable materials which optimize exciton generation but are also electronically similar enough to minimize defects is an engineering challenge. Poor band gap matching can lead to exciton recombination losses which lower the open-circuit voltage and efficiency. With symmetry-breaking charge separation (SB-CS), only one material is needed to generate excitons and separate charges. SB-CS occurs when one molecule is photoexcited while coupled to a neighbor and the complex undergoes charge transfer (CT) resulting in one anion and one cation. We show SB-CS in films of tetraphenoxy-PDI (tpPDI) with different tails at imide positions: octyl-tpPDI and H-tpPDI. Single crystal and grazing incidence X-ray diffraction reveal pi-pi stacking in both derivatives and intermolecular hydrogen-bonding only in H-tpPDI, resulting in H-tpPDI films being most ordered. A mixture of singlet and CT character is observed by transient absorption spectroscopy, resulting in confirmation of an excimer-like state. Stronger CT character is present in H-tpPDI suggesting it is the strongest candidate for OPVs.   

Large Bulk Piezophotovoltaic Effect of Monolayer 2H-MoS2

The bulk photovoltaic effect in noncentrosymmetric materials is an intriguing physical phenomenon that holds potential for high-efficiency energy harvesting. Here, we study the shift current bulk photovoltaic effect in the transition-metal dichalcogenide MoS₂. We present a simple automated method to guide materials design and use it to uncover a distortion to monolayer 2H-MoS₂ that dramatically enhances the integrated shift current. Using this distortion, we show that overlap in the Brillouin zone of the distributions of the shift vector (a quantity measuring the net displacement in real space of coherent wave packets during excitation) and the transition intensity is crucial for increasing the shift current. The distortion pattern is related to the material polarization and can be realized through an applied electric field via the converse piezoelectric effect. This finding suggests an additional method for engineering the shift current response of materials to augment previously reported methods using mechanical strain.   

Probing Carrier Dynamics of CdTe/ZnCdSe Intermediate Band Solar Cells by Time Resolved Reflection

Intermediate band photovoltaic have the potential to overcome the Shockley Queisser limit and potentially can achieve efficiencies as high 63%.  In this work we present time resolved reflectance measurements of intermediate band solar cells made from CdTe fractional monolayer QDs embedded in a ZnCdSe host matrix. The solar cells are grown by molecular beam epitaxy. An intermediate band is formed by incorporating CdTe fractional monolayer QDs embedded in a ZnCdSe host matrix. The type-II band alignment and the lack of a wetting layer make these materials very suitable for IB-devices. A superlattice structure of CdTe/ZnCdSe QDs (100 periods) is presented in which the size and the strain of the QDs are analyzed by X-ray diffraction and is analyzed spectroscopically.   

Assessment of the structural and electronic properties of GaxIn1-xP semiconductors for betavoltaic energy conversion systems

Ga_xIn_1-xP systems are suitable for the semiconductor absorber in betavoltaic batteries. It is well known that betavoltaic batteries offer unique advantages over conventional batteries, including long-term operation lifetime and high energy densities. However, radiation-induced damage in the semiconducting absorber has detrimental effects in the conversion efficiency and longevity of betavoltaic batteries. In this context, the structural and electronic properties of Ga_xIn_1-xP systems for 0 ≤ x ≤ 1 were investigated using first-principles calculations. The minimum energy structures were determined by systematic examination of a series of atomic configurations. The dynamical stability of the Ga_xIn_1-xP systems was confirmed by the absence of negative phonon frequencies in the phonon band structure. This work provides a detailed analysis of the electronic properties and structural properties. The effect that radiation-induced damage in Ga_0.5In_0.5P system were investigated in ab-initio molecular dynamics simulation of recoil atoms.   

Tackling Disorder in γ-Ga2O3

Ga₂O₃ and its polymorphs are attracting increasing attention. The rich structural space of polymorphic oxide systems such as Ga₂O₃ offers promising potential for electronic structure engineering, which is of particular interest for a range of energy applications, such as power electronics. γ-Ga₂O₃ presents a particular challenge across synthesis, characterisation, and theory due to its inherent disorder and resulting complex structure -- electronic structure relationship. Here, density functional theory is used in combination with a machine learning approach to screen a large number of potential structures, and thereby develop a robust atomistic model of the γ-phase. Theoretical results are compared with surface and bulk sensitive soft and hard X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, and photoluminescence excitation spectroscopy experiments representative of the occupied and unoccupied states of γ-Ga₂O₃. This work presents a leap forward in the treatment of complex, disordered oxides and is a crucial step towards exploring how their electronic structure can be understood in terms of local coordination and overall structure.   

The Influence of polymorphism on the electronic structure of Ga2O3

New wide and ultra wide band gap materials are necessary to develop more advanced generations of power electronic devices, ultimately improving their energy-efficiency and reliability. Ga₂O₃ is an interesting alternative to established materials such as SiC and GaN triggering renewed interest in its fundamental properties. In this work, three polymorphs of Ga₂O₃ are investigated using X-ray diffraction, soft and hard X-ray photoelectron and absorption spectroscopy, and ab initio theoretical approaches to gain insights into their structure–electronic structure relationships. The polymorphism of Ga₂O₃ and the related differences in local coordination environments are shown to affect both core levels as well as valence and conduction electronic structure. By combining state-of-the-art materials characterisation and electronic structure theory this work provides detailed insights into how tuning the structure of Ga₂O₃ results in differences in the electronic structure. Ultimately, the fundamental understanding of this relationship enables targeted tuning of materials properties which help to move towards integrating Ga₂O₃ in future power electronic device generations.   

Growth and analysis of rutile-structured (Ge,Sn)O2 alloy thin films

Ultra-wide bandgap (UWBG) semiconductors, whose bandgap is wider than 3.4 eV, have been gaining attentions as promising materials for future power-electronics devices. As a novel UWBG material, rutile-structured germanium oxide (r-GeO₂) has emerged due to its theoretically excellent properties^[1], that is, ambipolar dupability, high carrier mobility, and high thermal conductivity as well as its wide bandgap of 4.68 eV^[2]. In addition, by realizing an alloy system of rutile-structured r-SnO₂-r-GeO₂-r-SiO₂, bandgap engineering and an application of the heterostructures to power devices can be possible in the future. Recently, growth of r-GeO₂ thin film has been reported^[3,4].

In this work, we demonstrate r-(Ge_x,Sn_1-x)O₂ alloys in an entire Ge composition range (x=0.00, 0.37, 0.55, 0.82, and 1.00) by mist chemical vapor deposition. Physical and chemical properties at each x are measured by spectroscopic ellipsometry and X-ray photoelectron spectroscopy, respectively. These achievements advance the application of r-(Ge,Sn)O₂/r-GeO₂ heterostructures.

 

[1] Chae et al., APL 118, 260501 (2021).

[2] Stapelbroek et al., SSC 25, 959 (1978).

[3] Chae et al., APL 117, 072105 (2020).

[4] Takane et al, APL 119, 062104 (2021).   

Routes to barocaloric materials: importance of rotational dynamics in ammonium sulfate

Solid-state cooling using barocaloric materials is a promising avenue for eco-friendly, inexpensive and efficient cooling. However, in order to design barocaloric compounds it is essential to understand the mechanisms behind this group's large pressure-driven entropy change. To this end, we studied the rotational dynamics in the giant inverse-barocaloric ammonium sulfate. Using a newly developed low-background, high-pressure gas cell, quasi-elastic neutron scattering experiments under pressure have afforded detailed insight into the origin of the barocaloric effect. In the low-entropy phase, jump-rotations of the ammonium cations increase with pressure. This is the result of pressure destabilising the structure and driving the material to the high-entropy phase, where rotations are maximally activated. We argue that this mechanism is the result of competing hydrogen bond networks between the two phases; this feature can be a guide in the search for new caloric materials.   

A combined INS and DFT exploration of the lattice dynamics in barocaloric Ammonium Sulfate

As global warming prompts more people to use air conditioning, it is imperative that new environmentally-friendly cooling technologies are developed that break the vicious heating cycle. Ammonium sulfate is a giant inverse barocaloric material, which is cheaply and commercially available, with potential for cooling applications. Its efficient technological exploitation requires an understanding of the mechanism driving the entropy change and heat flow through the material. 

To resolve the question of the origin of the entropy change, we have performed a comprehensive investigation combining neutron scattering with DFT simulations. Our total scattering measurements indicate that the previously favoured simple Boltzmann model of the entropy, where each molecular ion is disordered across the mirror plane in the high temperature phase, is untenable, while our QENS results demonstrate that pressure facilitates the rotation of the ammonium cations, destabilising the crystallographic structure, driving the high-entropy phase transition.

In this presentation we will report an inelastic neutron scattering and density functional theory investigation under working temperature and pressure conditions, revealing that the subtle differences between the Pna2₁ and Pnam structures produce a distinct change in the phonon spectrum, particularly in terms of the ammonium ion librational modes. Going from low to high temperature these modes become more dispersive, lower in energy and have more negative Gruneisen parameters, indicative of a flatter energy surface in their high entropy phase. The demonstrated benefits of a flat energy landscape due to competing hydrogen bonds may provide a new route to caloric materials.