Session P20: Recent Advances in Solar Photovoltaics

An analytical model for polycrystalline photovoltaics

Despite decades of research, the role of grain boundaries in the photovoltaic behavior of thin film polycrystalline solar cells remains poorly understood. The high defect density of grain boundaries generally promotes recombination and reduces photovoltaic efficiency. However, thin film polycrystalline photovoltaics such as CdTe and Cu(In; Ga)Se2 exhibit high efficiencies despite a large density of grain boundaries.

I’ll present our analysis of recombination in a 2-d model of a pn+ junction containing columnar grains with positively charged grain boundaries. We restrict our attention to dark recombination at forward bias and derive the dark J-V relation. We formulate a physical picture of the electron/hole currents and recombination, and translate this picture into a simplified effective model which describes the essential features of the full system. A crucial simplification follows from that fact that electrons are electrostatically confined to the grain boundary core, so that parts of the problem can be reduced to 1 dimension.

The effective model we construct is amenable to simple analytic solutions. Under reasonable assumptions (such as non-depleted grains and reasonably high hole mobilities), we find that these analytic solutions successfully describe the results of full 2-d numerical simulations for a range of grain boundary orientations and defect structures. Numerical simulations further show that the dark J-V relation can be used to determine the open-circuit potential Voc of an illuminated junction for a given short-circuit current density Jsc. We can therefore provide a precise relation between grain boundary properties and Voc. This relation enables one to rationalize the (relatively) low Voc observed in polycrystalline CdTe and to formulate strategies to optimize grain boundary properties. I conclude by discussing approaches to experimentally test our results.
  

Synthesis and Characterization of Aluminum-Zinc Based Layered Double Hydroxides for Electrochemical Energy Storage Applications

Recently, Layered Double Hydroxides (LDH) such as Al-Zn based LDH have shown promise in several energy related applications including electrochemical energy storage applications. In this study we will report on the synthesis and characterization of Aluminum and Zinc based LDH (Al:Zn LDH) grown directly on thin foils of aluminum using a chemical bath deposition (CBD) technique. A detail structural and compositional analysis of these materials will be presented. We will also report on the electrochemical properties of these materials in order to determine the utility of these material in development of electrochemical double layer capacitior (super capacitor)
  

Ultrafast Phonon-assisted Exciton Transfer in Carbon Nanotube Films

Carbon-nanotube (CNT) aggregates are promising light-absorbing materials for photovoltaics due to their tunable optical bandgap and high optical. However, the hopping rate of excitons between CNTs directly related to the efficiency of these devices. Here, we theoretically investigate phonon-assisted exciton hopping, where excitons scatter with phonons into a same-tube transition state, followed by intertube Coulomb scattering into the final state. Second-order hopping between bright excitonic states is as fast as the first-order process (∼1 ps). For perpendicular CNTs, the high rate stems from the high density of phononic states; for parallel CNTs, the reason lies in relaxed selection rules. Moreover, second-order exciton transfer between dark and bright states, facilitated by phonons with large angular momentum, has rates comparable to bright-to-bright transfer, so dark excitons provide an additional pathway for energy transfer in CNT composites. These results are important as optically inactive excitons are invisible in the most experiments so one needs to complement measurements with theoretical calculations in order to get a complete picture of exciton dynamics in these types of nanostructures.   

Sn2P2S6 and Sn2P2Se6: Ferroelectric Visible Light Absorbing Semiconductors with High Polarization and Photovoltaic Potential

Ferroelectrics with suitable band gaps, in which the inversion symmetry breaking may promote the separation of photo-excited carriers and allow voltages higher than the band gap, have recently attracted attention as candidate solar absorbing materials for photovoltaics. However, these effects are not fully understood. Here, we report properties of ferroelectric Sn₂P₂S₆ and Sn₂P₂Se₆ using first principles calculations, which may serve as useful model systems for understanding photovoltaic effects in ferroelectric semiconductors. Results are given for the electronic structure, carrier pocket shapes, optical absorption and transport. We find favorable band structures for carrier transport, including both holes and electrons. Strong absorption is found above the direct gaps (2.43 eV and 1.76 eV respectively) .   

Hot carrier-enhanced interlayer electron–hole pair multiplication in 2D semiconductor heterostructure photocells

Two-dimensional heterostructures composed of atomically thin transition metal dichalcogenides (TMDs) provide an excellent opportunity to study electron–hole (e–h) pair multiplication. Here, we report the highly efficient multiplication of interlayer e–h pairs in 2D semiconductor heterostructure photocells composed of 2L-WSe₂ and 1L-MoSe₂. The carrier multiplication process, which is made more efficient through hot carrier transfer at the interface between WSe₂ and MoSe₂, can be utilized to significantly increase the photoresponse in these TMD heterojunction photocells. By exploiting this highly efficient interlayer e–h pair multiplication process, we demonstrate near-infrared optoelectronic devices that exhibit 350% enhancement of the optoelectronic responsivity at microwatt power levels. Our findings, which demonstrate efficient carrier multiplication in TMD-based optoelectronic devices, make 2D semiconductor heterostructures viable for a new class of ultra-efficient photodetectors based on layer-indirect e–h excitations.   

Manipulating hot-electron based photovoltage generations at oxide interfaces

To maximize efficiencies of photovoltaic devices and photodetectors, it is highly desirable to enable carrier excitation from low energy photons and extract excessive energy from hot carriers. At the LaAlO3/SrTiO3 interface contacted by metal electrodes, substantial photovoltages can be generated by photons with energy below the oxide bandgaps. This originates from the photoexcitation of hot carriers in metal and the subsequent charge separations through either hole filtering at the metal/oxide junction or the photothermoelectric effect produced in the interfacial two-dimensional electron gas. The two light-to-voltage conversion mechanisms can be locally selected and reconfigured by patterning of 2DEG using conductive atomic force microscope. With tunable band alignment, improved light absorption and giant Seebeck coefficient at oxide interface, the hot-electron enabled photovoltage generations make LaAlO3/SrTiO3 viable for programmable solar energy harvesting and thermoelectric device applications.   

Designing Photon Absorbing Materials by Cation Substitutions for Photovoltaics

Recently, significant research efforts have focused on emerging solar cell technologies such as organic solar cells, perovskite photovoltaics, etc. However, these technologies face many challenges. In this talk, we will present a new family of light absorbing materials so-called pyroxene silicates. These materials are abundantly present in the earth crust, however, bandgaps of naturally formed pyroxene silicates such as NaAlSi₂O₆ are quite high ~5 eV. Therefore, it is important to find a way to reduce bandgaps below 3 eV to make them usable for optoelectronic applications¹. Using first-principles calculations we investigated the possibility of band structure engineering of pyroxene silicates with chemical formula A⁺¹B⁺³Si₂O₆ by proper cation substitution (A⁺= Na, PH₄⁺, SH₃⁺, CH₃PH₃⁺, CH₃SH₂⁺ and B³⁺ = Al³⁺ Ga³⁺, Tl³⁺). We found that appropriate substitutions of both A⁺ and B³⁺ cations can reduce the electronic bandgaps of these materials to as low as 1.31 eV. In this talk, we will also discuss in details how the bandgap is tailored in this class of materials with emphasis on the impact of our bandgap engineering on thermodynamic stabilities.
1. F. El Mellouhi, et al., ChemSusChem 10 1931 (2017).   

Optimized Multi-layer Interference for Color-tuned and Transparent Colloidal Quantum Dot Solar Cells

Due to their band gap tunability and near infrared responsivity, colloidal quantum dots (CQDs) are promising active materials for mitigating absorption and photocurrent losses in color-tuned and transparent solar cells. Typical CQD solar cells are multi-layered structures comprised of optically thin layers that are optimized for their electrical properties. In this work, we develop an optimization algorithm to explore the multi-dimensional thickness space that controls multi-layer optical interference in CQD solar cells to simultaneously optimize devices for their electrical and optical performance. The tradeoffs between attainable color or transparency and available photocurrent are quantified, and the effects of non-ideal interference on apparent color are taken into account. Using our new designs, we fabricated blue, green, yellow and red CQD solar cells with photocurrents ranging from 10 mA/cm² to 15 mA/cm² and semitransparent devices with visible transparencies ranging from 27% to 32%. In addition, our optimization method can be adapted for custom-design of multi-layer structured optoelectronic devices with arbitrary spectral profiles.   

Morphology and device physics of ternary polymer solar cells with power conversion efficiencies over 11%.

Fabricating ternary solar cells (TSCs) is a promising strategy to improve the device efficiency of organic photovoltaics without introducing sophisticated processing procedures. We report high efficiency TSCs with PCEs over 11% in a few polymer solar cell systems. We demonstrate contrasting effects of two crystallizable polymers in determining efficiency improvements in PTB7-Th:PC₇₁BM host blends when they are added in. Whilst negligible charge transfer was determined in binary blends of each polymer mixture, effective energy transfer was identified from PffBT4T-2OD to PTB7-Th that contributes to an improvement in ternary device efficiency. In contrast, energy transfer from PTB7-Th to PDPP2TBT worsened the device efficiency. In another ternary system, the thrid component PCDTBT8 was found locating at the donor/acceptor interface without disrupting the crystallization of PffBT4T-2OD to maintain decent charge mobility, and loosens the fullerene networks to facilitate exciton dissociation. Device physics studies support that the additive can enhance the built-in voltage and suppress the trap-assisted charge recombination, leading to improved FF and V_OC to boost efficiency.   

Carrier Dynamics in an Intermediate-Band Semiconductor by THz Transient Photoconductivity

Multiband semiconductors may form the basis of efficient intermediate band solar cells, if sufficiently long carrier lifetimes can be engineered. THz Time-Resolved Photoconductivity was used to probe carrier dynamics in GaP_0.5 As_0.5-x N_x bulk intermediate-band semiconductors with x<0.05. The decay of photoconductivity after excitation is consistent with bimolecular electron-hole recombination with recombination constant r = 3 10^-8 cm³/s. This rapid recombination poses a challenge for solar energy applications. The carrier mobility is observed to decrease from 280 to 65 cm²/Vs as the nitrogen concentration is increased from x=0 to 0.036. We also observe a suppression of the low-frequency conductivity (f<1THz) that may be due to sample inhomogeneity. In these measurements a femtosecond optical pump pulse excites electron-hole pairs, and a delayed THz pulse measures the change in conductivity.   

Enhanced multiple exciton generation in PbS|CdS Janus nanocrystals

Creating multiple excitons from a single high-energy photon is a promising third generation solar energy conversion strategy. Multiple exciton generation (MEG) is particularly efficient in lead chalcogenide nanocrystals (NCs). A recent study showed that heterostructuring nanocrystals, e.g. by making core-shell NCs, can further enhance MEG.[1] However, in core-shell structures, either the photo-generated hole or the electron is difficult to extract.[2] Here, we show that MEG is as efficient in PbS|CdS Janus NCs as in core-shell ones; however, in Janus NCs both electrons and holes can be easily extracted. We also demonstrate that MEG is retained in conductive Janus-particle arrays, with power conversion efficiencies of nearly 3% in proof-of-principle solar cells. Using first-principles simulations we provide insights into the mechanism enhancing MEG in Janus NCs and we propose design rules for next generation nanostructured solar cells.
[1] C. M. Cirloganu et al., Nat. Commun. 5, 4148 (2014).
[2] F. Giberti, M. Vörös, and G. Galli, Nano Lett. 17, 2547 (2017).   

Increasing the Area of a White Scattering Background can Increase the Power Output of a Luminescent Solar Concentrator

Luminescent Solar Concentrators (LSCs) are fluorescent sheets of glass or plastic that absorb sunlight and concentrate their fluorescence using total internal reflection onto photovoltaic solar cells for energy conversion. LSCs have the potential to generate electricity at a lower cost than standard solar panels. Some light incident on an LSC passes through without being absorbed. To increase efficiency, an inexpensive white background is placed underneath to scatter this light back into the LSC for a second chance of absorption. Traditionally, this white background is placed very close to the LSC. I have discovered the LSC’s power output increases by increasing the area of the white background and separating it from the LSC by an optimal air gap. Using an Arduino-controlled apparatus, the LSC’s power output was measured while varying the size of the air gap for nine different areas of square backgrounds. By optimally separating a background 16 times the area of the LSC, its overall power output was increased 28.5% compared to using an optimized white background of equal size.   

Understanding Nanoscale Heterogeneity of Photogenerated Charge Carriers in BiVO4 Photoanodes by Conductive AFM

Bismuth vanadate (BiVO₄) is a promising oxide semiconductor for photoelectrochemical water splitting due to its moderate bandgap, favorable conduction band position, and long photocarrier lifetimes. However, the performance is limited by poor majority carrier transport, stoichiometry deviations, and structural defects leading to interfacial charge trapping and non-uniform energy landscapes. Identifying and controlling these nanoscale phenomena will enable the design of the next generation of highly efficient materials.
Here, we use photoconductive atomic force microscopy to resolve the correlation between local photocurrent, charge transport mechanisms, and morphology of polycrystalline BiVO₄ films. We find that by tuning the probe coating selective sensitivity to spatial surface or bulk heterogeneity can be achieved. In this context, we study the impact of temperature, environmental atmosphere, excitation energy and power on the local charge carrier transport. This careful analysis allows to identify the local charge transport and loss mechanisms in BiVO₄ which ultimately contribute to desired photocurrent generation or undesired side reactions.