In Situ Optical Measurement of Charge Transport Dynamics in Organic Photovoltaics

We present a novel experimental approach which allows extraction of both spatial and temporal information on charge dynamics in organic solar cells. Using the wavelength dependence of the photonic structure in these devices, we monitor the change in spatial overlap between the photogenerated hole distribution and the optical probe profile as a function of time. In a model system we find evidence for a buildup of the photogenerated hole population close to the hole-extracting electrode on a nanosecond time scale and show that this can limit charge transport through space-charge effects under operating conditions

Spatially Resolved Optical Efficiency Measurements of Luminescent Solar Concentrators

Luminescent solar concentrators (LSCs) are able to concentrate both direct and diffuse solar radiation, and this ability has led to great interest in using them to improve solar energy capture when coupled to traditional photovoltaics (PV). In principle, a large-area LSC could concentrate light onto a much smaller area of PV, thus reducing costs or enabling new architectures. However, LSCs suffer from various optical losses which are hard to quantify using simple measurements of power conversion efficiency. Here, we show that spatially resolved photoluminescence quantum efficiency measurements on large-area LSCs can be used to resolve various loss processes such as out-coupling, self-absorption via emitters, and self-absorption from the LSC matrix. Further, these measurements allow for the extrapolation of device performance to arbitrarily large LSCs. Our results provide insight into the optimization of optical properties and guide the design of future LSCs for improved solar energy capture

In Situ Optical Measurement of Charge Transport Dynamics in Organic Photovoltaics

We present a novel experimental approach which allows extraction of both spatial and temporal information on charge dynamics in organic solar cells. Using the wavelength dependence of the photonic structure in these devices, we monitor the change in spatial overlap between the photogenerated hole distribution and the optical probe profile as a function of time. In a model system we find evidence for a buildup of the photogenerated hole population close to the hole-extracting electrode on a nanosecond time scale and show that this can limit charge transport through space-charge effects under operating conditions

Charge Dynamics in Solution-Processed Nanocrystalline CuInS₂ Solar Cells

We investigate charge dynamics in solar cells constructed using solution-processed layers of CuInS₂ (CIS) nanocrystals (NCs) as the electron donor and CdS as the electron acceptor. By using time-resolved spectroscopic techniques, we are able to observe photoinduced absorptions that we attribute to the mobile hole carriers in the NC film. In combination with transient photocurrent and photovoltage measurements, we monitor charge dynamics on time scales from 300 fs to 1 ms. Carrier dynamics are investigated for devices with CIS layers composed of either colloidally synthesized 1,3-benzenedithiol-capped nanocrystals or <i>in situ</i> sol–gel synthesized thin films as the active layer. We find that deep trapping of holes in the colloidal NC cells is responsible for decreases in the open-circuit voltage and fill factor as compared to those of the sol–gel synthesized CIS/CdS cell

Synthesis, Purification, and Characterization of Well-Defined All-Conjugated Diblock Copolymers PF8TBT-<i>b</i>-P3HT

We present the synthesis, purification, and characterization of all-conjugated block copolymers comprising poly­((9,9-dioctylfluorene)-2,7-diyl-<i>alt</i>-[4,7-bis­(3-hexylthien-5-yl)-2,1,3-benzothiadiazole]-2′,2″-diyl) (PF8TBT) and poly­(3-hexylthiophene) (P3HT). Suzuki step-growth polycondensation is used for the synthesis of PF8TBT, which is subsequently terminated via the addition of narrow-distributed, monobrominated P3HT-Br. Purification via preparative GPC is carried out to reduce polydispersity and to remove excess P3HT. Wavelength-dependent GPC and careful NMR end group analysis, assisted by model compounds, reveal pure diblock copolymers of PF8TBT-<i>b</i>-P3HT. Insight into structure formation is given by temperature-dependent UV–vis absorption, DSC, and X-ray scattering. These indicate that PF8TBT-<i>b</i>-P3HT does not microphase-separate within the investigated range of composition and molecular weight. The critical role of introducing sufficient dissimilarity between the segments in all-conjugated block copolymers in order to induce phase separation is discussed, with the conclusion that careful tuning of side chains is crucial for achieving self-organization

Ultrafast Charge- and Energy-Transfer Dynamics in Conjugated Polymer: Cadmium Selenide Nanocrystal Blends

Hybrid nanocrystal–polymer systems are promising candidates for photovoltaic applications, but the processes controlling charge generation are poorly understood. Here, we disentangle the energy- and charge-transfer processes occurring in a model system based on blends of cadmium selenide nanocrystals (CdSe-NC) with poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylene vinylene] (MDMO-PPV) using a combination of time-resolved absorption and luminescence measurements. The use of different capping ligands (<i>n</i>-butylamine, oleic acid) as well as thermal annealing allows tuning of the polymer–nanocrystal interaction. We demonstrate that energy transfer from MDMO-PPV to CdSe-NCs is the dominant exciton quenching mechanism in nonannealed blends and occurs on ultrafast time scales (<1 ps). Upon thermal annealing electron transfer becomes competitive with energy transfer, with a transfer rate of 800 fs independent of the choice of the ligand. Interestingly, we find hole transfer to be much less efficient than electron transfer and to extend over several nanoseconds. Our results emphasize the importance of tuning the organic–nanocrystal interaction to achieve efficient charge separation and highlight the unfavorable hole-transfer dynamics in these blends

Device Performance of Small-Molecule Azomethine-Based Bulk Heterojunction Solar Cells

Recently, we demonstrated that small-molecule azomethines are promising candidates as electron-donating materials for organic photovoltaic (OPV) devices. Azomethines can be prepared via well-known condensation chemistry, with water being the sole byproduct. Here we present a record power conversion efficiency for azomethine-based small-molecule OPV devices of 2.2%. To understand the underlying physics limiting device performance, devices of the small-molecule azomethine TPA–TBT–TPA were characterized using a range of spectroscopic and electro-optical techniques. Light-intensity-dependent current-density measurement showed the presence of nongeminate charge recombination, which is most likely the result of poor charge mobility. In addition, transient absorption measurements showed a relatively short lifetime for the exciton (120 ps). However, due to the very fast charge dissociation (<300 fs), charge separation is relatively efficient. This knowledge presents a guideline for preparing subsequent generations of compounds with improved device performance

Solution-Processable Singlet Fission Photovoltaic Devices

We demonstrate the successful incorporation of a solution-processable singlet fission material, 6,13-bis­(triisopropylsilylethynyl)­pentacene (TIPS-pentacene), into photovoltaic devices. TIPS-pentacene rapidly converts high-energy singlet excitons into pairs of triplet excitons via singlet fission, potentially doubling the photocurrent from high-energy photons. Low-energy photons are captured by small-bandgap electron-accepting lead chalcogenide nanocrystals. This is the first solution-processable singlet fission system that performs with substantial efficiency with maximum power conversion efficiencies exceeding 4.8%, and external quantum efficiencies of up to 60% in the TIPS-pentacene absorption range. With PbSe nanocrystal of suitable bandgap, its internal quantum efficiency reaches 170 ± 30%

Does Heat Play a Role in the Observed Behavior of Aqueous Photobatteries?

Light-rechargeable photobatteries have emerged as an elegant solution to address the intermittency of solar irradiation by harvesting and storing solar energy directly through a battery electrode. Recently, a number of compact two-electrode photobatteries have been proposed, showing increases in capacity and open-circuit voltage upon illumination. Here, we analyze the thermal contributions to this increase in capacity under galvanostatic and photocharging conditions in two promising photoactive cathode materials, V2O5 and LiMn2O4. We propose an improved cell and experimental design and perform temperature-controlled photoelectrochemical measurements using these materials as photocathodes. We show that the photoenhanced capacities of these materials under 1 sun irradiation can be attributed mostly to thermal effects. Using operando reflection spectroscopy, we show that the spectral behavior of the photocathode changes as a function of the state of charge, resulting in changing optical absorption properties. Through this technique, we show that the band gap of V2O5 vanishes after continued zinc ion intercalation, making it unsuitable as a photocathode beyond a certain discharge voltage. These results and experimental techniques will enable the rational selection and testing of materials for next-generation photo-rechargeable systems

Does Heat Play a Role in the Observed Behavior of Aqueous Photobatteries?

Light-rechargeable photobatteries have emerged as an elegant solution to address the intermittency of solar irradiation by harvesting and storing solar energy directly through a battery electrode. Recently, a number of compact two-electrode photobatteries have been proposed, showing increases in capacity and open-circuit voltage upon illumination. Here, we analyze the thermal contributions to this increase in capacity under galvanostatic and photocharging conditions in two promising photoactive cathode materials, V2O5 and LiMn2O4. We propose an improved cell and experimental design and perform temperature-controlled photoelectrochemical measurements using these materials as photocathodes. We show that the photoenhanced capacities of these materials under 1 sun irradiation can be attributed mostly to thermal effects. Using operando reflection spectroscopy, we show that the spectral behavior of the photocathode changes as a function of the state of charge, resulting in changing optical absorption properties. Through this technique, we show that the band gap of V2O5 vanishes after continued zinc ion intercalation, making it unsuitable as a photocathode beyond a certain discharge voltage. These results and experimental techniques will enable the rational selection and testing of materials for next-generation photo-rechargeable systems