Origin of low sensitizing efficiency of quantum dots in organic solar cells

Subpicosecond Photon-Energy-Dependent Hole Transfer from PbS Quantum Dots to Conjugated Polymers

We use transient absorption (TA) spectroscopy to study the origin of photon-energy dependent hole transfer yields in blends of PbS quantum dots with the conjugated polymer poly(3-hexylthiophene-2,5-diyl) (P3HT). We selectively excite only the quantum dots at two different wavelengths and measure the polymer ground state bleach resulting from the transfer of photoexcited holes. The higher photon-energy pump shows a greater prompt yield of hole transfer compared to the lower photon-energy excitation, on time scales sufficient to out-compete hot carrier cooling in lead chalcogenide quantum dots. We interpret the results as evidence that the excess energy of nonthermalized, or "hot," excitons resulting from higher photon-energy excitation allows more efficient charge transfer to the polymer in these systems. The data also demonstrate slow charge transfer rates, up to ∼1 ns, of the relaxed excitations on the PbS dots. These findings help to clarify the role of excess photon energy and carrier relaxation dynamics on free carrier generation in donor/acceptor solar cells.

Dielectric Effects at Organic/Inorganic Interfaces in Nanostructured Devices

Dielectric interfaces are important in organic electronic devices, as they dominate charge generation and recombination dynamics and set the tone for efficiency of the device. In a charge separation scenario across the interface, we calculate the binding energy of a charge carrier for variations in dielectric mismatch (i.e., the ratio of the dielectric constant of materials forming the interface), interface shape and size, and dielectric anisotropy. We find that dielectric mismatch results in binding of the charge carrier to the interface with energies on the order of several kT. For the variation in interface shape and size, epitomized by the device morphology, we show that the assumption of a planar interface overestimates the attractive potential. The change in the interface curvature affects the binding energy of the charge carrier by order of kT. Anisotropy is shown to affect critically the electric field along the principal axis, while the binding energy of the charge is altered by more than 5 kT. We are able to give an upper limit on the change in the binding energy for the variations in the above interfacial factors. These limits can serve as guidelines for optimization, interface engineering, and design of high efficiency organic electronic devices.

Generating free charges by carrier multiplication in quantum dots for highly efficient photovoltaics

CONSPECTUS: In a conventional photovoltaic device (solar cell or photodiode) photons are absorbed in a bulk semiconductor layer, leading to excitation of an electron from a valence band to a conduction band. Directly after photoexcitation, the hole in the valence band and the electron in the conduction band have excess energy given by the difference between the photon energy and the semiconductor band gap. In a bulk semiconductor, the initially hot charges rapidly lose their excess energy as heat. This heat loss is the main reason that the theoretical efficiency of a conventional solar cell is limited to the Shockley-Queisser limit of ∼33%. The efficiency of a photovoltaic device can be increased if the excess energy is utilized to excite additional electrons across the band gap. A sufficiently hot charge can produce an electron-hole pair by Coulomb scattering on a valence electron. This process of carrier multiplication (CM) leads to formation of two or more electron-hole pairs for the absorption of one photon. In bulk semiconductors such as silicon, the energetic threshold for CM is too high to be of practical use. However, CM in nanometer sized semiconductor quantum dots (QDs) offers prospects for exploitation in photovoltaics. CM leads to formation of two or more electron-hole pairs that are initially in close proximity. For photovoltaic applications, these charges must escape from recombination. This Account outlines our recent progress in the generation of free mobile charges that result from CM in QDs. Studies of charge carrier photogeneration and mobility were carried out using (ultrafast) time-resolved laser techniques with optical or ac conductivity detection. We found that charges can be extracted from photoexcited PbS QDs by bringing them into contact with organic electron and hole accepting materials. However, charge localization on the QD produces a strong Coulomb attraction to its counter charge in the organic material. This limits the production of free charges that can contribute to the photocurrent in a device. We show that free mobile charges can be efficiently produced via CM in solids of strongly coupled PbSe QDs. Strong electronic coupling between the QDs resulted in a charge carrier mobility of the order of 1 cm(2) V(-1) s(-1). This mobility is sufficiently high so that virtually all electron-hole pairs escape from recombination. The impact of temperature on the CM efficiency in PbSe QD solids was also studied. We inferred that temperature has no observable effect on the rate of cooling of hot charges nor on the CM rate. We conclude that exploitation of CM requires that charges have sufficiently high mobility to escape from recombination. The contribution of CM to the efficiency of photovoltaic devices can be further enhanced by an increase of the CM efficiency above the energetic threshold of twice the band gap. For large-scale applications in photovoltaic devices, it is important to develop abundant and nontoxic materials that exhibit efficient CM.

Tandem and triple-junction polymer:nanocrystal hybrid solar cells consisting of identical subcells

Tandem and triple-junction polymer:nanocrystal hybrid solar cells with identical subcells based on P3HT:CdSe nanocrystal bulk heterojunctions (BHJs) are reported for the first time showing 2-fold and 3-fold increases of open-circuit voltage (VOC), respectively, relative to the single-junction cell. A combination of nanocrystalline ZnO and pH-neutral PEDOT:PSS is used as the interconnecting layer, and the thicknesses of subcells are optimized with the guidance of optical simulations. As a result, the average power conversion efficiency (PCE) exhibits a significant increase from 2.0% (VOC = 0.57 V) in single-junction devices to 2.7% (champion 3.1%, VOC = 1.28 V) in tandem devices and 2.3% (VOC = 1.98 V) in triple-junction devices.

A comprehensive review of the application of chalcogenide nanoparticles in polymer solar cells

In this review the use of solution-processed chalcogenide quantum dots (CdS, CdSe, PbS, etc.) in hybrid organic-inorganic solar cells is explored. Such devices are known as potential candidates for low-cost and efficient solar energy conversion, and compose the so-called third generation solar cells. The incorporation of oxides and metal nanoparticles has also been successfully achieved in this new class of photovoltaic devices; however, we choose to explore here chalcogenide quantum dots in light of their particularly attractive optical and electronic properties. We address herein a comprehensive review of the historical background and state-of-the-art comprising the incorporation of such nanoparticles in polymer matrices. Later strategies for surface chemistry manipulation, in situ synthesis of nanoparticles, use of continuous 3D nanoparticles network (aerogels) and ternary systems are also reviewed.

Hot Hole Transfer Increasing Polaron Yields in Hybrid Conjugated Polymer/PbS Blends

We use quasi-steady-state photoinduced absorption (PIA) to study charge generation in blends of poly(3-hexylthiophene-2,5-diyl) (P3HT) with PbS nanocrystal quantum dots as a function of excitation energy. We find that, per photon absorbed, the yield of photogenerated holes present on the conjugated polymer increases with pump energy, even at wavelengths where only the quantum dots absorb. We interpret this result as direct evidence for transfer of hot holes in these conjugated polymer/quantum dot blends. These results help understand the operation of hybrid organic/inorganic photovoltaics.

Probing surface states in PbS nanocrystal films using pentacene field effect transistors: controlling carrier concentration and charge transport in pentacene

We used a bilayer field effect transistor (FET) consisting of a thin PbS nanocrystals (NCs) film interfaced with vacuum-deposited pentacene to probe trap states in NCs.

Charge Generation Dynamics in CdS:P3HT Blends for Hybrid Solar Cells

Development of design rules for hybrid inorganic-organic solar cells through understanding charge generation and recombination dynamics is an important pathway for the improvement of solar cell conversion efficiencies. In this Letter, we study the dynamics of charge generation in CdS:polymer blends by transient absorption spectroscopy. We show that charge generation following excitation of the inorganic component is highly efficient and can occur up to a few nanoseconds after excitation, allowing for diffusion of charges within the inorganic component to an interface. In contrast, charge generation following excitation of the organic component occurs on subpicosecond time scales but suffers from two loss processes, incomplete exciton dissociation and geminate recombination.

Charge trapping dynamics in PbS colloidal quantum dot photovoltaic devices

The efficiency of solution-processed colloidal quantum dot (QD) based solar cells is limited by poor charge transport in the active layer of the device, which originates from multiple trapping sites provided by QD surface defects. We apply a recently developed ultrafast electro-optical technique, pump-push photocurrent spectroscopy, to elucidate the charge trapping dynamics in PbS colloidal-QD photovoltaic devices at working conditions. We show that IR photoinduced absorption of QD in the 0.2-0.5 eV region is partly associated with immobile charges, which can be optically detrapped in our experiment. Using this absorption as a probe, we observe that the early trapping dynamics strongly depend on the nature of the ligands used for QD passivation, while it depends only slightly on the nature of the electron-accepting layer. We find that weakly bound states, with a photon-activation energy of 0.2 eV, are populated instantaneously upon photoexcitation. This indicates that the photogenerated states show an intrinsically bound-state character, arguably similar to charge-transfer states formation in organic photovoltaic materials. Sequential population of deeper traps (activation energy 0.3-0.5 eV) is observed on the ~0.1-10 ns time scales, indicating that most of carrier trapping occurs only after substantial charge relaxation/transport. The reported study disentangles fundamentally different contributions to charge trapping dynamics in the nanocrystal-based optoelectronic devices and can serve as a useful tool for QD solar cell development.

Two similar near-infrared (IR) absorbing benzannulated aza-BODIPY dyes as near-IR sensitizers for ternary solar cells

Ternary composite inverted organic solar cells based on poly(3-hexylthiophen-2,5-diyl) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) blended with two different near-infrared absorbing benzannulated aza-BODIPY dyes, difluoro-bora-bis-(1-phenyl-indoyl)-azamethine (1) or difluoro-bora-bis-(1-(5-methylthiophen)-indoyl)-azamethine (2), were constructed and characterized. The amount of these two aza-BODIPY dyes, within the P3HT and PCBM matrix, was systematically varied, and the characteristics of the respective devices were recorded. Although the addition of both aza-BODIPY dyes enhanced the absorption of the blends, only the addition of 1 improved the overall power conversion efficiency (PCE) in the near-infrared (IR) region. The present work paves the way for the integration of near-infrared absorbing aza-BODIPY derivatives as sensitizers in ternary composite solar cells.