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Perovskite solar cells have recently seen rapid improvements in performance with certified efficiencies of above 23%. Fullerene compounds are a very popular electron-transfer material in these devices. In a previous report, it has been shown that while an ultrathin fullerene layer of just 1 nm is sufficient to achieve good device performance, removal of this layer causes a drastic decrease in performance. We provide an explanation to these observed effects by use of a numerical device model. This work provides theoretical support to the experimental understanding of the dominant role of fullerenes in perovskite solar cells.

In the United States, approximately 65 × 109 square-feet of underutilized rooftop space could be appropriated for the installation of solar photovoltaic systems. Although several programs are available that assist in designing a photovoltaic (PV) system, a planning tool that can take an adaptive modeling approach towards capitalizing on the complex geometry of a rooftop does not exist. We therefore focus on the development of a parametric planning tool for retrofitting rooftops with solar photovoltaic systems. The solar planning tool exploits the existing blueprint of a building's rooftop, and via image processing, the layouts of the solar photovoltaic arrays are developed based on the building's geographical location and typical weather patterns. The resulting energy generation of a PV system is estimated and is utilized to determine the leveled energy costs. The advantage of incorporating image processing in the design of a PV system not only reduces the time required for performing a robust solar PV analysis but also enables a high-level of dimensional precision when modeling the solar arrays, thus making a rooftop solar photovoltaic system installation ultimately more cost-effective. This paper demonstrates the planning tool and verifies the output array layout, sizing of the balance of system components, and expected energy generation with the existing rooftop photovoltaic systems.

We demonstrate efficient zinc sulfide cathode window layers in thin-film organic photovoltaics enabled by n-type doping zinc sulfide (ZnS) with aluminum sulfide (Al2S3) directly through co-deposition. By optimizing the Al2S3 concentration, the power conversion efficiency is improved from 0.6% ± 0.2% in undoped ZnS window layer devices to 1.8% ± 0.1%, identical to control devices. The mechanism for this performance enhancement is shown to stem from the enhanced conductivity and interface energetics of ZnS upon n-type doping. This work expands the catalog of efficient, inorganic, non-toxic, cathode side window layers that could be effective in a range of thin-film photovoltaic technologies.

Integration of transparent photovoltaics into the building envelope creates unique opportunities to reduce the levelized electricity cost of solar power. However, this integration warrants consideration of the angular dependence of these devices as illumination around the building envelope is rarely at normal incidence. Here we correctly update transfer-matrix and equations to accurately model the quantum efficiency and optical properties under oblique illumination. We use this model to demonstrate the various angular performance characteristics possible for proof-of-concept optimizations of transparent planar-heterojunction solar cells and discuss considerations needed to fully account for optical, electrical, and positional configurations in this optimization.

Massachusetts Institute of Technology. Institute for Soldier Nanotechnologies (Contract W911NF-07-D-0004)

Transparent photovoltaics (PVs) provide a potentially facile route to building-integrated PVs and seamless energy-harvesting within non-window surfaces such as electronic displays, autonomously powered electronic-glazings, and mobile-electronic accessories. Such devices have been enabled by manipulation of excitons in organic and molecular semiconductors that allow for selective ultraviolet and near-infrared solar conversion. Here, the theoretical efficiency limits of transparent photovoltaics are determined as a function of transparency. Power-production from ultraviolet and near-infrared photons alone leads to a theoretical single-junction efficiency of 21% in transparent structures, compared to 33% for opaque-junctions. Reducing thermal losses via transparent multi-junction stacking these limits increase to 37%.

We fabricate near-infrared absorbing organic photovoltaics that are highly transparent to visible light. By optimizing near-infrared optical-interference, we demonstrate power efficiencies of 1.3±0.1% with simultaneous average visible transmission of >65%. Subsequent incorporation of near-infrared distributed-Bragg-reflector mirrors leads to an increase in the efficiency to 1.7±0.1%, approaching the 2.4±0.2% efficiency of the opaque cell, while maintaining high visible-transparency of >55%. Finally, we demonstrate that a series-integrated array of these transparent cells is capable of powering electronic devices under near-ambient lighting. This architecture suggests strategies for high-efficiency power-generating windows and highlights an application uniquely benefiting from excitonic electronics.

We demonstrate that material utilization efficiencies of 50% and deposition nonuniformities ≤2.5% are achievable over substrate diameters of 200 mm using a simplified, organic vapor phase deposition (OVPD) system. The OVPD system is used to demonstrate doped electrophosphorescent organic light emitting diodes whose performance is comparable to those grown by vacuum thermal evaporation. Through continuum modeling, we demonstrate that analogous systems whose chamber dimensions are comparable to the substrate width are scalable to substrate sizes of at least 1500×1800 mm2 with deposition nonuniformities between 1.5% and 2.5%. These results indicate that OVPD is useful in the large area deposition of displays, lighting, and other organic electronic devices.

We demonstrate spectrally resolved photoluminescence quenching as a means to determine the exciton diffusion length of several archetype organic semiconductors used in thin film devices. We show that aggregation and crystal orientation influence the anisotropy of the diffusion length for vacuum-deposited polycrystalline films. The measurement of the singlet diffusion lengths is found to be in agreement with diffusion by Forster transfer, whereas triplet diffusion occurs primarily via Dexter transfer.

We demonstrate high open circuit voltage photovoltaic cells achieved by reducing the electron leakage current through the introduction of both organic and inorganic electron blocking layers between the donor layer and the anode contact. As an example, the blocking layers reduce the dark current in tin (II) phthalocyanine (SnPc)∕C60 solar cells with response across the visible and near infrared spectral region up to a wavelength of 1000nm, is decreased by two orders of magnitude compared to cells lacking the layers, resulting in a doubling of the open circuit voltage. The structure: indium tin oxide/electron blocker/SnPc (100A)∕C60 (400A)/bathocuproine (100A)∕Al, has a power conversion efficiency of (2.1±0.1)% at 1sun, standard AM1.5G solar illumination. This work demonstrates the importance of reducing dark current to achieve high organic thin film photovoltaic cell efficiencies.

We demonstrate organic solar cells incorporating two simultaneously active donor-acceptor heterojunctions contained within a single film with sensitivity extending across the visible solar spectrum into the near infrared (NIR). The NIR absorbing material, tin(II)-phthalocyanine (SnPc), is grown into nanocrystalline islands using organic vapor phase deposition. Using an optimized copper phthalocyanine (CuPc)∕SnPc∕C60 nanocrystalline network, we obtain a power conversion efficiency of (2.9±0.2)% under 100mW∕cm2, AM1.5G illumination. We find that the open circuit voltage of the simultaneous heterojunction is determined by the lowest voltage of the separate donor-acceptor constituent junctions.

Organic-vapor jet printing (OVJP) is used to directly pattern small molecular weight electrophosphorescent white organic light-emitting devices (WOLEDs) consisting of parallel stripes of separate red, green, and blue OLEDs. The spatial patterning of the WOLEDs with peak forward viewing external quantum and luminous efficiencies of 7.1±0.3% and 7.0±0.4lm∕W, respectively, is accomplished by transporting the vapors of organic semiconductors through a nozzle to a translating sample substrate. Using multiple color segments to generate white light allows for the individual optimization of each color, is continuously color tunable, and can be used to compensate for differential aging of the constituent dopants. The theory and experimental results of WOLED printing via OVJP are discussed.

The crystalline thin film growth of the organic material, copper phthalocyanine (CuPc), by organic vapor-phase deposition (OVPD) is studied using high-pressure reflection high-energy electron diffraction (HP-RHEED). In situ growth of this material was monitored, in real time, on both highly oriented pyrolytic graphite and native SiO2 on Si(100) substrates. The growth of the first several monolayers on both substrates was found to be independent of the growth conditions; however, the crystalline texture of thicker films was controlled through changes in the substrate temperature and deposition rate. Higher substrate temperatures lead to an increase in crystalline ordering for growth on both substrates. This work shows that HP-RHEED is a powerful tool for real-time monitoring of growth morphology in the low-vacuum OVPD environment, ultimately leading to in situ control of thin film crystalline order.