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In this review, we discuss the physics underlying the operation of single and multiple heterojunction, vacuum-deposited organic solar cells based on small molecular weight thin films. For single heterojunction cells, we find that the need for direct contact between the deposited electrode and the active organics leads to quenching of excitons. An improved device architecture, the double heterojunction, is shown to confine excitons within the active layers, allowing substantially higher internal efficiencies to be achieved. A full optical and electrical analysis of the double heterostructure architecture leads to optimal cell design as a function of the optical properties and exciton diffusion lengths of the photoactive materials. Combining the double heterostructure with novel light trapping schemes, devices with external efficiencies approaching their internal efficiency are obtained. When applied to an organic photovoltaic cell with a power conversion efficiency of 1.0%±0.1% under 1 sun AM1.5 illumination, devices with external power conversion efficiencies of 2.4%±0.3% are reported. In addition, we show that by using materials with extended exciton diffusion lengths LD, highly efficient double heterojunction photovoltaic cells are obtained, even in the absence of a light trapping geometry. Using C60 as an acceptor material, double heterostructure external power conversion efficiencies of 3.6%±0.4% under 1 sun AM1.5 illumination are obtained. Stacking of single heterojunction devices leads to thin film multiple heterojunction photovoltaic and photodetector structures. Thin bilayer photovoltaic cells can be stacked with ultrathin (∼5 Å), discontinuous Ag layers between adjacent cells serving as efficient recombination sites for electrons and holes generated in the neighboring cells. Such stacked cells have open circuit voltages that are n times the open circuit voltage of a single cell, where n is the number of cells in the stack. In optimized structures, the short circuit photocurrent remains approximately constant upon stacking thin cells, leading to higher achievable power conversion efficiencies, as confirmed by modelling optical interference effects and exciton migration. A 2.5%±0.3% power efficiency under 100 mW/cm2 AM1.5 illumination conditions is obtained by stacking two ∼1% efficient devices. Alternatively, when the contact layers between the stacked cells are eliminated, a multilayer structure consisting of alternating films of donor and acceptor-type materials is obtained. Since the thicknesses of the individual layers (∼5 Å) can be substantially smaller than the exciton diffusion length, nearly 100% of the photogenerated excitons are dissociated, and the resulting free charges are detected. In addition, the ultrathin organic layers facilitate electron and hole transport through the multilayer stack by tunneling. When these devices are operated as photodetectors under applied fields >106 V/cm, the carrier collection efficiency reaches 80%, leading to external quantum efficiencies of 75%±1% across the visible spectrum in cells containing the thinnest layers. We find that due to the fast carrier tunneling process, the temporal response of these multilayer detectors is a direct measure of exciton dynamics. Response times of 720±50 ps are achieved, leading to a 3 dB bandwidth of 430±30 MHz. A summary of representative results obtained for both polymer and small molecule photovoltaic cells and photodetectors is included in this review. Prospects for further improvements in organic solar cells and photodetectors are considered.

We demonstrate high open circuit voltage (Voc) organic photovoltaic (PV) cells that incorporate two, three, or five stacked, thin heterojunctions (HJs) consisting of Cu-phthalocyanine as a donor, and 3,4,9,10 perylenetetracarboxylic bis-benzimidazole as an acceptor. Ultrathin (∼5 Å average thickness) layers of Ag clusters are placed between each HJ to serve as recombination centers for unpaired charges that are photogenerated in the device’s interior. High power conversion efficiencies with high Voc have been demonstrated in cells containing up to five optically active HJs and four interstitial metallic recombination regions. The power conversion efficiencies of the two and three HJ cells under one sun, AM 1.5 illumination are ηp=2.5±0.1% and ηp 2.3±0.1%, with Voc=0.93±0.05 and 1.20±0.05 V, respectively. These values of ηp are more than twice that of a comparable single-junction cell based on the same materials, where ηp=1.1±0.1%.

Space charge effects in polymer-based light-emitting diodes studied by means of a polarization sensitive electroreflectance technique

Conjugated polymers such as poly(phenylenevinylene) are an attractive target for electroluminescent (EL) devices because of their ease of preparation, processability, and color tunability. The authors review theoretical and experimental progress in polymer EL devices with special emphasis on device degradation under strong pulses. The Figure shows the morphology of a degraded Al cathode.

Polycrystalline CdTe thin films for CdTe solar cell devices are typically grown using physical vapor techniques such as close‐spaced sublimation or vapor transport deposition with, in some cases, a small amount of oxygen in the process gas. In this report, we discuss the results of growing CdTe films in an environment of pure oxygen using close‐space sublimation. The results show that oxygen has a positive impact on CdTe film quality as it increases charge carrier mobility and lifetime in the absorber layer of CdTe‐based devices. This has resulted in a 10–20 mV improvement in open‐circuit voltages when growing the CdTe in pure oxygen. Copyright © 2014 John Wiley & Sons, Ltd.

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