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Abstract: We have developed semi-transparent n–i–p organic solar cells (OSC) with free-standing multi-wall carbon nanotube (f-CNT) sheets as transparent top electrodes. By a simple and damage-free room temperature orthogonal liquid solution assisted self-laminating process, f-CNT top electrodes are successfully deposited on top of ZnPc:C60 bulk heterojunction small molecule OSCs. The cells show high fill factors above 58% as well as efficiencies up to 1.5% and greater long-term stability compared to the device having a metal electrode. For the given cell structure with f-CNT semi-transparent electrodes, the influence of an optical spacer on light absorption is studied by a systematic variation of the hole transport layer thickness, supported by optical simulations. The results strongly indicate that OSCs with f-CNT top electrodes and optimized thin film stack are highly promising for semi-transparent OSCs, which can be used in tandem devices, in tinted smart windows, and similar applications by a simple and damage-free process in roll-to-roll configuration that can be scaled to large area manufacturing. [Copyright &y& Elsevier]

Recently, organic solar cells have demonstrated high power conversion efficiencies, but extrinsic device stability remains to be a challenge. Often, delamination of the cathode upon water ingress leads to a massive active area loss and to a strong decrease in efficiency. We present highly efficient organic solar cells with an oligothiophene-based donor and two different cathodes. Devices are aged at four different climates under continuous illumination. Al and MoO 3 /Cr/Al cathodes are compared. Additional interlayers are introduced for enhanced cathode adhesion. Aging in different climates reveals a strong dependency of the degradation speed on absolute humidity for standard Al cathodes. However, using additional interlayers, degradation seems to be almost decoupled from the applied climate and device degradation is slowed immensely. Parallel WVTR measurements of the AlO x barrier allow for the assessment of absolute water quantities until devices are degraded to half of their initial efficiency: 24 g m 2 are found for standard Al cathodes, while devices with additional interlayers reach 58 g m 2 . This difference is related to a completely different aging behavior, which trades strong short-circuit current density losses for moderate fill-factor losses. [ABSTRACT FROM AUTHOR]

Abstract: We report on transparent stand alone C60 top electrodes for small molecule based organic photovoltaic cells. All device layers (including bottom and top electrode) are thermally evaporated in vacuum, guaranteeing fast, reliable, high-purity and low-cost processing. Employing an organic compound as electrode avoids refractive index mismatch between contact and photo-active layer which results in an increased number of photons reaching the absorber layers. The conductivity of C60 is improved from 10−8 S/cm to remarkable 4.97S/cm by optimized molecular n-type doping. Furthermore, it is shown that high doping concentrations change the absorption behavior of C60, leading to transmittance values over 70% for wavelengths larger than 600nm with a weak dependence on layer thickness. Organic solar cells employing such an optimized n-doped C60 top electrode (thickness 200nm, doping concentration 16wt%) exhibit power conversion efficiencies of 1.53%. Electrical device simulations based on a discretized equivalent circuit model reveal an efficient lateral charge transport in n-doped C60 electrodes over a few 100µm. In combination with a supporting grid electrode (usually employed for large area photovoltaic applications), n-doped C60 might replace ITO and fill the lack of transparent conductive top electrodes required for roll-to-roll processed semi-transparent organic devices or devices on opaque substrates. [Copyright &y& Elsevier]

Abstract: In this work, we use different encapsulations to protect vacuum-evaporated small molecule organic solar cells with a simple p-i-i-stack for lifetime studies. Our devices use ZnPc and C60 as active materials. Lifetimes (T50) in a range from 300h for un-encapsulated devices to 4000h for glass-encapsulated have been observed. We use a model to distinguish between the water vapor transmission rate (WVTR) of the barrier and an additional WVTR of the aluminum top electrode. For all observed devices a loss of 50% of initial efficiency is observed when 10mgm−2 water entered the device. The losses are related to a reduction of short circuit current density only, whereas open circuit voltage and fill factor remains unaffected. We relate this to an interaction of the water molecules with C60. [Copyright &y& Elsevier]