The power conversion effi ciency of organic photovoltaic (OPV) cells has increased from 4-5% in 2005 [ 1 , 2 ] to 7.4% [ 3 ] and 8.3% [ 4 ] in 2010. The goal of a 10% single junction OPV device seems attainable [ 5 ] making the commercialization of OPV more realistic. With advances made on the effi ciency front, the lifetime and reliability of OPV devices has come into focus. [ 6 , 7 ] To date there has been considerable work done in understanding and quantifying the lifetime and degradation of bulk heterojunction solar cells (BHJs) based on poly( para -phenylene vinylene) (PPV) [8][9][10][11] and poly(3-hexylthiophene) (P3HT) polymers. [12][13][14][15] A comparison of OPV lifetime experimental results across different research groups has posed challenges due to the lack of standardized testing and reporting procedures; however, great strides were made in this regard during the most recent International Summit on OPV Stability (ISOS-3). Modules based on P3HT/fullerene BHJs have shown lifetimes of 5000 h when state-of-the-art encapsulation with a glass-on-glass architecture is used. [ 16 ] Assuming negligible degradation in the dark and 5.5 h of one-sun intensity per day, 365 days per year, this translates into an operating lifetime approaching three years. More recently P3HT/PCBM devices utilizing an inverted architecture have been shown to retain more than 50% of their initial effi ciency after 4700 h of continuous exposure to one-sun intensity at elevated temperatures [ 17 ] and have exhibited a long shelf-life when stored in the dark in ambient conditions. [ 18 , 19 ] However, results to date have yet to show polymer based OPV lifetimes greater than 3-4 years.Here we present a detailed operating lifetime study of encapsulated solar cells comprising poly[9'-hepta-decanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole) (PCDTBT) in BHJ composites with the fullerene derivative [6,6]-phenyl C 70 -butyric acid methyl ester (PC 70 BM). PCDTBT/ PC 70 BM solar cells achieved an effi ciency greater than 6%, [ 20 ] making this one of a small number of polymers able to achieve this level of performance. We describe an experimental set-up that is capable of testing large numbers of solar cells simultaneously, holding each device at its maximum power point while controlling and monitoring the temperature and light intensity. Using this set-up we were able to compare the PCDTBT/PC 70 BM system with the well-studied P3HT/PCBM system and demonstrate a lifetime for PCDTBT devices that approaches 7 years, which is the longest reported operating lifetime for a polymer-based solar cell. Figure 1 shows a typical effi ciency decay pattern for polymer/ fullerene BHJs employing a standard architecture with an organic hole-transporting layer as the anode (e.g., PEDOT:PSS) and a metal cathode (e.g., Ca/Al). [ 14 , 15 ] One typically observes a burn-in period characterized by an exponential loss in effi ciency whose magnitude and duration can vary by polymer system, followed by a linear decay period that someti...

Degradation in a high efficiency polymer solar cell is caused by the formation of states in the bandgap. These states increase the energetic disorder in the system. The power conversion efficiency loss does not occur when current is run through the device in the dark but occurs when the active layer is photo-excited.

Understanding the stability and degradation mechanisms of organic solar materials is critically important to achieving long device lifetimes. Here, an investigation of the photodegradation of polymer:fullerene blend films exposed to ambient conditions for a variety of polymer and fullerene derivative combinations is presented. Despite the wide range in polymer stabilities to photodegradation, the rate of irreversible polymer photobleaching in blend films is found to consistently and dramatically increase with decreasing electron affinity of the fullerene derivative. Furthermore, blends containing fullerenes with the smallest electron affinities photobleached at a faster rate than films of the pure polymer. These observations can be explained by a mechanism where both the polymer and fullerene donate photogenerated electrons to diatomic oxygen to form the superoxide radical anion which degrades the polymer.

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Accurately measuring the bulk minority carrier lifetime is one of the greatest challenges in evaluating photoactive materials used in photovoltaic cells. One-photon time-resolved photoluminescence decay measurements are commonly used to measure lifetimes of direct bandgap materials. However, because the incident photons have energies higher than the bandgap of the semiconductor, most carriers are generated close to the surface, where surface defects cause inaccurate lifetime measurements. Here we show that two-photon absorption permits sub-surface optical excitation, which allows us to decouple surface and bulk recombination processes even in unpassivated samples. Thus with two-photon microscopy we probe the bulk minority carrier lifetime of photovoltaic semiconductors. We demonstrate how the traditional one-photon technique can underestimate the bulk lifetime in a CdTe crystal by 10× and show that two-photon excitation more accurately measures the bulk lifetime. Finally, we generate multi-dimensional spatial maps of optoelectronic properties in the bulk of these materials using two-photon excitation.

Using two-photon tomography, carrier lifetimes are mapped in polycrystalline CdTe photovoltaic devices. These 3D maps probe subsurface carrier dynamics that are inaccessible with traditional optical techniques. They reveal that CdCl treatment of CdTe solar cells suppresses nonradiative recombination and enhances carrier lifetimes throughout the film with substantial improvements particularly near subsurface grain boundaries and the critical buried p-n junction.

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The concept of a nanowire solar cell with photon-harvesting shells is presented. In this architecture, organic molecules which absorb strongly in the near infrared where silicon absorbs weakly are coupled to silicon nanowires ͑SiNWs͒. This enables an array of 7-m-long nanowires with a diameter of 50 nm to absorb over 85% of the photons above the bandgap of silicon. The organic molecules are bonded to the surface of the SiNWs forming a thin shell. They absorb the low-energy photons and subsequently transfer the energy to the SiNWs via Förster resonant energy transfer, creating free electrons and holes within the SiNWs. The carriers are then separated at a radial p-n junction in a nanowire and extracted at the respective electrodes. The shortness of the nanowires is expected to lower the dark current due to the decrease in p-n junction surface area, which scales linearly with wire length. The theoretical power conversion efficiency is 15%. To demonstrate this concept, we measure a 60% increase in photocurrent from a planar silicon-on-insulator diode when a 5 nm layer of poly͓2-methoxy-5-͑2Ј-ethyl-hexyloxy͒-1,4-phenylene vinylene is applied to the surface of the silicon. This increase is in excellent agreement with theoretical predictions.