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Spatially resolved 3D carrier-lifetime maps were measured using 2P tomography to uncover carrier dynamics at buried structures in CdTe thin-films photovoltaics. When thin-film CdTe solar cells are initially deposited, we observe increased nonradiative carrier recombination at GBs throughout the 3D volume of the film, which is most pronounced in the critical junction region where small nucleation grains form at the buried CdTe/CdS interface. The data on CdTe films treated by exposure to CdCl2vapor reveal that a critical function of the treatment is to reduce surface, interface and GB recombination throughout the entirety of these polycrystalline films. The CdCl2treatment may also compensate CdTe films, making it difficult to exceed a hole density of 1015m-3and achieve higher current density and efficiency.

The inverted organic photovoltaic (OPV) device architecture represents an important advancement due to the relative environmental stability of the electron transport layer (ETL) and hole-collecting contact. We investigated the initial and long-term behavior of inverted devices to identify changes taking place at the Ag hole-collecting contact. We show that efficient hole collection can be obtained after modifying the Ag contact by thermal annealing, long-term exposure to ambient atmosphere, or employing a high work function organic hole-transport layer (HTL). We find that whether or not the device employs an organic HTL, degradation of the photocurrent initially follows a simple exponential decay. After prolonged illumination (>500 h), devices with an organic HTL fail catastrophically due to a precipitous drop in photocurrent. Based on evidence for pinhole-induced degradation observed in photocurrent maps, we propose a nucleation and island growth mechanism and a model for the photocurrent behavior employing a modified Johnson–Mehl–Avrami–Kolmogorov (JMAK) equation. Devices that do not contain an HTL appear to degrade by a mechanism other than pinhole ingress resulting in a more uniform degradation of the photocurrent across the active area.

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) [ 8–11 ] and poly(3-hexylthiophene) (P3HT) polymers. [ 12–15 ] 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 ]

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.

Tandem solar cells (TSCs), which use two or more materials to absorb sunlight, have achieved power conversion efficiencies of >25% versus 11-20% for commercialized single junction solar cell modules. The key to widespread commercialization of TSCs is to develop the wide-band, top solar cell that is both cheap to fabricate and has a high open-circuit voltage (i.e. >1V). Previous work in TSCs has generally focused on using expensive processing techniques with slow growth rates resulting in costs that are two orders of magnitude too expensive to be used in conventional solar cell modules. The objective of the PLANT PV proposal was to investigate the feasibility of using Ag(In,Ga)Se2 (AIGS) as the wide-bandgap absorber in the top cell of a thin film tandem solar cell (TSC). Despite being studied by very few in the solar community, AIGS solar cells have achieved one of the highest open-circuit voltages within the chalcogenide material family with a Voc of 949mV when grown with an expensive processing technique (i.e. Molecular Beam Epitaxy). PLANT PV's goal in Phase I of the DOE SBIR was to 1) develop the chemistry to grow AIGS thin films via solution processing techniques to reduce costs and 2) fabricate new devicemore » architectures with high open-circuit voltage to produce full tandem solar cells in Phase II. PLANT PV attempted to translate solution processing chemistries that were successful in producing >12% efficient Cu(In,Ga)Se2 solar cells by replacing copper compounds with silver. The main thrust of the research was to determine if it was possible to make high quality AIGS thin films using solution processing and to fully characterize the materials properties. PLANT PV developed several different types of silver compounds in an attempt to fabricate high quality thin films from solution. We found that silver compounds that were similar to the copper based system did not result in high quality thin films. PLANT PV was able to deposit AIGS thin films using a mixture of solution and physical vapor deposition processing, but these films lacked the p-type doping levels that are required to make decent solar cells. Over the course of the project PLANT PV was able to fabricate efficient CIGS solar cells (8.7%) but could not achieve equivalent performance using AIGS. During the nine-month grant PLANT PV set up a variety of thin film characterization tools (e.g. drive-level capacitance profiling) at the Molecular Foundry, a Department of Energy User Facility, that are now available to both industrial and academic researchers via the grant process. PLANT PV was also able to develop the back end processing of thin film solar cells at Lawrence Berkeley National Labs to achieve 8.7% efficient CIGS solar cells. This processing development will be applied to other types of thin film PV cells at the Lawrence Berkeley National Labs. While PLANT PV was able to study AIGS film growth and optoelectronic properties we concluded that AIGS produced using these methods would have a limited efficiency and would not be commercially feasible. PLANT PV did not apply for the Phase II of this grant.« less

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.

Bulk heterojunction solar cells (BHJs) based on poly[N-9″-hepta-decanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT) can have internal quantum efficiencies approaching 100% but require active layers that are too thin to absorb more than ∼70% of the above bandgap light.[1] In this work, we demonstrate that PCDTBT films contain a high density of hole-traps, which correlate with polymer morphology and are important for BHJ performance.

Silicon nanowire Schottky junction solar cells have been fabricated using n-type silicon nanowire arrays and a spin-coated conductive polymer (PEDOT). The polymer Schottky junction cells show superior surface passivation and open-circuit voltages compared to standard diffused junction cells with native oxide surfaces. External quantum efficiencies up to 88% were measured for these silicon nanowire/PEDOT solar cells further demonstrating excellent surface passivation. This process avoids high temperature processes which allows for low-cost substrates to be used.

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 (SiNWs). This enables an array of 7-μm-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 Forster 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 photocurr...