Researchers at Lawrence Berkeley National Laboratory have developed air-stable all-inorganic nanocrystal solar cells processed from solution. These ultrathin, solution-processed solar cells are composed entirely of inorganic nanocrystals and demonstrate power conversion efficiencies approaching 3% in initial tests. The devices are stable in air and offer a new class of photovoltaic devices with potential for stable, low-cost power generation. The solar cells utilize rod-shaped cadmium selenide (CdSe) and cadmium telluride (CdTe) nanocrystals, synthesized and prepared separately. A schematic energy diagram illustrates the staggered band alignment of this prototypical donor-acceptor pair. The nanocrystals were spin-cast from a filtered pyridine solution, allowing for the creation of ultra-thin, flexible films of densely packed nanocrystals on virtually any substrate. The devices were fabricated by sequentially spin-casting films of CdTe followed by CdSe on indium tin oxide (ITO) glass. Thermally deposited aluminum was used as a reflective top contact. The photoaction spectrum of a typical bilayer cell reveals features from both the CdSe and CdTe absorption spectra, demonstrating that both components contribute to the photocurrent. Current-voltage characteristics of this device in the dark and at simulated AM1.5G full-sun illumination are presented. The device exhibits strong photo-response and diode rectification in the dark and light. The device also exhibits a significant photovoltaic effect with a short circuit current of 0.58 mA/cm², open circuit voltage of 0.41 V, and fill factor of 0.40. The solar cells are distinguished from conventional thin film heterojunction cells by their lack of organic components. The devices are electrically insulating in the dark and show a dramatic rise in conductivity under illumination. The photoconductive effect suggests that these materials, like their organic counterparts, have an extremely limited number of untrapped carriers in the dark and are better characterized by a rigid band model than one employing band bending. The researchers propose a mechanism for photovoltaic conversion based on donor-acceptor charge transfer. The photoexcitations that probe the CdTe/CdSe junction experience an energetic driving force for charge transfer, with holes finding lower energy states in the CdTe and electrons finding lower states in the CdSe. Carrier extraction is driven not by means of a built-in field created from a depletion region of substitutional dopants; rather, extraction is primarily driven by directed diffusion, as dictated by the type II heterojunction. The solar cells demonstrate a significant enhancement in carrier creation and extraction due to the presence of a charge transfer interface within the device. The results show that the photoaction of these devices is based on a donor-acceptor junction rather than a conventional planar p-n junction. The researchers also found that sintering the nanocrystal films enhances the performance of these devices, allowing forResearchers at Lawrence Berkeley National Laboratory have developed air-stable all-inorganic nanocrystal solar cells processed from solution. These ultrathin, solution-processed solar cells are composed entirely of inorganic nanocrystals and demonstrate power conversion efficiencies approaching 3% in initial tests. The devices are stable in air and offer a new class of photovoltaic devices with potential for stable, low-cost power generation. The solar cells utilize rod-shaped cadmium selenide (CdSe) and cadmium telluride (CdTe) nanocrystals, synthesized and prepared separately. A schematic energy diagram illustrates the staggered band alignment of this prototypical donor-acceptor pair. The nanocrystals were spin-cast from a filtered pyridine solution, allowing for the creation of ultra-thin, flexible films of densely packed nanocrystals on virtually any substrate. The devices were fabricated by sequentially spin-casting films of CdTe followed by CdSe on indium tin oxide (ITO) glass. Thermally deposited aluminum was used as a reflective top contact. The photoaction spectrum of a typical bilayer cell reveals features from both the CdSe and CdTe absorption spectra, demonstrating that both components contribute to the photocurrent. Current-voltage characteristics of this device in the dark and at simulated AM1.5G full-sun illumination are presented. The device exhibits strong photo-response and diode rectification in the dark and light. The device also exhibits a significant photovoltaic effect with a short circuit current of 0.58 mA/cm², open circuit voltage of 0.41 V, and fill factor of 0.40. The solar cells are distinguished from conventional thin film heterojunction cells by their lack of organic components. The devices are electrically insulating in the dark and show a dramatic rise in conductivity under illumination. The photoconductive effect suggests that these materials, like their organic counterparts, have an extremely limited number of untrapped carriers in the dark and are better characterized by a rigid band model than one employing band bending. The researchers propose a mechanism for photovoltaic conversion based on donor-acceptor charge transfer. The photoexcitations that probe the CdTe/CdSe junction experience an energetic driving force for charge transfer, with holes finding lower energy states in the CdTe and electrons finding lower states in the CdSe. Carrier extraction is driven not by means of a built-in field created from a depletion region of substitutional dopants; rather, extraction is primarily driven by directed diffusion, as dictated by the type II heterojunction. The solar cells demonstrate a significant enhancement in carrier creation and extraction due to the presence of a charge transfer interface within the device. The results show that the photoaction of these devices is based on a donor-acceptor junction rather than a conventional planar p-n junction. The researchers also found that sintering the nanocrystal films enhances the performance of these devices, allowing for