Enhanced Output from Biohybrid Photoelectrochemical Transparent Tandem Cells Integrating Photosynthetic Proteins Genetically Modified for Expanded Solar Energy Harvesting

DOI: 10.1002/aenm.201601821 chemical or biochemical linkers for immobilizing proteins on electrodes has been the primary approach to improving electron-transfer, along with the use of alternative electrode/ electrolyte combinations based on energy-level considerations to achieve current enhancement (see refs. [7,11] for reviews). In contrast, there have been almost no attempts to enhance photocurrents by augmenting the natural light-harvesting abilities of photosynthetic bacteria and therefore the spectral range covered in biohybrid devices. A characteristic of natural photosynthetic pigments is that they have strong absorbance bands in some regions of the UV–visible–near-IR spectrum but little or no absorbance in other regions. Plasmonic enhancement of photocurrent generation by purple bacterial RC– LH1 complexes on nanostructured metal electrodes has been achieved,[16] but this does not change the wavelengths of light absorbed. A few attempts have been made to increase the optical absorption cross-section of photosynthetic RCs or LH complexes by attaching tailored molecular fluorophores and photoluminescent quantum dots, but these have not been scaled up for photocurrent generation at a device-level.[17] Fabrication of such partially-synthetic photovoltaic proteins complicates device construction, adds to cost, and can involve the use of materials that are not environmentally friendly or constitute a limited resource. An alternative approach to enhancing spectral coverage is to employ a stacked tandem device architecture in which photovoltaic proteins with natural pigments that have complementary absorption characteristics incorporated into different layers of the device. Here we employ two variants of RC–LH1 complexes which incorporate either the native red carotenoid spheroidenone (RC–LH1red) or the green carotenoids neurosporene, hydroxyneurosporene, and methoxyneurosporene (RC–LH1green) (Figure 1a–c). This is achieved using a “green strain” of Rba. sphaeroides containing a spontaneous mutation in the crtD gene encoding methoxyneurosporene dehydrogenase, which halts carotenoid synthesis prematurely.[18] The two, otherwise identical, pigment–proteins have different absorption characteristics in the blue to yellow region of the visible spectrum (Figure 1d,e and Figure S1 in the Supporting Information), 17 molecules of spheroidenone per complex giving rise to a single broad band between 400 and 600 nm, and 17 molecules of neurosporene and its derivatives to narrower, more intense absorbance between 400 and 500 nm with distinctive maxima at 429, 454, and 485 nm.[19] As the tandem cell architecture requires a transparent rear electrode for the front cell, we have explored the option of using poly(3,4-ethylenedioxythiophene): polystyrene sulfonate (PEDOT:PSS). This transparent polymer Increased solar energy utilization has been forecast as the principal way to meet the growing energy demands of the 21st century in an environmentally benign way. In response, significant efforts are being made to develop novel and costeffective approaches to solar energy conversion that do not compromise environmental security.[1] Given that photosynthesis is the prime process that powers the biosphere,[2,3] the strategies of emulating and directly exploiting natural photosynthetic machineries for solar energy harvesting have attracted substantial interest in recent years.[4,5] At the heart of the photosynthetic process, reaction center (RC) and light harvesting (LH) pigment–protein complexes accomplish the transduction of absorbed light energy through a photochemical charge separation in which an electron is generated for almost every photon absorbed.[5,6] The photosynthetic pigment–proteins of plants, algae, and bacteria have therefore been evaluated for a range of potential optoelectronic, bioelectronic, and photobioelectrochemical devices and applications.[7,8] RCs from purple photosynthetic bacteria such as Rhodobacter (Rba.) sphaeroides,[9] and the larger RC–LH1 complexes they form with the LH1 light harvesting protein,[10] are a popular choice for the construction of photo-bioelectrochemical cells.[7,11,12] Previous studies utilizing these proteins have been aimed at enhancing photocurrent generation by improving the effectiveness of protein–electrode electron transfer processes in three-electrode cells[13] or, to a lesser extent, in two-electrode cells.[14,15] Manipulating protein orientation and the nature of