The two major losses that occur in solar cells are the subband-gap transmission and the thermalization of the hot charge carriers.[1] One way to circumvent both effects simultaneously is the realization of tandem solar cells. Indeed, stacking series-connected sub-cells allows to achieve efficiencies as high as 32 %, [2] which is higher than the Shockley-Queisser limitation for single p-n solar cells. [3] Alike its inorganic counterpart, the organic solar cell community is seeking for highperformance devices and, hence, is investigating the tandem approach.

Bulk heterojunction solar cells have attracted considerable attention over the past several years due to their potential for low-cost photovoltaic technology. The possibility of manufacturing modules via a standard printing/coating method in a roll-to-roll process in combination with the use of low-cost materials will lead to a watt-peak price of less than 1 US$ within the next few years. [1] Despite the low-cost potential, the power conversion efficiency of bulk heterojunction devices is low compared to inorganic solar cells. Efficiencies in the range of 5-6% have been certified at NREL and AIST usually on devices with small active areas.[2]The current understanding of bulk heterojunction solar cells suggests that the maximum efficiency is in the range of 10-12%.[3] Several reasons for the power conversion efficiency limitation have been identified.[1] Some of the prerequisites for achieving highest efficiencies are donor and acceptor materials with optimized energy levels [highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO)], efficient charge transport in the donor-acceptor blend, efficient charge generation and limited recombination losses. Power conversion efficiency is strongly dependent on charge transport and charge generation, which are dominated by the phase behavior of the donor and acceptor molecules. The resulting, and often unfavorable, nanomorphology of this two-component blend limits the power conversion efficiency of bulk heterojunction solar cells. Precise control of the nanomorphology is very difficult and has been achieved only for a few systems. [4][5][6] The relation between the chemical structure of donor and acceptor materials and the nanomorphology that they form when they are blended is currently not well understood, and as will be shown in this paper, minor changes in the chemical structure can cause major changes in the performance of the materials in organic solar cells.

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Organic photovoltaics in a flexible wire format has potential advantages that are described in this paper. A wire format requires long-distance transport of current that can be achieved only with conventional metals, thus eliminating the use of transparent oxide semiconductors. A phase-separated, photovoltaic layer, comprising a conducting polymer and a fullerene derivative, is coated onto a thin metal wire. A second wire, coated with a silver film, serving as the counter electrode, is wrapped around the first wire. Both wires are encased in a transparent polymer cladding. Incident light is focused by the cladding onto to the photovoltaic layer even when it is completely shadowed by the counter electrode. Efficiency values of the wires range from 2.79% to 3.27%.

In this work, we report efficient semitransparent perovskite solar cells using solution-processed silver nanowires (AgNWs) as top electrodes. A thin layer of zinc oxide nanoparticles is introduced beneath the AgNWs, which fulfills two essential functionalities: it ensures ohmic contact between the PC 60 BM and the AgNWs and it serves as a physical foundation that enables the solution-deposition of AgNWs without causing damage to the underlying perovskite. The as-fabricated semitransparent perovskite cells show a high fill factor of 66.8%, V oc = 0.964 V, J sc = 13.18 mA cm −2 , yielding an overall efficiency of 8.49% which corresponds to 80% of the reference devices with reflective opaque electrodes.Inorganic-organic halide perovskite solar cells have recently emerged as a promising photovoltaic technology due to their high efficiencies and low-cost processing potential. [1][2][3][4] The exceptional optoelectronic properties of the perovskite crystals such as high carrier mobility and long charge diffusion length promise highly efficient charge separation. 5,6 These intriguing characteristics make perovskites ideal materials for photovoltaic applications. Since the first device demonstration in 2009, power conversion efficiency (PCE) of perovskite solar cells processed by both vacuum-deposition and solutionprocessing has surged to over 15%. 2,4,[7][8][9] The continuous and fast progress in the research related to perovskite solar devices has established them as a serious contestant to the traditional silicon-based panels.Together with the considerable efforts devoted to pursuing high efficiencies via improved crystallization of perovskite and searching for low-cost interface materials, 4,10-13 aesthetic semitransparent perovskite solar cells have been simultaneously receiving growing attention because of their specific application in transparent architectures, 14-17 such as windows, rooftops, greenhouses and other fashion elements. To achieve efficient semitransparent perovskite devices, both the anode and the cathode of the devices should be highly transparent and conductive in order to minimize the optical and resistance losses. To date, several studies have reported semitransparent perovskite solar cells, but most of these devices employed thin metal films (Al, Ag, Au) as top electrodes which were fabricated based on energy-intensive evaporation processes. [15][16][17] It is well known that, in addition to low-cost materials, the cost reduction of photovoltaic devices substantially depends on the ability to use high-throughput coating techniques in combination with roll-to-roll processing. 18 Despite its importance, however, less attention has been paid to the exploration of solution-processable transparent electrodes for perovskite solar cells. Carbon based materials have received much attention for use as conducting electrodes for perovskite solar cells, due to their low-cost and high stability. 14,19,20 For example, Li et al.have recently reported semitransparent perovskite solar cells using carbon nanotub...

Carbon bridged (C‐PCPDTBT) and silicon‐bridged (Si‐PCPDTBT) dithiophene donor–acceptor copolymers belong to a promising class of low bandgap materials. Their higher field‐effect mobility, as high as 10−2 cm2 V−1 s−1 in pristine films, and their more balanced charge transport in blends with fullerenes make silicon‐bridged materials better candidates for use in photovoltaic devices. Striking morphological changes are observed in polymer:fullerene bulk heterojunctions upon the substitution of the bridging atom. XRD investigation indicates increased π–π stacking in Si‐PCPDTBT compared to the carbon‐bridged analogue. The fluorescence of this polymer and that of its counterpart C‐PCPDTBT indicates that the higher photogeneration achieved in Si‐PCPDTBT:fullerene films (with either [C60]PCBM or [C70]PCBM) can be correlated to the inactivation of a charge‐transfer complex and to a favorable length of the donor–acceptor phase separation. TEM studies of Si‐PCPDTBT:fullerene blended films suggest the formation of an interpenetrating network whose phase distribution is comparable to the one achieved in C‐PCPDTBT:fullerene using 1,8‐octanedithiol as an additive. In order to achieve a balanced hole and electron transport, Si‐PCPDTBT requires a lower fullerene content (between 50 to 60 wt%) than C‐PCPDTBT (more than 70 wt%). The Si‐PCPDTBT:[C70]PCBM OBHJ solar cells deliver power conversion efficiencies of over 5%.

Perovskite hybrid solar cells (pero-HSCs) were demonstrated to be amongst the most promising candidates within the emerging photovoltaic materials with respect to their power conversion efficiency (PCE) and inexpensive fabrication. Further PCE enhancement mainly relies on minimizing the interface losses via interface engineering and the quality of the perovskite film. Here we demonstrate that the PCEs of pero-HSCs is significantly increased to 14.0% by incorporation of a solution-processed perylene diimide (PDINO) as cathode interface layer between [6,6]-phenyl-C61 butyric acid methyl ester (PCBM) layer and the top Ag electrode. Notably, for PDINO based devices, prominent PCEs over 13% are achieved within a wide range of the PDINO thickness (5-24 nm). Without the PDINO layer, the best PCE of the reference PCBM/Ag device was only 10.0%. The PCBM/PDINO/Ag devices also outperformed the PCBM/ZnO/Ag devices (11.3%) with the well-established zinc oxide (ZnO) cathode interface layer. This enhanced performance is due to the formation of a highly qualitative contact between PDINO and the top Ag electrode, leading to reduced series resistance (R s ) and enhanced shunt resistance (R sh ) values. This study opens the door for the integration of a new class of easy-accessible, solution-processed high performance interfacial materials for pero-HSCs.

substrates accounts for 90% of the total energy required for manufacturing OPV. [ 8 ] To achieve ITO-free and fully solutionprocessed organic solar cells, the choice and processing of the anode and cathode electrodes are two challenges which require different considerations. Solutionprocessed bottom electrodes based on poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), AgNW and metallic grid electrodes are constantly narrowing the gap to ITO. [9][10][11][12][13][14][15][16] The main challenge for fully solution-processed devices remains the combination of a bottom and a top electrode, because the very thin and/ or soft underlying layers have to resist the solution deposition of the top electrode. Recently, solution-processed graphene, [ 17 ] AgNW, [ 18 , -21 ] silver nanoparticles [ 22 ] and PEDOT:PSS [ 23,24 ] were investigated as transparent top electrodes for organic solar cells. However, devices with incorporation of these top electrodes suffered either from the inferior optoelectronic properties of the electrodes or their multi-transfer processing procedure. On the other hand, aesthetic (semi-)transparent solar cells with special applications in windows, foldable curtains, buildings and clothes, etc., have recently gained much scientifi c attention and are considered to be the highest priority market for OPV. [ 25 ] We report here on materials and processes for reliable and cost-effi cient processing of ITO-free semitransparent organic solar cells from solution. Fully solution-processed organic solar Organic photovoltaic (OPV) solar cells that can be simply processed from solution are in the focus of the academic and industrial community because of their enormous potential to reduce cost. One big challenge in developing a fully solution-processed OPV technology is the design of a well-performing electrode system, allowing the replacement of ITO. Several solution-processed electrode systems were already discussed, but none of them could match the performance of ITO. Here, we report effi cient ITO-free and fully solution-processed semitransparent inverted organic solar cells based on silver nanowire (AgNW) electrodes. To demonstrate the potential of these AgNW electrodes, they were employed as both the bottom and top electrodes. Record devices achieved fi ll factors as high as 63.0%, which is comparable to ITO based reference devices. These results provide important progress for fully printed organic solar cells and indicate that ITO-free, transparent as well as non-transparent organic solar cells can indeed be fully solution-processed without losses.