Scaling Up ITO‐Free Solar Cells

Abstract

A recent development, which happens to be also one of the most important conclusions, in the fi eld of organic photovoltaics (OPVs) is that indium tin oxide (ITO) should be omitted as the transparent electrode in organic solar cells. The high price of ITO is driven by the limited amount of indium available on the planet, thereby hindering low-cost photovoltaic energy conversion. Moreover, the brittleness of ITO limits the mechanical stability of fl exible organic solar cells. There are several alternatives for ITO that have been investigated; these include graphite, [1][2][3][4] carbon nanotubes, [ 1,[5][6][7] and highly conducting polymers, e.g., poly(3,4ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). [8][9][10] However, the conductivity of these materials is not high enough for usage in large-area devices. To decrease resistive losses during up-scaling, metallic grids can be included in the structure of organic solar cells. The combination of a metal grid and a layer of highly conductive PEDOT:PSS seems to be a very promising alternative for the commonly used ITO electrode. [11][12][13][14][15] The metal current-collecting grids can be produced by different methods; for example, they can be printed, [ 11,14,[16][17][18] produced using evaporation with a shadow mask, [ 19 ] or structured via photolithography. [ 12 ] These production methods enable a design freedom of the grid geometry. In order to reduce resistive losses, the geometry of the grids needs to be optimized. The design of current-collecting grids is a compromise between shading losses and resistive losses, caused by series resistances. This trade-off has been theoretically optimized for several device architectures, including ITO-free concepts that have been presented by the Fraunhofer Institute for Solar Energy Systems (ISE). [ 18,[20][21][22] Cheknane [ 23 ] has performed a comparative study between a linear and a circular geometry and has demonstrated an enhanced effi ciency for the circular geometry. From another study [ 13 ] comparing linear and honeycomb grid structures, it was clear that with the same grid spacing the honeycomb structures provide higher shading losses and reduced short-circuit current densities ( J SC ). With identical grid surface coverage for lines and honeycomb patterns, the line pattern has a smaller grid spacing, providing a better fi ll factor (FF). Experimental and theoretical studies on the relationship between grid spacing and effi ciency of polymer solar cells with linear grid structures has also been presented; [ 12 ] these studies showed that the efficiency of the cells increases with decreasing grid spacing, and at some point, the effi ciency starts to drop due to shading losses as expected from the theoretical work. Thus, for high-conductivity PEDOT:PSS, with a sheet resistance of 500 Ω / ᮀ in combination with 325-μ m-wide grid lines, the optimal grid spacing is 2.5-3.3 mm for a cell size of 2 cm × 2 cm. Decreasing the width of the grid to 180 μ m increases the effi ciency and shifts the optimum grid ...

Describing the light intensity dependence of polymer:fullerene solar cells using an adapted Shockley diode model

“…Next the third order term was ignored and the constant terms, the terms linear with I ill and the terms quadratic in I ill , were collected. The expressions for the constants a, b and c are too large to display them in this paper, but using these expressions the current density could indeed be expressed in terms of I ill and I ill 2 similar to the right side of eqn (10). To show the validity of the result, Fig.…”

Investigation of morphology and physical properties of modified PEDOT: PSS films made via in-situ grafting method

“…This is because the most frequently-used transparent electrode in emerging solar cells, indium tin oxide (ITO), despite its intrinsic advantages, such as high transparency and low sheet resistance ($10 V/sq) [9], needs the scarce and expensive element indium restricting its industry-scale production. Moreover, the brittleness of ITO [10], and the need for vacuum-based deposition techniques, such as sputtering, are not in-line with the concept of roll-to-roll processing from a solution, something which is expected from the OSCs and other similar emerging solar cells to offer [11]. Besides, it has been argued that indium can diffuse into the active layer of OSCs causing degradation of the active layer and deterioration of the solar cell performance [12].…”

A Silver‐Free, Reflective Substrate Electrode for Electron Extraction in Top‐Illuminated Organic Photovoltaics

“…[2] This is because doped conducting oxide electrodes are relatively costly to fabricate, being poorly compatible with highspeed roll-to-roll manufacturing processes, which require compatibility with flexible substrates. [1] One promising approach to address this problem is to fabricate OPVs with a top-illuminated architecture by using an optically thin Ag electrode as the window electrode. [17,18] This approach not only removes the need for a conducting oxide window electrode but also gives more flexibility in terms of the materials used as the supporting substrate, thereby increasing the range of potential applications.…”

“…[1,2] In conventional OPV architectures electrons are extracted to the external circuit by the reflective metal electrode, which is deposited onto a photoactive organic semiconductor layer supported on a transparent electrode, most commonly tin-doped indium oxide (ITO)-coated glass. [3] For efficient electron extraction the reflective metal electrode is preferably a low work function (F) metal such as Ca or Mg, although parasitic optical absorption by these metals is known to reduce the photocurrent in OPVs, so Al and Ag are most often used.…”