Temperature Dependence of Ideality Factors in Organic Solar Cells and the Relation to Radiative Efficiency

Abstract

provides information on which types of recombination are ruling the loss of voltage at a specifi c illumination intensity. Several efforts in literature aim at determining the ideality factor by steady state techniques with the objective to also understand the dominant recombination mechanism. Currently there is, however, a substantial disagreement of the reported values of the ideality factor for several identical materials studied [ 3 ] and literature related to organic photovoltaic cells (OPV) ideality factors currently provides values ranging from 0.6 [ 4 ] via unity [ 5 ] up to values as high as 20. [ 6 ] This large range is surely doubtful to be correct. The Shockley diode equation is of fundamental importance for solar cells as it solely describes the general voltage and temperature dependent recombination processes.The ideal diode equationwhere q is the electronic charge and k the Boltzmann constant, is therefore actually ruled only by just one material dependent factor, that is the prefactor to the exponential Boltzmann term, frequently referred to as the dark saturation current J 0 . For the ideal diode, J 0 therefore must entail all information related to charge carrier recombination in the photovoltaic material used. The term "dark saturation current" originates from earlier belief of experimentally assigning its value by simply reverse biasing the diode, and measuring the saturated low current value. However, the prefactor J 0 has in fact a much deeper physical meaning, which also bears very little relation to just reverse biasing of a diode. A much better term for J 0 would in the ideal diode case instead be "thermal equilibrium recombination current" as pointed out recently by Cuevas. [ 7 ] Its value is then exclusively determined by the material properties (i.e., bandgap) and the temperature of the material. Hence, in the ideal description of Equation ( 1) the value of the equilibrium recombination current J 0 is therefore always independent of external factors such as illumination or applied voltage, but on the other hand strongly dependent on temperature. However, if it is the case that J 0 in itself actually also depends on said external conditions, the term "thermal equilibrium recombination current" becomes less useful, as it is no longer a real constant. As the signifi cance of J 0 partly lied in it being a constant, Complementary electroluminescence measurements attest that the current density dependence of the ability of the solar cell to emit light is better correlated to the series resistance free ideality factor. For the studied set of OPV devices it is observed that the ideality factors are quite low, and with very weak temperature dependence. The J SC -V OC method to determine ideality factors further allows good estimates of activation energies as well as recombination current prefactors J 00 . The fi ndings imply that the principal OPV non-radiative recombination mechanism is not recombination of free carriers with trapped carriers in an exponential density of tail states as previou...

Origin of Open-Circuit Voltage Loss in Polymer Solar Cells and Perovskite Solar Cells

“…24,[62][63][64][65] In this analysis, J 0,rad was calculated by Equation (3) with E g eff (E CT = E g eff ) because qV OC converges E g eff in accordance with Equation (1). As shown in Figure 4 and Table 1, the ∆qV temp at room temperature is evaluated to be ≈0.…”

“…24,[62][63][64][65] On the other hand, ∆qV non was evaluated to be ≈0.2-0.3 eV for polymer solar cells and ≈0.2 eV for perovskite solar cells. In summary, ∆qV non is the major loss channel for polymer solar cells while ∆qV rad is the major loss channel for perovskite solar cells.…”

“…In the modified SQ theory, η A (E) consists of not only the absorption of bulk materials for photon energies above the E g (η A Bulk ≈ 1), but also the absorption of the CT state in donor/acceptor blend for E CT < E < E g (typically, η A CT ≈ 10 −6 -10 −4 ). 24,[62][63][64][65] Hence, the absorption profile can be given by two step functions:…”

Reduced voltage losses yield 10% efficient fullerene free organic solar cells with >1 V open circuit voltages

“…As such, a key challenge for organic solar cells is increased the EQE EL which is viable with the suppression of such non-radiative recombination losses. 19,21,22 Finding a donor:acceptor combination which simultaneously has minimized energetic offset for charge separation (Δ E CS ) and high EQE EL (>10 –4 ) with suppressed non-radiative recombination losses (<0.3 V) along with a favourable nanoscale morphology for reduced non-geminate recombination to give high EQE and fill factors (FFs), is still a challenge in OPV. Small molecule acceptors (SMA) have emerged as strong alternatives to fullerene derivatives in organic solar cells in the last couple of years.…”

Discriminating between surface and bulk recombination in organic solar cells by studying the thickness dependence of the open-circuit voltage

“…In many cases, it may be possible to identify the dominant recombination mechanism, i.e. Shockley-Read-Hall, radiative or Auger, from an analysis of luminescence yield 12 , luminescence transients 13,14 , ideality factor [15][16][17][18][19] , doping level 17,20,21 , and numerical simulations 22,23 . However, distinguishing between recombination at the interface between absorber and electrodes (surface recombination) and recombination in the bulk of the absorber (bulk recombination) can be extremely challenging 24,25 .…”