Nongeminate and Geminate Recombination in PTB7:PCBM Solar Cells

Abstract

A combination of transient photovoltage (TPV), voltage dependent charge extraction (CE) and time delayed collection field (TDCF) measurements is applied to poly [[4,8-bis[(2-ethylhexyl) [3,4-b]thiophenediyl]] (PTB7):[6,6]phenyl-C71-butyric acid (PC 71 BM) bulk heterojunction solar cells to analyze the limitations of photovoltaic performance. Devices are processed from pure chlorobenzene (CB) solution and a subset was optimized with 1,8-diiodooctane (DIO) as co-solvent. The dramatic changes in device performance are discussed with respect to the dominating loss processes. While in the devices processed from CB solution, severe geminate and nongeminate recombination is observed, the use of DIO facilitates efficient polaron pair dissociation and minimizes geminate recombination. Thus, from the determined charge carrier decay rate under open circuit conditions and the voltage dependent charge carrier densities n(V ), the nongeminate loss current j loss of the samples with DIO alone enables us to reconstruct the current/voltage ( j/V ) characteristics across the whole operational voltage range. Geminate and nongeminate losses are considered to describe the j/V response of cells prepared without additive, but lead to a clearly overestimated device performance. We attribute the deviation between measured and reconstructed j/V characteristics to trapped charges in isolated domains of pure fullerene phases.

The Effect of Diiodooctane on the Charge Carrier Generation in Organic Solar Cells Based on the Copolymer PBDTTT-C

“…Recently, Foster et al reported on a similarly reduced geminate recombination but enhanced nongeminate losses, i.e., a reduced charge carrier lifetime, for PTB7:PC 61 BM devices optimized through the use of 3% DIO 53 . The comparable result was found by Foertig et al for PTB7:PC 71 BM devices 54 .…”

“…The occurrence of a fast and slow regime can be understood within the scenario of a recombination dominated by spatial trapping at short times and slowed down thermal detrapping at long times. Thus, the maximum value of λ f in the case of the 0% DIO device agrees well with the scenario of charge carriers spatially trapped inside isolated PC 71 BM devices as suggested by Foertig et al for PTB7:PC 71 BM 54 . The constant λ s for 0%, 1%, and 3% DIO can be interpreted as the slope of an exponential density of states (DOS) E U = λ s · k B T ≈ 140 meV 60 61 , which is independent of the blend morphology.…”

Barrierless Slow Dissociation of Photogenerated Charge Pairs in High-Performance Polymer–Fullerene Solar Cells

“…This indicates that the mobility of dissociating charge pairs is independent of electric field above 12 V/μm and implies a low potential barrier for dissociation of photogenerated charge pairs. This finding is compatible with previous observations that the free carrier yield is independent of electric field. − Assuming a uniform electric field across the active layer which will be discussed in the next section, we can relate the field screening in the time interval τ with the integrated discharge photocurrent of the capacitor and hence with the density of generated charge pairs N ( t ) and the sum of electron and hole mobilities μ­( t ) as where Δ Q = ∫ 0 τ J ( t ) d t is the charge transported by the photocurrent J ( t ), C is the capacitance of the blend, L is its thickness between electrodes, e is the elementary charge, and A is the illuminated area of the sample. The Δ F / F kinetics measured at F = 28 V/μm are well described with eq using the time-dependent carrier mobility μ­( t ) ∝ t –0.6 and only one fitted parameter μ­(1 ps) = 0.12 cm 2 V –1 s –1 .…”

“…This implies a low potential barrier for dissociation of photogenerated charge pairs. This finding is compatible with previous observations that the free carrier yield is independent of electric field. − Because of field-independent mobility, the TREFISH experiment does not distinguish between charge pairs and free carriers.…”

Electron Collection as a Limit to Polymer:PCBM Solar Cell Efficiency: Effect of Blend Microstructure on Carrier Mobility and Device Performance in PTB7:PCBM

“…We have therefore established that the effect of DIO is not to improve the mobility‐lifetime product (and hence charge collection) in PTB7:PCBM. Instead, our findings offer support for the hypothesis in the literature that the effect of DIO is to reduce geminate recombination . Thus, whereas non‐geminate recombination is the primary limiting factor for films both with and without DIO at large film thicknesses, geminate recombination is an important additional loss pathway for films without DIO.…”