Advanced Functional Materials and Devices (AFMD) Group

Abstract:

We present 2,3,10,11-tetrapropyl-1,4,9,12-tetraphenyl-diindeno[1,2,3-cd:1′,2′,3′-lm]perylene (P4-Ph4-DIP) as new green donor for small molecule organic solar cells (SMOSC). P4-Ph4-DIP absorbs in the green spectral range of 490-580 nm and thus is a decisive building block towards broad coverage of the sun spectrum. SMOSC with P4-Ph4-DIP as donor and C60 as blue-absorbing acceptor display very high fill factors FF > 76% and open circuit voltages of 0.99 V, reaching η = 1.92% for 12 nm P4-Ph4-DIP. Current voltage and external quantum efficiency spectra suggest that an exciton diffusion length ED < 20 nm is the main limiting factor. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Abstract:

We detect and measure the trap charges in a small molecular bulk heterojunction solar cell under operating conditions. The trap-charge density is estimated from capacitance measurements with light incident on the sample. At high intensities (∼1sun, 100mW/ cm2), the trapped charge concentration leads to a spatial distortion of the electric field in the device. The lower limit of the trap-charge density is estimated to be 6× 10 16 cm-3. The frequency dependence of the capacitance suggests that the charges are trapped in a manifold of deep states present in the energy gap of the semiconductors. The distortion of the electric field by this trap charge affects the charge-carrier collection efficiency. © 2010 The American Physical Society.

Abstract:

We present small molecule solar cells with α,ω-bis- (dicyanovinylene)-sexithiophene: C60 mixed heterojunctions, reaching power conversion efficiencies of 4.9±0.2%. We use substrate heating during deposition of the mixed layer to achieve an optimized morphology and show that this significantly improves the internal quantum efficiencies (IQEs) to values approaching 70%. By optical modeling, we evaluate the amount of loss due to absorption in inactive layers and show that IQE of the active layer itself is about 80%. © 2010 American Institute of Physics.