Fluorinated Copolymer PCPDTBT with Enhanced Open-Circuit Voltage and Reduced Recombination for Highly Efficient Polymer Solar Cells

Abstract

A novel fluorinated copolymer (F-PCPDTBT) is introduced and shown to exhibit significantly higher power conversion efficiency in bulk heterojunction solar cells with PC(70)BM compared to the well-known low-band-gap polymer PCPDTBT. Fluorination lowers the polymer HOMO level, resulting in high open-circuit voltages well exceeding 0.7 V. Optical spectroscopy and morphological studies with energy-resolved transmission electron microscopy reveal that the fluorinated polymer aggregates more strongly in pristine and blended layers, with a smaller amount of additives needed to achieve optimum device performance. Time-delayed collection field and charge extraction by linearly increasing voltage are used to gain insight into the effect of fluorination on the field dependence of free charge-carrier generation and recombination. F-PCPDTBT is shown to exhibit a significantly weaker field dependence of free charge-carrier generation combined with an overall larger amount of free charges, meaning that geminate recombination is greatly reduced. Additionally, a 3-fold reduction in non-geminate recombination is measured compared to optimized PCPDTBT blends. As a consequence of reduced non-geminate recombination, the performance of optimized blends of fluorinated PCPDTBT with PC(70)BM is largely determined by the field dependence of free-carrier generation, and this field dependence is considerably weaker compared to that of blends comprising the non-fluorinated polymer. For these optimized blends, a short-circuit current of 14 mA/cm(2), an open-circuit voltage of 0.74 V, and a fill factor of 58% are achieved, giving a highest energy conversion efficiency of 6.16%. The superior device performance and the low band-gap render this new polymer highly promising for the construction of efficient polymer-based tandem solar cells.

Impact of Backbone Fluorination on π-Conjugated Polymers in Organic Photovoltaic Devices: A Review

“…Indeed, the fluorine atoms need to be introduced before the functionalization step [10,[19][20][21][22][23][24][25]. For instance, if we consider two of the most common building blocks in the conjugated polymer field, namely the thiophene electron-donating group [26][27][28][29][30][31][32] and the 2,1,3-benzothiadiazole [11,[32][33][34][35][36][37][38][39][40][41][42] electron-accepting group, their fluorinated derivatives requires a minimum of three and one additional steps, respectively (see Scheme 1).…”

“…Polymers 2016, 8,11 3 of 27 [11,[32][33][34][35][36][37][38][39][40][41][42] electron-accepting group, their fluorinated derivatives requires a minimum of three and one additional steps, respectively (see Scheme 1).…”

Understanding the Impact of Hierarchical Nanostructure in Ternary Organic Solar Cells

“…All of the ternary blends exhibit purer BDT‐3T‐CNCOO:PC 71 BM and PBDTTT‐C‐T:PC 71 BM domains than the corresponding binary blend. Purer domain results in a reduced bimolecular recombination 47, 53. This finding also explains why ternary blends show higher hole mobility and FF than binary blends.…”

Low band-gap polymers incorporating benzotriazole and 5,6-dialkoxy-benzothiadiazole as solution processable electrochromic materials

“…To tune the physical properties of the functional polymer, e.g., varying the frontier orbital energy level and absorption behaviour, a large number of electron-withdrawing building blocks have been synthesized based on the similar chemical structure of benzothiadiazole. Prevailing chemical strategies to modify the electronic property of BT unit include a) replacement of the S atoms to O [12][13][14][15], -NR [10,16], -CR 2 [17,18], Se [19,20], Te [20]; b) replacement of the C-H part with N [21][22][23], C-F [24][25][26], C-NO 2 [27], C-OR [28,29] and c) extension of the !-conjugation [30][31][32][33][34][35][36][37]. Out of all the building blocks available in the literatures, those with flexible chains for enhanced solubility of corre- [38].…”